Current Protocols in Molecular Biology
Frederick M. Ausubel, Roger Brent, Robert E. Kingston, David D. Moore, J.G. Seidman, John A. Smith, Kevin Struhl (eds.) Current Protocols in Molecular Biology Copyright © 2003 John Wiley & Sons, Inc. All rights reserved.
John Wiley & Sons Inc; ringbou edition (December 4, 2003) ISBN: 047150338X
Editors & Contributors
EDITORIAL BOARD Frederick M. Ausubel Massachusetts General Hospital & Harvard Medical School Roger Brent The Molecular Sciences Institute, Berkeley, California Robert E. Kingston Massachusetts General Hospital & Harvard Medical School David D. Moore Baylor College of Medicine J.G. Seidman Harvard Medical School John A. Smith University of Alabama at Birmingham Kevin Struhl Harvard Medical School
GUEST EDITORS Lisa M. Albright Austin, Texas Donald M. Coen Harvard Medical School Carolyn L. Smith Baylor College of Medicine Ajit Varki University of California San Diego Mary C. Wildermuth University of California Berkeley
R. Donald Allison University of Florida Gainesville, Florida Roland Annan SmithKline Beecham King of Prussia, Pennsylvania Anthony T. Annunziato Boston College Chestnut Hill, Massachusetts Oscar M. Aparicio University of Southern California Los Angeles, California
Lonnie D. Adams The Upjohn Company Kalamazoo, Michigan Lisa M. Albright Austin, Texas
E.L. Bearer Brown University Providence, Rhode Island C.R. Bebbington Celltech Slough, United Kingdom Daniel M. Becker Pennie & Edwards Menlo Park, California
Carmen A. Argmann Institut de Génétique et de Biologie Moléculaire et Cellulaire Illkirch, France
Sabine D. Bell University of Texas Austin, Texas
Alejandro Aruffo Bristol-Myers Squibb Princeton, New Jersey
Timothy P. Bender University of Virginia Charlottesville, Virginia
Frederick M. Ausubel Massachusetts General Hospital and Harvard Medical School Boston, Massachusetts
Claude Besmond Hôpital Robert Debré Paris, France
Johan Auwerx Institut de Génétique et de Biologie Moléculaire et Cellulaire and Institut Clinique de la Souris Illkirch, France Omar Bagasra Thomas Jefferson University Philadelphia, Pennsylvania Jim Bahre Fuji Medical Systems, USA Stamford, Connecticut
CONTRIBUTORS Susan M. Abmayr Pennsylvania State University University Park, Pennsylvania
Andreas D. Baxevanis National Human Genome Research Institute Bethesda, Maryland
Albert S. Baldwin, Jr. University of North Carolina Chapel Hill, North Carolina Cynthia Bamdad Clinical Micro Sensors Pasadena, California David Baulcombe John Innes Centre Norwich, United Kingdom
Stephen M. Beverley Harvard Medical School Boston, Massachusetts Michael Bittner National Human Genome Research Institute, NIH Bethesda, Maryland Seth Blackshaw Harvard Medical School Boston, Massachusetts Justin Blethrow University of California at San Francisco San Francisco, California Kenneth D. Bloch Massachusetts General Hospital Boston, Massachusetts Mark S. Boguski National Center for Biotechnology Information Bethesda, Maryland
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Juan S. Bonifacino National Institute of Child Health and Human Development Bethesda, Maryland Angie L. Bookout Howard Hughes Medical Institute University of Texas Southwestern Medical Center Dallas, Texas Ann Boyle Current Protocols Madison, Connecticut Reinhard I. Boysen Monash University Victoria, Australia Allan R. Brasier Univerity of Texas Galveston, Texas Michael Brenowitz Albert Einstein College Bronx, New York Roger Brent The Molecular Sciences Institute Berkeley, California Terry Brown University of Manchester Institute of Science and Technology Manchester, United Kingdom Bernard H. Brownstein Howard Hughes Medical Institute and Washington University School of Medicine St. Louis, Missouri Mark Brunswick Uniformed Services University of the Health Sciences Bethesda, Maryland Mikail Bubunenko National Cancer Institute at Frederick Frederick, Maryland Kim Budelier Qiagen, Inc. Chatsworth, California William Buikema University of Chicago Chicago, Illinois Linda Buonocore Yale University School of Medicine New Haven, Connecticut
Stephen Buratowski Harvard Medical School Boston, Massachusetts Richard R. Burgess University of Wisconsin Madison, Wisconsin Dean Burgi Genomyx Foster City, California Michael C. Byrne Genetics Institute Cambridge, Massachusetts Steven A. Carr SmithKline Beecham King of Prussia, Pennsylvania Andrés E. Carrasco University of Buenos Aires-CONICET School of Medicine Buenos Aires, Argentina Miles W. Carroll Oxford BioMedica Oxford, United Kingdom A. Nigel Carter The Salk Institute La Jolla, California Anthony Celeste Genetics Institute Cambridge, Massachusetts Constance Cepko Harvard Medical School Boston, Massachusetts Bulbul Chakravarti Keck Graduate Institute of Applied Life Sciences Claremont, California Deb N. Chakravarti Keck Graduate Institute of Applied Life Sciences Claremont, California Marie-France Champy Institut Clinique de la Souris Illkirch, France Lawrence Chan Baylor College of Medicine Houston, Texas Yuan Chang Hillman Cancer Center University of Pittsburgh Pittsburgh, Pennsylvania
David D. Chaplin Howard Hughes Medical Institute and Washington University School of Medicine St. Louis, Missouri Claudia A. Chen National Institute of Mental Health Bethesda, Maryland Jin-Long Chen Tularik, Inc. South San Francisco, California J. Michael Cherry Stanford University School of Medicine Stanford, California Jonathan D. Chesnut Invitrogen Corporation Carlsbad, California Cheng-Ming Chiang Case Western Reserve University School of Medicine Cleveland, Ohio Lewis A. Chodosh University of Pennsylvania Philadelphia, Pennsylvania Piotr Chomczynski University of Cincinnati College of Medicine Cincinnati, Ohio Joanne Chory The Salk Institute for Biological Studies La Jolla, California Valentina Ciccarone Life Technologies, Inc. Rockville, Maryland Donald M. Coen Harvard Medical School Boston, Massachusetts Nelson B. Cole University of Pennsylvania Philadelphia, Pennsylvania Robert N. Cole The Johns Hopkins University Medical School Baltimore, Maryland Thomas Coleman Fox Chase Cancer Center Philadelphia, Pennsylvania John E. Coligan National Institute of Allergy and Infectious Diseases Bethesda, Maryland
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Donald Coling University of California, Berkeley Berkeley, California
University of Texas Southwestern Medical Center Dallas, Texas
Dennis Dowhan Baylor College of Medicine Houston, Texas
Martine A. Collart Harvard Medical School Boston, Massachusetts
Rita Das Harvard Medical School Boston, Massachusetts
Tingting Du University of Massachusetts Medical School Worcester, Massachusetts
James F. Collawn The Salk Institute La Jolla, California
Simanti Datta National Cancer Institute at Frederick Frederick, Maryland
Allan Duby Medical City Dallas, Texas
David A. Conner Harvard Medical School Boston, Massachusetts
Pieter J. de Jong Children’s Hospital Oakland Research Institute Oakland, California
Barbara Dunn GenPharm International Mountain View, California
H. Edward Conrad University of Illinois Urbana, Illinois Nina Costantino National Cancer Institute at Frederick Frederick, Maryland Darryl Conte Jr. University of Massachusetts Medical School Worcester, Massachusetts Helen M. Cooper Ludwig Institute for Cancer Research Melbourne, Australia Norman Cooper National Institute of Allergy and Infectious Diseases Bethesda, Maryland Lynn M. Corcoran Walter and Eliza Hall Institute Victoria, Australia Brendan Cormack Harvard Medical School Boston, Massachusetts Nina Costantino National Cancer Institute at Frederick Frederick, Maryland Michele Cottler-Fox University of Maryland Greenebaum Cancer Center Baltimore, Maryland Donald L. Court National Cancer Institute at Frederick Frederick, Maryland Francis Cruz Brigham and Women’s Hospital Boston, Massachusetts Carolyn L. Cummins Howard Hughes Medical Institute
Job Dekker University of Massachusetts Medical School Worcester, Massachusetts Esteban C. Dell’Angelica National Institute of Child Health and Human Development Bethesda, Maryland Joseph DeRisi University of California, San Francisco San Francisco, California Sandra Diaz University of California, San Diego La Jolla, California Andree Dierich Institut de Génétique et de Biologie Moléculaire et Cellulaire and Institut Clinique de la Souris Illkirch, France Michael P. DiGiovanna Yale University School of Medicine New Haven, Connecticut Jack E. Dixon University of California at San Diego La Jolla, California Tamara L. Doering Cornell University Medical College New York, New York Robert L. Dorit Yale University New Haven, Connecticut David Dorris Harvard Medical School Boston, Massachusetts Josée Dostie University of Massachusetts Medical School Worcester, Massachusetts
Ben Dunn University of Florida Gainesville, Florida Christopher D. Earl Plant Resources Venture Fund Cambridge, Massachusetts Patricia L. Earl National Institute of Allergy and Infectious Diseases Bethesda, Maryland Richard L. Eckert Case Western Reserve School of Medicine Cleveland, Ohio Diane G. Edmondson M.D. Anderson Cancer Center Houston, Texas Karen L. Elbing Clark and Elbing LLP Boston, Massachusetts Elaine A. Elion Harvard Medical School Boston, Massachusetts Andrew Ellington University of Texas Austin, Texas Orna Elroy-Stein Tel Aviv University Tel Aviv, Israel Isam-Eldin Eltoum University of Alabama at Birmingham Birmingham, Alabama JoAnne Engebrecht State University of New York Stony Brook, New York Paul T. Englund Johns Hopkins Medical School Baltimore, Maryland
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Douglas J. Epstein University of Pennsylvania School of Medicine Philadelphia, Pennsylvania Jeffrey D. Esko University of Alabama Birmingham, Alabama Michael J. Evelegh ADI Diagnostics Rexdale, Ontario Hua-Ying Fan Massachusetts General Hospital Boston, Massachusetts Rhonda Feinbaum Massachusetts General Hospital Boston, Massachusetts Gregg B. Fields Florida Atlantic University Boca Raton, Florida Russell L. Finley, Jr. Wayne State University School of Medicine Detroit, Michigan Michael Finney MJ Research Watertown, Massachusetts Achim Fischer F. Hoffmann-La Roche AG Basel, Switzerland Thomas A. Fleisher Warren Grant Magnuson Clinical Center Bethesda, Maryland Maximillian T. Follettie Genetics Institute Cambridge, Massachusetts Susan L. Forsburg Molecular and Cell Biology Laboratory The Salk Institute for Biological Studies La Jolla, California John J. Fortin Tropix, Inc. Bedford, Massachusetts Cecil H. Fox Molecular Histology Laboratory Gaithersburg, Maryland Nicole Francis Massachusetts General Hospital Boston, Massachusetts Paula Franco University of Buenos Aires-CONICET
School of Medicine Buenos Aires, Argentina Verna Frasca Amersham Pharmacia Biotech Piscataway, New Jersey Hudson H. Freeze The Burnham Institute La Jolla, California Eirik Frengen The Biotechnology Centre of Oslo University of Oslo Oslo, Norway Andra R. Frost University of Alabama at Birmingham Birmingham, Alabama Michiko N. Fukuda La Jolla Cancer Research Foundation La Jolla, California Minoru Fukuda La Jolla Cancer Research Foundation La Jolla, California Steven A. Fuller Nabi Rockville, Maryland Dmitry V. Fyodorov University of California, San Diego La Jolla, California Sean R. Gallagher UVP, Inc. Upland, California Subinay Ganguly SmithKline Beecham King of Prussia, Pennsylvania Yuan Gao Deakin University Victoria, Australia Henrik Garoff Karolinska Institute, Novum Research Center Huddinge, Sweden Paul A. Garrity University of California, Los Angeles Los Angeles, California Anke Geick Ribopharma AG Kulmbach, Germany Joseph V. Geisberg Harvard Medical School Boston, Massachusetts
David H. Gelfand Roche Molecular Systems Alameda, California C. Ronald Geyer University of Florida Gainesville, Florida Nele Gheldof University of Massachusetts Medical School Worcester, Massachusetts Chris Gilbert Cancer Research UK London Research Institute Clare Hall Laboratory Hertfordshire, U.K. Michael Gilman Cold Spring Harbor Laboratory Cold Spring Harbor, New York Warren Gish Washington University School of Medicine St. Louis, Missouri Arthur Glatfelter National Human Genome Research Institute, NIH Bethesda, Maryland Jane Glazebrook University of Minnesota St. Paul, Minnesota Erica A. Golemis Fox Chase Cancer Center Philadelphia, Pennsylvania Chris Gooden National Human Genome Research Institute, NIH Bethesda, Maryland Gregory A. Grant Washington University School of Medicine St. Louis, Missouri Michael E. Greenberg Harvard Medical School Boston, Massachusetts John M. Greene Massachusetts General Hospital Boston, Massachusetts David Greenstein Massachusetts General Hospital Boston, Massachusetts Mitchell S. Gross SmithKline Beecham King of Prussia, Pennsylvania
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Barbara Grossmann Amersham Life Science Cleveland, Ohio
Milton T. W. Hearn Monash University Victoria, Australia
Charles B-C. Hwang Harvard Medical School Boston, Massachusetts
Stefan Grünwald Pharmingen San Diego, California
Olaf Heidenreich University of Tübingen Tübingen, Germany
James Gulick Cincinnati Children’s Hospital University of Cincinnati Cincinnati, Ohio
Joseph S. Heilig University of Colorado Boulder, Colorado
Panayiotis A. Ioannou The Murdoch Institute for Research into Birth Defects Royal Children’s Hospital Melbourne, Australia
Tod Gulick Massachusetts General Hospital Charleston, Massachusetts Jeno Gyuris Mitotix, Inc. Cambridge, Massachusetts Philipp Hadwiger Ribopharma AG Kulmbach, Germany Lars Hagel Amersham Pharmacia Biotech Uppsala, Sweden John Hanson MJ Research Watertown, Massachusetts Anna Haramis EMBL Heidelberg, Germany Gerald W. Hart The Johns Hopkins University Medical School Baltimore, Maryland Grant Hartzog Harvard Medical School Boston, Massachusetts Robert G. Hawley American Red Cross Rockville, Maryland and The George Washington University Washington, D.C. Pamela Hawley-Nelson Life Technologies, Inc. Rockville, Maryland Bradley K. Hayes University of Alabama Birmingham, Alabama Jeffrey J. Hayes University of Rochester Medical Center Rochester, New York
Peter Heinrich Consortium für Elektrochemische Industrie Munich, Germany William J. Henzel Genentech, Inc. South San Francisco, California David E. Hill Applied Biotechnology Cambridge, Massachusetts James P. Hoeffler Invitrogen Corporation Carlsbad, California Timothy Hoey University of California, Berkeley Berkeley, California Charles S. Hoffman Boston College Chestnut Hill, Massachusetts Diane Hollenbaugh Bristol-Myers Squibb Seattle, Washington Shelley B. Hoover Molecular Histology Laboratory Gaithersburg, Maryland Peter Hornbeck University of Maryland Baltimore, Maryland Hayden Huang Brigham and Women’s Hospital Boston, Massachusetts Helen M. Huls Baylor College of Medicine Houston, Texas Tony Hunter The Salk Institute for Biological Studies La Jolla, California John G.R. Hurrell FluorRx Carmel, Indiana
Nina Irwin Cambridge, Massachusetts Vishwanath R. Iyer University of Texas at Austin Austin, Texas Kenneth A. Jacobs Genetics Institute Cambridge, Massachusetts Yongsu Jeong University of Pennsylvania School of Medicine Philadelphia, Pennsylvania Timm Jessen Hoechst AG Frankfurt am Main, Germany Sulay D. Jhaveri University of Texas Austin, Texas Yuan Jiang National Human Genome Research Institute, NIH Bethesda, Maryland Matthias John Ribopharma AG Kulmbach, Germany Stephen Albert Johnston University of Texas Southwestern Medical Center Dallas, Texas Bryan Jones Bristol-Myers Squibb Seattle, Washington Jingyue Ju Incyte Pharmaceuticals Palo Alto, California Bechara Kachar National Institute on Deafness and Other Communication Disorders Bethesda, Maryland Mustak A. Kaderbhai University College of Wales Penglais, United Kingdom
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James T. Kadonaga University of California, San Diego La Jolla, California
Joan H.M. Knoll Harvard Medical School Boston, Massachusetts
Jason A. Kahana Dana-Farber Cancer Institute Boston, Massachusetts
Mikhail G. Kolonin Wayne State University School of Medicine Detroit, Michigan
Steven R. Kain Clontech Laboratories Palo Alto, California
Elena Kotova Fox Chase Cancer Center Philadelphia, Pennsylvania
Fumiaki Katagiri University of Minnesota St. Paul, Minnesota
Martha F. Kramer Harvard Medical School Boston, Massachusetts
Randal J. Kaufman University of Michigan Ann Arbor, Michigan
Nandini Krishnamurthy University of California Berkeley, California
Ning Ke Iowa State University Ames, Iowa
Anuj Kumar Yale University New Haven, Connecticut
Marie Keaveney Harvard Medical School Boston, Massachusetts
Thomas A. Kunkel National Institute of Environmental Health Sciences Research Triangle Park, North Carolina
Anthony D. Keefe Massachusetts General Hospital Boston, Massachusetts Leslie A. Kerrigan University of California, San Diego La Jolla, California Jae Bum Kim Harvard Medical School Boston, Massachusetts Jonghwan Kim University of Texas at Austin Austin, Texas Jung-Sik Kim Georgetown University School of Medicine Washington, D.C.
David M. Kurnit University of Michigan Medical Center Ann Arbor, Michigan Joshua LaBaer Harvard Medical School Cambridge, Massachusetts Mariana Lagos-Quintana The Rockefeller University New York, New York Edward R. LaVallie Genetics Institute Cambridge, Massachusetts Gretchen Lawler Purdue University West Lafayette, Indiana
Robert E. Kingston Massachusetts General Hospital and Harvard Medical School Boston, Massachusetts
Carolyn Lee Georgetown University School of Medicine Washington, D.C.
Carol M. Kissinger Millipore Burlington, Massachusetts
Chung-Sheng Brian Lee Harvard Medical School Boston, Massachusetts
Christoph A. Klein Ludwig-Maximilians-University of Munich Munich, Germany
Kyu-Min Lee Massachusetts General Hospital Boston, Massachusetts
Lloyd B. Klickstein Brigham and Women’s Hospital Boston, Massachusetts
Richard T. Lee Brigham and Women’s Hospital Boston, Massachusetts
Michael Lenardo National Institute of Allergy and Infectious Diseases Bethesda, Maryland Mark E. Levenstein University of California, San Diego La Jolla, California Peng Liang Vanderbilt-Ingram Cancer Center Nashville, Tennessee Peter Lichter Deutsches Krebsforschungs-zentrum Heidelberg, Germany Peter Liljeström Karolinska Institute, Novum Research Center Huddinge, Sweden Paul D. Ling Baylor College of Medicine Houston, Texas Robert J. Linhardt University of Iowa Iowa City, Iowa Peter Linsley Bristol-Myers Squibb Seattle, Washington Silvia López University of Buenos Aires-CONICET School of Medicine Buenos Aires, Argentina John Lueders National Human Genome Research Institute, NIH Bethesda, Maryland Victoria Lundblad Baylor College of Medicine Houston, Texas George Lunn Baltimore, Maryland Ming-Juan Luo Harvard Medical School Boston, Massachusetts Karol Mackey Molecular Research Cincinnati, Ohio Thomas L. Madden National Center for Biotechnology Information Bethesda, Maryland
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David D. Moore Baylor College of Medicine Houston, Texas
Osamu Ohara Shionogi Research Laboratories Osaka, Japan
Adriana E. Manzi University of California, San Diego La Jolla, California
Malcolm Moos, Jr. Center for Biologics Evaluation and Research Food and Drug Administration Bethesda, Maryland
Kazuhiro Oka Baylor College of Medicine Houston, Texas
Chris D. Martin Tropix Inc. Bedford, Massachusetts
Zarmik Moqtaderi Harvard Medical School Boston, Massachusetts
David Martin National Institute of Allergy and Infectious Diseases Bethesda, Maryland
Richard Mortensen University of Michigan Medical Center Ann Arbor, Michigan
David J. Mangelsdorf Howard Hughes Medical Institute University of Texas Southwestern Medical Center Dallas, Texas
John M. McCoy Genetics Institute Cambridge, Massachusetts Michael McPhaul University of Texas Southwestern Medical Center Dallas, Texas Scott Medberry Amersham Pharmacia Biotech San Francisco, California Annemarie H. Meijer Leiden University Leiden, The Netherlands Jill Meisenhelder The Salk Institute La Jolla, California Craig C. Mello University of Massachusetts Medical School Worcester, Massachusetts Andrew R. Mendelsohn The Molecular Sciences Institute Berkeley, California Adriana Miele University of Massachusetts Medical School Worcester, Massachusetts James J. Mond Uniformed Services University of the Health Sciences Bethesda, Maryland Rebecca I. Montgomery Northwestern University Chicago, Illinois Butch Moomaw Hamamatsu Photonic Systems Spring Branch, Texas
Bernard Moss National Institute of Allergy and Infectious Diseases Bethesda, Maryland Paul R. Mueller California Institute of Technology Pasadena, California Cheryl Isaac Murphy Aquila Biopharmaceuticals Worcester, Massachusetts Richard M. Myers Stanford University School of Medicine Stanford, California Geeta Narlikar Massachusetts General Hospital Boston, Massachusetts Robert J. Nelson Dakota Scientific Sioux Falls, South Dakota Rachael L. Neve McLean Hospital Belmont, Massachusetts Tagvor G. Nishanian Georgetown University School of Medicine Washington, D.C. Paul E. Nisson Life Technologies, Inc. Gaithersburg, Maryland
Hiroto Okayama Osaka University Osaka, Japan Salvatore Oliviero Harvard Medical School Boston, Massachusetts Amos Oppenheim National Cancer Institute at Frederick Frederick, Maryland Kazutoyo Osoegawa Children’s Hospital Oakland Research Institute Oakland, California B.F. Francis Ouellette University of British Columbia Vancouver, British Columbia Pieter B.F. Ouwerkerk Leiden University Leiden, The Netherlands Alejandra Paganelli University of Buenos Aires-CONICET School of Medicine Buenos Aires, Argentina Nicholas M. Papadopoulos Warren Grant Magnuson Clinical Center Bethesda, Maryland Ophelia Papoulas Harvard Medical School Boston, Massachusetts Arthur B. Pardee Dana Farber Cancer Institute Boston, Massachusetts Jaehong Park Harvard Medical School Cambridge, Massachusetts
B. Tracy Nixon Pennsylvania State University University Park, Pennsylvania
Mukesh Patel Whitehead Institute for Biomedical Research Cambridge, Massachusetts
Marjorie Oettinger Massachusetts General Hospital Boston, Massachusetts
Yvonne Paterson University of Pennsylvania Philadelphia, Pennsylvania
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Scott D. Patterson Amgen, Inc. Thousand Oaks, California
Huntington Potter Harvard Medical School Boston, Massachusetts
Phil Robakiewicz Worcester Polytechnic Institute Worcester, Massachusetts
Warren Pear University of Pennsylvania Philadelphia, Pennsylvania
Leland D. Powell University of California, San Diego School of Medicine La Jolla, California
Jeffrey Robbins Cincinnati Children’s Hospital University of Cincinnati Cincinnati, Ohio
Pedro A. Prieto Ross Laboratories Columbus, Ohio
Melissa Rogers University of South Florida Tampa, Florida
Thomas Quertermous Massachusetts General Hospital Boston, Massachusetts
Sharon Rogers Lakeside Biotechnology Chicago, Illinois
Elisabeth Raleigh New England Biolabs Beverly, Massachusetts
M.R. Rolfe Celltech Ltd. Slough, United Kingdom
Ali Ramezani American Red Cross Rockville, Maryland
William G. Romanow Corning Costar Portsmouth, New Hamphire
Ayoub Rashtchian Life Technologies, Inc. Gaithersburg, Maryland
John K. Rose Yale University School of Medicine New Haven, Connecticut
Hervé E. Recipon National Center for Biotechnology Information Bethesda, Maryland
Martin Rosenberg SmithKline Beecham King of Prussia, Pennsylvania
Fred A. Pereira Baylor College of Medicine Houston, Texas Heather Perry-O’Keefe Millipore Burlington, Massachusetts Jean M. Pesola Harvard Medical School Boston, Massachusetts Karina Petat-Dutter Ludwig-Maximilians-University of Munich Munich, Germany Kevin J. Petty University of Texas Southwestern Medical Center Dallas, Texas Sébastien Pfeffer The Rockefeller University New York, New York Mary C. Phelan Molecular Pathology Laboratory Network Maryville, Tennessee Helen Piwnica-Worms Washington University School of Medicine St. Louis, Missouri Anne Pizard Harvard Medical School Boston, Massachusetts Jack D. Pollard, Jr. Mintz Levin Cohn Glovsky Ferris and Popeo Boston, Massachusetts Roy M. Pollock Ariad Pharmaceuticals, Inc. Cambridge, Massachusetts Kornelia Polyak Harvard Medical School Boston, Massachusetts Roger Pomerantz Thomas Jefferson University Philadelphia, Pennsylvania
K.J. Reddy State University of New York Binghamton, New York Robin Reed Harvard Medical School Boston, Massachusetts Mark Reichardt Lakeside Biotechnology Chicago, Illinois Ann Reynolds University of Washington Seattle, Washington Randall K. Ribaudo National Institute of Allergy and Infectious Diseases Bethesda, Maryland Eric J. Richards Washington University St. Louis, Missouri Paul Riggs New England Biolabs Beverly, Massachusetts
Sharon Y. Roth M.D. Anderson Cancer Center Houston, Texas Robert R. Roussel Dartmouth College Hanover, New Hampshire Hadara Rubinfeld The Weizmann Institute of Science Rehovot, Israel Colleen A. Ryan Boston College Chestnut Hill, Massachusetts Elizabeth F. Ryder Worcester Polytechnic Institute Worcester, Massachusetts Nicoletta Sacchi National Cancer Institute Frederick, Maryland Brad St. Croix Harvard Medical School Boston, Massachusetts Thomas P. St. John ICOS Corporation Bothwell, Washington
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Joachim Sasse Shriners Hospital for Crippled Children Tampa, Florida
Richard F. Selden TKT, Inc. Cambridge, Massachusetts
Michael H. Simonian Beckman Instruments, Inc. Fullerton, California
N.M. Sawtell Children’s Hospital Medical Center Cincinnati, Ohio
Kentaro Semba The Salk Institute La Jolla, California
Harinder Singh University of Chicago Chicago, Illinois
Stephen J. Scharf Cetus Corporation Emeryville, California
Donald Senear University of California, Irvine Irvine, California
Hazel Sive Whitehead Institute for Biomedical Research Cambridge, Massachusetts
David G. Schatz Howard Hughes Medical Institute and Yale University School of Medicine New Haven, Connecticut
Ilya Serebriiskii Fox Chase Cancer Center Philadelphia, Pennsylvania
Paul Schendel Genetics Institute Cambridge, Massachusetts Timothy D. Schlabach Applied Biosystems Foster City, California Gavin R. Schnitzler Tufts University School of Medicine Boston, Massachusetts
Thikkavarapu Seshamma Thomas Jefferson University Philadelphia, Pennsylvania Raj Shankarappa University of Pittsburgh School of Medicine Pittsburgh, Pennsylvania Allan Shatzman SmithKline Beecham King of Prussia, Pennsylvania
Kimmen V. Sjölander University of California Berkeley, California Barton E. Slatko New England Biolabs Beverly, Massachusetts Alan J. Smith Stanford University Medical Center Stanford, California Carolyn L. Smith National Institute of Neurological Disorders and Stroke Bethesda, Maryland
Joachim Schorr Diagen GmbH Hilden, Germany
Yoav Shaul Department of Biological Regulation The Weizmann Institute of Science Rehovot, Israel
Gregory D. Schuler National Center for Biotechnology Information Bethesda, Maryland
Jen Sheen Massachusetts General Hospital and Harvard Medical School Boston, Massachusetts
Donald B. Smith University of Edinburgh Edinburgh, Scotland
R.K. Scopes La Trobe University Bundoora, Australia
Shirish Shenolikar Duke University Medical Center Durham, North Carolina
John A. Smith University of Alabama at Birmingham Birmingham, Alabama
Brian Seed Massachusetts General Hospital and Harvard Medical School Boston, Massachusetts
L.A. Sherman Purdue University West Lafayette, Indiana
Michael Snyder Yale University New Haven, Connecticut
Penny Shockett Yale University School of Medicine New Haven, Connecticut
Timothy A. Springer Center for Blood Research Harvard Medical School Boston, Massachusetts
Bartholomew M. Sefton The Salk Institute San Diego, California Rony Seger Department of Biological Regulation The Weizmann Institute of Science Rehovot, Israel Christine E. Seidman Harvard Medical School Boston, Massachusetts Edward Sekinger Harvard Medical School Boston, Massachusetts
Kevan M. Shokat University of California at San Francisco San Francisco, California Gene P. Siegal University of Alabama at Birmingham Birmingham, Alabama Pam A. Silver Dana-Farber Cancer Institute Boston, Massachusetts
David F. Smith University of Georgia Atlanta, Georgia
Patrick Stanssens Keygene N.V. The Netherlands David F. Stern Yale University School of Medicine New Haven, Connecticut Julie M. Stone University of Missouri Columbia, Missouri
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William M. Strauss Whitehead Institute Cambridge, Massachusetts
Peter van der Geer University of California, San Diego La Jolla, California
Maryann Z. Whitley Genetics Institute Cambridge, Massachusetts
Kevin Struhl Harvard Medical School Boston, Massachusetts
Ajit P. Varki University of California, San Diego La Jolla, California
Michael Whitt Yale University School of Medicine New Haven, Connecticut
John T. Stults Genentech, Inc. South San Francisco, California
Baruch Velan Israel Institute of Biological Research Ness Ziona, Israel
Alan Williams Amersham Pharmacia Biotech Piscataway, New Jersey
Cynthia Sung Bioengineering and Physical Science Program/NIH Bethesda, Maryland
Jacob Villegas University of Texas Southwestern Medical Center Dallas, Texas
David S. Wilson Zyomyx Hayward, California
Jesper Q. Svejstrup Cancer Research U.K. London Research Institute Clare Hall Laboratory Hertfordshire, U.K.
Joseph M. Vitolo University of Rochester Medical Center Rochester, New York
Jonathan C. Swaffield University of Texas Southwestern Medical Center Dallas, Texas Stanley Tabor Harvard Medical School Boston, Massachusetts Tomoko M. Tabuchi University of Massachusetts Medical School Worcester, Massachusetts Miyoko Takahashi Spectral Diagnostics, Inc. Toronto, Ontario Christophe Thiriet University of Rochester Medical Center Rochester, New York Lynn Thomason National Cancer Institute at Frederick Frederick, Maryland Nancy E. Thompson University of Wisconsin Madison, Wisconsin Douglas A. Treco TKT, Inc. Cambridge, Massachusetts Steven J. Triezenberg Michigan State University East Lansing, Michigan Thomas Tuschl The Rockefeller University New York, New York
Hans-Peter Vornlocher Ribopharma AG Kulmbach, Germany Pieter Vos Keygene N.V. The Netherlands Daniel Voytas Iowa State University Ames, Iowa Todd Waldman Georgetown University School of Medicine Washington, D.C. Simon Watkins University of Pittsburgh Medical School Pittsburgh, Pennsylvania John H. Weis Harvard Medical School Boston, Massachusetts Jane M. Weisemann National Center for Biotechnology Information Bethesda, Maryland Douglas C. Weiser Duke University Medical Center Durham, North Carolina Eric L. Weiss Northwestern University Evanston, Illinois Nicole Wendler Ludwig-Maximilians-University of Munich Munich, Germany
Helen Wilson National Cancer Institute at Frederick Frederick, Maryland Kate Wilson Australian Institute of Marine Science Townsville, Australia Kenneth J. Wilson Applied Biosystems Foster City, California Fred Winston Harvard Medical School Boston, Massachusetts Scott E. Winston Nabi Rockville, Maryland C. Richard Wobbe Harvard Medical School Boston, Massachusetts Barbara Wold California Institute of Technology Pasadena, California Paula Wolf Massachusetts Institute of Technology Cambridge, Massachusetts Tyra G. Wolfsberg National Center for Biotechnology Information Bethesda, Maryland Carolyn A. Worby University of California at San Diego La Jolla, California Jerry L. Workman Pennsylvania State University University Park, Pennsylvania Shwu-Yuan Wu Case Western Reserve University School of
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Medicine Cleveland, Ohio Linda S. Wyatt National Institute of Allergy and Infectious Diseases Bethesda, Maryland Dong Xu Digital Biology Laboratory University of Missouri-Columbia Columbia, Missouri Jianming Xu Baylor College of Medicine Houston, Texas Ying Xu Department of Biochemistry and Molecular Biology University of Georgia Athens, Georgia
Annie Yang Harvard Medical School Boston, Massachusetts
Chao Zhang University of California at San Francisco San Francisco, California
Yanhai Yin Iowa State University Ames, Iowa
Ning Zhang Tularik, Inc. South San Francisco, California
Wayne M. Yokoyama University of California School of Medicine San Francisco, California
Heng Zhou Harvard Medical School Boston, Massachusetts
Natasha E. Zachara The Johns Hopkins University Medical School Baltimore, Maryland
Zhaolan Zhou Children’s Hospital and Harvard Medical School Boston, Massachusetts
Ken Zaret Fox Chase Cancer Center Philadelphia, Pennsylvania
Dietlind Zohlnhöfer Ludwig-Maximilians-University of Munich Munich, Germany
Rolf Zeller University of Utrecht The Netherlands
Lou Zumstein Baylor College of Medicine Houston, Texas
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i Contents Chapter 1 Escherichia coli, Plasmids, and Bacteriophages Introduction
Section I Escherichia coli Unit 1.1 Media Preparation and Bacteriological Tools Unit 1.2 Growth in Liquid Media Unit 1.3 Growth on Solid Media Unit 1.4 Selected Topics from Classical Bacterial Genetics
Section II Vectors Derived from Plasmids Unit 1.5 Introduction to Plasmid Biology Unit 1.6 Minipreps of Plasmid DNA Unit 1.7 Large-Scale Preparation of Plasmid DNA Unit 1.8 Introduction of Plasmid DNA into Cells
Section III Vectors Derived from Lambda and Related Bacteriophages Unit 1.9 Introduction to Lambda Phages Unit 1.10 Lambda as a Cloning Vector Unit 1.11 Plating Lambda Phage to Generate Plaques Unit 1.12 Growing Lambda-Derived Vectors Unit 1.13 Preparing Lambda DNA from Phage Lysates
Section IV Vectors Derived from Filamentous Phages Unit 1.14 Introduction to Vectors Derived from Filamentous Phages Unit 1.15 Preparing and Using M13-Derived Vectors
Section V Specialized Techniques Unit 1.16 Recombineering: Genetic Engineering in Bacteria Using Homologous Recombination
Chapter 2 Preparation and Analysis of DNA Introduction
Section I Manipulation of DNA Unit 2.1A Purification and Concentration of DNA from Aqueous Solutions Unit 2.1B Purification of DNA by Anion-Exchange Chromatography Unit 2.2 Preparation of Genomic DNA from Mammalian Tissue Unit 2.3 Preparation of Genomic DNA from Plant Tissue Unit 2.4 Preparation of Genomic DNA from Bacteria
Section II Resolution and Recovery of Large DNA Fragments Unit 2.5A Agarose Gel Electrophoresis Unit 2.5B Pulsed-Field Gel Electrophoresis Unit 2.6 Isolation and Purification of Large DNA Restriction Fragments from Agarose Gels
Section III Resolution and Recovery of Small DNA Fragments Unit 2.7 Separation of Small DNA Fragments by Conventional Gel Electrophoresis Unit 2.8 Capillary Electrophoresis of DNA
Section IV Analysis of DNA Sequences by Blotting and Hybridization Unit 2.9A Southern Blotting Unit 2.9B Dot and Slot Blotting of DNA
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Unit 2.10 Hybridization Analysis of DNA Blots
Section V Synthesis and Purification of Oligonucleotides Unit 2.11 Synthesis and Purification of Oligonucleotides Unit 2.12 Purification of Oligonucleotides Using Denaturing Polyacrylamide Gel Electrophoresis
Chapter 3 Enzymatic Manipulation of DNA and RNA Introduction
Section I Restriction Endonucleases Unit 3.1 Digestion of DNA with Restriction Endonucleases
Section II Restriction Mapping Unit 3.2 Mapping by Multiple Endonuclease Digestions Unit 3.3 Mapping by Partial Endonuclease Digestions
Section III Enzymes for Modifying and Radioactively Labeling Nucleic Acids Unit 3.4 Reagents and Radioisotopes Used to Manipulate Nucleic Acids Unit 3.5 DNA-Dependent DNA Polymerases Unit 3.6 Template-Independent DNA Polymerases Unit 3.7 RNA-Dependent DNA Polymerases Unit 3.8 DNA-Dependent RNA Polymerases Unit 3.9 DNA-Independent RNA Polymerases Unit 3.10 Phosphatases and Kinases Unit 3.11 Exonucleases Unit 3.12 Endonucleases Unit 3.13 Ribonucleases Unit 3.14 DNA Ligases Unit 3.15 RNA Ligases
Section IV Construction of Hybrid DNA Molecules Unit 3.16 Subcloning of DNA Fragments Unit 3.17 Constructing Recombinant DNA Molecules by the Polymerase Chain Reaction
Section V Specialized Applications Unit 3.18 Labeling and Colorimetric Detection of Nonisotopic Probes Unit 3.19 Chemiluminescent Detection of Nonisotopic Probes Unit 3.20 Recombinational Cloning
Chapter 4 Preparation and Analysis of RNA Introduction
Section I Preparation of RNA from Eukaryotic and Prokaryotic Cells Unit 4.1 Preparation of Cytoplasmic RNA from Tissue Culture Cells Unit 4.2 Guanidine Methods for Total RNA Preparation Unit 4.3 Phenol/SDS Method for Plant RNA Preparation Unit 4.4 Preparation of Bacterial RNA Unit 4.5 Preparation of Poly(A)+ RNA
Section II Analysis of RNA Structure and Synthesis
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Unit 4.6 S1 Analysis of Messenger RNA Using Single-Stranded DNA Probes Unit 4.7 Ribonuclease Protection Assay Unit 4.8 Primer Extension Unit 4.9 Analysis of RNA by Northern and Slot Blot Hybridization Unit 4.10 Identification of Newly Transcribed RNA
Chapter 5 Construction of Recombinant DNA Libraries Introduction
Section I Overview of Recombinant DNA Libraries Unit 5.1 Genomic DNA Libraries Unit 5.2 cDNA Libraries
Section II Preparation of Insert DNA from Genomic DNA Unit 5.3 Size Fractionation Using Sucrose Gradients Unit 5.4 Size Fractionation Using Agarose Gels
Section III Preparation of Insert DNA from Messenger RNA Unit 5.5 Conversion of mRNA into Double-Stranded cDNA Unit 5.6 Ligation of Linkers or Adapters to Double-Stranded cDNA
Section IV Production of Genomic DNA and cDNA Libraries Unit 5.7 Production of a Genomic DNA Library Unit 5.8A Production of a Complete cDNA Library Unit 5.9 Construction of Bacterial Artificial Chromosome (BAC/PAC) Libraries
Section V Amplification of Transformed or Packaged Libraries Unit 5.10 Amplification of a Bacteriophage Library Unit 5.11 Amplification of Cosmid and Plasmid Libraries
Chapter 6 Screening of Recombinant DNA Libraries Introduction
Section I Plating Libraries and Transfer to Filter Membranes Unit 6.1 Plating and Transferring Bacteriophage Libraries Unit 6.2 Plating and Transferring Cosmid and Plasmid Libraries
Section II Hybridization with Radioactive Probes Unit 6.3 Using DNA Fragments as Probes Unit 6.4 Using Synthetic Oligonucleotides as Probes
Section III Purification of Bacteriophage, Cosmid, and Plasmid Clones Unit 6.5 Purification of Bacteriophage Clones Unit 6.6 Purification of Cosmid and Plasmid Clones
Section IV Screening with Antibodies Unit 6.7 Immunoscreening of Fusion Proteins Produced in Lambda Plaques Unit 6.8 Immunoscreening after Hybrid Selection and Translation
Section V Yeast Artificial Chromosome Libraries
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Unit 6.9 Overview of Strategies for Screening YAC Libraries and Analyzing YAC Clones Unit 6.10 Analysis of Isolated YAC Clones
Section VI Specialized Strategies for Screening Libraries Unit 6.11 Use of Monoclonal Antibodies for Expression Cloning Unit 6.12 Recombination-Based Assay (RBA) for Screening Bacteriophage Lambda Libraries
Chapter 7 DNA Sequencing Introduction Unit 7.1 DNA Sequencing Strategies Unit 7.2 Constructing Nested Deletions for Use in DNA Sequencing Unit 7.3 Preparation of Templates for DNA Sequencing Unit 7.4A DNA Sequencing by the Dideoxy Method Unit 7.4B Dideoxy DNA Sequencing with Chemiluminescent Detection Unit 7.5 DNA Sequencing by the Chemical Method Unit 7.6 Denaturing Gel Electrophoresis for Sequencing Unit 7.7 Computer Manipulation of DNA and Protein Sequences
Chapter 8 Mutagenesis of Cloned DNA Introduction Unit 8.1 Oligonucleotide-Directed Mutagenesis without Phenotypic Selection Unit 8.2A Mutagenesis with Degenerate Oligonucleotides: Creating Numerous Mutations in a Small DNA Sequence Unit 8.2B Gene Synthesis: Assembly of Target Sequences Using Mutually Priming Long Oligonucleotides Unit 8.3 Random Mutagenesis by PCR Unit 8.4 Linker-Scanning Mutagenesis of DNA Unit 8.5 Directed Mutagenesis Using the Polymerase Chain Reaction
Chapter 9 Introduction of DNA into Mammalian Cells Introduction
Section I Transfection of DNA into Eukaryotic Cells Unit 9.1 Calcium Phosphate Transfection Unit 9.2 Transfection Using DEAE-Dextran Unit 9.3 Transfection by Electroporation Unit 9.4 Transfection of Cultured Eukaryotic Cells Using Cationic Lipid Reagents Unit 9.5 Selection of Transfected Mammalian Cells
Section II Uses of Fusion Genes in Mammalian Transfection Unit 9.6 Overview of Genetic Reporter Systems Unit 9.7A Isotopic Assays for Reporter Gene Activity Unit 9.7B Nonisotopic Assays for Reporter Gene Activity Unit 9.7C Use of the A. Victoria Green Fluorescent Protein to Study Protein Dynamics in Vivo Unit 9.8 Direct Analysis of RNA after Transfection
Section III Transduction of Genes Using Retrovirus Vectors Unit 9.9 Overview of the Retrovirus Transduction System Unit 9.10 Preparation of a Specific Retrovirus Producer Cell Line
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Unit 9.11 Transient Transfection Methods for Preparation of High-Titer Retroviral Supernatants Unit 9.12 Large-Scale Preparation and Concentration of Retrovirus Stocks Unit 9.13 Detection of Helper Virus in Retrovirus Stocks Unit 9.14 Retrovirus Infection of Cells In Vitro and In Vivo
Section IV Inactivation of Genes in Mammalian Cells Unit 9.15 Human Somatic Cell Gene Targeting
Chapter 10 Analysis of Proteins Introduction
Section I Quantitation of Proteins Unit 10.1A Spectrophotometric and Colorimetric Determination of Protein Concentration Unit 10.1B Quantitative Amino Acid Analysis
Section II Electrophoretic Separation of Proteins Unit 10.2A One-Dimensional SDS Gel Electrophoresis of Proteins Unit 10.2B One-Dimensional Electrophoresis Using Nondenaturing Conditions Unit 10.3 Two-Dimensional Gel Electrophoresis Using the ISO-DALT System Unit 10.4 Two-Dimensional Gel Electrophoresis Unit 10.5 Overview of Digital Electrophoresis Analysis
Section III Detection of Proteins Unit 10.6 Staining Proteins in Gels Unit 10.7 Detection of Proteins on Blot Transfer Membranes Unit 10.8 Immunoblotting and Immunodetection
Section IV Purification of Proteins by Conventional Chromatography Unit 10.9 Gel-Filtration Chromatography Unit 10.10 Ion-Exchange Chromatography Unit 10.11A Immunoaffinity Chromatography Unit 10.11B Metal-Chelate Affinity Chromatography
Section V Purification of Proteins by High-Performance Liquid Chromatography Unit 10.12 HPLC of Peptides and Proteins: Preparation and System Set-Up Unit 10.13 HPLC of Peptides and Proteins: Standard Operating Conditions Unit 10.14 Reversed-Phase Isolation of Peptides
Section VI Specialized Applications Unit 10.15 Purification of Recombinant Proteins and Study of Protein Interaction by Epitope Tagging Unit 10.16 Immunoprecipitation Unit 10.17 Synthesizing Proteins In Vitro by Transcription and Translation of Cloned Genes Unit 10.18 Metabolic Labeling with Amino Acids Unit 10.19 Isolation of Proteins for Microsequence Analysis Unit 10.20 Capillary Electrophoresis of Proteins and Peptides Unit 10.21 Overview of Peptide and Protein Analysis by Mass Spectrometry Unit 10.22 Protein Identification and Characterization by Mass Spectrometry Unit 10.23 Difference Gel Electrophoresis (DIGE) Using CyDye DIGE Fluor Minimal Dyes Unit 10.24 Solution Radioimmunoassay of Proteins and Peptides
vi Chapter 11 Immunology Introduction
Section I Immunoassays Unit 11.1 Conjugation of Enzymes to Antibodies Unit 11.2 Enzyme-Linked Immunosorbent Assays (ELISA) Unit 11.3 Isotype Determination of Antibodies
Section II Preparation of Monoclonal Antibodies Unit 11.4 Immunization of Mice Unit 11.5 Preparation of Myeloma Cells Unit 11.6 Preparation of Mouse Feeder Cells for Fusion and Cloning Unit 11.7 Fusion of Myeloma Cells with Immune Spleen Cells Unit 11.8 Cloning of Hybridoma Cell Lines by Limiting Dilution Unit 11.9 Freezing and Recovery of Hybridoma Cell Lines Unit 11.10 Production of Monoclonal Antibody Supernatant and Ascites Fluid Unit 11.11 Purification of Monoclonal Antibodies
Section III Preparation of Polyclonal Antisera Unit 11.12 Production of Polyclonal Antisera Unit 11.13 In Vitro Antibody Production Unit 11.14 Purification of Immunoglobulin G Fraction from Antiserum, Ascites Fluid, or Hybridoma Supernatant
Section IV Preparation of Antipeptide Antibodies Unit 11.15 Introduction to Peptide Synthesis Unit 11.16 Synthetic Peptides for Production of Antibodies that Recognize Intact Proteins
Section V Determination of Specific Antibody Titer and Isotype Unit 11.17 Determination of the Specific Antibody Titer
Section VI Preparation and Use of Specialized Antibodies Unit 11.18 Identification of Polyol-Responsive Monoclonal Antibodies for Use in Immunoaffinity Chromatography
Chapter 12 DNA-Protein Interactions Introduction Unit 12.1 Preparation of Nuclear and Cytoplasmic Extracts from Mammalian Cells Unit 12.2 Mobility Shift DNA-Binding Assay Using Gel Electrophoresis Unit 12.3 Methylation and Uracil Interference Assays for Analysis of Protein-DNA Interactions Unit 12.4 DNase I Footprint Analysis of Protein-DNA Binding Unit 12.5 UV Crosslinking of Proteins to Nucleic Acids Unit 12.6 Purification of DNA-Binding Proteins Using Biotin/Streptavidin Affinity Systems Unit 12.7 Detection, Purification, and Characterization of cDNA Clones Encoding DNA-Binding Proteins Unit 12.8 Rapid Separation of Protein-Bound DNA from Free DNA Using Nitrocellulose Filters Unit 12.9 Analysis of DNA-Protein Interactions Using Proteins Synthesized In Vitro from Cloned Genes Unit 12.10 Purification of Sequence-Specific DNA-Binding Proteins by Affinity Chromatography Unit 12.11 Determination of Protein-DNA Sequence Specificity by PCR-Assisted Binding-Site Selection Unit 12.12 Yeast One-Hybrid Screening for DNA-Protein Interactions
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Chapter 13 Yeast Introduction
Section I Basic Techniques of Yeast Genetics Unit 13.1 Preparation of Yeast Media Unit 13.2 Growth and Manipulation of Yeast Unit 13.3 Genome-Wide Transposon Mutagenesis in Yeast
Section II Yeast Vectors Unit 13.4 Yeast Cloning Vectors and Genes Unit 13.6 Yeast Vectors and Assays for Expression of Cloned Genes
Section III Manipulation of Yeast Genes Unit 13.7 Introduction of DNA into Yeast Cells Unit 13.8 Cloning Yeast Genes by Complementation Unit 13.9 Manipulation of Plasmids from Yeast Cells Unit 13.10 Manipulation of Cloned Yeast DNA
Section IV Preparation of Yeast DNA, RNA, and Proteins Unit 13.11 Preparation of Yeast DNA Unit 13.12 Preparation of Yeast RNA Unit 13.13 Preparation of Protein Extracts from Yeast
Section V Schizosaccharomyces pombe Unit 13.14 Overview of Schizosaccharomyces pombe Unit 13.15 S. pombe Strain Maintenance and Media Unit 13.16 Growth and Manipulation of S. pombe Unit 13.17 Introduction of DNA into S. pombe Cells
Chapter 14 In Situ Hybridization and Immunohistochemistry Introduction Unit 14.1 Fixation, Embedding, and Sectioning of Tissues, Embryos, and Single Cells Unit 14.2 Cryosectioning Unit 14.3 In Situ Hybridization to Cellular RNA Unit 14.4 Detection of Hybridized Probe Unit 14.5 Counterstaining and Mounting of Autoradiographed In Situ Hybridization Slides Unit 14.6 Immunohistochemistry Unit 14.7 In situ Hybridization and Detection Using Nonisotopic Probes Unit 14.8 In situ Polymerase Chain Reaction and Hybridization to Detect Low-Abundance Nucleic Acid Targets Unit 14.9 Whole-Mount In Situ Hybridization and Detection of RNAs in Vertebrate Embryos and Isolated Organs Unit 14.10 Principles and Application of Fluorescence Microscopy Unit 14.11 Basic Confocal Microscopy Unit 14.12 Measurement of In Situ Hybridization Unit 14.13 Morphological, Biochemical, and Flow Cytometric Assays of Apoptosis Unit 14.14 Whole-Mount Histochemical Detection of -Galactosidase Activity Unit 14.15 Overview of Image Analysis, Image Importing, and Image Processing using Freeware Unit 14.16 Three-Dimensional Reconstruction of Tissues
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Chapter 15 The Polymerase Chain Reaction Introduction Unit 15.1 Enzymatic Amplification of DNA by PCR: Standard Procedures and Optimization Unit 15.2 Direct DNA Sequencing of PCR Products Unit 15.3 Ligation-Mediated PCR for Genomic Sequencing and Footprinting Unit 15.4 Molecular Cloning of PCR Products Unit 15.5 Enzymatic Amplification of RNA by PCR (RT-PCR) Unit 15.6 cDNA Amplification Using One-Sided (Anchored) PCR Unit 15.7 Quantitation of Rare DNAs by PCR Unit 15.8 High-Throughput Real-Time Quantitative Reverse Transcription PCR
Chapter 16 Protein Expression Introduction
Section I Expression of Proteins in Escherichia coli Unit 16.1 Overview of Protein Expression in E. coli Unit 16.2 Expression Using the T7 RNA Polymerase/Promoter System Unit 16.3 Expression Using Vectors with Phage Regulatory Sequences Unit 16.4A Introduction to Expression by Fusion Protein Vectors Unit 16.4B Enzymatic and Chemical Cleavage of Fusion Proteins Unit 16.5 Expression and Purification of lacZ and trpE Fusion Proteins Unit 16.6 Expression and Purification of Maltose-Binding Protein Fusions Unit 16.7 Expression and Purification of Glutathione-S-Transferase Fusion Proteins Unit 16.8 Expression and Purification of Thioredoxin Fusion Proteins
Section II Expression of Proteins in Insect Cells Using Baculovirus Vectors Unit 16.9 Overview of the Baculovirus Expression System Unit 16.10 Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses Unit 16.11 Expression and Purification of Recombinant Proteins Using the Baculovirus System
Section III Expression of Proteins in Mammalian Cells Unit 16.12 Transient Expression of Proteins Using COS Cells Unit 16.13 Expression and Purification of Epitope-Tagged Multisubunit Protein Complexes from Mammalian Cells Unit 16.14 Inducible Gene Expression Using an Autoregulatory, Tetracycline-Controlled System Unit 16.15 Overview of the Vaccinia Virus Expression System Unit 16.16 Preparation of Cell Cultures and Vaccinia Virus Stocks Unit 16.17 Generation of Recombinant Vaccinia Viruses Unit 16.18 Characterization of Recombinant Vaccinia Viruses and Their Products Unit 16.19 Gene Expression Using the Vaccinia Virus/T7 RNA Polymerase Hybrid System Unit 16.20 Expression of Proteins Using Semliki Forest Virus Vectors Unit 16.21 Overview of the HIV-1 Lentiviral Vector System Unit 16.22 Generation of HIV-1-Based Lentiviral Vector Particles Unit 16.23 Amplification Using CHO Cell Expression Vectors Unit 16.24 Helper-Dependent Adenoviral Vectors
Chapter 17 Preparation and Analysis of Glycoconjugates Introduction
ix Section I Special Considerations for Glycoproteins and Their Purification Unit 17.1 Special Considerations for Glycoproteins and Their Purification Unit 17.2 Special Considerations for Proteoglycans and Glycosaminoglycans and Their Purification Unit 17.3 Special Considerations for Glycolipids and Their Purification
Section II Detection of Saccharides on Glycoconjugates Unit 17.4 Metabolic Radiolabeling of Animal Cell Glycoconjugates Unit 17.5 Chemical Labeling of Carbohydrates by Oxidation and Sodium Borohydride Reduction Unit 17.6 Detection and Analysis of Proteins Modified by O-Linked N-Acetylglucosamine Unit 17.7 Lectin Analysis of Proteins Blotted onto Filters Unit 17.8 Detection of Glycophospholipid Anchors on Proteins Unit 17.9 Direct Chemical Analysis of Glycoconjugates for Carbohydrates Unit 17.10A Inhibition of N-Linked Glycosylation Unit 17.10B Inhibition of Glycolipid Biosynthesis Unit 17.11 Synthetic Glycosides as Primers of Oligosaccharide Biosynthesis and Inhibitors of Glycoprotein and Proteoglycan Assembly
Section III Release of Saccharides from Glycoconjugates Unit 17.12 Sialidases Unit 17.13A Endoglycosidase and Glycoamidase Release of N-Linked Oligosaccharides Unit 17.13B Analysis of Glycosaminoglycans with Polysaccharide Lyases Unit 17.14A Preparation of Glycopeptides Unit 17.14B Detection of Individual Glycosylation Sites on Glycoproteins Unit 17.15A -Elimination for Release of O-Linked Glycosaminoglycans from Proteoglycans Unit 17.15B -Elimination for Release of O-GalNAc-Linked Oligosaccharides from Glycoproteins and Glycopeptides Unit 17.16 Acid Hydrolysis for Release of Monosaccharides Unit 17.17A Enzymatic Release of Oligosaccharides from Glycolipids Unit 17.17B Endo- -Galactosidases and Keratanase
Section IV Analysis of Saccharides Released from Glycoconjugates Unit 17.18 Analysis of Monosaccharides Unit 17.19A Total Compositional Analysis by High-Performance Liquid Chromatography or Gas-Liquid Chromatography Unit 17.19B Composition of Labeled Monosaccharides from Glycosaminoglycans Unit 17.20 Analysis of Oligosaccharide Negative Charge by Anion-Exchange Chromatography Unit 17.21A HPLC Methods for the Fractionation and Analysis of Negatively Charged Oligosaccharides and Gangliosides Unit 17.21B Fractionation and Analysis of Neutral Oligosaccharides by HPLC Unit 17.22A Nitrous Acid Degradation of Glycosaminoglycans Unit 17.22B Analysis of Disaccharides and Tetrasaccharides Released from Glycosaminoglycans Unit 17.23 Analysis of Sulfate Esters by Solvolysis or Hydrolysis
Chapter 18 Analysis of Protein Phosphorylation Introduction Unit 18.1 Overview of Protein Phosphorylation Unit 18.2 Labeling Cultured Cells with 32Pi and Preparing Cell Lysates for Immunoprecipitation Unit 18.3 Phosphoamino Acid Analysis Unit 18.4 Analysis of Phosphorylation of Unlabeled Proteins Unit 18.5 Detection of Phosphorylation by Enzymatic Techniques Unit 18.6 Production of Antibodies That Recognize Specific Tyrosine-Phosphorylated Peptides Unit 18.7 Assays of Protein Kinases Using Exogenous Substrates Unit 18.8 Permeabilization Strategies to Study Protein Phosphorylation
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Unit 18.9 Phosphopeptide Mapping and Identification of Phosphorylation Sites Unit 18.10 Use of Protein Phosphatase Inhibitors Unit 18.11 Design and Use of Analog-Sensitive Protein Kinases Unit 18.12 The Detection of MAPK Signaling
Chapter 19 Informatics for Molecular Biologists Introduction Unit 19.1 Internet Basics for Biologists Unit 19.2 Sequence Databases: Integrated Information Retrieval and Data Submission Unit 19.3 Sequence Similarity Searching Using the BLAST Family of Programs Unit 19.4 Protein Databases on the Internet 1 Unit 19.5 Basic Protein Sequence Analysis
Chapter 20 Analysis of Protein Interactions Introduction Unit 20.1 Interaction Trap/Two-Hybrid System to Identify Interacting Proteins Unit 20.2 Affinity Purification of Proteins Binding to GST Fusion Proteins Unit 20.3 Phage-Based Expression Cloning to Identify Interacting Proteins Unit 20.4 Surface Plasmon Resonance for Measurements of Biological Interest Unit 20.5 Detection of Protein-Protein Interactions by Coprecipitation Unit 20.6 Identification of Protein Interactions by Far Western Analysis Unit 20.7 Two-Hybrid Dual Bait System Unit 20.8 Interaction Trap/Two-Hybrid System to Identify Loss-of-Interaction Mutant Proteins
Chapter 21 Chromatin Assembly and Analysis Introduction Unit 21.1 Micrococcal Nuclease Analysis of Chromatin Structure Unit 21.2 Separation of Histone Variants and Post-Translationally Modified Isoforms by Triton/Acetic Acid/Urea Polyacrylamide Gel Electrophoresis Unit 21.3 Chromatin Immunoprecipitation for Determining the Association of Proteins with Specific Genomic Sequences In Vivo Unit 21.4 DNase I and Hydroxyl Radical Characterization of Chromatin Complexes Unit 21.5 Isolation of Histones and Nucleosome Cores from Mammalian Cells Unit 21.6 Assembly of Nucleosomal Templates by Salt Dialysis Unit 21.7 Chromatin Assembly Using Drosophila Systems Unit 21.8 Analysis of Protein Co-Occupancy by Quantitative Sequential Chromatin Immunoprecipitation Unit 21.9 Defining In Vivo Targets of Nuclear Proteins by Chromatin Immunoprecipitation and Microarray Analysis Unit 21.10 Identifying Chromosomal Targets of DNA-Binding Proteins by Sequence Tag Analysis of Genomic Enrichment (STAGE) Unit 21.11 Mapping Chromatin Interactions by Chromosome Conformation Capture
Chapter 22 Nucleic Acid Arrays Introduction Unit 22.1 Overview of Nucleic Acid Arrays Unit 22.2 Preparation of mRNA for Expression Monitoring
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Unit 22.3 Profiling Human Gene Expression with cDNA Microarrays Unit 22.4 Overview of mRNA Expression Profiling Using Microarrays Unit 22.5 Pattern Discovery in Expression Profiling Data
Chapter 23 Manipulating the Mouse Genome Introduction Unit 23.1 Overview of Gene Targeting by Homologous Recombination Unit 23.2 Mouse Embryo Fibroblast (MEF) Feeder Cell Preparation Unit 23.3 Mouse Embryonic Stem (ES) Cell Culture Unit 23.4 Mouse Embryonic Stem (ES) Cell Isolation Unit 23.5 Production of a Heterozygous Mutant Cell Line by Homologous Recombination (Single Knockout) Unit 23.6 Production of a Homozygous Mutant Embryonic Stem Cell Line (Double Knockout) Unit 23.7 Chimeric Mouse Production by Microinjection Unit 23.8 Mouse Colony Management Unit 23.9 Transgenic Mouse Production By Zygote Injection Unit 23.10 Transgenic Mouse Colony Management Unit 23.11 Modification and Production of BAC Transgenes Unit 23.12 Regulation of Transgene Expression Using Tetracycline
Chapter 24 Generation and Use of Combinatorial Libraries Unit 24.1 Overview of Agents from Combinatorial Nucleic Acid and Protein Libraries Unit 24.2 Design, Synthesis, and Amplification of DNA Pools for Construction of Combinatorial Pools and Libraries Unit 24.3 In Vitro Selection of RNA Aptamers to a Protein Target by Filter Immobilization Unit 24.4 Peptide Aptamers: Dominant "Genetic" Agents for Forward and Reverse Analysis of Cellular Processes Unit 24.5 Protein Selection Using mRNA Display
Chapter 25 Discovery and Analysis of Differentially Expressed Genes in Single Cells and Cell Populations Introduction
Section A Nucleic Acid Amplification from Individual Cells Unit 25A.1 Laser Capture Microdissection Unit 25A.2 Preparation of Single Cells from Solid Tissues for Analysis by PCR
Section B Molecular Methods for Discovery of Differentially Expressed Genes Unit 25B.1 Production of a Subtracted cDNA Library Unit 25B.2 PCR-Based Subtractive cDNA Cloning Unit 25B.3 Differential Display of mRNA by PCR Unit 25B.4 Restriction-Mediated Differential Display (RMDD) Unit 25B.5 AFLP-Based Transcript Profiling Unit 25B.6 Serial Analysis of Gene Expression (SAGE) Unit 25B.7 Representational Difference Analysis Unit 25B.8 Gene Expression Analysis of a Single or Few Cells
xii Chapter 26 Gene Silencing Introduction Unit 26.1 Overview of RNA Interference and Related Processes Unit 26.2 Gene Silencing by RNAi in Mammalian Cells Unit 26.3 RNA Interference in Caenorhabditis Elegans Unit 26.4 Cloning of Small RNA Molecules Unit 26.5 RNA Interference in Cultured Drosophila Cells Unit 26.6 RNAi in Transgenic Plants
Chapter 27 RNA-Protein Interactions Introduction Unit 27.1 Agarose Gel Separation/Isolation of RNA-Protein Complexes Unit 27.2 Identification of RNA Binding Proteins by UV Cross-Linking Unit 27.3 Purification of Functional RNA-Protein Complexes using MS2-MBP Unit 27.4 RNA Immunoprecipitation for Determining RNA-Protein Associations In Vivo
Chapter 28 Mammalian Cell Culture Introduction Unit 28.1 Preparation, Culture, and Immortalization of Mouse Embryonic Fibroblasts Unit 28.2 Isolation and Immortalization of Lymphocytes Unit 28.3 Establishment and Culture of Human Skin Fibroblasts
Chapter 29 Mouse Phenotyping Introduction
Section A General Considerations in Mouse Phenotyping Unit 29A.1 Uses of Forward and Reverse Genetics in Mice to Study Gene Function Unit 29A.2 Minimizing Variation Due to Genotype and Environment
Section B Metabolic Exploration of the Mouse Unit 29B.1 Evaluation of Energy Homeostasis
Appendix 1 Standard Measurements, Data, and Abbreviations 1A Common Abbreviations 1B Useful Measurements and Data 1C Characteristics of Amino Acids 1D Characteristics of Nucleic Acids 1E Radioactivity 1F Safe Use of Radioisotopes 1G Centrifuges and Rotors 1H Safe Use of Hazardous Chemicals 1I Commonly Used Detergents 1J Common Conversion Factors 1K Compendium of Drugs Commonly Used in Molecular Biology Research
xiii Appendix 2 Commonly Used Reagents and Equipment 2 Commonly Used Reagents and Equipment
Appendix 3 Commonly Used Techniques in Biochemistry and Molecular Biology 3A Detection and Quantitation of Radiolabeled Proteins and DNA in Gels and Blots 3B Silanizing Glassware 3C Dialysis and Ultrafiltration 3D Quantitation of DNA and RNA with Absorption and Fluorescence Spectroscopy 3E Introduction of Restriction Enzyme Recognition Sequences by Silent Mutation 3F Techniques for Mammalian Cell Tissue Culture 3G Importing Biological Materials 3H Kinetic Assay Methods 3I Statistics for the Molecular Biologist: Group Comparisons
Appendix 4 Suppliers 4 Selected Suppliers of Reagents and Equipment
Appendix 5 Vectors 5 Vectors
CHAPTER 1 Escherichia coli, Plasmids, and Bacteriophages INTRODUCTION
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astery of current DNA technology requires familiarity with a small number of basic concepts and techniques. The goal of this chapter is to present this information concisely, yet in enough detail to be useful when a procedure goes wrong. Section I is devoted to Escherichia coli. Recipes are provided for media that support E. coli growth, as well as instructions for making the simple tools needed to work with bacterial cells. Growth of E. coli in liquid and solid media is then detailed. The final unit in Section I describes a few detailed aspects of E. coli biology learned from classical bacterial genetic studies, the understanding of which is especially relevant to the techniques used in modern DNA work. The remainder of the chapter discusses vectors used to introduce foreign DNA into E. coli. For the purposes of this chapter, vectors are said to be derived from plasmids, from bacteriophage lambda and related phages, or from filamentous phages. (Many modern vectors incorporate elements from more than one of these classes, and it is likely that this classification scheme will be hopelessly outdated by the time this chapter is revised.) Section II is concerned with plasmid vectors. Following a brief introduction to plasmid biology, procedures are described for purifying small and large amounts of plasmid DNA (“minipreps” and large preps). Finally, procedures for reintroducing plasmid DNA into bacterial cells are described. Section III covers vectors derived from bacteriophages. The biology of bacteriophage lambda is first introduced, followed by detailed aspects of biology that are especially significant when lambda derivatives are used as cloning vectors. Protocols in this section describe techniques for manipulating lambda-derived vectors, making single plaques, making and titering phage stocks, and isolating phage DNA. Finally, Section IV covers the biology and manipulation of vectors derived from filamentous phages. This chapter will be meaningful primarily to readers with some knowledge of the principles of molecular biology. Several books on molecular biology are recommended in the preface. For further advanced reading in the topics of this chapter, we recommend five books, all from Cold Spring Harbor Laboratory: Methods in Molecular Genetics (Miller, 1972), Advanced Bacterial Genetics (Davis et al., 1980), The Bacteriophage Lambda (Hershey, 1971), Lambda II (Hendrix et al., 1983), and Experiments with Gene Fusions (Silhavy et al., 1984). Many terms and jargon used by molecular biologists are introduced in this chapter. These terms are italicized at their first mention, and are defined below.
alpha fragment peptide containing the amino terminus of β-galactosidase, the lacZ gene product. Alpha fragments lack enzymatic activity, but can associate with omega fragments (see below) to form proteins whose β-galactosidase activity has been restored. alpha-complementation restoration of β-galactosidase activity to omega fragments by association with alpha fragments. Current Protocols in Molecular Biology (2002) 1.0.1-1.0.3 Copyright © 2002 by John Wiley & Sons, Inc.
amplification increase in copy number of some plasmids that occurs when host protein synthesis is inhibited. cloning site site on a vector into which foreign DNA is inserted. competent state in which bacterial or yeast cells are able to take up foreign DNA (for example, as the result of calcium treatment).
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cos site of action of phage lambda ter function. Cos site is cut by ter to yield two cohesive ends (cos ends). dilution, 10x-fold a solution or suspension that contains 1⁄10x as much (10−x as much) of the dissolved or suspended species as does the starting liquid. For example, to “do a 102-fold dilution” is to dilute a solution 100×. early-log phase period during the growth of a culture after the lag period. During early log phase growth, cells have begun exponential growth. efficiency of plating (EOP) titer of bacterial colonies or phage plaques under some experimental conditions divided by the titer of bacteria or phage obtained by growth on some reference medium. exponential growth period during which the number of cells in the culture increases as an exponential function of time, that is, during which cell number = ket. F factor genetic element found in some strains of E. coli and related species. F encodes proteins used in formation of sex pili which allow its transfer from bacterium to bacterium. female strain strain that does not contain the F factor and that receives genetic information when crossed with a strain containing F. helper phages bacteriophages that encode essential proteins and that allow other phages which do not encode these essential proteins to grow. incompatible phenomenon in which two plasmids cannot replicate in the same cell without continual selection for both of them. incompatibility group consists of plasmids that cannot be maintained together in the same cell. Compatible plasmids belong to different incompatibility groups. induction the onset of transcription of a new gene or operon, usually in response to some environmental stimulus. Phage induction or lysogenic induction describe the process in which prophage excise from the chromosome of bacteria that harbor them and begin to grow lytically. inoculation introduction of cells into a container of sterile growth medium. lag period period just after inoculation of a culture when cells have not yet begun to grow exponentially. late-log phase last period of exponential growth of a culture, after which growth slows and then stops altogether due to nutrient exhaustion or accumulation of waste products. lawn uniform layer of bacteria that covers the surface of a plate.
log phase period during growth of a culture in which cells are growing exponentially. low-copy-number plasmids plasmids found in less than about 20 copies per cell when cells containing them are grown in rich medium. lysogen E. coli cell or strain that harbors a dormant bacteriophage. male strain strain of bacteria that contains the F factor. male-specific phages bacteriophages that only grow on male strains because they adsorb to sex pili. marker detectable genetic difference between one organism and another (usually wild-type) organism of the same species. minimal medium growth medium for cells that contains only salts, vitamins, trace elements, and simple compounds which serve as carbon, nitrogen, and phosphorous sources. miniprep small-scale preparation or purification of some desired species, usually of plasmid or phage DNA. mobilization transfer of DNA from one cell to another caused by a mobile genetic element such as the F factor. multiplicity of infection (MOI) ratio of infecting bacteriophage to host cells. nonsense suppression the insertion of amino acids into proteins at positions where translation would normally not occur because the mRNA contains a UAG (amber), UAA (ochre) or UGA nonsense codon. nonsense suppressor tRNA that inserts amino acids at nonsense codons. The term is sometimes used for the genes encoding these tRNAs. omega fragment protein containing the carboxy-terminal fragment of β-galactosidase. This protein lacks enzymatic activity, but βgalactosidase activity can be restored when the peptide is complexed with an alpha fragment. ori (origin) site on genome at which DNA replication begins. outgrowth the growth of freshly transformed cells under nonselective conditions for enough time to allow proteins encoded by the foreign DNA to be expressed. overnight a small, freshly saturated liquid culture of bacteria. packaging extract extract from special strains of E. coli that contains bacteriophage lambda head proteins, tail proteins, and packaging proteins. Phage DNA added to such an extract is assembled into phage particles. par site on some plasmids which ensures that each daughter cell receives a plasmid copy. pilot protein protein in the coat of filamentous phages that helps phage DNA enter the cell.
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plates petri dishes filled with solid medium, used to grow separated bacterial colonies or plaques. The term is sometimes used to refer to 96-well microtiter dishes. plating out the placement of bacteria or phage on plates so that colonies or plaques are formed. polylinker stretch of DNA that contains contiguous restriction sites. prophage dormant bacteriophage, usually integrated into the host chromosome, that replicates with the host bacterium. relaxed control applies to plasmids whose replication does not depend on the bacterial cell cycle. replicative form double-stranded circular filamentous phage DNA found inside infected cells. replicator stretch of DNA on a phage or plasmid that enables the phage or plasmid to replicate. rich medium growth medium that contains complex organic molecules (peptides, nucleotides, etc.). Typical components of rich media include tryptone (made from beef) and yeast extract (made from yeast). rolling-circle replication mechanism of replication sometimes used by circular molecules in which DNA polymerase continually circumnavigates the template, and thus synthesizes a long tail. satellite colonies small colonies that grow around a large colony on a plate containing selective medium. These are usually composed of cells unable to grow on selective medium, but which are able to grow near the large colony because the cells in the large colony neutralize the selective agent. saturated culture culture of cells in liquid medium that has stopped growing because nutri-
ents are exhausted or because waste products have accumulated. SOS response response of E. coli to DNA damage or other treatments that inhibit DNA replication. Lambda-derived phages are induced during this response. stringent control applies to plasmids whose replication is synchronized with the E. coli cell cycle. temperate describes bacteriophages capable of lysogenic growth. transfection introduction of bacteriophage DNA into competent E. coli cells. Also describes the introduction of any DNA (including plasmid DNA) into cells of higher eukaryotes. transformation introduction of plasmid DNA into E. coli or yeast. Also used to denote any of a number of changes in cultured higher eukaryotic cells to characteristics more typical of cancer cells (immortal growth, loss of contact inhibition, etc.).
LITERATURE CITED Davis, R., Botstein, D., and Roth, J.R. 1980. Advanced Bacterial Genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Hendrix, R., Roberts, J., Stahl, F., and Weisberg, R. 1983. Lambda II. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Hershey, A.D. 1971. The Bacteriophage Lambda. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Miller, J. 1972. Methods in Molecular Genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Silhavy, T., Berman, M.L., and Enquist, L.W. 1984. Experiments with Gene Fusions. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.
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ESCHERICHIA COLI
SECTION I
Escherichia coli is a rod-shaped bacterium with a circular chromosome about 3 million base pairs (bp) long. It can grow rapidly on minimal medium that contains a carbon compound such as glucose (which serves both as a carbon source and an energy source) and salts which supply nitrogen, phosphorus, and trace metals. E. coli grows more rapidly, however, on a rich medium that provides the cells with amino acids, nucleotide precursors, vitamins, and other metabolites that the cell would otherwise have to synthesize. The purpose of this first section is to provide basic information necessary to grow E. coli. A more detailed introduction to certain aspects of E. coli biology may be found in UNIT 1.4. When E. coli is grown in liquid culture, a small number of cells are first inoculated into a container of sterile medium. After a period of time, called the lag period, the bacteria begin to divide. In rich medium a culture of a typical strain will double in number every 20 or 30 min. This phase of exponential growth of the cells in the culture is called log phase (sometimes subdivided into early-log, middle-log, and late-log phases). Eventually the cell density increases to a point at which nutrients or oxygen become depleted from the medium, or at which waste products (such as acids) from the cells have built up to a concentration that inhibits rapid growth. At this point, which, under normal laboratory conditions, occurs when the culture reaches a density of 1–2 × 109 cells/ml, the cells stop dividing rapidly. This phase is called saturation and a culture that has just reached this density is said to be freshly saturated. With very few exceptions, bacterial strains used in recombinant DNA work are derivatives of E. coli strain K-12. Most advances in molecular biology until the end of the 1960s came from studies of this organism and of bacteriophages and plasmids that use it as a host. Much of the cloning technology in current use exploits facts learned during this period.
Media Preparation and Bacteriological Tools
UNIT 1.1
Recipes are provided below for minimal liquid media, rich liquid media, solid media, top agar, and stab agar. Tryptone, yeast extract, agar (Bacto-agar), nutrient broth, and Casamino Acids are from Difco. NZ Amine A is from Hunko Sheffield (Kraft). MINIMAL MEDIA Ingredients for these media should be added to water in a 2-liter flask and heated with stirring until dissolved. The medium should then be poured into separate bottles with loosened caps and autoclaved at 15 lb/in2 for 15 min. Do not add nutritional supplements or antibiotics to any medium until it has cooled to 15 kb, do not transform well and frequently give lower DNA yields. Consider the final size of the vector plus insert when planning an experiment and wherever possible use smaller vectors. The higher the copy number the more vector DNA is produced, but high-copy-number vectors may not be applicable to all situations (see Mechanism of Replication and Copy-Number Control). When choosing a plasmid vector, consider both what sites are present in the polylinker and the order of the sites. If it will be necessary to manipulate the cloned sequence subsequent to insertion into the polylinker, plan ahead to ensure that the necessary sites will remain available in the polylinker. Selections or screens for identification of recombinant clones are useful for experiments where cloning efficiency is expected
to be low or when generation of a large number of clones is necessary. However, for routine subcloning experiments the advent of PCR (UNIT 15.1) has made it possible to rapidly screen through a large number of transformants to identify potential recombinant molecules, obviating the need for histochemical and genetic screening methods. The primary factor in choosing a plasmid vector is to understand and anticipate the experiments for which the recombinant clone will be used. The specialized functions of plasmid vectors are generally the key to selecting the correct vector for an experiment. For example, completely different types of vectors would be used for generating large quantities of DNA, expressing a fusion protein in bacteria, or for undertaking a two-hybrid screen in yeast. Once the type of vector required is determined then deciding upon a particular vector is dependent on both the details of the ancillary vector features, for example the type of promoter used to express a recombinant protein, and the properties of the replicator, polylinker, and selectable marker.
PLASMID VECTORS FOR PRODUCTION OF SINGLE-STRANDED DNA Plasmids have been developed that contain a filamentous phage origin of replication in addition to a plasmid ori. These “phagemid” vectors (UNIT 1.14) can be grown and propagated as plasmids. However, upon super-infection of a plasmid-containing cell with a wild-type helper phage, the phage ori becomes active, and single-stranded DNA (ssDNA) is produced and secreted. There are usually (+) and (−) versions of these vectors where the phage ori is in opposite orientation so that it is possible to produce ssDNA from either DNA strand. For many years, ssDNA was the substrate of choice for DNA sequencing (UNIT 7.4) and oligonucleotide-directed mutagenesis (UNIT 8.1). Development of sequencing and mutagenic protocols that use double-stranded templates has made the production of ssDNA a less frequently utilized featu re of plasmid vectors. The pBluescriptI, pBluescriptII, and pBS phagemid vectors derived from the general cloning vector pUC19 (Fig. 1.5.2) are examples of phagemids that incorporate the f1 filamentous phage ori.
PLASMID VECTORS FOR CLONING LARGE INSERTS Cosmid vectors, plasmids carrying a lambda phage cos site (e.g., pWE15, Fig. 1.5.3), were
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Eco O 109 2674
396 EcoRI SacI KpnI SmaI XmaI BamHI XbaI HincII Pst I SphI HindIII 447
Aat lI 2617 Nde l 183 Nar l 235 Ssp l 2501 Xmn l 2294 lac Z′ Scal 2177 Plac r
Ap
′lac l
pUC19 2686 bp ori
AfI III 806
Gsu l 1784 Cfr10I 1779 PpaI 1766
420
400
440
460
AGTGAATTCGAGCTCGGTACCCGGGGATCCTCTAGAGTCGACCTGCAGGCATGCAAGCTTGGCGTAATCATGGTCAT
EcoRI
Sac I
KpnI
SmaI XmaI
BamHI XbaI
HincII Acc I Sal I
Pst I
SphI
HindIII lac Z′
1 ThrlleMetThr(Met)
T3
EcoRI
NotI
Bam HI
NotI
EcoRI
Figure 1.5.2 Map of pUC19.
T7
Apr cos
ori
pWE15 8.2 kb
SV40 ori Neor
Introduction to Plasmid Biology
Figure 1.5.3 Map of pWE15 (adapted from Wahl et al., 1987, with permission).
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T7 SaII
NotI cosN IoxP
SP6 HB
NotI
IacZ
par C
EcoRI CMr
par B SnaBI pBeloBAC11 7.3 kb
XhoI
par A ori S XbaI rep E EcoRV EcoRV Figure 1.5.4 Map of pBeloBAC11 (adapted from Shizuya et al., 1992, with permission). Abbreviation: CM, chloramphenicol.
developed to facilitate cloning of large DNA fragments (UNIT 1.10). Cosmids can be transformed into cells like plasmids and once in the cells, replicate using their plasmid ori. The ColE1 type replicators are the most commonly used in these vectors, and cosmids can generally be maintained at high-copy-number in E. coli. Cosmids can also be packaged into lambda phage heads. In order for packaging to occur the cos sites must be separated by 40 to 50 kb, the approximate length of the wild-type lambda genome. Many cosmid vectors are between 5 to 10 kb in size and therefore can accept inserts of 30 to 45 kb. Because of the size of inserts they can accept, the high efficiency of packaging recombinant molecules into phage, and the efficiency of infection of cosmid-containing phage heads, cosmid vectors are frequently used for making genomic libraries. Unfortunately, propagation of insert-containing cosmid vectors in E. coli sometimes results in the deletion of all or a portion of the insert. To address this problem, a new set of cosmid vectors have been developed that replace the ColE1 replica-
tor with the F factor replicator. These “fosmid” vectors are maintained at low-copy-number, 1 to 2 copies per cell, and are more stable than higher-copy-number cosmid vectors when grown in E. coli (Kim et al., 1992). Another type of plasmid cloning vector, called bacterial artificial chromosome (BAC; Fig. 1.5.4), has been developed using the F factor replicator for propagation of very large pieces of DNA (100 to 500 kb). The vectors are used in a similar manner to yeast artificial chromosome (YAC) vectors but have the advantage of being manipulated solely in E. coli.
PLASMID VECTORS FOR EXPRESSION OF LARGE QUANTITIES OF RECOMBINANT PROTEINS There are a wide variety of vectors for expressing high levels of recombinant proteins in E. coli, insect, and mammalian cells (Chapter 16). The general goal of expressing proteins in any of these systems is to produce large quantities of a particular protein upon demand.
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Introduction to Plasmid Biology
While the features of the protein expression systems vary considerably, the basic properties outlined for expressing proteins in E. coli are common to all of them. Expression vectors are usually designed such that production of the foreign protein is tightly regulated. This is necessary because the host cellular machinery is co-opted to produce large quantities of the foreign protein, the shear amount of which may be toxic to the cell, and/or the foreign protein may encode a function that will inhibit cell growth or kill the host cell. Generally, expression vectors are configured so the polylinker cloning site is downstream of an inducible promoter. One of the promoters commonly used in E. coli expression vectors is the hybrid trp/lac promoter (trc) which contains the lacO operator site. This promoter is turned off in the presence of the lacIq repressor; the repressor gene is either carried by the bacterial host or is encoded on the expression vector itself. Expression of the foreign protein from the trc promoter is induced by the addition of IPTG. The quantity of foreign protein produced will be determined by both the rate of transcription of the gene and the efficiency of translation of the mRNA. Therefore, in addition to regulated highly inducible promoters, many E. coli protein expression vectors are designed to optimize translation of the foreign protein in bacterial cells. These expression vectors include an efficient ribosome binding site and an ATG start codon uptsream of the polylinker cloning site. Usually the cloning sites in the polylinker are designed so that it is possible to make an inframe fusion to the protein of interest in all three reading frames. For many experiments, high levels of pure recombinant protein are required, and some expression vectors are designed to create tagged or fusion proteins that facilitate purification of the recombinant protein. Tag sequences may be located 5′ or 3′ to the polylinker, creating either amino- or carboxy-terminal-tagged fusion proteins. Six polyhistidine residues (UNIT 10.11B), the FLAG epitope, and glutathione-S-transferase protein (UNIT 16.7) are some of the sequences that are appended to proteins to assist in purification. Tagged or fusion proteins can be rapidly and efficiently purified using an appropriate affinity column designed to tightly bind the tag or fusion region. Many protein expression vectors are created with specific protein cleavage sites adjacent to the tag or fusion sequences to allow for removal of these sequences from the purified protein. This feature may prove to be essential if the tag
or fusion sequence impairs the function of the protein in the relevant assays.
PLASMID VECTORS FOR REPORTER GENE FUSIONS Plasmid vectors have been designed to simplify the construction and manipulation of reporter gene fusions, where a promoter of interest is used to drive an easily scored marker gene (UNIT 9.6). Gene fusions provide a rapid and simple method for following the expression pattern conferred by a particular promoter. There are generally two types of reporter fusions, transcriptional and translational fusions. For transcriptional fusions, the ATG start codon is provided by the marker gene. In translational fusions, the 5′ untranslated region and ATG are provided by the gene of interest; in fact these constructs may fuse a large portion, or even entire coding region, to the amino terminus of a marker. In the reporter vector, the polylinker cloning site is located directly upstream of the reporter gene for insertion of the promoter fragment. In vectors that are designed for expression in eukaryotic cells, a polyadenylation signal is located downstream of the reporter gene. There are a variety of reporter genes used including chloramphenicol acetyltransferase, luciferase, β-galactosidase, secreted alkaline phosphatase, human growth hormone, β-glucuronidase, and green fluorescent protein (also see UNITS 9.6-9.7C). An example of a reporter vector pEGFP, is shown in Figure 1.5.5. The choice of reporter gene will depend on the cell type or organism, whether the assay will be done in vivo or in vitro, and whether quantitative or qualitative data is desired. The replicator, selectable marker, and other elements of the reporter vector will also have to be compatible with the system.
PLASMID VECTORS FOR YEAST Yeast plasmid vectors contain the same basic features as E. coli vectors—replicator, selectable marker, and cloning site (UNIT 13.4). There are two primary types of replicators used in yeast plasmid vectors, autonomously replicating sequences (ARS) derived from the yeast chromosomes and the natural yeast 2µm plasmid replicator. ARS-containing plasmids frequently contain yeast centromeric sequences to ensure their stable maintenance in the population of cells. Both ARS and 2µm vectors average 10 to 30 copies per cell. Many yeast vectors are “shuttle” vectors that can be maintained in both S. cerevisae and E. coli (e.g., pRS303; see Fig. 1.5.6). These vectors have an E. coli plas-
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tions in a biosynthetic pathway. For example, the URA3 marker carried on a plasmid is used to restore the ability to grow in the absence of uracil to a ura3− yeast. Therefore, yeast selectable markers, unlike most bacterial selectable markers, are strain dependent. There are a wide variety of yeast vectors available that have a range of specialized func-
P
f1 ori
SV40 poly A
NotI
Figure 1.5.5
HSV TK poly A
Map of pEGFP-1.
Kanr/ Neor
pEGFP-1 4.2 kb
e
SV40 ori PSV40
EGFP pUC ori
MCS
EGFP MCS
TA GCG CTA CCG GAC TCA GAT CTC GAG CTC AAG CTT CGA ATT CTG CAG TCG ACG GTA CCG CGG GCC CGG GAT CCA CCG GTC GCC ACC ATG GTG XmaI BamHI AgeI Eco47III SalI KpnI SacII ApaI EcoRI PstI XhoI SacI HindIII BglII Bsp1201 SmaI Ecl136II AccI Asp7181
mid replicator, frequently pMB1 derived, and a yeast replicator. Conversely, the yeast integrating plasmid vectors, used for introduction of genes into the yeast chromosome, have a bacterial replicator but no yeast replicator. The selectable markers in yeast are for the most part recessive markers, usually cloned yeast genes that are used to complement muta-
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CEN6 pRS 313 4967
ARSH4
AatII 4384 ScaI 3944 PvuI 3833
0
HindIII 808
4
HIS3
Apr
Bst XI 886 1
pRS303 4453 bp
HindIII 995 KpnI 1122 PstI 1183
3
ori
f1(+) lacZ 2
PvuII 2395 2175 KpnI
NaeI 1748 PvuI 1920 PvuII1950 2073
XhoI ClaI EcoRV PstI BamHI XbaI EagI Bst XI
ApaI SaII HindIII EcoRI SmaI SpeI NotI SacII SacI
T3
T7
Figure 1.5.6 Map of pRS303 (adapted from Sikorski and Heiter, 1989, with permission).
tions—e.g., expression of recombinant proteins in yeast, integration of sequences into the yeast genome, and cloning of very large fragments (hundreds of kilobases) of genomic DNA.
PLASMID VECTORS FOR EXPRESSION IN CULTURED MAMMALIAN CELLS
Introduction to Plasmid Biology
The type of vector that is used in mammalian cells depends on whether the experiment involves transient transfection into mammalian cells or the generation of stable mammalian cell lines carrying the construct of interest (UNITS 9.1-9.5). Virtually any plasmid vector that contains an appropriate construct for expression in mammalian cells can be used in transient assays. In transient assays, the plasmid vector carrying the DNA of interest is transfected into mammalian cells, the cells are harvested some time later (24 to 96 hr), and the pertinent assay
is performed. It is not necessary for plasmid vectors used in these assays to carry a mammalian selectable marker or to replicate in mammalian cells; therefore easily manipulatable bacterial plasmids, like pUC, are usually the vectors of choice. Plasmid vectors carrying a selectable marker that functions in mammalian cells are necessary for the generation of stable transgenic lines (UNITS 9.5; e.g., pcDNA3.1, Fig. 1.5.7). In order to generate a stable mammalian cell line, the plasmid DNA is transfected into the mammalian cells, and over a period of several weeks the DNA of interest is selected for based on expression of the vector-borne marker. Many mammalian vectors cannot replicate in mammalian cells, and the only way to maintain the DNA of interest and the selectable marker is for the vector to randomly integrate into the mammalian genome. However, there are some plasmid vectors that carry the simian
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T7
NheI PmeI AflII HindIII Asp718I KpnI BamHI Bst XI EcoRI EcoRV Bst XI NotI XhoI XbaI ApaI PmeI
pcDNA3.1(+)
T7
NheI PmeI ApaI XbaI XhoI NotI Bst XI EcoRV EcoRI Bst XI BamHI Asp718I KpnI HindIII AflII PmeI
pcDNA3.1(–)
PCMV
BGH polyA f1 ori
SV40 ori
Apr
pcDNA3.1 Vectors Neor
or
Zeor
SV40 polyA ColE1 Figure 1.5.7 Map of pcDNA3.1.
virus (SV40) or bovine papilloma virus (BPV) ori that can replicate in mammalian cells if the necessary viral replication proteins are provided; in most cases the viral replication proteins must be provided by the host cell line, limiting the range of cell types in which these vectors are useful. The most critical general feature of plasmid vectors (and also viral vectors) for creation of stable mammalian cell lines is the selectable marker (see UNIT 9.5). Unlike transient assays, formation and maintenance of stable cell lines requires selection. The number of selectable markers available for mammalian cells is limited; resistance to hygromycin, puromycin, G418, and neomycin are the predominant markers used. Since expression of foreign proteins or assaying the expression of a mammalian gene may require multiple plasmids and/or integration of constructs into the mammalian chromosome, careful planning must take place to ensure that all constructs can be selected for with the limited number markers. Further, since selection may be necessary over a long period
of time—weeks for generation of the lines and years for their maintenance—it is important to recognize that some of the antibiotics used in selection are very expensive and thus cost may be a factor in experimental design. Specialized plasmid vectors are also used for production of viruses that can infect mammalian cells. Plasmid vectors have been designed to produce infectious retroviral particles when transfected into the appropriate packaging cell line (UNIT 9.9). These retroviral vectors are then used to create stable transgenic lines in mammalian cell types not amenable to transfection. In addition, plasmid vectors have been designed to allow easy insertion of DNA sequences into vaccinia virus for the purpose of creating recombinant viruses that overexpress recombinant proteins (UNIT 16.15).
PLASMID VECTORS FOR NON–E. COLI BACTERIA Three features required of all bacterial plasmid vectors are that they replicate (unless they are suicide vectors), carry a selectable marker,
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trfA oriV
pRR54 8.1 kb bla par
ori T
H3 SphI XbaI BamHI Figure 1.5.8 Map of pRR54 (adapted from Roberts et al., 1990, with permission).
Introduction to Plasmid Biology
and can be easily introduced into host cells. The first thing to consider when selecting a plasmid vector for use in a non-E. coli host is whether or not it can replicate and be stably maintained in the particular strain. Different plasmid replicators have different host ranges, some have a narrow host range and can only replicate in a specific strain while others are promiscuous and can replicate in a wide variety of host (e.g., pRR54, Fig. 1.5.8). Unfortunately, the ColE1type replicators have a narrow host range, thus many standard E. coli vectors cannot be maintained in other bacteria. However, a number of broad-host-range replicators, such as RK2 and RSF1010, have been well characterized and used to construct vectors that can replicate in many gram-negative bacterial species (and in the case of RSF1010, some gram-positive species as well). Antibiotic resistance genes are used as selectable markers in non-E. coli bacterial hosts; however, the quantity of an antibiotic used to select against non-plasmid-containing cells is usually higher than the quantity used for E. coli selection. Furthermore, some bacterial strains are inherently resistant to particular antibiotics, thus it is important to determine whether the selectable marker carried on a plasmid vector is functional in a particular strain.
The favored method for introduction of plasmid DNA into bacterial host cells varies widely with the bacterial strain. Unlike E. coli, many bacteria cannot be efficiently transformed by chemical procedures or electroporation. In these cases bacterial mating is used to introduce plasmid DNA into the desired bacterial host. Mating to transfer a plasmid vector from an E. coli host where the vector is maintained and manipulated to a recipient bacterium requires both cis- and trans-acting functions. The tra (or mob) genes encode the trans-acting proteins necessary to transfer the plasmid DNA from one bacterium to another, and they are usually located on a helper plasmid that is distinct from the plasmid vector. Any plasmid vector that is to be mobilized must contain the cis-acting site called oriT where the DNA is cleaved and transfer is initiated.
MAPS OF PLASMIDS Figures 1.5.2-1.5.11 present maps of plasmids that are in widespread use or are examples of plasmids whose special functions make them useful for particular techniques described in this manual. Note that the trend in development of vectors is to include multiple features on a single vector, and many of these examples span the vector categories described.
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EcoRI 4361 ClaI 23 HindIII 29 AatII 4286 EcoRV 185 SspI 4170 NheI 229 BamHI 375 SphI 562 Sal I 651
ScaI 3846 PvuI 3735 PstI 3609 r
r
Ap
PpaI 3435
Tet
EagI 939 NruI 972 BspMI 1063
pBR322 4363bp
ori
BsmI 1353 Sty I 1369 AvaI 1425 Bal I 1444 BspMII 1664
Afl lII 2475
PvuII 2066
Ndel 2297
Tth111I 2219 Figure 1.5.9 Map of pBR322 (Bolivar et al. 1977; sequence in Sutcliffe, 1978).
pBR322 is one of the classic cloning vectors from which many other vectors are derived. It contains an amplifiable pMB1 replicator and genes encoding resistance to ampicillin and tetracycline. Insertion of DNA into a restriction site in either drug-resistance gene usually inactivates it and allows colonies bearing plasmids with such insertions to be identified by their inability to grow on medium with that antibiotic (see Fig. 1.5.9; Bolivar et al., 1977; sequence in Sutcliffe, 1978). pUC19 belongs to a family of plasmid vectors that contains a polylinker inserted within the alpha region of the lacZ gene. The polylinkers are the same as those used in the m13mp series (Fig. 1.14.2). pUC19 and pUC18 have the same polylinker but in opposite orientations. Under appropriate conditions (see UNIT 1.4 for a description), colonies that bear plasmids containing a fragment inserted into the polylinker form white colonies instead of blue ones. These pMB1-derived plasmids (see Fig. 1.5.2) maintain a very high-copy-number (1000 to 3000 per genome). Wild-type and recombinant plasmids confer ampicillin resistance and can be amplified with chloramphenicol (Norrander et al., 1983). In addition wildtype plasmids confer a LacZ+ phenotype to appropriate cells (e.g., JM101 cells, UNIT 1.4). pBluescript is a commonly used phagemid cloning vector that contains a polylinker in-
serted into the alpha region of the lacZ gene and T3 and T7 promoter sequences flanking the cloning sites. The f1 (+) filamentous phage origin of replication in pBluescript SK+ allows for the recovery of the sense strand of the lacZ gene as ssDNA; the pBluescript SK(−) vector (Fig. 1.5.10) with the f1 origin in the opposite orientation, f1(−), facilitates recovery of the other strand. The position of the polylinker in the alpha region of the lacZ gene allows for identification of inserts based on a blue/white color screen under the appropriate conditions. The T3 and T7 promoters are recognized by bacteriophage RNA polymerases. Transcription from these promoters reads into the polylinker from either side. RNA transcripts of any DNA cloned into the polylinker can thus be produced by run-off transcription in vitro. pWE15 is an example of a cosmid vector used for cloning DNA fragments ∼35 to 45 kb (see Fig. 1.5.3; Wahl et al., 1987). The cos sites allow the DNA to be cut and packaged into phage heads by the appropriate lambda proteins. There is a single unique BamHI cloning site flanked by T3 and T7 promoter sequences. These promoters are particularly useful for production of labeled RNA probes corresponding to the ends of the insert DNA, and these can be used to identify overlapping cosmids for chromosome walking and construction of cosmid contigs. Not1 sites flanking the cloning site can
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lacZ
T3 primer 5′–AATTAACCCTCACTAAAGGG– 3′
ColE1ori
2958 bp
AflIII 1153
pBluescript SK (+/–) MCS
Apr
f1(+)ori
SK primer 5′–CGCTCTAGAACTAGTGGATC– 3′
PvuII 977
SacI 759
KpnI 657
PvuI 503 PvuII 532
NaeI 333
SspI 445
Figure 1.5.10
759 Bsp106I HindII Eco 0109I ClaI AccI DraII Eco RV HindIII Sal I KpnI XhoI ApaI AATTCGATATCA AGCTTATCGATACCGTCGACC TCGAGGGGGGGCCCGGTACC CCAAT TCGCCCTATAGTGAGTCGTAT TACAAT TCACTGGCCGTCGT T T TACAA – 3′ (+) GCTATAGT TCGAATAGCTATGGCAGCTGGAGCTCCCCCCC GGGCCATGG GGTTAAGCGGGATAT CACTCAGCATAATGTTAAGTGACCGGCAGCAAAATGT T – 5′ (–) +1 T 7 promoter 657 3′–CGGGATATCACTCAGCATA A T G – 5′3′–TGACCGGCAGCAAAATG– 5′ 3′– C TATGGCAGCTGGAGCT– 5′ T7 primer M1320 primer KS primer
Map of pBluescript SK (+/−).
816 β-Galactosidase
EcoRI Bst XI Eag I SpeI SmaI MET T3 promoter +1 XbaI Sac I Sac II Not I BamHI Pst I 5′– GGAAACAGCTATGACCATGAT TACGCCAAGCTCGAAAT TAACCCTCACTAAAGGGAACAA AAGCTGGAGCTCCACCGCGGTGGCGGC CGCTCTAGAACTAGTGGATCCCCCGGGCTGCAGG 3′–CCTT TGTCGATACTGGTACTA ATGCGGT TCGAGCTTTAATTGGGAGTGATT TCCCTTGT T T TCGACCTCGAGGTGGCGCCAC CGCCGGCGAGATCTTGATCACCTAGGGGGCCCGACGTC CT TAA
reverse primer 5′– GGAAACAGCTATGACCATG–3′
PvuI 2416
ScaI 2526
XmnI 2645
SspI 2850 SspI 22
NaeI 134
4.0
3.5
lacI q
NcoI EcoRI SstI KpnI SmaI 4.37/0 BamHI XbaI 0.5 SalI PstI Ptrc 5S SphI T1 HindIII T2
1.0
pTrc99A,B,C 3.0
ori
Apr PvuI 1.5
2.5 2.0 pTrc99A
1
2
Met Glu
3
4
Phe Glu
5
6
7
8
9
Leu
Gly
Thr
Arg
Gly
11
12
13
14
15
16
17
18
19
Ser Ser
10
Arg
Val
Asp
Leu
Gln
Ala
Cys
Lys Leu
20
rrnB RBS TCACACAGGAAACAGACC ATG GAA TTC GAG CTC GGT ACC CGG GGA TCC TCT AGA GTC GAC CTG CAG GCA TGC AAG CTT GGCTGTTTTGGC
Nco l
pTrc99B
EcoRI
Sst I
XmaI SmaI
KpnI
1
2
3
4
5
6
Met
Gly
Ile
Arg
AIa
Arg
7 Tyr
8 Pro
BamHI
SaI I Acc I HincII
XbaI
9
10
11
Gly
IIe
Leu
Pst I
SphI
HindIII
rrnB RBS TCACACAGGAAACAGACC ATG GGA ATT CGA GCT CGG TAC CCG GGG ATC CTC TAG A GTCGACCTGCAGGCATGCAAGCTTGGCTGTTTTGGC Nco l
pTrc99C
EcoRI
1
2
3
Met
Gly
Asn
Sst I
KpnI
4
5
6
Ser
Ser
Ser Val
7
XmaI SmaI
BamHI
8
9
Pro
Gly
10
SaI I Acc I HincII
XbaI
11
Pst I
Sph I
HindIII
12
13
14
15
16
17
18
19
20
21
Asp Pro Leu
Glu
Ser
Thr
Cys
Arg
His
Ala
Ser
Leu
rrnB RBS TCACACAGGAAACAGACC ATG GGG AAT TCG AGC TCG GTA CCC GGG GAT CCT CTA GAG TCG ACC TGC AGG CAT GCA AGC TTG GCTGTTTTGGC Nco l
EcoRI
Sst I
KpnI
XmaI SmaI
BamHI
XbaI
SaI I Acc I HincII
Pst I
SphI
HindIII
Figure 1.5.11 Map of pTrc99A,B,C.
potentially be used to excise an intact insert fragment from the vector. The ColE1-derived ori and ampicillin resistance gene allow for replication and selection in bacteria. The SV40 promoter (included in SV40 ori) which drives the neomycin phosphotransferase gene enables selection in eukaryotic cells. pBeloBAC11 is an example of the family of bacterial-artificial-chromosome vectors based on the low-copy-number F factor replicator (see Fig. 1.5.4; Shizuya et. al., 1992). BAC vectors are used for cloning large DNA fragments (100 to 500 kb) in E. coli and are used commonly in genome-mapping strategies. oriS, repE, parA, parB, and parC genes are the
essential genes that compose the F factor replicator. oriS and repE genes are required for unidirectional replication of the plasmid, and parABC loci stably maintain the copy number at one to two per E. coli genome. There are two unique cloning sites (HindIII and BamHI) inserted into the lacZ alpha region. Other useful features of the cloning region are (1) T7 and SP6 promoter sequences flanking the cloning sites, (2) NotI restriction sites flanking the cloning sites for potential excision of the insert, and (3) and presence of the loxP and cosN sites that can be cleaved by specific enzymes. The ends generated by cleavage at loxP or cosN can be used as fixed reference points in building an
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Introduction to Plasmid Biology
ordered restriction map by end labeling and partial restriction digestion. pEGFP-1 is a selectable vector for monitoring promoter activity in mammalian cells via fluorescence of a green fluorescent protein (GFP) derivative (Clontech; see Fig. 1.5.5; Yang et al., 1996). The vector contains a neomycin resistance gene downstream of the SV40 early promoter for selection of stably transformed mammalian cells. It has a polylinker located upstream of the EGFP gene, so that the function of promoter sequences introduced into the polylinker can be assessed based on EGFP activity. The EGFP gene is modified from wildtype GFP to ensure expression in mammalian cells, it has silent base mutations that correspond to human codon-usage preferences, and sequences flanking the coding region have been converted to a Kozak consensus translation initiation signal. The vector backbone contains an f1 ori for production of ssDNA, a pUC-derived ori for propagation in E. coli and a kanamycin resistance gene for selection in bacteria. A series of yeast shuttle vectors (pRS304, 305, and 306) has been created to facilitate manipulation of DNA in Saccharomyces cerevisiae (see Fig. 1.5.6; Sikorski and Hieter, 1989). These vectors have a backbone derived from pBLUESCRIPT into which the features necessary for replication and maintenance in yeast have been introduced. The members of this series of plasmids differ only in the yeast selectable marker incorporated; pRS303 carries the HIS3 marker that complements a nonreverting his3 chromosomal mutation in specific yeast strains. These plasmids contain an autonomously replicating sequence as well as a centromere sequence, CEN6, that ensures stable maintenance in yeast cells. pcDNA3.1 is a selectable cloning and expression vector for use in mammalian cells. The features of this vector include a neomycin resistance gene driven by the SV40 early promoter (contained within the SV40 ori) and terminated by an SV40 polyadenylation signal for selection in mammalian cells (see Fig. 1.5.7). In addition, due to the inclusion of the SV40 ori, the vector can replicate as an episome in cells expressing the SV40 large T antigen. The polylinker cloning site is located downstream of strong cytomegalovirus enhancerpromoter sequences and upstream of the bovine growth hormone gene termination signals for high-level expression of protein-coding sequences cloned into this vector. This vector also contains some of the more standard features of
other plasmid vectors, including a ColE1 replicator for propagation in E. coli, the ampicillin resistance gene for selection in E. coli, the f1 ori for production of ssDNA, and the T7 promoter sequence for in vitro transcription of DNA inserted into the polylinker. pRR54 is an example of a broad-host-range mobilizable plasmid vector. This vector contains replicator and stablilization sequences derived from the natural RK2 broad-host-range plasmid (see Fig. 1.5.8; Roberts et al., 1990). oriV is the vegetative origin of replication, trfA encodes trans-acting functions necessary for replication, and par encodes a locus that enhances stability of the plasmid. This plasmid can be mated into diverse gram-negative species as long as the appropriate mobilization machinery is provided in trans because it contains the origin of conjugal transfer, oriT. The plasmid carries the β-lactamase gene, allowing for ampicillin/carbenecillin selection of plasmid containing bacteria. The pTrc series of plasmid expression vectors facilitates regulated expression of genes in E. coli. These vectors carry the strong hybrid trp/lac promoter, the lacZ ribosome-binding site (RBS), the MCS of pUC18 that allows insertion in three reading frames, and the rrnB transcription terminators (see polylinker sequences given below the vector diagram in Fig. 1.5.11). These vectors are equally useful for expression of unfused proteins (resulting from insertion in the NcoI site) or for expression of fusion proteins (using one of the cloning sites in the correct translational frame). The presence of the lacIq allele on the plasmid ensures complete repression of the hybrid trp/lac promoter during cloning and growth in any host strain (see Amann et al., 1988, for further details).
LITERATURE CITED Amann, E., Ochs, B., and Abel, K.-J. 1988. Tightly regulated tac promoter vectors useful for the expression of unfused and fused proteins in Escherichia coli. Gene 69:301-315. Bolivar, F., Rodriguez, R.L., Greene, P.J., Betlach, M.C., Heynecker, H.L, and Boyer, H.W. 1977. Construction of useful cloning vectors. Gene 2:95-113. Chang, A.C.Y. and Cohen, S.N. 1978. Construction and characterization of amplifiable mulitcopy DNA cloning vehicles derived from the P15A cryptic miniplasmid. J. Bacteriol. 134:11411156. Gerhart, E., Wagner, H., and Simmons, R.W. 1994. Antisense RNA control in bacteria, phages, and plasmids. Annu. Rev. Microbiol. 48:713-742.
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Jackson, D.A., Symons, R.M., and Berg, P. 1972. Biochemical method for inserting new genetic information into DNA of Simian Virus 40 circular DNA molecules containing lambda phage genes and the galactose operon of Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 69:2904-2909. Kahn, M., Kolter, R., Thomas, C., Figurski, D., Meyer, R., Remaut, E., and Helinski, D.R. 1979. Plasmid cloning vehicles derived from plasmids ColE1, F, R6K, RK2. Methods Enzymol. 68:268280.
Sikorski, R.S. and Hieter, P.I. 1989. A system of shuttle vectors and yeast host strains designed for efficient manipulation of DNA in Saccharomyces cerevisiae. Genetics 122:19-27. Stoker, N.G., Fairweather, N., and Spratt, B.G. 1982. Versatile low-copy-number plasmid vectors for cloning in Escherichia coli. Gene 18:335-341. Sutcliffe, J.G. 1978. Complete nucleotide sequence of the Escherichia coli plasmid pBR322. Cold Spring Harbor Symp. Quant. Biol. 43:77-90.
Kim, U.J., Shizuya, H., de Jong, P.J., Birren, B., and Simon, M.I. 1992. Stable propagation of cosmidsized human DNA inserts in an F-factor based vector. NAR 20:1083-1085.
Uhlin, B.E., Schweickart, V., and Clark, A.J. 1983. New runaway-replication-plasmid cloning vectors and suppression of runaway replication by novobiocin. Gene 22:255-265.
Norrander, J., Kempe, T., and Messing, J. 1983. Construction of improved M13 vectors using oligonucleotide-directed mutagenesis. Gene 26:101-106.
Wahl, G.M., Lewis, K.A., Ruiz, J.C., Rothenberg, B., Zhao, J., and Evans, G.A. 1987. Cosmid vectors for rapid genomic walking, restriction mapping, and gene transfer. Proc. Natl. Acad. Sci. U.S.A. 84:2160-2164.
Roberts, R.C., Burioni, R., and Helinski, D.R. 1990. Genetic characterization of the stabilizing functions of a region of broad-host-range plasmid RK2. J. Bacteriol. 172:6204-6216. Shizuya, H., Birren, B., Kim U.J., Mancin, V., Slepak, T., Tachiiri, Y., and Simon, M.I. 1992. A bacterial system for cloning large human DNA fragments. Proc. Natl. Acad. Sci. U.S.A. 89:8794-8797.
Yang, T., Cheng, L., and Kain, S.R. 1996. Optimized codon usage and chromophore mutations provide enhanced sensitivity with the green fluorescent protein. Nucl. Acids Res. 24:4592-4593.
Contributed by Rhonda Feinbaum Massachusetts General Hospital Boston, Massachusetts
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Minipreps of Plasmid DNA
UNIT 1.6
Although there are a large number of protocols for the isolation of small quantities of plasmid DNA from bacterial cells (minipreps), this unit presents four procedures based on their speed and success: the alkaline lysis prep, a modification of the alkaline lysis prep that is performed in 1.5-ml tubes or 96-well microtiter dishes, the boiling method, and a lithium-based procedure. A support protocol provides information on storing plasmid DNA. ALKALINE LYSIS MINIPREP The alkaline lysis procedure (Birnboim and Doly, 1979, and Birnboim, 1983) is the most commonly used miniprep. Plasmid DNA is prepared from small amounts of many different cultures (1 to 24) of plasmid-containing bacteria. Bacteria are lysed by treatment with a solution containing sodium dodecyl sulfate (SDS) and NaOH (SDS denatures bacterial proteins, and NaOH denatures chromosomal and plasmid DNA). The mixture is neutralized with potassium acetate, causing the covalently closed plasmid DNA to reanneal rapidly. Most of the chromosomal DNA and bacterial proteins precipitate—as does the SDS, which forms a complex with potassium—and are removed by centrifugation. The reannealed plasmid DNA from the supernatant is then concentrated by ethanol precipitation.
BASIC PROTOCOL
Materials LB medium (UNIT 1.1) containing appropriate antibiotic (Table 1.4.1) Glucose/Tris/EDTA (GTE) solution TE buffer (APPENDIX 2) NaOH/SDS solution Potassium acetate solution 95% and 70% ethanol 10 mg/ml DNase-free RNase (optional; UNIT 3.13) 1.5-ml disposable microcentrifuge tubes 1. Inoculate 5 ml sterile LB medium with a single bacterial colony. Grow to saturation (overnight). 2. Spin 1.5 ml of cells 20 sec in a microcentrifuge at maximum speed to pellet. Remove the supernatant with a Pasteur pipet. The spins in steps 2 and 6 can be performed at 4°C or at room temperature. Longer spins make it difficult to resuspend cells.
3. Resuspend pellet in 100 µl GTE solution and let sit 5 min at room temperature. Be sure cells are completely resuspended.
4. Add 200 µl NaOH/SDS solution, mix by tapping tube with finger, and place on ice for 5 min. 5. Add 150 µl potassium acetate solution and vortex at maximum speed for 2 sec to mix. Place on ice for 5 min. Be sure mixing is complete.
6. Spin 3 min as in step 2 to pellet cell debris and chromosomal DNA. 7. Transfer supernatant to a fresh tube, mix it with 0.8 ml of 95% ethanol, and let sit 2 min at room temperature to precipitate nucleic acids. Contributed by JoAnne Engebrecht, Roger Brent, and Mustak A. Kaderbhai Current Protocols in Molecular Biology (1991) 1.6.1-1.6.10 Copyright © 2000 by John Wiley & Sons, Inc.
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8. Spin 1 min at room temperature to pellet plasmid DNA and RNA. 9. Remove supernatant, wash the pellet with 1 ml of 70% ethanol, and dry pellet under vacuum. 10. Resuspend the pellet in 30 µl TE buffer and store as in support protocol. Use 2.5 to 5 µl of the resuspended DNA for a restriction digest. Contaminating RNA may interfere with detection of DNA fragments on the agarose gel; it can be destroyed by adding 1 ìl of a 10 mg/ml RNase solution (DNase-free) to the digestion mixture. ALTERNATE PROTOCOL
ALKALINE LYSIS IN 96-WELL MICROTITER DISHES Escherichia coli cells that contain plasmids are grown and lysed, and the plasmid DNA is precipitated—all in the wells of 96-well microtiter dishes. This procedure makes it possible to perform hundreds of rapid plasmid preps in a day. It is based on an unpublished procedure by Brian Seed of Massachusetts General Hospital. Additional Materials TYGPN medium (UNIT 1.1) 70% ethanol, ice-cold Isopropanol 96-well microtiter plates (Dynatech PS plates or equivalent) Multichannel pipetting device (8-prong Costar; 12-prong Titer Tek) Multitube vortexer Sorvall RT-6000 low-speed centrifuge, or equivalent, with microplate carrier in H-1000B rotor 1. Add 0.3 ml sterile TYGPN medium to each well of a 96-well microtiter plate (see sketch 1.6A). Inoculate each well with a single plasmid-containing colony. The 96-well microtiter plates used must have U-shaped bottoms. To take full advantage of this protocol, one should perform all pipetting steps with a multichannel pipetting device.
adding solution to 96-well microtiter dish
Minipreps of Plasmid DNA
Sketch 1.6A
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2. Grow bacteria to saturation at 37°C (∼48 hr). All subsequent steps are performed at room temperature unless otherwise noted. Potassium nitrate in the TYGPN medium presumably acts as a terminal electron acceptor when the bacteria in the wells are growing anaerobically, resulting in high cell densities.
3. Spin saturated cultures in H-1000B rotor with microplate carrier for 10 min at 2000 rpm (600 × g), 4°C. Decant supernatant with brief flick. A microplate carrier is available for the Beckman JS-4.2 rotor. The same rpm values can be used for the JS-4.2 as those given here for the Sorvall H-1000B (see APPENDIX 1 for rotor conversion values).
4. Resuspend cells in the well bottoms by clamping the plate in a multitube vortexer and running it 20 sec at setting 4. 5. Add 50 µl GTE solution to each well. 6. Add 100 µl NaOH/SDS solution to each well. Wait 2 min. 7. Add 50 µl potassium acetate solution to each well. 8. Cover with plate tape or parafilm. Agitate vigorously in vortexer 20 sec at setting 4. Spin 5 min at 2000 rpm (600 × g), 4°C. 9. Insert a pipet tip just at the edge of the U in the bottom of the well. Remove 200 µl from each well and transfer to a new plate. Do not try to recover all the fluid in each well.
10. Add 150 µl isopropanol to each well of the new plate. Cover with plate tape, agitate, and chill 30 min at −20°C. 11. Spin 25 min at 2000 rpm, 4°C, and decant supernatant. Wash pellets with cold 70% ethanol, gently decant supernatant, wash with 95% ethanol, and again gently decant supernatant. Pellets often shrink visibly during the 70% ethanol wash, as impurities in them are dissolved. Restriction enzymes will not cut well if the DNA is contaminated with even very small amounts of NaOH/SDS or potassium acetate solutions. It is therefore very important to decant the supernatants from the isopropanol precipitation and ethanol washes thoroughly, but not so vigorously that the pellets are flung out of the well bottoms. If pellets become detached during either washing step, the plate should be respun at 2500 rpm for 5 min to bring the pellets to the bottom of the wells again.
12. Air dry pellets for 30 min, then resuspend in 50 µl TE buffer. Store as in support protocol and use 10-µl aliquots for digestion.
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BASIC PROTOCOL
BOILING MINIPREP Bacteria that contain plasmid DNA are broken open by treatment with lysozyme, Triton (a nonionic detergent), and heat. The chromosomal DNA remains attached to the bacterial membrane and is pelleted to the bottom of a centrifuge tube during a brief spin. Plasmid DNA is precipitated from the supernatant with isopropanol (Holmes and Quigley, 1981). This procedure is recommended for preparing small amounts of plasmid DNA from 1 to 24 cultures. It is extremely quick, but the quality of DNA produced is lower than that from the alkaline lysis miniprep. Materials LB medium (UNIT 1.1) containing appropriate antibiotic (Table 1.4.1) STET solution Hen egg white lysozyme Isopropanol, ice-cold TE buffer (APPENDIX 2) 10 mg/ml DNase-free RNase (optional; UNIT 3.13) 1.5-ml disposable microcentrifuge tubes Boiling water bath (100°C) 1. Inoculate 5 ml sterile LB medium with a single bacterial colony. Grow at 37°C at least until mid-log phase ∼6 hr, (a freshly saturated overnight culture works even better; see UNIT 1.2). 2. Transfer 1.5 ml of the saturated culture to a 1.5-ml microcentrifuge tube and pellet the cells by spinning 20 sec in microcentrifuge at maximum speed. Discard supernatant with a Pasteur pipet. The spins in steps 2, 6, and 7 can be performed at 4°C or room temperature. Longer spins make it difficult to resuspend cells.
3. Resuspend the bacteria in 300 µl of STET solution containing 200 µg lysozyme. Vortex to achieve complete suspension. Be sure cells are completely resuspended in order to maximize the number of cells exposed to the lysozyme and consequently the yield of plasmid DNA.
4. Place tube on ice for 30 sec to 10 min. The time required for this step can vary between the limits indicated without affecting the yield or quality of the plasmid DNA.
5. Place tube in a boiling water bath (100°C) 1 to 2 min. Heat and detergents cause the weakened cell walls to break, releasing plasmid DNA and RNA, but not the larger bacterial chromosome which remains attached to or trapped inside the lysed cells.
6. Spin in microcentrifuge 15 to 30 min at maximum speed. The pellet, which should be fairly gummy, contains bacterial debris as well as chromosomal DNA. The supernatant contains plasmid DNA and RNA.
7. Pipet off supernatant into a new tube, carefully, without dislodging pellet. Mix with 200 µl (an equal volume) of cold isopropanol. Place at −20°C for 15 to 30 min. Spin 5 min in microcentrifuge at maximum speed. The cold isopropanol precipitates the plasmid DNA and cellular RNA. Considerably shorter incubation periods (e.g., 2 to 5 min) may be sufficient for precipitation. Minipreps of Plasmid DNA
8. Remove the supernatant by inverting the tube and flicking it several times. Dry the
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pellet by placing under a vacuum until it looks flaky. If a vacuum source is unavailable, the pellet can be air dried.
9. Resuspend the pellet in 50 µl TE buffer and store as in the support protocol. Use 5 µl of the resuspended DNA for a restriction digest. Contaminating RNA may interfere with detection of DNA fragments on the agarose gel; it can be destroyed by adding 1 ìl of a 10 mg/ml RNase solution (DNase-free) to the digestion mixture.
LITHIUM MINIPREP Plasmid DNA is obtained from E. coli grown on plates as colonies or in liquid cultures. Bacterial cells harboring plasmid DNA are sequentially treated with Triton X-100/LiCl and phenol/chloroform. These steps solubilize plasmid DNA while precipitating chromosomal DNA with cellular debris. The debris is removed by centrifugation. This isolation procedure yields preparations of plasmid DNA that are virtually devoid of chromosomal DNA.
BASIC PROTOCOL
The procedure described here (originally presented by He et al., 1990) for small-scale isolation of plasmid DNA can also be readily extended for large-scale preparations as described in the annotation to the final step. The merit of the approach is that it is extremely reliable and rapid—requiring no more than 20 min of simple and economical operations for a preparation. The final plasmid DNA preparations are of a purity and quality usable for most biological applications. Materials TELT solution LB medium (UNIT 1.1) containing appropriate antibiotic (Table 1.4.1) l:l (w/v) phenol/chloroform (UNIT 2.1) 100% ethanol, prechilled to −20°C TE buffer (APPENDIX 2) 10 mg/ml DNase-free RNase A (optional; UNIT 3.13) 1.5-ml disposable microcentrifuge tubes NOTE: All steps are performed at room temperature. 1. To isolate plasmid DNA from transformant colonies grown on agar plates, prepare the cells as follows: a. Using a microspatula, scoop out an entire bacterial colony grown to 2- to 5-mm diameter on an LB agar plate. Transfer the colony to a 1.5-ml microcentrifuge tube containing 100 µl TELT solution. b. Vortex thoroughly to suspend the cells. Proceed to step 3. 2. To isolate plasmid DNA from liquid cultures, prepare the cells as follows: a. Inoculate a colony of bacteria into 1.8 ml of sterile LB medium supplemented with appropriate antibiotic. Grow to saturation with shaking for 18 to 24 hr at 30°C (see UNIT 1.2). Glass test tubes with plastic caps are suitable. Place the tubes at a suitably inclined angle to achieve good agitation.
b. Carefully transfer the entire culture volume into a 1.5-ml microcentrifuge tube. At this stage the volume of the liquid culture will have been reduced to ∼1.5 ml.
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c. Pellet the cells by spinning in a microcentrifuge (10,000 × g) for 20 sec. The spins in steps 2, 4, 7, and 9 can be performed at 4°C or room temperature. Centrifugation for longer periods or at higher speeds makes it difficult to resuspend the cells in the following step.
d. With the tube held in a vertical position, aspirate the supernatant using a longtipped Pasteur pipet connected to a vacuum line. e. Add 100 µl of TELT solution to the pellet and resuspend by vortexing. Ensure that the cells are thoroughly suspended. See annotation to step 11 for scaled-up DNA preparations.
3. Add 100 µl of 1:1 phenol/chloroform and thoroughly vortex for 5 sec. This mixture may be left at room temperature for ≤15 min. Plasmid yield will elevate with increasing duration of incubation; however, incubation periods >15 min may result in phenol-mediated modification of DNA.
4. Microcentrifuge 1 min at 15,000 × g (maximum speed). 5. Using a pipettor or similar device, carefully withdraw 75 µl of the upper aqueous phase and transfer the contents into a clean microcentrifuge tube. Do not agitate the resolved phases. If mixing occurs, recentrifuge. When collecting the top layer avoid picking the debris at the interface.
6. To the supernatant, add 150 µl of chilled 100% ethanol. Mix the contents well to precipitate the plasmid DNA. 7. Pellet the nucleic acids by microcentrifuging 5 min at maximum speed. 8. Discard the supernatant by inverting the tube. When all the supernatant has drained, hold the tube in the same position for a few seconds and wipe off the last droplet from the rim of the tube by touching the edge of a Kleenex paper tissue. 9. Wash the pellet with 1 ml of cold 100% ethanol and harvest the nucleic acid pellet as in steps 6 and 7. After centrifugation, decant the supernatant carefully as the pellet may be loose.
10. Cap the tube. Stab a small hole in the cap with a thumbtack or a syringe needle. Place the tube in a vacuum desiccator (without desiccant). Apply vacuum until the nucleic acid pellet appears completely dry. A water-pumped vacuum line suffices for the purpose and usually takes ≤15 min.
11. Dissolve the pellet in 30 µl TE buffer. Vortex the contents well, capturing most of the DNA around the inner surface of the microcentrifuge tube. Store as in the support protocol and use 2 to 5 µl of DNA solution in a final 20-µl reaction volume for restriction digestion. Contaminating RNA may interfere with detection of DNA fragments on the agarose gel; it can be destroyed by adding 1 ìl of a 10 mg/ml RNase solution (DNase-free) to the digestion mixture. For scaled-up plasmid DNA preparations (He et al., 1991), increase the amounts of TELT solution and 1:1 phenol/chloroform in direct proportion to the culture volume used. For cultures ≤5 ml, transfer the cells after suspension in TELT buffer into a microcentrifuge tube. Wash the final nucleic acids pellet twice with 1 ml of 100% ethanol. For cultures between 5 and 100 ml, use Corex glass tubes for treatment with TELT and phenol/chloroform and for centrifugations (Sorvall RC-5C centrifuge at 6000 × g). Minipreps of Plasmid DNA
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STORAGE OF PLASMID DNA Plasmids can be maintained for a short period (up to 1 month) in bacterial strains simply by growing on selective plates and storing at 4°C. For permanent storage, bacteria harboring the plasmid should be grown to saturation in the presence of the appropriate selective agent. An equal volume (∼1 to 2 ml) of bacteria should be added to sterile 100% glycerol or a DMSO-based solution (recipe in UNIT 1.3) and frozen at −70°C in sterile vials. Cells taken from storage should again be grown on a selective plate (UNIT 1.1), and the plasmid DNA should be checked by restriction analysis (UNIT 3.1).
SUPPORT PROTOCOL
Plasmid DNA can be stored in TE buffer at 4°C for several weeks or preserved for several years by storing at −20° or −70°C. Most investigators prefer to store plasmids as frozen DNA, due to the widely held belief that plasmids stored in bacteria are sometimes lost, are rearranged, or accumulate insertion sequences and transposons during storage or on revival. Although such rearrangements certainly occur during storage of plasmids in bacteria in stab vials, we are unaware of any report of rearrangements affecting plasmids stored in frozen cells. REAGENTS AND SOLUTIONS Glucose/Tris/EDTA (GTE) solution 50 mM glucose 25 mM Tris⋅Cl, pH 8.0 10 mM EDTA Autoclave and store at 4°C NaOH/SDS solution 0.2 N NaOH 1% (wt/vol) sodium dodecyl sulfate (SDS) Prepare immediately before use 5 M potassium acetate solution, pH 4.8 29.5 ml glacial acetic acid KOH pellets to pH 4.8 (several) H2O to 100 ml Store at room temperature (do not autoclave) STET solution 8% (wt/vol) sucrose 5% (wt/vol) Triton X-100 50 mM EDTA 50 mM Tris⋅Cl, pH 8.0 Filter sterilize and store at 4°C TELT solution 2.5 M LiCl 50 mM Tris⋅Cl, pH 8.0 62.5 mM Na2 EDTA 4% (wt/vol) Triton X-100 Store as 1- to 5-ml aliquots frozen at −20°C (do not filter sterilize or autoclave)
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COMMENTARY Background Information Isolation of small quantities of plasmid DNA from bacterial cells is essential for the analysis of recombinant clones (UNITS 3.2 & 6.3). A myriad of plasmid DNA miniprep methods now exist and investigators are generally remarkably loyal to his or her own particular protocol. This unit presents three of the most widely used and reliable methods—alkaline lysis, the boiling method, and a lithium-based miniprep. The success of any of these procedures is largely a function of the expertise of the investigator; choice of method is therefore determined by personal preference as well as the size and type of the plasmid and the host strain of E. coli. With practice, all three protocols yield plasmid DNA of sufficient quantity and quality for use in most enzymatic manipulations (Chapters 3 & 7), and in most bacterial (UNIT 1.8) and yeast (UNIT 13.7) transformation procedures. The procedures presented in this unit allow for the preferential recovery of circular plasmid DNA over linear chromosomal DNA. Treatments with either base or detergent (used in all three procedures) disrupt base pairing and cause the linear chromosomal DNA to denature and separate. In contrast, because of its supercoiled configuration, covalently closed circular plasmid DNA is unable to separate and readily reforms a correctly paired superhelical structure under renaturing conditions. In the alkaline lysis miniprep, treatment with SDS and NaOH breaks open bacterial cells. Subsequent addition of potassium acetate preferentially reanneals covalently closed plasmid DNA, while chromosomal DNA and proteins are trapped in a complex formed between the potassium and SDS. The lysis treatment in the boiling miniprep causes chromosomal DNA to remain attached to the bacterial membrane, while plasmid DNA remains in the supernatant. In the lithium method, treatment of bacterial cells with Triton X-100/LiCl results in dissolution of the inner bacterial plasma membrane. However, this treatment has no effect on the overall morphology of the cells as observed microscopically, nor does it lead to release of plasmid DNA from the cells. Subsequent addition of phenol/chloroform leads to denaturation and precipitation of intracellular proteins. The concomitant rapid shrinkage of the cells preferentially expels soluble, super-
coiled plasmid DNA into the medium while retaining chromosomal DNA and denatured cellular protein with the bulk of the cell mass. Bacterial morphology, particularly of the cell wall envelope, is preserved under these conditions for at least 30 min, at which point cell lysis ensues. In all preparations, chromosomal DNA is removed with cellular debris by centrifugation, and soluble, supercoiled plasmid DNA is concentrated by ethanol precipitation. In general, all three methods provide plasmid DNA of comparable yield and quality suitable for most biological applications. Yield of DNA is determined more by the type of plasmid than by method of isolation. Plasmids derived from pBR322 generally give lower (but, for most applications, sufficient) yields compared with the more recently derived pUC-like vectors, which contain a lesion in the plasmid-encoded rop gene, causing the plasmid to be maintained in high copy number in the cell (see UNIT 1.5). All three methods are successful for the isolation of small (10 µg/ml. This is not a concern when applied to plasmid DNA purification, where the concentrations should be orders of magnitude greater than that figure. Plasmid DNA purification by anion-exchange or size-exclusion chromatography. The columns available in kit form from a number of manufacturers are quite reliable. Almost all necessary reagents are provided, including common buffers such as TE buffer. Because the composition of the matrix is undisclosed, it is impossible to evaluate the procedure carefully and attempt to optimize it. Therefore, users of kits are strongly encouraged to follow the manufacturer’s recommended procedures. Additional discussion of one type of anion-exchange procedure (Qiagen) can be found in the Commentary of UNIT 2.1B. A major drawback to some prepared columns is the limited capacity of the matrix. A 500-ml culture of plasmid-containing bacteria can often yield 2 to 8 mg of plasmid DNA in the crude lysate. Some columns routinely yield only 500 to 1000 µg purified plasmid DNA, which is adequate for most purposes. Recovering the column flowthrough and rechromatographing it is the most practical method of increasing recovery of plasmid DNA. Not all columns can be reused, however, and recovery may require use of additional columns. Alter-
natively, smaller culture volumes can be used as suggested. If optimal recovery of DNA is desired, CsCl/ethidium bromide centrifugation or PEG precipitation should be used. DNA obtained from chromatographic purification of plasmid DNA is comparable in quality to that prepared by the other methods. It is of sufficient purity for virtually any procedure for which it can be used. For example, plasmid DNA prepared using Qiagen-tips is at least equivalent in purity to that obtained following two rounds of CsCl-gradient centrifugation. It is suitable for all applications from cloning to transfection, radioactive and fluorescent sequencing, and gene therapy research. Qiagen also offers an endotoxin-free plasmid DNA kit which yields plasmid DNA containing lower levels of endotoxins than DNA purified by two rounds of CsCl centrifugation, which is essential in gene therapy studies. The two most frequent contaminants are chromosomal DNA and high-molecularweight RNA. These contaminants may be detected by the presence of large, diffuse ethidium bromide–binding material in agarose gel electrophoresis of purified plasmid DNA. To prevent such contamination, follow the manufacturer’s suggestions for the use of RNase. Only common laboratory equipment is required for chromatographic purification of plasmid DNA. Another consideration with commercial kits is the large amount of packaging material and waste. In addition to the plastic columns and excess packaging, kits contain standard reagents supplied in plastic bottles. These reagents are solutions of buffers, salts, ethanol, and detergent—all of which can be, and usually are, prepared in the lab. Most suppliers will provide the column without the reagents. However, the major advantage of using prepared anion-exchange column kits includes the absence of organic extractions and exposure to toxic chemicals such as phenol, chloroform, ethidium bromide, and CsCl. The toxicity of DNA prepared by several methods has been assessed by performing a biological assay. Crude lysate from a 1-liter culture of plasmid-containing bacteria was prepared by the alkaline lysis procedure and divided into four equal aliquots. The aliquots were then subjected to purification by CsCl/ ethidium bromide centrifugation, PEG precipitation, or chromatography on Qiagen and pZ523 columns. Purified plasmid DNA was injected into Drosophila embryos, and the frequency of germline transformation and killing
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of injected embryos was determined. No significant differences attributable to the method of purification were observed.
Anticipated Results Most plasmids currently used are derivatives of the pUC series (Fig. 1.5.2). These plasmids contain an origin of replication significantly more efficient than that of the previous generation of pBR322-derived plasmids. This allows recovery of 1 to 5 mg of plasmid DNA (free of contaminating bacterial products) from a 500ml culture following any of the crude lysate preparation methods or PEG precipitation. Purification by CsCl/ethidium bromide density gradient centrifugation yields 75% to 90% of the amount of plasmid DNA obtained using PEG precipitation. Yields obtained from column chromatography are limited by the capacity of the column and are generally 1 ìg of DNA in the ligation reaction, or if the ligation reaction is from low gelling/melting temperature agarose, it is wise to dilute the ligation mix (see UNIT 3.16).
12. Rapidly thaw competent cells by warming between hands and dispense 100 µl immediately into test tubes containing DNA. Gently swirl tubes to mix, then place on ice for 10 min. Introduction of Plasmid DNA into Cells
Competent cells should be used immediately after thawing. Remaining cells should be discarded rather than refrozen.
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13. Heat shock cells by placing tubes into a 42°C water bath for 2 min. Alternatively, incubate at 37°C for 5 min.
14. Add 1 ml LB medium to each tube. Place each tube on a roller drum at 250 rpm for 1 hr at 37°C. 15. Plate aliquots of transformation culture on LB/ampicillin or other appropriate antibiotic-containing plates. It is convenient to plate several different dilutions of each transformation culture. The remainder of the mixture can be stored at 4°C for subsequent platings.
16. When plates are dry, incubate 12 to 16 hr at 37°C. ONE-STEP PREPARATION AND TRANSFORMATION OF COMPETENT CELLS
ALTERNATE PROTOCOL 1
This procedure is considerably easier than Basic Protocol 1 because it eliminates the need for centrifugation, washing, heat shock, and long incubation periods (Chung et al., 1989). Moreover, competent cells made by this simple procedure can be directly frozen at −70°C for long-term storage. A variety of strains can be made competent by this procedure, and the transformation frequency can be as high as that achieved by Basic Protocol 1. However, frequency is considerably lower than can be obtained by electroporation. Additional Materials (also see Basic Protocol 1) 2× transformation and storage solution (TSS; see recipe), ice cold LB medium (UNIT 1.1) containing 20 mM glucose 1. Dilute a fresh overnight culture of bacteria 1:100 into LB medium and incubate at 37°C until the cells reach an OD600 of 0.3 to 0.4. The procedure will work if cells are harvested at other stages of the growth cycle (including stationary phase), but with reduced efficiency.
2. Add a volume of ice-cold 2× TSS equal to that of the cell suspension, and gently mix on ice. For long-term storage, freeze small aliquots of the suspension in a dry ice/ethanol bath and store at −70°C. To use frozen cells for transformation, thaw slowly and then use immediately. Cells can also be used if pelleted by centrifugation 10 min at 1000 × g, 4°C, and this may increase the frequency of transformation (according to Chung et al., 1989). Discard supernatant and resuspend cell pellet at one-tenth of original volume in 1× TSS (prepared by diluting 2× TSS). Proceed with transformation as in step 3.
3. Add 100 µl competent cells and 1 to 5 µl DNA (0.1 to 100 ng) to an ice-cold polypropylene or glass tube. Incubate 5 to 60 min at 4°C. As is the case for related procedures, the transformation frequency as measured by transformants/ìg DNA is relatively constant at amounts of DNA 3-fold.
Transform the cells 10. Set the electroporation apparatus to 2.5 kV, 25 µF. Set the pulse controller to 200 or 400 ohms. The pulse controller is necessary when high-voltage pulses are applied over short gaps in high-resistance samples (see Background Information).
11. Add 5 pg to 0.5 µg plasmid DNA in 1 µl to tubes containing fresh or thawed cells (on ice). Mix by tapping the tube or by swirling the cells with the pipettor. 12. Transfer the DNA and cells into a cuvette that has been chilled 5 min on ice, shake slightly to settle the cells to the bottom, and wipe the ice and water from the cuvette with a Kimwipe. The volume of DNA added to the cells should be kept small. Adding DNA up to one-tenth of the cell volume will decrease the transformation efficiency 2- to 3-fold. Also, since the resistance of the sample should be high, make sure that addition of the DNA to the cells does not increase the total salt concentration in the cuvette by >1 mM.
13. Place the cuvette into the sample chamber. If using a homemade apparatus, connect the electrodes to the cuvette.
14. Apply the pulse by pushing the button or flipping the switch. 15. Remove the cuvette. Immediately add 1 ml SOC medium and transfer to a sterile culture tube with a Pasteur pipet. Incubate 30 to 60 min with moderate shaking at 37°C. If the actual voltage and time constant of the pulse are displayed on the electroporation apparatus, check this information. Verify that the set voltage was actually delivered, and record the time constant of the pulse so that you may vary it later if necessary (see Critical Parameters).
16. Plate aliquots of the transformation culture on LB plates containing antibiotics.
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ALTERNATE PROTOCOL 2
DIRECT ELECTROPORETIC TRANSFER OF PLASMID DNA FROM YEAST INTO E. COLI The use of “shuttle vectors”—plasmids that can be grown successfully in at least two different organisms—facilitates the transfer of DNA between, for example, yeast and E. coli. In this adaptation of the electroporation protocol, plasmid DNA from a shuttle vector is transformed directly from yeast into E. coli. Components of the interaction trap/twohybrid system (UNIT 20.1) are used as an example in this protocol. The transfer and selection of a “prey” plasmid from the yeast strain EGY48 into the E. coli strain KC8 is described here, but the approach can be adapted for use with other yeast and E. coli strains. Additional Materials (also see Basic Protocol 2) Single colony of E. coli KC8 cells (UNIT 20.1) Streak colony of Trp− plasmid–harboring EGY48 yeast cells on Gal/Raff/Xgal/CM plates (UNIT 20.1), no older than 2 weeks M9 minimal medium and plates (UNIT 1.1) containing 100 µg/ml ampicillin (Table 1.4.1) and standard concentrations of leucine, histidine, and uracil Additional reagents and equipment for growth and manipulation of yeast (UNIT 13.2) and for plasmid DNA miniprep (UNIT 1.6) or PCR (UNIT 15.1) 1. Prepare electrocompetent KC8 cells (see Basic Protocol 2, steps 1 to 9a), resuspending the final cell pellet in ice-cold water to obtain an OD600 of 100. Fresh KC8 cells work better in this electroporation method than frozen ones. To measure OD, dilute 5 ìl of the cell suspension with water to 1 ml and measure the OD600. If necessary add more water to the suspension to get an OD600 of 100.
2. Distribute 65-µl aliquots of the electrocompetent E. coli KC8 cells into ice-cold microcentrifuge tubes. 3. With a sterile wooden or plastic stick, scrape off ∼10 µl of yeast from a streak colony of EGY48 harboring the respective “prey” plasmid derivative of pJG4-5 and grown on Gal/Raff/Xgal/CM plates. Resuspend the yeast cells in the KC8 suspension by swirling the stick used for scraping off the cells. Avoid scraping off plate medium when collecting the yeast streak cells. Keep the microcentrifuge tube on ice as much as possible. Try to get an even distribution of the two cell types but do not vortex. Yeast grown on plates other than Gal/Raff/Xgal/CM will probably work as well; do not worry if the yeast colony used is blue.
4. Set the electroporation apparatus to 1.5 kV, 25 µF, and the pulse controller to 100 ohms. Transfer the cell suspension into a 0.2-cm cuvette that has been chilled 5 min on ice, shake slightly to settle the cells to the bottom, and wipe the ice and water from the cuvette with a Kimwipe. The use of Pasteur pipettes will facilitate placing the cell suspension at the bottom of the cuvette. Avoid any air bubbles.
5. Place the cuvette in the sample chamber of the apparatus and pulse. Take the cuvette out and place it on ice for ≥45 sec. Meanwhile, change the settings in preparation for the second pulse. The expected time constant for the first pulse is 2.2 to 2.4 msec.
6. Set the electroporation apparatus to 2.5 kV, 25 µF, and the pulse controller to 200 ohms. Wipe the cuvette again, place it in the sample chamber, and pulse. Introduction of Plasmid DNA into Cells
The expected time constant for the second pulse is 4.2 to 4.8 msec.
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7. Remove the cuvette, immediately add 1 ml LB medium, and transfer the suspension into a microcentrifuge tube. Incubate 45 min at room temperature. Incubation of the suspension after electroporation for 1 hr at 37°C decreases the yield of transformants, probably due to prolonged adhesion of the E. coli cells to the yeast cell debris.
8. Spread 150 µl of the suspension evenly onto M9 minimal medium plates containing 100 µg/ml ampicillin and leucine, histidine, and uracil. Incubate ≥24 hr at 37°C. A slight yeast background might appear on the plates but single E. coli colonies are easy to pick. Between 50 and 200 KC8 colonies have been obtained per plate employing 150 ìl out of the 1 ml LB suspension.
9. Pick a single KC8 colony, inoculate it into 1.5 to 5 ml M9 minimal medium (Leu+, His+, Ura+, 100 µg/ml Amp) or LB (100 µg/ml Amp) and grow at 37°C. Harvest at an appropiate OD to prepare miniprep DNA (UNIT 1.6) or perform PCR analysis (UNIT 15.1). Using M9 minimal medium to grow KC8 in liquid culture is an additional safety measure but not absolutely necessary. It ensures isolation of the plasmid whose marker complements the auxotrophic defect in KC8; in addition, slightly increased plasmid copy number and improved DNA quality have been reported.
REAGENTS AND SOLUTIONS CaCl2 solution 60 mM CaCl2 15% glycerol 10 mM PIPES [piperazine-N,N′-bis(2-hydroxypropanesulfonic acid)], pH 7 Filter sterilize using a disposable filter unit, or autoclave Store at room temperature (stable for years) SOC medium 0.5% yeast extract 2% tryptone 10 mM NaCl 2.5 mM KCl 10 mM MgCl2 10 mM MgSO4 20 mM glucose Store at room temperature (stable for years) Transformation and storage solution (TSS), 2× Dilute sterile (autoclaved) 40% (w/v) polyethylene glycol (PEG) 3350 to 20% PEG in sterile LB medium containing 100 mM MgCl2. Add dimethyl sulfoxide (DMSO) to 10% (v/v) and adjust to pH 6.5. COMMENTARY Background Information Calcium and one-step transformation Transformation of E. coli was first described by Mandel and Higa (1970). Subsequent modifications to improve transformation efficiencies have included prolonged exposure of cells to CaCl2 (Dagert and Ehrlich, 1974), substitution of calcium with other cations such as Rb+
(Kushner, 1978), Mn2+, and K+, and addition of other compounds such as dimethyl sulfoxide, dithiothreitol, and cobalt hexammine chloride (Hanahan, 1983). Basic Protocol 1 given here provides good transformation efficiencies, permits long-term storage of competent cells, and is relatively uncomplicated to perform. Variations on this protocol can be obtained from the references provided. Alternate Proto-
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col 1, a one-step preparation and transformation procedure, is considerably faster. Transformation by electroporation Electroporation has become a valuable technique for transfer of nucleic acids into eukaryotic and prokaryotic cells. The method can be applied to many different E. coli strains and to other gram-negative and gram-positive bacteria. In this technique, a high-voltage electric field is applied briefly to cells, apparently producing transient holes in the cell membrane through which plasmid DNA enters (Shigekawa and Dower, 1988). The field strength used for mammalian cell and plant protoplast electroporation is usually 0.5 to 1 kV/cm. The field strength needed for high-efficiency transformation of E. coli is much greater, usually ∼12.5 kV/cm. Under these conditions up to 1010 transformants/µg plasmid DNA have been reported (Calvin and Hanawalt, 1988; Miller et al., 1988; Dower et al., 1988). Recently, field strength up to 8 kV/cm was also used successfully to electroporate both mammalian cells and plant protoplasts. The capacitor discharge circuit of the electroporation apparatus typically generates an electrical pulse with an exponential decay waveform. The voltage across the electrodes rises rapidly to a peak voltage, which then declines over time as follows: −t/T
Vt = V0 [e
Introduction of Plasmid DNA into Cells
]
where Vt = voltage at a given time t after the time of V0, V0 = initial voltage, t = time (sec), T = pulse time constant = RC, R = resistance of circuit (ohms); and C = capacitance of circuit (Farads). The pulse time constant is ∼5 to 10 msec for electroporating E. coli cells and ranges from 5 µsec to 50 msec for higher eukaryotic cells. The pulse controller contains a number of different-sized resistors, any one of which is placed in parallel with the sample, and one resistor of fixed (20-ohm) resistance, which is placed in series with the sample. The resistor placed in parallel with the sample (usually 200 or 400 ohms) swamps out the effect of changes in the resistance of the sample on the total resistance of the circuit, thus determining the total resistance across the capacitor, and controlling the time constant (T) of the capacitor discharge. The 20-ohm resistor in series with the sample protects the circuitry by limiting the current should a short circuit (arc) occur and the capacitor discharge instantly.
The pulse controller is required when highvoltage electroporation pulses are delivered to high-resistance samples across narrow electrode gaps. In this procedure, the resistance of washed E. coli in the 0.2-cm cuvette is ∼5000 ohms. A pulse controller is not necessary when samples of low (3 hr at 30◦ to 32◦ C, then transfer the filter to the appropriate drug plate using sterile forceps. The number of recombinants is generally less than 500 per electroporation mix. Usually, approximately half of the surviving cells will have spontaneously lost the prophage. It is possible to screen for non-lysogens by testing candidate colonies for their ability to plate λ, since the prophage renders the cells immune to phage infection; the cured cells will also be viable at 42◦ C while those containing the prophage will not (see Commentary).
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4a. Use PCR (UNIT mutation.
15.1)
and subsequent DNA sequencing (Chapter 7) to confirm the
If mutations have been targeted to the bacteriophage itself In this case, the 15-min induction time can be used. 3b. Dilute the electroporation mix into 5 ml LB medium and aerate by shaking in a shaking water bath 90 min at 39◦ C. Add 0.25 ml chloroform to complete cell lysis and release the phage particles. Dilute the lysate and plate for single plaques (see UNIT 1.11). 4b. Plaque-purify the positive candidates (UNIT 1.11). Resuspend a plaque in 50 µl of sterile water and use 20 µl of this suspension as template for a PCR reaction (UNIT 15.1; reduce the amount of water accordingly). Resuspend another plaque from the same plate in 1 mM of 1× TM buffer to grow a stock. Reconfirm the mutation after growing the stock. ALTERNATE PROTOCOL 2
TARGETING AN INFECTING PHAGE λ WITH THE DEFECTIVE PROPHAGE STRAINS Sometimes it may be useful to target genetic changes to bacteriophage λ derivatives. A strain carrying the Red system on the prophage or a plasmid can be infected with a phage and recombineering can then be targeted to the incoming phage chromosome. A procedure for this method follows. Ideally the construction should be designed so that the plaque morphology of the recombinant phage will differ from that of the parent. For example, a PCR product able to both introduce the mutation of interest and correct a known mutation (such as an amber or temperature-sensitive allele) can be recombined onto a phage containing the known mutation. Select for correction of the known mutation and screen among these recombinants for the mutation of interest. If no selection exists, plaque hybridization can sometimes be used to identify recombinant phages. While the authors have only tested this method for phage λ, theoretically it may be possible to introduce genetic changes onto the chromosome of any phage able to propagate in the defective prophage host (Oppenheim et al., 2004).
Additional Materials (also see Basic Protocol 1) 10% (w/v) maltose stock solution, filter sterilized Tris-magnesium (TM) buffer: 10 mM Tris·Cl, pH 7.4 (APPENDIX 2)/10 mM MgSO4 High-titer lysate of the bacteriophage to be engineered PCR product with desired sequence changes and flanking homology to the target on the phage chromosome Lambda plates and lambda top agar (see UNIT 1.1, adjust NaCl to 5 g per liter in both the plates and the top agar) Chloroform Appropriate bacterial indicator strain Additional reagents and equipment for working with λ bacteriophages (UNITS 1.9-1.13)
Genetic Engineering in Bacteria Using Homologous Recombination
Grow host strain 1. Grow strain with defective prophage to mid-log phase at 32◦ C (see Basic Protocol 1, step 6), except supplement the LB medium with 0.4% maltose (added from 10% w/v maltose stock solution). Maltose induces the phage receptor on the bacterial cell surface, ensuring efficient adsorption of the phage to the host cells.
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Adsorb bacteriophage 2. Harvest the cells by centrifuging 7 min at 4600 × g, 4◦ C. Resuspend pellet in 1 ml Tris-magnesium (TM) buffer. Infect the cells with the phage to be engineered at a multiplicity of 1 to 3 phages per cell. The cells will be at ∼1 × 108 /ml before concentration. UNITS 1.9-1.13 contain protocols for working with λ bacteriophages.
3. Let the phage adsorb to the cells for 15 min at room temperature. Phages other than λ may require different adsorption conditions.
Induce recombination functions and phage DNA entry; electroporate cells 4. Transfer the infected cells to 5 ml of 42◦ C LB medium and shake vigorously for 15 min. At end of incubation, chill rapidly on ice. This serves to both induce the Red functions and allow phage infection to proceed.
5. Make electrocompetent cells and introduce transforming DNA (i.e., introduce PCR product with mutation of interest; see Basic Protocol 1, steps 8 to 14).
Outgrow infected cells to allow phage production 6. Dilute electroporation mix into 5 ml warm LB medium and shake vigorously for 90 min at 42◦ C to allow the phage to complete a lytic cycle. Add 0.25 ml chloroform to completely lyse infected cells. Plate phage on lambda plates with lambda top agar (see UNIT 1.11) using the appropriate bacterial indicator strain as a host; apply a selection if possible; or plate non-selectively and screen for the desired mutation with plaque hybridization. Amber mutations can be specifically selected (Oppenheim et al., 2004).
REAGENTS AND SOLUTIONS Use Milli-Q purified water or equivalent in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
M63 minimal glucose plates with biotin Per liter: 3 g KH2 PO4 7 g K2 HPO4 2 g (NH4 )SO4 0.5 ml of 1 mg/ml FeSO4 0.2% (v/v) glucose 0.001% (w/v) biotin (omit for control) 1 ml 1% (w/v) vitamin B1 (thiamine) 1 ml 1 M MgSO4 15 g agar Pour 40 ml of the agar-containing medium per plate. Store up to several months at 4◦ C. M63 minimal glycerol plates with sucrose Per liter: 3 g KH2 PO4 7 g K2 HPO4 2 g (NH4 )SO4 0.5 ml of 1 mg/ml FeSO4 0.2% (v/v) glycerol continued
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5% (w/v) sucrose 0.001% (w/v) biotin 1 ml 1% (w/v) vitamin B1 (thiamine) 1 ml 1 M MgSO4 15 g agar Pour 40 ml of the agar-containing medium per plate. Store up to several months at 4◦ C.
COMMENTARY Background Information
Bacteriophage λ encodes three genes important for recombineering. The exo and bet genes, respectively, encode a 5 to 3 double-strand exonuclease, Exo, and a single strand annealing protein, Beta, which together can recombine a double-stranded PCR product with short flanking homologies into the desired genetic target. The λ gam gene encodes a protein, Gam, that inhibits the RecBCD enzyme, which will otherwise degrade linear DNA introduced into the bacterial cell. Only the Beta single-strand annealing function is required to recombine single-stranded oligos containing the desired alterations. A cryptic lambdoid prophage, rac, is present in some strains of E. coli, and encodes RecE and RecT functions that are analogous to Exo and Beta, respectively. Unlike some other in vivo genetic engineering methods (Russell et al., 1989), recombineering does not require the host recA function. A strain mutant for recA provides more controllable recombination, since the strain is recombination proficient only when the phage functions are induced. A more detailed discussion of the molecular mechanism of recombineering can be found in Court et al. (2002). Recently the Red genes have been moved to several plasmids (Table 1.16.3; S. Datta, manuscript in preparation) having different DNA replication origins. Here, the essential control elements of the prophage system are retained and recombination functions are
induced by a temperature shift as in Basic Protocol 1. The plasmids are especially useful when one wants to create a mutation in a particular bacterial strain, rather than create the mutations in the prophage-containing strains and subsequently move them into a different background.
Critical Parameters and Troubleshooting Induction times When inducing the recombination functions from the defective prophage, the proper induction time is essential. Longer induction times will cause decreased cell viability, since the prophage Kil function is also induced by the temperature shift. The Gam function, necessary for efficient transformation of dsDNA, is also toxic to the cells (Sergueev et al., 2001). Storage of induced cells It is also important to use the induced cells promptly, since the induced phage functions will decay over time, especially at 32◦ C. Although one pioneering laboratory member reports that induced cells can be successfully stored on ice for several hours before electroporation, the authors do not recommend this procedure. As detailed in the Basic Protocol 1, competent cells may be frozen in 15% glycerol, although their recombineering efficiency has been less than that of the freshly prepared cells.
Table 1.16.3 Plasmids Containing the Red System Under cI857 Control
Genetic Engineering in Bacteria Using Homologous Recombination
Plasmid designation
Plasmid origin
Approximate copy number/cell
pSIM5(CmR ) pSIM6(AmpR )
pSC101ts
16
pSIM7(CmR ) pSIM8(AmpR )
pBBR1
30-40
pSIM9(CmR )
pRK2ts
20-40
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Amplification of drug cassettes The authors strongly recommend that each drug cassette be amplified with only one standard set of primers. Confusion has arisen in the authors’ laboratory when individuals have designed their own primers for the drug cassettes. Remember that it is best to have a promoter, the open reading frame, and a transcriptional terminator on each cassette. Standard priming sequences for the commonly used cassettes are listed in Table 1.16.2. The cat-sacB element The cat-sacB DNA segment described in Support Protocol 1 is amplified from plasmid pEL04, previously known as both pKO4 and pcat-sacB, or from the strain DY441, using colony PCR. If a different antibiotic resistance gene is required, the cat gene on pcat-sacB can be replaced with another drug marker. Use 50 bases of homology upstream of the 5 cat primer and downstream of the 3 cat primer to cross in the other marker. The sequence of pEL04 is available from the Court Laboratory (
[email protected]). Since the catsacB PCR product is 3264 bp, it is not always easy to amplify. The authors have used the PCR conditions detailed above, but other conditions and any other high-fidelity enzyme should also work. This is really an empirical problem, and, in the opinion of the authors, the biggest factors, if sufficient extension times and appropriate temperatures are used, are the PCR machine itself and the polymerase. An MJ Research PTC-100 machine is used in the authors’ laboratory. Adjusting the Mg2+ concentration may also improve yield. There has been some confusion regarding the primers for amplification of the cat-sacB cassette. The authors have now settled on an optimal primer pair for amplification of this DNA, and strongly recommend that only this set of primers be used. Please note that both primer sequences are different than those previously given in this protocol. Detecting recombinants If one has engineered a drug marker into the DNA, drug resistance will serve as the selection. The two-step cat-sacB replacement allows precise modification without leaving an antibiotic-resistance cassette or other marker behind. The authors have also had success using colony hybridization with a labeled oligo probe, although a unique sequence hybridizing only to the probe must be inserted (or a unique sequence must be created by a deletion) for this method to be an option.
Mismatch repair minus conditions for oligonucleotide recombination The authors have recently found (Costantino and Court, 2003) that extremely high levels of recombinants can be obtained when recombineering with oligos if the bacterial strain is mutant for the host methyl-directed mismatch repair system (the mut HLS system). Under these conditions ∼20% to 25% of the total viable cells are recombinant. Strains with the prophage and mutations in the mismatch repair system are available from the Court laboratory: these are HME63 and HME64 and contain a mutSamp and an uvrDkan mutation, respectively. Either mutation will eliminate mismatch repair; the latter strain can be used for engineering onto plasmids expressing ampicillin resistance. The same elevated level of recombination is obtained in mismatch repair proficient strains if the incoming oligo creates a C-C mismatch when annealed to the target DNA, since C-C mispairs are not subject to mismatch repair. Counter-selections The cat-sacB cassette is a counterselection; it can be selected either for (by chloramphenicol resistance) or against (sucrose sensitivity). Several such counter-selections are available and may prove useful for one’s particular situation; tetracycline resistance is another example (Bochner et al., 1980; Maloy and Nunn, 1981). If removing a bacteriostatic drug marker (such as tetracycline), enrichment for recombinants is possible using ampicillin. Nonrecombinant bacteria will still express resistance to the tet marker encoded on the chromosome; when the electroporated culture is propagated in the presence of both tetracycline and ampicillin, only nonrecombinant cells will grow; these will be killed by the ampicillin (Murphy et al., 2000). Recombinant cells will not grow but will not be killed. 2-deoxy Dgalactose is a toxic analog of galactose on which only mutants of galactokinase, the product of the galK gene, will grow (Alper and Ames, 1975). Other counter-selections may be devised. In vivo assembly with overlapping oligos Complementary oligos will anneal in vivo when introduced by electroporation into cells expressing Exo and Beta protein (Yu et al., 2003). Multiple overlapping oligos can be used to build moderately sized DNA products (100 to 150 bp) in an in vivo reaction similar to PCR assembly (Stemmer et al., 1995). By adding
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flanking homologies to chromosomal targets, these overlapping oligos are restored to linear dsDNA in vivo and recombined into the target.
of foreign DNA. DY380 is the bacterial host of choice for use with BACs; it is a DH10B derivative containing the defective prophage. The plasmids listed in Table 1.16.3 may also be useful in manipulating BACs.
Anticipated Results
Genetic Engineering in Bacteria Using Homologous Recombination
As discussed above, ideally, transformation with dsDNA and ssDNA can approach frequencies of 0.1%, and 25%, respectively. Because of this high efficiency, there should rarely if ever be experiments where no recombinants are found. However, in practice, recombineering frequency may be partially context dependent; certain areas of the chromosome appear “hotter” for recombination than others (Ellis et al., 2001). If recombinants are not obtained, check the construct design and redesign primers if necessary, as this is one reason for failure. If the recombineering reaction does not work, the authors strongly recommend doing a control experiment using known strains and oligos or PCR products, as described in Yu et al. (2000), Ellis et al. (2001), and Costantino and Court (2003) to verify that the predicted number of recombinants are obtained in these control reactions. It has been the authors’ experience that most recombineering failures occur because the protocol is not executed carefully or properly.
Mutating essential genes Mutation of an essential gene usually results in cell death. Such a mutation can be recovered in a diploid state; however, the cell will maintain a wild-type copy of the gene as well as a mutant version. This diploid state can usually be detected by PCR analysis.
Recombineering onto plasmids Genetic alterations can also be targeted to intact circular plasmids. If the desired change can be engineered with an oligonucleotide, the mismatch repair–deficient strains can be used and unselected clones screened for the mutation. Otherwise, it is helpful to have a selection for the desired recombinant. If one has a selection, the plasmid may either be already resident in the bacterial cell or may be introduced by coelectroporation. The former method is less optimal, because expression of the λ Gam function can lead to formation of linear multimers of some plasmid replicons (Cohen and Clark, 1986). Hence, co-electroporation is preferable. If introducing an unselected mutation, co-electroporation is essential, since it is unlikely that more than one resident plasmid in a cell will become modified. In either case, the recombinant plasmid clones should be purified and the DNA isolated, characterized, and used to transform a new host.
The basic recombineering protocol can be executed in one day, with bacterial cultures started the evening before. The recombinants may take several days to grow on the selective plates, and it may take another day or so to confirm the recombinants.
Engineering onto BACs Red recombineering has also been optimized for engineering bacterial artificial chromosomes (Copeland et al., 2001; Lee et al., 2001; Swaminathan and Sharan, 2004), which are able to accommodate hundreds of kilobases
Moving the prophage to a different background If a different host is required for recombineering, the defective prophage can be moved by P1 transduction (Miller, 1972; Yu et al., 2000). Grow a P1 lysate on DY329 and transduce the strain of choice. Select for tetracycline resistance and screen for temperature sensitivity or an inability to grow in the absence of biotin. Check the genotype of the strain first and supplement the minimal plates to complement any additional auxotrophies when performing the biotin screen.
Time Considerations
Literature Cited Alper, M.D. and Ames, B.N. 1975. Positive selection of mutants with deletions of the gal-chl region of the Salmonella chromosome as a screening procedure for mutagens that cause deletions. J. Bacteriol. 121:259-266. Altschul, S.F., Gish, W., Miller, W., Myers, E.W. and Lipman, D.J. 1990. Basic local alignment search tool. J. Mol. Biol. 215:403-410. Arber, W., Enquist, L., Hohn, B., Murray, N.E., and Murray, K. 1983. Experimental methods for use with lambda. In Lambda II (R. Hendrix, J. Roberts, F. Stahl, and R. Weisberg, eds.) pp. 433471. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York. Blomfield, I.C., Vaughn, V., Rest, R.F., and Eisenstein, B.I. 1991. Allelic exchange in Escherichia coli using the Bacillus subtilis sacB gene and a temperature-sensitive pSC101 replicon. Mol. Microbiol. 5:1447-1457. Bochner, B.R., Huang, H.C., Schieven, G.L., and Ames, B.N. 1980. Positive selection for loss of tetracycline resistance. J. Bacteriol. 143:926933.
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Cha, R.S., Zarbl, H., Keohavong, P., and Thilly, W.G. 1992. Mismatch amplification mutation assay (MAMA): Application to the c-H-ras gene. PCR Methods Appl. 2:14-20. Cohen, A. and Clark, A. J. 1986. Synthesis of linear plasmid multimers in Escherichia coli K-12. J. Bacteriol. 167:327-35. Copeland, N.G., Jenkins, N.A, and Court, D.L. 2001. Recombineering: A powerful new tool for mouse functional genomics. Nat. Rev. Genet. 2:769-779. Costantino, N. and Court, D.L. 2003. Enhanced levels of lambda Red-mediated recombinants in mismatch repair mutants. Proc. Natl. Acad. Sci. U.S.A. 100:15748-15753. Court, D.L., Sawitzke, J.A., and Thomason, L.C. 2002. Genetic engineering using homologous recombination. Annu. Rev. Genet. 36:361-388. Court, D.L., Swaminathan, S., Yu, D., Wilson, H., Baker, T., Bubunenko, M., Sawitzke, J., and Sharan, S.K. 2003. Mini-lambda: A tractable system for chromosome and BAC engineering. Gene 315:63-69. Datsenko, K.A. and Wanner, B.L. 2000. Onestep inactivation of chromosomal genes in Escherichia coli K-12 using PCR products. Proc. Natl. Acad. Sci. U.S.A. 97:6640-6645. Ellis, H.M., Yu, D., DiTizio, T., and Court, D.L. 2001. High efficiency mutagenesis, repair, and engineering of chromosomal DNA using singlestranded oligonucleotides. Proc. Natl. Acad. Sci. U.S.A. 98:6742-6746. Gay, P., Le Coq, D., Steinmetz, M., Berkelman, T., and Kado, C.I. 1985. Positive selection procedure for entrapment of insertion sequence elements in Gram-negative bacteria. J. Bacteriol. 164:918-921. Lee, E.C., Yu, D., Martinez de Velasco, J., Tessarollo, L., Swing, D.A., Court, D.L., Jenkins, N.A., and Copeland, N.G. 2001. A highly efficient Escherichia coli–based chromosome engineering system adapted for recombinogenic targeting and subcloning of BAC DNA. Genomics 73:56-65. Maloy, S.R. and Nunn, W.D. 1981. Selection for loss of tetracycline resistance by Escherichia coli. J. Bacteriol. 145:1110-1111. Miller, J.M. 1972. Generalized transduction: Use of P1 in strain construction. In Experiments in Molecular Genetics (J.M. Miller, ed.) pp. 201- 205. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York. Murphy, K.C., Campellone, K.G., and Poteete, A.R. 2000. PCR-mediated gene replacement in Escherichia coli. Gene 246:321-330.
Oppenheim A.B., Rattray, A.J., Bubunenko, M., Thomason, L.C., and Court, D.L. 2004. In vivo recombineering of bacteriophage λ by PCR fragments and single-strand oligonucleotides. Virology 319:185-189. Russell, C.B., Thaler, D.S., and Dahlquist, F.W. 1989. Chromosomal transformation of Escherichia coli recD strains with linearized plasmids. J. Bacteriol. 171:2609-2613. Sergueev, K., Yu, D., Austin, S., and Court, D. 2001. Cell toxicity caused by products of the PL operon of bacteriophage lambda. Gene 272:227-235. Stemmer, W.P., Crameri, A., Ha, K.D., Brennan, T.M., and Heyneker, H.L. 1995. Single-step assembly of a gene and entire plasmid from large numbers of oligodeoxyribonucleotides. Gene 164:49-53. Swaminathan, S. and Sharan, S.K. 2004. Bacterial artificial chromosome engineering. Methods Mol. Biol. 256:89-106. Swaminathan, S., Ellis, H.M., Waters, L.S., Yu, D., Lee, E-C., Court, D.L., and Sharan, S.K. 2001. Rapid engineering of bacterial artificial chromosomes using oligonucleotides. Genesis 29:1421. Thomason, L.C., Myers, R.S., Openheim, A., Costantino, N., Sawitzke, J.A., Datta, S., Bubunenko, M., and Court, D.L. 2005. Recombineering in prokaryotes. In Phages: Their Role in Bacterial Pathogenisis and Biotechnology. pp. 383-399. ASM Press, Herndon, Virginia. Yu, D., Ellis, H.M., Lee, E.C., Jenkins, N.A., Copeland, N.G., and Court, D.L. 2000. An efficient recombination system for chromosome engineering in Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 97:5978-5983. Yu, D., Sawitzke, J.A., Ellis, H., and Court D.L. 2003. Recombineering with overlapping singlestranded DNA oligonucleotides: Testing a recombination intermediate. Proc. Natl. Acad. Sci. U.S.A. 100:7207-7212. Zhang, Y., Muyrers, J.P., Testa, G., and Stewart, A.F. 2000. DNA cloning by homologous recombination in Escherichia coli. Nature Biotechnol. 12:1314-1317.
Contributed by Lynn Thomason, Donald L. Court, Mikail Bubunenko, Nina Costantino, Helen Wilson, Simanti Datta, and Amos Oppenheim National Cancer Institute at Frederick Frederick, Maryland
Escherichia coli, Plasmids, and Bacteriophages
1.16.21 Current Protocols in Molecular Biology
Supplement 70
CHAPTER 2 Preparation and Analysis of DNA INTRODUCTION
T
he ability to prepare and isolate pure DNA from a variety of sources is an important step in many molecular biology protocols. Indeed, the isolation of genomic, plasmid, or DNA fragments from restriction digests and polymerase chain reaction (PCR) products has become a common everyday practice in almost every laboratory. This chapter therefore begins with protocols for purification of genomic DNA from bacteria, plant cells, and mammalian cells (UNITS 2.1-2.4). These protocols consist of two parts: a technique to lyse the cells gently and solubilize the DNA, followed by one of several basic enzymatic or chemical methods to remove contaminating proteins, RNA, and other macromolecules. The basic approaches described here are generally applicable to a wide variety of starting materials. A brief collection of general protocols for further purifying and concentrating nucleic acids is also included.
The last decade has shown a dramatic departure from the use of traditional DNA purification methods outlined in UNITS 2.2-2.4, with a concomitant increase in the use of purpose-specific kits for the isolation and purification of DNA. For example, kits for purification of DNA using pre-made anion-exchange columns packaged with all necessary solutions to lyse the cells and solubilize the DNA are available from many molecular biology companies. A variety of kits based on binding of DNA to glass beads are also available. The uses of both types of kits are discussed in UNIT 2.1B. The use of kits has two main advantages: it saves time and makes the process of DNA purification a relatively easy and straightforward process. The purification of DNA by anion-exchange chromatography (UNIT 2.1B) is readily becoming the accepted standard for quick and efficient large-scale (more than 100 µg of DNA) production of DNA from bacteria, mammalian tissue, and plant tissue. In most cases, the cell lysis and solubilization of DNA is relatively unchanged compared to traditional methods, with anion-exchange chromatography columns having replaced labor and time-intensive techniques such as cesium chloride centrifugation for the isolation of relatively pure DNA. Purification kits are usually available in several sizes and configurations, allowing the researcher to have variability concerning the processing and purification of their DNA. A variety of techniques exist for the isolation of small amounts of plasmid DNA from minipreps and for DNA fragments from restriction digests/PCR products from agarose gels (with removal of unincorporated nucleoside triphosphates, reaction products, and small oligonucleotides from PCR reactions). These are detailed in UNITS 2.1A, 2.1B, 2.6 & 2.7. Likewise, kits are available from several molecular biology companies, usually based on silica-gel technology, for each of these applications (UNIT 2.1B). As with large-scale DNA isolation and purification, these kits provide a quick and efficient means to recover purified DNA that can be used for subsequent cloning or other modifications. Virtually all protocols in molecular biology require, at some point, fractionation of nucleic acids. Chromatographic techniques are appropriate for some applications and may be used for separation of plasmid from genomic DNA as well as separation of genomic DNA from debris in a cell lysate (UNIT 2.1B). Gel electrophoresis, however, has much greater Preparation and Analysis of DNA Contributed by David D. Moore and Dennis Dowhan Current Protocols in Molecular Biology (2002) 2.0.1-2.0.3 Copyright © 2002 by John Wiley & Sons, Inc.
2.0.1 Supplement 58
resolution than alternative methods and is generally the fractionation method of choice. Gel electrophoretic separations can be either analytical or preparative, and can involve fragments with molecular weights ranging from less than 1000 Daltons to more than 108. A variety of electrophoretic systems have been developed to accommodate such a large range of applications. In general, the use of electrophoresis to separate nucleic acids is simpler than its application to resolve proteins. Nucleic acids are uniformly negatively charged and, for double-stranded DNA, reasonably free of complicating structural effects that affect mobility. A variety of important variables affect migration of nucleic acids on gels. These include the conformation of the nucleic acid, the pore size of the gel, the voltage gradient applied, and the salt concentration of the buffer. The most basic of these variables is the pore size of the gel, which dictates the size of the fragments that can be resolved. In practice, this means that larger-pore agarose gels are used to resolve fragments >500 to 1000 bp (UNITS 2.5A & 2.6) and smaller-pore acrylamide or sieving agarose gels (UNIT 2.7) are used for fragments >G. As a result, different termini will be degraded at different rates. In addition to the 3′→5′ exonuclease, exo III has three other activities: a 3′ phosphatase, an RNase H, and an endonuclease specific for apurinic or apyrimidinic sites.
Enzymatic Manipulation of DNA and RNA
3.11.3 Current Protocols in Molecular Biology
Reaction conditions For 50-ìl reaction: 50 mM Tris⋅Cl, pH 7.5 5 mM MgCl2 5 mM DTT 2 µg DNA 50 µg/ml BSA 10 U exo III Incubate at 37°C for 1 to 30 min, depending upon the extent of digestion required. Stop the reaction by adding 2 µl of 0.5 M EDTA or by heating to 75°C for 10 min. One unit of exo III will remove ∼ 200 nucleotides from each 3′ recessed end of 1 µg of a 5000-bp linear double-stranded DNA template in 10 min at 37°C. Applications All of the applications described below utilize the nonprocessive 3′→5′ double-stranded exonuclease activity of exo III to generate uniform single-stranded regions in doublestranded DNA. 1. Preparing strand-specific radioactive probes, in conjunction with the Klenow fragment; this procedure is analogous to replacement synthesis using the exonuclease and polymerase functions of T4 DNA polymerase (Fig. 3.5.5). 2. Preparing single-stranded DNA templates for sequencing by the dideoxy technique (Sanger et al., 1977) (UNIT 7.4). 3. Constructing unidirectional deletions; since exo III is double-strand specific, it will degrade preferentially from duplex DNA having a 3′-recessed end as opposed to one with a 5′-recessed end (Henikoff, 1984). This property is useful for constructing a set of unidirectional deletions from a given position in cloned DNA, that are used for DNA sequencing without prior restriction site mapping. LITERATURE CITED Berk, A.J. and Sharp, P.A. 1977. Sizing and mapping of early adenovirus mRNAs by gel electrophoresis of S1 endonuclease digested hybrids. Cell 12:721-732. Goff, S. and Berg, P. 1978. Excision of DNA segments introduced into cloning vectors by the poly(dA-dT) joining method. Proc. Natl. Acad. Sci. U.S.A. 75:1763-1767. Henikoff, S. 1984. Unidirectional digestion with exonuclease III creates targeted breakpoints for DNA sequencing. Gene 28:351-359. Kerr, C. and Sadowski, P.D. 1972. Gene 6 exonuclease of bacteriophage T7. I. Purification and properties of the enzyme. J. Biol. Chem. 247:305-310. Little, J.W. 1981. Lambda exonuclease. In Gene Amplification and Analysis, Vol. 2: Structural analysis of nucleic acids (J.G. Chirikjian and T.S. Papas, eds.) pp. 135-145. Elsevier/North Holland, NY, Amsterdam, Oxford. Rogers, S.G. and Weiss, B. 1980. Exonuclease III of Escherichia coli K-12, an AP exonuclease. Meth. Enzymol. 65:201-211. Sanger, F., Nicklen, S., and Coulson, A.R. 1977. DNA sequencing with chain-terminating inhibitors. Proc. Natl. Acad. Sci. U.S.A. 74:5463-5467. Thomas, K.R. and Olivera, B.M. 1978. Processivity of DNA exonucleases. J. Biol. Chem. 253:424-429. Vales, L.D., Rabin, B.A., and Chase, J.W. 1982. Subunit structure of Escherichia coli exonuclease VII. J. Biol. Chem. 257:8799-8805.
Exonucleases
Contributed by Stanley Tabor Harvard Medical School Boston, Massachusetts
3.11.4 Current Protocols in Molecular Biology
Endonucleases
UNIT 3.12
BAL 31 NUCLEASE
ENZYME
Bal 31 nuclease from Alteromonas espejiana is a single strand–specific endodeoxyribonuclease (Gray et al., 1981; Wei et al., 1983) (Fig. 3.12.1). On duplex circular DNA, it degrades at nicks or at transient single-stranded regions created by supercoiling (Lau and Gray, 1979). On duplex linear DNA, it degrades from both the 5′ and 3′ termini at both ends, resulting in a controlled shortening of the DNA. Bal 31 also acts as a ribonuclease, catalyzing the hydrolysis of ribosomal and tRNA (Gray et al., 1975). Bal 31 nuclease requires both Ca++ and Mg++. Digestions can be terminated with EGTA, a specific chelator of Ca++, without affecting the Mg++ concentration. Bal 31 nuclease is active in SDS and urea (Gray et al., 1975).
I. Activity on single stranded DNA or RNA:
5′
3′
Ca++
5′ dNMPs or 5′ rNMPs
ss DNA or RNA II. Activity at a nick or gap in duplex DNA or RNA:
5′ 3′
3′ 5′
Ca++
5′ 3′
3′ 5′
5′ 3′
3′ 5′
nicked DNA or RNA III. Activity at the ends of duplex DNA fragments:
Ca++
Figure 3.12.1 Bal 31 activities.
Reaction Conditions For 50-ìl reaction: 50 mM Tris⋅Cl, pH 7.5 10 mM CaCl2 10 mM MgCl2 600 mM NaCl 2 µg DNA 50 µg/ml BSA 10 U Bal 31 nuclease Incubate at 30°C for 1 to 30 min, depending upon extent of digestion required. Stop the reaction by adding 5 µl of 0.5 M EDTA or by heating to 75°C for 10 min, and analyze products by agarose gel electrophoresis (UNIT 2.5). Due to the high NaCl concentration in the Bal 31 buffer, small volumes of the reaction mixture should be loaded. For subsequent enzymatic treatments, it is usually necessary to remove the NaCl by precipitating the DNA with ethanol (UNIT 2.1). One unit is defined as the amount of enzyme that catalyzes the removal of 200 bp from each end of linearized pBR322 in 10 min at 30°C using a DNA concentration of 50 µg/ml in a 50-µl reaction. Contributed by Stanley Tabor and Kevin Struhl Current Protocols in Molecular Biology (1989) 3.12.1-3.12.6 Copyright © 2000 by John Wiley & Sons, Inc.
Enzymatic Manipulation of DNA and RNA
3.12.1 Supplement 8
Critical Parameters 1. Bal 31 activity is inhibited by RNA that contaminates crude DNA preparations. Thus, DNA should be purified using CsCl gradients or PEG precipitation (UNIT 1.7), or gel-purified (UNIT 2.6). 2. The unit definition for Bal 31 nuclease should be used only as as a rough estimate. For each preparation of DNA, the extent of digestion should be monitored by agarose gel electrophoresis (UNIT 2.5). 3. To reduce the rate of digestion, lower the temperature of the reaction. A temperature of 20°C reduces the rate of Bal 31 nuclease 3-fold from that observed at 30°C. 4. Bal 31 activity will vary with different DNA templates. GC-rich regions are degraded more slowly than AT-rich regions. Consequently, the two ends of a given DNA fragment can be degraded at different rates. Also, a series of deletions of a given DNA fragment will be nonuniform at the nucleotide level; some sequences will have a very low frequency of appearance at the junction of the deletion. 5. Bal 31-digested DNA fragments can be ligated directly at low frequency. To increase the efficiency of ligation, digested DNA should be extracted with phenol and precipitated with ethanol (UNIT 2.1). Then the ends of the DNA should be repaired with Klenow fragment or T4 DNA polymerase (UNIT 3.5), prior to the ligation reaction with T4 DNA ligase (UNIT 3.16). Applications 1. Cloning for creating deletions of different sizes in a controlled manner. The cloned DNA is resected at specific sites with a restriction enzyme, and the resulting linear DNA is digested with Bal 31 nuclease for various intervals of time (UNIT 7.3). 2. Mapping restriction sites in a DNA fragment (Legerski et al., 1978). 3. Investigating secondary structure of supercoiled DNA (Lau and Gray, 1979) and alterations in the helix structure of duplex DNA caused by treatment with mutagenic agents (Legerski et al., 1977). 4. Creating successive deletions needed for linker-scanning mutagenesis (UNIT 8.4). ENZYME
S1 NUCLEASE S1 nuclease from Aspergillus oryzae is a highly specific single-stranded endonuclease (Vogt, 1980; Fig. 3.12.2). The rate of digestion of single-stranded DNA is 75,000 times faster than that of double-stranded DNA. I. Activity on single stranded DNA or RNA:
Zn++ 5′
3′
ss DNA or RNA
pH 4.5
5′ dNMPs or 5′ rNMPs
II. Activity at a nick or gap in duplex DNA or RNA:
5′ 3′
3′ 5′ nicked DNA or RNA
Endonucleases
Zn++ pH 4.5
5′ 3′
3′ 5′
5′ 3′
3′ 5′
Figure 3.12.2 S1 nuclease activities.
3.12.2 Supplement 8
Current Protocols in Molecular Biology
The pH optimum is 4.0 to 4.3, and the rate drops 50% at pH 4.9. Reactions are normally run at pH 4.6 to avoid depurination of the DNA. The enzyme requires low levels of Zn++, and it is largely unaffected by NaCl concentrations in the range of 10 to 300 mM. It is stable to urea, SDS, and formamide. Reaction Conditions For 100-ìl reaction: 50 mM sodium acetate, pH 4.6 1 mM zinc acetate 250 mM NaCl 2 µg DNA 50 µg/ml BSA 10 U S1 nuclease Incubate at 37°C for 30 min. Stop reaction by adding 1 µl of 0.5 M EDTA. The volume of reaction, amount of DNA, units of S1 nuclease, temperature, and time of reaction will vary with different applications of S1 nuclease. Applications Most applications of S1 nuclease make use of its ability to trim protruding single-stranded ends of DNA and RNA without significant nibbling of blunt duplex ends. 1. Mapping the 5′ and 3′ ends of RNA transcripts by the analysis of S1-resistant RNA:DNA hybrids (UNIT 4.6). 2. Mapping the location of introns by digesting a hybrid of mature mRNA with 32 P-labeled genomic DNA. S1 will cleave at the single-stranded loops created by introns within these hybrid molecules. 3. Digesting the hairpin structures formed during the synthesis of cDNA by reverse transcriptase (UNIT 5.5). 4. Removing single-stranded termini of DNA fragments to produce blunt ends for ligation (UNIT 3.16). 5. Creating small deletions at restriction sites. It will nibble at the ends of a linear duplex fragment at a very slow rate. 6. Rendering the ends flush after successive deletions by exo III for linker-scanning mutagenesis (UNIT 8.4). MUNG BEAN NUCLEASE
ENZYME
Mung bean nuclease (from sprouts of mung bean) is a highly specific single-stranded endonuclease with properties similar to those of S1 nuclease (Kroeker et al., 1976). Reactions are normally performed at pH 5.0, which is optimal for degrading singlestranded DNA. Activity against double-stranded DNA is essentially negligible at pH 5.0, but it increases as the pH is reduced below this point. Mung bean nuclease activity is significantly decreased at NaCl concentrations >200 mM. One unit is defined as the amount of enzyme that produces 1 µg of acid-soluble material in 1 min at 37°C using single-stranded salmon sperm DNA as the substrate. Enzymatic Manipulation of DNA and RNA
3.12.3 Current Protocols in Molecular Biology
Supplement 8
Reaction Conditions For 100-ìl reaction: 30 mM sodium acetate, pH 5.0 50 mM NaCl 1 mM zinc acetate 5% (vol/vol) glycerol 1 µg DNA 50 µg/ml BSA 15 U mung bean nuclease Incubate 30 min at 37°C. Stop reaction by adding 1 µl of 0.5 M EDTA. The volume of reaction, amount of DNA, units of enzyme, temperature, and time of reaction will vary depending on the application. Applications Mung bean nuclease is used for most of the same purposes as described for S1 nuclease. However, mung bean nuclease is more precise than S1 nuclease for cleaving immediately adjacent to the last hybridized base pair without removing any of the base-paired nucleotides. For this reason, mung bean nuclease will typically generate single bands in transcript mapping experiments (UNIT 4.6), whereas S1 often results in multiple bands. In addition, mung bean nuclease is preferred over S1 for precisely deleting overhanging bases that result from restriction endonuclease cleavage. Unlike S1 nuclease, mung bean nuclease will not cleave the strand opposite a nick in duplex DNA. The disadvantages of mung bean nuclease are that it is more expensive than S1, and that its activity is more sensitive to reaction conditions. ENZYME
MICROCOCCAL NUCLEASE Micrococcal nuclease from Staphylococcus aureus is a relatively nonspecific nuclease that cleaves single- and double-stranded DNA and RNA to oligo- and mononucleotides with 3′ phosphates (Alexander et al., 1961). The enzyme is more active on single- stranded nucleic acids. Cleavage of DNA or RNA occurs preferentially at AT- or AU-rich regions although all sequences are ultimately cleavable. The enzyme is strictly dependent on calcium for activity and hence can be inactivated by Ca++-specific chelating agents such as EGTA. One unit is defined as the amount of enzyme that hydrolyzes 1 µmol of acid-soluble oligonucleotides from native DNA per minute at 37°C, pH 8.8. Reaction Conditions Typical micrococcal nuclease digestions are performed in solutions containing 10 mM Tris⋅Cl, pH 8.0, and 1 mM CaCl2. However, the enzyme is somewhat more active at higher pH. The volume of reaction, amount and source of nucleic acid, units of enzyme, temperature, ionic strength, and time of reaction will vary greatly depending on the application. The reactions can be stopped by EDTA or the Ca++-specific chelator EGTA. Applications 1. Studies of chromatin structure (Kornberg, 1977).
Endonucleases
2. Removing nucleic acid from crude cell-free extracts without destroying enzyme activities. Digestion of nucleic acid is performed under mild buffer and ionic conditions in the presence of CaCl2. When digestion is complete, micrococcal nuclease is inactivated with EGTA (Pelham and Jackson, 1976). Such treated extracts are generally active for all processes except those requiring calcium.
3.12.4 Supplement 8
Current Protocols in Molecular Biology
DEOXYRIBONUCLEASE I (DNase I)
ENZYME
DNase I from bovine pancreas is an endonuclease that degrades double-stranded DNA to produce 3′-hydroxyl oligonucleotides (Moore, 1981; Fig. 3.12.3). The enzyme requires divalent cations; the specificity of the reaction differs depending upon which divalent cation is present (Campbell and Jackson, 1980). In the presence of Mg++, DNase I produces nicks in duplex DNA, while in the presence of Mn++ the enzyme produces double-stranded breaks in the DNA. I. Activity in the presence of Mg++:
P 5′ 3′
3′ 5′
5′ 3′
P
P
P 3′ 5′
P P
II. Activity in the presence of Mn++:
5′ 3′
3′ 5′
5′ 3′
P 5′ OH 3′
3′ OH 5′ P
3′ OH 5′ P
P 5′ OH 3′
P 5′ OH 3′
3′OH 5′ P
3′ 5′
Figure 3.12.3 DNase I activities.
Reaction Conditions For 100-ìl reaction: 50 mM Tris⋅Cl, pH 7.5 10 mM MgCl2 (for single-strand nicks; to create double-strand breaks, replace with 10 mM MnCl2) 2 µg DNA 50 µg/ml BSA 1 µl DNase I (the concentration depends on the application) Incubate at 37°C for 1 to 30 min, depending upon the degree of digestion desired. Stop the reaction by adding 5 µl of 0.5 M EDTA. For nick translations, the DNase I reaction is carried out simultaneously with the DNA polymerase I reaction (UNIT 3.5). Preparation and Storage of DNase I Solutions DNase I is usually purchased as a lyophilized powder at a concentration of 2000 to 3000 U/mg protein. For almost all applications, it is useful to make up a solution containing DNase I that can be stored for long periods of time without loss of enzyme activity. One method is to dissolve 1 mg DNase I in 1 ml of a 50% (wt/vol) solution of glycerol containing 20 mM Tris⋅Cl, pH 7.5, plus 1 mM MgCl2; this solution can be stored in liquid form at −20°C. Alternatively, 1 mg DNase I can be dissolved in 1 ml of a solution containing 20 mM Tris⋅Cl, pH 7.5, plus 1 mM MgCl2, then aliquotted into small microcentrifuge tubes (100 10-µl aliquots are convenient). The aliquots should be quick-frozen on dry ice, then stored at −80°C. For use, an aliquot of DNase I is thawed on ice and an appropriate amount is added to the reaction mixture (for nick translations, the enzyme must first be diluted; see UNIT 3.5). Aliquots are not refrozen; they are simply discarded after use. DNase I solutions prepared by either method are stable for at least 1 year. It is important that DNase I be dissolved without vortexing (to minimize denaturation) and at a concentration of at least 1 mg/ml (to improve long-term stability).
Enzymatic Manipulation of DNA and RNA
3.12.5 Current Protocols in Molecular Biology
Supplement 8
RNase-Free DNase I For many purposes, DNase I should be free of RNase. High-grade commercial preparations such as Worthington grade DPRF can be satisfactory. Alternatively, DNase I can be dissolved at 1 mg/ml in 0.1 M iodoacetic acid plus 0.15 M sodium acetate at a final pH of 5.3. The solution is then heated 40 min at 55°C and cooled. Finally, 1 M CaCl2 is added to the solution to 5 mM. This homemade preparation of RNase-free DNase I should be stored frozen in small aliquots. Applications 1. Nick translation (UNIT 3.5). Introduces nicks in duplex DNA which then serve as primer sites to initiate DNA synthesis by E. coli DNA polymerase I. 2. Cloning random DNA fragments by catalyzing double-stranded cleavage of DNA in the presence of Mn++ (Anderson, 1981). LITERATURE CITED Alexander, M., Heppel, L.A., and Hurwitz, J. 1961. The purification and properties of micrococcal nuclease. J. Biol. Chem. 236:3014-3019. Anderson, S. 1981. Shotgun DNA sequencing using cloned DNase I-generated fragments. Nucl. Acids Res. 9:3015-3027. Campbell, V.W. and Jackson, D.A. 1980. The effect of divalent cations on the mode of action of DNase I. The initial reaction products produced from covalently closed circular DNA. J. Biol. Chem. 255:37263735. Gray, H.B., Ostrander, D.A., Hodnett, J.L., Legerski, R.J., and Robberson, D.L. 1975. Extracellular nucleases of Pseudomonas Bal 31. I. Characterization of single strand-specific deoxyriboendonuclease and doublestrand deoxyriboexonuclease activites. Nucl. Acids Res. 2:1459-1492. Gray, H.B., Winston, T.P., Hodnett, J.L., Legerski, R.J., Nees, D.W., Wei, C.-F., and Robberson, D.L. 1981. The extracellular nuclease from Alteromonas espejiana: An enzyme highly specific for nonduplex structure in nominally duplex DNAs. In Gene Amplification and Analysis, Vol. 2: Structural analysis of nucleic acids (J.G. Chirikjian and T.S. Papas, eds.) pp. 169-203. Elsevier/North Holland, New York, Amsterdam, Oxford. Kornberg, R.D. 1977. Structure of chromatin. Ann. Rev. Biochem. 46:931-954. Kroeker, W.D., Kowalski, D., and Laskowski, M. 1976. Mung bean nuclease I. Terminally directed hydrolysis of native DNA. Biochemistry 15:4463-4467. Lau, P.P. and Gray, H.B. 1979. Extracellular nucleases of Alteromonas espejiana Bal 31. IV. The single strand-specific deoxyriboendonuclease activity as a probe for regions of staltered secondary structure in negatively and positively supercoiled closed circular DNA. Nucl. Acids Res. 6:331-357. Legerski, R.J., Gray, H.B. and Robberson, D.L. 1977. A sensitive endonuclease probe for lesions in DNA helix structure produced by carcinogenic or mutagenic agents J. Biol. Chem. 252:8740-8746. Legerski, R.J., Hodnett, J.L., and Gray, H.B. 1978. Extracellular nucleases of Pseudomonas Bal 31. III. Use of the double-strand deoxyriboexonuclease activity as a basis of a convenient method for the mapping of fragments of DNA produced by cleavage with restriction enzymes. Nucl. Acids Res. 5:1445-1464. Moore, S. 1981. Pancreatic DNase. In The Enzymes, Vol. 14A (P.D. Boyer, ed.) pp. 281-298. Academic Press, San Diego. Pelham, H.R.B. and Jackson, R.J. 1976. An efficient mRNA-dependent translation system from reticulocyte lysates. Eur. J. Biochem. 67:247-256. Vogt, V.M. 1980. Purification and properties of S1 nuclease from Aspergillus. Methods Enzymol. 65:248254. Wei, C.-F., Alianell, G.A., Bencen, G.H., and Gray, H.B. 1983. Isolation and comparison of two molecular species of the Bal 31 nuclease from Alteromonas espejiana with distinct kinetic properties. J. Biol. Chem. 258:13506-13512.
Contributed by Stanley Tabor and Kevin Struhl Harvard Medical School Boston, Massachusetts Endonucleases
3.12.6 Supplement 8
Current Protocols in Molecular Biology
Ribonucleases
UNIT 3.13
Ribonucleases (RNases) with different sequence specificities are used for a variety of analytical purposes, including RNA sequencing, mapping, and quantitation. One very common application for RNase A (see below) is hydrolyzing RNA that contaminates DNA preparations. Two other commonly used RNases, RNase H and RNase T1, are also described below. Many commercially available RNases are sequence-specific endoribonucleases. This property has been used for enzymatic sequencing of RNA. For example, a combination of three different RNases and Staphylococcus aureus nuclease can be used in RNA sequence determination (see Table 3.13.1). Table 3.13.1
RNases Used in RNA Sequencinga
RNase
Sequence specificity
T1 U2 CL3 S. aureus nucleaseb
Gp↓N Ap↓N C(A/G)p↓N Np↓A/U
aAdapted from Boehringer Mannheim. bStaphylococcus aureus nuclease cleaves both RNA and DNA.
RIBONUCLEASE A
ENZYME
Ribonuclease A (RNase A) from bovine pancreas is an endoribonuclease that specifically hydrolyzes RNA after C and U residues (Richards and Wyckoff, 1971). Cleavage occurs between the 3′-phosphate group of a pyrimidine ribonucleotide and the 5′- hydroxyl of the adjacent nucleotide. The reaction generates a 2′:3′ cyclic phosphate which then is hydrolyzed to the corresponding 3′-nucleoside phosphates. RNase A activity can be inhibited specifically by an RNase inhibitor (e.g., RNasin from Promega), a protein isolated from human placenta. Reaction Conditions RNase A is active under an extraordinarily wide range of reaction conditions, and it is extremely difficult to inactivate. At low salt concentrations (0 to 100 mM NaCl), RNase A cleaves single-stranded and double-stranded RNA as well the RNA strand in RNA:DNA duplexes. However, at NaCl concentrations of 0.3 M or above, RNase A becomes specific for cleavage of single-stranded RNA. Removal of RNase A from a reaction solution generally requires treatment with proteinase K followed by multiple phenol extractions and ethanol precipitation. DNase-Free RNase A To prepare RNase A free of DNase, dissolve RNase A in TE buffer at 1 mg/ml, and boil 10 to 30 min. Store aliquots at −20°C to prevent microbial growth. Applications 1. Mapping and quantitating RNA species using the ribonuclease protection assay (UNIT 4.7). It is used in conjunction with RNase T1. 2. Hydrolyzing RNA that contaminates DNA preparations (UNITS 1.6 & 1.7). 3. RNA sequencing. Contributed by Kevin Struhl Current Protocols in Molecular Biology (1989) 3.13.1-3.13.3 Copyright © 2000 by John Wiley & Sons, Inc.
Enzymatic Manipulation of DNA and RNA
3.13.1 Supplement 8
4. Blunt-ending double-stranded cDNA (UNIT 5.5). It is used in conjunction with RNase H. ENZYME
RIBONUCLEASE H Ribonuclease H (RNase H) from E. coli is an endoribonuclease that specifically hydrolyzes the phosphodiester bonds of RNA in RNA:DNA duplexes to generate products with 3′ hydroxyl and 5′ phosphate ends (Berkower et al., 1973). It will not degrade singlestranded or double-stranded DNA or RNA. RNase H cleavage can be directed to specific sites by hybridizing short deoxyoligonucleotides to the RNA (Donis-Keller, 1979). One unit is defined as the amount of enzyme that produces 1 nmol of acid-soluble ribonucleotides from poly(A)⋅poly(dT) in 20 min at 37°C. Reaction Conditions For 100-ìl reaction: 20 mM HEPES⋅KOH, pH 8.0 50 mM KCl 4 mM MgCl2 1 mM DTT 2 µg RNA:DNA duplex 50 µg/ml BSA 1 U ribonuclease H Incubate 20 min at 37°C. Stop reaction by adding 1 µl of 0.5 M EDTA. The volume of reaction, amount of DNA, units of enzyme, temperature, time, and method of stopping the reaction will vary depending on the application. Applications 1. Facilitating the synthesis of double-stranded cDNA by removing the mRNA strand of the RNA:DNA duplex produced during first strand synthesis of cDNA (UNIT 5.5). It is also used later in the protocol to degrade residual RNA in conjunction with RNase A. 2. Creating specific cleavages in RNA molecules by using synthetic deoxyoligonucleotides to create local regions of RNA:DNA duplexes (Donis-Keller, 1979).
ENZYME
RIBONUCLEASE T1 Ribonuclease T1 (RNase T1) from Aspergillus oryzae is an endoribonuclease that specifically hydrolyzes RNA after G residues (Uchida and Egami, 1971). Cleavage occurs between the 3′-phosphate group of a guanine ribonucleotide and the 5′-hydroxyl of the adjacent nucleotide. The reaction generates a 2′:3′ cyclic phosphate which then is hydrolyzed to the corresponding 3′-nucleoside phosphates. Reaction Conditions The enzyme is active under a wide range of reaction conditions, and it is difficult to inactivate. At low salt concentrations (0 to 100 mM NaCl), it cleaves single-stranded and double-stranded RNA as well as the RNA strand in RNA:DNA duplexes. However, at NaCl concentrations of 0.3 M or above, it becomes specific for cleavage of single-stranded RNA. Removal of RNase T1 from a reaction solution generally requires treatment with proteinase K followed by multiple phenol extractions and ethanol precipitation.
Ribonucleases
3.13.2 Supplement 8
Current Protocols in Molecular Biology
Applications 1. Mapping and quantitating RNA species using the ribonuclease protection assay (UNITS 4.7 & 9.8). It is used in conjunction with RNase A. 2. RNA sequencing. 3. Determining the level of RNA transcripts synthesized in vitro from DNA templates containing a “G-less cassette.” Because accurately initiated transcripts contain no G residues, treatment of the reaction products with RNase T1 preferentially destroys all nonspecific transcripts (Sawadogo and Roeder, 1985). LITERATURE CITED Berkower, I., Leis, J., and Hurwitz, J. 1973. Isolation and characterization of an endonuclease from Escherichia coli specific for ribonucleic acid in ribonucleic acid⋅deoxyribonucleic acid hybrid structures. J. Biol. Chem. 248:5914-5921. Boehringer Mannheim Biochemicals. Biochemicals for Molecular Biology (catalog). Indianapolis. Donis-Keller, H. 1979. Site specific enzymatic cleavage of RNA. Nucl. Acids Res. 7:179-192. Richards, F.M. and Wyckoff, H.W. 1971. Bovine pancreatic ribonuclease. In The Enzymes, Vol. IV (P.D. Boyer, ed.) pp. 647-806. Academic Press, San Diego. Sawadogo, M. and Roeder, R.G. 1985. Factors involved in specific transcription by human RNA polymerase II: Analysis by a rapid and quantitative in vitro assay. Proc. Natl. Acad. Sci. U.S.A. 82:4394-4398. Uchida, T. and Egami, F. 1971. Microbial ribonucleases with special reference to RNases T1, T2, N1, and U2. In The Enzymes, Vol. IV (P.D. Boyer, ed.) pp. 205-250. Academic Press, San Diego.
Contributed by Kevin Struhl Harvard Medical School Boston, Massachusetts
Enzymatic Manipulation of DNA and RNA
3.13.3 Current Protocols in Molecular Biology
Supplement 8
UNIT 3.14
DNA Ligases DNA ligases catalyze the formation of phosphodiester bonds between juxtaposed 5′ phosphate and a 3′-hydroxyl terminus in duplex DNA. This activity can repair singlestranded nicks in duplex DNA (Fig. 3.14.1) and join duplex DNA restriction fragments having either blunt ends (Fig. 3.14.2) or homologous cohesive ends (Fig. 3.14.3).Two ligases are used for nucleic acid research—E. coli ligase and T4 ligase. These enzymes differ in two important properties. One is the source of energy: T4 ligase uses ATP, while E. coli ligase uses NAD. Another important difference is their ability to ligate blunt ends; under normal reaction conditions, only T4 DNA ligase will ligate blunt ends.
3′ Example:
3′ 5′
5′ 3′ ATP (T4, T7 ligase)
5′
P
A
G
C
T
A
G
G
A
T
C
G
A
T
C
P
P
P
P
P
P
5′
5′
3′
P
NAD (E. coli ligase) AMP + NMN
(T4, T7 ligase) AMP + PP i 3′ 5′
3′
P
T
ATP
NAD (E. coli ligase)
P
P
C
3′ 5′ 3′
5′ 3′
5′
P OH P
P
P
P
P
P
P
P
C
T
A
G
C
T
A
G
G
A
T
C
G
A
T
C
P
P
P
P
P
P
3′
P
P
5′
Figure 3.14.1 DNA ligase activity at a nick.
ENZYME
T4 DNA LIGASE T4 DNA ligase, the product of gene 30 of phage T4, was originally purified from phage-infected cells of E. coli. The phage T4 gene 30 has been cloned, and the enzyme is now prepared from overproducing strains. Using ATP as a cofactor, T4 DNA ligase catalyzes the repair of single-stranded nicks in duplex DNA and joins duplex DNA restriction fragments having either blunt or cohesive ends. It is the only ligase that efficiently joins blunt-end termini under normal reaction conditions. It appears that T4 DNA ligase activity may be stimulated by T4 RNA ligase (UNIT 3.15). See UNIT 3.16 for a detailed ligation protocol.
DNA Ligases
3.14.1 Supplement 8
Contributed by Stanley Tabor Current Protocols in Molecular Biology (1987) 3.14.1-3.14.4 Copyright © 2000 by John Wiley & Sons, Inc.
Reaction Conditions For 50-ìl reaction: 40 mM Tris⋅Cl, pH 7.5 10 mM MgCl2 10 mM DTT 1 µg DNA 0.5 mM ATP 50 µg/ml BSA 1 “Weiss” U T4 DNA ligase Incubate at 12° to 30°C for 1 to 16 hr. Stop reaction by adding 2 µl of 0.5 M EDTA or by heating to 75°C for 10 min. The volume of reaction, concentration of DNA, and the temperature and time of the reaction will vary, depending upon the individual application. One Weiss unit is equivalent to 60 cohesive-end units. Ligation of cohesive ends is usually carried out at 12° to 15°C to maintain a good balance between annealing of the ends and activity of the enzyme. Higher temperatures make it difficult for the ends to anneal, whereas lower temperatures diminish ligase activity. Blunt-end ligations are typically performed at room temperature since annealing is not a factor (the enzyme is not particularly stable above 30°C). Blunt-end ligations require about 10 to 100 times more enzyme than cohesive-end ligations to achieve an equal efficiency. T4 DNA ligase is not inhibited by tRNA, but it is strongly inhibited by NaCl concentrations >150 mM. Macromolecular exclusion molecules (e.g., PEG 8000) have been shown to greatly increase the rate of both cohesive-end and blunt-end joining by T4 DNA ligase (Pfeiffer and Zimmerman, 1983). An inherent consequence of macromolecular crowding is that all ligations are intermolecular; thus, this technique is not suitable for the ligation and circularization of inserts and vectors that are required for most cloning experiments.
3′ 5′ Example:
5′ 3′
5′
P
P
OH P
P
3′
P
C
T
A
G
C
T
A
G
G
A
T
C
G
A
T
C
3′ 5′ 5′ 3′
3′ 5′ 3′
P
P
P
P OH
3′ 5′
P
P
P
5′
5′ 3′ ATP
ATP (T4 ligase) 5′ 3′
P
P
(T4 ligase)
AMP +PP i
5′
P
P
P
P
P
P
P
C
T
A
G
C
T
A
G
G
A
T
C
G
A
T
C
P
P
3′
Figure 3.14.2 DNA ligase activity at blunt ends.
P
P
P
P
3′
P
5′
Enzymatic Manipulation of DNA and RNA
3.14.2 Current Protocols in Molecular Biology
Applications T4 DNA ligase is by far the most commonly used DNA ligase. It can be used for virtually any application requiring a DNA ligase. Importantly, it efficiently ligates blunt-end termini, a reaction that other ligases do not carry out in the absence of macromolecular exclusion molecules. ENZYME
ESCHERICHIA COLI DNA LIGASE DNA ligase from E. coli is the product of the lig gene. The lig gene has been cloned, and the enzyme is obtained from an overproducing strain. E. coli DNA ligase catalyzes the repair of single-stranded nicks in duplex DNA and joins restriction fragments having homologous cohesive ends. E. coli DNA ligase does not join termini with blunt ends under normal reaction conditions. Unlike the other ligases, it uses NAD as a cofactor. 3′ 5′
Example:
P OH P
5′
5′ 3′
3′ 5′
P
P
P
P
3′
P
C
T
A
G
C
T
A
G
G
A
T
C
G
A
T
C
3′ 5′
5′ 3′ P P P (T4, T7 ligase) (E. coli ligase) 3′ ATP ATP NA (T4, T7 ligase) D AMP +PP i 5′ 3′ 3′ 5′ 5′ P P P
P
P
OH P
P
5′ NAD5′ 3′ (E. coli ligase) AMP + NMN
P
P
P
C
T
A
G
C
T
A
G
G
A
T
C
G
A
T
C
P
P
P
3′
P
P
P
3′
P
P
5′
Figure 3.14.3 DNA ligase activity at cohesive ends.
Reaction Conditions For 50-ìl reaction: 40 mM Tris⋅Cl, pH 8 10 mM MgCl2 5 mM DTT 1 µg DNA 0.1 mM NAD 50 µg/ml BSA 10 “Modrich-Lehman” U E. coli DNA ligase Incubate at 10° to 25°C for 2 to 16 hr. Stop reaction by adding 2 µl of 0.5 M EDTA or by heating to 75°C for 10 min. The volume of reaction, concentration of DNA, and temperature and time of reaction will vary, depending upon the individual application. DNA Ligases
E. coli DNA ligase, in contrast to T4 DNA ligase, does not require reducing agents
3.14.3 Current Protocols in Molecular Biology
(e.g., DTT) in the reaction. PEG 8000 greatly increases the rate of cohesive end joining by E. coli DNA ligase (Harrison and Zimmerman, 1983). Interestingly, the presence of macromolecular exclusion molecules also enables E. coli DNA ligase to efficiently join blunt-end termini, a reaction it is unable to carry out in their absence. Modrich-Lehman units measure the ability to form poly d(A-T) circles. One ModrichLehman unit is equivalent to 6 Weiss units (Modrich and Lehman, 1975). Applications E. coli DNA ligase can be used as an alternative to T4 DNA ligase when blunt-end ligations are not required. Transformation using DNA ligated with E. coli DNA ligase has a lower background that results from aberrant ligations compared with T4 DNA ligase, since T4 DNA ligase has a much lower specificity for the structure of the termini. LITERATURE CITED Harrison, B. and Zimmerman, S.B. 1983. Macromolecular crowding allows blunt-end ligation by DNA ligases from rat liver or Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 80:5852-5856. Modrich, T. and Lehman, I.R. 1975. Enzymatic joining of polynucleotides. J. Biol. Chem. 245:3626-3631. Pfeiffer, B.H. and Zimmerman, S.B. 1983. Polymer-stimulated ligation: Enhanced blunt- or cohesive-end ligation of DNA or deoxyribonucleotides by T4 DNA ligase in polymer solutions. Nucl. Acids Res. 11: 7853-7871.
KEY REFERENCE Engler, M.J. and Richardson, D.C. 1982. DNA ligases. In The Enzymes, Vol. 15B (P.D. Boyer, ed.) pp. 3-30. Academic Press, San Diego.
Contributed by Stanley Tabor Harvard Medical School Boston, Massachusetts
Enzymatic Manipulation of DNA and RNA
3.14.4 Current Protocols in Molecular Biology
UNIT 3.15 ENZYME
RNA Ligases T4 RNA LIGASE (Uhlenbeck and Gumport, 1982) T4 RNA ligase, the product of the phage gene 63, is purified from phage-infected cells. It catalyzes the ATP-dependent covalent joining of single-stranded 5′-phosphoryl termini of DNA or RNA to single-stranded 3′-hydroxyl termini of DNA or RNA (Fig. 3.15.1).
Example: 3′ RNA [or DNA] 5′
RNA [or DNA] 3′
5′
5′
P
P
5′
OH
P
P
P
P
3′
P
3′
C
T
A
G
C
T
A
G
ATP ATP 5′
AMP + PP i
3′ 5′
P
C
T
P
A
P
G
P
C
P
T
P
A
P
3′
G
Figure 3.15.1 RNA ligase activity.
Reaction Conditions For 50-ìl reaction: 50 mM HEPES, pH 8.3 10 mM MgCl2 5 mM DTT 2 µg single-stranded DNA or RNA 2 mM ATP 50 µg/ml BSA 1 U T4 RNA ligase Incubate at 17°C for 10 hr. Stop the reaction by adding 2 µl of 0.5 M EDTA. Applications 1. Radioactive labeling of 3′ termini of RNA (Uhlenbeck and Gumport, 1982). The reaction contains the RNA molecule as the acceptor, a [5′-32P]nucleoside 3′, 5′bis(phosphate) (e.g., 5′-32P-Cp) as the donor, and ATP as the energy source. The product formed by this reaction is the RNA molecule containing one additional nucleotide at the 3′ terminus and a 32P-labeled phosphate in the last internucleotide linkage. 2. Circularizing deoxy- and ribo-oligonucleotides (Brennan et al., 1983). 3. Ligating oligomers for oligonucleotide synthesis (Romaniuk and Uhlenbeck, 1983). One such use is to synthesize oligomers that contain internally labeled oligomers at specific residues. RNA Ligases
3.15.1
4. Stimulating the blunt-end ligation activity of T4 DNA ligase (Sugino et al., 1977). Contributed by Stanley Tabor Current Protocols in Molecular Biology (1987) 3.15.1-3.15.2 Copyright © 2000 by John Wiley & Sons, Inc.
LITERATURE CITED Brennan, C.A., Manthey, A.E., and Gumport, R.I. 1983. Using T4 RNA ligase with DNA substrates. Meth. Enzymol. 100:38-52. Romaniuk, P.J. and Uhlenbeck, O.C. 1983. Joining of RNA molecules with RNA ligase. Meth. Enzymol. 100:52-59. Sugino, A., Goodman, H.M., Heyneker, H.L., Shine, J., Boyer, H.W., and Cozzarelli, N.R. 1977. Interaction of bacteriophage T4 RNA and DNA ligases in joining of duplex DNA at base-paired ends. J. Biol. Chem. 252:3987-3994.
KEY REFERENCE Uhlenbeck, O.C. and Gumport, R.I. 1982. T4 RNA ligase. In The Enzymes, Vol. 15B (P.D. Boyer, ed.) pp. 31-60. Academic Press, NY.
Contributed by Stanley Tabor Harvard Medical School Boston, Massachusetts
Enzymatic Manipulation of DNA and RNA
3.15.2 Current Protocols in Molecular Biology
Supplement 13
SECTION IV
CONSTRUCTION OF HYBRID DNA MOLECULES Given the large number of restriction endonucleases and other enzymes for manipulating DNA, it is possible to create any DNA molecule of interest. As it is impossible to cover all possible ways of manipulating DNA molecules, this section will discuss the general principles involved along with several specific techniques that are frequently used. Chapter 8 describes methods for creating point mutations in DNA.
UNIT 3.16 BASIC PROTOCOL
Subcloning of DNA Fragments In order to construct new DNA molecules, the starting DNAs are treated with appropriate restriction endonucleases and other enzymes if necessary. The individual components of the desired DNA molecule are purified by agarose or polyacrylamide gel electrophoresis, combined, and treated with DNA ligase. The products of the ligation mixture (along with control mixtures) are introduced into competent E. coli cells, and transformants are identified by an appropriate genetic selection. DNA is prepared from the colonies or plaques and subjected to restriction endonuclease mapping in order to determine if the desired DNA molecule was created. All cloning experiments follow the steps outlined below. Materials Calf intestine phosphatase (CIP) and buffer (optional; UNIT 3.10) dNTP mix (0.5 mM each; UNIT 3.4) Klenow fragment of E. coli DNA polymerase I or T4 DNA polymerase (optional; UNIT 3.5) Oligonucleotide linkers (optional) 10 mM ATP 0.2 mM dithiothreitol (DTT) T4 DNA ligase (measured in cohesive-end units; UNIT 3.14) 2× T4 DNA ligase buffer Additional reagents and equipment for restriction endonuclease digestions (UNIT 3.1), transformation of E. coli cells (UNIT 1.8), DNA minipreps (UNIT 1.6), and agarose or polyacrylamide gel electrophoresis (UNIT 2.5 or 2.7). 1. In a 20-µl reaction mixture, cleave the individual DNA components with appropriate restriction endonuclease. After the reaction is complete, inactivate the enzymes by heating 15 min to 75°C. If no further enzymatic treatments are necessary, proceed to step 6. Reaction mixtures can be done in any volume; 20 ìl is convenient for gel electrophoresis (step 6). Many of the subsequent enzymatic manipulations (steps 2 to 5) can be carried out sequentially without further buffer changes.
2. If the 5′ phosphates of one of the DNAs are to be removed (see Example 3.16.1), add 2 µl of 10× CIP buffer and 1 U CIP; incubate 30 to 60 min at 37°C as described in UNIT 3.10. After the reaction is complete, inactivate CIP by heating 15 min to 75°C. If no further enzymatic treatments are necessary, proceed to step 6.
Subcloning of DNA Fragments
3.16.1 Supplement 13
3. If one or both ends generated by a restriction endonuclease must be converted to blunt ends (see Example 3.16.4), add 1 µl of a solution containing all 4 dNTPs (0.5 mM each) and an appropriate amount of the Klenow fragment of E. coli DNA polymerase I or T4 DNA polymerase; carry out the filling-in or trimming reaction as described in UNIT 3.5. After the reaction is complete, inactivate the enzymes by Contributed by Kevin Struhl Current Protocols in Molecular Biology (1987) 3.16.1-3.16.2 Copyright © 2000 by John Wiley & Sons, Inc.
heating 15 min to 75°C. If oligonucleotide linkers are to be added (see Example 3.16.6), proceed to step 4. If a DNA fragment containing only one blunt end is desired, cleave the reaction products with an appropriate restriction endonuclease. If no further enzymatic treatments are necessary, proceed to step 6. 4. Add 0.1 to 1.0 µg of an appropriate oligonucleotide linker, 1 µl of 10 mM ATP, 1 µl of 0.2 M DTT, and 20 to 100 cohesive-end units of T4 DNA ligase; incubate overnight at 15°C. Inactivate the ligase by heating 15 min to 75°C. In general, the oligonucleotide linkers are phosphorylated prior to ligation (UNIT 3.10). However, the linkers do not have to be phosphorylated, in which case the products will contain only one linker on each end.
5. Cleave the products from step 4 with the restriction endonuclease recognizing the oligonucleotide linker, adjusting the buffer conditions if necessary. If only one of the two ends is to contain a linker, cleave the products with an additional restriction enzyme (see Examples 3.16.2 and 3.16.6). 6. Isolate the desired DNA segments by gel electrophoresis, or by other methods if appropriate. Electrophoresis in agarose gels is the most common method. Purification is not essential for many cloning experiments, but it is usually very helpful. See critical parameters.
7. Using longwave UV light for visualization of the DNA, cut out the desired band(s) and purify the DNA away from the gel material using the procedures described in UNIT 2.6. It is critical to use longwave UV light sources to prevent damage to the DNA. If low gelling/melting temperature agarose is used, ligation reactions can usually be performed directly in the gel slice (see alternate protocol).
8. Set up the following ligation reaction: 9 µl component DNAs (0.1 to 5 µg) 10 µl 2× ligase buffer 1 µl 10 mM ATP 20 to 500 U (cohesive end) T4 DNA ligase Incubate 1 to 24 hr at 15°C. Simple ligations with two fragments having 4-bp 3′ or 5′ overhanging ends require much less ligase than more complex ligations or blunt-end ligations. The quality of the DNA will also affect the amount of ligase needed. Controlled cloning experiments usually involve several different ligation reactions. For simplicity, appropriate DNAs should be added to each tube (adding water if necessary) so that the volume is 9 ìl. Immediately prior to use, premix on ice the remaining ingredients (2× buffer, ATP, enzyme) in sufficient amounts for all the reactions. To start the ligation reactions, add 11-ìl aliquots of the premix to each tube containing DNA. The amounts and ratios of the component DNAs and the controls for these ligation reactions are discussed below.
9. Introduce 1 to 10 µl of the ligated products into competent E. coli cells and select for transformants by virtue of the genetic marker present on the vector. 10. From individual E. coli transformants, purify plasmid or phage DNAs by miniprep procedures and determine their structures by restriction mapping. If the frequency of transformants containing the desired hybrid DNA molecule is too low, initial screening by filter hybridization to immobilized colonies (UNIT 6.2) or plaques (UNIT 6.1) or by genetic techniques is necessary.
Enzymatic Manipulation of DNA and RNA
3.16.2 Current Protocols in Molecular Biology
Supplement 9
ALTERNATE PROTOCOL
LIGATION OF DNA FRAGMENTS IN GEL SLICES This alternate protocol saves considerable time in comparison to the basic protocol because it eliminates purification of the DNA fragments away from the gel matrix (Struhl, 1985). It is suitable for most simple cloning experiments and is particularly valuable for carrying out a set of hybrid constructions involving a variety of different DNA fragments. However, as the cloning efficiency may be reduced, this method should be employed only when it is desired to make one (or relatively few) specific molecules. Additional Materials Low gelling/melting temperature agarose (SeaPlaque, FMC Marine Colloids) TAE buffer (APPENDIX 2) 1. Treat the starting DNAs with appropriate restriction endonucleases and other enzymes, as described in steps 1 to 6 of the basic protocol. 2. Subject the treated DNAs to electrophoresis in low gelling/melting temperature agarose using TAE buffer. It is critical that the agarose be of high quality. SeaPlaque is a good choice. The agarose concentration should be kept as low as possible (0.7% is suitable for most applications).
3. Cut out the desired band(s) in the smallest possible volume (20 to 50 µl) using a clean razor blade, and place the gel slice in a microcentrifuge tube. 4. Melt the gel slices containing DNA at 70°C for at least 10 min. This temperature is hot enough to melt the agarose without denaturing the DNA.
5. In separate tubes for each ligation reaction, combine the gel slices containing appropriate DNAs (and water if necessary) for a total volume of 9 µl. Place the tubes at 37°C for a few minutes. The gel slices should remain molten at 37°C.
6. To each tube containing DNA, add 11 µl of an ice-cold mixture containing 2× buffer, ATP, and T4 DNA ligase. Mix immediately by flicking the tube and place on ice. Then incubate the reaction mixtures 1 to 48 hr at 15°C. DNA fragments can still be ligated even though the reaction mixture has resolidified into a gel.
7. After the ligation reaction is complete, remelt the gel slices 5 to 10 min at 73°C and add 5 µl of the ligated products to 200 µl of competent E. coli cells (UNIT 1.8). The remelted gel must be diluted at least 30-fold so that it does not resolidify when the cells are placed on ice.
8. Carry out steps 9 and 10 of the basic protocol. Ligation in gel slices is suitable for most cloning applications. If the method does not work, DNA fragments can be purified from the gel by various techniques discussed in UNIT 2.6. Ligations are then carried out by the basic protocol.
REAGENTS AND SOLUTIONS
Subcloning of DNA Fragments
2× T4 DNA ligase buffer 100 mM Tris⋅Cl, pH 7.5 20 mM MgCl2 20 mM DTT
3.16.3 Supplement 9
Current Protocols in Molecular Biology
COMMENTARY Background Information The essence of recombinant DNA technology is the joining of two or more separate segments of DNA to generate a single DNA molecule that is capable of autonomous replication in a given host. The simplest constructions of hybrid DNA molecules involve the cloning of insert sequences into plasmid or bacteriophage cloning vectors. The insert sequences can derive from essentially any organism, and they may be isolated directly from the genome (UNITS 5.3 and 5.4), from mRNA (UNIT 5.5), or from previously cloned DNA segments (in which case, the procedure is termed subcloning). Alternatively, insert DNAs can be created directly by DNA synthesis.
Critical Parameters Many different factors must be considered before embarking on any specific DNA manipulation. These include the number, type, and concentration of DNA fragments, the preparation and purification of DNA fragments, and the selection or screening systems that might be necessary for identifying the DNA molecule of interest. Another practical consideration is the number of hybrid DNA molecules that are desired for any particular experiment. For example, when generating genomic or cDNA libraries, it is critical to obtain the maximum number of recombinants. Strategies for producing large libraries are discussed elsewhere (UNITS 5.1 and 5.2). On the other hand, when it is desired to make one (or relatively few) specific DNA molecule, it is important to maximize the frequency of generating the “correct” molecule rather than the absolute cloning efficiency. In other words, it is better to obtain a low number of colonies, mostly containing the desired DNA rather than a large number of colonies few of which contain the desired DNA. In this way, screening for the correct colonies or plaques is unnecessary, and preparation and analysis of DNA from E. coli transformants is minimized; these represent the major time-consuming steps in the overall cloning process. Although it is obvious that the experimental conditions should be designed to favor the desired ligation events, the crucial factor for optimizing the frequency of correct molecules is to reduce the “background” of undesired events that will result in transformable DNA molecules. One major cause of background involves
“errors” in the preparation of the starting DNA components (e.g., incomplete digestion by restriction endonucleases). Such errors can be significantly reduced, but never entirely eliminated. The other major source of background is undesired ligation between molecules in the reaction mixture. This problem is addressed by considering all possible ways in which the starting components (and probable contaminants) can be joined to produce DNA molecules that can transform E. coli via the genetic selection being imposed. By appropriate “tricks,” the major “side reactions” can often be minimized; these are discussed below. DNA preparation and the purification of specific fragments. The efficiency of obtaining the desired recombinant DNA molecule is greatly improved by purifying the individual components by electrophoresis in agarose gels. The extra effort involved is more than compensated by the elimination (in most cases) of the need to screen the recombinants and the resulting preparation and analysis of fewer DNAs. Equally importantly, gel purification makes it possible to use crude miniprep DNAs (UNIT 1.6) as the initial starting material instead of CsClpurified DNAs, thus eliminating time, expense, and tedium. Purified DNA fragments are relatively free of minor contaminants that can cause major background problems. For example, if a circular plasmid vector is cleaved at 99% efficiency with a single restriction enzyme, the uncleaved 1% of the molecules are fully infectious even though they cannot be visualized by ethidium bromide staining. This background, which is unacceptably high for many experiments, can be greatly reduced by gel purification of the linearized DNA. Gel purification is extremely useful in situations where only one of several fragments is needed or where fragments are generated by partial digestion. It is also useful for removal of small oligonucleotide linkers or other low-molecular-weight contaminants. Electrophoretically separated DNA fragments should be isolated in the smallest possible volume of gel slice. Purity is more important than yield. DNA fragments can be purified away from the agarose (or acrylamide) by any of the procedures described in UNIT 2.6. It is important to use high-quality agarose such as SeaKem or SeaPlaque, since other preparations may contain inhibitors of DNA ligase that are hard to remove. DNA purified from acrylamide
Enzymatic Manipulation of DNA and RNA
3.16.4 Current Protocols in Molecular Biology
Subcloning of DNA Fragments
gels does not usually contain such inhibitors. Finally, for most applications, ligation reactions can be performed directly in slices of low gelling/melting temperature agarose (Struhl, 1985; see alternate protocol on p. 3.16.3). By eliminating the need to purify the DNA away from the agarose, considerable time and effort is saved. Intramolecular versus intermolecular ligation of DNA fragments. The absolute and relative concentrations of the input DNA fragments influence the frequencies of specific ligation events. The first consideration is whether the end of a particular DNA molecule will be joined to the other end of the same molecule (intramolecular ligation) or to a separate DNA molecule (intermolecular ligation). The second consideration is whether intermolecular events will be between molecules of the same type (vector–vector or insert–insert) or between molecules of a different type (vector–insert). Most hybrid constructions involve one or more intermolecular events followed by a final intramolecular event to generate a circular plasmid molecule. Intramolecular events. Intramolecular events are a major source of background in many cloning experiments primarily because self-ligation of vector molecules produces undesired E. coli transformants. However, since intramolecular ligation can occur only if the ends of the molecule are cohesive or blunt, it can be almost completely eliminated by using molecules with heterologous ends. This is achieved by cleaving the vector with two restriction endonucleases that produce incompatible ends. For situations where the vector must be cleaved with a single endonuclease, intramolecular joining can be prevented by removing both 5′ phosphates of each molecule with calf intestinal phosphatase. These dephosphorylated vector molecules can be ligated to phosphorylated insert DNAs even though at each end only 1 of the 2 strands will be covalently joined in vitro. Escherichia coli can repair the single-stranded nicks in vivo to generate the desired DNA molecules. Intramolecular ligation of insert fragments does not generally affect the background of unwanted colonies. However, it strongly influences the frequency of obtaining the correct colonies because self-ligated insert DNA cannot be joined to the vector. Thus, in situations where the insert DNA fragments can cyclize, relatively high concentrations of DNA (20 to 100 µg/ml) should be used in order to favor
intermolecular ligation events. This is particularly true for short DNA fragments which are much more likely to cyclize than large fragments (for a theoretical discussion, see Dugaiczyk et al., 1975). Intermolecular events. Almost all cloning experiments using plasmid vectors require the joining of two or more separate DNA molecules followed by circularization of the product. Obviously, intermolecular ligation events become increasingly favored over self-joining as the total concentration of the relevant ends becomes higher. Thus, optimal cloning efficiency occurs at DNA concentrations that are high enough to permit sufficient intermolecular joining, but not so high as to reduce intramolecular ligation. For situations when the background is low, DNA concentrations between 1 and 50 µg/ml are acceptable. If we exclude self-ligation events, the frequency of joining different DNA segments is directly related to the molarity of each component in terms of specific ends to be joined. For example, if equimolar amounts of vector (V) and insert (I) are present, 50% of the bimolecular products will be V–I, 25% will be V–V, and 25% will be I–I. However, because in most cases only molecules containing vector sequences will generate E. coli transformants, it is preferable to use higher molar concentrations of insert fragment(s) because this will favor the desired V–I products over the V–V background. This consideration becomes less important if V–V events are precluded by phosphatase treatment or if V–V events cannot generate viable molecules. Amount of DNA. In general, it is not necessary to use very much DNA. For simple hybrid constructions, roughly 1 to 10% of the vector DNA molecules are converted to the recombinant DNA of interest. With normal transformation efficiencies, 106 colonies per microgram of pure vector DNA, ligated products containing as little as 1 ng of treated vector DNA should generate 10 to 100 transformants. This is more than enough if the background is not a problem. In practical terms, if starting DNA fragments are easily visualized by conventional ethidium bromide staining, there is enough for most experiments. This is true even when performing ligation in gel slices (see alternate protocol) where only a fraction of the electrophoretically separated DNA is used. More difficult constructions such as those involving blunt ends, three fragments, or Bal 31 nuclease usually require more DNA. How-
3.16.5 Current Protocols in Molecular Biology
ever, even in these cases, there is sufficient DNA in a fraction of a gel slice such that the alternate protocol can be used. In general, 25 to 100 ng of each DNA component (in a standard 20-µl reaction) is usually enough to obtain the desired recombinants. Of course, the cloning efficiency will be increased by using higher concentrations of the components. If this proves to be necessary, the DNA can be concentrated easily after purifying it away from the agarose (UNIT 2.6). Selection, screening, and enrichment procedures. For some hybrid constructions, colonies containing the desired DNA molecule can be distinguished from background colonies by genetic methods. In such cases, techniques for background reduction become less important, which often means that time can be saved. As an extreme but relatively infrequent example, if one fragment contains the gene for ampicillin resistance and another fragment contains the gene for tetracycline resistance, the recombinant containing both fragments can be selected directly on plates containing both drugs; there is no need to purify the component DNAs prior to ligation. Cloning vectors have been designed to select or screen for recombinants. Perhaps those most commonly used are the pUC or M13mp series of vectors which contain the α-complementation region of the E. coli lacZ gene and produce blue plaques or colonies on appropriate indicator plates (UNIT 1.4). Insertion of a DNA fragment into the lacZ region generally causes a white colony or plaque, which is easily seen among a background of thousands of blue transformants. Another common vector, pBR322, contains two selectable markers, ampicillin and tetracycline resistance. Insertion into one of these marker genes renders the colony sensitive to that drug; such colonies can be screened by replica plating (UNIT 1.5). In many cases, the desired recombinant contains (or lacks) specific regions of DNA that are present in either or both of the starting molecules. Such recombinants can be identified by colony or plaque filter hybridization (UNITS 6.1 and 6.2) using an appropriate 32P-labeled probe (UNIT 3.5). These screening procedures can identify the correct recombinant from a background of hundreds or thousands of transformants. However, they take several days to perform and are unnecessary if the background is acceptably low. Finally, the proportion of correct colonies can be increased by appropriate enzymatic
treatment after the ligation reaction. For example, if the desired product lacks an EcoRI site, but undesired products contain such a site, the background can be reduced simply by EcoRI cleavage after the ligation reaction. Most restriction endonucleases are active in ligase buffer. Controls. It is critical to perform appropriate control experiments simultaneously with the reaction of interest. In this way, it is often possible to determine the relative success of the hybrid construction. Such knowledge is particularly important because preparation and analysis of prospective recombinant DNAs is the most time-consuming part of the overall process. Obviously, it is worth minimizing the number of colonies to be examined from successful experiments as well as avoiding further analysis from cloning attempts that were clearly unsuccessful. NOTE: The basic controls for any experiment are to set up and analyze parallel ligation reactions, each lacking a single DNA component. In order to keep the volume of the ligation reactions constant, the omitted DNA should be replaced by an equal volume of water. In a successful hybrid construction, the number of colonies obtained when all DNA components are present should be higher than any of the control reactions. If this is the case, many or most of the colonies should contain the molecule of interest. Two additional controls should be carried out as part of the transformation procedure. First, an aliquot of competent cells should be incubated in the absence of DNA. The appearance of transformants is indicative of contamination in the cells or transformation buffers. Second, another aliquot of competent cells should be incubated with 1 ng of a control DNA (either plasmid or phage depending on the experiment) to measure the transformation efficiency; 1000 colonies or plaques should be achieved routinely.
Anticipated Results For simple hybrid constructions, approximately 100 to 10,000 colonies or plaques should be generated, most of which should be the desired molecule. For more complex situations, the expected number of colonies should be between 1 and 1000; depending on the individual situation 5 to 80% of the molecules should contain the desired structure. For many cloning experiments, one correct colony is enough.
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Time Considerations The enzyme reactions can usually be performed in 1 to 4 hr (except in experiments involving ligation of oligonucleotide linkers). Gel electrophoresis and DNA fragment purification should take an additional 1 to 4 hr; shorter times are obtained by using minigels (UNIT 2.5) and by using the alternate protocol. Thus, in one day, it should be possible to go from uncleaved starting DNAs to the ligation reaction. On day 2, the ligated products can be introduced into E. coli, and the resulting transformants will be obtained on day 3. After colony or plaque purification, DNA from the transformants can be prepared and analyzed on day 4.
EXAMPLES Example 3.16.1: Subcloning DNA Fragments with Homologous and Cohesive Ends Consider the cloning of a 2-kb EcoRI fragment into a vector containing a single EcoRI site. The major source of background is uncleaved or self-ligated vector DNA. In most cases, self-ligation should be reduced by treating the EcoRI cleaved vector DNA with calf intestinal alkaline phosphatase (see Fig. 3.16.1 below and UNIT 3.10). Although phosphatase treatment does lower the absolute efficiency of obtaining the desired DNA molecule, the great
reduction in background is necessary to obtain a decent relative frequency of the desired molecule. The concentration of insert DNA(s) should be relatively high in order to facilitate ligation to the vector. It should also be noted that the insert DNA can contribute to background transformants by self-ligation, especially when it is difficult to purify away from “vector” fragments derived from the original plasmid. Because all EcoRI ends are equivalent, the 2-kb insert can be cloned in either of the two possible orientations with respect to the vector sequences. These two outcomes can be distinguished only by restriction mapping using an enzyme that cleaves asymmetrically within the insert DNA. In addition, it is possible to insert multiple copies of the fragment into the vector. Generally, multiple insertion is infrequent unless the DNA concentrations are very high. When multiple insertions occur, the inserts are essentially always oriented in the same direction. Molecules containing tandem copies oriented in the opposite direction are incapable of replication in E. coli unless the insert fragment is very short (less than 30 bp). For all the reasons mentioned above, cloning of fragments with homologous and cohesive ends is best avoided when possible. Directional cloning, discussed below, is preferable.
vector 5′ pAATTC 3′ G
insert 3′ G CTTAAp 5′
5′ pAATTC 3′ G
G 3′ CTTAAp 5′
phosphatase ligase AATTC G
G CTTAA ligase
GAATTC CTTAAG
AA C TT G
G CTTAA
GAATTC CTTAAG
recombinant DNA (2 orientations)
no recircularization
Subcloning of DNA Fragments
Figure 3.16.1 Treatment of cleaved vector with CIP in order to reduce background.
3.16.7 Current Protocols in Molecular Biology
Example 3.16.2: Directional Cloning Using Fragments with Heterologous Ends This is by far the most efficient method for cloning, and it should be used wherever possible. In the simplest example, consider the joining of two fragments, each produced by cleavage with enzymes A and B. Self-ligation of either fragment is theoretically eliminated, and ligation of two vector fragments produces a “perfect inverted repeat” molecule that cannot be stably maintained in E. coli. Thus, the background is very low, and if enzymes A and B produce 5′ or 3′ overhangs, the efficiency is very high. Moreover, almost all the transformants will contain recombinant molecules in which one copy of the insert DNA is oriented in a defined direction with respect to the vector. Molecules containing two copies of the insert cannot be formed, and those with three copies will be very rare. The major problems associated with directional cloning come from errors in generating the DNA segments, particularly incomplete cleavage of the vector by one of the restriction endonucleases. Vector molecules that are singly cleaved can self-ligate and generate background. Ideally, molecules cleaved by both enzymes can be purified away from those that are incompletely cleaved by gel electrophoresis. However, this is not possible when the sites for cleavage by enzymes A and B are close together. Moreover, if the two sites are extremely close together, as is the case for the polylinkers in the pUC and M13mp vectors, it can be difficult to cleave the molecules with both enzymes. The high efficiency of directional cloning makes it possible to create molecules composed of three or more segments. A typical three-piece ligation will consist of fragments generated by enzymes A + B, B + C, and A + C. The desired recombinant will occur less frequently than in the case for two-fragment ligation. However, the low background makes it possible to obtain the desired molecule at high enough frequency to permit individual analysis of colonies.
Example 3.16.3: Blunt-End Ligation Although T4 DNA ligase can join fragments with blunt ends, ligation efficiency is greatly reduced as compared to fragments with cohesive ends. Blunt-end ligation is facilitated by using higher concentrations of DNA and ∼10 times more ligase. Unlike the ligation of cohesive ends which requires compatible termini, blunt fragments are equally ligatible no matter which endonuclease is used to produce the
ends. The joining of blunt ends produced by different enzymes almost always results in a product that cannot be cleaved by either enzyme. The most difficult ligations are those in which all the relevant ends are blunt (e.g., cloning an SmaI fragment into an SmaI cleaved vector). The problems are similar to those described in Example 3.16.1 and are compounded by the inefficiency of blunt-end ligation. Cloning involving blunt ends is considerably easier if one of the ligation events involves cohesive ends. For example, two fragments generated by SmaI and EcoRI are much more easily joined than two SmaI-generated segments. In addition, such heterologous ends permit directional cloning and the background is very low (see Example 3.16.2).
Example 3.16.4: Joining DNA Fragments with Incompatible Ends Consider the case of joining an EcoRI end (overhang 5′ AATT) to an SacI end (overhang 3′ AGCT). To join such incompatible termini, it is necessary to convert them to blunt ends. For 5′ overhangs, this is accomplished by “filling in” the ends with the Klenow fragment of E. coli DNA polymerase I in the presence of all 4 dNTPs (see Fig. 3.16.2 below and UNIT 3.5). This reaction should be carried out in the presence of all 4 dNTPs in order to prevent more extensive exonuclease action. For 3′ overhangs, the 3′ to 5′ exonuclease activity of T4 DNA polymerase or Klenow enzyme is used to remove the protruding nucleotides (UNIT 3.5). The T4 enzyme is preferred because it has a much more active exonuclease. However, the Klenow enzyme will do the job, and it is considerably less expensive than T4 DNA polymerase. Both polymerases are active in all restriction endonuclease buffers and hence can be added (along with the 4 dNTPs) directly to the reaction mixture after cleavage. Once the relevant ends have been blunted, the ligation procedure proceeds as in Example 3.16.3. However, the additional step for generating the blunt ends usually results in a lowered efficiency of ligation; the more ends to be blunted, the worse the problem. The fill-in reaction for 5′ overhangs is usually more efficient than the trimming reaction for 3′ extensions. Thus, when incompatible ends must be joined, it is preferable if some of the required ligation reactions involve joining of cohesive ends; e.g., the joining of a BamHI–EcoRI (filled) fragment to a BamHI–SacI (trimmed) fragment.
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SacI end
EcoRI end 5′ 3′
Klenow fragment DNA polymerase I
5′ C 3′ TCGAG
3′ G CTTAA 5′
3′ 5′
T4 DNA
4 dNTPs
polymerase
4 dNTPs
C G
GAATT CTTAA ligase
GAATT C CTTAA G regenerated EcoRI site
Figure 3.16.2 Joining DNA fragments with incompatible ends.
In many cases, the joining of incompatible ends generates a product that cannot be cleaved by either of the restriction enzymes used to produce the ends. In such cases, the structure of the molecule at the fusion point can be determined only by DNA sequencing. Sometimes, however, one of enzyme sites should be regenerated, thus making it possible to analyze the prospective molecules by restriction mapping. For example, a filled in EcoRI site has the terminal sequence GAATT. If this is joined to a fragment whose 5′ terminal nucleotide is C, the EcoRI site will be restored. In principle, a SacI site (GAGCTC) that has been trimmed will regenerate an EcoRI site that has been filled in. Products where the EcoRI and SacI ends have been joined aberrantly will not result in the regeneration of the EcoRI site.
Example 3.16.5: Ligation of Fragments Generated by Partial Cleavage
Subcloning of DNA Fragments
Sometimes a given DNA fragment will contain internal restriction sites that are the same as those on one (or both) of the ends. In this case, the fragment can be obtained only by partial restriction endonuclease digestion. Contrary to popular opinion, it is not difficult to obtain hybrid DNA molecules when one or more of the components is generated by partial cleavage by a restriction endonuclease(s). By gel purifying the desired partial cleavage product(s), the ligation reactions can be set up in the normal way, and they should be equally efficient. The main difficulty with partial cleavage,
obtaining enough of the specific fragment, can usually be overcome by starting with more DNA (1 to 5 µg should be enough) prior to gel electrophoresis. To ensure the proper degree of partial digestion, the method of serial enzyme dilution should be performed (UNIT 3.1). In some cases, it will be difficult to purify the desired partial cleavage product from undesired products. This is especially true if the starting molecule contains many cleavage sites or if cleavage sites are close together. However, if the ligation reaction has a low background, such impurities are only a minor problem. For example, if the “band” that is cut from the gel contains four different clonable fragments, roughly 25% of the transformants should be the ones of interest. Moreover, if the fragment is generated by partial cleavage at one end and complete cleavage by another enzyme at the other end, many of the contaminating fragments cannot be cloned into the vector and hence will not contribute to the background.
Example 3.16.6: Ligations Involving Oligonucleotide Linkers Synthetic linkers are self-complementary oligonucleotides, typically 8 to 12 bases in length, that anneal to form blunt-ended, double-stranded DNA which contains a site for restriction endonuclease cleavage. For example, GGAATTCC is an 8-bp EcoRI linker (the underlined nucleotides represent the recognition site). Oligonucleotide linkers facilitate
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restriction enzyme). If possible, the ligation products should be cleaved with a second restriction enzyme to produce a fragment with heterologous ends suitable for cloning (see Example 3.16.2). Third, the fragment is purified by gel electrophoresis and then ligated by conventional methods. The gel purification step also removes unligated linkers, which would interfere with this second ligation step. Linkers can also be removed by other methods (chromatography on Sepharose CL-4B or Sephacryl S-300; see UNITS 2.6 and 5.6), but electrophoresis is more efficient and allows for purification of the desired fragment away from undesired fragments.
the cloning of blunt-ended DNA fragments, and they are valuable for “introducing” new restriction sites at desired positions. A wide variety of linkers, each containing a specific recognition sequence, is available from commercial suppliers. Linkers are usually obtained in the nonphosphorylated form. For most applications, they are phosphorylated using ATP and T4 polynucleotide kinase (UNIT 3.10). After the kinase reaction, the phosphorylated linkers can be added directly to the ligation reaction. Ligations involving linkers are performed in three stages. First, the DNA of interest, which must contain blunt ends, is joined to the linker, as illustrated in Figure 3.16.3. This reaction is performed with 0.1 to 1 µg of linker, which represents a 100- to 1000-fold molar excess of linker over fragment. The high concentration of ends makes this blunt-end ligation reaction more efficient than normal. Generally, the linker is phosphorylated prior to ligation. In this case, the ligation products will typically contain several linkers on each end of the fragment. Some experiments involve ligation of nonphosphorylated linkers, in which case the products will contain only one linker on each end. Second, after heat treatment to inactivate the ligase, the products of this reaction are cleaved with the restriction enzyme that recognizes the linker, as illustrated in Figure 3.16.3. Because of the high concentration of restriction sites due to the linker, this step usually requires incubation for several hours with a large amount of restriction enzyme (20 to 50 U). It is important that the fragment does not contain an internal site that is recognized by the restriction enzyme (unless the fragment has been methylated previously by the methylase corresponding to the
Example 3.16.7: Cloning Synthetic Oligonucleotides Double-stranded oligonucleotides suitable for cloning can be obtained by annealing two single-stranded oligonucleotides that have been synthesized chemically. Generally, such oligonucleotides are phosphorylated with T4 kinase (UNIT 3.10) prior to annealing. Doublestranded and phosphorylated oligonucleotides can also be generated by “mutually primed synthesis” of appropriate single-stranded oligonucleotides (UNIT 8.2). Oligonucleotides obtained by either method can be cloned by standard ligation procedures using cohesive or blunt ends. The main problem associated with cloning oligonucleotides is that of multiple insertion. This is because small amounts of oligonucleotide represent a large molar excess. This problem is best dealt with by making a series of 10-fold serial dilutions of oligonucleotide and setting up parallel ligation reactions using
blunt-ended DNA
EcoRI linker pGGAATTCC CCTTAAGGp
p p ligase
pGGAATTCCGGAATTCCGGAATTCC CCTTAAGGCCTTAAGGCCTTAAGG
GGAATTCCGGAATTCCGGAATTCC CCTTAAGGCCTTAAGGCCTTAAGGp
EcoRI AATTCC GG
GG CCTTAA
Figure 3.16.3 Joining blunt-ended DNA to EcoRI linker.
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these various concentrations. If single insertions are desired, transformants should be picked off plates that represent the lowest amount of oligonucleotide needed to increase the number of colonies significantly above the background (i.e., the absence of oligonucleotide). In general, optimal conditions for single insertion occur when the oligonucleotide is present in a 5- to 20-fold molar excess over the other fragments in the ligation mixture.
Literature Cited Dugaiczyk, A., Boyer, H.W., and Goodman, H.M. 1975. Ligation of EcoRI endonuclease-generated DNA fragments into linear and circular structures. J. Mol. Biol. 96:174-184. Struhl, K. 1985. A rapid method for creating recombinant DNA molecules. Biotechniques 3:452453.
Contributed by Kevin Struhl Harvard Medical School Boston, Massachusetts
Subcloning of DNA Fragments
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Current Protocols in Molecular Biology
Constructing Recombinant DNA Molecules by the Polymerase Chain Reaction
UNIT 3.17
Any two segments of DNA can be ligated together into a new recombinant molecule using the polymerase chain reaction (PCR). The DNA can be joined in any configuration, with any desired junction-point reading frame or restriction site, by incorporating extra nonhomologous nucleotides within the PCR primers. Cloning by PCR is often more rapid and versatile than cloning with standard techniques that rely on the availability of naturally occurring restriction sites and require microgram quantities of DNA. It is not necessary to know the nucleotide sequence of the DNA being subcloned by this technique, other than the two short flanking regions (∼20 bp) that serve as anchors for the two oligonucleotide primers used in the amplification process. Moreover, PCR can be performed on low-abundance or even degraded DNA (or RNA) sources. This unit describes using PCR to construct hybrid DNA molecules. The main objective is to give an overview of how PCR can be exploited to accomplish numerous cloning strategies; it is assumed that the reader is already familiar with basic molecular biology techniques including PCR amplification (UNIT 15.1) and subcloning (UNIT 3.16). The basic protocol outlines the PCR amplification and cloning strategies. A troubleshooting guide for problems most frequently encountered in PCR cloning, and three specific examples of this technique—for creating (1) in-frame fusion proteins, (2) recombinant DNA products, and (3) deletions and inversions by inverse PCR—are presented in the Commentary. SUBCLONING DNA FRAGMENTS In this protocol, synthetic oligonucleotides incorporating new unique restriction sites are used to amplify a region of DNA to be subcloned into a vector containing compatible restriction sites. The amplified DNA fragment is purified, subjected to enzymatic digestion at the new restriction sites, and then ligated into the vector. Individual subclones are analyzed by restriction endonuclease digestion and either sequenced or tested in a functional assay. The procedure is summarized in Figure 3.17.1.
BASIC PROTOCOL
Materials Template DNA (1 to 10 ng of plasmid or phage DNA; 20 to 300 ng of genomic or cDNA) Oligonucleotide primers (0.6 to 1.0 mM; UNIT 8.5) Mineral oil TE-buffered phenol (UNIT 2.1) and chloroform 100% ethanol TE buffer, pH 8.0 (APPENDIX 2) Klenow fragment of E. coli DNA polymerase I (UNIT 3.5) Vector DNA Calf intestinal phosphatase (UNIT 3.10) Additional reagents and equipment for phosphorylating synthetic oligonucleotides (UNIT 3.10), enzymatic amplification of DNA by PCR (UNIT 15.1), agarose and polyacrylamide gel electrophoresis (UNITS 2.5 & 2.7), DNA extraction and precipitation (UNIT 2.1), purification of DNA by glass beads, electroelution from agarose gels, or from low-gelling/melting temperature agarose gels (UNIT 2.6), restriction endonuclease digestion (UNIT 3.1), ligation of DNA fragments (UNIT 3.16), transformation of E. coli (UNIT 1.8), plasmid DNA minipreps (UNIT 1.6), and DNA sequence analysis (UNIT 7.4) Contributed by Elaine A. Elion Current Protocols in Molecular Biology (1993) 3.17.1-3.17.10 Copyright © 2000 by John Wiley & Sons, Inc.
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Amplify the target DNA 1. Prepare the template DNA. If using an impure DNA preparation (i.e., not purified by CsCl gradients), heat sample 10 min at 100°C to inactivate nucleases. Plasmid, phage, genomic, or cDNA, obtained from either rapid preparations or purified on CsCl gradients, can be used as the source of target DNA.
2. Prepare the oligonucleotide primers. If the PCR product is to be cloned by blunt-end ligation, phosphorylate the 5′ hydroxyl of the oligonucleotide primers. A 5′ phosphate on the ends of the PCR products will be needed to form the phosphoester linkage to the 3′OH of the vector during ligation. This step is essential if the vector has been treated with a phosphatase. Because the purity of the oligonucleotides does not seem to affect the PCR reaction, primer purification (as detailed in UNIT 8.5) may not be necessary.
3. Set up a standard amplification reaction and overlay with mineral oil as described in UNIT 15.1. Carry out PCR in an automated thermal cycler for 20 to 25 cycles under the following conditions: denature 60 sec at 94°C, hybridize 1 min at 50°C, and extend 3 min at 72°C. Extend an additional 10 min at 72°C in the last cycle to make products as complete as possible. Include negative controls of no template DNA and each oligonucleotide alone, as well as several different oligonucleotide:template ratios. For a discussion of optimization of amplification conditions see UNIT 15.1. A thermostable DNA polymerase with 3′→5′ exonuclease proofreading activity can be used instead of Taq DNA polymerase to reduce the amount of nucleotide misincorporation during amplification. Pfu DNA polymerase (Stratagene) and Vent DNA polymerase (New England Biolabs) have this activity (follow manufacturers’ instructions).
Recover the amplified fragment 4. Analyze an aliquot (e.g., 4 to 8 µl) of each reaction mix by agarose or polyacrylamide gel electrophoresis to verify that the amplification has yielded the expected product. 5. Recover amplified DNA from PCR reaction mix. Remove mineral oil overlay from each sample, then extract sample once with buffered chloroform to remove residual mineral oil. Extract once with buffered phenol and then precipitate DNA with 100% ethanol. Carrier tRNA may be added during precipitation if desired.
6. Microcentrifuge DNA 10 min at high speed, 4°C. Dissolve pellet in 20 µl TE buffer. Purify desired PCR product from unincorporated nucleotides, oligonucleotide primers, unwanted PCR products, and template DNA using glass beads, electroelution, or phenol extraction of low gelling/melting temperature agarose. Unused oligonucleotide primers can inhibit the ability of the restriction enzymes to digest the amplified PCR product. Amplified DNA that is greater than 100,000 Da (>150 bp) can be rapidly separated from the primers using a Centricon 100 microconcentrator unit from Amicon (follow manufacturers’ instructions). If the gel analysis in step 4 shows that amplification yielded only the desired PCR product, this microconcentrated DNA can be used directly for cloning.
Constructing Recombinant DNA Molecules by PCR
Prepare amplified fragment and vector for ligation 7a. If the PCR fragment is to be cloned by blunt-end ligation, repair the 3′ ends with DNA polymerase I (Klenow fragment). This step is necessary because Taq DNA polymerase adds a nontemplated nucleotide (usually dA) to the 3′ ends of PCR fragments.
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7b. If primers contain unique restriction sites, digest half the amplified DNA in 20 µl with the appropriate restriction enzyme(s). Use an excess of enzyme, and digest for several hours. Reserve the undigested half for future use, if necessary.
8. Prepare the recipient vector for cloning by digesting 0.2 to 2 µg in 20 µl with compatible restriction enzymes. If necessary, treat vector DNA with calf intestinal phosphatase (UNIT 3.1) to prevent recircularization during ligation. 9. Separate the linearized vector from uncut vector by agarose or low-gelling/melting temperature gel electrophoresis. Recover linearized vector from the gel by adsorption to glass beads, electroelution, or phenol extraction of low-gelling/melting temperature agarose. Ligate amplified fragment and vector 10. Ligate the PCR fragment into the digested vector following the procedure outlined in UNIT 3.16. 11. Transform an aliquot of each ligation into E. coli. Prepare plasmid miniprep DNA from a subset of transformants. Analyze recombinant plasmids 12. Digest the plasmid DNA of the selected transformants with the appropriate restriction endonuclease. Analyze the digestions by agarose gel electrophoresis to confirm fragment incorporation. 13. Sequence the amplified fragment portion of the plasmid DNA to check for mutations. Alternatively, screen the subset of transformants using a biochemical or genetic functional assay if available. This analysis is critical because the Taq DNA polymerase can introduce mutations into the amplified fragment.
COMMENTARY Background Information The main benefit of cloning by PCR is that unique restriction sites can be introduced on either side of any segment region of amplified DNA to allow its ligation into a recipient vector (Mullis and Faloona, 1987; Chapter 15) in any configuration. The incorporation of additional nucleotides at the 5′ ends of the oligonucleotide primers permits the creation of novel restriction sites or changes in reading frame and coding sequence. The oligonucleotide primers can also be designed to contain mismatches, deletions, or insertions in the region of homology (UNIT 8.5). However, it is not necessary to always incorporate a new restriction site in the primer. Amplified PCR fragments can also be subcloned by blunt-end or sticky-end ligation using preexisting restriction sites within the amplified DNA. For example, PCR might be used to amplify a target DNA that already contains appropriate restriction sites, but is available in limited quantities. Finally, sequential polym-
erase chain reactions can be used to generate more complex recombinant PCR products which can subsequently be subcloned into a recipient vector. The most obvious disadvantage of PCR cloning is the need to verify that the subcloned PCR product does not contain mutations generated during the polymerase chain reaction. In cases where longer DNA segments (i.e., >1 kb) are being amplified, it may be more advantageous to use a polymerase which has a 3′→5′ exonuclease activity (e.g., Pfu polymerase, Stratagene) to reduce the chances of generating mutations. After subcloning, several independent PCR products should be analyzed by DNA sequencing to be sure that the recombinant DNA molecule is not mutated. Sequencing can be laborious when a large fragment of DNA is subcloned. However, subcloned PCR products can be prescreened by either biological or biochemical functional assays if they are available. In some instances, it may be more desir-
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able to break down the cloning into several steps that might involve the introduction of a needed restriction site within a short piece of DNA first.
Critical Parameters
Constructing Recombinant DNA Molecules by PCR
In general, the DNA preparation, purification, and ligation guidelines outlined in UNIT 3.16 should be applied to PCR cloning to ensure recovery of the desired ligation products. However, the following points deserve special consideration. Design of oligonucleotide primers. Primers should only hybridize to the sequence of interest. This can be predicted in instances where sequence information is available. In general, primers with homology of 16 to 20 nucleotides to the target DNA and a GC content of ∼50% should be chosen. A longer oligonucleotide of ∼25 nucleotides should be used for AT-rich regions. In instances where genomic DNA is the source of the target DNA, the oligonucleotide primers should contain at least 20 nucleotides of homology to the target DNA to ensure that they anneal specifically (Arnheim and Erlich, 1992). When using primers to introduce a specific restriction site, a sequence within the target DNA should be selected that requires the addition of the fewest noncomplementary nucleotides to create the new site, if possible. Special consideration should be given to the choice of site itself, as restriction endonucleases vary in their ability to cleave recognition sequences within ten nucleotides of the end of a DNA duplex (consult Table 8.5.1 for the efficacies of different restriction enzymes in cleaving terminal recognition sequences). It is also recommended that four to five additional nucleotides be added on the 5′ side of the restriction site in the primer. Because DNA duplexes “breathe” at termini, potentially interfering with the ability of a restriction enzyme to cleave (Innis et al., 1990), it is useful to use the GCGC “clamp” sequence that is most thermostable (Sheffield et al., 1989). Finally, the sequence of the primer should be checked for internal complementarity to avoid secondary structure formation that will interfere with hybridization of the primer to the target DNA. The 3′ ends of the two primers being used must not be complementary, so that the formation of primer-dimers that will compete with the synthesis of the desired PCR product will be avoided. Additional details on primer design are discussed in UNIT 15.1.
DNA polymerase. Commercially available Taq DNA polymerase (Perkin-Elmer Cetus) lacks the 3′→5′ proofreading exonuclease activity used by DNA polymerase I Klenow fragment and T4 DNA polymerase to reduce error frequency (Kornberg, 1992). This absence of proofreading activity in Taq DNA polymerase is thought to result in a heightened error frequency. Old estimates indicate that the average rate of misincorporation is 8.5 × 10−6 nucleotides per cycle (Goodenow et al., 1989; Fucharoen et al., 1989). Two other thermostable DNA polymerases possessing proofreading 3′→5′ exonuclease activity have recently become commercially available: Pfu DNA polymerase, purified from Pyrococcus furiosus (Stratagene) and Vent DNA polymerase, purified from Thermococcus litoralis (New England Biolabs and Promega). Both are more thermostable than Taq DNA polymerase. Pfu DNA polymerase is 12-fold more accurate than Taq DNA polymerase, as assayed by the method of Kohler et al. (1991). Vent DNA polymerase is 4-fold more accurate than Taq DNA polymerase (Cariello et al., 1991). Although it is difficult to compare the relative error frequencies of three enzymes because they were assayed by different methods, the use of either Vent or Pfu DNA polymerases may reduce the amount of misincorporation. Removal of unincorporated nucleotide triphosphates. It is recommended that the amplified PCR fragment be purified from unincorporated nucleotides and primers. Any method of purification that involves electrophoresis can also separate the desired PCR product from any undesired DNA species produced during amplification. Typical methods of DNA purification include electrophoresis through lowgelling/melting temperature agarose or electrophoresis through agarose followed by DNA purification by electroelution or adsorption to glass beads (UNIT 2.6). However, amplified DNA can be more rapidly purified from unincorporated nucleotide triphosphates and primers using a Centricon microconcentration unit (Amicon). The disadvantage of using the microconcentrator is that undesirable PCR products and the starting template DNA will copurify with the amplified PCR fragment.
Troubleshooting PCR amplification. The use of appropriately designed primers should allow the amplification of the DNA segment of interest. Occasionally, however, primers may not be specific, leading to the amplification of undesired DNA
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A
homologous to target DNA
primer 1: 5 ′ GCGCGAAT TCAATGNNNNNNNNNNN
EcoRI
homologous to target DNA
primer 1: 5 ′ GCGCGGATCC NNNNNNNNNNNNNNN
Bam HI
B target DNA primer 1
E
vector DNA primer 2
B
amplify
E ••••••••••••••••••••• ••••••••••••••••••••• B
E
B
open reading frame digest with Eco RI and Bam HI
E
digest with Eco RI and Bam HI B
••••••••••••••• •••••••••••••••
B purify digested fragment and vector; ligate and transform E. coli
E
original DNA
chimeric open reading frame
• • • • • • PCR DNA vector DNA
E
B
restriction site overhang
Figure 3.17.1 Introducing unique restriction sites and creating an in-frame fusion protein by PCR. Abbreviations: E, EcoRI; B, BamHI. For a full description, see Example 3.17.1 in Commentary.
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Constructing Recombinant DNA Molecules by PCR
segments. The specificity of primer to template hybridization will depend upon temperature and salt (see UNITS 15.1 & 6.4 for a thorough discussion). The highest annealing temperature possible should be used to reduce nonspecific associations. Some nonspecific amplifications can be avoided by employing a “hot-start” technique—i.e., adding the DNA polymerase to a prewarmed sample (D’Aquila et al., 1991). In addition, purifying the PCR product by gel electrophoresis will help ensure that the proper DNA fragment is subcloned. If the synthesized primer does not bind with specificity, it may be simplest to have another one synthesized. In setting up the amplification cycle, keep in mind that for cloning, fidelity is more important than yield, so it is better to keep a low cycle number and not to raise the MgCl2 concentration too much. 1.5 mM MgCl2 in the amplification buffer should be sufficient for most primers. Cloning of the amplified fragment. “Stickyend” ligation of the amplified DNA can sometimes be difficult, due to poor cutting of the terminal restriction site by the desired restriction endonuclease. Careful choice of restriction sites and the addition of extra nucleotides to the 5′ end of the primer (see critical parameters) will facilitate digestion. Other common explanations for poor cutting by restriction endonucleases include: (1) Blockage of the duplex terminus by bound Taq DNA polymerase. If this is the case, the amplified DNA can be treated with proteinase K to remove associated protein. This is done by adding 50 µg/ml proteinase K in 10 mM Tris⋅Cl (pH 7.8)/5 mM EDTA/0.5% (v/v) SDS to the sample and incubating 30 min at 37°C. The sample must be extracted with phenol/chloroform (UNIT 2.1) to remove the proteinase K. (2) Inefficient extension by Taq DNA polymerase. The nonduplex ends generated in this fashion can be repaired by filling in with Klenow fragment (UNIT 3.5). (3) An insufficient number of extra nucleotides 5′ to the restriction site in the primer. In this case, the primer should be resynthesized. Prolonged (typically overnight) DNA digestion should also be tried. (4) 5′ terminal breathing of duplex DNA. Duplex formation can be stabilized by the inclusion of 0.1 mM spermidine in the digestion reaction. An alternative approach is to internalize the restriction site by concatemerizing the PCR products prior to restriction endonuclease di-
gestion (Jung et al., 1990). To do this, 5′ phosphorylated oligonucleotide primers are used in the PCR amplification, or following amplification, the DNA fragment is phosphorylated using T4 polynucleotide kinase (UNIT 3.10). The amplified fragments are then concatemerized by ligation using T4 DNA ligase (UNIT 3.16) prior to restriction endonuclease treatment. Aliquots of the amplified DNA before and after concatemerization, and after digestion, can be compared on an agarose gel to confirm that the procedure worked. Blunt-end ligations are often inhibited by nonflushed ends in the PCR fragment, due to the presence of a nontemplate-directed nucleotide (usually dATP) added by Taq DNA polymerase (Clark, 1988). Treatment of the PCR fragment with Klenow polymerase in the presence of dNTPs (UNIT 3.5) to make the ends flush should circumvent this problem. In the event that this simple approach does not work, the PCR fragment can be subcloned into a MstII (or Bsu36 I) site (CC↓TNAGG) which leaves a 5′ dT overhang. This approach requires a recipient vector with a unique MstII site.
Anticipated Results All of the cloning approaches outlined are reliable and should result in efficient recovery of the desired recombinant molecules.
Time Considerations Once the oligonucleotides have been synthesized, the PCR amplification, purification, ligation, and transformation steps can all be done within 2 days. The appropriate subclones can then be sequenced or tested in a functional assay immediately thereafter.
EXAMPLES Example 3.17.1: Creating In–Frame Fusion Proteins by PCR PCR cloning is particularly useful for creating in-frame fusions between two open reading frames, as is often done for synthesizing fusion proteins with E. coli expression vectors (Chapter 16). The essence of this type of subcloning involves incorporating additional noncomplementary nucleotides within the oligonucleotide primer that will encode the junction sequences of the amplified PCR fragment. Consider the introduction of a unique EcoRI site into a piece of target DNA that is to be fused with an open reading frame in the recipient vector, as depicted in Figure 3.17.1. For this experiment, the primers should be
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designed as indicated in panel A. Each primer is designed to contain a unique restriction site not present within the target DNA. The primer carrying the EcoRI site contains an additional nucleotide (shown in bold) to allow the ATG of the amplified target DNA to be in-frame with the open reading frame in the vector (bold bracket). The second primer contains a unique BamHI site. Both oligonucleotide primers are designed to be homologous to and anneal with ∼20 nucleotides of DNA flanking the target DNA. They are oriented such that their 3′ hydroxyl ends point toward the target DNA. These unique restriction sites are 5′ to the region of the primer that is homologous to the target DNA. Each new restriction site is separated from the 5′ end of the oligonucleotide by four additional nucleotides to facilitate enzymatic digestion of the amplified DNA. Shown in this example is a GC clamp (Myers et al., 1985) which favors duplex formation at the ends of the amplified fragment. Panel B depicts the sequence of events in this experiment. Following primer annealing and PCR amplification, the amplified DNA is first digested with restriction endonucleases that cleave at the new restriction sites, then purified by gel electrophoresis. The recipient vector DNA is digested with either the same or compatible restriction endonucleases and is purified before being ligated to the recipient vector. In this example, one oligonucleotide contains additional bases to create an in-frame fusion with the plasmid-borne open reading frame. More elaborate primers can be designed to include additional restriction sites in different reading frames to allow subcloning of the same PCR fragment into multiple recipient fusion vectors that may have nonidentical cloning sites. This is efficient both in terms of labor and the cost of having to synthesize a new oligonucleotide primer. Note that an ATG can also be incorporated into the 5′ oligonucleotide primer to create an open reading frame with a new translational start (e.g., in the construction of a promoter-exon fusion).
Example 3.17.2: Creating a Recombinant DNA Molecule by Sequential PCR Amplifications Consider creating a chimeric DNA molecule by sequential polymerase chain reactions rather than by ligation. This technique is useful for complex cloning schemes that involve fusing together more than two pieces of DNA, as depicted in the creation of the gene fusion
shown in Figure 3.17.2. In this example, two PCR products are made from noncontiguous regions of DNA (that are also nonhomologous) in separate reactions. Two of the first-round amplification primers are designed to contain 5′ extensions that are homologous to a portion of the other target gene (see primers 1b and 1c). In this example, the primers for target gene 1 are labeled as 1a and 1b. Primer 1a contains a unique EcoRI site; primer 1b contains a 5′ extension that is homologous to a region in target gene 2 that will be amplified (thin line of arrow). Primers 1c and 1d are for amplifying target gene 2: primer 1c contains a 5′ extension that is homologous to a portion of target gene 1 that will be amplified (bold line of arrow), and primer 1d contains a unique BamHI restriction site. Because primers 1b and 1c contain complementary 5′ extensions, two PCR products containing a region of overlapping homology are generated. The two PCR fragments are purified away from the primers, then mixed together and annealed by denaturation and renaturation. Four DNA species are generated in this reaction: two heteroduplexes associating at the region of overlapping homology and two parental homoduplexes. The recessed 3′ ends of the heteroduplexes are extended by Taq DNA polymerase to produce a single fragment that is equal in length to the sum of the two overlapping fragments. In a second round of amplification, the combined heteroduplex DNA species is amplified by adding the outside set of primers (1b and 1d) to the PCR assay. These primers will now have complete homology to the amplified heteroduplex DNA species. (Note that the parental homoduplexes will not be amplified because only one of each outside primers will anneal to each parental homoduplex.) The complementary primers used in the first polymerase chain reaction step can be designed to either insert a restriction site at the junction between the joined PCR products, or alter a reading frame.
Example 3.17.3: Inserting a Restriction Site by Inverse PCR Consider deleting a segment of DNA from a plasmid by inserting a unique restriction site by inverse PCR as depicted in Figure 3.17.3. Divergent primers containing the same novel restriction site (R) are annealed to two portions of the plasmid. (Note that R can be any restriction site not found in the plasmid.) The primers are oriented with their 3′ ends facing away from
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target gene 2
target gene 1
E
gene 1 homology
primer 1a
primer 1c
gene 2 homology
primer 1b
amplify each target DNA in separate reactions
E • • • • • • • • • • • • • • • ••• • • • • • • • • • • • • • • • •••
B
primer 1d • • ••••••••••••••••••••••••••• • • ••••••••••••••••••••••••••• B
remove primers mix amplified DNA together denature and anneal
E • • • • • • • • • • • • • • • • • • ••• B • • • ••••••••••••••••••••••••••••••• B • • • ••••••••••••••••••••••••••••••• • • • • • • • • • • • • • • • • • • ••• E E
not shown in rest of reactions
B extend with Taq DNA polymerase
E B • • • • • • • • • • • • • • • • • • ••••••••••••••••••••••••••••••• • • • • • • • • • • • • • • • • • • ••••••••••••••••••••••••••••••• E B • • • • • • • • • • • • • • • • • • ••••••••••••••••••••••••••••••• • • • • • • • • • • • • • • • • • • ••••••••••••••••••••••••••••••• anneal outside primers (1a,1d)
1a
E ••••••••••••••••••• •••••••••••••••••••
B ••••••••••••••••••••••••••••••• •••••••••••••••••••••••••••••••
1d
amplify hybrid PCR products
E
gene 1
gene 2
B
original target gene 1 original target gene 2 • • • • • • • • • • • PCR DNA, target gene 1 • • • • • • • • • • • • • • • • • • • • • PCR DNA, target gene 2 PCR DNA, hybrid gene
Constructing Recombinant DNA Molecules by PCR
Figure 3.17.2 Creating a recombinant DNA molecule by sequential PCR amplifications. Primer 1b has a region of homology to target gene 2 (open box); primer 1c has a region of homology to target gene 1 (closed box). Abbreviations: E, EcoRI; B, BamHI. For a complete description, see Example 3.17.2 in Commentary.
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5′
R
deletion/ fusion region
R
5′
amplify target DNA
R
R
remove primers; digest DNA with R; circularize by ligation
R
Figure 3.17.3 Inserting a restriction site by inverse PCR. Abbreviation: R, restriction enzyme not found in target plasmid. For a complete description, see Example 3.17.3 in Commentary.
each other so that sequences flanking the region to be deleted will be amplified. The full-length PCR product is purified and digested at the new restriction site and the ends are ligated in a unimolecular reaction. The amount of spacing between the two primers will determine whether the final product contains all of the original sequence or a deletion. Insertions can also be made at specific sites by including 5′ extensions in the primers. Mismatches within the body of the primer can also be used to introduce mutations. This method is useful for rapid introduction of desired restriction sites, and is limited only by the size of the plasmid and the ability of
DNA polymerase to synthesize complete products. This methodology allows the amplification of DNA flanking a region of known sequence. It is also useful for cloning DNA that has not yet been sequenced and for making hybridization probes (Ochman et al., 1990).
Literature Cited Arnheim, N. and Erlich, H. 1992. Polymerase chain reaction strategy. Annu. Rev. Biochem. 61:131156. Cariello, N.F., Swenberg, J.A., and Skopek, T.R. 1991. Fidelity of Thermococcus litoralis DNA polymerase (Vent) in PCR determined by denaturing gradient gel electrophoresis. Nucl. Acids Res. 19:4193-4198.
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Clark, J.M. 1988. Novel nontemplated nucleotide addition reactions catalyzed by procaryotic and eucaryotic DNA polymerases. Nucl. Acids Res. 16:9677-9689. D’Aquila, R.T., Bechtel, L.J., Videler, J.A., Eron, J.J., Gorczyca, P., and Kaplan, J.C. 1991. Maximizing sensitivity and specificity of PCR by pre-amplification heating. Nucl. Acids Res. 19:3749. Fucharoen, S., Fucharoen, G., Fucharoen, P., and Fukamaki, Y. 1989. A novel ochre mutation in the beta-thalassemia gene of a Thai identified by direct cloning of the entire beta-globin gene amplified using polymerase chain reactions. J. Biol. Chem. 264:7780-7783. Goodenow, M., Huet, T., Saurin, W., Kwok, S., Sninsky, J., and Wain-Hobson, S. 1989. HIV-1 isolates are rapidly evolving quasi-species: Evidence for viral mixtures and preferred nucleotide substitutions. J. Acquired Immunol. Defic. Syndr. 2:344-352. Innis, M.A., Gelfand, D.H., Sninsky, J.J., and White, T.J. (eds.) 1990. PCR Protocols. Academic Press, San Diego. Jung, V., Pestka, S.B., and Pestka, S. 1990. Efficient cloning of PCR-generated DNA containing terminal restriction endonuclease sites. Nucl. Acids Res. 18:6156.
Kornberg, A. and Baker, T.A. 1992. DNA Replication, 2nd ed. W.H. Freeman, New York. Mullis, K.B. and Faloona, F.A. 1987. Specific synthesis of DNA in vitro via a polymerase-catalyzed chain reaction. Methods Enzymol. 155:335-350. Ochman, H., Medhora, M.M., Garza, D., and Hartl, D.L. 1990. Amplification of flanking sequences by inverse PCR. In PCR Protocols. (M.A. Innis, D.H. Gelfand, J.J Sninsky, and T.J. White, eds.) pp. 219-227. Academic Press, San Diego, Calif. Sheffield, V.C., Cox, D.R., Lerman, L.S., and Myers, R.M. 1989. Attachment of a 40 base pair G+Crich sequence (GC clamp) to genomic DNA fragments by the polymerase chain reaction results in improved detection of single-base changes. Proc. Natl. Acad. Sci. U.S.A. 86:232-236.
Key Reference Innis et al., 1990. See above. Provides an in-depth analysis of PCR methods and techniques.
Contributed by Elaine A. Elion Harvard Medical School Boston, Massachusetts
Constructing Recombinant DNA Molecules by PCR
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SPECIALIZED APPLICATIONS
SECTION V
Labeling and Colorimetric Detection of Nonisotopic Probes
UNIT 3.18
Although a number of different nonisotopic labels have been described in the literature, biotin and digoxigenin are used most frequently and are commercially available. Either label can be easily incorporated into DNA probes and be detected colorimetrically; a number of fluorochromes, as well as alkaline phosphatase and horseradish peroxidase (which produce colored precipitates) are available directly conjugated to anti-digoxigenin antibodies and to avidin. Chemiluminescent detection methods (UNIT 3.19) and indirect immunofluorescent techniques (UNIT 14.6) also provide sensitive alternatives for many molecular biology applications. Because one of the advantages of nonisotopically labeled probes is their long shelf life (≥2 years), many micrograms of DNA can be labeled in one reaction to provide probes of constant quality for multiple experiments. The two basic protocols describe incorporating biotinylated nucleotides into DNA probes by nick translation and random-primed synthesis. The support protocol describes colorimetric detection of the probes, which also serves to check the extent of nucleotide incorporation. An alternate protocol describes adaptations of the basic protocols for incorporation of digoxigenin-modified nucleotides. PREPARATION OF BIOTINYLATED PROBES BY NICK TRANSLATION Nick translation is frequently used to label DNA probes for nonisotopic detection procedures. The DNA to be labeled can be an isolated fragment or—as is often the case for in situ hybridization probes—the intact phage, cosmid, or plasmid clone. The biotinavidin system is the most widely used nonisotopic labeling method and the biotinylated deoxynucleotides and detection reagents required are commercially available from several sources.
BASIC PROTOCOL
The protocol is quite similar to the standard nick translation procedure (UNIT 3.5) except for some modifications—the biotin-11-dUTP is substituted for dTTP in a standard nick translation reaction mixture and the DNase I concentration is adjusted to ensure a size range of 100 to 500 nucleotides. A minimum of 2 hr incubation time is necessary for optimal incorporation of the modified deoxynucleotide. The reaction provides sufficient probe for 15 to 50 in situ hybridizations or 2 to 5 Southern blots. Materials E. coli DNA polymerase I (UNIT 3.5) and 10× buffer (UNIT 3.4) 0.5 mM 3dNTP mix (minus dTTP; UNIT 3.4) 0.5 mM biotin-11-dUTP stock 100 mM 2-mercaptoethanol (2-ME) Test DNA 1 mg/ml DNase I stock (UNIT 3.12) prepared in 0.15 M NaCl/50% glycerol DNA molecular weight markers (UNIT 2.5A) 0.5 M EDTA, pH 8.0 (APPENDIX 2) 10% (w/v) SDS 100% ethanol SDS column buffer 1-ml syringe Additional reagents and equipment for agarose gel electrophoresis (UNIT 2.5) and removal of unincorporated nucleotides by Sephadex G-50 spin columns (UNIT 3.4) Contributed by Ann Boyle and Heather Perry-O’Keefe Current Protocols in Molecular Biology (1992) 3.18.1-3.18.9 Copyright © 2000 by John Wiley & Sons, Inc.
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1. Prepare a 100-µl reaction mix as follows: 10 µl 10× E. coli DNA polymerase I buffer 10 µl 0.5 mM 3dNTP mix 10 µl 0.5 mM biotin-11-dUTP stock 10 µl 100 mM 2-ME 2 µg DNA 20 U E. coli DNA polymerase I DNase I stock diluted 1:1000 in cold H2O immediately before use H2O to 100 µl. Incubate reaction 2 to 2.5 hr at 15°C. Other biotinylated nucleotides such as biotin-14-dATP and biotin-16-dUTP can be used. Adjust the mixture of unsubstituted deoxynucleotides accordingly. The quantity of DNase I depends on the individual lot used. The enzyme can be diluted in water as described, or in a buffer that is compatible with the nick translation reaction. See notes on DNase I titration in critical parameters.
2. Place reaction on ice. Remove a 6-µl aliquot, boil it 3 min and place on ice 2 min. 3. Load aliquot on an agarose minigel, along with suitable size markers (0.1- to 10-kb range). Run gel quickly (15 V/cm) in case additional incubation is necessary. 4. If the digested DNA is between 100 and 500 nucleotides, proceed to step 5. If the probe size is between 500 and 1000 nucleotides (or larger) add a second aliquot of DNase I and incubate further. Additional DNase I is added in a more concentrated form than in the initial reaction to avoid significant volume changes. For example, if 10ìl of a 1:1000 dilution of DNase I is added initially, the second addition might be 1 ìl of a 1:100 dilution incubated for ~1 hr. Monitor the additional incubation carefully to avoid complete digestion of the probe. If little or no digestion occurs, purify a new DNA sample by phenol extraction and ethanol precipitation (UNIT 2.1) and repeat the nick translation.
5. Add 2 µl of 0.5 M EDTA, pH 8.0 (10 mM final) and 1 µl of 10% SDS (0.1% final) to the reaction. Heat 10 min at 68°C to stop the reaction and inactivate the DNase I. 6. Prepare a Sephadex G-50 spin column in a 1-ml syringe. Wash syringe and silanized glass wool plug with 2 ml of 100% ethanol, then 4 ml water. Pack G-50 resin to the 1-ml mark. Wash column 3 to 4 times with 100 µl SDS column buffer before loading the sample. An additional wash after loading the sample is not necessary. Alternatively, the G-50 resin can be stored in SDS column buffer, reducing the number of times the column should be washed to 2 to 3 times. SDS prevents the biotinylated DNA from sticking nonspecifically to the resin and the glass wool plug.
7. Separate the biotinylated probe from unincorporated nucleotides. The eluted probe concentration should be ∼20 ng/µl (for 2 µg nick-translated DNA) and is ready to use without further treatment. Probe can be stored at −20°C for years without loss of activity. Biotinylated DNA should not be subjected to phenol extraction, as biotin causes the probe to partition to the phenol/water interface or completely into the phenol if heavily biotinylated.
Labeling and Colorimetric Detection of Nonisotopic Probes
8. Assess the extent of the biotinylation reaction and the probe quality by colorimetric (support protocol) or chemiluminescent (UNIT 3.19) detection. Chemiluminescent detection is 5 × 103 times more sensitive than colorimetric detection (Beck and Koster, 1990).
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PREPARATION OF BIOTINYLATED PROBES BY RANDOM OLIGONUCLEOTIDE–PRIMED SYNTHESIS
BASIC PROTOCOL
This protocol outlines a priming reaction using random octamers that have been biotinylated at the 5′ end. Octamers are the optimal length for efficient hybridization of 5′-labeled oligonucleotides because the additional two bases act to minimize the steric hindrance from the 5′ end label. This ensures that every probe molecule generated contains at least one biotin, but biotin-16-dUTP is also included in the reaction so that additional biotin is incorporated into some of the probe molecules. Other biotinylated nucleotides (e.g., biotin-14-dATP and biotin-11-dUTP) can be used in place of the biotin-16-dUTP. The starting DNA template should be linear. Best results are obtained with isolated insert DNA that is ≥200 bp. The use of shorter templates will lead to ineffective probe synthesis because of the inability for many random primers to bind on a short target. The amount of template that can be labeled by this procedure ranges from 25 ng to 2 µg. Materials Linear template DNA Biotinylated random octamers dNTP/biotin mix 5 U/µl Klenow fragment (UNIT 3.5) TE buffer, pH 7.5 (APPENDIX 2) 0.5 M EDTA, pH 8.0 (APPENDIX 2) 4 M LiCl 100% and 70% ethanol, ice-cold 1. Place 500 ng to 1 µg template DNA in a 1.5-ml microcentrifuge tube. Add nucleasefree water to a total volume of 34 µl. Up to 2 ìg template DNA can be biotinylated without scaling up the reaction volumes.
2. Denature the DNA for 5 min in boiling water. Place on ice 5 min and microcentrifuge briefly. 3. Add the following to the sample in the order listed: 10 µl biotinylated random octamers 5 µl dNTP/biotin mix 1 µl (5 U) Klenow fragment. Incubate 1 hr at 37°C. If desired, an additional 2.5 U of Klenow fragment can be added after 30 min of incubation to “boost” the reaction. Continuing the incubation for up to 6 hr can increase the yield.
4. Terminate reaction by adding 3 µl of 0.5 M EDTA, pH 8.0. Precipitate the probe by adding 5 µl of 4 M LiCl and 150 µl ice-cold 100% ethanol. Place 30 min on dry ice. 5. Microcentrifuge 10 min at top speed, room temperature, and wash DNA pellet with ice-cold 70% ethanol. 6. Resuspend DNA pellet in 20 µl TE buffer, pH 7.5. Assess the quality of the biotinylation reaction by colorimetric (support protocol) or chemiluminescent (UNIT 3.19) detection. Chemiluminescent detection is 5 × 103 times more sensitive than colorimetric detection (Beck and Koster, 1990). Enzymatic Manipulation of DNA and RNA
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SUPPORT PROTOCOL
COLORIMETRIC DETECTION OF BIOTINYLATED PROBES This support protocol provides a method for checking the extent of the biotinylated incorporation into the labeled probe and serves as the basis for detecting biotinylated DNA probes hybridized to Southern blots. Because the detectability of biotinylated probes is a function of the number of biotin molecules per kilobase, rather than the specific activity of a corresponding radiolabeled probe, the extent of biotinylated deoxynucleotide incorporation is checked by a colorimetric assay (similar assessment can be made by chemiluminescent detection; UNIT 3.19). Under the standard nick translation conditions provided, ∼50 biotin molecules are incorporated per kilobase of DNA. In this procedure, biotinylated DNA is spotted on a nitrocellulose filter. The streptavidin– alkaline phosphatase conjugate binds strongly to the biotin and is visualized when the enzyme is provided with substrates that produce a colored precipitate. As little as 2 pg biotinylated DNA should be visible. If the protocol is being utilized as a method for detecting DNA probes by Southern analysis, follow standard Southern blot protocols (UNIT 2.9) substituting the biotinylated probe (∼100 ng/ml) for the radiolabeled probe and proceed to steps 3 to 8 below. Additional Materials Biotinylated standard DNA (basic protocols or GIBCO/BRL) and test DNA DNA dilution buffer Alkaline phosphatase pH 7.5 (AP 7.5) buffer Blocking buffer: 3% (w/v) BSA fraction V in AP 7.5 buffer 1 mg/ml streptavidin–alkaline phosphatase (AP) conjugate (GIBCO/BRL) Alkaline phosphatase pH 9.5 (AP 9.5) buffer 75 mg/ml nitroblue tetrazolium (NBT) 50 mg/ml 5-bromo-4-chloro-3-indoyl phosphate (BCIP) TE buffer, pH 8.0 (APPENDIX 2) Small piece of membrane (e.g., 5 × 3 cm2): nitrocellulose or uncharged nylon Sealable bags NOTE: To avoid nonspecific background, wear powder-free gloves when handling the membranes. 1. Prepare biotinylated standard DNA in concentrations of 0, 1, 2, 5, 10 and 20 pg/µl in dilution buffer. Dilute the biotinylated test DNA in a similar fashion. 6× SSC in the dilution buffer prevents the DNA spot from spreading on the nitrocellulose, which diminishes the color intensity of the spot.
2a. For nitrocellulose membrane: Spot 1 µl of each dilution on a small piece of nitrocellulose. Bake the membrane ∼1 hr at 80°C. Proceed directly to step 3. Nitrocellulose membranes cannot be used in chemiluminescent detection procedures (UNIT 3.19).
2b. For nylon membrane: Spot 1 µl of each dilution on the nylon membrane. Air-dry and cross-link DNA to the membrane by UV illumination (UNIT 3.19). Proceed to step 3 or develop membrane as described for chemiluminescent detection (UNIT 3.19). This cross-linking step is critical. If done incorrectly, ≤50% of the DNA can be washed off the membrane during the detection steps, resulting in inaccurate probe assessment. Labeling and Colorimetric Detection of Nonisotopic Probes
If chemiluminescent detection is used, the test DNA should be diluted in a series from 10−1 to 10−6, and must be visible at a 10−3 dilution. If it is not, the probe is not sufficiently biotinylated.
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3. Float membrane in a small volume of AP 7.5 buffer for 1 min to rehydrate. Place membrane and 10 ml blocking buffer in a sealable bag (cut to size). Avoid trapping air bubbles when sealing. Block for 1 hr at 37°C. 4. Dilute 10 µl streptavidin–AP conjugate with 10 ml AP 7.5 buffer (1 µg/ml final). Cut corner of bag and squeeze out blocking buffer. Replace with the streptavidin–AP solution and reseal. Incubate 10 min at room temperature with agitation on a platform shaker. Do not allow the membrane to dry out at any stage.
5. Remove membrane from bag and transfer to a shallow dish. Wash in 200 ml AP 7.5 buffer (twice, 15 min each time) and in 200 ml AP 9.5 buffer (once, 10 min) with gentle agitation. 6. Add 33 µl of 75 mg/ml NBT to 7.5 ml AP 9.5 buffer and invert to mix. Add 25 µl of 50 mg/ml BCIP and mix gently. Be sure to add reagents in this order and mix gently to prevent precipitation of the reagents.
7. Incubate membrane with NBT/BCIP solution in a shallow dish in low light, checking periodically until color development is satisfactory (usually 15 to 60 min). 8. Stop reaction by washing with TE buffer, pH 8.0. Check incorporation of biotinylated dUTP by comparing the intensities of standard and test DNA. If the probe is at least half as intense as the standard DNA at the corresponding dilution, it should be suitable as an in situ hybridization probe. PREPARATION AND DETECTION OF DIGOXIGENIN-LABELED DNA PROBES
ALTERNATE PROTOCOL
The digoxigenin-based detection system is an alternative nonisotopic labeling method offered by Boehringer Mannheim. Detection is achieved by incubation with antidigoxigenin antibodies coupled directly to one of several fluorochromes or enzymes, or by indirect immunofluorescence (UNIT 14.6). The availability of uncoupled antibodies also permits signal-amplification protocols to be employed. Biotin- and digoxigenin-labeled probes can be visualized simultaneously using a different fluorochrome for each probe. Digoxigenin-11-dUTP can be incorporated into DNA by either of the nick translation or random oligonucleotide–primed synthesis protocols. Boehringer Mannheim advises that nick translation incorporation is not as efficient as random-priming; however, nick translation affords greater control over the final probe size. Nick translated probes perform well with in situ hybridizations, with no apparent sensitivity problems. The protocol is virtually identical to that for biotin-11-dUTP incorporation by nick translation. The standard 100-µl reaction is set up as described in the basic protocol, but 10 µl of a 10× digoxigenin-11-dUTP/dTTP stock solution (see reagents and solutions) is substituted for 10 µl of a 10× biotin-11-dUTP stock solution. The amount of DNase I added to the sample and the time of incubation remain the same and do not depend on the modified deoxynucleotide in the reaction. After incubation at 15°C for 2 hr, an aliquot is run on a minigel to check the probe size. When the correct size has been obtained, the reaction is stopped and the probe is separated from the unincorporated nucleotides using a G-50 spin column. It is advisable to avoid phenol extraction of digoxigenin-labeled probes. The support protocol for colorimetric detection of biotinylated probes can be modified for use with digoxigenin-labeled probes. If incorporation is to be checked, one can use as a control either digoxigenin-labeled standard DNA (obtained from Boehringer Mannheim as part of the Genius kit) or digoxigenin-labeled DNA that has already been used
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successfully. Anti-digoxigenin antibody conjugated to alkaline phosphatase is substituted for streptavidin–alkaline phosphatase. Boehringer Mannheim recommends a 1:5000 dilution. The other steps in the support protocol are unchanged and can be followed exactly. A protocol for chemiluminescent detection of digoxigenin-labeled probes is discussed in UNIT 3.19. REAGENTS AND SOLUTIONS Alkaline phosphatase pH 7.5 (AP 7.5) buffer 0.1 M Tris⋅Cl, pH 7.5 0.1 M NaCl 2 mM MgCl2 Autoclave or filter sterilize and store at room temperature Alkaline phosphatase pH 9.5 (AP 9.5) buffer 0.1 M Tris⋅Cl, pH 9.5 0.1 M NaCl 50 mM MgCl2 Autoclave or filter sterilize and store at room temperature ≤1 yr Biotin-11-dUTP, 0.5 mM Prepare a 0.5 mM stock of lyophilized biotin-11-dUTP (e.g., Sigma) in 20 mM Tris⋅Cl, pH 7.5 (at 289 nm, the molar extinction coefficient ε = 7100; see Table A.3D.1). Check the pH of the stock; if 1 × 108 cfu/µg DNA is recommended. Low efficiencies may result in failed reactions. Do not use more than 5 µl of diluted reaction mix for 50 µl competent cells. Refer to UNIT 1.8 for more information about bacterial transformation.
7. Plate the transformation mix on an LB plate supplemented with 100 µg/ml ampicillin and 40 µg/ml Xgal. Antibiotics should be carefully chosen to be appropriate to the master vector (in this case, pDNR).
8. Incubate the plate overnight at 37◦ C. 9. Pick several (one to four) white colonies and inoculate each into 1 ml LB medium containing 100 µg/ml ampicillin. Incubate overnight at 37◦ C. 10. Proceed as in Basic Protocol 2, steps 9 through 12, to create glycerol stock cultures and sequence plasmid DNA covering the ORF and recombinational sequence region. Even though the blue-white selections work well, it is necessary to verify the identity of the master clone insert by sequencing. BASIC PROTOCOL 4
GENERATING GATEWAY EXPRESSION CLONES Expression clones are generated by the LR reaction between the attL-containing master clone and attR-containing Gateway-compatible expression vectors (see Fig. 3.20.4). Either the commercial Gateway expression vectors or Gateway-compatible expression vectors generated in-house can be used. Conversion to a Gateway-compatible vector involves a simple insertion of a blunt-ended conversion cassette by ligation into the blunt-ended expression vector that is to be adapted to the Gateway system. The conversion cassette, commercially available for the Gateway system, includes a ccdB death gene and a chloramphenicol resistance (Cmr ) gene flanked by attR sites at the ends (Fig. 3.20.6).
Materials
Recombinational Cloning
Expression vector (e.g., pDual; Stratagene; or any expression vector with a different selectable marker than the master clone) Appropriate restriction enzymes (e.g., NEB) T4 DNA polymerase or Klenow fragment (e.g., NEB) Calf intestine alkaline phosphatase (CIAP) and buffer (e.g., NEB; optional) TE buffer, pH 8.0 (APPENDIX 2) T4 DNA ligase and buffer (e.g., NEB) Gateway recombination cassette (rfA, rfB, or rfC; Invitrogen), for making in-house Gateway-compatible expression vectors Sterile H2 O E. coli competent cell cultures (e.g., DB3.1 and DH5α; Invitrogen) LB plates supplemented with 30 µg/ml chloramphenicol (or other antibiotic appropriate for the expression vector; UNIT 1.1)
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Figure 3.20.6 Conversion of an expression vector into a Gateway-compatible vector. An expression vector of choice is linearized and blunted at favorable restriction sites. It is then ligated with an available blunt-ended cassette containing ccdB and Cmr genes. The Gateway-compatible expression vector sequence should be analyzed to confirm the correct orientation.
LB medium supplemented with 30 µg/ml chloramphenicol (or other antibiotic appropriate for the expression vector; UNIT 1.1) 50% (v/v) glycerol, sterile 150 ng/µl Gateway expression vector (e.g., pDEST27; Invitrogen), for commercial expression vector LR Clonase (Invitrogen) 5× LR Clonase buffer (Invitrogen) 150 ng/µl master clone plasmid DNA (Basic Protocol 2) 2 µg/µl proteinase K (Invitrogen) LB plates supplemented with 100 µg/ml ampicillin (or other appropriate antibiotic; UNIT 1.1) LB medium supplemented with 100 µg/ml ampicillin (or other appropriate antibiotic; UNIT 1.1) 96-well plates or 1.5-ml microcentrifuge tubes Multichannel pipettor 25◦ C incubator Additional reagents and equipment for growth of E. coli competent cells (UNIT 1.8), linearizing the expression vector at the multiple cloning sites using restriction enzymes (UNIT 3.1), generating blunt ends using T4 DNA polymerase or Klenow fragment (UNITS 3.5 & 3.16), and analyzing plasmid DNA using restriction enzymes (UNIT 3.1) Prepare a Gateway-compatible expression vector (optional) 1. Linearize the expression vector at the multiple cloning sites using restriction enzymes (UNIT 3.1), and generate blunt ends using T4 DNA polymerase or Klenow fragment (UNITS 3.5 & 3.16). If necessary, perform dephosphorylation using CIAP (see UNIT 3.10) to decrease background from self-ligation of the vector.
2. Adjust the linearized expression vector to a final concentration of 20 to 50 ng/µl in TE buffer, pH 8.0. 3. Prepare the ligation reaction mix in a microcentrifuge tube as follows:
1 to 5 µl of 20 to 50 ng/µl dephosphorylated linearized vector 1 µl 10× T4 DNA ligase buffer 2 µl 5 ng/µl Gateway recombination cassette
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1 µl 1 U/µl T4 DNA ligase Sterile H2 O to a final volume of 10 µl. If a polypeptide fusion is to be added at either end of the gene, ensure that the Gateway recombination cassette is in the appropriate reading frame.
4. Incubate 1 hr at room temperature. 5. Add 1 to 2 µl reaction mixture to 50 µl E. coli DB3.1 competent cell culture and perform transformation. DB3.1 is a strain modified to allow growth of plasmids containing ccdB. Cells other than DB3.1 will be killed by the new expression vector that will carry a copy of the ccdB death gene. Transformation efficiency of >1 × 108 cfu/µg DNA is recommended. Refer to UNIT 1.8 for more information about bacterial transformation.
6. Plate the transformation mix on an LB plate supplemented with 30 µg/ml chloramphenicol. Incubate overnight at 37◦ C. Antibiotics should be carefully chosen to be appropriate to the recombination cassette (in this case chloramphenicol).
7. Pick several (one to four) colonies and inoculate each into 1 ml LB supplemented with 30 mg/ml chloramphenicol. Incubate overnight at 37◦ C. 8. Combine 0.7 ml overnight culture with 0.3 ml 50% glycerol and store up to 6 months at −80◦ C. 9. Isolate plasmid DNA from the remainder of the culture using standard miniprep methods (e.g., UNIT 1.6) or a commercial kit (e.g., Qiagen). Analyze plasmid DNA using appropriate restriction enzymes (UNIT 3.1) or by sequencing to identify clones with the proper orientation. Quantify plasmid DNA from a selected clone (APPENDIX 3D). Tranformants need to be screened for orientation because both orientations will be generated after blunt-end ligation. Appropriate restriction enzymes will cut the vector and cassette in an asymmetric fashion, yielding fragment lengths that depend on orientation.
Transfer an ORF into the expression vector using the Gateway LR reaction 10. Prepare the master reaction mix on ice for the 20-µl LR reaction (multiply the volume for each component for the number of reactions plus two or three to allow for losses during transfer) as follows (18 µl per tube or well): 2 µl 150 ng/µl commercial Gateway (e.g., pDEST27) or Gateway-compatible (from step 9) expression vector 4 µl LR Clonase 4 µl 5× LR Clonase buffer 8 µl TE buffer, pH 8.0. 11. Mix well and dispense 18 µl master reaction mix into each well of a 96-well plate or 1.5-ml microcentrifuge tube. For a 96-well plate format, a multichannel pipettor should be used.
12. Add 2 µl master clone plasmid DNA to the 18 µl master reaction mix. 13. Incubate the reaction 1 hr at 25◦ C. Recombinational Cloning
Although, the manual calls for room temperature, reactions incubated at a controlled temperature of 25◦ C are recommended.
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14. Add 2 µl proteinase K solution and incubate at 37◦ C for 10 min. The reaction mixture can be stored up to 6 monts at −20◦ C after this step.
15. Add 2 to 3 µl of reaction mixture to 50 µl E. coli competent cell culture (e.g., DH5α) and perform transformation. Any standard cloning strain is fine. Transformation efficiency of >1 × 108 cfu/µg DNA is recommended. Refer to UNIT 1.8 for more information about bacterial transformation.
16. Plate the transformation mix on an LB plate supplemented with 100 µg/ml ampicillin. Incubate overnight at 37◦ C. Antibiotics should be carefully chosen to be appropriate to the expression vector, in this case, ampicillin for the commercial vector pDEST27.
17. Pick colonies and analyze the plasmid DNA using restriction enzymes. Typically, the transfer reaction is very efficient (90% to 95%). For large-scale projects, in many cases, it is not necessary to pick several colonies and analyze them.
18. Choose the best clone and prepare for the desired use or store as a 15% glycerol stock up to 6 months at −80◦ C.
GENERATING CREATOR EXPRESSION CLONES Cre recombinase facilitates the transfer reaction of the ORF from the master clone to a Creator-compatible expression vector in generating expression clones (Fig. 3.20.5). The process can be performed using the commercially available vectors or Creatorcompatible vectors generated in-house. The generation of Creator-compatible expression vectors involves (1) the amplification of the Creator conversion cassette with primers that contain restriction sites, (2) restriction of expression vector, and (3) ligation of the prepared cassette into the target expression vector (Fig. 3.20.7).
BASIC PROTOCOL 5
Materials Creator expression vector (e.g., pLP-CMVneo; BD Clontech) or other expression vector to make Creator-compatible vector LoxP primers (BD Clontech) LoxP template (e.g., Creator acceptor vector; BD Clontech) Restriction enzymes and buffers (NEB) T4 DNA ligase and buffer (NEB) E. coli competent cell culture (e.g., DH5α; Invitrogen) LB plates supplemented with 100 µg/ml kanamycin (or other appropriate antibiotic; UNIT 1.1) LB plates supplemented with 30 µg/ml chloramphenicol (or other appropriate antibiotic; UNIT 1.1) and 7% (w/v) sucrose Cre recombinase (New England Biolabs) 10× Cre reaction buffer (New England Biolabs) 20 ng/µl master clone plasmid DNA (Basic Protocol 3) 96-well plate or 1.5-ml microcentrifuge tubes Multichannel pipettor Additional reagents and equipment for PCR (Basic Protocol 1), agarose gel electrophoresis (Support Protocol; UNIT 2.5A), growth of E. coli competent cells (UNIT 1.8), and analyzing the plasmid DNA using restriction enzymes (UNIT 3.1) or recombination-specific PCR methods (BD Clontech)
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Figure 3.20.7 Conversion of an expression vector into a Creator-compatible vector. The region flanking the loxP site and prokaryotic promoter is amplified with specific PCR primers containing specific restriction enzyme sites. Then, the PCR products and an expression vector are digested with appropriate restriction enzyme. A ligation reaction generates a Creator-compatible expression vector.
Prepare Creator-compatible expression vector (optional) 1. Design PCR primers to amplify the loxP template as follows: 5 primer: 5 -RS-ATA ACT TCG TAT AGC ATA CAT TAT-3 3 primer: 5 -RS-CAC GTC AGG TGG CAC TTT TCG-3 . Restriction sites (RS) should be added, allowing the ligation of the cassette into the target vector. The target vector should have restriction enzyme sites that are not present in the loxP cassette. Available restriction sites can be found in the Creator Acceptor vector construction manual.
2. Amplify the loxP cassette by PCR (see Basic Protocol 1, steps 1 to 3 and 4b). 3. Digest the PCR loxP fragment and the expression vector with the selected restriction enzymes. 4. Purify the digested PCR fragment (161 bp) by agarose gel electrophoresis (UNIT 2.5A). 5. Ligate (see Basic Protocol 4, step 3) the PCR-amplified loxP cassette and the linearized expression vector. Use of the appropriate matched restriction enzymes for preparing the loxP cassette and the linearized expression vector are necessary for the ligation reaction to occur.
6. Add 2.5 µl of ligation mixture to 50 µl E. coli competent cell culture, e.g., DH5α, and perform transformation. Recombinational Cloning
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Transformation efficiency of >1 × 108 cfu/µg DNA is recommended. Refer to UNIT 1.8 for more information about bacterial transformation. Current Protocols in Molecular Biology
7. Plate the transformation mix on an LB plate supplemented with 100 µg/ml kanamycin. Incubate overnight at 37◦ C. Antibiotics should be carefully chosen to be appropriate to the expression vector.
8. Pick colonies and prepare overnight cultures and glycerol stocks (see Basic Protocol 4, steps 7 and 8). Prepare plasmid DNA (UNIT 1.6 or Qiagen) and quantify (APPENDIX 3D).
Transfer an ORF into the expression vector using Creator Cre reaction 9. Prepare the master reaction mix on ice for the 20-µl LR reaction (multiply the volume for each component for the number of reactions plus two or three to allow for losses during transfer) as follows (10 µl per tube or well): Reaction mix per well includes: 200 ng commercial Creator (e.g., pLP-CMVneo) or Creator-compatible (step 8) expression vector 1 µl Cre recombinase 2 µl 10× Cre reaction buffer Bring volume to 10 µl with H2 O. 10. Dispense 10 µl master reaction mix into each well of a 96-well plate. For a 96-well plate format, a multichannel pipettor should be used.
11. Add 10 µl 20 ng/µl master clone plasmid DNA to the 10 µl master reaction mix. Master clone plasmid DNA (150 to 200 ng) per reaction works well.
12. Incubate 30 min at 37◦ C. 13. Heat inactivate the reaction 15 min at 70◦ C. 14. Add 2 to 3 µl reaction mixture to 50 µl E. coli competent cell culture, e.g., DH5α, and perform transformation. Transformation efficiency of >2 × 108 cfu/µg DNA is recommended. More than 5 µl of reaction mix can be toxic to bacterial competent cells. Refer to UNIT 1.8 for more information about bacterial transformation.
15. Plate the transformation mix on an LB plate supplemented with 30 µg/ml chloramphenicol and 7% (w/v) sucrose. Incubate the plates overnight at 37◦ C. Antibiotics should be carefully chosen to be appropriate to the expression vector (in this case, pLP-CMVneo).
16. Pick good-sized colonies. It is important to pick good-sized colonies rather than small ones. It has been reported that the small colonies could be background and may harbor undesirable recombinants (Creator DNA cloning kit manual).
17. Analyze the plasmid DNA of picked colonies using restriction enzymes (UNIT 3.1) or recombination-specific PCR methods (BD Clontech). Typically, the transfer reaction is very efficient (90% to 95%). For large-scale projects, in many cases, it is not necessary to pick several colonies and analyze them.
18. Choose the best clone and and prepare for the desired use or store as a 15% glycerol stock up to 6 months at −80◦ C.
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COMMENTARY Background Information
Recombinational Cloning
Overview of universal cloning strategy As molecular biology has matured, the number of experimental approaches that can be conceived and executed with a cloned cDNA copy of a gene has grown exponentially. Proteins can be expressed in vitro or in vivo in a wide variety of organisms, ranging from bacteria to human cells; their expression can drive the production of purified protein for structural studies, or their regulated expression can be used for complementing genetic phenotypes; epitope tags (e.g., green fluorescent protein, GST, or HA) can be added as markers for localization, for use in purification, or as part of two-hybrid assay systems. Each of the myriad applications that utilize protein expression demands the subcloning of the relevant cDNA into an appropriate plasmid vector that adds the relevant selectable markers, transcriptional and/or translational signals, any needed polypeptide tags, and other relevant sequences. Using traditional subcloning methods based on restriction enzymes and ligation, the effort required to transfer cDNAs from vector to vector can be substantial and time consuming. If one aspires to do large-scale experiments involving the examination of thousands of genes simultaneously using automated and parallel approaches, it would be impossible using standard methods. To move the focus of the community’s efforts away from the mechanics of subcloning and put it back on biology, recombinational cloning has been developed. This approach facilitates the transfer of DNA molecules from one vector to another in a single concerted step that is highly efficient, in-frame, and does not introduce mutations. Recombinational cloning is based on sitespecific recombination to transfer DNA sequences into multiple expression systems (Walhout et al., 2000; Reboul et al., 2001, 2003; Brizuela et al., 2002; Yamada et al., 2003; Marsischky and LaBaer, 2004). It is independent of the insert DNA sequence to be cloned, separating it from the limitations of classical restriction enzyme–based cloning. It enables rapid and efficient transfer of single inserts into multiple different expression systems for protein expression and functional analysis. It also allows the simultaneous transfer of a many different DNA sequences into an expression vector in parallel, enabling high throughput applications (Fig. 3.20.1). Presently, the Gateway system (Invitrogen)
and the Creator system (BD Clontech) are commercially available. The strategy employs the use of a set of master clones from which numerous different expression clones can be easily produced. The master clones contain the coding sequences (also called open reading frames, ORFs) situated in inert (non-protein-expressing) vectors, which are endowed with recombinational sequences. The use of inert vectors for the master clones avoids inadvertent protein expression and consequent negative selection of clones that contain inserts toxic to the propagating bacteria. Using recombination, the coding sequences can then be transferred to any protein expression vector that has the recombination sequences situated in the appropriate position and reading frame. Virtually any existing expression vector can be easily adapted to contain recombinational sequences for either of the existing commercial systems. Based upon the design of the original master clones, the expression clones can either produce native protein (when master clones contain a naturally positioned stop codon) or carboxyl-terminal tagged proteins (when the stop codon is omitted from the master clone). The transfer process is a conservative molecular reaction. Once the sequence of the master clones has been verified, there is no need to verify the daughter clones. Thus, one library of clones in master vectors becomes many libraries of clones in expression vectors. The steps involved in making expressionready master clones include (1) identifying the genes that will be cloned, (2) obtaining sequence information regarding the coding sequences, (3) determining whether to include or exclude the stop codon, (4) designing genetargeted oligos for PCR, (5) constructing master clones using high-fidelity PCR and capture into the master vector, (6) bacterial transformation and picking colonies, (7) DNA sequencing of the insert and important flanking sequences, and (8) sequence analysis and selection of master clones that meet the acceptance criteria. Once the master clones are constructed and their sequences verified, they can be transferred to appropriately constructed expression vectors to generate expression-ready clones. As a matter of a common lexicon, the term vector is used to describe plasmids without an insert, whereas the term clone refers to a vector containing an insert. For example, one
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might state that the coding sequences from a set of master clones were transferred to an expression vector to produce expression clones. Each commercial system also uses its own terminologies. The Gateway system refers to master vectors as entry vectors and expression vectors as destination vectors. The Creator system refers to master vectors as donor vectors and expression vectors as acceptor vectors. Each system supports the insertion of a simple cassette that adds the appropriate recombinational signals and relevant selectable markers into virtually any existing expression vector adapting it for use in that system. The recombination reactions themselves are equilibrium reactions that result in a mixture of end products, starting substrates, and intermediates. The desired product may only be present as a small fraction of this mix, but by using the appropriate positive and negative selections, it is the only viable product that is recovered. This results in an overall efficiency that exceeds 96% in most cases, which can enable high throughput automation. Experiments can be easily expanded to genome scale once a complete high-quality collection has been constructed in a recombinational cloning system. The Gateway system Gateway technology is based on the bacteriophage lambda (λ) site-specific recombination system, which facilitates the integration of λ genome into the Escherichia coli (E. coli) chromosome and the switch between the lytic and lysogenic pathways (Ptashne, 1992; Hartley et al., 2000; Walhout et al., 2000). Lambda-based recombination involves the DNA recombination sequences and the proteins that mediate the recombination reaction (i.e., Clonase enzyme mix). Recombination occurs between specific attachment (att) sites on the interacting DNA molecules. The DNA segments flanking the recombination sites are changed such that after recombination the att sites become hybrid sequences from the att sequences of each parental vector. For example, attL sites comprise sequences from attB and attP sites. Strand exchange occurs within a core region that is common to all att sites. For more detailed information about λ recombination, see published references and reviews (Landy, 1989; Ptashne, 1992; http://www.invitrogen.com/gateway). The att sites serve as the binding site for recombination proteins and have been well characterized (Weisberg and Landy, 1983). Upon λ integration, recombination occurs between
attB (bacteria) and attP (phage) sites to give rise to attL (left) and attR (right) sites. The attB and attP sites are different sizes, and because the attL and attR sites are hybrids, their sizes fall in between. The actual crossover occurs between homologous core regions (15 bp) on the two sites, but surrounding sequences are required as they contain the binding sites for the recombination proteins (Landy, 1989). In developing the Gateway system, mutations were introduced into the core of the att sites to create different sets that are incompatible with each other, generating attB1 and attP1 sites and attB2 and attP2 sites. The attB1 recombine only with attP1, not with attP2. This allows a gene to be flanked by attL1 and attL2 sites on either side that can recombine with the corresponding attR sites and maintain the correct orientation. The different species of recombination sites (attB, attP, attL, and attR) are found in different vector constructs. The system is designed so that the attB site, the smallest of the four, appears in the final expression clone introducing an additional 9 amino acids into any fusion proteins. Lambda recombination is catalyzed by a mixture of enzymes that bind to specific att sequences. The recombination proteins differ depending upon which direction the reaction is run. When the reaction is run to mix attB and attP sites, such as when making master clones, it is catalyzed by the bacteriophage λ integrase (Int) and E. coli integration host factor (IHF) proteins (BP Clonase enzyme mix), whereas, if the reaction is between attL and attR sites it is catalyzed by the bacteriophage λ Int and excisionase (Xis) proteins, and the E. coli IHF protein (LR Clonase enzyme mix). For more information about the recombination enzymes, see published references and reviews (Nash, 1977; Nash and Robertson, 1981; Landy, 1989; Ptashne, 1992). Two recombination reactions constitute the basis of the Gateway technology. BP reaction, catalyzed by BP Clonase enzyme mix, facilitates recombination of an attB substrate (attB-PCR product or a linearized attB-containing expression clone) with an attP substrate (master vector) to create an attL-containing master clone (Fig. 3.20.4). The BP cloning efficiency tends to drop rapidly with increasing insert DNA size larger than 2 kb (Marsischky and LaBaer, 2004). On the other hand, LR Reaction, catalyzed by LR Clonase enzyme mix, facilitates recombination of an attL substrate (master clone) with an attR substrate (expression vector) to create an attB-containing expression clone (Fig. 3.20.4).
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Recombination reaction is an equilibrium reaction; therefore tight selection is required to ensure that only the desired product is obtained. In the Gateway system, positive selection for the correct master clone is mediated by an antibiotic resistance marker. To avoid contamination from starting vectors, there is counter-selection by the death gene (ccdB), which is toxic to general cloning bacteria (Bernard and Couturier, 1992). Thus, only bacteria that contain master clones where the death gene has been successfully replaced by the desired insert will survive selection. Vectors carrying the death gene can be propagated using a special E. coli strain (DB3.1), which is resistant to the ccdB gene. The combined positive and negative selection provides an overall BP or LR reaction efficiency of 95% to%. For more detail refer to the Gateway manual from the manufacturer.
Recombinational Cloning
The Creator system The BD Clontech Creator system relies on homologous recombination and Cre-loxP recombination generating master clone and expression clone, respectively. PCR amplified ORF containing homologous sequences in each ends will be transferred into a master vector in the In-Fusion reaction. Because it is simply a homologous recombination event, many thermostable DNA polymerase could be adapted in place of the In-Fusion enzyme. The Cre-loxP site-specific recombination reaction catalyzes the transfer of ORF from a master clone to an expression vector creating expression clone (Fig. 3.20.5). Cre, a 38-kDa recombinase protein from bacteriophage P1, mediates recombination between or within DNA sequences at specific locations called loxP sites (Sternberg et al., 1981; Abremski and Hoess, 1984; Sauer, 1994). These sites consist of two 13-bp inverted repeats separated by an 8-bp spacer region that provides directionality to the recombination reaction (http://www.clontech.com/clontech/products/ families/creator). The 8-bp spacer region in the loxP site has a defined orientation that forces a gene to be transferred in a fixed orientation and reading frame. Master vectors contain two loxP sites that flank a multicloning site (MCS) and the open reading frame for the chloramphenicol resistance gene (Cmr ; Fig. 3.20.5). Master vectors also contain the ampicillin gene for propagation and selection in E. coli, and the sucrase gene (sacB) from B. subtilis for negative selec-
tion of correct recombinants. Expression vectors contain a single loxP site, followed by a bacterial promoter, which drives expression of the chloramphenicol marker after Cre-loxmediated recombination. After transfer, the gene of interest will become linked to the specific expression elements for which the expression vector was designed. Furthermore, if the coding sequence for the gene of interest is in frame with the upstream loxP site in the master vector, it will automatically be in frame in the expression vector. The expression of C-terminal fusion proteins is complicated because the chloramphenicol resistance gene blocks access to C-terminal fusion tags. To enable C-terminal fusions, the gene of interest should be collected into a master vector that has a splice donor site (pDNR-Dual), and any C-terminal tag will be spliced in-frame to the gene of interest. During the recombination reaction, the DNA sequences between the loxP sites on the master clone are inserted at the loxP site of the expression vector. This reaction will put the chloramphenicol resistance gene under control of the constitutively active bacterial promoter and the gene of interest under control of the heterologous promoter (Fig. 3.20.5). Because the recombination is site-specific, the reading frame is always maintained. Correct recombinants can be selected using positive selection on chloramphenicol (selects for the recombinant) and counter selection on sucrase (selects against the master vector). Sucrose in the presence of endogenous sucrase is toxic to general E. coli strains (Pelicic et al., 1996). For more details, refer to the Creator manual from the manufacturer. Modification for use with other vectors The Gateway and Creator recombinational cloning technology platforms provide various forms of expression systems (e.g., bacterial, mammalian, insect, and in vitro transcription/translation). In many cases, however, users may want to modify their own expression vectors into recombination-compatible systems (Figs. 3.20.6 and 3.20.7). For that purpose, those commercials systems also provide a way to convert other expression vectors into recombination-compatible ones by simply inserting the recombination cassette, using PCR and restriction-mediated cloning steps. For this purpose, refer to standard cloning manuals or to the Gateway and Clontech vector conversion manuals for more details.
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Critical Parameters and Troubleshooting Amplification of target DNA Primer mapping. When performing agarose gel electrophoresis in handling multiple genes or large cloning projects using a 96-well plate format, it is helpful if the resulting gel bands form a saw tooth pattern in which the expected fragments progressively increase in size across the gel lanes, but alternate in size to avoid lane-to-lane contamination (Fig. 3.20.8). This effect is most easily planned at the stage of primer design, so that the genes are amplified in an order that will result in this pattern. DNA polymerase. Fidelity and PCR success rate (% of genes with positive PCR product) are crucial parameters in high-throughput PCR cloning. The choice of polymerase has a dramatic effect on the success rate. In the authors’ experience, Pfu Turbo (Stratagene) and Pfx (Invitrogen) were comparable and gave the highest success rates in cloning up to 4 kbp from human brain and placenta first-strand cDNA mix. In addition, Herculase (Strategene), GC-rich polymerase (Roche), and Elongase (Invitrogen) were successfully used in other systems (Walhout et al., 2000; Reboul et al., 2001). Running pilot experiments on small subsets of clones is recommended before executing a large-scale experiment in order to
adjust reaction parameters, e.g., the amount of polymerase, melting temperature, cycle number, Mg2+ concentration, and GC content of the primers. Template. PCR reactions using plasmid template are highly efficient. As little as 200 ng of template plasmid DNA is enough to reduce the number of PCR cycles and the rate of mutations. When the plasmid form for the target gene is not available, the cDNAs must be amplified from cDNA libraries, first-strand cDNA, or genomic DNA. Genomic DNA is an excellent source for a simple organism because it is essentially a normalized library where each gene is represented equally. For eukaryotes, the authors have found increased success rates from first-strand cDNA with highest rates in the brain and placenta firststrand cDNA mixture (1:4) for general human gene cloning purposes; however, for cloning certain cell- or tissue-type-specific genes, use of template DNA generated from those cells or tissues is recommended. Generation of master clones Pilot experiments. The concentration of purified PCR product can vary from gene to gene, especially in large-scale projects, and can influence the success rate of generating master clones, even though all were successfully amplified. Consequently, running a pilot
Figure 3.20.8 Examples of a saw-tooth pattern in agarose gel analysis of PCR product. PCR mixtures (96 samples) were run on a 1% agarose gel. A primer-ordering algorithm generates a map of oligonucleotide primers in 96-well format with alternating expected product size, so that the expected PCR bands would appear in a saw-tooth fashion. The letter M indicates DNA size markers.
Enzymatic Manipulation of DNA and RNA
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experiment with a small number of genes is strongly recommended, using positive and negative controls for estimating the range of PCR product concentration against the fixed master vector concentration. Transformation. The efficiency of transformation into competent E. coli cells is critical, especially in dealing with large-scale projects in a 96-well format. Transformation efficiencies of >1 × 108 cfu/µg DNA are recommended. For high-throughput projects LB agar can be prepared in 48-well grid square bioassay trays (Genetix), and the bacterial colonies can be automatically picked using robotics. Selection. Because the recombination reaction does not go to completion, a mixture containing the starting plasmids, one or more intermediates, the desired product, and one or more byproducts is obtained. Thus, stringent antibiotic selection conditions are required to select only the desired recombinant containing the target gene in its correct orientation. Verification. Enzymatic amplification using PCR can introduce unwanted random mutations in the DNA sequence. In addition, primer mutations can be introduced during synthesis of the oligonucleotide primers. The enzymatic contribution to this mutation rate can be decreased by reducing the number of amplification cycles. Nevertheless, each clone should be sequenced in its full length to verify the correct sequence of the target gene and the recombination site has been obtained. Once a master clone is verified, it is not necessary to resequence after it is transferred into an expression vector. Generation of expression clones Transformation. As in generation of the master clone, the efficiency of transformation into competent E. coli cells is critical in dealing with large-scale projects in a 96-well format. Bacterial competent cells with transformation efficiencies of >2 × 108 cfu/µg DNA are recommended for the Creator system pipeline. Pilot experiments. Unusual recombination events occasionally occur, especially in retroviral expression vectors. It is advisable to confirm that the expression clones harbor correct inserts in proper orientation by analyzing plasmid DNA of some expression clones using specific restriction enzymes in a pilot experiment.
Anticipated Results Recombinational Cloning
Amplification of target DNA From both plasmid DNA or genomic DNA (especially prokaryotic cells) as a template,
PCR should amplify only the gene of interest, and the overall success rate is >95%. In the case of first-strand cDNA, the PCR success rate is ∼70% because of the complexity and relatively low representation of each gene. Upon electrophoresis of first-strand PCR product the authors frequently observe a few bands in addition to those for the desired products, most likely because of the complexity of the mixture. Purification of the desired products alleviates that problem except in cases where the product bands are smears. In those cases PCR conditions should be optimized again. Generation of master clones In the authors’ experience, a >95% success rate for generating master clones from plasmid DNA can be expected, but with firststrand cDNA the situation is a bit more complicated. Generally, upon electrophoresis, the PCR product band is faint compared to those from plasmid DNA template because of a lower DNA concentration. This is a result of the lower success rate in PCR steps (∼70%) for first-strand DNA templates. Moreover, after DNA sequencing and analysis, more clones may be eliminated for the first-strand cDNA template because of more random mutations introduced during higher numbers of PCR cycles required as a result of the more complex and heterogeneous DNA template. Overall, based on the authors’ experience, a >50% success rate is expected from a first-strand cDNA cloning scheme, compared to >95% overall success rate using plasmid DNA as template. Generation of expression clones The transfer reaction is robust, and a ∼95% success rate is expected. The few ineffective transfers should be easily rescuable after an additional round of transfer reaction. Their failure is most often due to random losses during plasmid DNA preparation in the 96well format. Using highly competent cells (2 × 108 cfu/µg DNA competency), at least fifty colonies in a 10-cm petri dish are normally obtained from a standard bacterial transformation protocol. Because the entire process of transferring inserts into expression vectors is very robust, it is easily automated. In addition, because the reaction does not involve any amplification or rearrangements, if the insert DNA sequence has been fully verified and is in-frame with the recombination sites, it is not necessary to resequence the insert DNA sequence again after constructing expression clones.
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Any functioning expression vector can be converted to accept inserts in the Gateway or Creator systems. It should be noted that the final expression vector is in the same reading frame, which will ensure that all fusion proteins will be in-frame. The expression vector should not use the same antibiotic marker as those of the master vector.
Time Considerations The basic procedures can be completed in ∼1 week with activities broken down as follows: 1 day for PCR and gel plugging, 1 day for capture reactions and transformation, 1 day for colony picking and DNA minipreps, 1 to 2 days for sequencing, 1 day for transfer of ORF into expression vector, and 1 to 2 days for DNA preparation and analysis.
Literature Cited Abremski, K. and Hoess, R. 1984. Bacteriophage P1 site-specific recombination. Purification and properties of the Cre recombinase protein. J. Biol. Chem. 259:1509-1514. Bernard, P. and Couturier, M. 1992. Cell killing by the F plasmid CcdB protein involves poisoning of DNA-topoisomerase II complexes. J. Mol. Biol. 226:735-745. Brizuela, L., Richardson, A., Marsischky, G., and LaBaer, J. 2002. The FLEXGene repository: Exploiting the fruits of the genome projects by creating a needed resource to face the challenges of the post-genomic era. Arch. Med. Res. 33:318324. Hartley, J.L., Temple, G.F., and Brasch, M.A. 2000. DNA cloning using in vitro site-specific recombination. Genome Res. 10:1788-1795. Landy, A. 1989. Dynamic, structural, and regulatory aspects of lambda site-specific recombination. Annu. Rev. Biochem. 58:913-949. Marsischky, G. and LaBaer, J. 2004. Many paths to many clones: A comparative look at highthroughput cloning methods. Genome Res. 14:2020-2028. Nash, H.A. 1977. Integration and excision of bacteriophage lambda. Curr. Top. Microbiol. Immunol. 78:171-199. Nash, H.A. and Robertson, C.A. 1981. Purification and properties of the Escherichia coli protein factor required for lambda integrative recombination. J. Biol. Chem. 256:9246-9253. Pelicic, V., Reyrat, J.M., and Gicquel, B. 1996. Expression of the Bacillus subtilis sacB gene confers sucrose sensitivity on mycobacteria. J. Bacteriol. 178:1197-1199. Ptashne, M. 1992. A Genetic Switch: Phage (Lamda) and Higher Organisms. Cell Press, Cambridge, MA.
Reboul, J., Vaglio, P., Tzellas, N., Thierry-Mieg, N., Moore, T., Jackson, C., Shin-I, T., Kohara, Y., Thierry-Mieg, D., Thierry-Mieg, J., Lee, H., Hitti, J., Doucette-Stamm, L., Hartley, J.L., Temple, G.F., Brasch, M.A., Vandenhaute, J., Lamesch, P.E., Hill, D.E., and Vidal, M. 2001. Open-reading-frame sequence tags (OSTs) support the existence of at least 17,300 genes in C. elegans. Nat. Genet. 27:332-336. Reboul, J., Vaglio, P., Rual, J.F., Lamesch, P., Martinez, M., Armstrong, C.M., Li, S., Jacotot, L., Bertin, N., Janky, R., Moore, T., Hudson, J.R. Jr, Hartley, J.L., Brasch, M.A., Vandenhaute, J., Boulton, S., Endress, G.A., Jenna, S., Chevet, E., Papasotiropoulos, V., Tolias, P.P., Ptacek, J., Snyder, M., Huang, R., Chance, M.R., Lee, H., Doucette-Stamm, L., Hill, D.E., and Vidal, M. 2003. C. elegans ORFeome version 1.1: Experimental verification of the genome annotation and resource for proteome-scale protein expression. Nat. Genet. 34:35-41. Sauer, B. 1994. Site-specific recombination: Developments and applications. Curr. Opin. Biotechnol. 5:521-527. Sternberg, N., Hamilton, D., Austin, S., Yarmolinsky, M., and Hoess, R. 1981. Site-specific recombination and its role in the life cycle of bacteriophage P1. Cold Spring Harb. Symp. Quant. Biol. 45:297-309. Walhout, A.J., Temple, G.F., Brasch, M.A., Hartley, J.L., Lorson, M.A, van den Heuvel, S., and Vidal, M. 2000. GATEWAY recombinational cloning: Application to the cloning of large numbers of open reading frames or ORFeomes. Methods Enzymol. 328:575592. Weisberg, R.A. and Landy, A. 1983. Site-Specific Recombination in Phage Lamda. In Lamda II (R.A. Weisberg, ed.) pp. 211-250. Cold Spring Harbor Press, Cold Spring Harbor, NY. Yamada, K., Lim, J., Dale, J.M., Chen, H., Shinn, P., Palm, C.J., Southwick, A.M., Wu, H.C., Kim, C., Nguyen, M., Pham, P., Cheuk, R., KarlinNewmann, G., Liu, S.X., Lam, B., Sakano, H., Wu, T., Yu, G., Miranda, M., Quach, H.L., Tripp, M., Chang, C.H., Lee, J.M., Toriumi, M., Chan, M.M., Tang, C.C., Onodera, C.S., Deng, J.M., Akiyama, K., Ansari, Y., Arakawa, T., Banh, J., Banno, F., Bowser, L., Brooks, S., Carninci, P., Chao, Q., Choy, N., Enju, A., Goldsmith, A.D., Gurjal, M., Hansen, N.F., Hayashizaki, Y., Johnson-Hopson, C., Hsuan, V.W., Iida, K., Karnes, M., Khan, S., Koesema, E., Ishida, J., Jiang, P.X., Jones, T., Kawai, J., Kamiya, A., Meyers, C., Nakajima, M., Narusaka, M., Seki, M., Sakurai, T., Satou, M., Tamse, R., Vaysberg, M., Wallender, E.K., Wong, C., Yamamura, Y., Yuan, S., Shinozaki, K., Davis, R.W., Theologis, A., and Ecker, J.R. 2003. Empirical analysis of transcriptional activity in the Arabidopsis genome. Science 302:842846. Enzymatic Manipulation of DNA and RNA
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Internet Resources http://www.invitrogen.com/gateway Highly recommended website for more information and troubleshooting related to Invitrogen Gateway Technology. http://www.clontech.com/clontech/products/ families/creator Highly recommended website for more information and troubleshooting related to BD Clontech Creator Technology.
Contributed by Jaehong Park and Joshua LaBaer Harvard Medical School Cambridge, Massachusetts
Recombinational Cloning
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CHAPTER 4 Preparation and Analysis of RNA INTRODUCTION
T
he ability to isolate clean intact RNA from cells is essential for experiments that measure transcript levels, for cloning of intact cDNAs, and for functional analysis of RNA metabolism. RNA isolation procedures frequently must be performed on numerous different cell samples, and therefore are designed to allow processing of multiple samples simultaneously. This chapter begins by describing several methods commonly used to isolate RNA, and concludes with methods used to analyze RNA expression levels and synthesis rates. The difficulty in RNA isolation is that most ribonucleases are very stable and active enzymes that require no cofactors to function. The first step in all RNA isolation protocols therefore involves lysing the cell in a chemical environment that results in denaturation of ribonuclease. The RNA is then fractionated from the other cellular macromolecules under conditions that limit or eliminate any residual RNase activity. The cell type from which RNA is to be isolated and the eventual use of that RNA will determine which procedure is appropriate. No matter which procedure is used, it is important that the worker use care (e.g., wearing gloves) not to introduce any contamination that might include RNase during work up of the samples, and particularly when the samples are prepared for storage at the final step. While the RNA isolation protocols describe methods that can be performed using common laboratory reagents, several kits for RNA isolation are commercially available. These kits offer the dual advantage of ease of use and (at least in theory) of reagents that have been tested for effectiveness. These kits frequently work well and are widely used. The disadvantages of using kits are that they are more expensive per sample than isolations that are done using “home made” solutions, and that the kits do not offer flexibility for cell types that require special conditions. The cost disadvantage is frequently outweighed in situations where only a few RNA isolations are performed; however, preparing reagents from scratch can take time, and in the event that any of the reagents are not working properly, troubleshooting will require further time. In situations where numerous samples are routinely processed, significant cost savings can be realized by avoiding the use of kits. One of the primary uses of RNA isolation procedures is the analysis of gene expression. In order to elucidate the regulatory properties of a gene, it is necessary to know the structure and amount of the RNA produced from that gene. The second part of this chapter is devoted to techniques that are used to analyze RNA. Procedures such as S1 nuclease analysis and ribonuclease protection can be used to do fine-structure mapping of any RNA. These techniques allow characterization of 5′ and 3′ splice junctions as well as the 5′ and 3′ ends of RNA. Both of these procedures, as well as northern analysis, can also be used to accurately determine the steady-state level of any particular message. After determining the steady-state level of a message, many investigators wish to examine whether that level is set by the rate of transcription of the gene. Alterations Preparation and Analysis of RNA Contributed by Robert E. Kingston Current Protocols in Molecular Biology (2002) 4.0.1-4.0.2 Copyright © 2002 by John Wiley & Sons, Inc.
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in steady-state level might also reflect changes in processing or stability of the RNA. The final section of the chapter describes the “nuclear run-off” technique, which determines the number of active RNA polymerase molecules that are traversing any particular segment of DNA. This procedure is used to analyze directly how the rate of transcription of a gene varies when the growth state of a cell is changed. Robert E. Kingston
Introduction
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PREPARATION OF RNA FROM EUKARYOTIC AND PROKARYOTIC CELLS
SECTION I
Three methods are presented for preparing RNA from eukaryotic cells. The first two are rapid and can be used to prepare several RNA samples at the same time. As written, they are intended for production of RNA to be used for analysis by S1 nuclease or ribonuclease protection. Modifications of each protocol are given that should be used if intact fulllength RNA is a priority. Both of these protocols require limited hands-on time. The first (UNIT 4.1) utilizes a gentle detergent to lyse the cell. Its main advantage is that it requires no high-speed centrifuge spins, thus allowing preparation of numerous samples without having to find several high-speed rotors. In the second protocol (UNIT 4.2) cells are lysed using 4 M guanidinium isothiocyanate. This protocol requires very few manipulations, gives clean RNA from many sources, and is the method of choice when working with tissues that have high levels of endogenous RNase. It does require a high-speed centrifuge run, which limits the number of samples that can be prepared at the same time. In the third protocol (UNIT 4.3) the cell is lysed with phenol and SDS. It produces clean, full-length RNA from large quantities of plant cells. This protocol also works well with several mammalian cells and tissues. All three protocols can be used with cells from any higher eukaryote. In particular, many laboratories use the guanidinium procedure when preparing RNA from plant tissue. These protocols produce total RNA, which contains primarily ribosomal RNA and transfer RNA. Many techniques require messenger RNA that is largely free of contaminating rRNA and tRNA. The isolation of poly(A)+ RNA, which is highly enriched for mRNA, is described in UNIT 4.5. A protocol is also presented for extracting RNA from gram-negative and gram-positive bacteria (UNIT 4.4). The major source of failure in any attempt to produce RNA is contamination by ribonuclease. RNases are very stable enzymes and generally require no cofactors to function. Therefore, a small amount of RNase in an RNA preparation will create a real problem. To avoid contamination problems, the following precautions can be taken: 1. Solutions. Any water or salt solutions used in RNA preparation should be treated with the chemical diethylpyrocarbonate (DEPC). This chemical inactivates ribonucleases by covalent modification. Solutions containing Tris cannot be effectively treated with DEPC because Tris reacts with DEPC to inactivate it. See Reagents and Solutions for instructions regarding DEPC treatment. 2. Glassware and plastic. Labware used in the preparation of RNA should be treated to remove residual RNase activity. Autoclaving will not fully inactivate many RNases. Glassware can be baked at 300°C for 4 hr. Certain kinds of plasticware (e.g., some conical centrifuge tubes and pipets) can be rinsed with chloroform to inactivate RNase. When done carefully, this rinse is an effective treatment. Keep in mind, however, that many plastics (e.g., gel boxes) will melt when treated with chloroform. Plasticware straight out of the package is generally free from contamination and can be used as is. 3. Hands are a major source of contaminating RNase. Wear gloves.
Preparation and Analysis of RNA Contributed by Michael Gilman Current Protocols in Molecular Biology (2002) 4.1.1-4.1.5 Copyright © 2002 by John Wiley & Sons, Inc.
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UNIT 4.1
BASIC PROTOCOL
Preparation of Cytoplasmic RNA from Tissue Culture Cells Cells are washed with ice-cold phosphate-buffered saline and kept on ice for all subsequent manipulations. The pellet of harvested cells is resuspended in a lysis buffer containing the nonionic detergent Nonidet P-40. Lysis of the plasma membranes occurs almost immediately. The intact nuclei are removed by a brief microcentrifuge spin, and sodium dodecyl sulfate (SDS) is added to the cytoplasmic supernatant to denature protein. Protein is digested with protease and removed by extractions with phenol/chloroform and chloroform. Cytoplasmic RNA is recovered by ethanol precipitation and quantitated by measuring its absorbance at 260 and 280 nm. Materials Cell monolayer or suspension Phosphate-buffered saline, ice cold (PBS; APPENDIX 2) Lysis buffer, ice cold (see recipe) 20% (w/v) sodium dodecyl sulfate (SDS) 20 mg/ml proteinase K 25:24:1 phenol/chloroform/isoamyl alcohol (UNIT 2.1A) 24:1 chloroform/isoamyl alcohol 3 M DEPC-treated (see recipe) sodium acetate, pH 5.2 (APPENDIX 2) 100% ethanol 75% ethanol/25% 0.1 M DEPC-treated (see recipe) sodium acetate, pH 5.2 Beckman JS-4.2 rotor or equivalent Rubber policeman Additional reagents and equipment for removing contaminating DNA (see Support Protocol) CAUTION: Diethylpyrocarbonate (DEPC) is a suspected carcinogen and should be handled carefully (APPENDIX 1H). IMPORTANT NOTE: Water and sodium acetate should be treated with DEPC (see recipe). Wash cells 1a. For monolayer cells: Rinse cell monolayer three times with ice-cold PBS. Scrape cells into a small volume of cold PBS with a rubber policeman. Transfer to a centrifuge tube on ice. Collect cells by centrifuging in a Beckman JS-4.2 rotor 5 min at 300 × g (1000 rpm), 4°C, or in a microcentrifuge 15 sec at maximum speed, 4°C. For a 10-cm dish, collect cells in 1 ml PBS. For a 15-cm dish collect in 3 to 5 ml PBS.
1b. For suspension cultures: Pellet by centrifuging in a Beckman JS-4.2 rotor 5 min at 300 × g (1000 rpm), 4°C. Resuspend pellet in one-half original culture volume ice-cold PBS. Repeat once. This procedure, as written, is used for up to 2 × 107 cells (two 10-cm dishes or ∼20 ml suspension culture). The procedure can be scaled up for larger cell quantities by increasing volumes appropriately and using larger, conical tubes.
Preparation of Cytoplasmic RNA from Tissue Culture Cells
Lyse cells 2. Resuspend cells in 375 µl ice-cold lysis buffer. Incubate 5 min on ice. The suspension should clear rapidly, indicating cell lysis.
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Cells are best suspended by careful but vigorous vortexing. Avoid foaming.
3. If the cells are not already in a microcentrifuge tube, transfer them into one. Microcentrifuge 2 min at maximum speed, 4°C. 4. Transfer supernatant to a clean tube containing 4 µl of 20% SDS. Mix immediately by vortexing. The supernatant is the cytoplasmic extract. It is usually slightly cloudy and yellow-white, depending on the cells. The pellet, which contains nuclei and some cell debris, should be considerably smaller than the whole cell pellet obtained in step 1a or 1b and white in color.
5. Add 2.5 µl of 20 mg/ml proteinase K. Incubate 15 min at 37°C. Extract with phenol/chloroform/isoamyl and chloroform/alcohol 6. Add 400 µl of 25:24:1 phenol/chloroform/isoamyl alcohol, room temperature. Vortex thoroughly—i.e., at least 1 min. Microcentrifuge ≥5 min at maximum speed, room temperature. With protease treatment, there should be only a small amount of precipitate at the interface between the two phases, although this can vary depending on the cell type. For some cells, the protease step can be safely omitted. In this case, the white precipitate at the interface can be considerable. If a very large precipitate forms after the first organic extraction and little or no aqueous phase can be recovered, first try spinning for a few minutes more. If the precipitate fails to collapse to the interface, remove and discard the organic phase from the bottom of the tube. Add 400 ìl chloroform/isoamyl alcohol. Vortex well and spin ∼2 min. The precipitate should have largely disappeared. Recover the upper aqueous phase and proceed.
7. Transfer the aqueous (upper) phase to a clean tube, avoiding precipitated material from the interface. Add 400 µl phenol/chloroform/isoamyl alcohol and repeat the extraction. 8. Transfer the aqueous phase to a clean tube. Add 400 µl of 24:1 chloroform/isoamyl alcohol. Vortex 15 to 30 sec and microcentrifuge 1 min at maximum speed, room temperature. 9. Again, transfer the aqueous (upper) phase to a clean tube. Precipitate RNA 10. Add 40 µl of 3 M DEPC-treated sodium acetate, pH 5.2, and 1 ml of 100% ethanol. Mix by inversion. Incubate 15 to 30 min on ice or store at −20°C overnight. 11. Recover the RNA by microcentrifuging for 15 min at maximum speed, 4°C. 12. If necessary, remove contaminating DNA (see Support Protocol). 13. Rinse the pellet with 1 ml of 75% ethanol/25% 0.1 M sodium acetate, pH 5.2 solution. Analyze purity 14. Dry and resuspend in 100 µl DEPC-treated water. Dilute 10 µl into 1 ml water to determine the A260 and A280. Store the remaining RNA at −70°C.
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SUPPORT PROTOCOL
REMOVAL OF CONTAMINATING DNA If RNA is isolated from cells transiently transfected with cloned DNA, substantial amounts of this DNA will copurify with the RNA in this procedure. This contaminating DNA will interfere with analysis of the RNA by nuclease protection assays, especially if uniformly labeled probes are used. To remove this DNA, perform the following steps after step 11 of the preparation (see Basic Protocol). Additional Materials (also see Basic Protocol) TE buffer, pH 7.4 (APPENDIX 2) 100 mM MgCl2/10 mM dithiothreitol (DTT; see APPENDIX 2 for both components) 2.5 mg/ml RNase-free DNase I (see recipe) 25 to 50 U/µl placental ribonuclease inhibitor (e.g., RNAsin from Promega Biotec) or vanadyl-ribonucleoside complex DNase stop mix (see recipe) 1. Redissolve the RNA in 50 µl TE buffer, pH 7.4. 2. Prepare on ice a cocktail containing (per sample) 10 µl of 100 mM MgCl2/10 mM DTT, 0.2 µl of 2.5 mg/ml RNase-free DNase, 0.1 µl of 25 to 50 U/µl placental ribonuclease inhibitor or vanadyl-ribonucleoside complex, and 39.7 µl TE buffer. Add 50 µl of this cocktail to each RNA sample. Mix and incubate 15 min at 37°C. 3. Stop the DNase reaction by adding 25 µl DNase stop mix. 4. Extract once with phenol/chloroform/isoamyl alcohol and once with chloroform/ isoamyl alcohol. 5. Add 325 µl of 100% ethanol and precipitate 15 to 30 min on ice or overnight at −20°C. Resume Basic Protocol at step 13.
REAGENTS AND SOLUTIONS Diethylpyrocarbonate (DEPC) treatment of solutions Add 0.2 ml DEPC to 100 ml of the solution to be treated. Shake vigorously to get the DEPC into solution. Autoclave the solution to inactivate the remaining DEPC. Many investigators keep the solutions they use for RNA work separate to ensure that “dirty” pipets do not go into them. CAUTION: Wear gloves and use a fume hood when using DEPC, as it is a suspected carcinogen (also see APPENDIX 1H).
DNase stop mix 50 mM EDTA 1.5 M sodium acetate 1% (w/v) sodium dodecyl sulfate (SDS) The SDS may come out of solution at room temperature. Heat briefly to redissolve.
Lysis buffer 50 mM Tris⋅Cl, pH 8.0 (APPENDIX 2) 100 mM NaCl 5 mM MgCl2 0.5% (v/v) Nonidet P-40 Prepare with DEPC-treated H2O (see recipe) Filter sterilize Preparation of Cytoplasmic RNA from Tissue Culture Cells
If the RNA is to be used for northern blot analysis or the cells are particularly rich in ribonuclease, add ribonuclease inhibitors to the lysis buffer: 1000 U/ml placental ribonuclease inhibitor (e.g., RNAsin) plus 1 mM DTT or 10 mM vanadyl-ribonucleoside complex.
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RNase-free DNase I Commercially prepared enzymes such as Worthington grade DPRF are satisfactory. If supplied as a powder, redissolve in TE buffer containing 50% (v/v) glycerol and store at −20°C. See also UNIT 4.10, reagents and solutions, for homemade preparation of RNase-free DNase. COMMENTARY Background Information Most procedures for isolating RNA from eukaryotic cells involve lysing and denaturing cells to liberate total nucleic acids. Additional steps are then required to remove DNA. This procedure allows rapid preparation of total cytoplasmic RNA by using a nonionic detergent to lyse the plasma membrane, leaving the nuclei intact. The nuclei and hence the bulk of the cellular DNA are then removed with a simple brief centrifugation. Variations of this technique are in wide use, and its precise origins are obscure. Versions of this procedure were used in some of the early S1 nuclease mapping papers (Berk and Sharp, 1977; Favoloro et al., 1980). The protocol described here is a considerable simplification of the earlier methods, omitting, for example, removal of nuclei by centrifugation through sucrose. It is fast and streamlined, designed for preparing total cytoplasmic RNA from many cultures simultaneously for nuclease protection analysis. It is scaled for small cultures—i.e., 1 to 2 dishes of adherent cells or 10 to 20 ml of suspension culture. The Basic Protocol works well for many cell types. The protocol takes no special precautions for ribonucleases and may not yield northern blot-quality RNA from some cells. If fulllength RNA is required, ribonuclease inhibitors should be added to the lysis buffer (as described; see Reagents and Solutions) or the guanidinium isothiocyanate method should be used (UNIT 4.2). Finally, if RNA is isolated from transiently transfected cells, the RNA should be further treated with deoxyribonuclease to remove transfected DNA (see Support Protocol). This modification is especially critical if the RNA is to be assayed by nuclease protection using uniformly labeled probes.
Troubleshooting Degradation of RNA by ribonuclease is best avoided by working quickly and keeping everything cold until SDS is added to the cytoplasmic
extract. For cells with which ribonuclease is a problem, inhibitors can be added to the lysis buffer (see Reagents and Solutions), but in most cases this is unnecessary. Note that for some cell lines, it may be possible to omit the proteinase K step and to proceed directly to organic extraction after removal of the nuclei and addition of SDS. DNA contamination is only a problem when preparing RNA from cells transfected with cloned DNA in a transient expression assay. In this case, add the DNase digestion steps outlined in the Support Protocol.
Anticipated Results Yields vary widely, depending on the cell line. Expect 30 to 100 µg from a confluent 10-cm dish of most fibroblast lines or 1 × 107 lymphoid cells. Ratios of A260 to A280 should fall in the range 1.7 to 2.0. RNA at 1 mg/ml has an A260 of 25.
Time Considerations Depending on the number of samples being processed, it is possible to proceed from harvesting the cells to the ethanol precipitation step in 1 to 2 hr. This is the best interim stopping point. The RNA may be recovered, redissolved, and quantitated later the same day or the following day.
Literature Cited Berk, A. J. and Sharp, P. A. 1977. Sizing and mapping of early adenovirus mRNAs by gel electrophoresis of S1 endonuclease-digested hybrids. Cell 12: 721-732. Favoloro, J., Treisman, R., and Kamen, R. 1980. Transcription maps of polyoma virus-specific RNA: Analysis by two-dimensional nuclease S1 gel mapping. Meth. Enzymol. 65:718-749.
Contributed by Michael Gilman Cold Spring Harbor Laboratory Cold Spring Harbor, New York
Preparation and Analysis of RNA
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Guanidine Methods for Total RNA Preparation
UNIT 4.2
Three different methods for RNA preparation using guanidine are presented in this unit — a single-step isolation method employing liquid-phase separation to selectively extract total RNA from tissues and cultured cells (see Basic Protocol) and two methods that rely on a CsCl step gradient to isolate total RNA (see Alternate Protocols 1 and 2). SINGLE-STEP RNA ISOLATION FROM CULTURED CELLS OR TISSUES Cultured cells or tissues are homogenized in a denaturing solution containing 4 M guanidine thiocyanate. The homogenate is mixed sequentially with 2 M sodium acetate (pH 4), phenol, and finally chloroform/isoamyl alcohol or bromochloropropane. The resulting mixture is centrifuged, yielding an upper aqueous phase containing total RNA. In this single-step extraction the total RNA is separated from proteins and DNA that remain in the interphase and in the organic phase. Following isopropanol precipitation, the RNA pellet is redissolved in denaturing solution (containing 4 M guanidine thiocyanate), reprecipitated with isopropanol, and washed with 75% ethanol.
BASIC PROTOCOL
Materials Denaturing solution (see recipe) 2 M sodium acetate, pH 4 (see recipe) Water-saturated phenol (see recipe) 49:1 (v/v) chloroform/isoamyl alcohol or bromochloropropane 100% isopropanol 75% ethanol (prepared with DEPC-treated water; UNIT 4.1) DEPC-treated water (UNIT 4.1) or freshly deionized formamide (see recipe) Glass Teflon homogenizer 5-ml polypropylene centrifuge tube Sorvall SS-34 rotor (or equivalent) CAUTION: Phenol is a poison and causes burns. When handling phenol, use gloves and eye protection. NOTE: Carry out all steps at room temperature unless otherwise stated. Homogenize cells 1a. For tissue: Add 1 ml denaturing solution per 100 mg tissue and homogenize with a few strokes in a glass Teflon homogenizer. 1b. For cultured cells: Either centrifuge suspension cells and discard supernatant, or remove the culture medium from cells grown in monolayer cultures. Add 1 ml denaturing solution per 107 cells and pass the lysate through a pipet seven to ten times. Do not wash cells with saline. Cells grown in monolayer cultures can be lysed directly in the culture dish or flask. The procedure can be carried out in sterile, disposable, round-bottom polypropylene tubes with caps; no additional treatment of the tubes is necessary. Before using, test if the tubes can withstand centrifugation at 10,000 × g with the mixture of denaturing solution and phenol/chloroform.
2. Transfer the homogenate into a 5-ml polypropylene tube. Add 0.1 ml of 2 M sodium acetate, pH 4, and mix thoroughly by inversion. Add 1 ml water-saturated phenol, Contributed by Robert E. Kingston, Piotr Chomczynski, and Nicoletta Sacchi Current Protocols in Molecular Biology (1996) 4.2.1-4.2.9 Copyright © 2000 by John Wiley & Sons, Inc.
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mix thoroughly, and add 0.2 ml of 49:1 chloroform/isoamyl alcohol or bromochloropropane. Mix thoroughly and incubate the suspension 15 min at 0° to 4°C. Make sure that caps are tightly closed when mixing. The volumes used are per 1 ml denaturing solution. Bromochloropropane is less toxic than chloroform and its use for phase separation decreases possibility of contaminating RNA with DNA (Chomczynski and Mackey, 1995).
3. Centrifuge 20 min at 10,000 × g (9000 rpm in SS-34 rotor), 4°C. Transfer the upper aqueous phase to a clean tube. The upper aqueous phase contains the RNA, whereas the DNA and proteins are in the interphase and lower organic phase. The volume of the aqueous phase is ∼1 ml, equal to the initial volume of denaturing solution.
Isolate RNA 4. Precipitate the RNA by adding 1 ml (1 vol) of 100% isopropanol. Incubate the samples 30 min at −20°C. Centrifuge 10 min at 10,000 × g, 4°C, and discard supernatant. For isolation of RNA from tissues with a high glycogen content (e.g., liver), a modification of the single-step method is recommended to diminish glycogen contamination (Puissant and Houdebine, 1990). Following this isopropanol precipitation, wash out glycogen from the RNA pellet by vortexing in 4 M LiCl. Sediment the insoluble RNA 10 min at 5000 × g. Dissolve the pellet in denaturing solution and follow the remainder of the protocol.
5. Dissolve the RNA pellet in 0.3 ml denaturing solution and transfer into a 1.5-ml microcentrifuge tube. 6. Precipitate the RNA with 0.3 ml (1 vol) of 100% isopropanol for 30 min at −20°C. Centrifuge 10 min at 10,000 × g, 4°C, and discard supernatant. 7. Resuspend the RNA pellet in 75% ethanol, vortex, and incubate 10 to 15 min at room temperature to dissolve residual amounts of guanidine contaminating the pellet. 8. Centrifuge 5 min at 10,000 × g, 4°C, and discard supernatant. Dry the RNA pellet in a vacuum for 5 min. Do not let the RNA pellet dry completely, as this greatly decreases its solubility. Avoid drying the pellet by centrifugation under vacuum. Drying is not necessary for solubilization of RNA in formamide.
9. Dissolve the RNA pellet in 100 to 200 µl DEPC-treated water or freshly deionized formamide by passing the solution a few times through a pipe tip. Incubate 10 to 15 min at 55° to 60°C. Store RNA dissolved in water at −70°C and RNA dissolved in formamide at either −20° or −70°C. RNA dissolved in formamide is protected from degradation by RNase and can be used directly for formaldehyde-agarose gel electrophoresis in northern blotting (Chomczynski, 1992). However, before use in RT-PCR, RNA should be precipitated from formamide by adding 4 vol ethanol and centrifuging 5 min at 10,000 × g.
Quantitate RNA 10. Quantitate RNA by diluting 5 µl in 1 ml alkaline water and reading the A260 and A280 (APPENDIX 3D).
Guanidine Methods for Total RNA Preparation
Water used for spectrophotometric measurement of RNA should have pH >7.5. Acidic pH affects the UV absorption spectrum of RNA and significantly decreases its A260/A280 ratio (Willfinger et al., 1996). Typically, distilled water has pH 8 hr). Because the RNA probe cannot reanneal as can a double-stranded DNA probe in an S1 nuclease assay, hybridization temperature is not absolutely critical. For each probe, however, there is an optimal temperature, perhaps because of secondary structures that can form in the probe. A good temperature to try is 45°C, but it is advisable to test a range of temperatures from 30° to 60°C. For some RNAs, hybridization may be complete in as little as 4 to 6 hr.
Digest the ribonuclease 16. Add to each hybridization reaction 350 µl ribonuclease digestion buffer containing 40 µg/ml ribonuclease A and 2 µg/ml ribonuclease T1. Incubate 30 to 60 min at 30°C. The final hybridization signal is relatively insensitive to changes in these incubation conditions. However, if problems are suspected with this step, incubation temperatures ranging from 15° to 37°C should be tested. If internal cleavage of the RNA duplexes is observed, omit ribonuclease A (see commentary).
17. Add 10 µl of 20% (w/v) SDS and 2.5 µl of 20 mg/ml proteinase K. Incubate 15 min at 37°C. 18. Extract once with 400 µl phenol/chloroform/isoamyl alcohol, removing the aqueous phase to a clean microcentrifuge tube containing 1 µl of 10 mg/ml yeast tRNA. 19. Add 1 ml ethanol and precipitate. 20. Dry pellet and redissolve in 3 to 5 µl RNA loading buffer. 21. Incubate 3 min at 85°C to denature. 22. Analyze on a denaturing polyacrylamide/urea (sequencing) gel. RNA has a lower mobility in these gels than DNA of the same length. If DNA markers are used to estimate the size of a protected RNA fragment, the correct size is 5% to 10% smaller than this estimate. For example, if an RNA species runs with a DNA marker of 100 nucleotides, its actual length is 90 to 95 nucleotides.
GEL PURIFICATION OF RNA PROBES The following protocol is used for purification of full-length probe, which may be necessary under certain conditions.
SUPPORT PROTOCOL
Additional Materials TBE buffer (APPENDIX 2) Elution buffer 1. Dry the RNA after the first ethanol precipitation and redissolve in 10 µl RNA loading buffer. It is important that the RNA be completely dried and then fully redissolved. Preparation and Analysis of RNA
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2. Heat 5 min at 85°C to denature the RNA. 3. Load onto a gel containing 6% polyacrylamide (29:1 acrylamide/bisacrylamide) and TBE buffer. The gel is 0.4 mm thick and 14 cm long (sequencing-length gels may also be used). Run at 300 V (higher for longer gels) until the bromphenol blue dye has run one-half to two-thirds down the gel. Nondenaturing gels are used in the interest of speed, but it is important that the RNA be fully denatured before loading. Denaturing gels containing urea may be used, if desired.
4. Disassemble the gel, leaving it on one plate. Wrap in plastic wrap, mark with hot ink, and expose to film for 30 sec. Using the film as a template, excise the full-length RNA band. Don’t be greedy. The RNA actually occupies a narrower band than the band on the film. Cleaner hybridizations result from smaller gel slices.
5. Elute the RNA in 400 µl elution buffer and shake 2 to 4 hr at 37°C. The elution buffer generally precipitates at room temperature. Warm briefly before use. Small probes (400 nucleotides) require longer elution times, but amounts sufficient for the experiment may elute in 2 hr.
6. Remove the eluate to a fresh microcentrifuge tube and add 1 ml of 100% ethanol. 7. Incubate 15 min on ice and spin 15 min in a microcentrifuge. Monitor the eluate and the gel slice. There should be more counts in the former. All counts in the eluate should precipitate with ethanol.
8. Redissolve the RNA pellet in 50 µl hybridization buffer and count 1 µl in a liquid scintillation counter. Yields will be lower than those obtained without gel purification, but this procedure should yield sufficient probe for more than 50 hybridizations. Background will be substantially lower. SUPPORT PROTOCOL
PREPARATION OF TEMPLATE DNA Template DNA is prepared by inserting the sequences of interest into a plasmid vector carrying a bacteriophage promoter. Strategies for constructing such plasmids are discussed in the commentary. Vectors containing bacteriophage promoters are commercially available. Digest the DNA with a restriction enzyme (Table 3.1.1) that cuts immediately downstream of the probe sequence. This allows the generation of a uniquely sized runoff transcript. The enzyme chosen may cut the plasmid in several places as long as it does not cut within the phage promoter, the probe sequence, or intervening vector DNA. Restriction enzymes that generate 5′ overhangs are best. (Do not cut with enzymes that leave 3′ overhangs because these overhangs serve as initiation sites for the polymerase, leading to synthesis of RNA complementary to the probe.) Cut the DNA to completion but do not grossly overdigest. Extract cut DNA with phenol/chloroform, precipitate with ethanol, and redissolve at 0.5 mg/ml in RNase-free TE buffer. As little as 100 ng of template DNA will yield a reasonable amount of probe.
Ribonuclease Protection Assay
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Current Protocols in Molecular Biology
REAGENTS AND SOLUTIONS Bacteriophage RNA polymerase SP6, T3, or T7, depending on the vector in which probe sequences are cloned. All are functionally equivalent. […-32P]CTP Purchased commercially as an aqueous solution at a concentration of 10 mCi/ml. Specific activities of 400 to 800 Ci/mmol should be used. Higher specific activities yield a probe too unstable to use. Labeled GTP or UTP may be substituted (see commentary). Be sure to use an appropriate mix of the three unlabeled nucleoside triphosphates.
Elution buffer 2 M ammonium acetate 1% SDS 25 µg/ml tRNA Hybridization buffer 5× stock solution: 200 mM PIPES, pH 6.4 2 M NaCl 5 mM EDTA
Working solution: 4 parts formamide 1 part 5× stock buffer
Prepare hybridization buffer fresh as needed from frozen 5× stock and formamide freshly deionized by vortexing with mixed bed resin beads. Alternatively, store in small aliquots at −70°C.
3NTP mix 4 mM each ATP, UTP, and GTP 0.5 mM EDTA, pH 8 Store at −20°C Ribonuclease digestion buffer 10 mM Tris⋅Cl, pH 7.5 300 mM NaCl 5 mM EDTA Add 1⁄50 vol of 50× ribonuclease mix: 2 mg/ml ribonuclease A 0.1 mg/ml ribonuclease T1 RNase digestion buffer may be stored at room temperature. Add RNases from frozen stocks as needed. Be sure to use disposable tubes to make up this reagent.
RNA loading buffer 80% (v/v) formamide 1 mM EDTA, pH 8.0 0.1% bromphenol blue 0.1% Xylene Cyanol Do not use Maxam-Gilbert loading buffer or any buffer containing NaOH.
5× transcription buffers T7, T3 RNA polymerases: 200 mM Tris⋅Cl, pH 8 40 mM MgCl2 10 mM spermidine 250 mM NaCl
SP6 RNA polymerase: 200 mM Tris⋅Cl, pH 7.5 30 mM MgCl2 10 mM spermidine
Use freshly prepared DEPC-treated water (see UNIT 4.1).
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COMMENTARY Background Information Bacteriophage RNA polymerases possess several properties that make them well suited for the preparation of high-specific-activity hybridization probes (for review, see Chamberlin and Ryan, 1982). They are single subunit enzymes that are relatively stable and easy to purify. Moreover, because their genes reside on phage genomes, they have been fairly straightforward to clone and express in uninfected E. coli, thereby increasing the ease and economy of purification. Second, they polymerize RNA at an exceedingly high rate—200 to 300 nucleotides per minute—approximately 10 times faster than E. coli RNA polymerase and faster than DNA polymerases. Thus, large amounts of probe are easily prepared. Third, they are very specific in their action, recognizing fairly long promoter sequences that are unlikely to appear fortuitously in other DNA. Therefore, the probes are very homogeneous in sequence and usually require little further purification. To make a probe for use in this assay, it is first necessary to subclone a fragment containing the sequences of interest downstream of a phage promoter (see Fig. 4.7.1). The sequence to be analyzed must be cloned such that the RNA produced by the phage polymerase is complementary to the RNA to be analyzed. Ideally, this construct should be able to be digested with a restriction enzyme to produce a linear template that will be transcribed into a 100- to 300-base runoff transcript. For example, if one wished to analyze the level of appropriately initiated transcription from a promoter, a DNA fragment from that promoter with endpoints at +150 and −100 could be cloned such that the +150 site is immediately adjacent to the
SP6 or T7 polymerase promoter
phage promoter. Cleaving the resultant clone at a restriction site adjacent to the −100 site would then allow runoff synthesis of a 250-base probe that would give a 150-base signal in the assay.
Literature Review This protocol is a substitute for the widely used S1 nuclease mapping technique (Berk and Sharp, 1977; Weaver and Weissman, 1979; UNIT 4.6, this manual). It was first reported in the literature by Zinn et al. (1983) and subsequently described in detail by Melton et al. (1984). These developments were made possible by the isolation and characterization of the phage-encoded RNA polymerase of the S. typhimurium phage SP6 (Butler and Chamberlin, 1982) and the mapping and cloning of a bacteriophage promoter sequence (Kassevetis et al., 1982; Melton et al., 1984). Subsequently, the RNA polymerase produced by the related E. coli phages T3 and T7 have been similarly employed. The advantages of this technique over classical S1 nuclease mapping are as follows: (1) the ease of probe preparation—gel purification is not usually required; (2) unlike end-labeled or nicktranslated probes, the specific activities of these RNA probes are fixed by the specific activity of the labeled ribonucleoside triphosphate, not by the efficiency of the enzymatic reaction; (3) the probes are prepared in large quantities and at much higher specific activity than classical end-labeled S1 probes, dramatically increasing the sensitivity of detection; (4) the probes are single-stranded and therefore cannot reanneal, and they generate more stable duplexes than would a DNA probe; and (5) treatment of RNARNA duplexes with ribonuclease is a more
test promoter
+150
-100
probe 250 nt protected 150 nt
Ribonuclease Protection Assay
Figure 4.7.1 Structure of a typical transcription template for synthesizing an RNA probe.
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reliable and reproducible reaction than is treatment of RNA-DNA hybrids with S1 nuclease, which can be sensitive to temperature and enzyme concentration. Many of these advantages may also be achieved, however, by carrying out S1 nuclease mapping with body-labeled singlestranded DNA probes (Ley et al., 1982). RNA probes may also be substituted for DNA probes in blot hybridizations (UNIT 2.9).
Troubleshooting The most common problems encountered with this procedure are probes that do not reach full length and high background in the hybridizations (i.e., excessive signal in the tRNA control). Incomplete transcripts may be caused by ribonuclease contamination or by pausing or termination of the RNA polymerase before completion of the transcript. Pausing and termination are sequence-specific phenomena which are further exacerbated by the low concentration of the labeled ribonucleoside triphosphate. They are best avoided by choosing a relatively small probe (100 to 300 nucleotides in length). The appearance of incomplete transcripts can also depend on the choice of labeled ribonucleoside triphosphate. Therefore, changing the labeled nucleotide may help. In addition, specific activity may be sacrificed by adding unlabeled ribonucleoside triphosphate to increase the absolute concentration. If the problem persists, full-length probe may be gel purified (see support protocol), taking care to avoid ribonuclease contamination. Background hybridization is most commonly caused by incomplete digestion of the template DNA in the transcription reaction. Residual DNA fragments copurify with the probe and will hybridize efficiently to the probe, generating a smear of bands which will appear in all lanes. This problem can usually be solved by trying a fresh preparation of DNase, by preparing a new batch of template DNA, or by gel purifying the probe. While the hybridization and digestion conditions listed in the protocol work satisfactorily for most probes, parameters that should be varied to optimize the signal-to-noise ratio include the hybridization temperature, amount of probe, and ribonuclease digestion conditions. A second source of background is the presence of traces of sense RNA in the probe. This RNA can arise from excess enzyme in the transcription reaction or from the use of a template cleaved with a restriction enzyme that leaves 3′ overhangs. These overhangs are in-
itiation sites for RNA polymerase. Even exceedingly small amounts of sense RNA in the antisense probe will create very high background in the hybridizations. Sense RNA contamination in the probe, whatever the cause, may be the explanation for persistent background problems. This situation is best resolved simply by gel purifying the probe (see support protocol). Another problem that occasionally arises is internal cleavage of RNA duplexes during ribonuclease digestion. This leads to a loss of the full-length protected product and the generation of small subfragments. The bacterial CAT gene, for example, contains a long run of A residues near its 5′ end. This region can “breathe” during ribonuclease treatment and then get cut by RNase A. This problem is solved simply by omitting RNase A from the digestions. Treatment with 2 µg/ml RNase T1 alone is sufficient for most reactions, although it does not allow mapping of RNA ends at highest resolution.
Anticipated Results Over half the label in the transcription reaction is typically incorporated into probe (∼108 cpm), often more. If substantially lower incorporation is achieved, the specific activity of the probe is just as high, but the yield is reduced. In principle, therefore, this probe should work just as well as probe from an efficient transcription reaction. In practice, however, greater backgrounds usually result when probes from low-incorporation reactions are used. When the probe is purified by gel electrophoresis (support protocol), yields are lower—typically 2-5 × 107 cpm—enough for at least 40 typical hybridizations. The hybridization signal resulting from a fairly low abundance mRNA in 10 µg of total RNA should be detectable with an overnight exposure using an intensifying screen. Often, shorter exposures or exposures without a screen will suffice.
Time Considerations
Probe synthesis takes ∼3 hr, mostly incubation time. With gel purification of probes, elapsed time—including probe electrophoresis and elution—is 6 to 8 hr. RNA samples for hybridization may be ethanol precipitated during this time. The probes are best used the day they are synthesized. Thus, a convenient arrangement is to prepare the probes and set up the hybridizations the same day. The following morning, nuclease digestions and gel
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electrophoresis are performed. Allow 2 to 3 hr for the digestions and subsequent purification steps.
Literature Cited Berk, A.J. and Sharp, P.A. 1977. Sizing and mapping of early adenovirus mRNAs by gel electrophoresis of S1 endonuclease-digested hybrids. Cell 12:721-732. Butler, E.T. and Chamberlin, M.J. 1982. Bacteriophage SP6-specific RNA polymerase. I. Isolation and characterization of the enzyme. J. Biol. Chem. 257:5772-5778. Chamberlin, M. and Ryan, T. 1982. Bacteriophage DNA-dependent RNA polymerase. In The Enzymes, Vol. XV (P. Boyer, ed.), pp. 87-108. Academic Press, NY. Kassevetis, G.A., Butler, E.T., Roulland, D., and Chamberlin, M.J. 1982. Bacteriophage SP6-specific RNA polymerase. II. Mapping of SP6 DNA and selective in vitro transcription. J. Biol. Chem. 257:5779-5787. Ley, T.J., Anagnou, N.P., Pepe, G., and Nienhuis, A.W. 1982. RNA processing errors in patients with β-thalassemia. Proc. Natl. Acad. Sci. U.S.A. 79:4775-4779. Melton, D.A., Krieg, P.A., Rebagliati, M.R., Maniatis, T., Zinn, K., and Green, M.R. 1984. Efficient in vitro synthesis of biologically active RNA and RNA hybridization probes from plasmids con-
taining a bacteriophage SP6 promoter. Nucl. Acids Res. 12:7035-7056. Weaver, R.F. and Weissman, C. 1979. Mapping of RNA by modification of the Berk-Sharp procedure: The 5′ termini of 15S β-globin mRNA and mature 10S β-globin mRNA have identical map coordinates. Nucl. Acids Res. 7:1175-1193. Zinn, K., DiMaio, D., and Maniatis, T. 1983. Identification of two distinct regulatory regions adjacent to the human β-interferon gene. Cell 34:865-879.
Key References Chamberlin and Ryan, 1982. See above. Summarizes properties of bacteriophage RNA polymerases and provides references to the original literature. Melton et al., 1984. See above. Describes the construction of vectors carrying promoters for SP6 RNA polymerase, use of the polymerase for preparing RNA probes and large quantities of biologically active RNA, and biochemical properties of the transcription reaction.
Contributed by Michael Gilman Cold Spring Harbor Laboratory Cold Spring Harbor, New York
Ribonuclease Protection Assay
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Primer Extension This protocol can be used to map the 5′ terminus of an RNA and to quantitate the amount of a given RNA by extending a primer using reverse transcriptase. The primer is an oligonucleotide (or restriction fragment) that is complementary to a portion of the RNA of interest. The primer is end-labeled, hybridized to the RNA, and extended by reverse transcriptase using unlabeled deoxynucleotides to form a single-stranded DNA complementary to the template RNA. The resultant DNA is analyzed on a sequencing gel. The length of the extended primer maps the position of the 5′ end of the RNA, and the yield of primer extension product reflects the abundance of the RNA.
UNIT 4.8 BASIC PROTOCOL
Materials Diethylpyrocarbonate (DEPC; UNIT 4.1) 10× T4 polynucleotide kinase buffer (UNIT 3.4) 0.1 M and 1 M dithiothreitol (DTT; APPENDIX 2) 1 mM spermidine 50 to 100 ng/µl oligonucleotide primer (5 to 10 µM; UNIT 2.11) 10 µCi/µl [γ-32P]ATP (3000 Ci/mmol) 20 to 30 U/µl T4 polynucleotide kinase (UNIT 3.10) 0.5 M EDTA, pH 8.0 (APPENDIX 2) TE buffer, pH 8.0 (APPENDIX 2) Cation-exchange resin (e.g., Bio-Rad AG 50W-X8), equilibrated in 0.1 M Tris⋅Cl (pH 7.5)/0.5 M NaCl Anion-exchange resin (e.g., Whatman DE-52), equilibrated in TEN 100 TEN 100 buffer: 100 mM NaCl in TE buffer, pH 7.5 (APPENDIX 2) TEN 300 buffer: 300 mM NaCl in TE buffer, pH 7.5 (APPENDIX 2) TEN 600 buffer: 600 mM NaCl in TE buffer, pH 7.5 (APPENDIX 2) Total cellular RNA (UNITS 4.1-4.3) 10× hybridization buffer 0.1 M Tris⋅Cl, pH 8.3 (APPENDIX 2) 0.5 M MgCl2 1 mg/ml actinomycin D (store at 4°C protected from light; UNIT 1.4) 10 mM 4dNTP mix (UNIT 3.4) 25 U/µl AMV reverse transcriptase (UNIT 3.7) RNase reaction mix 3 M sodium acetate (APPENDIX 2) 25:24:1 (v/v/v) phenol/chloroform/isoamyl alcohol (UNIT 2.1) 100% and 70% ethanol Stop/loading dye (UNIT 7.4) 9% acrylamide/7 M urea gel (UNIT 2.12) Silanized glass wool and 1000-µl pipet tip (APPENDIX 3) 65°C water bath Additional reagents and equipment for denaturing gel electrophoresis (UNITS 2.12 & 7.6), phenol extraction and ethanol precipitation of DNA (UNIT 2.1), and autoradiography (APPENDIX 3) NOTE: Water should be treated with DEPC to inhibit RNase activity. See UNIT 4.1, reagents and solutions, for instructions. CAUTION: DEPC is a suspected carcinogen and should be handled carefully. Preparation and Analysis of RNA Contributed by Steven J. Triezenberg Current Protocols in Molecular Biology (1992) 4.8.1-4.8.5 Copyright © 2000 by John Wiley & Sons, Inc.
4.8.1 Supplement 20
Label and purify the oligonucleotide 1. Mix the following reagents in the order indicated (10 µl final): 2.5 µl H2O 1 µl 10× T4 polynucleotide kinase buffer 1 µl 0.1 M DTT 1 µl 1 mM spermidine 1 µl 50-100 ng/µl oligonucleotide primer 3 µl 10 µCi/µl [γ-32P]ATP 0.5 µl 20-30 U/µl T4 polynucleotide kinase. Incubate 1 hr at 37°C. In this step, 5 to 10 pmol of oligonucleotide are radiolabeled. Oligonucleotides used as primers should be 20 to 40 nucleotides long; shorter primers may not hybridize efficiently under the conditions described in step 7. Generally, primers should be selected to yield an extended product of 5 hr) is not recommended for northern transfers as this necessitates more formaldehyde in the gel (e.g., the recipe for a gel run overnight would be 1.0 g agarose, 60 ml water, 10 ml of 10× MOPS, and 30 ml of 12.3 M formaldehyde). Increasing the amount of formaldehyde causes the gel to become more brittle and prone to breakage during transfer and also increases the health hazard from volatilization of formaldehyde during electrophoresis.
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Stain and photograph gel 7a. Remove the gel and cut off the lanes that are to be stained. Place this portion of the gel in an RNase-free glass dish, add sufficient 0.5 M ammonium acetate to cover, and soak for 20 min. Change solution and soak for an additional 20 min (to remove the formaldehyde). Pour off solution, replace with 0.5 µg/ml ethidium bromide in 0.5 M ammonium acetate, and allow to stain for 40 min. If necessary (i.e., if background fluorescence makes it difficult to visualize RNA fragments), destain in 0.5 M ammonium acetate for up to 1 hr.
7b. Alternatively, remove gel, cut off lanes, and stain 2 min in 1.1 M formaldehyde/10 mM sodium phosphate containing 10 µg/ml acridine orange. If necessary, destain 20 min in the same buffer without acridine orange.
8. Examine gel on a UV transilluminator to visualize the RNA and photograph with a ruler laid alongside the gel so that band positions can later be identified on the membrane. Molecular weight markers are not usually run on RNA gels as the staining causes rRNA molecules present in cellular RNA to appear as sharp bands that can be used as internal markers. In mammalian cells, these molecules are 28S and 18S (corresponding to 4718 and 1874 nucleotides respectively; Fig. 4.9.1). Mitochondrial rRNAs (16S and 12S in mammalian cells) may also be visible in some extracts, and plant extracts usually contain chloroplast rRNAs (23S and 16S). Bacterial rRNAs are smaller than the eukaryotic nuclear counterparts (E. coli: 23S and 16S, 2904 and 1541 nucleotides) and in some species one or both of the molecules may be cleaved into two or more fragments. If poly(A)+ RNA is being fractionated, commercial RNAs (e.g., 0.24- to 9.5-kb RNA ladder from Life Technologies) can be used as molecular weight markers.
Prepare gel for transfer 9. Place unstained portion of gel in an RNase-free glass dish and rinse with several changes of sufficient deionized water to cover the gel. The rinses remove formaldehyde, which would reduce retention of RNA by nitrocellulose membranes and hinder transfer onto nylon. The portion of the gel that will be blotted is not stained with ethidium bromide as this can also reduce transfer efficiency.
28S
18S
Analysis of RNA by Northern and Slot Blot Hybridization
Figure 4.9.1 Rat liver RNA (5 µg) was electrophoresed on a formaldehyde 1% agarose gel containing ethidium bromide (left), transferred to a hybridization membrane and stained with methylene blue stain (Molecular Research Center; Herrin and Schmidt, 1988, right). Shown are 28S (4.7 kb) and 18S (1.9 kb) ribosomal RNAs, as well as 4S to 5S (0.10 to 0.15 kb) RNA containing mix of tRNA and 5S ribosomal RNA.
4-5S
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10. Add ∼10 gel volumes of 0.05 M NaOH/1.5 M NaCl to dish and soak for 30 min. Decant and add 10 gel volumes of 0.5 M Tris⋅Cl (pH 7.4)/1.5 M NaCl. Soak for 20 min to neutralize. This step is optional. It results in partial hydrolysis of the RNA which in turn leads to strand cleavage; the length reduction improves transfer of longer molecules. However, RNA is extremely sensitive to alkaline hydrolysis and smaller molecules may be fragmented into lengths too short for efficient retention by the membrane. Neutralization should be carried out only if efficient transfer of molecules >5 kb is required from a gel that has an agarose concentration of >1.0% and is >5 mm thick.
11. Replace solution with 10 gel volumes of 20× SSC and soak for 45 min. This step is also optional but improves transfer efficiency with some brands of membrane.
Transfer RNA from gel to membrane 12. Place an oblong sponge slightly larger than the gel in a glass or plastic dish (if necessary, use two or more sponges placed side by side). Fill the dish with enough 20× SSC to leave the soaked sponge about half-submerged in buffer. Refer to Figure 2.9.1A for a diagram of the transfer setup. The sponge forms the support for the gel. Any commercial sponge will do, but before a sponge is used for the first time, it should be washed thoroughly with distilled water to remove any detergents that may be present. Two or more sponges can be placed side by side if necessary. As an alternative, a solid support with wicks made out of Whatman 3MM paper (Fig. 2.9.1B) may be substituted. Do not use an electrophoresis tank, as the high-salt transfer buffer will corrode the electrodes. If using a nylon membrane, a lower concentration of SSC (e.g., 10×) may improve transfer of molecules >4 kb; reduction of the SSC concentration is not recommended for a nitrocellulose membrane as the high salt is needed for retention of RNA.
13. Cut three pieces of Whatman 3MM paper to the same size as the sponge. Place them on the sponge and wet them with 20× SSC. 14. Place the gel on the filter paper and squeeze out air bubbles by rolling a glass pipet over the surface. 15. Cut four strips of plastic wrap and place over the edges of the gel. This is to prevent buffer from “short-circuiting” around the gel rather than passing through it.
16. Cut a piece of nylon or nitrocellulose membrane just large enough to cover the exposed surface of the gel. Pour distilled water ∼0.5 cm deep in an RNase-free glass dish and wet the membrane by placing it on the surface of the water. Allow the membrane to submerge. For nylon membrane, leave for 5 min; for nitrocellulose membrane, replace the water with 20× SSC and leave for 10 min. Avoid handling nitrocellulose and nylon membranes even with gloved hands—use clean blunt-ended forceps instead.
17. Place the wetted membrane on the surface of the gel. Try to avoid getting air bubbles under the membrane; remove any that appear by carefully rolling a glass pipet over the surface. 18. Flood the surface of the membrane with 20× SSC. Cut five sheets of Whatman 3MM paper to the same size as the membrane and place on top of the membrane. 19. Cut paper towels to the same size as the membrane and stack on top of the Whatman 3MM paper to a height of ∼4 cm. Preparation and Analysis of RNA
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20. Lay a glass plate on top of the structure and add a weight to hold everything in place. Leave overnight. The weight should be sufficient to compress the paper towels to ensure good contact throughout the stack. Excessive weight, however, will crush the gel and retard transfer. An overnight transfer is sufficient for most purposes. Make sure the reservoir of 20× SSC does not run dry during the transfer.
Prepare membrane for hybridization 21. Remove paper towels and filter papers and recover the membrane and flattened gel. Mark in pencil the position of the wells on the membrane and ensure that the up-down and back-front orientations are recognizable. Pencil is preferable to pen, as ink marks may wash off the membrane during hybridization. With a nylon membrane only, the positions of the wells can be marked by slits cut with a razor blade (do not do this before transfer or the buffer will short-circuit). The best way to record the orientation of the membrane is by making an asymmetric cut at one corner.
22. Rinse the membrane in 2× SSC, then place it on a sheet of Whatman 3MM paper and allow to dry. The rinse has two purposes: to remove agarose fragments that may adhere to the membrane and to leach out excess salt.
Immobilize the RNA and assess transfer efficiency 23a. For nitrocellulose membranes: Place between two sheets of Whatman 3MM filter paper and bake in a vacuum oven for 2 hr at 80°C. Baking results in noncovalent attachment of RNA to the membrane; the vacuum is needed to prevent the nitrocellulose from igniting.
23b. For nylon membranes: Bake as described above or wrap the dry membrane in UV-transparent plastic wrap, place RNA-side-down on a UV transilluminator (254-nm wavelength), and irradiate for the appropriate length of time (determined as described in UNIT 2.9A, Support Protocol). CAUTION: Exposure to UV irradiation is harmful to the eyes and skin. Wear suitable eye protection and avoid exposure of bare skin. UV cross-linking is recommended for a nylon membrane as it leads to covalent attachment and enables the membrane to be reprobed several times. The membrane must be completely dry before UV cross-linking; check the manufacturer’s recommendations, which may suggest baking for 30 min at 80°C prior to irradiation. The plastic wrap used during irradiation must be UV transparent—e.g., polyvinylidene (Saran Wrap). A UV light box (e.g., Stratagene Stratalinker) can be used instead of a transilluminator (follow manufacturer’s instructions).
24. If desired, check transfer efficiency by either staining the gel in ethidium bromide or acridine orange as in steps 7 and 8 or (if using nylon membrane) staining the membrane in 0.03% (w/v) methylene blue in 0.3 M sodium acetate, pH 5.2, for 45 sec and destaining in water for 2 min. If significant fluorescence is observed in the gel, not all the RNA has transferred. RNA bands on a nylon membrane will be stained by the methylene blue (Herrin and Schmidt, 1988).
Analysis of RNA by Northern and Slot Blot Hybridization
Membranes can be stored dry between sheets of Whatman 3MM filter paper for several months at room temperature. For long-term storage they should be placed in a desiccator at room temperature or 4°C.
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25. Prepare DNA or RNA probe labeled to a specific activity of >108 dpm/µg and with unincorporated nucleotides removed. Probes are ideally 100 to 1000 bp in length. DNA for a double-stranded probe is obtained as a cloned fragment (Chapter 1) and purified from the vector by restriction digestion (UNIT 3.1) followed by recovery from an agarose gel (UNIT 2.6). The DNA is labeled by nick translation or random oligonucleotide priming (UNIT 3.5) to create the radioactive probe. A single-stranded DNA probe is created in the same fashion but using a single-stranded vector; the probe should be antisense so it will hybridize to the sense RNA strands that are bound to the membrane. An RNA probe, which should also be antisense, is created by in vitro synthesis from a single-stranded sense DNA fragment (UNIT 2.10).
26. Wet the membrane carrying the immobilized RNA (from step 23) in 6× SSC. 27. Place the membrane RNA-side-up in a hybridization tube and add ∼1 ml formamide prehybridization/hybridization solution per 10 cm2 of membrane. Prehybridization and hybridization are usually carried out in glass tubes in a commercial hybridization oven. Alternatively, a heat-sealable polyethylene bag and heat-sealing apparatus can be used. The membrane should be placed in the bag, all edges sealed, and a corner cut off. Hybridization solution can then be pipetted into the bag through the cut corner and the bag resealed.
28. Place the tube in the hybridization oven and incubate with rotation 3 hr at 42°C (for DNA probe) or 60°C (for RNA probe). If using a bag, it can be shaken slowly in a suitable incubator or water bath. If using a nylon membrane, the prehybridization period can be reduced to 15 min.
29. If the probe is double-stranded, denature by heating in a water bath or incubator for 10 min at 100°C. Transfer to ice. 30. Pipet the desired volume of probe into the hybridization tube and continue to incubate with rotation overnight at 42°C (for DNA probe) or 60°C (for RNA probe). The probe concentration in the hybridization solution should be 10 ng/ml if the specific activity is 108 dpm/ìg or 2 ng/ml if the specific activity is 109 dpm/ìg. For denatured probe, add to hybridization tube as soon after denaturation as possible. If using a bag, a corner should be cut, the probe added, and the bag resealed. It is very difficult to do this without contaminating the bag sealer with radioactivity. Furthermore, the sealing element (the part that gets contaminated) is often difficult to clean. Hybridization in bags is therefore not recommended.
Wash membrane and perform autoradiography 31. Pour off hybridization solution and add an equal volume of 2× SSC/0.1% SDS. Incubate with rotation 5 min at room temperature, change wash solution, and repeat. CAUTION: Hybridization solution and all wash solutions must be treated as radioactive waste and disposed of appropriately. To reduce background, it may be beneficial to double the volume of the wash solutions. If using a bag, transfer the membrane to a plastic box for the washes.
32. Replace wash solution with an equal volume of 0.2× SSC/0.1% SDS and incubate 5 min with rotation at room temperature. Change wash solution and repeat (this is a low-stringency wash; see UNIT 2.10 Commentary). 33. If desired, carry out two further washes using prewarmed (42°C) 0.2× SSC/0.1% SDS for 15 min each at 42°C (moderate-stringency wash). Preparation and Analysis of RNA
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34. If desired, carry out two further washes using prewarmed (68°C) 0.1× SSC/0.1% SDS for 15 min each at 68°C (high-stringency wash). 35. Remove final wash solution and rinse membrane in 2× SSC at room temperature. Blot excess liquid and cover in UV-transparent plastic wrap. Do not allow membrane to dry out if it is to be reprobed.
36. Perform autoradiography. If the membrane is to be reprobed, the probe can be stripped from the hybridized membrane without removing the bound RNA (see Support Protocol). Do not add NaOH. The membrane must not be allowed to dry out between hybridization and stripping, as this may cause the probe to bind to the matrix. ALTERNATE PROTOCOL 1
NORTHERN HYBRIDIZATION OF RNA DENATURED BY GLYOXAL/DMSO TREATMENT In this procedure denaturation of the RNA is achieved by treating samples with a combination of glyoxal and DMSO prior to running in an agarose gel made with phosphate buffer. The glyoxal/DMSO method produces sharper bands after northern hybridization than do formaldehyde gels, but is more difficult to carry out as the running buffer must be recirculated during electrophoresis. Additional Materials (also see Basic Protocol 1) 10 mM and 100 mM sodium phosphate, pH 7.0 (see recipe) Dimethyl sulfoxide (DMSO) 6 M (40%) glyoxal, deionized immediately before use (see recipe) Glyoxal loading buffer (see recipe) 20 mM Tris⋅Cl, pH 8.0 (APPENDIX 2) Apparatus for recirculating running buffer during electrophoresis 50° and 65°C water baths NOTE: All solutions should be prepared with sterile deionized water that has been treated with DEPC as described in UNIT 4.1; see unit introduction for further instructions and precautions regarding establishment of an RNase-free environment. Denature and carry out agarose gel electrophoresis 1. Prepare a 1.0% agarose gel by dissolving 1.0 g agarose in 100 ml of 10 mM sodium phosphate, pH 7.0. Cool to 60°C in a water bath, pour gel, and allow to set. Remove comb, place gel in gel tank, and add 10 mM sodium phosphate (pH 7.0) until gel is submerged to a depth of ∼1 mm (see UNIT 2.5A). provides details on preparing, pouring, and running the agarose gel; vary as described here.
UNIT 2.5A
A 1.0% gel is suitable for RNA molecules 500 bp to 10 kb in size. A higher-percentage gel (1.0 to 2.0%) should be used to resolve smaller molecules or a lower percentage (0.7 to 1.0%) for longer molecules. The recipe may be scaled up or down depending on the size of gel desired; the gel should be 2 to 6 mm thick after it is poured and the wells large enough to hold 60 ìl of sample.
2. Adjust volume of each RNA sample to 11 µl with water, then add:
Analysis of RNA by Northern and Slot Blot Hybridization
4.5 µl 100 mM sodium phosphate, pH 7.0 22.5 µl DMSO 6.6 µl 6 M glyoxal. Mix samples by vortexing, spin briefly (5 to 10 sec) in a microcentrifuge to collect the liquid, and incubate 1 hr at 50°C.
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3. Cool samples on ice and add 12 µl glyoxal loading buffer to each sample. Load samples onto gel. 0.5 to 10 ìg of RNA should be loaded per lane (see Commentary). Duplicate samples should be loaded at one side of gel for ethidium bromide staining.
4. Run the gel at 4 V/cm with constant recirculation of running buffer for ∼3 hr or until bromphenol blue dye has migrated one-half to two-thirds the length of the gel. Recirculation is needed to prevent an H+ gradient forming in the buffer. If a gradient forms, the pH in parts of the gel may rise to >8.0, resulting in dissociation of the glyoxal from the RNA followed by renaturation. If no recirculation apparatus is available, electrophoresis should be paused every 30 min and the tank shaken to remix the buffer.
5. Remove the gel, cut off lanes, and stain with ethidium bromide (see Basic Protocol 1, steps 7a and 8). The RNA transfer (using the remaining portion of the gel) should be set up as soon as the gel is cut, before starting the staining.
Carry out northern transfer and hybridization analysis 6. Transfer RNA (see Basic Protocol 1, steps 9 to 24). 7. Immediately before hybridization, soak the membrane in 20 mM Tris⋅Cl (pH 8.0) for 5 min at 65°C to remove glyoxal. 8. Continue with hybridization analysis (see Basic Protocol 1, steps 25 to 36). NORTHERN HYBRIDIZATION OF UNFRACTIONATED RNA IMMOBILIZED BY SLOT BLOTTING
ALTERNATE PROTOCOL 2
RNA slot blotting is a simple technique that allows immobilization of unfractionated RNA on a nylon or nitrocellulose membrane. Hybridization analysis is then carried out to determine the relative abundance of target mRNA sequences in the blotted samples. The technique is based on the DNA dot- and slot-blotting procedure (UNIT 2.9B), the main difference being the way in which the samples are denatured prior to immobilization. RNA dot blots can be prepared by hand but slot blots constructed using a manifold apparatus are preferable because the slots make it easier to compare hybridization signals by densitometry scanning. Additional Materials (also see Basic Protocol 1) 0.1 M NaOH 10× SSC (APPENDIX 2) 20× SSC (APPENDIX 2), room temperature and ice-cold Denaturing solution (see recipe) 100 mM sodium phosphate, pH 7.0 (see recipe) Dimethyl sulfoxide (DMSO) 6 M (40%) glyoxal, deionized immediately before use (see recipe) Manifold apparatus with a filtration template for slot blots (e.g., Bio-Rad Bio-Dot SF, Schleicher and Schuell Minifold II) 50° and 60°C water baths NOTE: All solutions should be prepared with sterile deionized water that has been treated with DEPC as described in UNIT 4.1; see unit introduction for further instructions and precautions regarding establishment of an RNase-free environment. Preparation and Analysis of RNA
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Set up membrane for transfer 1. Clean the manifold with 0.1 M NaOH and rinse with distilled water. 2. Cut a piece of nylon or nitrocellulose membrane to the size of the manifold. Pour 10× SSC (for nylon membrane) or 20× SSC (for nitrocellulose membrane) into a glass dish; place membrane on top of liquid and allow to submerge. Leave for 10 min. Avoid handling nitrocellulose and nylon membranes even with gloved hands—use clean blunt-ended forceps instead.
3. Place the membrane in the manifold. Assemble the manifold according to manufacturer’s instructions and fill each slot with 10× SSC. Ensure there are no air leaks in the assembly. Denature RNA samples 4a. Add 3 vol denaturing solution to RNA sample. Incubate 15 min at 65°C, then place on ice. Up to 20 ìg of RNA can be applied per slot. Total cellular RNA (UNITS 4.1-4.4) or poly(A)+ RNA (UNIT 4.5) can be used, although the latter is preferable (see Commentary).
4b. Alternatively, mix: 11 µl RNA sample 4.5 µl 100 mM sodium phosphate, pH 7.0 22.5 µl DMSO 6.6 µl 6 M glyoxal. Mix by vortexing, spin briefly in a microcentrifuge to collect liquid, and incubate 1 hr at 50°C. 5. Add 2 vol ice-cold 20× SSC to each sample. Pass samples through manifold 6. Switch on the suction to the manifold device and allow the 10× SSC added in step 3 to filter through. Leave the suction on. The suction should be adjusted so that 500 ìl buffer takes ∼5 min to pass through the membrane. Higher suction may damage the membrane. Slots that are not being used can be blocked off by placing masking tape over them or by applying 500 ìl of 3% (w/v) gelatin to each one. The former method is preferable as use of gelatin may lead to a background signal after hybridization. Alternatively, keep all slots open and apply 10× SSC instead of sample to the slots not being used.
7. Load each sample to the slots and allow to filter through, being careful not to touch the membrane with the pipet tip. 8. Add 1 ml of 10× SSC to each slot and allow to filter through. Repeat. 9. Dismantle the apparatus, place the membrane on a sheet of Whatman 3MM paper, and allow to dry. Immobilize RNA and carry out hybridization 10. Immobilize the RNA (see Basic Protocol 1, step 23). If glyoxal/DMSO denaturation has been used, immediately before hybridization soak the membrane in 20 mM Tris⋅Cl (pH 8.0) for 5 min at 65°C to remove glyoxal. Analysis of RNA by Northern and Slot Blot Hybridization
11. Carry out hybridization analysis as described in steps 25 to 36 of Basic Protocol 1.
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REMOVAL OF PROBES FROM NORTHERN BLOTS Hybridization probes can be removed from northern blots on nylon membranes without damage to the membrane or loss of the transferred RNA. Some probes (particularly RNA probes) are more resistant to stripping. In these cases, higher temperatures, longer incubation periods, or the inclusion of formamide may be necessary for complete probe removal. The following stripping procedures are appropriate for both radioactive and chemiluminescent probes. Begin with the mildest conditions (step 1a) and monitor results to determine the extent of stripping. If the hybridization signal is still evident, proceed with the more stringent treatments (steps 1b and 1c) until stripping is complete.
SUPPORT PROTOCOL
Materials Northern hybridization membrane containing probe (see Basic Protocol 1, Alternate Protocol 1, or Alternate Protocol 2) Stripping solution (see recipe) Hybridization bags 65°, 80°, or 100° (boiling) water bath UV-transparent plastic wrap (e.g., Saran Wrap or other polyvinylidene wrap) Additional reagents and equipment for autoradiography (APPENDIX 3A) CAUTION: If hybridization probes include a radioactive label, dispose of stripping solutions as radioactive waste. Observe appropriate caution when working with the toxic compound formamide. 1a. To remove probes at 80°C: Place membrane in a hybridization bag containing stripping solution without formamide. Place bag in water preheated to 80°C for 5 min. Pour out solution, then repeat this washing process three to four times. Add sufficient solution to cover the membrane completely when using a bag; alternatively, stripping can be done in an open container, again with sufficient solution to cover the membrane.
1b. To remove probes at 100°C: Place membrane in a hybridization bag containing stripping solution without formamide. Place bag in boiling water for 5 min. Pour out solution, then repeat this washing process three to four times. 1c. To remove probes with formamide: Place membrane in a hybridization bag containing stripping solution with formamide. Place bag in water preheated to 65°C for 5 min. Pour out solution, then repeat this washing process three times using stripping solution with formamide and once using stripping solution without formamide. 2. Place membrane on filter paper to remove excess solution. Wrap membrane in plastic wrap and perform autoradiography to verify probe removal. If a chemiluminscent probe was used, verify probe removal by chemiluminscent detection (UNIT 3.19). The membrane may be immediately rehybridized or air-dried and stored for future use.
NORTHERN HYBRIDIZATION OF SMALL RNA FRACTIONATED BY POLYACRYLAMIDE GEL ELECTROPHORESIS This protocol is adapted for analysis of small RNAs. The major differences between this procedure and the traditional northern hybridization procedure are the fractionation system and the transfer system applied. Fractionation using denaturing polyacrylamide gel electrophoresis (PAGE) allows better separation of small RNAs. The introduction of a semidry transfer system reduces the time of the experimental procedure to 2 days.
BASIC PROTOCOL 2
Preparation and Analysis of RNA
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Tissue or cell samples TRIzol reagent (Invitrogen) RNase-free H2O (UNIT 4.1) 15% denaturing polyacrylamide sequencing (urea/TBE) gel (UNIT 7.6) 0.5× TBE electrophoresis buffer (APPENDIX 2) Formamide loading dye (see recipe) 2× SSC (APPENDIX 2) 50 µM probe oligonucleotide (DNA or RNA; UNIT 2.11) in RNase-free H2O ≥10 mCi/ml [γ-32P]ATP (6000 Ci/mmol; ICN Biomedicals) 10× T4 polynucleotide kinase buffer (New England Biolabs) 200 U/µl T4 polynucleotide kinase (New England Biolabs) Prehybridization/hybridization solution (see recipe), prewarmed to 37°C 2× SSC (APPENDIX 2) containing 0.1× (w/v) SDS, prewarmed to 37°C 95°C heating block or water bath Hybond N+ Nylon Transfer Membrane (Amersham Biosciences) Extra-thick blotting paper (Bio-Rad), slightly larger than the gel being blotted Semi-dry transfer apparatus (e.g., Bio-Rad Trans-Blot SD cell) Sephadex G-25 spin column Hybridization oven with rotating glass hybridization bottles, 37°C Image-analysis software (also see UNIT 10.5): e.g., QuantityOne (Bio-Rad) or ImageGauge (Fuji) Additional reagents and equipment for denaturing polyacrylamide gel electrophoresis (UNIT 7.6), phosphor imaging (APPENDIX 3A), and digital electrophoresis analysis (UNIT 10.5) Prepare RNA sample and run the gel 1. Isolate total RNA from tissue or cell samples with TRIzol reagent according to the manufacturer’s instructions. At the end of the procedure, dissolve the RNA to a final concentration of 10 µg/µl in RNase-free water. 2. Prerun 15% denaturing polyacrylamide sequencing (urea/TBE) gel for 15 min at 25 W in 0.5× TBE electrophoresis buffer. Mix 5 µl RNA sample (10 µg/µl) with equal volume of formamide loading dye. Heat 2 min at 95°C, and load onto gel (UNIT 7.6). 3. Run the gel at 25 W until the bromophenol blue dye has migrated to the bottom of the gel. Transfer RNA from gel to membrane 4. Cut a piece of Hybond N+ nylon membrane slightly larger than the gel. Soak the membrane and four pieces of blotting paper of appropriate size in 0.5× TBE buffer for 10 min. 5. Stack two pieces of blotting paper on the anode platform of the transfer cell. Avoid getting air bubbles under or between the papers; remove any that appear by carefully rolling a glass pipet over the surface. 6. Place the membrane on top of the blotting paper and squeeze out air bubbles by rolling a glass pipet over the surface. 7. Carefully transfer the gel from glass plate to the top of the membrane and squeeze out air bubbles. 8. Stack another two pieces of blotting paper on the gel and squeeze out air bubbles. Analysis of RNA by Northern and Slot Blot Hybridization
9. Set the cathode assembly and the safety lid on the sandwich. Transfer for 1 hr at 300 mA.
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Prepare membrane for hybridization 10. Disassemble the transfer cell. Remove the paper and the gel. Rinse the membrane in 2× SSC, then place it on a sheet of filter paper and allow it to air dry. 11. Place membrane RNA-side-down on a UV transilluminator (254-nm wavelength) or in a UV light box for the appropriate length of time to covalently attach the RNA to the membrane. Prepare probe The authors typically use a chemically synthesized 21-22 nt DNA or RNA oligonucleotide (see UNIT 2.11 for oligonucleotide synthesis) perfectly complementary to the small RNA to be detected. 12. Set up the 5′-end-labeling reaction by combining the following reagents: 1 µl of 50 µM probe oligonucleotide (DNA or RNA) 1 µl of [γ- 32P]ATP (6000 Ci/mmol, ≥10 mCi/ml) 4 µl of 10× T4 polynucleotide kinase buffer H2O to a final volume of 40 µl 1 µl of 200 U/µl T4 polynucleotide kinase. Incubate reaction 1 hr at 37°C. 13. Pass reaction mixture through Sephadex G-25 spin column (centrifuging per manufacturer’s instructions) to remove unincorporated [γ- 32P]ATP. Perform prehybridization and hybridization 14. Place membrane (from step 11) RNA-side-up in a hybridization bottle and add ∼1 ml prewarmed (37°C) prehybridization/hybridization solution per 10 cm2 of membrane. Place the bottle in a hybridization oven and incubate with rotation for 30 min at 37°C. 15. Pipet the entire reaction mix (from step 13) into the hybridization bottle and continue to incubate with rotation overnight at 37°C. Wash membrane 16. Pour off hybridization solution and wash the membrane briefly with 2× SSC for 5 min. Add prewarmed (37°C) 2× SSC containing 0.1% SDS to the bottle. Incubate with rotation for 15 min at 37°C. Replace solution with fresh solution and repeat. Remove final wash solution, blot excess liquid, and wrap with plastic wrap. Do not allow membrane to dry out if it is to be reprobed.
Perform phosphor imaging and analyze the hybridization signals 17. Visualize the hybridization signals by phosphor imaging (APPENDIX 3A). Analyze hybridization result using appropriate software (UNIT 10.5). Subtract the background from the original signal to obtain the specific hybridization. 18. To compare the amount of small RNA in different samples, normalize the amount of small RNA detected to the nonspecific hybridization of the probe to 5S rRNA. Alternatively, the blot can be reprobed with a probe specific for 5S rRNA (for Drosophila 5′-CAA CAC GCG GTG TTC CCA AGC CG-3′) or, for Drosophila, the 2S RNA (5′-TAC AAC CCT CAA CCA TAT GTA GTC CAA GCA-3′). The precise amount (pmol or molecules) of small RNA can be determined if concentration standards of synthetic RNA are included on the blot. The assay is typically linear with respect to concentration over a 10,000-fold range.
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ALTERNATE PROTOCOL 3
NORTHERN HYBRIDIZATION OF RNA USING CHURCH’S HYBRIDIZATION BUFFER Church’s hybridization buffer can be used as an alternative for the standard prehybrization/hybridization solution in this assay. It provides similar sensitivity. Additional Materials (also see Basic Protocol 2) Church’s hybridization buffer without BSA (see recipe) 1. Perform northern blotting and prepare probe (see Basic Protocol 2, steps 1 through 13). 2. Perform prehybridization and hybridization (see Basic Protocol 2, steps 14 to 15) using Church’s hybridization buffer (without BSA) in place of the prehybridization/hybridization solution. 3. Wash membrane and proceed with development and analysis (see Basic Protocol 2, steps 16 to 18). REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Church’s hybridization buffer without BSA 0.5 M sodium phosphate buffer, pH 7.2 (APPENDIX 2) 1 mM EDTA, pH 8.0 (APPENDIX 2) 7% (w/v) SDS Store up to 1 year at room temperature Denaturing solution 500 µl formamide 162 µl 12.3 M (37%) formaldehyde 100 µl MOPS buffer (see recipe) Make fresh from stock solutions immediately before use If formamide has a yellow color, deionize as follows: add 5 g of mixed-bed ion-exchange resin (e.g., Bio-Rad AG 501-X8 or X8(D) resins) per 100 ml formamide, stir 1 hr at room temperature, and filter through Whatman #1 filter paper. CAUTION: Formamide is a teratogen. Handle with care.
Formaldehyde loading buffer 1 mM EDTA, pH 8.0 (APPENDIX 2) 0.25% (w/v) bromphenol blue 0.25% (w/v) xylene cyanol 50% (v/v) glycerol Store up to 3 months at room temperature Formamide loading dye 98% (v/v) deionized formamide 10 mM EDTA pH 8.0 (APPENDIX 2) 0.025% (w/v) xylene cyanol 0.025% (w/v) bromphenol blue Store indefinitely at −20°C
Analysis of RNA by Northern and Slot Blot Hybridization
Glyoxal, 6 M, deionized Immediately before use, deionize glyoxal by passing through a small column of mixed-bed ion-exchange resin (e.g., Bio-Rad AG 501-X8 or X8(D) resins) until the pH is >5.0.
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Glyoxal loading buffer 10 mM sodium phosphate, pH 7.0 (see recipe) 0.25% (w/v) bromphenol blue 0.25% (w/v) xylene cyanol 50% (v/v) glycerol Store up to 3 months at room temperature MOPS buffer 0.2 M MOPS [3-(N-morpholino)-propanesulfonic acid], pH 7.0 0.5 M sodium acetate 0.01 M EDTA Store up to 3 months at 4°C Store in the dark and discard if it turns yellow.
MOPS running buffer, 10× 0.4 M MOPS, pH 7.0 0.1 M sodium acetate 0.01 M EDTA Store up to 3 months at 4°C Prehybridization/hybridization solution 5× SSPE (see recipe) 5× Denhardt solution (APPENDIX 2) 50% (v/v) formamide 0.5% (w/v) SDS 72 µg/ml denatured herring sperm DNA (Promega) Make fresh from stock solutions immediately before use The herring sperm DNA is denatured by heating 10 min at 75°C just before it is added.
Sodium phosphate, pH 7.0, 100 mM and 10 mM 100 mM stock solution: 5.77 ml 1 M Na2HPO4 4.23 ml 1 M NaH2PO4 H2O to 100 ml Store up to 3 months at room temperature 10 mM solution: Dilute 100 mM stock 1/10 with H2O Store up to 3 months at room temperature SSPE, 10× 1.5 M NaCl 50 mM NaH2PO4⋅H2O 5 mM EDTA Store indefinitely at room temperature Stripping solution 1% (w/v) SDS 0.1× SSC (APPENDIX 2) 40 mM Tris⋅Cl, pH 7.5 to 7.8 (APPENDIX 2) Store up to 1 year at room temperature Where formamide stripping is desired, prepare the above solution and add an equal volume of formamide just before use. Preparation and Analysis of RNA
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COMMENTARY Background Information The development of Southern blotting (UNIT 2.9A; Southern, 1975) was quickly followed by an equivalent procedure for the immobilization of gel-fractionated RNA (Alwine et al., 1977). The term northern blotting, initially used in a humorous fashion, has become enshrined in molecular biology jargon. Northern hybridization is a standard procedure for identification and size analysis of RNA transcripts and RNA slot blotting is frequently used to assess the expression profiles of tissue-specific genes (Kafatos et al., 1979). Procedures for the removal of hybridization probes from northern blots are similar to those for Southern blots, except that NaOH is omitted to prevent hydrolysis of the RNA, and formamide may be included. The recent discovery of microRNAs (miRNAs) revealed an entire new class of molecules that regulate gene expression. miRNAs are small noncoding RNAs that range from 20 to 30 nucleotides, making traditional formaldehyde-agarose gel electrophoresis unsuitable for their size fractionation. The modified northern protocol here (Basic Protocol 2) combines denaturing polyacrylamide gel electrophoresis (PAGE), which is ideal for the separation of small RNAs, with standard blotting and hybridization procedures.
Critical Parameters
Analysis of RNA by Northern and Slot Blot Hybridization
Gel electrophoresis and northern blotting The main distinction between northern and Southern blotting lies with the initial gel fractionation step. Because single-stranded RNA can form secondary structures, samples must be electrophoresed under denaturing conditions to ensure good separation. A variety of denaturants for RNA gels have been used, including formaldehyde (Basic Protocol 1; Lehrach et al., 1977), glyoxal/DMSO (Alternate Protocol 1; Thomas, 1980), and the highly toxic methylmercuric chloride (Bailey and Davidson, 1976). Because of the substantial health risks, use of methylmercuric chloride is not advised. Formaldehyde gels are recommended, as they are easy to run and reasonably reliable. The formaldehyde must be rinsed from the gel before the transfer is set up, but this is a minor inconvenience compared to assembling the buffer recircularization system required for electrophoresis of glyoxal-denatured RNA.
Total cellular RNA (UNITS 4.1-4.4) or poly(A)+ RNA (UNIT 4.5) can be used for northern transfers and slot blots. Total RNA is less satisfactory because nonspecific hybridization, however slight, to one or both of the highly abundant rRNA molecules will lead to a substantial hybridization signal. Any hybridizing band that appears in the vicinity of an rRNA should be treated with suspicion and its identity confirmed by blotting with poly(A)+ RNA. Under ideal conditions, a band that contains as little as 1 pg of RNA can be detected by northern hybridization with a probe labeled to a specific activity of 109 dpm/µg. In practice, the effective detection limit with an overnight exposure is ∼5 pg RNA. An mRNA is usually considered to be abundant if it constitutes >1% of the mRNA fraction. In a typical mammalian cell, the mRNA fraction makes up about 0.5% of total RNA, so >5 pg of an abundant mRNA should be present in just 100 ng of total RNA. If 10 µg of total RNA is transferred, abundant mRNAs should give strong hybridization signals and less abundant ones (down to 0.01% of the mRNA population) should be detectable with an overnight exposure. For rarer molecules, the poly(A)+ fraction must be prepared. In this sample 3 µg is sufficient for detecting an mRNA that makes up 0.0002% of the polyadenylated population. Unlike probing for mRNA, which often requires enrichment by poly(A) selection prior to analysis, total cellular RNA can always be used for the detection of miRNA, because individual miRNA species can be present in thousands to tens of thousands of copies per cell. The highly abundant rRNA and microRNAs are well separated in a 15% denaturing polyacrylamide gel; thus, the nonspecific hybridization to rRNA will not affect the interpretation of the desired hybridizing signal. RNA slot blots Although easy to perform, RNA slot-blot hybridization is one of the most problematic techniques in molecular biology. A number of criteria must be satisfied if slot blotting is to be used to make meaningful comparisons of mRNA abundance in different extracts. The first requirement is that equal amounts of RNA must be loaded in each slot. In practice this is difficult to achieve, especially if RNA concentrations are estimated by absorbance spectroscopy (APPENDIX 3D), which is subject to errors
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due to the small quantities being measured and the presence of contaminants such as protein and DNA. Even if equal amounts of RNA are loaded, a difference in hybridization signal does not necessarily mean that the gene whose transcript is being studied is more active in a particular tissue. The analysis provides information on the abundance of an mRNA (i.e., the fraction of total RNA that it constitutes), not its absolute amount. To illustrate this point, consider a tissue in which a highly active gene is switched on at time t, where the transcripts of this gene constitute 0% of the mRNA at t − 1 but 20% of the mRNA at t + 1. If the slot blots of RNA from t − 1 and t + 1 are probed with the highly active gene, there will be a clear increase in hybridization signal after time t. In contrast, hybridization of the same slot blots with a second gene whose transcription rate is unchanged will show a decreased hybridization signal at t + 1. Transcripts of this gene are present in the same absolute amounts at t − 1 and t + 1, but their abundance decreases as the total mRNA population becomes larger due to activation of the highly expressed gene. To the unwary, the result of the hybridization analysis could appear to indicate down-regulation of a gene whose expression rate in fact remains constant. Choice of membrane and transfer system General information relating to the choice of membrane for a nucleic acid transfer is given in the Commentary to UNIT 2.9A. Because of the greater tensile strength of nylon, together with the fact that the RNA can be bound covalently by UV cross-linking, most transfers are now carried out using nylon rather than nitrocellulose. Nylon has the added advantage of being able to withstand the highly stringent conditions (50% formamide at 60°C) that may be required during hybridization with an RNA probe; nitrocellulose tends to disintegrate under these conditions. For DNA transfer, a major advantage of positively charged nylon is that nucleic acids become covalently bound to the membrane if the transfer is carried out with an alkaline buffer. RNA can also be immobilized on positively charged nylon by alkaline transfer, but the procedure is not recommended as the alkaline conditions result in partial hydrolytic degradation of the RNA. This hydrolysis is difficult to control and smaller molecules are easily broken down into fragments too short for efficient retention by the membrane (see Table 2.9.1). This results in a loss of signal after hybridiza-
tion, a problem that is exacerbated by the increased background caused by lengthy exposure of the membrane to the alkaline solution. Only if the signals are expected to be strong should an alkaline transfer be considered. In this case, Basic Protocol 1 should be modified as follows: omit the pre-transfer alkaline hydrolysis (step 10), use 8 mM NaOH rather than 20× SSC as the transfer buffer, do not transfer for more than 6 hr, and rinse the membrane in 2× SSC/0.1% (w/v) SDS rather than plain SSC immediately after transfer (step 22). If using Alternate Protocol 1, omit step 7 as well, as the alkaline transfer buffer removes glyoxal from the RNA. The standard northern transfer system can be modified as described for Southern blotting (UNIT 2.9A). Aqueous transfers onto nylon can be performed using a variety of buffers, although SSC is still most frequently used. Changes can be made to the transfer time and architecture of the blot (e.g., downward transfer; Chomczynski, 1992), and alternative methods such as electroblotting (Smith et al., 1984) and vacuum transfer (Peferoen et al., 1982) can be used. Hybridization procedures Hybridization analysis of an RNA blot is subject to the same considerations as DNA hybridization (see UNIT 2.10 Commentary). The factors that influence sensitivity and specificity are the same, and incubation times, hybridization solutions, probe length, and mechanics of hybridization all have similar effects. There are just two additional points that need to be made with respect to RNA blots. The first point is that formamide is almost always used in RNA hybridization solutions. The primary reason for this is to permit a lower hybridization temperature to be used, minimizing RNA degradation during the incubations. The second point concerns the stability of the hybrids formed between the immobilized RNA and the probe molecules. For a DNA probe the relevant equation is (Casey and Davidson, 1977): Tm = 79.8°C + 18.5(log M) + 0.58(%GC) + 11.8(%GC)2 − 0.50(%form) − 820⁄L
and for an RNA probe (Bodkin and Knudson, 1985): Tm = 79.8°C + 18.5(log M) + 0.58(%GC) + 11.8(%GC)2 − 0.35(%form) − 820⁄L
where Tm is the melting temperature, M is the molarity of monovalent cations, %GC is the
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percentage of guanosine and cytosine nucleotides in the DNA, %form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. What these equations indicate is that an RNA-RNA hybrid is more stable than a DNA-RNA hybrid: if %form is 50%, the Tm for an RNA-RNA hybrid is 7.5°C higher than that for an equivalent RNA-DNA hybrid. The greater stability of the RNA-RNA hybrid means that an RNA probe requires a more stringent hybridization and washing regime than a DNA probe (e.g., hybridization at 60°C in 50% formamide and final wash at 68°C in 0.1× SSC/0.1% SDS). Reprobing conditions Nitrocellulose presents problems regarding both membrane integrity and RNA retention (see UNIT 2.10). UV cross-linking of RNA to a neutral nylon membrane presents optimal conditions for northern blot reprobing. Because of the sensitivity of RNA to alkaline hydrolysis, NaOH, which is included in protocols for probe removal from Southern blots, should not be used when removing probes from northern blots. It is recommended that probes be removed prior to membrane storage, because unstripped probes remain permanently attached if the blot dries.
Troubleshooting
Analysis of RNA by Northern and Slot Blot Hybridization
The appearance of the agarose gel after staining gives a first indication of how successful a northern experiment is likely to be. If total RNA has been used, the rRNA bands should be clear and sharp (Fig. 4.9.1) with no “smearing” toward the positive electrode. The only exception is when the RNA has been prepared by the guanidinium isothiocyanate procedure (UNIT 4.2), in which case some smearing is normal. If the rRNA bands are not sharp, the RNA preparation may be of poor quality (usually because insufficient care has been taken in establishing an RNase-free environment) or the denaturing gel electrophoresis system may not have worked adequately. If the latter problem is suspected, make sure that the formaldehyde concentration in the gel is 2.2 M or, if glyoxal denaturation has been used, that the buffer recircularization is sufficient to maintain the gel pH at 7.0. Whatever the problem, if the rRNAs are not distinct, there is no point in proceeding with the transfer as the bands obtained after hybridization will also be fuzzy. In fact, even if the rRNA bands are clear there is no guarantee that the mRNAs are intact.
An indication of the efficiency of transfer onto nylon can be obtained by staining the membrane with methylene blue (see Basic Protocol 1, step 24), but often a problem with transfer is not recognized until after hybridization. If poor signals are obtained, the troubleshooting section of UNIT 2.10 (including Table 2.10.4) should be consulted to identify the likely cause. Note that it is relatively easy to detach RNA from a membrane before immobilization, so some loss may occur when the membrane is rinsed in 2× SSC to wash off agarose fragments and leach out salt (see Basic Protocol 1, step 22). If necessary, this rinse can be postponed until immediately before hybridization, after the RNA has been immobilized. Other problems, such as high backgrounds, extra bands, and difficulties with probe stripping, should be dealt with by referring to Table 2.10.4. To increase the sensitivity of the miRNA assay, RNA probes can be used instead of DNA probes. In some cases, RNA probes work better to detect miRNA precursors (∼60 to 70 nt long) which contain the immature miRNAs in a stemloop whose structure can prevent the hybridization of DNA probes.
Anticipated Results Using either a nylon or nitrocellulose membrane and a probe labeled to ≥5× 108 dpm/µg, it should be possible to detect transcripts that represent 0.01% of the mRNA population with a blot of 10 µg total mammalian RNA or 0.0002% of the population with a blot of 3 µg poly(A)+ RNA. It should be possible to detect small RNAs at levels as low as 0.3 fmol miRNA by following the Basic Protocol 2. Small RNAs that differ by as little as 1 nt (or even by a single phosphate group) can be separated on 15% denaturing polyacrylamide gel (50 to 100 cm), particularly when a long gel is used. For long gels, only the lower portion of the gel is used for transfer to the membrane.
Time Considerations Traditional blots A northern experiment can be completed in 3 days. The agarose gel is prepared and electrophoresed during the first day and the transfer carried out overnight. On the second day the blot is prehybridized and then hybridized overnight. Washes are completed early on the third day. A slot-blot experiment takes only two days, as the blot can be prepared and prehybridized
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on the first day, hybridized overnight, and washed on the second day. The length of time needed for the autoradiography depends on the abundance of the target sequences in the blotted RNA. Adequate exposure can take anything from overnight to several days. With blots that are intended for reprobing, stripping procedures can be completed in ∼1 hr, not including the verification steps. miRNA blots Using a semidry transfer system, the northern experiment can be completed in 2 days. The length of time needed for the autoradiography depends on the abundance of the small RNA sequence and the detection system used. Optimal exposure can range from overnight to several days. Under ideal conditions, a band that contains as little as 0.3 fmol of small RNA can be detected by northern hybridization with a 1-day exposure to a phosphor imager plate when scanned at 25 µm resolution. The amount of RNA loaded and the exposure time may vary with the abundance of the individual miRNA species. For abundant miRNAs, loading of 5 µg total RNA and overnight exposure will give strong hybridization signals.
Literature Cited Alwine, J.C., Kemp., D.J., and Stark, G.R. 1977. Method for detection of specific RNAs in agarose gels by transfer to diazobenzyloxymethyl-paper and hybridization with DNA probes. Proc. Natl. Acad. Sci. U.S.A. 74:5350-5354. Bailey, J.M. and Davidson, N. 1976. Methylmercury as a reversible denaturing agent for agarose gel electrophoresis. Anal. Biochem. 70:75-85. Bodkin, D.K. and Knudson, D.L. 1985. Assessment of sequence relatedness of double-stranded RNA genes by RNA-RNA blot hybridization. J. Virol. Methods 10:45-52. Casey, J. and Davidson, N. 1977. Rates of formation and thermal stabilities of RNA:DNA and DNA:DNA duplexes at high concentrations of formamide. Nucl. Acids Res. 4:1539-1552.
Chomczynski, P. 1992. One-hour downward alkaline capillary transfer for blotting of DNA and RNA. Anal. Biochem. 201:134-139. Herrin, D.L. and Schmidt, G.W. 1988. Rapid, reversible staining of Northern blots prior to hybridization. BioTechniques 6:196-200. Kafatos, F.C., Jones, C.W., and Efstratiadis, A. 1979. Determination of nucleic acid sequence homologies and relative concentrations by a dot hybridization procedure. Nucl. Acids Res. 7:1541-1552. Lehrach, H., Diamond, D., Wozney, J.M., and Boedtker, H. 1977. RNA molecular weight determinations by gel electrophoresis under denaturing conditions: A critical reexamination. Biochemistry 16:4743-4751. Peferoen, M., Huybrechts, R., and De Loof, A. 1982. Vacuum-blotting: A new simple and efficient transfer of proteins from sodium dodecyl sulfate–polyacrylamide gels to nitrocellulose. FEBS Lett. 145:369-372. Smith, M.R., Devine, C.S., Cohn, S.M., and Lieberman, M.W. 1984. Quantitative electrophoretic transfer of DNA from polyacrylamide or agarose gels to nitrocellulose. Anal. Biochem. 137:120124. Southern, E.M. 1975. Detection of specific sequences among DNA fragments separated by gel electrophoresis. J. Mol. Biol. 98:503-517. Thomas, P.S. 1980. Hybridization of denatured RNA and small DNA fragments transferred to nitrocellulose. Proc. Natl. Acad. Sci. U.S.A. 77:5201-5205. Wilkinson, M. 1991. Purification of RNA. In Essential Molecular Biology: A Practical Approach, Vol. 1 (T.A. Brown, ed.) pp. 69-87. IRL Press, Oxford.
Contributed by Terry Brown University of Manchester Institute of Science and Technology Manchester, United Kingdom Karol Mackey (probe removal) Molecular Research Cincinnati, Ohio Tingting Du (miRNA blots) University of Massachusetts Medical School Worcester, Massachusetts
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UNIT 4.10
Identification of Newly Transcribed RNA Newly transcribed RNA can be identified using the nuclear runoff transcription assay. Isolated nuclei, free of membranes and cytoplasmic debris, are required for the assay. Cell lysis that does not allow the isolation of nuclei free of cell membranes and cytoplasmic material often results in poor incorporation of 32P-labeled UTP into nascent transcripts. Although there is no way to predict what cell types present this problem, many adherent cell lines and lymphocytes isolated from murine spleen or thymus do. Interestingly, very few nonadherent cell lines have posed this problem. Isolating nuclei by detergent lysis of cells (basic protocol) works well for many tissue culture cell lines but may not be appropriate for all cell lines and many tissues. Detergent lysis and Dounce homogenization (first alternate protocol) or cell lysis in an isoosmotic solution and centrifugation through a sucrose cushion (second alternate protocol) are alternative methods for preparing nuclei. The support protocol describes preparation of the cDNA nitrocellulose filter strips that are used to detect the presence of specific transcripts in the nuclear runoff transcription assay. NOTE: Experiments involving RNA require careful technique to prevent RNA degradation; see Chapter 4 Section I introduction.
BASIC PROTOCOL
NUCLEAR RUNOFF TRANSCRIPTION IN MAMMALIAN CELLS Nuclear runoff transcription is currently the most sensitive procedure for measuring specific gene transcription as a function of cell state. Nuclei are first isolated from cultured cells or tissues and frozen in liquid nitrogen. Thawed nuclei are incubated with 32P-labeled UTP and unlabeled NTPs to label nascent RNA transcripts. 32P-labeled RNA is purified and used to detect specific RNA transcripts by hybridization to cDNAs immobilized on nitrocellulose membranes. Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
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Cultures of mammalian cells (UNIT 9.0) or freshly isolated lymphoid cells Phosphate-buffered saline (PBS; APPENDIX 2), made fresh and ice cold Nonidet P-40 (NP-40) lysis buffer A (see recipe) Glycerol storage buffer (see recipe), ice cold 2× reaction buffer with and without nucleotides (see recipes) 10 mCi/ml [α-32P]UTP (760 Ci/mmol) 1 mg/ml DNase I (RNase-free; see recipe) HSB buffer (see recipe) SDS/Tris buffer (see recipe) 20 mg/ml proteinase K 25:24:1 (v/v/v) buffered phenol/chloroform/isoamyl alcohol (UNIT 2.1) 10% (v/v) trichloroacetic acid (TCA)/60 mM sodium pyrophosphate 10 mg/ml tRNA (UNIT 4.6) 5% (v/v) TCA/30 mM sodium pyrophosphate DNase I buffer (see recipe) 0.5 M EDTA, pH 8.0 20% (w/v) SDS Elution buffer (see recipe) 1 M NaOH 1 M HEPES (free acid) Contributed by Michael E. Greenberg and Timothy P. Bender Current Protocols in Molecular Biology (1997) 4.10.1-4.10.11 Copyright © 1997 by John Wiley & Sons, Inc.
3 M sodium acetate, pH 5.2 100% ethanol TES solution (see recipe) TES/NaCl solution (see recipe) cDNA plasmid(s) immobilized on nitrocellulose membrane (support protocol) 2× SSC (APPENDIX 2) 10 mg/ml heat-inactivated RNase A (UNIT 7.3) Rubber policeman 15- and 50-ml conical polypropylene centrifuge tubes Beckman JS-4.2 and JA-20 rotors or equivalent 30° and 65°C shaking water baths 42° and 65°C water baths 0.45-µm HA filters (Millipore) 30-ml Corex tube, silanized (APPENDIX 3) Whatman GF/F glass fiber filters 5-ml plastic scintillation vials Whatman 3MM filter paper NOTE: Keep cells and nuclei on ice until the nuclei are frozen. Isolate nuclei 1a. For cultures of adherent cells: Remove medium from monolayer cultures (5 × 107 cells per assay) and place cells on ice. Rinse twice with 5 ml ice-cold PBS. Scrape flask with a rubber policeman and collect cells in a 15-ml centrifuge tube. Centrifuge 5 min at 500 × g (1500 rpm in JS-4.2 rotor), 4°C. Remove supernatant. 1b. For cultures of nonadherent cells: Pipet up and down several times to resuspend cells (5 × 107 cells per assay) and transfer cells and medium to a 50-ml conical centrifuge tube. Centrifuge 5 min at 500 × g (1500 rpm in JS-4.2 rotor), 4°C, and remove supernatant. Wash by gently resuspending pellet in 5 ml ice-cold PBS, adding 45-ml ice-cold PBS, and collecting cells by centrifuging 5 min at 500 × g. Remove supernatant. Wash cells once more with PBS and remove supernatant. 1c. For freshly isolated lymphoid cells: Transfer lymphoid cells (5 × 107 cells per assay) removed directly from organ to a 50-ml conical centrifuge tube. Centrifuge 5 min at 500 × g (1500 rpm in JS-4.2 rotor), 4°C, and remove supernatant. Wash by resuspending pellet in 5 ml ice-cold PBS, adding 45 ml PBS, and collecting cells by centrifuging 5 min at 500 × g, 4°C. Remove supernatant. Wash cells once more with PBS and remove supernatant. Lymphoid cell nuclei are more fragile than other cell types, so a gentle procedure (e.g., second alternate protocol) may be required to isolate intact nuclei. It is not necessary to eliminate erythrocytes from lymphoid cells prior to preparation of nuclei. 5 × 107 cells are required for each nuclear runoff transcription assay.
2. Loosen cell pellet by gently vortexing 5 sec. Add 4 ml NP-40 lysis buffer A, continuing to vortex as buffer is added. After lysis buffer is completely added, vortex cells 10 sec at half maximal speed. Gentle vortexing (at a setting of six) uniformly resuspends cells and inhibits clumping. The same method is used to resuspend nuclei.
3. Incubate lysed cells 5 min on ice. Examine a few microliters of cell lysate on a hemacytometer with a phase-contrast microscope to ensure that cells have uniformly lysed and nuclei appear free of cytoplasmic material. Centrifuge 5 min at 500 × g, 4°C. Remove supernatant.
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Supernatant contains cytoplasmic RNA that can be purified as described in UNITS 4.1 & 4.5, if desired.
4. Resuspend the nuclear pellet in 4 ml NP-40 lysis buffer A by vortexing as described in step 2. Centrifuge 5 min at 500 × g, 4°C. Discard supernatant and resuspend nuclei in 100 to 200 µl glycerol storage buffer by gently vortexing. Freeze resuspended nuclei in liquid nitrogen. Nuclei are stable in liquid nitrogen for >1 year.
Perform nuclear runoff transcription 5. Thaw 200 µl frozen nuclei at room temperature and transfer to a 15-ml conical polypropylene centrifuge tube. Immediately add 200 µl of 2× reaction buffer with nucleotides plus 10 µl of 10 mCi/ml [α-32P]UTP. Incubate 30 min at 30°C with shaking. This reaction is done in a 15-ml polypropylene tube rather than a microcentrifuge tube to reduce the possibility of spilling radioactive materials.
6. Mix 40 µl of 1 mg/ml RNase-free DNase I and 1 ml HSB buffer. Add 0.6 ml of this solution to labeled nuclei and pipet up and down 10 to 15 times with a Pasteur pipet to mix thoroughly. Incubate 5 min at 30°C. 7. Add 200 µl SDS/Tris buffer and 10 µl of 20 mg/ml proteinase K. Incubate for 30 min at 42°C. DNA and protein should be well digested and a fairly uniform solution should be obtained. The presence of a substantial amount of particulate matter usually indicates that either DNase I or proteinase K treatment was not effective and should be repeated. It may be necessary first to ethanol precipitate the RNA, then to repeat the treatment with DNase I and proteinase K with fresh reagents.
Extract and precipitate RNA 8. Extract sample with 1 ml 25:24:1 buffered phenol/chloroform/isoamyl alcohol. Centrifuge 5 min at 800 × g (2000 rpm in JS-4.2 rotor), at or below room temperature. Transfer aqueous phase to a clean 15-ml polypropylene centrifuge tube. 9. Add 2 ml water, 3 ml of 10% TCA/60 mM sodium pyrophosphate, and 10 µl of 10 mg/ml E. coli tRNA carrier to aqueous phase. Incubate 30 min on ice. 10. Filter TCA precipitate onto 0.45-µm Millipore HA filter. Wash filter three times with 10 ml of 5% TCA/30 mM sodium pyrophosphate. If the HA filter clogs and filters very slowly, a Whatman GF/A glass fiber filter can be used instead.
11. Transfer filter to a glass scintillation vial. Incubate with 1.5 ml DNase I buffer and 37.5 µl of 1 mg/ml RNase-free DNase I for 30 min at 37°C. Quench the reaction by adding 45 µl of 0.5 M EDTA and 68 µl of 20% SDS. 12. Heat sample 10 min at 65°C to elute the RNA. Remove supernatant and save. Add 1.5 ml elution buffer to filter and incubate 10 min at 65°C. Remove supernatant and combine with original supernatant. This procedure removes >95% of the radioactivity from the filter. Identification of Newly Transcribed RNA
13. Add 4.5 µl of 20 mg/ml proteinase K to 3 ml supernatant containing 32P-labeled RNA. Incubate 30 min at 37°C.
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14. Extract 3 ml RNA solution once with 3 ml of 25:24:1 buffered phenol/chloroform/isoamyl alcohol. 15. Remove aqueous phase to a silanized 30-ml Corex tube. Add 0.75 ml of 1 M NaOH to aqueous phase. Let stand 10 min on ice. Quench reaction by adding 1.5 ml of 1 M HEPES. 16. Precipitate RNA by adding 0.53 ml of 3 M sodium acetate and 14.5 ml of 100% ethanol. Incubate 30 min on dry ice or overnight at −20°C. 17. Centrifuge RNA 30 min at 10,000 × g (9000 rpm in JA-20 rotor), 4°C. Remove ethanol and resuspend pellet in 1 ml TES solution. Shake 30 min at room temperature. RNA should be completely dissolved.
18. Count a 5-µl aliquot of each sample in duplicate by spotting onto Whatman GF/F glass fiber filters. If necessary, dilute sample by adding TES solution to adjust 32 P-labeled RNA to ≥5 × 106 cpm/ml. Hybridize RNA to cDNA 19. Mix 1 ml RNA solution with 1 ml TES/NaCl solution. In a 5-ml plastic scintillation vial, hybridize to cDNA immobilized on nitrocellulose membrane strip for 36 hr at 65°C with shaking. Use a vial rather than a plastic bag for hybridization to ensure reproducible quantitative hybridization of the same number of cpm to each sample within a given experiment. Coil the nitrocellulose strip before inserting it into the scintillation vial and be sure the strip is completely immersed in hybridization solution.
20. After hybridization, transfer the strips to a 50-ml tube and wash filter in 25 ml of 2× SSC 1 hr at 65°C. Repeat wash once with fresh 2× SSC. 21. Remove filter to a glass scintillation vial containing 8 ml of 2× SSC and 8 µl of 10 mg/ml RNase A. Incubate without shaking 30 min at 37°C. 22. Wash filter once more in 25 ml of 2× SSC 1 hr at 37°C. Blot filter dry on Whatman 3MM filter paper. Unravel the strips, tape them to Whatman 3MM filter paper, and expose to X-ray film. Appropriate exposure time and conditions will vary depending on the experiment.
ISOLATION OF NUCLEI BY DOUNCE HOMOGENIZATION This protocol is used for isolation of nuclei from cell types that do not give clean nuclear preparations after lysis in NP-40 lysis buffer A. If cells are not lysed by treatment with NP-40 lysis buffer A (basic protocol), the addition of Dounce homogenization in NP-40 lysis buffer B is usually sufficient to lyse the cells.
ALTERNATE PROTOCOL
Additional Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Lysis buffer (see recipe), ice cold Nonidet P-40 (NP-40) lysis buffer B (see recipe) Glycerol storage buffer (see recipe), ice cold Dounce homogenizer with type B pestle, ice cold 1.5-ml microcentrifuge tubes, chilled on dry ice NOTE: Keep cells and nuclei on ice until the nuclei are frozen.
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Harvest cells 1. Harvest and wash cells as in basic protocol step 1. 2. Remove supernatant and loosen cell pellet by gently vortexing 5 sec. Resuspend cell pellet to a single-cell suspension in 5 to 10 ml ice-cold lysis buffer. Add ice-cold lysis buffer to a total of 40 ml and rock tube back and forth for several seconds to distribute the cells. 3. Pellet cells at 500 × g (1500 rpm in JS-4.2 rotor), 4°C. Remove and discard supernatant. Resuspend pellet in 1 ml lysis buffer per 5 × 107 cells and vortex gently to mix. After centrifugation, the pellet should appear to be two to three times its initial size.
4. Add 1 ml NP-40 lysis buffer B per 5 × 107 cells and mix by gently rocking the tube. Break cells and collect nuclei 5. Transfer cells to an ice-cold Dounce homogenizer and break them with ten strokes of a B pestle or until nuclei appear free of membrane components by phase-contrast microscopy. 6. Transfer homogenized cells to a plastic 50-ml conical centrifuge tube and pellet nuclei by centrifugation 5 min at 500 × g, 4°C. Pellet should now be approximately one-third to one-half the starting volume and appear opaque white.
7. Carefully remove supernatant with a Pasteur pipet attached to a vacuum supply. Tilt the tube sideways so supernatant is pulled away from the pellet. Remove any bubbles or liquid that remain on the side of the tube. Return pellet to an ice bucket. 8. Loosen pelleted nuclei by gentle vortexing. Add 200 µl ice-cold glycerol storage buffer per 5 × 107 nuclei and resuspend pellet by pipetting up and down. Nuclei will be clumped at first but will disperse with continued pipetting. Pipetting should be steady but not hard enough to cause bubbles.
9. Aliquot 210 µl (∼5 × 107 nuclei) into chilled 1.5-ml microcentrifuge tube and immediately return tube to dry ice. Store nuclei at −70°C or in liquid nitrogen. Frozen nuclei are stable for at least 1 year.
10. Proceed with nuclear runoff transcription assay starting with basic protocol step 5.
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ISOLATION OF NUCLEI BY SUCROSE GRADIENT CENTRIFUGATION Quality of nuclei used in nuclear runoff protocols is a major determinant in the success of the experiment. Normal lymphocytes in particular can present a problem because the nuclei are more fragile. In this protocol, cells are resuspended in an isoosmotic buffer containing nonionic detergent, then lysed by Dounce homogenization. Nuclei are collected by ultracentrifugation through a sucrose cushion and are quite clean and free of contaminating membranes and cytoplasmic components. Typically, 10% to 30% more [α-32P]UTP is incorporated into nascent transcripts of nuclei prepared by this method. The density of nuclei varies with cell type so a pilot experiment should be performed to verify that these conditions (which work well for murine splenic lymphocytes and other vertebrate cells) result in a nuclear pellet.
ALTERNATE PROTOCOL
Additional Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Sucrose buffer I (see recipe), ice cold Sucrose buffer II (see recipe) Dounce homogenizer with B pestle, ice cold Polyallomer centrifuge tubes (9⁄16 × 33⁄4 in., Beckman) for SW 40.1 rotor Ultracentrifuge and SW 40.1 rotor or equivalent 1.5-ml microcentrifuge tubes, chilled on dry ice NOTE: Keep cells and nuclei on ice until the nuclei are frozen. Harvest and lyse cells 1. Harvest and wash cells as in basic protocol step 1. 2. Loosen cell pellet by gently vortexing 5 sec. Resuspend cell pellet in 4 ml ice-cold sucrose buffer I. Examine a small aliquot of cells for lysis with a phase-contrast microscope. Many cell types will lyse at this point and do not require Dounce homogenization. If cells have lysed, proceed directly to step 4.
3. Transfer cells to an ice-cold Dounce homogenizer and break the cells with five to ten strokes of a B pestle or until the nuclei appear free of cytoplasmic tags. Check a few microliters of cells with a phase-contrast microscope to be sure they are uniformly lysed. 4. Transfer nuclei to a clean 50-ml conical polypropylene centrifuge tube and add 4 ml sucrose buffer II. Mix by gentle pipetting and inversion. The final concentration of sucrose in cell homogenate should be sufficient to prevent a large buildup of debris at the interface between homogenate and sucrose cushion. The amount of sucrose buffer II added to cell homogenate may need to be adjusted.
Collect nuclei 5. Add 4.4 ml sucrose buffer II to polyallomer SW 40.1 tube. Sucrose buffer II serves as the sucrose cushion. Unlysed cells will not sediment through the sucrose cushion. If these conditions do not result in a nuclear pellet, adjust the concentration of sucrose in sucrose buffer II.
6. Carefully layer nuclei (from step 4) onto the sucrose cushion. Use sucrose buffer I to top off the gradient. Do not centrifuge more than 2 × 108 nuclei per tube.
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7. Centrifuge the gradient 45 min at 30,000 × g (15,500 rpm in SW 40.1 rotor), 4°C. 8. Remove supernatant by vacuum aspiration. Tilt the tube sideways so supernatant is pulled away from the pellet and remove any bubbles or liquid that remain on the side of the tube. Return tube to an ice bucket. Nuclei should form a tight pellet at the bottom of the tube and there may be some debris caught at the interface between sucrose buffers I and II. If the cells did not lyse during Dounce homogenization, nuclei will not pellet. Thus, it is important to be sure that the majority of the cells are clearly lysed in step 3. If the pellet appears as a gelatinous mass, nuclei have lysed and the pellet should be discarded.
9. Loosen nuclear pellet by gently vortexing 5 sec. Add 200 µl ice-cold glycerol storage buffer per 5 × 107 nuclei and resuspend nuclei by pipetting up and down. Nuclei will be clumped at first but will disperse with continued pipetting. Pipetting should be steady but should not create air bubbles.
10. Aliquot 210 µl (∼5 × 107 nuclei) into chilled microcentrifuge tube and immediately return tube to dry ice. Store frozen nuclei at −70°C or in liquid nitrogen. Frozen nuclei are stable for at least 1 year.
11. Proceed with nuclear runoff transcription assay starting at basic protocol step 5. SUPPORT PROTOCOL
PREPARATION OF NITROCELLULOSE FILTERS FOR NUCLEAR RUNOFF TRANSCRIPTION ASSAY cDNA plasmids are linearized and immobilized on nitrocellulose membrane filters for hybridization in the nuclear runoff transcription assay. Filters are prepared in advance and may be stored at least 6 months. Prior to their use for hybridization, filters are cut into strips that contain the cDNA plasmids of interest. Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
cDNA plasmid 1 M NaOH 6× SSC (APPENDIX 2) 0.45-µm nitrocellulose membrane Slot blot apparatus 80°C vacuum oven Additional reagents and equipment for restriction endonuclease digestion (UNIT 3.1) NOTE: Wear gloves and handle membranes with blunt-ended forceps. 1. Linearize 200 µg cDNA plasmid by digestion with an appropriate restriction enzyme. It is usually not necessary to phenol extract or ethanol precipitate the DNA after digestion if BSA is absent from the restriction enzyme digestion buffer. If the buffer contains BSA, extract plasmid DNAs with phenol/chloroform/isoamyl alcohol, ethanol precipitate, and resuspend in TE or similar buffer prior to denaturation (UNIT 2.1).
Identification of Newly Transcribed RNA
2. Add 49 µl of 1 M NaOH to linearized DNA (200 µg in 440 µl). Incubate 30 min at room temperature to denature DNA. 3. Add 4.9 ml of 6× SSC to DNA and place on ice to neutralize the sample.
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4. Set up slot blot apparatus with 0.45-µm nitrocellulose membrane. Apply 125 µl of sample (∼5 µg cDNA plasmid) to each slot under a low vacuum provided by a water aspirator. Rinse each slot with 500 µl of 6× SSC. 5. Use a blue pencil to mark the location on the membrane of slots containing DNA. It is difficult to detect the location of slots once nitrocellulose has dried. Mark the edge of the slot so that the nitrocellulose strip can be trimmed very close to the edge of the slot. It is possible to minimize the volume of hybridization solution if a narrow filter strip is used.
6. Air dry nitrocellulose filter overnight. Bake filter 2 hr in an 80°C vacuum oven. Store filter in a vacuum desiccator at either room temperature or 4°C. REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
DNase I, RNase-free (1 mg/ml) Adjust pH of 0.1 M iodoacetic acid/0.15 M sodium acetate to 5.3 and filter sterilize. Add sterile solution to lyophilized RNase-free DNase I (Worthington) to give a final concentration of 1 mg/ml. Heat 40 min at 55°C. Cool and add 1 M CaCl2 to a final concentration of 5 mM. Store 0.3-ml aliquots at −20°C. DNase I buffer 20 mM HEPES, pH 7.5 5 mM MgCl2 1 mM CaCl2 Sterilize by autoclaving Elution buffer 1% (w/v) SDS 10 mM Tris⋅Cl, pH 7.5 5 mM EDTA Sterilize by autoclaving Glycerol storage buffer 50 mM Tris⋅Cl, pH 8.3 40% (v/v) glycerol 5 mM MgCl2 0.1 mM EDTA HSB buffer 0.5 M NaCl 50 mM MgCl2 2 mM CaCl2 10 mM Tris⋅Cl, pH 7.4 Sterilize by autoclaving Lysis buffer 10 mM Tris⋅Cl, pH 7.4 3 mM CaCl2 2 mM MgCl2 Sterilize by autoclaving
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Nonidet P-40 (NP-40) lysis buffers Buffer A (basic protocol) 10 mM Tris⋅Cl, pH 7.4 10 mM NaCl 3 mM MgCl2 0.5% (v/v) NP-40
Buffer B (alternate protocol) 10 mM Tris⋅Cl, pH 7.4 3 mM CaCl2 2 mM MgCl2 1% (v/v) NP-40
Autoclave first three components and cool before adding NP-40. Reaction buffer, 2× 10 mM Tris⋅Cl, pH 8.0 5 mM MgCl2 0.3 M KCl Sterilize by autoclaving Reaction buffer with nucleotides, 2× 1 ml 2× reaction buffer (see recipe) 10 µl 100 mM ATP 10 µl 100 mM CTP 10 µl 100 mM GTP 5 µl 1 M DTT Prepare immediately prior to use Separate 100 mM solutions of each nucleotide should be prepared in 0.5 M EDTA (pH 8.0) and the pH of each one checked to be sure it is between 7.0 and 8.0. The solutions should be stored in aliquots at −20°C.
SDS/Tris buffer 5% (w/v) SDS 0.5 M Tris⋅Cl, pH 7.4 0.125 M EDTA Sterilize by autoclaving Sucrose buffer I 0.32 M sucrose 3 mM CaCl2 2 mM magnesium acetate 0.1 mM EDTA 10 mM Tris⋅Cl, pH 8.0 1 mM DTT 0.5% (v/v) Nonidet P-40 (NP-40) Prepare without DTT and NP-40. Autoclave and cool to room temperature. Add DTT and NP-40 just prior to use. Sucrose buffer II 2 M sucrose 5 mM magnesium acetate 0.1 mM EDTA 10 mM Tris⋅Cl, pH 8.0 1 mM DTT Prepare without DTT. Autoclave buffer and cool to room temperature. Add DTT just prior to use. Identification of Newly Transcribed RNA
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TES solution 10 mM N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES), pH 7.4 10 mM EDTA 0.2% (w/v) SDS Sterilize by autoclaving TES/NaCl solution 10 mM N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES), pH 7.4 10 mM EDTA 0.2% (w/v) SDS 0.6 M NaCl Sterilize by autoclaving COMMENTARY Background Information The nuclear runoff transcription assay allows direct measurement and comparison of specific gene transcription in cells in various states of growth or differentiation. It takes advantage of the fact that newly synthesized RNA can be labeled to high specific activity in isolated nuclei, something that is difficult to accomplish in intact cells. The protocol described here has the advantage that it facilitates measurement of the level of transcription for many different genes in a single experiment. A point of controversy is whether some initiation of new RNA synthesis occurs in isolated nuclei during the runoff transcription reaction. What is clear is that transcripts that have initiated prior to cell lysis are faithfully elongated. Elongation of previously initiated transcripts most likely accounts for the bulk of the radioactivity incorporated into RNA in these reactions. Therefore, the method gives a reasonably accurate measure of the level of transcription occurring at the time of cell lysis. The runoff transcription assay is often used to assess whether changes in mRNA levels of a particular gene that occur as a function of cell state reflect a change in its synthesis as opposed to a change in mRNA degradation or transport from the nucleus to the cytoplasm. A variety of different nuclear runoff transcription protocols have been described. The procedures differ primarily in the method of isolation of 32P-labeled RNA. The protocol described here is advantageous because very low levels of background transcription are obtained. This is attributed in part to the TCA precipitation step, which allows effective removal of unincorporated 32P. Limited digestion of 32P-labeled RNA with NaOH appears to facilitate hybridization. Excellent reviews of
the nuclear runoff transcription method have been published by Marzluff (1978) and by Marzluff and Huang (1985).
Critical Parameters The most critical step in the nuclear runoff assay is isolation of nuclei. For many different types of tissue culture cells, the procedure described in the basic protocol works well. However, the isolation protocol may have to be altered somewhat for isolating nuclei from tissues and lymphocytes (Marzluff and Huang, 1985). Poor incorporation of 32P-labeled UTP into RNA in isolated nuclei may reflect damage to the nuclei during isolation or failure to isolate nuclei free of cytoplasmic and membrane contaminants. Lymphocyte nuclei are quite fragile, and it is necessary to employ an alternative isolation procedure to obtain nuclei that incorporate significant levels of radioactive isotope. In this alternative protocol, cells are lysed by Dounce homogenization in an isoosmotic buffer with nonionic detergent and nuclei are collected by centrifugation through a sucrose cushion. A minimum of 5 × 106 nuclei is required for a successful assay, but 5 × 107 is recommended. With fewer nuclei the level of incorporation of 32P-labeled UTP into RNA drops significantly, and the level of specific transcription is usually difficult to distinguish from background radioactivity. In this protocol the cpm/ml of 32P-labeled RNA is equalized in each sample prior to hybridization based on the assumption that the overall level of RNA synthesis is not changing as a function of the cell state. When analyzing gene transcription under a new set of conditions, it is critical to determine if this is a reasonable assumption. Particular care must be taken in these experiments to ensure that the
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same number of nuclei is used for each cell state analyzed. Incorporation of 32P-labeled UTP into RNA appears to increase when the number of nuclei is increased between 5 and 50 × 106 nuclei. Increasing the amount of 32P-labeled UTP beyond 100 µCi per sample appears to be less useful. Increasing the incubation time of nuclei with 32P-labeled UTP beyond 30 min is also ineffective for increasing the amount of 32P incorporated into RNA.
Anticipated Results
Beginning with 5 × 106 nuclei and 100 µCi of 32P-labeled UTP, the nuclear runoff transcription protocol allows incorporation of 1–10 × 106 cpm into total RNA. Good results have been obtained when as few as 1 × 106 cpm were used in the hybridization reaction.
Time Considerations Nuclei to be tested are isolated and stored in liquid nitrogen. NP-40 lysis and Dounce homogenization take 3 mm thick. Resolution in the Bull’s-eye apparatus is determined by several factors, including the concentration of the gel (Southern, 1979a,b). As the concentration of the gel increases, so does the time required for a DNA fragment of a certain size to elute. There is an upper limit to gel concentration, however, because with higher concentrations the time required for separation becomes unacceptably long, and the DNA concentration in each sample is reduced. Generally, a gel concentration of 1.2% is optimal for separation of fragments for cloning.
Literature Review The concept of simplifying cloning by gene enrichment has been reviewed by Edgell et al. (1979). They describe a two-step DNA fractionation procedure involving RPC5 column chromatography followed by preparative electrophoresis. Bott et al. (1980) have used preparative agarose electrophoresis to fractionate the Bacillus subtilis genome to produce DNA fragments for transformation analysis and cloning. The degree of enrichment achieved with preparative electrophoresis alone is adequate and allows one to start with a smaller quantity of DNA. Methods for constructing an apparatus that allows collection of multiple samples of sizefractionated DNA from a slab horizontal gel have been described (Polsky et al., 1978; Southern, 1979a,b). The resolution on a preparative agarose slab gel is adequate (although not as good as the Bull’s-eye apparatus). The primary difficulty with some preparative agarose gels is that the DNA that is eluted contains impurities that inhibit DNA ligase and prevent cloning. The Bull’s-eye apparatus is discussed here because it is commercially available, yields DNA that can be ligated, and produces large numbers
Troubleshooting Using the preparative agarose gel technique, fragments can generally be identified and eluted. Problems sometimes arise in that not enough clones can be obtained to make a subgenomic library (see below). Generally, if this occurs, it is best to start over with another fractionation technique. The Bull’s-eye apparatus is not easy for the beginner. Setting up the apparatus can be difficult, and instruction from someone skilled in the use of this machine is advised. In particular, placement of the dialysis tubing over the central anode is a tricky and crucial step in assembly. However, once assembled correctly the machine rarely malfunctions. Occasionally, bubbles will impede flow of buffer in one of the circuits. This is prevented by clearing the tubing of bubbles before starting a run. Failure of the tubing leading to the fraction collector to run dry in less than 7 min usually means that either the peristaltic pump in this circuit is too slow or that there is a leak into the sample collection chamber. Such a leak occurs through the dialysis tubing that covers the anode and is corrected by replacing the
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5.4.3 Current Protocols in Molecular Biology
dialysis tubing. DNA fractionated in this fashion and cleaned by ethanol precipitation and chloroform extractions usually can be ligated efficiently into bacteriophage or plasmid vectors. If appropriate controls point to the fractionated DNA as the source of difficulty, it may help to repeat the phenol and chloroform extractions or to pass the DNA over an Elutip column (UNIT 2.6). If the DNA still cannot be ligated it probably means that the restriction fragment ends were damaged by exonuclease activity.
Anticipated Results Up to 80% of the DNA applied to a gel (either a slab gel or a Bull’s-eye gel) should be recovered. If 1 mg DNA is digested and loaded onto the gel, and 100 fractions collected, each fraction will contain several micrograms of DNA. This is enough DNA to produce a number of subgenomic libraries. The size distribution of the fragments will vary, depending on the enzyme used, but fractions around 4 kb will usually contain the most DNA.
Time Considerations Fractionation in a preparative agarose slab gel is rapid, generally 5 kb) cDNA.
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3. In a separate tube add in the following order (180 µl total): 20 µl 5 mM dNTPs (500 µM final each) 40 µl 5× RT buffer (1× final) 10 µl 200 mM DTT (10 mM final) 20 µl 0.5 mg/ml oligo(dT)12-18 (50 µg/ml final) 60 µl H2O 10 µl (10 U) RNasin (50 U/ml final). Mix by vortexing, briefly microcentrifuge, and add the mixture to the tube containing the RNA. Add 20 µl (200 U) AMV reverse transcriptase for a final concentration of 1000 U/ml in 200 µl. Mix as above and remove 10 µl to a separate tube containing 1 µl of [α-32P]dCTP. Leave both tubes at room temperature 5 min, then place both tubes at 42°C for 1.5 hr. The aliquot is removed to determine incorporation and permit an estimation of recovery. The remainder of the cDNA will be labeled during second-strand synthesis. Labeling cDNA during first-strand synthesis to a high enough specific activity to permit easy detection with a hand-held radiation monitor during all subsequent steps requires a relatively large amount of label, which may then interfere with reverse transcription due to buffer effects. Some investigators check the quality of the cDNA by fractionating the radiolabeled cDNA on an alkaline agarose gel and detecting it by autoradiography. Much of the cDNA should be >1000 bp long. For a specifically primed library, substitute an equal weight of antisense 15- to 40-mer primer for the oligo(dT)12-18. Expect a 100-fold enrichment of specific clones in the library. For a randomly primed library, substitute an equal weight of random-hexamer primers for oligo(dT)12-18 (or use a 50:50 mix), and perform the reverse transcription at 37° instead of 42°C.
4. Add 1 µl of 0.5 M EDTA, pH 8.0, to the radioactive reaction and freeze it at −20°C. It will be used later to estimate the amount of cDNA synthesized. 5. To the main reaction add 4 µl of 0.5 M EDTA, pH 8.0, and 200 µl buffered phenol. Vortex well, microcentrifuge at room temperature for 1 min to separate phases, and transfer the upper aqueous phase to new tube. Save the tube containing the phenol layer, too.
6. Add 100 µl TE buffer, pH 7.5, to the phenol layer and vortex and microcentrifuge as in step 5. Remove the aqueous layer and add it to the aqueous phase from the first extraction. The volume of aqueous phase is now about 300 ìl; the phenol may be discarded. Back extraction of the organic phase at each phenol extraction significantly improves the yield. See UNIT 2.1 for a discussion of phenol extraction and ethanol precipitation.
7. Add 1 ml diethyl ether, vortex, and microcentrifuge as in step 5. Remove and discard the upper (ether) layer with a glass pipet. Repeat the extraction with an additional 1 ml of ether. A single chloroform/isoamyl alcohol extraction followed by back extraction of the organic phase may be substituted for the two ether extractions; however, the yield is normally slightly lower.
Conversion of mRNA into Double-Stranded cDNA
8. Add 125 µl of 7.5 M ammonium acetate to the aqueous phase (final concentration 2.0 to 2.5 M) and 950 µl of 95% ethanol. Place in dry ice/ethanol bath 15 min, warm
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to 4°C, and microcentrifuge at 10 min at full speed, 4°C, to pellet nucleic acids. A small, yellow-white pellet may be visible. Precipitation from ammonium acetate leaves short oligonucleotides in the supernatant, thus removing the oligo(dT) primer and enriching the pellet in longer cDNAs. Do not substitute sodium acetate for ammonium acetate.
9. Remove the supernatant with a pipet, fill the tube with ice-cold 70% ethanol, and microcentrifuge 3 min at full speed, 4°C. Again remove the supernatant, then dry the tube containing the precipitated nucleic acids briefly in a vacuum desiccator. 10. Thaw the tube containing the radioactive aliquot of the first-strand synthesis reaction and spot the sample onto a nitrocellulose membrane filter. 11. Wash the membrane with ice-cold 10% TCA and determine the radioactivity bound to the filter with a fluor and scintillation counter. Use the specific activity of the radiolabel in the reaction, the amount of RNA used, the counts incorporated, and the efficiency of the β-counter to calculate the amount of cDNA synthesized (see Sample Calculation for Determining Amount of cDNA Synthesized, in Commentary). From 1 to 4 ìg is typical even though the theoretical maximum is 10 ìg.
Convert cDNA into double-stranded cDNA 12. Resuspend the pellet from the first-strand synthesis in 284 µl water and add to the tube in the following order (400 µl total): 4 µl 5 mM dNTPs (50 µM final each) 80 µl 5× second-strand buffer I (1× final) 12 µl 5 mM β-NAD+ (150 µM final) 2 µl 10 µCi/µl [α-32P]dCTP (50 µCi/ml final). Mix by vortexing, briefly microcentrifuge, and add: 4 µl (4 U) RNase H (10 U/ml final) 4 µl (20 U) E. coli DNA ligase, (not T4 DNA ligase; 50 U/ml final) 10 µl (100 U) E. coli DNA polymerase I (250 U/ml final). Mix by vortexing, briefly microcentrifuge, and incubate 12 to 16 hr at 14°C. Unrelated cDNA fragments are not ligated in this reaction because E. coli DNA ligase does not catalyze blunt ligations.
13. After second-strand synthesis, remove 4 µl of the reaction to a new tube and freeze at −20°C. Later, when time permits, determine the incorporation of radiolabel into acid-insoluble material as outlined in steps 10 and 11. Expect 1-10 × 106 cpm incorporated in the total reaction.
14. Phenol extract the second-strand synthesis reaction with 400 µl buffered phenol and back extract the phenol phase with 200 µl TE buffer, pH 7.5, as in steps 5 and 6. 15. Pool the two aqueous phases and extract twice with 900 µl ether, as in step 7. The volume of the aqueous phase is now ∼600 µl. 16. Divide the aqueous phase evenly between two tubes, add ammonium acetate, and ethanol precipitate as in steps 8 and 9. Unincorporated radioactive dCTP is removed by the ethanol precipitation and wash steps. From this point on, the cDNA may be followed with a hand-held radiation monitor. Construction of Recombinant DNA Libraries
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Create blunt ends on double-stranded cDNA 17. Complete second-strand synthesis and blunt the double-stranded cDNA by resuspending the pooled pellets in 42 µl water. Add in the following order (80 µl total): 5 µl 5 mM dNTPs (310 µM final each) 16 µl 5× TA buffer (1× final) 1 µl 5 mM β-NAD+ (62 µM final). Mix by vortexing, microcentrifuge briefly, and add: 4 µl of 2 µg/ml RNase A (100 ng/ml final) 4 µl (4 U) RNase H (50 U/ml final) 4 µl (20 U) E. coli DNA ligase (250 U/ml final) 4 µl (8 U) T4 DNA polymerase (100 U/ml final). Mix as above and incubate 45 min at 37°C. Volume of T4 DNA polymerase used to obtain 8 U may require adjustment depending on batch. If the library is to be screened with an antiserum, some investigators (see Tamkun et al., 1986) digest the cDNA at this point with AluI or HaeIII to prepare small inserts that may produce more stable fusion proteins in λgt11. Do this after the T4 polymerase step by diluting the 80-ìl reaction to 170 ìl with water; add 24 ìl of 5× TA buffer and 6 ìl of one of the above restriction enzymes. Incubate 1 hr at 37°C, then proceed to step 18, except do not add any TE buffer. An insert isolated from an immunological screen may then be used to screen a second library of full-length inserts.
18. Add 120 µl TE buffer, pH 7.5, and 1 µl of 10 mg/ml tRNA. Extract with 200 µl buffered phenol and back extract the phenol phase with 100 µl TE buffer as described in steps 5 and 6. 19. Pool the two aqueous phases and extract twice with 1 ml ether, as in step 7. 20. Ethanol precipitate the cDNA as in steps 8 and 9. The cDNA is now ready to be tailed or linkered to create compatible ends for subsequent cloning steps. ALTERNATE PROTOCOL
CONVERSION OF mRNA INTO DOUBLE-STRANDED cDNA FOR DIRECTIONAL CLONING Generation of cDNA that has unique ends for directional cloning is carried out in two parts, as in the basic protocol. First, the mRNA is hybridized to a linker-primer that incorporates a poly(dT) tract (at its 3′ end) as well as a restriction site for XhoI (Fig. 5.5.2). The linker-primer is extended using an RNase H− version of the Moloney murine leukemia virus reverse transcriptase (SuperScript) and a nucleotide mix in which dCTP is replaced with 5-methyl-dCTP. When first-strand synthesis is completed, the reaction mixture is transferred into a second tube that contains the prechilled second-strand mixture. The second strand is synthesized using RNase H and E. coli DNA polymerase I. Finally, a blunting step (consisting of treatments with mung bean nuclease and Klenow fragment) is carried out to prepare the cDNA for the EcoRI adaptor ligation.
Conversion of mRNA into Double-Stranded cDNA
Additional Materials (also see Basic Protocol) 5× SuperScript buffer (RNase-free; see recipe) 0.1 M DTT (RNase-free) 3dNTP/methyl-dCTP mix: 10 mM each dATP, dGTP, dTTP, and 5-methyl-dCTP (Pharmacia Biotech) continued
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AAAAAAAn TTT n GAGCTC (GA)10
reverse transcription AAAAAAA TTTTTTTGAGCTC (GA)10
Me
Me
second - strand synthesis AAAn TTT n GAGCTC (GA)10
Figure 5.5.2 Synthesis of cDNA with unique ends for directional cloning.
0.25 µg/µl oligonucleotide primer (UNIT 2.11) incorporating (from 5′ to 3′): (dGdA)10, XhoI restriction site, and (dT)18 200 U/µl SuperScript or SuperScript II (GIBCO/BRL) 5× second-strand buffer II (see recipe) 10 mM and 2 mM 4dNTP mix (Pharmacia Biotech) 10 µCi/µl [α-32P]dATP (3000 Ci/mmol) 0.8 U/µl RNase H (Pharmacia Biotech) 10 U/µl E. coli DNA polymerase I (New England Biolabs) 1:1 (w/v) phenol/chloroform 100% ethanol ice-cold 10× Klenow buffer (see recipe) 5 U/µl Klenow fragment of E. coli DNA polymerase I (New England Biolabs) 10× mung bean nuclease buffer (see recipe) 10 U/µl mung bean nuclease (New England Biolabs) 1 M Tris⋅Cl, pH 8.0 (APPENDIX 2) 16°C incubator Additional reagents and equipment for agarose gel electrophoresis (UNIT 2.5) Synthesize cDNA 1. Prepare 5 to 7 µg poly(A)+ RNA. If mRNA is prepared as in UNIT 4.5, store under ethanol in a microcentrifuge tube until ready to proceed. If mRNA is in aqueous solution, add 3 M sodium acetate (RNase-free) to 0.3 M and 3 vol of 100% ethanol, chill ≥20 min at −20°C, and microcentrifuge 20 min at maximum speed, 4°C, to collect mRNA. Rinse the pellet with 70% ethanol (RNase-free), and invert the tube to air dry.
2. Prepare first-strand premix (45 µl final by mixing in the order listed): 10 µl 5× SuperScript buffer 5 µl 0.1 M DTT 2.5 µl 3dNTP/methyl-dCTP mix 1 µl RNasin 12 µl 0.25 µg/µl oligonucleotide primer 14.5 µl H2O. 3. Resuspend RNA pellet directly into premix. Incubate 15 min at room temperature, then add 5 µl of 200 U/µl SuperScript and incubate 1 hr at 42°C.
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4. Shortly before end of incubation, prepare second-strand premix (336 µl final) containing: 80 µl 5× second-strand buffer II 6 µl 10 mM 4dNTP mix 2.5 µl 10 µCi/µl [α-32P]dATP 247.5 µl H2O. Place on ice. 5. Continuing to work on ice, add the first-strand mix from step 3 (50 µl total) to prechilled second-strand premix from step 4. 6. In a second microcentrifuge tube combine 4 µl of 0.8 U/µl RNase H (3.2 U) and 10 µl of 10 U/µl E. coli DNA polymerase I (100 U). Add combined enzymes to the tube from step 5 containing the cDNA and mix by rapidly inverting a few times. In order to prevent formation of “snapbacks” during second-strand synthesis it is important to prechill the premix and add the RNase H and DNA polymerase simultaneously.
7. Microcentrifuge 5 sec at maximum speed, then incubate reaction 1 hr at 16°C and 1 hr at room temperature. At this step, second-strand synthesis is completed.
8. Electrophorese ∼2% to 5% of total reaction on a 0.7% agarose gel, and monitor size range of labeled cDNA by autoradiography. Optimally, a smear ranging from ∼200 bp to >8 kb will be seen, with a peak at 1 to 2 kb. The reaction can be frozen overnight (or longer) at −20°C, or one can immediately proceed to blunting and linker ligation.
Create blunt-ends on double-stranded cDNA 9. Add 500 µl of 1:1 phenol/chloroform to reaction. Microcentrifuge 5 min at maximum speed to separate phases. If reaction was frozen, thaw by vortexing with the phenol/chloroform.
10. Remove upper phase to a fresh tube, add 200 µl of 7.5 M ammonium acetate, and mix by inversion. 11. Divide sample evenly between two tubes (300 µl in each). Add 600 µl ice-cold 100% ethanol to each tube, place both tubes 5 min at −80°C, then microcentrifuge 20 min at maximum speed, 4°C. This and the following series of steps involve transfer of labeled cDNA from tube to tube, and require a number of extractions and precipitations. After this initial precipitation, the bulk of the radiolabel is incorporated in the cDNA. It is important to monitor using a Geiger counter to ensure that all cDNA is transferred from step to step and not lost during the extractions.
12. Pour off supernatants and rinse pellets with ice-cold 70% ethanol. Dry briefly in a Speedvac evaporator. Add 30 µl water to one of the two tubes, vortex to resuspend cDNA pellet, then transfer contents to the second tube. 13. Add an additional 10 µl water, 5 µl of 10× Klenow buffer, and 5 µl of 2 mM 4dNTP mix to the first tube. Vortex and transfer to the second tube. Add 5 µl of 10 U/µl Klenow fragment to the second tube and incubate 30 min at 37°C. Conversion of mRNA into Double-Stranded cDNA
The aim of the first Klenow treatment is to ensure that the 3′ end of the cDNA is
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completely blunted so that the XhoI restriction site will not be harmed by the subsequent enzymatic manipulations.
14. Add 50 µl of 1:1 phenol/chloroform to reaction, extract, and transfer upper phase to new tube. Back extract lower phase with an additional 50 µl water and combine it with the first extraction. 15. Add 50 µl of 7.5 M ammonium acetate and 300 µl ice-cold 100% ethanol. Place at −80°C for 5 min, then microcentrifuge 20 min at maximum speed, 4°C. 16. Pour off supernatant and rinse pellet with cold 70% ethanol. Dry pellet briefly in a Speedvac evaporator. Resuspend cDNA in 225 µl water. 17. Add 25 µl of 10× mung bean nuclease buffer and 1 µl of 1 U/µl mung bean nuclease. Incubate 15 min at 37°C. This mung bean exonuclease treatment removes any residual single-stranded nucleic acid extensions (i.e., single-stranded DNA, RNA overhangs, or small hairpins) from the 5′ end of the cDNA.
18. Add 25 µl of 1 M Tris⋅Cl, pH 8.0. Extract with 200 µl 1:1 phenol/chloroform and back extract with 50 µl water, pooling the upper phases in a new tube. 19. Add 175 µl of 7.5 M ammonium acetate and fill tube with ice-cold 100% ethanol. Place at −80°C for 5 min, then microcentrifuge 20 min at maximum speed, 4°C. 20. Rinse pellet with 70% ethanol and air dry briefly. Resuspend in 20 µl water, then add 2.5 µl of 10× Klenow buffer and 1 µl of 5 U/µl Klenow fragment. Incubate 5 min at 37°C. Add 2.5 µl of 2 mM dNTP mix to the reaction and incubate an additional 25 min at room temperature. Do not overdry pellet (i.e., if Speedvac evaporation is used, do not dry >5 min). The goal of this second Klenow fragment treatment is to effectively blunt any short single-stranded extensions that might remain after the mung bean exonuclease treatment or be caused by the “sloppiness” of this enzyme. This series of steps effectively causes essentially all the cDNA to become blunt-ended.
21. Add 25 µl of 1:1 phenol/chloroform to the reaction and extract, removing upper phase to a new tube. Back extract lower phase with an additional 25 µl water and pool with first upper phase. Store sample frozen at −20°C until ready for the EcoRI adaptor ligation step. EcoRI adaptor ligation is carried out as it is described in UNIT 5.6. Subsequently, XhoI digestion of the EcoRI adaptor–ligated cDNA results in a 5′ end that has an EcoRI-compatible overhang and a 3′ end with XhoI-compatible overhang.
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REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Buffer stock solutions Evaluate water and buffers for nuclease activity by incubating 5 µl solution with 3 µg of intact total cellular RNA or 0.5 µg of a DNA fragment, whichever is relevant, for 30 min. Check the RNA or DNA by agarose gel electrophoresis to be sure it was not degraded. Prepare at least 10 ml of each of the following 1 M stock solutions. Use RNA-grade water (see UNIT 4.1) and pass each solution through a sterile 0.45-µm filter. Store at room temperature unless otherwise indicated. 1 M Tris⋅Cl, pH 8.2, 42°C 1 M Tris⋅acetate: titrate aqueous Tris base to pH 7.8 with acetic acid 1 M KCl 1 M MgCl2 1 M (NH4)2SO4 1 M potassium acetate 1 M magnesium acetate 1 M DTT, −20°C, in tightly capped tube 5 mg/ml nuclease-free BSA, −20°C The pH of Tris buffers varies considerably with temperature. Be sure to measure the pH at the indicated temperature and use an electrode that accurately measures the pH of Tris buffers.
Enzymes Enzymes should be of the highest quality available. Before starting a cDNA cloning experiment with a rare or hard-to-obtain RNA prep, it is advisable to check the activities of all the enzymes used. Klenow buffer, 10× 0.5 M Tris⋅Cl, pH 7.6 0.1 M MgCl2 Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C. Mung bean nuclease buffer, 10× 0.3 M sodium acetate, pH 4.5 0.5 M NaCl 10 mM ZnCl2 50% (v/v) glycerol Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C.
Conversion of mRNA into Double-Stranded cDNA
Reverse transcriptase (RT) buffer, 5× 250 µl 1 M Tris⋅Cl, pH 8.2 250 µl 1 M KCl 30 µl 1 M MgCl2 470 µl H2O Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C.
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RNase A (DNase-free) Dissolve 1 mg/ml RNase A (Sigma) in TE buffer (APPENDIX 2); boil 10 min to remove contaminating DNase and store in aliquots at −20°C. Dilute to 2 µg/ml in TE buffer when needed. Second-strand buffer I, 5× 100 µl 1 M Tris⋅Cl, pH 7.5 500 µl 1 M KCl 25 µl 1 M MgCl2 50 µl 1 M (NH4)2SO4 50 µl 1 M DTT 50 µl 5 mg/ml BSA 225 µl H2O Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C. Second-strand buffer II, 5× 94 mM Tris⋅Cl, pH 6.9 453 mM KCl 23 mM MgCl2 50 mM (NH4)2SO4 Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C. SuperScript buffer, 5× 250 mM Tris⋅Cl, pH 8.3 375 mM KCl 15 mM MgCl2 Prepare 1.0 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C. TA buffer, 5× 200 µl 1 M Tris⋅acetate, pH 7.8 400 µl 1 M potassium acetate 60 µl 1 M magnesium acetate 3 µl 1 M DTT 105 µl 5 mg/ml BSA 432 µl H2O Prepare 1.2 ml buffer using high-quality water and buffer stock solutions (see recipe), then check for nuclease activity as described in buffer stock solution recipe. Freeze in 200-µl aliquots in screw-cap microcentrifuge tubes at −80°C.
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COMMENTARY Background Information
Conversion of mRNA into Double-Stranded cDNA
cDNA synthesis and subsequent library preparation are necessary for the study of most mRNAs because no technology for conveniently manipulating and propagating RNA sequences is currently available. Of fundamental importance is that the cDNAs faithfully represent the sequence, size, and complexity of the mRNA population. The high quality of commercially available enzymes and reagents has dramatically changed the major concerns of the scientist preparing a cDNA library. In the past, considerable attention was paid to the source of reverse transcriptase, the quality of other enzymes, methods for their further purification, and conditions for optimum activity (Maniatis et al., 1982). These issues have been successfully addressed and full-length cDNAs of 5 to 8 kb mRNAs are now obtainable. The major focus currently is on the preparation of libraries of full-length cDNAs that will contain copies of even the rarest mRNA. There are several relatively recent innovations incorporated into these protocols. First, it is recommended that size fractionation of the mRNA be omitted in order to minimize the opportunities for degradation of the mRNA template. Size fractionation of the cDNA, which is much more stable, is discussed in UNITS 5.3 & 5.4. Second-strand synthesis employs RNase H, E. coli DNA ligase, and E. coli DNA polymerase I as described by Okayama and Berg (1982) and modified by Gubler and Hoffman (1983). More recently, Neve et al. (1986) introduced during the blunting process the additional second-strand synthesis step, which increases the yield and size of the cDNA. Using this modification, S1 nuclease cleavage of the 5′ “hairpin loop” generated by reverse transcriptase, a problematic step in the past, is avoided. The cDNA produced is ready to be prepared for insertion into an appropriate vector. Variations such as omitting the DNA ligase, (e.g., as per Gubler and Hoffman, 1983), omitting the additional second-strand synthesis, or substituting Klenow fragment for the intact E. coli DNA polymerase I have been successfully employed by others. Finally, the basic protocol uses AMV (avian myeloblastosis virus) reverse transcriptase (RT), but the alternate protocol uses SuperScript, a genetically engineered derivative of Moloney murine leukemia viral (MoMuLV) RT that has had its RNase H activity removed (Kotewicz et al., 1988) and its polymerase activity enhanced, resulting in a
greater size and yield of cDNA from first-strand synthesis. A number of variations of the protocols in this unit exist for particular applications. Random hexamers can be used to prime first-strand synthesis rather than oligo(dT)-containing primers: resulting cDNAs will be enriched for the 5′-end sequences of very long transcripts, although average cDNA length will be shorter than with the methods described here. Another method that has gained popularity for a variety of analytical techniques is to perform a standard first-strand synthesis from an mRNA of interest, and then amplify specific rare transcripts using PCR. Over the last several years, a number of kits for cDNA synthesis have become commercially available (e.g., from GIBCO/BRL, Stratagene, Promega, Amersham, and Pharmacia Biotech), and generally yield excellent results.
Critical Parameters RNA used to generate the cDNA must be undegraded and of the highest quality obtainable. It must also be DNA-free. Buffers and solutions must be nuclease-free. These may be divided into aliquots, frozen, and stored for long periods; thus, it is worthwhile to check each by incubating intact RNA or DNA, whichever is appropriate, with a sample of the solution and checking for degradation by agarose gel electrophoresis. Enzymes (and all reagents) should be the highest quality available. If experimenting with a rare or hard-to-isolate RNA preparation, it is advisable to check the activities of all enzymes used. Enzymes should be relatively fresh and stored at −20°C in a freezer that is not frost-free (because the warming and cooling cycles of a frost-free freezer cause the enzymes to lose activity). The RNasin added in the first-strand synthesis is a potent though reversible inhibitor of RNase and must be added after the DTT, as its activity is dependent on the presence of a reducing agent.
Troubleshooting Once high-quality mRNA has been obtained, the cDNA synthesis usually proceeds well. Difficulties, if any, only occur later— when the cDNA is ligated to the vector. If the yield of cDNA is low, likely causes are inactive reverse transcriptase, inactive E. coli polymerase I, or deteriorated dNTP mixes. If the calculated yield is low, double-check the result
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Sample Calculation for Determining Amount of cDNA Synthesized 1. Assume 2 × 1012 cpm/Ci; thus, if [α-32P]dCTP is at a specific activity of 3000 Ci/mmol, there are 6 × 1015 cpm/mmol. 2. The second-strand synthesis aliquot representing 1% of the total contains 105 TCA-precipitable cpm, thus 105 cpm × 100 6 × 1015 cpm/mmol
= 1.67 × 10−9 mmol [alpha−32P]dCTP incorporated.
3. The initial concentration of [α-32P]dCTP was 50 µCi/ml = 1.67 × 10–2 µM. 3000 Ci/mmol
4. Thus, the total dCTP incorporated is 50 µM –2
1.67 × 10
µM
× 1.67 × 10–9 mmol ×
0.3 g dCMP = 1.5 µg. mmol
5. There were four deoxynucleotides present in the reaction, so the total cDNA synthesized in the second-strand reaction is 6 µg and since the cDNA is doublestranded, there are 12 µg of cDNA total.
by running 5% of the total sample on an agarose minigel and staining with ethidium bromide, or by determining the amount of DNA in the sample by optical density or DAPI fluorescence. If the radiolabel was old or deteriorated, incorporation will be low but the amount of cDNA synthesis will be correct. Very small cDNA is usually the result of DNase contamination. This is readily evaluated by adding a restriction fragment of DNA to the reaction mixes and checking the fragment after incubation by gel electrophoresis.
Anticipated Results The typical amount of poly(A)+ RNA used to generate a library is 10 µg; however, the protocol may be scaled down should RNA availability be limiting. The 10 µg of poly(A)+ RNA will yield 2 to 10 µg of double-stranded cDNA. An aliquot representing 2% to 5% of the total volume may be checked by agarose gel electrophoresis, ethidium bromide staining, and autoradiography. The aliquots removed from the first- and second-strand synthesis should contain 2 to 8 × 105 cpm and 2 to 10 × 104 cpm TCA-insoluble counts, respectively (basic protocol). 100 ng double-stranded cDNA should generate a full-complexity library (1 × 106 recombinants) if a phage vector such as λgt10 is employed.
To determine the amount of cDNA synthesized, see Sample Calculation for Determining Amount of cDNA Synthesized.
Time Considerations Either of these protocols can usually be completed in two days. In the basic protocol, first-strand synthesis should be started in the morning to permit the initial second-strand synthesis to proceed overnight. The final secondstrand synthesis should be carried out the following morning. In the alternate protocol, 4 hr should be allowed for synthesis of the doublestranded cDNA and 4 to 5 hr for generation of blunt ends. The cDNA may be linkered or tailed as described in UNIT 5.6 later the same day. If necessary, the protocol may be interrupted at any ethanol precipitation step.
Literature Cited Gubler, U. and Hoffman, B.J. 1983. A simple and very effective method for generating cDNA libraries. Gene 25:263-269. Kotewicz, M.L., Sampson, C.M., D’Alessio, J.M., and Gerard, G.F. 1988. Isolation of cloned Moloney murine leukemia virus reverse transcriptase lacking ribonuclease H activity. Nucl. Acids Res. 16:265-277. Maniatis, T., Fritsch, E.F., and Sambrook, J. 1982. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.
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Neve, R.L., Hanis, P., Kosik, K.S., Kurnit, D.M., and Donlon, T.A. 1986. Identification of the gene for the human microtubule-associated protein tau and chromosomal localization of the genes for tau and microtubule-associated protein 2. Brain Res. 387(3):271-80. Okayama, H. and Berg, P. 1982. High-efficiency cloning of full-length cDNA. Mol. Cell. Biol. 2:161-170. Tamkun, J.W., DeSimone, D.W., Fonda, D., Patel, R.S., Buck, D., Horwitz, A.F., and Hynes, R.O. 1986. Structure of integrin, a glycoprotein involved in the transmembrane linkage between fibronectin and actin. Cell 46:271-282.
Key References Gubler and Hoffman, 1983. See above. Huse, W.D. and Hansen, C. 1988. cDNA cloning redefined. Strategies (Stratagene) 1:1-3. First demonstrated use of linker primers and methylated nucleotides in cDNA synthesis.
Okayama and Berg, 1982. See above. These authors first developed the RNase H/E. coli DNA polymerase I alternative to S1 nuclease and Klenow fragment for second-strand synthesis.
Contributed by Lloyd B. Klickstein Brigham and Women’s Hospital Boston, Massachusetts Rachael L. Neve McLean Hospital Belmont, Massachusetts Erica A. Golemis Fox Chase Cancer Center Philadelphia, Pennsylvania Jeno Gyuris Mitotix, Inc. Cambridge, Massachusetts
Conversion of mRNA into Double-Stranded cDNA
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Ligation of Linkers or Adapters to Double-Stranded cDNA
UNIT 5.6
Linkers or adapters can be ligated to double-stranded cDNA (UNIT 5.5) to provide restriction endonuclease sites used in the production of a cDNA library (UNIT 5.8). For cloning purposes, only one linker or adapter must be present on each end of the cDNA. However, multiple linkers are usually ligated to the cDNA because both ends are phosphorylated and contain cohesive sequences. As a result, cDNA must first be methylated to protect it from a subsequent restriction digest designed to remove the multiple linkers (basic protocol). The procedure for ligating adapters to the cDNA is much simpler than that for linkers because only one end is phosphorylated, resulting in the ligation of just one adapter (alternate protocol). Linkered or adapted cDNA is then passed over a Sepharose CL-4B column to remove unligated linkers or adapters and other low-molecular-weight material (1.5 kb. Since only 50 to 100 ng of cDNA are required to produce a full complexity library with a phage vector, it is recommended that a library be produced at this stage in any event. A
A
B
pooled cDNA fractions 6 -10 inclusive
(origin)
32
P cpm /2 x 10
-2
µl
28 24 20 16
tracking d ye begins to elute
12 8
λ
ds cDNA
kb 21.4 9.6 6.7 4.5 2.1 1.9
4 2
4
6
8
10 12 14 16
CL - 4B fraction no.
Figure 5.6.1 Fractionation of EcoRI-digested cDNA by (A) Sepharose CL-4B chromatography and (B) agarose gel electrophoresis.
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library may be stored for years and may be useful in the future. For example, sequences related to the gene of interest may be identified that were excluded from a size-fractionated library.
Size-select the cDNA to obtain long inserts 16. Pour a 0.8% TBE agarose minigel—the gel should be thick enough such that all of the cDNA will fit into a single well. Rinse the gel box, tray, and comb thoroughly, and use fresh TBE electrophoresis buffer. High-quality, nuclease-free agarose that does not inhibit ligation is essential. Most commercial agarose advertised as molecular biology grade is adequate. The agarose may be checked by first carrying an EcoRI fragment of a plasmid through the procedure and comparing its cloning efficiency in the cDNA vector, expressed as recombinants/ng insert, to the cloning efficiency of the same fragment prior to fractionation. Wash the gel box thoroughly afterward!
17. Add 10× loading buffer to 1× final to the cDNA and load it into a well near the center. Load DNA molecular weight standards (e.g., an HindIII digest of λ phage) two wells away from the sample. Electrophorese at 70 V until adequate resolution is achieved as determined by ethidium bromide fluorescence, usually 1 to 4 hr. Be sure not to use standards with EcoRI ends.
18. Elute double-stranded cDNA of the desired size as estimated by comparison with the comigrated standards. λgt10 and λgt11 have a maximum insert size of 7 kb, so collecting cDNA larger than this won’t be useful unless a plasmid vector or a phage vector such as Charon 4A or EMBL 4 will be used.
19. Add 10 mg/ml tRNA to 20 µg/ml final, 1⁄10 vol of 3 M sodium acetate, pH 5.2 (APPENDIX 2), and 2.5 vol ice-cold 95% ethanol and place 15 min on dry ice. Microcentrifuge, wash and dry as in step 14, and resuspend pellet in 20 µl TE buffer. Ethanol precipitation also extracts the ethidium from the DNA.
20. Determine radioactivity in 1 µl using a fluor and scintillation counter and then calculate the recovery of double-stranded cDNA (see commentary). Proceed to library construction protocols in UNIT 5.8. ALTERNATE PROTOCOL
LIGATION OF BstXI SYNTHETIC ADAPTERS Blunt-ended, double-stranded cDNA is ligated to phosphorylated BstXI adapters and then purified as described in the basic protocol. Alternatively, EcoRI or EcoRI-NotI adapters may be used for cDNA to be cloned in vectors with the EcoRI site (Fig. 5.6.2). This protocol is simpler than that for linkers because the methylation and restriction digestion steps are unnecessary. Additional Materials BstXI adapters (UNIT 2.11; Invitrogen), EcoRI adapters (New England Biolabs), or EcoRI-NotI adapters (Invitrogen) 1. Dissolve blunt-ended, double-stranded cDNA pellet in 23 µl water and add in the following order (30 µl final volume):
Ligation of Linkers or Adapters to Double-Stranded cDNA
3 µl 10× T4 DNA ligase buffer containing 5 mM ATP (1× and 0.5 mM ATP final) 2 µl 1 µg/µl EcoRI, EcoRI-NotI, or phosphorylated BstXI adapters (67 µg/ml final).
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adapters
5′ CCATTGTG 3′ GGTA
CTCTAAAG
CTTTAGAGCACA
ACACGAGATTTC
GAAATCTC
ATGG 3′ GTGTTACC 5′
insert cDNA
vector cut with BstXI
Figure 5.6.2 Noncomplementary adapter strategy. The insert and vector ends are compatible with each other but cannot self-ligate because the cohesive ends are not self-complementary. The BstXI sites (underlined and bold) are not regenerated.
Mix gently by pipetting up and down with a pipettor. Add 2 µl (800 U) T4 DNA ligase to 27,000 U/ml final, mix as above, and incubate overnight at 4°C. It is helpful to use 32P-labeled cDNA to follow the DNA on the subsequent CL-4B column. If the cDNA is not 32P-labeled, it may be labeled at this step by using [32P]labeled adapters, prepared with [γ-32P]ATP as in the T4 polynucleotide kinase exchange reaction (UNIT 3.10). Alternatively, if the adapters are not yet phosphorylated, they may be labeled with [γ-32P]ATP as in the T4 polynucleotide kinase forward reaction (UNIT 3.10).
2. Add 100 µl TE buffer and remove excess adapters as in steps 10 to 15 of the basic protocol. If desired, size-select the cDNA as in steps 16 to 20 of the basic protocol. Resuspend purified cDNA pellet (obtained from either the CL-4B column or the gel) in 10 to 15 µl TE buffer. Proceed to library construction protocols in UNIT 5.8. Because adapter dimers formed during the ligation reaction will clone into the vector very efficiently, removal of the excess adapters is essential.
PREPARATION OF A CL-4B COLUMN The CL-4B column (Fig. 5.6.3) effectively removes linkers or adapters that would otherwise interfere in subsequent cloning steps; it also allows selection of cDNA ≥350 bp (see basic and alternate protocols). The column may be prepared while the cDNA is being digested with EcoRI, as described in steps 7 and 8 of the basic protocol.
SUPPORT PROTOCOL
Additional Materials Preswollen Sepharose CL-4B (Pharmacia), 4°C CL-4B column buffer Silanized glass wool Plastic tubing (new) with clamp
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disposable plastic pipet
Sepharose CL-4B
to ring stand clamp
silanized glass wool tubing clamp plastic tubing
Figure 5.6.3 CL-4B column used for removal of EcoRI-digested linkers and selection of cDNA ≥350 bp.
1. Transfer 10 ml of preswollen Sepharose CL-4B to a 50-ml polypropylene tube and fill the tube with CL-4B column buffer. Mix by inverting several times and let the Sepharose CL-4B settle by gravity for 10 to 15 min. Aspirate the buffer above the settled gel, removing also the unsettled “fines.” 2. Fill the tube two times with CL-4B column buffer—allow the Sepharose CL-4B to settle each time and remove the fines as in step 1. 3. Add 10 ml CL-4B column buffer and mix by inverting several times. Incubate the tube 10 min at 37°C, then proceed at room temperature. Outgassing may occur if the column is poured cold. The bubbles thus formed in the gel will interfere with the chromatography.
4. Break off the top of the 5-ml plastic pipet. Wearing gloves, use the 1-ml pipet to push a small piece (3- to 4-mm3) of silanized glass wool down to the tip of the 5-ml pipet. Push a 3-cm length of plastic tubing firmly onto the tip of the 5-ml pipet. Clamp the tubing and attach the column to the ring stand as shown in Figure 5.6.3. 5. With a pipet, carefully fill the column with the gel slurry from step 3. After a few minutes, release the clamp on the tubing and allow the column to flow. Periodically add more slurry to the column as the level drops until the volume of packed gel in the column is at the 5-ml mark. Ligation of Linkers or Adapters to Double-Stranded cDNA
6. Allow the level of buffer in the column to drop until it is just above the level of the gel and clamp the tubing to stop the flow. The column is ready to be loaded (see basic or alternate protocols).
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REAGENTS AND SOLUTIONS Buffer stock solutions Prepare ≥10 ml of each of the following stock solutions. Use autoclaved water and pass each solution through a sterile 0.45-µm filter. Store at room temperature unless otherwise indicated. 1 M Tris⋅Cl, pH 8.0 and pH 7.5 0.5 M EDTA, pH 8.0 3 M sodium acetate, pH 5.2 5 M NaCl (prepare 100 ml) 1 M MgCl2 7.5 M ammonium acetate 20% N-lauroylsarcosine (Sarkosyl) 1 M DTT; store at −20°C in tightly capped tube 0.1 M ATP, pH 7.0 (prepare 1.0 ml); neutralize as described in UNIT 3.4; store at −20°C From these stock solutions, prepare the following buffers, which should be checked for nuclease activity as described in UNIT 5.5. Enzyme buffers should be frozen in 200-µl aliquots at −80°C in screw-cap microcentrifuge tubes. Enzyme buffers prepared and stored as described will last for years. Several of these solutions are routine and may be already available. Nonetheless, to help ensure success, it is best to prepare separate stocks for critical applications such as library preparation.
CL-4B column buffer, 500 ml 5 ml 1 M Tris⋅Cl, pH 8.0 60 ml 5 M NaCl 1 ml 0.5 M EDTA, pH 8.0 2.5 ml 20% Sarkosyl 431.5 ml H2O Filter sterilize and store at room temperature 50× S-adenosylmethionine (SAM) 1 mg SAM 1.0 ml 50× SAM dilution buffer (see below) Prepare fresh just prior to use. Store dry SAM at −80°C for no longer than 2 months.
50× SAM dilution buffer, 7 ml 330 µl 3 M sodium acetate, pH 5.2 6.67 ml H2O Store in 1-ml aliquots Silanized glass wool Submerge the glass wool in a 1:100 dilution of a silanizing agent such as Prosil 28 (VWR) for 15 sec with shaking. Rinse the glass wool extensively with distilled H2O. Autoclave the glass wool for 10 min and store at room temperature.
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COMMENTARY Background Information
Ligation of Linkers or Adapters to Double-Stranded cDNA
The most common method currently employed to create compatible ends on cDNA prior to cloning is the attachment of synthetic linkers (basic protocol). This has for the most part superseded homopolymeric tailing (Maniatis et al., 1982), since linkering is relatively efficient, and its use eliminates the need for the sometimes tricky procedure of titrating the tailing reaction conditions. Other methods, such as the sequential ligation of two different linkers (Maniatis et al., 1982), are now rarely used. The linkered cDNA may be cloned into a plasmid or a phage vector. The use of synthetic adapters that ligate at high efficiency instead of linkers eliminates the methylation and restriction digestion steps (alternate protocol). The recently developed noncomplementary adapter strategy (see below) may ultimately supersede all the above methods as vectors with appropriate sites become available. Size fractionation of cDNA is preferable to size fractionation of the initial mRNA primarily because DNA is considerably less susceptible to degradation than is RNA. In addition, the small fragments that are generated during the cDNA synthesis process (e.g., due to contaminating nucleases) are removed. These would otherwise be preferentially inserted and decrease the yield of long cDNA clones in the library. RNA enrichment procedures were undertaken in the past because cloning efficiencies then were not high enough to ensure the representation of very rare mRNAs in the library. Currently, the most common reason for using mRNA fractionation or enrichment is in preparation of subtracted libraries (UNIT 5.8; Hedrick et al., 1984). The strategy of noncomplementary adapters (Fig. 5.6.2), developed by Brian Seed and coworkers (Seed, 1987), overcomes the loss of library complexity due to self-ligation of the cDNA inserts or the vector. This enables a high efficiency for the ligation of the vector to the insert since there are no competing reactions and the vector ends are similarly noncomplementary and do not self-ligate. The much higher yield of desirable ligation products ultimately results in a greater number of clones in the library. Furthermore, the occasional “scrambles” produced as a consequence of two unrelated cDNAs ligating together during the vector ligation step are eliminated; however, “scrambles” at the stage of adapter ligation
remain possible, though infrequent. Adapters are favored over linkers in general because the methylation and restriction digestion steps are bypassed, thus simplifying the procedure. adapters are required in the alternate protocol procedure to ensure that all of the cDNA will have the proper, noncomplementary “sticky ends.” The use of EcoRI-NotI adapters for cloning with EcoRI compatible vectors introduces the rare NotI site at both ends of the insert, allowing the insert to be cut out of the vector as one fragment, which is not always possible with EcoRI linkers if there are internal EcoRI sites present. The development of noncomplementary adapters has enabled the production of highcomplexity cDNA libraries in multifunctional plasmid vectors such as CDM8 (Seed, 1987). These vectors permit library screening by functional expression in eukaryotic cells and production of single-stranded DNA for mutagenesis or subtraction, in addition to conventional hybridization methods. For some specialized applications and for selectable markers other than the supF present in CDM8, other vectors that employ the noncomplementary linker strategy (and use the same BstXI sequence) include several available from Invitrogen with different antibiotic resistances and eukaryotic selectable markers. Other available vectors include AprM8, an ampicillin-resistant version of CDM8 (L.B. Klickstein, unpublished results), and retroviral vectors in the pBabe series (Morgenstern and Land, 1990).
Critical Parameters In order to maximize the length and cloning efficiency of the cDNA, it is essential that contamination by endo- and exonucleases be avoided. EDTA, an inhibitor of most nucleases, should be present whenever feasible. Reagents, solutions, and enzymes should be of the highest quality obtainable. The SAM reagent in the methylation step is very unstable, must be freshly dissolved prior to use, and should not be kept >2 months, even if stored dry at −80°C and never thawed. Stabilized SAM is supplied free of charge with methylases purchased from New England Biolabs. Methylation conditions may be checked by methylating λ DNA under the conditions described in step 1 of the basic protocol, digesting methylated and unmethylated DNA with
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EcoRI, and comparing the two samples by agarose gel electrophoresis. Methylated λ DNA should be protected from EcoRI digestion. Digested linkers must be completely removed. Because of their small size, even as a small weight percentage of the total DNA, they will comprise a large mole fraction; thus, if not completely removed, most clones in the library will contain only linkers. Some investigators remove digested linkers by three sequential precipitations from 2 M ammonium acetate. However, column chromatography provides better resolution of the cDNA from the linkers. The CL-4B column also removes small doublestranded cDNA molecules. Purification of cDNA by gel electrophoresis after ligation of the linkers or adapters may be performed as an alternative to the CL-4B column, but the yield usually is not as high and the separation of linkers or adapters from cDNA is not as complete. If noncomplementary adapters are employed, both 5′ ends should be phosphorylated for optimum results. If complementary adapters are used, only the blunt end should have a 5′ phosphate. The other end is phosphylated after ligation to the cDNA. Because BstXI produces a cohesive end with a sequence not specified by the restriction site sequence, the adapters and vector must correspond—e.g., the cohesive ends of the adapters used for the cDNA must be complementary to that of the vector. The vectors CDM8, AprM8, pCDNAI, pCDNAII, and the pBabe series all use the same sticky ends (Fig. 5.6.2).
Troubleshooting Methylation and addition of linkers to double-stranded DNA are important to the successful construction of a cDNA library, but cannot be evaluated immediately. Unmethylated cDNA will clone efficiently and the problem will only be detected if the isolated inserts all end at an internal EcoRI site. This possibility may be minimized by prior evaluation of the quality of the SAM used in the methylation reaction (see reagents and solutions), or by use of the stabilized SAM that accompanies New England Biolab’s EcoRI methylase. Incomplete addition of linkers or adapters will be detected as a poor cloning efficiency in the next step of ligating the inserts to the vector. The problem may be any of the following: (1) cDNA was not blunted properly; (2) linkers or adapters were not kinased well or not annealed; (3) multiple linkers were not cut off with
EcoRI; (4) ligation reaction did not work. However, the problem is usually improperly blunted cDNA or linkers that do not ligate well. Evaluate the linkered cDNA (after the CL-4B column) by ligating 5% of the sample with no vector and running the reaction on a 1% agarose minigel next to an equal amount of unligated cDNA. Properly linkered cDNA should significantly increase in size as determined by ethidium bromide staining or autoradiography of the dried gel. Poorly linkered cDNA may often be salvaged by repeating the blunting and linkering steps with fresh reagents. Inadequate separation of linkers or adapters from cDNA will be detected as cloning efficiency that is too high and as clones with no inserts detectable by gel electrophoresis (white plaques with no inserts in λgt11 or plaques on C600hflA with λgt10 that have no insert). The remainder of this cDNA may be salvaged by repeating the CL-4B column chromatography.
Anticipated Results Approximately 50% to 70% of the starting radioactivity present in the blunt-ended, double-stranded cDNA should be collected after the CL-4B column. This typically represents 1 to 3 µg of cDNA. In a human tonsil library prepared in λgt11 where the inserts were thoroughly evaluated, the mean insert size was 1.4 kb and actin clones represented 0.34% of all recombinants. Three-quarters of the actin cDNAs were nearly full length (actin mRNA = 2.1 kb).
Time Considerations The methylation can be done on the same day as the blunting step from the cDNA synthesis protocol. The linker ligation is then set up overnight. EcoRI digestion and CL-4B chromatography are performed the following day, and the cDNA is either ligated to the vector overnight (UNIT 5.8) or stored as an ethanol precipitate overnight and further size selected by agarose gel electrophoresis the next day. At any ethanol precipitation in the procedure, the cDNA may be stored for several days as an ethanol precipitate. The use of adapters rather than linkers eliminates the need for the methylation and linker digestion steps. The adapted cDNA is immediately loaded onto the CL-4B column after the adapter ligation and is used directly from the column after ethanol precipitation. The subsequent ligation of the adapted cDNA and the vector is performed overnight, requiring the same amount of time with adapters as with linkers.
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Literature Cited
Key References
Hedrick, S.M., Cohen, D.I., Nielsen, E.A., and Davis, M.M. 1984. Isolation of cDNA clones encoding T cell-specific membrane-associated proteins. Nature (Lond.) 308:149-153.
Neve, R.L., Hanis, P., Kosik, K.S., Kurnit, D.M., and Donlon, T.A. 1986. Identification of the gene for the human microtubule-associated protein tau and chromosomal localization of the genes for tau and microtubule-associated protein 2. Brain Res. 387:271-80.
Maniatis, T., Fritsch, E.F., and Sambrook, J. 1982. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.
Seed, B. 1987. See above.
Morgenstern, J.P. and Land, H. 1990. Advanced mammalian gene transfer: high titre retroviral vectors with multiple drug selection markers and a complementary helper-free packaging cell line. Nucl. Acids Res. 18:3587-3596.
Contributed by Lloyd B. Klickstein Brigham and Women’s Hospital Boston, Massachusetts
Seed, B. 1987. An LFA-3 cDNA encodes a phospholipid-linked membrane protein homologous to its receptor CD2. Nature (Lond.) 329:840842.
Rachael L. Neve McLean Hospital Belmont, Massachusetts
Ligation of Linkers or Adapters to Double-Stranded cDNA
5.6.10 Supplement 13
Current Protocols in Molecular Biology
PRODUCTION OF GENOMIC DNA AND cDNA LIBRARIES
SECTION IV
In this section the insert DNA (prepared in UNIT 5.2 or 5.3) is ligated to vector DNA and then the ligated DNA is propagated in Escherichia coli. The technical problems associated with joining DNA molecules together with ligase are described in UNIT 3.14. The creation of genomic and complete or subtracted cDNA libraries requires the production of large numbers of recombinant clones and this causes further complications. To recover large numbers of recombinant clones without using large amounts of packaging mixture, vector, or insert DNA, the optimal ratio of insert DNA to vector DNA must be determined prior to the production of the library. Without this optimization, the library will frequently not be large enough to contain the entire genome. After ligation of vector DNA to insert DNA the ligated DNA must be introduced into E. coli. The introduction of bacteriophage or cosmid DNA into E.coli is carried out most efficiently by packaging the DNA into bacteriophage particles then allowing these bacteriophage particles to infect E. coli (see UNIT 1.11 for a detailed description of this process). Unfortunately, plasmid DNAs cannot be packaged and thus must be introduced into E. coli by bacterial transformation (UNIT 1.8). Because of the high efficiency of introducing DNA into E. coli by the packaging procedure, we recommend producing genomic and complete or subtracted cDNA libraries using bacteriophage vectors whenever possible. In this section the protocols required to ligate vector DNA to insert DNA and to introduce these molecules into E. coli are described. The protocols are divided into two parts—one for genomic DNA libraries and the other for complete or subtracted cDNA libraries—because the amount of insert DNA required for genomic libraries is usually considerably more than for cDNA libraries.
Production of a Genomic DNA Library A number of small-scale ligations are performed using a set amount of vector and varying amounts of insert. Test ligations are transformed into bacteria (plasmid vectors) or packaged and plated on host bacteria (λ and cosmid vectors). The number of clones in the different ligations is compared, and the optimum ratio of vector to insert is indicated by the ligation with the most recombinant clones. A large-scale ligation is then set up using this optimum ratio. This protocol employs a bacteriophage vector; however, cosmid or plasmid vectors can be used with minor modifications (see commentary).
UNIT 5.7 BASIC PROTOCOL
Determining the number of clones required to make a genomic DNA library is discussed in the chapter introduction. A library with a base of about 700,000 clones is required for a complete bacteriophage library of mammalian DNA. Materials Vector DNA (phage arms, cosmid arms, or linearized plasmid) Insert fragment (UNITS 5.3-5.6) 10× DNA ligase buffer (UNIT 3.4) T4 DNA ligase (measured in cohesive-end units; New England Biolabs; UNIT 3.14) Packaging extract (phage and cosmid) or competent E. coli (plasmid) LB or LB/ampicillin plates (UNIT 1.1) Top agarose containing 10 mM MgSO4 (UNIT 1.1) Additional reagents and equipment for plating, packaging, and titering bacteriophage (UNIT 1.11) Contributed by Thomas Quertermous Current Protocols in Molecular Biology (1988) 5.7.1-5.7.4 Copyright © 2000 by John Wiley & Sons, Inc.
Construction of Recombinant DNA Libraries
5.7.1 Supplement 13
Ligating insert DNA to vector DNA 1. Perform a series of test ligations that bracket equimolar concentrations of vector and insert, i.e., insert/vector molar ratios of 5:1, 2:1, 1:1, 0.5:1, and 0.2:1. (Keep the vector constant and vary the insert DNA.) Since only the relative quantities of recombinants obtained are important, a small quantity of vector can be used for each reaction (40 to 100 ng). A control tube containing only ligated vector (tube no. 6 in Table 5.7.1.) is important to assess the yield of recombinants (see below). Conditions of the ligation reaction are as described in UNIT 3.14 (1× ligase buffer, 100 U T4 DNA ligase) and should be in a total volume of 5 to 10 µl. See Table 5.7.1 for a sample set of reactions. Vectors should be chosen as described in UNIT 5.1. Genomic DNA libraries should be constructed only with vectors that provide a selection for insert DNA. Table 5.7.1
Sample Set of Reactions for Ligating λ Vector to Insert DNA
Reaction component (µl) Vector DNA (100 ng/tube) Insert DNA (20 kb, 100 ng/µl) Insert DNA (20 kb, 50 ng/µl) Insert DNA (20 kb, 10 ng/µl) 10× ligation buffer Water T4 DNA ligase
Tube number 3 4
1
2
5
6
1
1
1
1
1
1
2
—
—
—
—
—
—
2
1
0.5
—
—
—
—
—
—
1
—
0.5 1 0.5
0.5 1 0.5
0.5 2 0.5
0.5 2.5 0.5
0.5 2 0.5
0.5 3 0.5
2. Package the phage recombinants as described in UNIT 1.11. Normally, commercially prepared packaging extract is designed for approximately 1 µg of λ DNA, so it is possible to divide such an aliquot of packaging extract among several test ligations. 3. Dilute packaged extract to 0.5 ml and plate 1 µl and 10 µl of packaged bacteriophage per plate. Grow overnight. 4. Compare the number of plaques on the plates to determine the optimum relative concentrations of arms and insert. The yield of plaques with insert DNA (from ligation tubes 1 to 5) should be at least twice and preferably 5 times the yield of colonies on the control plate (from tube 6) containing vector DNA alone. 5. Prepare a large-scale ligation reaction using the optimum ratio of insert DNA to vector DNA determined in step 4. The amount of DNA is determined by the size of the library that is desired. See UNIT 5.1 to determine total number of desired recombinants. If the expected number of recombinants that can be made uses too much insert DNA or vector DNA, either the insert DNA or the vector DNA is probably defective. The quality of the vector or insert DNA can be tested by ligating them together and fractionating the ligated DNA on a gel (see UNITS 3.14, 2.5A, & 2.5B) using commercially prepared extracts.
Production of a Genomic DNA Library
Introducing ligated DNA into E. coli 6. Package the ligated DNA. Remember that bacteriophage packaging extracts can be saturated by too much DNA.
5.7.2 Supplement 13
Current Protocols in Molecular Biology
Because producing extracts that efficiently package DNA can be quite difficult, we recommend the use of commercially available packaging kits (see UNIT 1.11). The number of recombinants that can be obtained with 1 ìg λ DNA from large-scale ligation will depend on the efficiency of the packaging extract or competent cells, but can be as high as 108 for phage vectors. For cloning from size-fractionated DNA, this represents a vast excess of recombinants. It is frequently possible to obtain over 106 recombinant phage from the test ligations, and this is often enough recombinants to clone a single-copy gene. Thus, it is useful to save the test ligations.
7. Package and titer the large-scale ligation and store at 4°C. Plate the bacteriophage for amplification or screening as soon as possible after the titer is known, as the titer will drop 3- to 10-fold within 48 hr of packaging.
COMMENTARY Background Information The theory of the ligation reaction is presented in UNIT 3.16 and is well outlined by Williams and Blattner (1980). From these theoretical considerations it is possible to predict the optimum concentration of vector and insert. However, because of the difficulty of estimating small quantities of DNA and the inability of some ends to ligate, it is helpful to conduct a series of test ligations as outlined here to determine the ratio of reagents that will result in the maximum number of clones. The theory of ligation reactions suggests that the optimum concentration for a ligation reaction varies for different sized molecules. However, the conditions suggested here are optimal for most vectors and inserts used for making genomic and cDNA libraries. The production of a cosmid library is identical to the procedure described here except that a cosmid vector is used rather than a bacteriophage vector. Different amounts of cosmid vector are used because cosmids are smaller (about 5 kb) than bacteriophages (50 kb). Thus the weight of vector DNA should be reduced 10fold. Production of a plasmid library involves the same procedure except that ligated DNA is introduced into E. coli cells by transformation (see UNIT 1.8) rather than by packaging.
Critical Parameters The quality of the insert DNA and vector DNA are critical to the success of these experiments. The most frequent cause of failure is that one of these DNAs has defective ends and thus the molecules are not able to ligate efficiently. Quality of packaging extracts is also critical. Normally, commercial packaging extracts are of high quality and yield large numbers of bacteriophage. However, an occasional bad lot
of packaging extract is obtained.
Troubleshooting A large number of clones in all the test ligations, or a large number of clones with extremely small quantities of insert, may indicate a high background of nonrecombinant clones. Background is determined by ligating the vector in the absence of insert. A large number of clones in this control ligation indicates inadequate dephosphorylation, inadequate purification from stuffer, or possibly that the bacterial strain being used for genetic selection is not correct. If few recombinants are obtained, there are a number of possible explanations. The packaging extract should be evaluated by appropriate control experiments. Usually, the commercially available packaging extract kits contain a suitable control fragment. At the same time a test ligation can be run on an agarose gel and compared to the unligated vector and insert. If both the insert and vector DNA appear intact (i.e., not degraded), either the sticky ends of the vector or insert have been damaged or one of the DNA samples (usually the insert) contains an inhibitor of the ligase reaction. Clonability of these two reagents can be evaluated by determining the ability of insert or vector DNA to ligate to itself or to an appropriate control. Inhibiting contaminants can sometimes be removed by phenol/ chloroform extraction followed by ethanol precipitation, or by passing the DNA over an Elutip column (UNIT 2.6).
Anticipated Results The number of clones obtained will depend on a number of factors including the efficiency of ligation, transformation, and packaging. At best, in a λ vector, over 108 recombinants per µg of insert DNA can be achieved. Cloning into
Construction of Recombinant DNA Libraries
5.7.3 Current Protocols in Molecular Biology
plasmid usually produces fewer recombinants per quantity of insert because transformation into competent E. coli is 10 to 100 times less efficient than the in vitro packaging of λ phage. In a plasmid, it may be necessary to use 1 to 2 µg insert DNA to achieve 106 recombinant clones. Cosmid clones usually are obtained at 105 to 106 colonies per µg insert DNA.
Time Considerations Test ligations are incubated overnight at 13°C (alternatively, incubate 2 hr at room temperature). The ligation reactions are packaged and then titered on the second day. Results are available late on the second day or the morning of the third day.
Literature Cited Kaiser, K. and Murray, N.E. 1984. The use of phage lambda replacement vectors in the construction of representative genomic DNA libraries. In DNA Cloning, Vol. 1: A Practical Approach (D. Glover, ed.) pp. 1-47. IRL Press, Oxford. Rodriguez, R.L. and Tait, R.C. 1983. Recombinant Techniques: An Introduction. Addison-Wesley, Reading, MA. Williams, B.G. and Blattner, F.R. 1980. Bacteriophage lambda vectors for DNA cloning. In Genetic Engineering, Vol. 2 (J.K. Setlow and A. Mullander, eds.) p. 201. Plenum, NY. Describes the ligation reaction as applied to cloning with λ vectors.
Contributed by Thomas Quertermous Massachusetts General Hospital Boston, Massachusetts
Production of a Genomic DNA Library
5.7.4 Current Protocols in Molecular Biology
Production of a Complete cDNA Library
UNIT 5.8A
A complete cDNA library is one that contains at least one cDNA clone representing each mRNA in the cell. There are about 34,000 different types of mRNA in a mammalian cell and about 500,000 mRNA molecules per cell. For any cDNA library there is a probability, P, that each mRNA will be represented at least once. A useful method for calculating the required base of a cDNA library is to calculate the probability, q, that a single cDNA clone will not be found in the library after choosing, at random, N clones: q = 1 − P = [1 − (n/T)]N
where n is the number of molecules of the rarest mRNA in a cell and T is the total number of mRNA molecules in a cell. The desired base, B, is the number of clones that should be screened to achieve a 99% probability that a cDNA clone will exist in the library, defined as: q = 1 − 0.99 = 0.01 = [1 − (n/T)]B
where n is 8 and T is 562,800 (for one mammalian cell), and the base B, is 324,000. In general, if 500,000 to 1,000,000 independent cDNA clones are obtained, the library should contain at least one copy of every mRNA. Either a phage or plasmid vector can be used in constructing a complete cDNA library (first basic and alternate protocols). Procedures for evaluating these cDNA libraries are described in the first support protocol. If necessary, the quality of the λ arms used in constructing complete phage cDNA libraries can be evaluated as described in the second support protocol. A subtracted DNA library is useful when a specific DNA or antibody probe for the desired clone is unavailable. A method for constructing a subtracted cDNA library is presented in UNIT 25B.1. LIGATION AND TRANSFECTION FOR A COMPLETE ë PHAGE LIBRARY Double-stranded cDNA with EcoRI ends is ligated to a 2-fold molar excess of phage vector DNA that has been digested with EcoRI and phosphatased to prevent ligation without a cDNA insert. The ligation mixture is packaged into infectious phage particles in vitro and transformed into host bacteria. Currently C600hf1A is used to propagate λgt10-based libraries and Y1088 is the host for λgt11 libraries (see Table 1.4.5).
BASIC PROTOCOL
Materials EcoRI-cut, phosphatased λgt10, λgt11, or λgt11 DNA or other phage vector DNA Double-stranded cDNA with EcoRI ends (UNIT 5.6) 10× DNA ligase buffer (UNIT 3.4) T4 DNA ligase (measured in cohesive-end units; UNIT 3.14) Cold 10 mM MgSO4 λ phage packaging extract (Stratagene) Suspension medium (SM; UNIT 1.11) Chloroform E. coli C600hflA (λgt10) or Y1088 (λgt11) IPTG (λgt11 only; UNIT 1.4) Xgal (λgt11 only; UNIT 1.4) Additional reagents and equipment for plating and titering bacteriophage (UNIT 1.11) Contributed by Lloyd B. Klickstein Current Protocols in Molecular Biology (1992) 5.8.1-5.8.8 Copyright © 2000 by John Wiley & Sons, Inc.
Construction of Recombinant DNA Libraries
5.8.1 Supplement 18
1. Resuspend EcoRI-digested, phosphatased λgt10 or λgt11 DNA at 1 µg/µl. λgt10 and λgt11 phosphatased phage arms are available commercially. Alternatively, prepare the phage DNA as described in UNIT 1.13 and digest with EcoRI and phosphatase (UNITS 3.1 & 3.10). Evaluate “homemade” phage arms as described in the second support protocol of this unit. Both λgt10 and λgt11 grow well and milligram quantities of DNA are routinely obtained from a 1-liter culture.
2. To a fresh tube add the following (final volume 19 µl): 100 ng cDNA with EcoRI ends (5 µg/ml final) 10 µl (10 µg) λgt10 or λgt11 DNA from step 1 (500 µg/ml final) 2 µl of 10× ligase buffer (1× final) H2O to 19 µl. Mix by gently pipetting up and down and add 1 µl (400 U) T4 DNA ligase to 20 U/µl final. Mix gently and incubate overnight at 4°C. Inoculate an overnight culture of plating bacteria. The objective is to maximize the number of recombinants obtained per microgram of cDNA and to minimize both the probability of two unrelated cDNA sequences ligating to each other and the probability of the cDNA forming an intramolecular loop. The 100-fold weight excess of vector used above is a 2-fold molar excess if the rough assumption is made that the average insert size is 1⁄50 that of the vector. Since the rate of intramolecular ligation is independent of cDNA concentration, the ligation is performed in a relatively small volume to favor intermolecular reactions.
3. Inoculate 50 ml broth with 0.5 ml of the overnight culture of host bacteria. When the OD600 = 0.5, pellet the bacteria by centrifugation 2 to 4 hr at 28,000 × g (3000 rpm in a tabletop centrifuge), 4°C, and resuspend in 5 ml cold 10 mM MgSO4. Store at 4°C prior to use. 4. While the culture is growing, incubate the ligation for 30 min at −20°C, and thaw at room temperature. Package the DNA mixture with a λ phage packaging extract. Use enough extracts such that the capacity of each is not saturated—typically 1 to 2 ìg ligated DNA per extract. Commercially available packaging extracts are recommended. High-efficiency in vitro packaging extracts (>108/ìg intact phage DNA) are critical to successful preparation of the library as are freshly prepared plating bacteria. A test packaging is not recommended as the components of ligation cannot be recovered if ligation fails.
5. Add SM to the packaging mixtures and pool into a single 5-ml polypropylene tube such that the total volume is 2 ml. Add a few drops of chloroform and shake by hand 3 sec. Place the tube containing the unamplified library on ice. 6. Determine the titer of the library. Use E. coli C600hflA as the host for a λgt10-based library or Y1088 as the host for a λgt11-based library. Include IPTG and Xgal in the case of a λgt11-based library to determine the fraction of the clones that represents recombinants. While the titer plates are incubating, store the library at 4°C. In vitro packaged phage are unstable. If stored for more than a day or so, the titer of the stock will drop noticeably. Thus, the library must be amplified at this time or no later than the following day (see UNIT 25B.2).
7. Calculate the cloning efficiency as recombinants/µg cDNA. Production of a Complete cDNA Library
Approximately 1–5 × 107 clones/ìg cDNA are typical. For a λgt11-based library, at least 90% of the plaques should be clear. The library is ready to be plated for amplification and/or screening.
5.8.2 Supplement 18
Current Protocols in Molecular Biology
LIGATION AND TRANSFORMATION FOR A COMPLETE PLASMID LIBRARY
ALTERNATE PROTOCOL
Double-stranded cDNA with noncomplementary BstXI ends is ligated to an equimolar amount of plasmid vector DNA that has been linearized at the two inverted BstXI sites and purified by gel electrophoresis. The ligation is introduced into bacteria either by transfection of competent cells or by electroporation, and the bacterial cells are plated and grown overnight. Additional Materials CDM8 plasmid DNA (Invitrogen) or similar plasmid vector DNA, CsCl-purified BstXI and 10× BstXI buffer (UNIT 3.1) Low melting temperature agarose for fragment purification (UNIT 2.6) TE buffer (APPENDIX 2) Double-stranded cDNA with BstXI ends (UNIT 5.6) Competent cells: MC1061/P3 (Invitrogen) is required for CDM8; MC1061, DK1, DH1, DH5, or HB101 are suitable for other vectors LB medium (UNIT 1.1) 150-mm LB agar plates (UNIT 1.1) with appropriate antibiotics (10 µg/ml ampicillin + 7.5 µg/ml tetracycline for CDM8 in MC1061/P3; 50 to 100 µg/ml ampicillin for the other vectors and strains above) 80% glycerol in H2O Additional reagents and equipment for plating and growth of E. coli (UNITS 1.1-1.4), isolation of DNA from agarose gels (UNIT 2.6), alkaline lysis/CsCl gradient preparation of plasmid DNA (UNIT 1.7), and introduction of plasmid DNA into cells (UNIT 1.8) Prepare vector DNA 1. Digest CDM8 plasmid DNA with BstXI restriction enzyme by adding to a 500-µl tube (final volume 100 µl): 30 µg CDM8 DNA (300 mg/ml final) 10 µl 10× BstXI buffer H2O to 92 µl 8 µl BstXI (60 to 100 U). Mix by vortexing; microcentrifuge at high speed briefly; incubate 3-4 hr at 55°C. 2. Purify the large vector fragment by electrophoresis through a low-melting-point agarose gel, by electroelution, or using DEAE-cellulose paper. Resuspend DNA at 1 µg/µl in TE buffer. A purification step is essential, because all vectors employing the noncomplementary adapter strategy have two nearby (inverted) BstXI sites, and the fragment between them (several basepairs in pcDNA1 and pBabeNeo, ∼300 bp in CDM8 and AprM8) must be completely removed for optimum cloning efficiency.
Ligate cDNA, transform of competent cells and store library 3. In a 500 µl microcentrifuge tube, add in the following order (final volume 20 µl): 1 µg cDNA with BstXI ends (100 µg/ml final) 5 µl (5 µg) vector DNA from step 2 (500 µg/ml final) 1.5 µl of 10× DNA ligase buffer H20 to 19 µl. Construction of Recombinant DNA Libraries
5.8.3 Current Protocols in Molecular Biology
Supplement 18
Mix by pipetting up and down with a micropipet and add 1 µl (400 U) T4 DNA ligase to 20 U/µl final. Mix gently and incubate overnight at 4°C. The above ligation employs equimolar amounts of vector and insert, assuming an average cDNA size of 1 kb and a vector size of 5 kb. An excess of vector is not required, because neither the vector nor the cDNA is able to self-ligate.
4. Transform 0.75 µl of the ligation mixture into each of twenty 100-µl aliquots of competent cells (as described in UNIT 1.8 or manufacturer’s protocol). High efficiency competent cells are critical to the successful production of a full complexity cDNA library. Whether competent cells are purchased commercially or are made in the laboratory, they should be tested prior to use. Efficiency yields should be ≥1 × 108 colonies per ìg supercoiled pUC18 DNA. Electroporation is an excellent alternative if the special apparatus is available; it yields even higher efficiencies and has been employed successfully for cDNA libraries (UNIT 1.8).
5. Pool the transformed cells in a single 50-ml polypropylene tube, grow with shaking 1 hr at 37°C, and plate on 10 to 20 15-cm agar plates containing the appropriate antibiotic(s) (UNIT 1.3). Incubate overnight at 37°C. The transformed bacteria may be grown in liquid culture, but there is a greater likelihood of altering the representation of the library due to overgrowth of relatively rapidly growing cells. For this reason, growth on agar plates is recommended. If the library is to be screened by hybridization without amplification, a detergent-free nitrocellulose filter may be placed on the plate prior to plating the bacteria. Replica filters can then be prepared after incubation (UNIT 6.2)
6. Count the number of colonies in a sector of a plate to determine the library complexity expressed as colonies per microgram cDNA. Add 5 ml LB medium to each plate and gently elute the bacteria with a spreader (UNIT 1.3) to form a dense suspension. Wash each plate with an additional 1 ml LB medium to recover residual bacteria. Pool bacteria in a single sterile tissue culture flask. Library complexities of 5 × 105 to 2 × 106 colonies per ìg input cDNA are typical for libraries prepared with competent cells. Efficiencies 10-fold higher are possible for libraries prepared by electroporation (UNIT 1.8).
7. Take half of the pooled bacteria and perform an alkaline lysis/CsCl gradient purification of the plasmid DNA. This DNA represents the unamplified library and is suitable for transfection of mammalian cells. To the remainder of the pooled bacteria, add 0.2 vol of 80% glycerol and mix thoroughly by inverting the flask. Dispense 1-ml aliquots into sterile 1.5-ml microcentrifuge tubes and freeze at −80°C. Up to 50 aliquots may be obtained with this procedure. Each aliquot will inoculate 500 ml of liquid culture for preparation of additional DNA.
8. Evaluate the library as described in the first support protocol. Note that the BstXI sites are not reconstituted with the adaptors commonly used, thus flanking restriction sites must be used to cut out the inserts.
Production of a Complete cDNA Library
5.8.4 Supplement 18
Current Protocols in Molecular Biology
EVALUATION OF A cDNA LIBRARY To ensure that a negative result upon screening is not the result of a technical error during the construction of the library, evaluate the library by (1) screening for an abundant mRNA sequence, such as actin, known to be present; (2) screening with a total mixed cDNA probe; or (3) examining the inserts from twelve or more randomly chosen clones. Evaluation of the library is carried out after amplification (UNIT 25B.2) or concurrently with screening if no amplification is done.
SUPPORT PROTOCOL
An amplified library is stable for years and may be screened thousands of times. Due to the expense and time involved in generating a cDNA library, amplification is recommended. If >2 × 106 clones are obtained, some investigators plate 4–5 × 105 clones for immediate screening and amplify the remainder of the library, against the possibility that the target clones will be underrepresented as a result of the amplification procedure. A cDNA cloning efficiency within the expected range is a good initial indication that all is well; however, further evaluation by at least one of the following methods is necessary because although screening a library is conceptually simple, in practice, identification of a positive clone is not always straightforward. Thus, in the case of an evaluated library, a negative result upon screening implies problems with the probe, the screening method, or the source of the RNA. Evaluate the library by screening duplicate lifts from a single plating of 10,000 to 50,000 clones (from a plasmid or phage library) with an actin hybridization probe, using the protocol in UNIT 6.3. Each duplicating signal on the film is scored as a positive. Table 5.8.1 gives a range of actin cDNAs for various tissues expressed as a percentage of total recombinant clones screened. Actin is a good choice of probes with which to evaluate the library because its sequence is highly conserved across species and probes are widely available; however, any abundant mRNA known to be present, for which a probe is available, will do. This probe can be labeled by methods described in UNIT 3.4. An alternative to a specific probe is to screen a single plating as above with a cDNA hybridization probe prepared from the same mRNA used to construct the library. Use the hybridization conditions for cloned probes outlined in the steps for screening libraries (UNIT 6.3). Approximately 50% to 90% of the clones in the plating should hybridize, depending in part upon the source of the RNA. A third means of evaluating the library is to pick 12 single recombinant clones, often from the initial titer plate from step 6 of the basic protocol, and carry them through the phage DNA miniprep described in UNIT 1.13. Resuspend the miniprep DNA in 25 µl water and digest 10 µl with EcoRI. Analyze the digest by electrophoresis through a 1.5% agarose gel in TBE buffer. At least 10 of the 12 recombinant clones should have inserts visible by ethidium fluorescence, with an average size of ∼1 kb or more. Table 5.8.1 Frequency of Actin cDNA Clones in Various Mammalian cDNA Libraries
Tissue or cell type
Actin-hybridizing clones (% of clones screened)
Liver Lymphoid tissue or cells Fibroblastoid cell line
0.02 to 0.06 0.1 to 0.5 1 to 4 Construction of Recombinant DNA Libraries
5.8.5 Current Protocols in Molecular Biology
Supplement 62
SUPPORT PROTOCOL
PREPARATION OF TEST INSERT FOR EVALUATION OF PHOSPHATASED PHAGE ARMS This protocol describes a procedure for testing the quality of λ arms. Bacteriophage DNA is ligated to test insert DNA and the efficiency of the vector is tested by packaging and plating the ligated DNA. Additional Materials Genomic DNA 1:1 phenol/chloroform (UNIT 2.1A) Additional reagents and equipment for restriction endonuclease digestions (UNIT 3.1) and plating bacteriophage libraries (UNIT 6.1) 1. Digest 20 µg genomic DNA to completion with EcoRI. Bring to 300 µl with TE buffer. 2. Extract with 300 µl phenol/chloroform, add 30 µl of 3 M sodium acetate and 700 µl of 95% ethanol, and place on dry ice for 15 min to precipitate the DNA. 3. Thaw, microcentrifuge 10 min at high speed, wash the pellet with 70% ethanol, and resuspend in 1.7 ml TE buffer. The test insert DNA concentration is ∼10 ng/µl. 4. Ligate 1 µg of arms to 10 µl (10 ng) of test insert in 10 µl as described in step 2 of the first basic protocol. Also ligate 1 µg of arms without a test insert as a control. Package and titer 2 µl of each ligation (UNIT 1.11). 5. To determine the efficiency (no. PFU per microgram test insert), multiply the number of plaques from the packaging mixture by 500. Usable arms will give 1 × 107 to 2 × 108 PFU/µg test insert. In the case of λgt11 plaques ≤10% of these should be blue on an Xgal plate. At least a 100-fold increase in the number of plaque-forming units should be observed in the presence of the test insert, compared with its absence from the ligation mixture. Perhaps because of a large number of short fragments or because of a greater fraction of fragments with the correct ends, an EcoRI genomic digest gives a higher cloning efficiency than does cDNA.
COMMENTARY Background Information
Production of a Complete cDNA Library
Significant technological advances over the past few years permit the construction of cDNA libraries that closely reflect the size and complexity of the corresponding mRNA. Historically the two most troublesome steps were S1 nuclease digestion of the cDNA and G-C tailing of the cDNA and vector, both of which require laborious adjustment of reaction conditions (see Sambrook et al., 1989). These, for the most part, have been superseded by the RNase H/E. coli polymerase I second-strand synthesis method of Okayama and Berg (1982) and the ligation of synthetic linkers (Huynh et al., 1984), respectively. The commercial availability of high-quality enzymes and reagents has made these newer methods quite reliable. The introduction of λgt10 and λgt11 phage vectors (Young and
Davis, 1983) allowed the high efficiencies of in vitro packaging systems to be applied to the preparation of cDNA libraries. It is now nearly routinetogeneratelibrariescontainingmillions of independent recombinant clones, many of which will contain the entire protein-coding region of interest. A limited cDNA library may also be obtained by following the basic protocol for the production of subtracted cDNA libraries (UNIT 25B.2). Compared with phage libraries, plasmid libraries usually contain fewer clones and are more complicated to screen by hybridization or with antibody. They are most useful when prepared for particular applications, such as functional screening in COS cells with the vector CDM8 (Seed, 1987; UNIT 16.12). One advantage of functional screening is that
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Current Protocols in Molecular Biology
clones may be identified when no antibody or hybridization probes are available. A second advantage is that, for the most part, only fulllength clones are detected, although some clones may contain residual introns. A disadvantage is that cDNAs encoding large proteins (>140 kDa) are unlikely to be found, as are cDNAs for proteins requiring other subunits for function.
Critical Parameters Vector DNA, whether phage or plasmid, must be completely digested and phosphatased. A small amount of intact vector will transform the host bacteria very efficiently and contribute to a high background of nonrecombinant clones in the library. Of course, phosphatasing is not necessary for vectors employing the noncomplementary linker strategy. Commercially available phosphatased λgt10 or λgt11 arms and EcoRI-cut, phosphatased pBR322 or BstXI-cut CDM8 work well in these procedures. If the vector is prepared by the investigator, the steps outlined to evaluate the vector should be followed before it is used. High-efficiency in vitro packaging extracts are crucial to the successful preparation of a high complexity cDNA library. Commercially available extracts work well. The CDM8 vector requires a bacterial host containing the P3 plasmid, e.g., MC1061/P3. P3 is a low copy number, ∼60-kb plasmid containing an origin of replication able to coexist with ColE1. The supF gene carried by CDM8 suppresses the amber mutations in the structural sequence for tetracycline and ampicillin carried on P3 and allows expression of the antibiotic resistance phenotype (UNIT 1.4). P3 also carries a functional kanamycin resistance to allow for its selection. If a particular host is necessary for an experiment, the P3 plasmid can be readily recovered by an alkaline lysis miniprep of MC1061/P3 (UNIT 1.6) and transformed into the desired host to enable propagation of CDM8. The supF selectable marker minimizes the size of the vector, and may allow isolation of genomic clones by an elegant recombination-based screening method (Seed, 1983). Alternate vectors that utilize a conventional antibiotic resistance and contain the noncomplementary BstXI cloning sites are mentioned above and in UNIT 5.6. High-efficiency transformation of bacteria with the ligated cDNA is probably the single most important step. Commercial competent cells may be worthwhile, if the investigator has difficulty preparing competent cells ≥1 × 108
CFU/µg supercoiled pUC18 DNA. Transformation of bacteria by electroporation is superior to the use of competent cells. Several methods of evaluating a library are presented; screening for a known abundant clone, such as actin, is probably the best of these. Thorough evaluation of a library is of particular value when a screen turns out negative. If a problem exists in an evaluated library it is usually due to the probe, the screening method, or the source of the RNA.
Troubleshooting The major problem at this stage is usually too few clones and is often due to a problem in previous steps. The second most likely problem is inefficient packaging extracts or competent cells, which can be tested by packaging or transforming, respectively, with uncut vector. The best diagnostic test at this point is ligation of a test insert to the vector and preparation of a test insert library. For phage or plasmid vectors with an EcoRI cloning site, use the EcoRI digest of genomic DNA as a test insert (second support protocol). For vectors with a noncomplementary BstXI cloning site, obtain new phosphorylated BstXI adapters and self-ligate 1 µg in 20 µl to form linker dimers. Precipitate the DNA by dilution to 300 µl with dH2O, add 3 µl of 1 M MgCl2, 20 µg of carrier tRNA and 4 vol ethanol, precipitate 30 min in dry ice and recover the oligonucleotide DNA by thawing, and then microcentrifuging at top speed for 15 min. Resuspend the adapter dimers (the test insert) in 10 µl TE, and ligate 1 µl (100 ng) to 1 µg of vector. Transform E. coli (UNIT 1.11) and plate an aliquot to determine the efficiency, which should be ≥1 × 105/100 ng test insert.
Anticipated Results A complete cDNA library constructed in either phage vector should contain in the range of 106 to 107 independent recombinants from 100 ng of cDNA. This is sufficiently complex to contain even the rarest mRNA. A cDNA library prepared in a plasmid vector employing the noncomplementary adapter strategy should contain 5 × 105 to 2 × 106 colonies per µg input cDNA if prepared by transformation of competent cells. Ten-fold higher complexities should be obtained with electroporation.
Time Considerations The initial ligation of cDNA to the phage arms is an overnight step. The ligation may be packaged or transformed the following day or stored at −20°C over the weekend. Once the
Construction of Recombinant DNA Libraries
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library is packaged in vitro it should be plated within 24 hr for amplification and/or screening since these in vitro packaged phage are unstable and the titer of the library will drop if stored at this stage. An amplified library is stable for years when stored at −80°C in 8% DMSO as outlined in the phage library amplification protocol (UNIT 25B.2), and evaluation and screening may be performed at the investigator’s convenience. For plasmid libraries, the ligation may be stored for several days at −20°C. Transformed competent cells should be plated the same day. As above, an amplified plasmid library may be stored for years at −80°C in glycerol.
Literature Cited Huynh, T.V., Young, R.A., and Davis, R.W. 1984. Constructing and screening cDNA libraries in λgt10 and λgt11. In DNA Cloning Techniques, Vol. 1: A Practical Approach (D. Glover, ed.) pp. 49-78. IRL Press, Oxford. Okayama, H. and Berg, P. 1982. High-efficiency cloning of full-length cDNA. Mol. Cell. Biol. 2:161-170.
Sambrook, J., Maniatis, T., and Fritsch, E.F. 1989. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. Seed, B. 1983. Purification of genomic sequence from bacteriophage libraries by recombination and selection in vivo. Nucl. Acids Res. 12:56275633. Seed, B. 1987. An LFA-3 cDNA encodes a phospholipid-linked membrane protein homologous to its receptor CD2. Nature (Lond.) 329:840842. Young, R.A. and Davis, R.W. 1983. Efficient isolation of genes by using antibody probes. Proc. Natl. Acad. Sci. U.S.A. 80:1194-1198.
Key References Huynh et al., 1984. See above. Describes production of a complete λ phage library. Seed, B. 1987. See above. Describes production and screening of a complete plasmid library.
Contributed by Lloyd B. Klickstein Brigham and Women’s Hospital Boston, Massachusetts
Production of a Complete cDNA Library
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Construction of Bacterial Artificial Chromosome (BAC/PAC) Libraries
UNIT 5.9
Large-insert genomic libraries are important reagents for physical mapping of large chromosomal regions, for isolation of complete genes including all regulatory sequences, and for use as intermediates in DNA sequencing of entire genomes. There is a need for libraries of large insert clones in which rearrangements and cloning artifacts are minimal. With recent developments in host/vector systems, it is now possible to efficiently create large-insert libraries using low-copy-number bacterial plasmid vectors. Suitable vector systems have been developed using Escherichia coli F factor or P1 phage replicons, which are capable of producing large insert clones by bacterial transformation. Construction of BAC and PAC libraries is detailed first (see Basic Protocol). Two vectors, pCYPAC2 (Ioannou et al., 1994) and pPAC4 (E. Frengen et al., unpub. observ.) have been used for preparing PAC libraries, and a new BAC vector pBACe3.6 (Frengen et al., 1999) has been developed for construction of BAC libraries. All vectors are illustrated in Figure 5.9.1. Support Protocol 1 describes preparation of PAC or BAC vector DNA for cloning by digestion with BamHI or EcoRI, simultaneous dephosphorylation with alkaline phosphatase, and subsequent purification through pulsed-field gel electrophoresis (PFGE). For the preparation of high-molecular weight DNA for cloning, Support Protocols 2 and 3 provide procedures for embedding total genomic DNA from lymphocytes or animal tissue cells, respectively, in InCert agarose. Support Protocol 4 details the next steps for the genomic DNA: partial digestion with MboI or with a combination of EcoRI endonuclease and EcoRI methylase (including appropriate methods for optimizing the extent of digestion), and subsequent size fractionation by preparative PFGE. Finally, Support Protocol 5 covers the isolation of BAC and PAC plasmid DNA for analyzing clones. Isolation of BAC and PAC plasmid DNA is somewhat more difficult than standard plasmid extraction, because it requires more attention to cell concentration during extraction and resuspension of ethanol-precipitated DNA. In order to construct, for example, a 10-fold redundant human library with 150-kb average insert size, 200,000 clones must be generated. The EcoRI-EcoRI ligation efficiency seems to be higher than the MboI-BamHI ligation. A reasonable transformation efficiency from EcoRI-EcoRI ligation is ∼2,000 colonies per transformation using 2 µl ligation mixture and 20 µl electrocompetent cells. In comparison, the efficiency of MboI-BamHI ligation is ∼800 colonies per transformation. Thus, it is necessary to perform at least 100 individual transformations for an EcoRI partially digested 10-fold redundant library, and 250 individual transformations for an MboI partially digested library. In order to perform 100 electroporations, 200 µl of concentrated ligation mixture is required, thus five individual 250-µl ligations must be prepared. For 250 transformations, 500 µl of ligation mixture is needed, so thirteen individual 250-µl ligations are required. Note that it will be necessary to repeat the Basic Protocol and Support Protocol 4 multiple times until enough clones are generated to complete the library. Steps 24 to 30 of the Basic Protocol are designed to handle the amount of size-fractionated genomic DNA that can reasonably and comfortably be generated from a single preparation as described in Support Protocol 4. It is advisable to prepare at least 20 DNA plugs (20 to 40 µg/plug) for the entire library construction procedure; up to four plugs may be needed to optimize the partial digestion, and as many as sixteen plugs may be needed for library construction. CAUTION: Radioactive, biological, and chemical substances require special handling; see APPENDIX 1F & 1H for guidelines.
Construction of Recombinant DNA Libraries
Contributed by Kazutoyo Osoegawa, Pieter J. de Jong, Eirik Frengen, and Panayiotis A. Ioannou Current Protocols in Molecular Biology (2001) 5.9.1-5.9.33
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Copyright © 2001 by John Wiley & Sons, Inc.
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A
BamHI (1)
NotI (18,720)
ScaI (21) ScaI (1789) T7 ScaI (2723) pUC-link BamHI (2739) P1 lytic SP6 NotI (2778) replicon sacBll pCYPAC2 loxP r 18.8 kb Km
P1 plasmid replicon
B
BamHI (1) SacII (8) EcoRI (10) SacI (20) MluI (22) NsiI (36) ApaLI (766)
NotI (11,456) Tn7att PI-SceI loxP 511
T7 Cmr
ApaLI (2012) pUC-link
pBACe3.6 11.5 kb F plasmid replicon
ApaLI (2509) Eco RI (2728) SacI (2738) NsiI (2784) MluI (2790) SacI (2800) EcoRI (2802) SacII (2810) BamHI (2811) NotI (2850)
SP6
sacBII
loxP
C
NotI (19,466) BamHI (1) Tn7 att PI-SceI ScaI (21) loxP 511 ScaI (1789) ScaI (2723) oriP T7 pUC-link BamHI (2739) SP6 NotI (2778) sacBII Kanr pPAC4 19.5 kb P1 plasmid replicon
bsrr cassette
cos hCMV loxG
Construction of BAC/PAC Libraries
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Figure 5.9.1 (at left) The (A) pCYPAC2, (B) pBACe3.6, and (C) pPAC4 vectors contain common features for positive selection of cloned inserts. The insert-containing E. coli cells can be grown in preference over cells containing plasmids without an insert by including sucrose in the medium. Sucrose is converted into toxic metabolites by levansucrase encoded by the sacBII gene present in all three vectors. The pUC-link segment contains a functional high-copy-number plasmid that includes the ampicillin resistance gene (Apr; not shown). The pUC-link sequence is removed in the cloning procedure and is not present in the PAC and BAC clones. The digested genomic DNA fragment replaces the fragment containing the pUC-link in the PAC or BAC vectors. The loxP recombination sites are not used in cloning. A detailed description of vector elements is given elsewhere (see, Background Information, vector elements). (A) The pCYPAC2 vector contains the P1 plasmid replicon, which maintains recombinant clones at around one copy per cell. Alternatively, the P1 lytic replicon can be induced to provide higher copy numbers by adding the lac inducer IPTG into the medium. The kanamycin resistance gene (Kmr) is also present in this vector. (B) The pBACe3.6 vector contains the chloramphenicol resistance gene (Cmr) and the F plasmid replicon, maintaining recombinant clones at one copy per cell. The multiple cloning sites flanking the pUC-link segment allow positive selection for cloned inserts using any of the restriction enzymes BamHI, SacII, EcoRI, SacI, MluI, or NsiI. The Tn7att sequence permits specific Tn7-based retrofitting of BAC clones. (C) The pPAC4 vector contains four elements from pCYPAC2: the P1 plasmid replicon, the kanamycin resistance gene, the sacBII gene, and the pUC-link. The P1 plasmid replicon ensures low-copy-number maintenance of recombinant clones; the P1 lytic replicon is removed. In addition, the oriP and bsrr cassette have been included to facilitate the use of PAC clones for functional analysis of genes carried on the cloned inserts. The Tn7att sequence is also present, enabling specific retrofitting of PACs.
PREPARATION OF BAC/PAC CLONES USING pCYPAC2, pPAC4, OR pBACe3.6 VECTOR
BASIC PROTOCOL
Preparation of large-insert bacterial and P1-derived artificial chromosome (BAC and PAC) clones is conceptually similar to procedures used for preparing small plasmid clones. A single vector fragment with dephosphorylated BamHI or EcoRI ends (see Support Protocol 1) is ligated to size-fractionated MboI- or EcoRI-digested genomic DNA fragments that are recovered from agarose slices using an electroelution procedure (see Support Protocol 4). Ligations are performed at low genomic DNA concentrations to favor the formation of circular molecules. A number of 120- to 300-kb fractions from the preparative gel are tested initially to identify the fraction giving the largest average insert size at an acceptable cloning efficiency. Once the optimal fraction is identified, a large-scale ligation is performed to make sufficient clones for a several-fold redundant library. Size fractionation of the partially digested DNA is important to select for the desired size fragments and to remove smaller digestion products, which otherwise would result in an overwhelming background of small BAC or PAC clones. Circular plasmid ligation products are introduced directly into host cells by electroporation. Although this procedure is conceptually easy, there are several aspects of the Basic and Support protocols that are critical for successful preparation of BAC and PAC libraries (see Critical Parameters). Materials ≥2 to 10 ng/µl size-fractionated MboI- or EcoRI-digested genomic DNA (see Support Protocol 4) 10 to 50 ng/µl pBACe3.6, pCYPAC2, or pPAC4 vector DNA prepared for cloning (see Support Protocol 1) 1 Weiss U/µl T4 DNA ligase (Life Technologies) and 5× buffer (see recipe) 0.5 M EDTA, pH 8.0 (APPENDIX 2) 10 mg/ml proteinase K
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100 mM phenylmethylsulfonyl fluoride solution (PMSF; see recipe) TE/PEG solution: 0.5× TE buffer, pH 8.0 (APPENDIX 2) containing 30% (w/v) polyethylene glycol 8000 (PEG 8000) Electrocompetent bacterial cells (ElectroMAX DH10B; Life Technologies) SOC medium (Life Technologies; also see UNIT 1.8, but reduce yeast extract to 0.5%) LB plates (UNIT 1.1) containing 5% (w/v) sucrose and either 25 µg/ml kanamycin (for PAC clones) or 20 µg/ml chloramphenicol (for BAC clones): 100 × 15–mm petri dishes for test transformation 22 × 22–cm trays for picking colonies LB medium (UNIT 1.1) containing 20 µg/ml chloramphenicol (BAC clones) or 25 µg/ml kanamycin (PAC clones) TE buffer, pH 8.0 (APPENDIX 2) NotI restriction endonuclease and buffer (New England Biolabs) 1% (w/v) ultrapure agarose solution (Life Technologies) 0.5× TBE buffer (APPENDIX 2) Low-range PFG markers in agarose containing a mixture of lambda HindIII fragments and lambda concatemers (New England Biolabs) 0.5 µg/ml ethidium bromide in 0.5× TBE buffer (APPENDIX 2) 80% (v/v) glycerol, sterile Dry ice/ethanol bath 16° and 37°C water baths 0.025-µm-pore-size microdialysis filters (Millipore): 25-mm diameter for small-scale test ligation and 47-mm diameter for large-scale ligation Wide-bore pipet tips, sterile Disposable microelectroporation cuvettes with a 0.15-cm gap (Life Technologies or equivalent) Electroporator (Cell Porator equipped with a voltage booster; Life Technologies or equivalent) 15-ml snap-cap polypropylene tubes, sterile Orbital shaker, 37°C Automated plasmid isolation system (AutoGen 740, Integrated Separation Systems, optional) Flexible plastic 96-well plate (Falcon or equivalent) Contour-clamped homogeneous electrical field (CHEF; UNIT 2.5B) apparatus (Bio-Rad) or field-inversion gel electrophoresis (FIGE; UNIT 2.5B) apparatus (Bio-Rad or equivalent) Digital imager (Alpha Innotech IS1000 or equivalent) 50-ml disposable centrifuge tube (Corning or equivalent) Additional reagents and equipment for modified alkaline lysis preparation of BAC or PAC clone DNA (see Support Protocol 5; optional) CAUTION: To prevent shearing, use sterile wide-bore pipet tips for all steps involving the handling of genomic DNA. Perform small-scale test ligation 1. Mix ∼50 ng of each 120- to 300-kb size fraction of MboI- or EcoRI-digested genomic DNA (see Support Protocol 4) with 25 ng BamHI- or EcoRI-digested and dephosphorylated pBACe3.6 vector DNA, or 50 ng BamHI-digested and dephosphorylated pCYPAC2 or pPAC4 vector DNA prepared for cloning (see Support Protocol 1). Construction of BAC/PAC Libraries
Suitable ligation controls include reactions with vector only or without ligase.
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The vector fragment is 8.7 kb for pBACe3.6, ∼15.5 kb for pCYPAC2, and 16.8 kb for pPAC4. The average size of fractionated genomic DNA is ∼150 to 200 kb. Thus, a 10/1 molar ratio of vector to insert is obtained by using approximately half quantity of pBACe3.6 vector to insert DNA in the ligation reaction, or equal quantities of vector and insert DNA for PAC cloning. The genomic DNA concentration in the final reaction is maintained at a low value (1 ng/ìl) to favor circle formation over concatemer formation. Ligation schedules will vary somewhat depending on the restriction endonuclease used in preparing the vector and insert DNA. For a typical EcoRI-EcoRI ligation schedule, perform step 1 through step 6 in a day and then dialyze the solution against TE/PEG solution at 4°C overnight. For BamHI-MboI cloning, incubate the ligation mixture in a thermal cycler at 16°C for 8 hr, then hold at 4°C overnight, and start step 4 the following day.
2. To each tube, add 10 µl of 5× T4 DNA ligase buffer (1× final) and sterile water to bring the total volume to 50 µl. Mix very gently. 3. Add 1 Weiss unit T4 DNA ligase and incubate at 16°C for 4 hr for EcoRI-EcoRI cloning or 8 hr for BamHI-MboI cloning. Keep in mind that the vector preparation can always contain a very low fraction of shortened fragments caused by BamHI or EcoRI degenerate (star) activities. Ligation could occur at those unspecific sites after long incubation, which could result in a small portion (200,000 clones. It is important to achieve as high a cloning efficiency as possible with the largest possible insert size, because this will reduce the number of electroporations that are subsequently required.
Isolate and characterize individual test clones 19. Pick 40 clones with a sterile toothpick and grow the cells overnight in 1.5 ml LB medium containing 20 µg/ml chloramphenicol (BAC clones) or 25 µg/ml kanamycin (PAC clones). 20. Extract DNA using an automated plasmid isolation system (AutoGen 740) or the modified alkaline lysis procedure (see Support Protocol 5). 21. Dissolve DNA (0.5 to 1 µg) in 100 µl TE buffer. Digest 5 to 10 µl DNA with 0.1 U NotI in a 20-µl volume to separate the vector and insert DNA fragments. It is convenient to perform the reaction in a flexible 96-well plate at 37°C for 2 hr.
22. Analyze digested DNA using a CHEF or a FIGE apparatus. Use a 1% agarose gel, 0.5× TBE buffer, low-range PFG markers, and the following conditions: For CHEF: 14°C, 6 V/cm, 16 hr, 0.1 to 40 sec pulse time, and 120° angle. For FIGE: Room temperature, 180 V forward voltage, 120 V reverse voltage, and 16 hr with 0.1 to 14 sec switch time linear shape. The FIGE system is much less expensive than CHEF, and is able to obtain enough resolution for this purpose.
23. Stain the gel with 0.5 µg/ml ethidium bromide solution and calculate the molecular weight of inserts using a digital imager. Most of the clones will give two bands—a vector fragment (8.7 kb for pBACe3.6, 15.5 kb for pCYPAC2, and 16.8 kb for pPAC4) and the insert DNA of variable size. If the insert contains internal NotI sites, additional bands will be seen. Incomplete digestion usually leads to a characteristic doublet, where the two bands differ by the size of the vector.
Create library 24. Repeat the ligation procedure using a size fraction of genomic DNA that gives the desired average insert size and cloning efficiency. Scale up the ligation reaction to 500 to 1000 µl using the entire remaining electroeluted insert DNA. The ligation reaction is usually performed in several microcentrifuge tubes at 250 ìl per tube. Otherwise, the increase in volume may cause the solution to be dispersed into the water during subsequent dialysis, even though a 47-mm-diameter filter is used.
25. Dialyze and concentrate the ligation mixture as described (steps 6 to 10), but use a 47-mm-diameter microdialysis filter for each 250-µl aliquot. After concentration, ∼40 ìl should be recovered from each 250-ìl ligation solution.
26. Add 12 µl concentrated DNA to tubes containing ∼110 µl each of electrocompetent bacterial cells. Mix well by gentle stirring using a wide-bore pipet tip. Thaw enough electrocompetent cells to complete all transformations using the entire recovered ligation mixture. Construction of Recombinant DNA Libraries
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27. Perform identical transformations at 20 µl per electroporation and collect the transformed cells from 10 electroporations into a 50-ml disposable centrifuge tube containing 5 ml SOC medium at room temperature. The transformation efficiency will drop if the medium containing transformed cells is kept at 4°C during the transformation experiment.
28. Incubate the cells on an oribital shaker at 200 rpm at 37°C for 1 hr. 29. Add 800 µl of 80% sterile glycerol solution into the bacterial culture and mix very well. Spread 200 µl on two individual 100 × 15–mm LB plates containing 5% sucrose and the appropriate antibiotic to examine the titer from each tube. The amount of culture spread per plate is determined based on the titer estimated from the test ligation and transformation (step 18). The number of colonies on a plate should be between 300 and 700 to estimate the accurate titer for colony picking. It is easier to homogeneously spread 200 ìl culture when 300 ìl SOC medium is also added to the plate.
30. Freeze the remaining cells in a dry ice/ethanol bath and keep at −80°C until colony picking can be scheduled. To pick colonies, spread culture at 1600 clones/plate on 22 × 22–cm LB plates containing 5% sucrose and the appropriate antibiotic. Grow overnight at 37°C. The colony picking is usually scheduled within a couple of months. The frozen cells should be able to be stored for at least 1 year. SUPPORT PROTOCOL 1
PREPARATION OF BAC/PAC VECTOR FOR CLONING The integrity of the vector’s cohesive ends and the absence of uncut vector molecules are prerequisites for success in preparing BAC and PAC libraries. This protocol describes steps for preparing the vectors to maximize cloning efficiency and to minimize the level of nonrecombinant clones. Preparation of pBACe3.6, pCYPAC2, and pPAC4 for cloning involves a single restriction endonuclease digestion with BamHI or EcoRI. These plasmids contain the pUC-link insert at the cloning site, thus inactivating the sacB gene and enabling clones containing the original plasmid to grow in the presence of sucrose. Clones containing uncut vectors cannot be distinguished from truly recombinant clones, and great care should be taken to ensure complete digestion of the original vector with BamHI or EcoRI during vector preparation. Depending on the source and activity of the different enzymes used, it is necessary to optimize the conditions for each digestion. The following conditions have been found to yield good vector preparations using the recommended enzymes.
Construction of BAC/PAC Libraries
Additional Materials (also see Basic Protocol) pBACe3.6, pCYPAC2, or pPAC4 stock in E. coli DH10B cells (P. de Jong;
[email protected]) LB plates (UNIT 1.1) containing: 25 µg/ml kanamycin (for PAC) or 20 µg/ml chloramphenicol (for BAC) 5% (w/v) sucrose and either kanamycin or chloramphenicol 5% (w/v) sucrose, 100 µg/ml ampicillin, and either kanamycin or chloramphenicol BamHI and EcoRI restriction endonucleases and 10× buffers (New England Biolabs or equivalent) 0.7% (w/v) agarose gels (for standard electrophoresis) Calf intestine alkaline phosphatase (AP; Boehringer Mannheim) 10 mg/ml proteinase K (Boehringer Mannheim) stock solution
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95% (v/v) ethanol 1.0% (w/v) ultrapure agarose solution (Life Technologies; for CHEF system) 6× loading buffer (UNIT 2.5A), not containing xylene cyanol FF or SDS 1-kb ladder or lambda HindIII markers T4 polynucleotide kinase (New England Biolabs) 30% (w/v) polyethylene glycol (PEG) 8000 1.5-mm-thick electrophoresis comb for CHEF apparatus Dialysis tubing of 3⁄4 in. diameter, mol. wt. exclusion limit of 12,000 to 14,000 daltons (Life Technologies or equivalent) Dialysis clip Additional reagents and equipment for agarose gel electrophoresis (UNIT 2.5A), plasmid extraction (e.g., UNIT 1.7), CsCl/ethidium bromide equilibrium centrifugation (e.g., UNIT 1.8), electroelution (see Support Protocol 4), and ethanol precipitation (UNIT 2.1A) Test vector DNA 1. Streak recombinant DH10B cells harboring BAC or PAC vectors onto an LB plate containing the appropriate antibiotic (chloramphenicol or kanamycin, respectively). Incubate overnight at 37°C. 2. Isolate five single colonies and inoculate into separate 15-ml snap-cap polypropylene tubes containing 3 ml LB medium with the appropriate antibiotic. Grow cultures overnight at 37°C. 3. Use 1.5 ml of each culture to prepare DNA using an automated plasmid isolation system (AutoGen 740) or the modified alkaline lysis protocol (see Support Protocol 5). Store the remainder of each culture at 4°C. 4. Resuspend DNA in 100 µl TE buffer. 5. Digest 5 µl DNA from each preparation with NotI, 5 µl with BamHI, and 5 µl with EcoRI, using the manufacturers’ recommended conditions. 6. Analyze digested DNA in a 0.7% agarose gel (UNIT 2.5A). Discard clones that contain any rearrangements of the vector. PAC vectors do not contain EcoRI sites. Otherwise all enzymes should liberate pUC19-link from the vectors, producing fragments of ∼2.7 kb (the pUC19 stuffer fragment) and ∼8.7 kb (pBACe3.6), 15.5 kb (pCYPAC2), or 16.8 kb (pPAC4).
Prepare vector for large-scale ligation 7. Dilute the selected culture into 1 liter LB medium containing the appropriate antibiotic. Grow to saturation at 37°C. This culture is used for making a large-scale preparation of the PAC or BAC vectors.
8. Prepare a crude lysate of the culture by the alkaline lysis or cleared lysate method. Any standard protocol for plasmid extraction may be used here (UNIT 1.7).
9. Purify vector plasmid from crude lysate by CsCl/ethidium bromide equilibrium centrifugation. Any standard protocol for CsCl/ethidium bromide equilibrium centrifugation may be applied (see UNIT 1.8). Construction of Recombinant DNA Libraries
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Digest vector with BamHI or EcoRI 10. To establish the minimal amount of restriction enzyme required for complete digestion of the vector, first digest 50 ng vector DNA with 0.1, 0.2, 0.5, and 1 U of either BamHI or EcoRI for pBACe3.6, and BamHI for PAC vectors, in separate 10-µl reactions for 1 hr at 37°C. Perform conventional agarose gel electrophoresis and ethidium bromide staining to view the digestion. It is essential to establish the minimal amount of enzyme required for complete digestion. If too little enzyme is used, many nonrecombinant clones containing undigested vector can be expected. On the other hand, excess enzyme results in a very high ratio of noninsert clones, which are most likely induced by star activity.
11. Next, to establish the maximal amount of DNA in the reaction (and the minimum scale up volume), digest 100, 200, and 400 ng vector DNA with 2, 4, and 8 times the minimal amount of enzyme in separate 10-µl reactions (total 9 reactions) for 1 hr at 37°C. Perform conventional agarose gel electrophoresis and ethidium bromide staining to view the digestion. High-quality vector preparation is difficult. If necessary, the vector can be digested with ApaLI for pBACe3.6 or ScaI for pCYPAC2 and pPAC4 to destroy the pUC-stuffer fragment in the vectors nearly completely. Note that a precipitation step is needed in order to change the reaction buffer and that optimal enzyme concentration must be determined.
12. Incubate ∼30 µg vector DNA with the defined amount of the appropriate enzyme at 37°C for 15 min. 13. Add 1 U calf intestine AP and continue incubating at 37°C for 1 hr. 14. To stop the reaction, add 0.5 M EDTA to a final concentration of 15 mM, add 10 mg/ml proteinase K to 200 µg/ml final, and incubate at 37°C for 1 hr. 15. To inactivate proteinase K, add 100 mM PMSF solution to 2 mM final and incubate at room temperature for 1 hr. Perform PFGE 16. During step 15, clean a 1.5-mm-thick electrophoresis comb with 95% ethanol and cover teeth with autoclave tape to create a large preparative slot with sufficient space to load the reaction mixture. Leave enough empty wells for a 1-kb ladder or lambda-HindIII markers on both sides and at least one empty well between the markers and the preparative slot. 17. Prepare a 1% agarose gel in a CHEF mold using the preparative comb. While the gel is solidifying, add 2 liters of 0.5× TBE buffer to the CHEF apparatus tank and equilibrate the unit at 14°C. 18. Place the gel in the precooled unit. Add 6× gel loading buffer to the samples (final 1×) and load samples in the large preparative well. Load 1-kb ladder or lambda HindIII markers in both side wells. Perform electrophoresis at 14°C and 6 V/cm for 16 hr with 0.1 to 40 sec pulse time at 120° angle. Use of xylene cyanol FF in the gel loading buffer is not recommended because this dye migrates with the vector band under these electrophoresis conditions.
Construction of BAC/PAC Libraries
Recover vector DNA 19. Cut away two flanking gel slices that contain the markers as well as 1 to 2 mm from each side of the central vector lane. Wrap the remaining middle portion of the preparative lane in plastic wrap and store at 4°C. 20. Stain the flanking slices with ethidium bromide to detect the major vector fragment.
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21. Cut linear vector DNA lacking the pUC-stuffer fragment from the central lane and recover by electroelution (see Support Protocol 4, steps 38 to 46). 22. Load sample into 3⁄4-in.-diameter dialysis tubing with a 12,000- to 14,000-Da mol. wt. exclusion limit, and dialyze once against 1 liter TE buffer at 4°C for >2 hr. 23. Recover solution from the dialysis tubing and precipitate DNA with sodium acetate and either ethanol or isopropanol (UNIT 2.1A). Dissolve DNA in 100 µl TE buffer. Check efficiency of dephosphorylation 24. Place four microcentrifuge tubes on ice and number them 1 to 4. Add 4 µl of 5× T4 DNA ligase buffer and 50 ng of digested and AP-treated vector DNA into the tubes, and adjust the volume to 20 µl with sterile distilled and deionized water. 25. Add 1 U T4 polynucleotide kinase to tubes 3 and 4. Incubate all four tubes at 37°C for 1 hr. T4 polynucleotide kinase will be active in 1× ligase buffer.
26. Heat at 65°C for 20 min, then place on ice to inactivate the enzyme. 27. Add 1 U T4 DNA ligase to tubes 2 and 4. Incubate all four tubes at 16°C for >4 hr. Thus, tube 1 contains no enzyme, tube 2 contains T4 DNA ligase, tube 3 contains T4 polynucleotide kinase, and tube 4 contains T4 polynucleotide kinase and T4 DNA ligase.
28. Run samples in a 0.7% agarose gel, stain the gel with ethidium bromide, and view. Closed circular DNA should not be seen in samples 1, 2, and 3. Sample 4 should have closed circular DNA but no linear DNA.
29. Perform a ligation using all the remaining vector DNA solution in a 200-µl reaction volume with 4 U T4 DNA ligase. 30. Purify the linear monomeric vector DNA away from the background of ligated vector by PFGE in a CHEF apparatus (see steps 16 through 23). Check ratio of background clones 31. Perform test ligations using vector DNA alone (25 ng pBACe3.6, 50 ng PAC vectors) and vector plus 50 ng control DNA (see Basic Protocol, steps 1 to 5). 32. Dialyze against water and 30% PEG 8000, and perform transformation as described (see Basic Protocol, steps 6 to 16). 33. Spread transformed cells on LB plates containing 5% sucrose and either kanamycin or chloramphenicol, as appropriate. Grow overnight at 37°C. Kanamycin/sucrose and chloramphenicol/sucrose plates are used to determine the level of recombinant clones and nonrecombinant clones. If the number of colonies obtained from vector DNA alone is high (>100), vector must be prepared again, beginning with enzyme titration (step 10). If the plasmid is isolated from these background clones and analyzed by PFGE after NotI digestion, smaller vector sizes will be observed. Presumably, star activity causes nonspecific digestion in the sacB gene (either promoter or coding sequence), thus inactivating the gene, and the nonspecifically digested, nondephosphorylated DNA is circularized by self-ligation. Thus, these colonies can grow on sucrose plates without insert DNA.
34. To examine the complete removal of stuffer fragment from the vector, spread the transformed cells with self-ligated vector on LB plates containing 5% sucrose, 100 µg/ml ampicillin, and either kanamycin or chloramphenicol, as appropriate. Grow overnight at 37°C. Kanamycin/ampicillin/sucrose and chloramphenicol/ampicillin/sucrose plates are used to determine the remaining level of undigested vector containing the pUC-link. Ideally, there should be zero AmpR colonies.
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SUPPORT PROTOCOL 2
PREPARATION OF HIGH-MOLECULAR-WEIGHT DNA FROM LYMPHOCYTES IN AGAROSE BLOCKS Construction of a total genomic library demands that some consideration be given to the source of DNA to be used. Although DNA from cultured cell lines has been extensively used in constructing cosmid libraries, it is well known that passage in tissue culture may result in chromosomal rearrangements; therefore, it is considered more desirable to use DNA obtained directly from human or animal sources. A convenient source of DNA for this purpose is whole blood, and more specifically, the circulating lymphocytes. Assuming an average of ∼5 × 106 lymphocytes per milliliter of blood and 6 pg DNA per cell, as much as 30 µg DNA can be recovered per ml of blood, making it a very convenient and inexpensive source of DNA. This method depends on preferential lysis of red blood cells (RBC) using a gentle procedure that does not affect lymphocytes. The method can be applied equally well to cultured cells by omitting the RBC-lysis step; however, the cells should be collected as quickly as possible, especially when using attached cells in culture. Other protocols for embedding high-molecular-weight mammalian DNA from tissues (see Support Protocol 3 and UNIT 2.5B) should also produce acceptable results. Materials Healthy human volunteer PBS (APPENDIX 2), ice cold 1× RBC lysis solution (see recipe) InCert agarose (FMC Bioproducts) Proteinase K lysis solution (see recipe) TE50 buffer: 10 mM Tris⋅Cl, pH 8.0 (APPENDIX 2)/50 mM EDTA 0.1 mM PMSF solution: 100 mM PMSF solution (see recipe) diluted 1/1000 in TE50 buffer immediately before use 0.5 M EDTA, pH 8.0 (APPENDIX 2) Blood-drawing equipment Blood collection tubes containing EDTA Automated hematology counter 50-ml conical screw-cap polypropylene tubes, sterile Refrigerated centrifuge with rotor/adapters for 50-ml tubes (e.g., Sorvall RI6000D centrifuge with H-1000B swinging-bucket rotor or equivalent) Roller mixer (Robbins Scientific or equivalent) 50°C water bath 10 × 5 × 1.5–mm disposable DNA plug mold (Bio-Rad) Collect blood and remove red blood cells 1. Use blood-drawing equipment to obtain 45 ml venous blood from a healthy human volunteer in blood collection tubes containing EDTA. Mix well to avoid clot formation. 2. Count the total number of lymphocytes using an automated hematology counter. The total number should be 2.25 to 3.3 × 108 in 45 ml.
3. Transfer blood to two 50-ml conical screw-cap polypropylene tubes, add 10 ml ice-cold PBS to each, and mix gently. 4. Centrifuge at 1876 × g for 5 min at 4°C. Discard the supernatant using a 10-ml disposable pipet, being careful not to remove any lymphocytes from the fuzzy coat layer. Construction of BAC/PAC Libraries
Centrifugation results in a large pellet of red blood cells at the bottom of the tube and a very thin layer of lymphocytes (the fuzzy coat layer) at the interface with the plasma layer.
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5. Repeat wash with ice-cold PBS ten times. 6. Discard the final supernatant, mix the cell suspension well, and divide into four 50-ml tubes. 7. Add 25 ml of 1× RBC lysis solution to each tube, mix samples gently on a roller mixer, and incubate on mixer ∼20 min at room temperature. During this time, an exchange process catalyzed by carbonic anhydrase results in accumulation of ammonium chloride inside the cells; this accumulation increases the internal osmotic strength and causes swelling and bursting of RBCs. Lymphocytes have much lower carbonic anhydrase activity and do not undergo lysis.
8. Visually monitor the progress of RBC lysis. When it appears complete (usually 200 kb) is always observed. This results from at least three factors: (1) DNA fragments and circular ligation products being increasingly more fragile, subject to shearing or nuclease action; (2) a lower efficiency in the formation of circular ligation intermediates; and (3) a dropoff in transformation efficiency of plasmids with increasing size, expressed as a molecular efficiency, and becoming more pronounced when expressed per DNA weight. It is imperative that highly competent bacterial cells are used to ensure that reasonable numbers of clones are obtained per electroporation. As a consequence of the dropoff in cloning efficiency with size, it is difficult to prepare BAC and PAC libraries with insert sizes averaging in 200 to 250 kb, and nearly impossible to construct highly redundant libraries with an average size >250 kb using the presented technology. Once a provisional BAC or PAC library has been constructed, it is essential to characterize the clones and isolate plasmid DNA. The use of Support Protocol 5 or an automatic plasmid isolation machine (AutoGen 740) will result in ∼0.5 to 1 µg PAC DNA (170 kb) per 1.5 ml of overnight culture (not induced with IPTG). When the pCYPAC2, pPAC4, or pBACe3.6 vector is used for library construction, two NotI sites flank the insert in recombinant plasmids; therefore, most clones should show only two bands when analyzed by PFGE: the vector (at ∼15.5 kb for pCYPAC2, 16.8 kb for pPAC4, and 8.7 kb for pBACe3.6) and the insert of variable size from clone to clone. Additional bands may be seen if the insert contains internal NotI sites. Incomplete digestion usually leads to a very characteristic doublet, where the two bands differ by the size of the vector.
Time Considerations Anticipated Results Under optimal conditions, one can obtain as many as 1800 transformants per 2 µl dialyzed solution with an average insert size of 160 kb, and as many as 250 transformants per 2 µl at 230 kb for BAC. Due to the low DNA concentrations in the ligation, a typical 10-fold redundant mammalian BAC or PAC library requires anywhere from a few hundred up to 2000 electroporations. Each electroporation should result in 300 to 2000 colonies for fragments in
It takes several days to prepare agarose-embedded high-molecular-weight DNA starting from a blood or animal tissue sample and at least another week for testing various conditions of partial digestion and analyzing the results using pulsed-field gel electrophoresis (weeks 1 and 2). Ligations and transformations take 3 to 5 days, and another 3 to 4 days are required to characterize candidate colonies by pulsed-field gel electrophoresis of NotI-restricted plasmids (weeks 2 and 3). If the par-
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ticular ligation gives desirable results (good cloning efficiency, low nonrecombinant levels, and large inserts), another week (week 4) should be dedicated to the 200 to 2000 electroporations required for a 10-fold redundant mammalian BAC or PAC library. It is often convenient, perhaps necessary, and possibly desirable to take a break at this point. The arraying of the BAC or PAC libraries can be done at a different time and place, as it requires other facilities. Picking of colonies (e.g., 100,000) will take about one week and will require at least two people to prepare and label microtiter dishes and spread all the cells, provided that there is access to a facility equipped with microtiter plate fillers and a colony-picking robot. Hence, under the most optimal laboratory conditions and with the appropriate skills, a 10-fold redundant mammalian BAC or PAC library can be constructed in 1 to 3 months, requiring at least two people; however, it is more realistic to expect that most of the time will be invested in acquiring the skills and optimizing the various critical parameters. Anytime between 6 months and a year should be dedicated by a single person, who should be able to count on assistance from others from time to time.
Literature Cited Chalker, A.F., Leach, D.R., and Lloyd, R.G. 1988. Escherichia coli sbcC mutants permit stable propagation of DNA replicons containing a long palindrome. Gene 71:201-205. Chu, G., Vollrath, D., and Davis, R.W. 1986. Separation of large DNA molecules by contourclamped homogeneous electric field. Science 234:1582-1585. Donovan, J. and Brown, P. 1995. Euthanasia. In Current Protocols in Immunology. (J.E. Coligan, A.M. Kruisbeek, D.H. Margulies, E.M. Shevock, and W. Strober, eds.) pp. 1.8.1-1.8.4. John Wiley & Sons, New York. Frengen, E., Weichenhan, D., Zhao, B., Osoegawa, K., van Geel, M., and de Jong, P.J. 1999. A modular, positive selection bacterial artificial chromosome vector with multiple cloning sites. Genomics. In press. Fukushige, S. and Sauer, B. 1992. Genomic targeting with a positive-selection lox integration vector allows highly reproducible gene expression in mammalian cells. Proc Natl Acad Sci U.S.A. 89:7905-7909.
Construction of BAC/PAC Libraries
Gibson, T.J., Coulson, A.R., Sulston, J.E., and Little, P.F. 1987. Lorist2, a cosmid with transcriptional terminators insulating vector genes from interference by promoters within the insert: Effect on DNA yield and cloned insert frequency. Gene 53:275-281.
Gibson, F.P., Leach, D.R., and Lloyd, R.G. 1992. Identification of sbcD mutations as cosuppressors of recBC that allow propagation of DNA palindromes in Escherichia coli K-12. J. Bacteriol. 174:1221-1228. Grant, S.G., Jessee, J., Bloom, F.R., and Hanahan, D. 1990. Differential plasmid rescue from transgenic mouse DNAs into Escherichia coli methylation-restriction mutants. Proc. Natl. Acad. Sci. U.S.A. 87:4645-4649. Hanahan, D., Jessee, J., and Bloom, F.R. 1991. Plasmid transformation of Escherichia coli and other bacteria. Methods Enzymol. 204:63-113. Ioannou, P.A., Amemiya, C.T., Garnes, J., Kroisel, P.M., Shizuya, H., Chen, C., Batzer, M.A., and de Jong, P.J. 1994. A new bacteriophage P1-derived vector for the propagation of large human DNA fragments. Nature Genet. 6:84-89. Osoegawa, K., Woon, P.-Y., Zhao, B., Frengen, E., Tateno, M., Catanese, J.J., and de Jong, P.J. 1998. An improved approach for construction of bacterial artificial chromosome libraries. Genomics 52:1-8. Pierce, J.C., Sauer, B., and Sternberg, N. 1992. A positive selection vector for cloning high molecular weight DNA by the bacteriophage P1 system: Improved cloning efficacy. Proc. Natl. Acad. Sci. U.S.A. 89:2056-2060. Raleigh, E.A. 1987. Restriction and modification in vivo by Escherichia coli K12. Methods Enzymol. 152:130-141. Schwartz, D.C., Li, X., Hernandez, L.I., Ramnarain, S.P., Huff, E.J., and Wang, Y.K. 1993. Ordered restriction maps of Saccharomyces cerevisiae chromosomes constructed by optical mapping. Science 262:110-114. Sheng, Y.L., Mancino, V., and Birren, B. 1995. Transformation of Escherichia coli with large DNA molecules by electroporation. Nucl. Acids Res. 23:1990-1996. Shizuya, H., Birren, B., Kim, U.-J., Mancino, V., Stepak, T., Tachiiri, Y., and Simon, M. 1992. Cloning and stable maintenance of 300-kilobase-pair fragments of human DNA in Escherichia coli using an f-factor-based vector. Proc. Natl. Acad. Sci. U.S.A. 89:8794-8797. Sternberg, N. 1990. Bacteriophage P1 cloning system for the isolation, amplification, and recovery of DNA fragments as large as 100 kilobase pairs. Proc. Natl. Acad. Sci. U.S.A. 87:103-107. Strong, S.J., Ohta, Y., Litman, G.W., and Amemiya C.T. 1997. Marked improvement of PAC and BAC cloning is achieved using electroelution of pulsed-field gel-separated partial digests of genomic DNA. Nucl. Acids Res. 25:3959-3961. Wang, M., Chen, X.N., Shouse, S., Manson, J., Wu, Q., Li, R., Wrestler, J., Noya, D., Sun, Z.G., Korenberg, J., and Lai, E. 1994. Construction and characterization of a human chromosome 2-specific BAC library. Genomics 24:527-534. Wang, G.L., Holsten, T.E., Song, W.Y., Wang, H.P., and Ronald, P.C. 1995. Construction of a rice bacterial artificial chromosome library and iden-
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tification of clones linked to the Xa-21 disease resistance locus. Plant J. 7:525-533. Woo, S.S., Jiang, J., Gill, B.S., Paterson, A.H., and Wing, R.A. 1994. Construction and characterization of a bacterial artificial chromosome library of Sorghum bicolor. Nucl. Acids Res. 22:4922-4931.
Osoegawa et al., 1998. See above. An improved BAC/PAC cloning approach. Pierce et al., 1992. See above. Development of an elegant and efficient P1 cloning vector, a direct predecessor to the pCYPAC vectors.
Woon, P.-Y., Osoegawa, K., Kaisaki, P.J., Zhao, B., Catanese, J.J., Gauguier, D., Cox, R., Levy, E. R., Lathrop, G.M., Monaco, A.P., and de Jong, P.J. 1998. Construction and characterization of a 10fold genome equivalent rat P1-derived artificial chromosomal library. Genomics 50:306-316.
Shizuya et al., 1992. See above.
Wyman, A.R. and Wertman, K.F. 1987. Host strains that alleviate underrepresentation of specific sequences: Overview. Methods Enzymol. 152:173180.
The first large-insert P1 plasmids constructed using in vitro packaging and viral infection to transform E. coli.
Key References Albertsen, H.M., Abderrahim, H., Cann, H.M., Dausset, J., Le Paslier, D., and Cohen, D. 1990. Construction and characterization of a yeast artificial chromosome library containing seven haploid equivalents. Proc. Natl. Acad. Sci. U.S.A. 87:4256-4260. The first description of use of PFGE to size select DNA for ligation. Ioannou et al., 1994. See above. The first paper describing PAC cloning. O’Connor, M., Peifer, M., and Bender, W. 1989. Construction of large DNA segments in Escherichia coli. Science 244:1307-1312.
The development of efficient procedures for constructing large-insert plasmids by bacterial transformation. Sternberg, 1990. See above.
Contributed by Kazutoyo Osoegawa and Pieter J. de Jong Children’s Hospital Oakland Research Institute Oakland, California Eirik Frengen The Biotechnology Centre of Oslo University of Oslo Oslo, Norway Panayiotis A. Ioannou The Murdoch Institute for Research into Birth Defects Royal Children’s Hospital Melbourne, Australia
Describes first large BAC clone constructed by a combination of in vivo recombination and recombinant DNA technology.
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AMPLIFICATION OF TRANSFORMED OR PACKAGED LIBRARIES
SECTION V
After libraries are packaged (or transformed) they can be amplified for storage. Amplification of a library involves replicating each clone in the library. This is frequently a worthwhile procedure for genomic or cDNA libraries because the libraries can then be used many times to isolate clones corresponding to different sequences. Procedures for amplifying bacteriophage, cosmid, and plasmid libraries are presented in this section.
Amplification of a Bacteriophage Library This protocol may be used for genomic DNA or cDNA libraries. A freshly packaged and titered library is adsorbed to log phase plating bacteria. The mixture is then plated at high density and allowed to grow until the plaques are just subconfluent. The phage are eluted from the plate by overnight incubation with phage buffer and the library is titered and stored at both 4°C and −80°C.
UNIT 5.10 BASIC PROTOCOL
Materials LB medium containing 0.2% maltose and 10 mM MgSO4 (UNIT 1.1) Suitable host (Table 5.10.1) In vitro packaged phage library (UNITS 5.7 & 5.8) Top agarose (UNIT 1.1), warmed to 47°C 150-mm H plates (UNIT 1.1), warmed to 37°C Suspension medium (SM; UNIT 1.11) Chloroform Dimethyl sulfoxide (DMSO) Additional reagents and equipment for titering bacteriophage (UNIT 1.11) 1. Inoculate 250 ml LB medium containing 0.2% maltose and 10 mM MgSO4 with 2.5 ml of a fresh overnight culture of host bacteria. Shake vigorously 2 to 4 hr at 37°C until the OD600 ≅ 0.5. 2. Prepare E. coli cells for plating as described in UNIT 1.11. The integrity of λ phage particles requires the presence of Mg2+. Maltose induces the expression of the λ phage receptor. To maximize the complexity of the library, it is important that the number of nonviable cells among the host bacteria be minimized; these represent “dead ends” for phage. Table 5.10.1 Suitable Escherichia coli Host Strains for Amplifying Lambda-Constructed Libraries
Vector
E. coli host
Relevant host genotype
λgt10 λgt11
C600hflA Y1088
EMBL 3 or 4
P2392, Q359, NM539 LE392
hflA SupF, lacIq (no antibiotics needed) P2 lysogen
Charon 4A
SupF Construction of Recombinant DNA Libraries
Contributed by Lloyd B. Klickstein Current Protocols in Molecular Biology (1996) 5.10.1-5.10.3 Copyright © 2000 by John Wiley & Sons, Inc.
5.10.1 Supplement 34
3. Shake by hand and then briefly microcentrifuge the tube containing the library to separate out the chloroform in the packaged library. Add the packaged, titered phage (but no chloroform) to plating bacteria, using 0.25 ml of host cells for every 1 × 105 phage. Incubate 15 min at 37°C to allow phage to adsorb to bacteria. 4. Add 0.5 ml of the host/phage mixture to 8 ml of 47°C top agarose, mix by inverting a few times, and pour onto a fresh 150-mm H plate, warmed to 37°C. Use as many plates as necessary to plate the entire mixture. Spread the top agarose by gently rotating the plate on the benchtop, and allow the agarose to harden for 5 min. Invert and incubate 6 to 7 hr at 37°C (39° to 41°C for λgt11 libraries amplified in Y1088). A myriad of tiny plaques should barely be visible at ∼4 to 5 hr. These should expand and be just less than confluent at 6 hr. If the plaques are growing slowly, continue the incubation for up to 8 hr.
5. Remove plates from incubator, cover each lawn with 10 ml SM, and place on a level surface at 4°C for 2 to 16 hr to elute the phage. 6. Using a pipet, combine the SM from all plates into a single glass or polypropylene tube. Centrifuge 5 min at 3000 rpm (2800 × g) and transfer the supernatant to a new tube. Add 0.5 ml chloroform and mix by inverting several times. Titer the amplified library as described in UNIT 1.11. Expect a titer of 1010 to 1011/ml. The volume of SM recovered from each plate will be nearly 1 ml less than what was added. If the plates were not fresh, greater losses of volume will occur. The total volume of the library will, of course, depend on the number of plates used in the amplification and may approach 100 ml in the case of large libraries.
7. Store the library at 4°C in Teflon-capped glass tubes with 0.5 ml chloroform. The library is stable in this form for years. (Storage in plastic results in a 100- to 1000-fold drop in titer.) For added security, transfer 930 µl-aliquots of amplified library (no chloroform) to screw-cap microcentrifuge tubes, add 70 µl of DMSO (final concentration 7%), mix thoroughly by inverting several times, and place at −80°C. COMMENTARY Background Information
Amplification of a Bacteriophage Library
Amplification is not essential to the successful creation and screening of a λ phage library; however, an unamplified library must be used right away and the number of times the library may be screened is limited by the life of the nitrocellulose filters or the primary plates which dry out in a few weeks to a month. In contrast, amplification allows the library to be stored nearly indefinitely and to be screened as many times as necessary. The principle of amplification is to allow each in vitro packaged phage to produce thousands of identical clones by a limited infection of a host. A potential disadvantage is that the composition of the library may change as a consequence of differences in growth rate during the amplification step, some clones being overrepresented compared to the corresponding mRNA abundance in the total cellular mRNA and others being underrepresented. The preadsorption of the li-
brary to the bacteria, the high plating density, and the relatively short incubation period all help minimize changes in the composition. An alternative approach used by some is amplification in liquid culture. This method is not recommended, since rapidly growing phage are overrepresented to a greater degree than when plates are used. Furthermore, there are more steps involved, since the phage must be purified from the relatively large volume by PEG precipitation and CsCl-gradient centrifugation.
Critical Parameters Amplification should be carried out as soon as possible after the library is packaged. Phage particles stored in the in vitro packaging mixture are unstable because the phage particles adsorb to bacterial debris in the mixture and are inactivated. A decrease in titer at this point decreases the complexity of the library. Fresh,
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log-phase host cells are important to ensure that each phage infects a viable cell and makes a phage burst and thus is successfully amplified. A phage that injects its DNA into a dead cell is lost from the library. For libraries constructed in vectors such as λgt10 or the EMBL series, which allow selection for recombinants, amplification should be done under selective conditions since this eliminates nonrecombinants. Subsequent platings of the amplified library may be done on either selective or nonselective hosts. The titer of a library stored as described in the basic protocol will usually drop only 2- to 3-fold over the years. If stored in plastic, then the titer may drop as much as 100- to 1000-fold.
that occurred during the elution of the plates. Storing the library in several tubes and freezing aliquots will circumvent loss of library due to this problem.
Anticipated Results The titer of an amplified phage library will be in the range of 1-10 × 1010 pfu/ml. The volume obtained will depend on the number of plates used, which in turn depends on the initial complexity of the library. For example, a library initially containing 2 × 106 clones in 1.5 ml requires ten 150-mm plates, so ∼100 ml of SM will be recovered in total. Thus, the library will have been amplified by a factor of roughly 105.
Time Considerations Troubleshooting The only difficulties that can occur here are a failure of the phage to infect the host or a failure of the host to grow. This may be avoided if the plates, media, and host strain are first tested with nonrecombinant phage prior to amplifying the library. Causes of problems include top agarose that is too hot, incorrect host strain, incorrect plates, incorrect incubator temperature, and so forth. Losing a library because a tube was dropped or contaminated may be avoided by splitting the packaged ligation into two tubes and amplifying in duplicate. A library stored over chloroform at 4°C may occasionally drop in titer due to fungal contamination
After in vitro packaging, the library is stored overnight at 4°C while a titer plate is incubating. The library should be amplified the next day by starting step 1 early in the day. The plaques should develop in 6 to 10 hr. Suspension medium is added and the plates elute overnight at 4°C. Alternatively, step 1 may be started in the late afternoon and the plates grown overnight for 7 hr.
Contributed by Lloyd B. Klickstein Brigham and Women’s Hospital Boston, Massachusetts
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Amplification of Cosmid and Plasmid Libraries Bacteria containing the recombinant clones are grown on agar plates, washed off the plates, and stored in glycerol. This procedure produces about a million-fold amplification of the library.
UNIT 5.11 BASIC PROTOCOL
Materials LB plates containing appropriate antibiotic (UNIT 1.1) LB medium (UNIT 1.1) Sterile glycerol Nitrocellulose membrane filters (Millipore HATF) Sterile rubber policeman 1. Plate drug-resistant bacteria on nitrocellulose filters that have been placed on LB plates (containing antibiotic to which the colonies are resistant), as described in UNIT 6.2. The filters are used so that the colonies remain small and do not grow together. Be sure to use commercially available triton-free filters to ensure bacterial growth.
2. Grow bacteria until colonies are just confluent. 3. Flood each plate with LB medium. Use ∼2 ml/10-cm plate and 4 ml/15-cm plate. 4. Using a sterile rubber policeman, rub colonies off from nitrocellulose filter, making a bacterial suspension. 5. Pool the suspensions from the different plates into one 50-ml plastic tube. 6. Add sterile glycerol to give a final glycerol concentration of 15%. 7. Mix the solution thoroughly and dispense 500 µl into 1-ml tubes. 8. Freeze the aliquots of the library at −70°C. They should remain viable without appreciable loss of bacteria for over 1 year. 9. To screen the library, simply thaw an aliquot, titer the bacteria concentration (UNIT 1.3), and plate the appropriate numbers of cosmid- or plasmid-containing bacteria on the screening filters. COMMENTARY Background Information Amplification of phage libraries is an established procedure, but the effective amplification of plasmid and especially cosmid libraries has proven to be more difficult. The major concern in amplifying a plasmid or cosmid library is that the doubling times of bacteria are often altered, depending upon the type of insert included within the plasmid or cosmid vector. This disproportionate growth of recombinant bacteria can easily be visualized by plating out a portion of a library and noting the variable colony size after 16 hr at 37°C. However, there is ample incentive to amplify the library. The advantages of obtaining thousands of copies of a cosmid or plasmid library from a single liContributed by John H. Weis Current Protocols in Molecular Biology (1996) 5.11.1-5.11.2 Copyright © 2000 by John Wiley & Sons, Inc.
brary are obvious. When amplifying a cosmid or bacterial library the individual must weigh the concern of the possible underrepresentation of particular clones because of overgrowth of some colonies, against the time required to create a library de novo for each screening or for the dissemination of the library to other researchers.
Critical Parameters The major concern with any amplification step is that each original recombinant be equally represented. Differences in the duplication rate of any recombinant bacteria will result in over- or underrepresentation in the amplified library. This concern is less for sim-
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5.11.1 Supplement 34
ple plasmid libraries than for cosmid libraries, which contain bacteria that grow at markedly different rates.
Anticipated Results The purpose of amplification is to provide a reagent library that can be used many times. An amplified cosmid or plasmid library will contain one library’s equivalence in 10 to 100 µl. By producing 100 µl of such a library frozen in 100 1-ml tubes, over 1000 platings can be expected.
Time Considerations After the transformation or transfection step in the preparation of the plasmid or cosmid libraries, respectively, the plating of the outgrowths will require 1 to 2 hr. The plates are then incubated overnight at 30° to 37°C, depending upon the desired colony size and the density of the platings. The filter wash, glycerol mix, and aliquoting into freezer tubes require an additional 2 hr.
Contributed by John H. Weis Harvard Medical School Boston, Massachusetts
Amplification of Cosmid and Plasmid Libraries
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CHAPTER 6 Screening of Recombinant DNA Libraries INTRODUCTION
T
he usual approach to isolating a recombinant DNA clone encoding a particular gene or mRNA sequence is to screen a recombinant DNA library. As described in Chapter 5, a recombinant DNA library consists of a large number of recombinant DNA clones, each one of which contains a different segment of foreign DNA. Since only a few of the thousands of clones in the library encode the desired nucleic acid sequence, the investigator must devise a procedure for identifying the desired clones. The optimal procedure for isolating the desired clone involves a positive selection for a particular nucleic acid sequence. If the desired gene confers a phenotype that can be selected in bacteria, then the desired clone can be isolated under selective conditions (UNIT 1.4). However, most eukaryotic genes and even many bacterial sequences do not encode a gene with a selectable function. Clones encoding nonselectable sequences are identified by screening libraries: the desired clone is identified either because (1) it hybridizes to a nucleic acid probe, (2) it expresses a segment of protein that can be recognized by an antibody, or (3) it promotes amplification of a sequence defined by a particular set of primers.
Screening libraries involves the development of a rapid assay to determine whether a particular clone contains the desired nucleic acid sequence. This assay is used first to identify the recombinant DNA clone in the library and then to purify the clone (see Fig. 6.0.1). Normally, this screening procedure is performed on bacterial colonies containing plasmids or cosmids or on bacteriophage plaques. To test a large number of clones at one time, the library is spread out on agarose plates (UNIT 6.1), then the clones are transferred to filter membranes (UNIT 6.2). The clones can be simultaneously hybridized to a particular probe (UNITS 6.3 & 6.4) or bound to an antibody (UNITS 6.7 & 6.11). When the desired clone is
bacteriophage, cosmid, or plasmid libraries
plate library (consider library base and insert size)
screen library by: hybridization to nick-translated DNA and synthetic oligonucleotidesor,
immunoreactivity or, hybrid selection of mRNA and translation
purify plaques or colonies
Figure 6.0.1
Flow chart for screening libraries.
Contributed by J.G. Seidman Current Protocols in Molecular Biology (1994) 6.0.3-6.0.5 Copyright © 2005 by John Wiley & Sons, Inc.
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first identified, it is usually found among many undesirable clones; an important feature of library screening is the isolation of the desired clones (UNITS 6.5, 6.6 & 6.12). Another method for identifying the desired clone involves hybrid selection (UNIT 6.8), a procedure by which the clone is used to select its mRNA. This mRNA is characterized by its translation into the desired protein. Libraries consisting of large genomic DNA fragments (∼1 Mb) carried in yeast artificial chromosome (YAC) vectors have proven to be tremendously useful for genome analysis. In general, these libraries (which are usually produced by large “core” laboratories) are intially screened using a locus-specific PCR assay (UNIT 6.9); the clone resulting from the initial round of screening is subsequently analyzed by more conventional hybridization methods (UNIT 6.10). To screen a DNA library, one must first devise the screening procedure. The next important choice is the selection of a recombinant DNA library. When choosing which library to screen the investigator should consider whether he or she wants to isolate clones encoding the gene or the mRNA sequence. cDNA clones encode the mRNA sequence and allow prediction of the amino acid sequence, whereas genomic clones may contain regulatory as well as coding (exon) and noncoding (intron) sequences. The differences between genomic and cDNA libraries are discussed in Chapter 5. Another critical parameter to be determined before proceeding with a library screen is the number of clones in the library that must be screened in order to identify the desired clone. That is, what is the frequency of the desired clone in the library? This frequency is predicted differently for genomic and cDNA libraries, as described below. Screening a genomic library. In general, genomic libraries can be made from DNA derived from any tissue, because only two copies of the gene are present per cell or per diploid genome. The predicted frequency of any particular sequence should be identical to the predicted frequency for any other sequence in the same genome. The formula for predicting the number of clones that must be screened to have a given probability of success is presented in UNIT 5.1. This number is a function of the complexity of the genome and the average size of the inserts in the library clones. For amplified libraries, the base (see UNIT 5.1) must exceed this number. Usually about 1 million bacteriophage clones or 500,000 cosmid clones must be screened to identify a genomic clone from a mammalian DNA library. Many of the clones that are screened from an amplified library will be screened more than once; the total number of clones that must be screened is 30 to 40% greater than the number calculated by the formula. Screening a cDNA library. The optimal cDNA library is one made from a particular tissue or cell that expresses the desired mRNA sequence at high levels. In highly differentiated cells, a particular mRNA may comprise as many as 1 of 20 of the poly(A)+ mRNA molecules, while some mRNAs are either not present at all or comprise as low as 1 molecule in 100,000 poly(A)+ mRNA molecules. When choosing a cDNA library the investigator must make every effort to obtain a library from a cell where the mRNA is being expressed in large amounts. Of course, the number of clones that must be screened is determined by the abundance of the mRNA in the cell. The amount of protein that is found in the cell is frequently a good indicator of the abundance of the mRNA. Thus, proteins that comprise 1% of the total cell protein are made by mRNAs that usually comprise 1% of the total poly(A)+ mRNA, and the desired cDNA clones should comprise about 1% of the clones in the cDNA library.
Introduction
Screening a YAC library. In the typical genomic libraries maintained in E. coli (described in Chapter 5), the size of the insert is limited to 20 to 25 kb for lambda vectors or to 40 to 45 kb for cosmid vectors. Yeast artifical chromosome (YAC) vectors, by contrast, are designed to carry much larger genomic DNA fragments and thereby facilitate genomic analysis, with inserts ranging from 0.3 to ∼1 Mb in size. Conventional screening of YAC
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libraries by hybridization is difficult, both because of the unfavorable signal-to-noise ratio and the sheer numbers of replica films required to represent an entire library. For example, a standard YAC library representing 5 to 8 genome equivalents requires over 500 microtiter plates (and corresponding filters for screening by hybridization). Thus, most core laboratories screen YAC libraries using a locus-specific PCR assay whose primers define a particular sequence. The PCR screening is initially performed using pools (representing up to 4 microtiter plates or 384 YAC clones) or superpools (representing up to 20 microtiter plates or nearly 2000 clones), followed by subsequent rounds of screening to narrow down the possible candidates. Specialized screening strategies. For particular applications, there exist specialized approaches to screening. For example, cloned cDNAs encoding cell surface or intracellular proteins can be identified by expression screening, involving rounds of transient expression of a library and subsequent screening by immunoselection (UNIT 6.11). The technique of recombination-based screening provides a rapid and efficient approach for screening a complex genomic library in bacteriophage lamba (UNIT 6.12). The library is screened for homology against a plasmid carrying a particular cloned target sequence. If homology exists, a recombination event occurs, resulting in integration of the plasmid into the phage, and the recombinant is isolated by genetic selection. General considerations. When selecting the library it is critical that the base be larger than the number of clones to be screened. One problem with predicting the number of clones to screen is that most libraries are amplified and in the process of amplifying the library some clones are lost while others may grow more rapidly. Thus, if the desired clone is not found in a particular library, another independent library should be screened. Having selected the library, the investigator is ready to begin screening for the desired clone. The technologies used to screen libraries are mostly extensions of the techniques that have been described earlier in the manual. Libraries are plated out, transferred to nitrocellulose filters, and hybridized to 32P-labeled probes or bound to antibodies. The major problem associated with this technique is that “false” positives can be identified: the probe may hybridize to clones that do not encode the desired sequence. Approaches to minimize this problem are discussed in UNIT 6.7. A second source of undesired clones arises from the power of the screening procedures that are normally used to screen these libraries. The investigator will be screening as many as one million clones. If the library contains any contaminating recombinant DNA clones that have been previously grown in the laboratory, it will be identified in the screening procedure. Thus, extreme care must be exercised to prevent contamination of the library with previously isolated recombinant clones. Despite these problems the ability to screen large DNA libraries to isolate the desired clone provides a powerful tool for molecular biologists. J.G. Seidman
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PLATING LIBRARIES AND TRANSFER TO FILTER MEMBRANES
SECTION I
The basic principle of screening recombinant DNA libraries is that bacteriophage plaques, or bacterial colonies containing plasmids or cosmids, contain relatively large amounts of insert DNA that can be detected either directly by hybridization (see below) or indirectly by the protein that may be expressed from the cloned segment (UNIT 6.7). The first step in the nucleic acid hybridization screening procedure is to grow large numbers of colonies or plaques on agar plates. Replica copies of these colonies are transferred to nitrocellulose filters, where they can be screened. In this section the techniques for producing large numbers of colonies and plaques, and for transferring these to filter membranes, are discussed. Prerequisites to these procedures are that the library must already be chosen and the number of clones to be screened must be determined (see introduction to this chapter).
Plating and Transferring Bacteriophage Libraries Bacteriophage are plated onto agar plates at high density so that as many as 1 million different plaques can be screened. The bacteriophage plaques are then transferred to nitrocellulose filters, denatured, and baked. The library and the number of clones to be screened are predetermined. Principles for choosing the plaque density and the number of plates to be used are outlined in the commentary.
UNIT 6.1
BASIC PROTOCOL
Materials Host bacteria, selection strain if applicable (UNIT 1.10; Table 1.4.5; Table 5.10.1) Recombinant phage (UNIT 5.10) 0.7% top agarose (prewarmed; UNIT 1.1) 82-mm or 150-mm LB plates; or 245 × 245–mm Nunc bioassay LB plates (UNIT 1.1) 0.2 M NaOH/1.5 M NaCl 0.4 M Tris⋅Cl, pH 7.6/2× SSC 2× SSC (APPENDIX 2) Nitrocellulose membrane filters (or equivalent) 20-G needle 46 × 57–cm Whatman 3MM or equivalent filter paper 80°C vacuum oven or 42°C oven Plating bacteriophage 1. Determine the titer of the library by serial dilution as described in UNITS 1.11 & 5.7. For λ vectors that allow genetic selection against nonrecombinants, plating should be done on the appropriate bacterial strain (e.g., P2 lysogen for EMBL vectors). LB plates should be poured several days in advance to allow them to dry prior to plating. The large Nunc plates are particularly prone to condensation on the surface of the agar, but this can be alleviated by allowing them to sit on the benchtop with covers removed for a few minutes to several hours before use.
2. Mix recombinant phage and plating bacteria (prepared as described in UNIT 1.11) in a culture tube as outlined in Table 6.1.1 and incubate 20 min at 37°C. 3. Add 0.7% top agarose to culture tube and transfer mixture to LB plates. Disperse bacteria and agarose on plates by tilting the plates back and forth. Mix cells and agarose for the large Nunc plates by gently inverting several times in a capped 50-ml tube prior to plating. Contributed by Thomas Quertermous Current Protocols in Molecular Biology (1996) 6.1.1-6.1.4 Copyright © 2000 by John Wiley & Sons, Inc.
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Top agarose rather than top agar should be used as agar tends to lift off with the nitrocellulose filter. Melt the top agarose and cool to 45° to 50°C before use. If top agarose is too hot it will kill the bacteria, while if it is too cold the library will solidify in the tube.
4. Incubate plates at 37°C until plaques cover the plate but are not confluent. Incubation time varies between 6 and 12 hr and depends on type of phage and bacteria used. Store at 4°C. Do not incubate unattended overnight, but rather place at 4°C and allow to continue growth the next day. Allowing phage plaques to incubate for the correct amount of time is critical. The object is to optimize two parameters. First, the plaques must be large enough to contain sufficient DNA to give a good signal. Second, if the plaques are too large and become confluent they are difficult to purify in subsequent steps. Because most nucleic acid probes give a very strong signal, we tend to prefer having smaller plaques and weaker signals.
5. Incubate plates at 4°C for at least 1 hr before applying filters. Transferring to nitrocellulose filters 6. Label nitrocellulose filters with a ballpoint pen and apply face down (ink side up) on cold LB plates bearing bacteriophage plaques. This is best accomplished by touching first one edge of the filter to the agarose and progressively laying down more of the filter as it wets. Bubbles should be avoided. If difficulties are encountered the filter should not be adjusted on the plate, but rather removed and replaced with a new filter. Nitrocellulose filters should be handled only with forceps or gloved hands.
7. Leave filters on plates for 1 to 10 min to allow transfer of phage particles to the filter. During this transfer period the orientation of the filter to the plate is recorded by stabbing a 20-G needle through the filter into the agar at several asymmetric points around the edge of the plate. Up to five replicas can be made from each plate. Remove the filter slowly from the plate with blunt, flat forceps and place face up on paper towels or filter paper. Some investigators dip the needle used to orient the filter in India ink to more clearly mark the filter and agar. Other investigators mark the back of the agar plate with a black marker. Making two replicas from each filter, hybridizing both to the DNA probe, and comparing the autoradiographs of the replica filters eliminates many possible artifacts. This is particularly helpful when screening with an oligonucleotide probe.
8. Dry the filters on the benchtop for at least 10 min. This drying process binds the plaques to the filter.
Table 6.1.1
Plating and Transferring Bacteriophage Libraries
Recommended Mixtures for Plating Bacteriophage Libraries
Plate size
LB plate ingredient
82 mm
Bacteriab (ml) Phage, pfu Top agarose, ml
0.2 5,000 3
245 × 245 mma
150 mm 0.5 20,000-30,000 7
2 150,000 30
aNunc Bioassay plates distributed by Vangard International. bPlating bacteria are prepared as described in Chapter 1.
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Denaturation and baking 9. Place 46 × 57–mm Whatman 3MM paper on the benchtop and saturate with 0.2 M NaOH/1.5 M NaCl. Place filters on the paper face up for 1 to 2 min. The 3MM paper should be wet enough to allow immediate saturation of the filters, but not so wet that the solution pools on the surface.
10. Transfer filters (face up) to 3MM paper saturated with 0.4 M Tris⋅Cl, pH 7.6/2× SSC for 1 to 2 min and then to 3MM paper saturated with 2× SSC for 1 to 2 min. Some investigators immerse the filters in all three solutions. This procedure can make the plaques detected by hybridization appear diffuse.
11. Dry filters in a vacuum oven 90 to 120 min at 80°C or overnight in a regular oven at 42°C. Store at room temperature in folded paper towels or other absorbent paper until needed for hybridization (described in UNIT 6.3 or 6.4). COMMENTARY Background Information There are two parts to this protocol—plating the library and preparing filters. The number of bacteriophage per plate determines the number of plates that must be poured. This number is defined by the number of recombinants in the library (i.e., base of the library) and the frequency of the expected clone in the library. There is no advantage to screening more than 3 to 5 times the base of the library. The frequency of the clone in the library is determined as follows. cDNA libraries: the expected frequency of the desired RNA among the total RNA of the cell, ranging from 1⁄100 to 1⁄50,000. Genomic libraries: the size of the insert divided by the total genome size. Subgenomic libraries: the size of insert per total genome size times the fold purification of the DNA fragment (usually 10- to 50-fold). The usefulness of a recombinant phage library depends on the ability to screen a large number of phage and identify the clone that carries the DNA sequence of interest. This has been made possible by the technique of in situ plaque hybridization described by Benton and Davis (1977). The phage are allowed to multiply in host bacteria in a thin layer of agarose on regular bacterial plates. When nitrocellulose is applied to the agarose, phage particles and unpackaged DNA adsorb to the filter to produce a replica of the plate surface. If the agarose surface is not excessively wet, there will be little spreading of the phage on the filter. Subsequent treatment of the filter with sodium hydroxide destroys the phage particles and denatures the phage DNA which then binds to the nitrocellulose. Neutralization of the filters is required to maintain the integrity of the nitrocellulose. Hy-
bridization of these filters to a DNA or RNA probe will identify the location of the phage plaque of interest, which can then be recovered from the plate. A common variation of this technique is the substitution of one of the nylon-based membranes for nitrocellulose (see UNIT 2.9). The advantage of nylon membranes is their durability, which allows multiple hybridizations to the same filter and allows one to sequentially clone several genes from the same library using a single set of filters. However, nylon filters do not offer an improvement in sensitivity and are often more expensive than nitrocellulose filter paper.
Literature Review
The molecular basis of λ phage replication and the adaptation of the λ genome for molecular cloning has been reviewed by Arber et al. (1983) and Williams and Blattner (1980). Principles governing the plating of λ phage have been outlined by Arber (1983); see also UNIT 1.10. Thorough understanding of these principles has led to a universal approach to plating phage libraries.
Critical Parameters To prevent recombination between different phage, do not allow them to overgrow, and grow them in recombination-minus hosts where possible. Calculations of the amount of phage stock to be used per plate should be based on a recent titration, and plating cells should be fresh. Filters must not become brittle during this procedure; brittle filters will be destroyed during the hybridization process. This can be avoided by limiting the time in the hydroxide solution to less than 5 min, making certain that
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the 0.4 M Tris⋅Cl, pH 7.6/2× SSC brings the filters to neutral pH, and limiting the baking to 2 hr.
Troubleshooting Plaques should be visible on the plate before filters are made. If there appears to be poor bacterial growth, it is possible that the top agarose was too warm and many bacteria were killed, or that the phage titer was higher than expected and most host cells were lysed. Lower than expected phage titer could be due to an inaccurate titration of the phage stock, poor host-cell preparation, or too little time for adsorption. The preparation of the nitrocellulose filters will only be tested after hybridization is complete. Occasionally, hybridization to a plaque will produce a streak instead of a discrete circle on the autoradiograph, making location of the correct plaque difficult. Steps that will often correct this problem include: (1) drying plates with the cover removed for 1 to 2 hr before applying the filter, (2) drying the filters well before the hydroxide treatment, and (3) making certain that the face (phage side) of the filters is not directly in contact with the solutions.
Anticipated Results This plating procedure characteristically produces plates with an even distribution of dense phage particles. It is sensitive enough to allow identification of a phage by hybridization even when the phage are plated at high density (>5000 plaques per 82-mm plate). A signal is easily visible after 18 to 24 hr, when filters are hybridized to a nick-translated DNA probe with activity of >107 counts/µg DNA.
Time Considerations Usually plaques will become visible within 6 to 10 hr after plating. Bacteriophage should generally not be allowed to grow longer than necessary to visualize the plaques. Using the procedure outlined, even a large number of filters can be processed in a single day.
Literature Cited Arber, W. 1983. A beginner’s guide to lambda biology. In Lambda II (R.W. Hendrix, J.W. Roberts, F.W. Stahl, and R.A. Weisberg, eds.) pp. 381395. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Arber, W., Enquist, L., Hohn, B., Murray, N., and Murray, K. 1983. Experimental methods for use with lambda. In Lambda II (R.W., Hendrix, J.W. Roberts, F.W. Stahl, and R.A. Weisberg, eds.) pp. 433-466. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Williams, B.G. and Blattner, F.R. 1980. Bacteriophage lambda vectors for DNA cloning. In Genetic Engineering, Vol. 2 (J.K. Setlow and A. Mullander, eds.) p. 201. Plenum, NY.
Key References
Benton, W.D. and Davis, R.W. 1977. Screening λgt recombinant clones by hybridization to single plaques in situ. Science 196:180-182. Describes the method of plaque hybridization developed by the authors to allow isolation of phage possessing specific cloned DNA sequences.
Contributed by Thomas Quertermous Massachusetts General Hospital Boston, Massachusetts
Plating and Transferring Bacteriophage Libraries
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Plating and Transferring Cosmid and Plasmid Libraries A bacterial suspension is suctioned through a porous membrane, leaving the bacteria bound to the membrane surface. The membrane is transferred, bacteria up, to an agar plate upon which the bacteria will receive enough nutrients to grow into colonies. These filters can then be used for replica platings and for hybridization with specific DNA probes.
UNIT 6.2
BASIC PROTOCOL
Materials LB plates containing antibiotic (UNIT 1.1) LB medium (UNIT 1.1) LB plates containing 50 µg/ml chloramphenicol (UNIT 1.1) 0.5 M NaOH 1 M Tris⋅Cl, pH 7.5 0.5 M Tris⋅Cl, pH 7.5/1.25 M NaCl 10- or 15-cm Whatman 3MM or equivalent filter paper discs Sintered glass filter with vacuum Nitrocellulose membrane filters (10- or 15-cm, Millipore HATF) 20 × 20–cm Whatman 3MM or equivalent filter paper 20 × 20–cm glass plate 20-G needle 46 × 57–cm Whatman 3MM or equivalent filter paper 80°C vacuum oven NOTE: All materials coming into contact with E. coli must be sterile. Plating cosmids 1. Start with plasmid or cosmid library produced after transformation, transfection, or amplification (UNIT 5.7). 2. Determine titer of the library by serial dilutions using plates containing antibiotics (see UNIT 1.3). Remaining library suspension can be held at 4°C overnight with only minimal loss of viable bacteria. A 10-cm nitrocellulose filter can accommodate 10,000 to 20,000 colonies, while a 15-cm filter can hold up to 50,000.
3. Calculate the appropriate amount of the bacterial suspension for plating and dilute the suspension in LB medium such that there is the desired amount of bacteria in 5 ml (10-cm filter) or 10 ml (15-cm filter) of solution. 4. Meanwhile, prepare a layer of 10- or 15-cm Whatman 3MM paper discs on either the bottom part of a sintered glass Buchner funnel or on a porcelain filter funnel. Pour 10 to 20 ml LB medium over two or three layers of 3MM paper discs to make a level bed. The same pad of discs can be used for many filters. Sterilize filter apparatus and filter paper before use. The 3MM and nitrocellulose filters can be sterilized by autoclaving them while wrapped in aluminum foil. The purpose of this step is to spread the bacteria uniformly across the surface of a nitrocellulose filter. The filtering apparatus must be level, it must create a uniform suction to all the surface of the filter, and it should be easy to move the filters to and from the apparatus.
Contributed by Thomas Quertermous Current Protocols in Molecular Biology (1987) 6.2.1-6.2.3 Copyright © 2004 by John Wiley & Sons, Inc.
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5. Label a nitrocellulose filter with a ballpoint pen on the side opposite that where the bacteria will be plated. Place the filter on the surface of the LB/antibiotic plate to wet it. The antibiotic plate must be permissive for cosmid- or plasmid-bearing bacterial cells and usually is ampicillin or tetracycline. Most ballpoint pen inks do not smudge during the hybridization reaction. If the one you choose runs, try another type.
6. Remove the wet filter from an antibiotic plate to the filtration apparatus. The suction should be off.
Carefully pipet the 5 to 10 ml of bacterial suspension onto the surface of the nitrocellulose filter, leaving the outer 4 to 5 mm of the filter free of solution. This outside bacteria-free ring leaves enough surface area to work with the filter without smearing or losing the colonies.
7. Slowly suction the solution down through the filter, taking care not to create any preferential suction pockets that would concentrate the bacteria. After suctioning all of the solution through the filter, transfer the filter back to the antibiotic plate on which it was wetted. In laying the filter down on the agar surface, take care to avoid trapping any air bubbles between the surface of the plate and the filter.
8. Plate the entire library in this way and incubate the plates upside down (agar side up) at 37°C until the colonies are ∼1 mm in diameter. Do not overgrow the filters, as smaller colonies can be lost beneath larger, faster-growing recombinant bacteria.
Preparing replica filters 9. Label and wet another set of nitrocellulose filters, as described in step 5. 10. Remove the initial library filter from its plate and place on several sheets of 20 × 20 cm 3MM paper, bacteria side up. While wearing gloves, carefully position the wetted replica filter above the bacterial lawn. Lay the second filter upon the first, leaving the two filters offset by 2 to 3 mm. This overlap will help in the separation of the two filters after the replica transfer. Do not allow air bubbles to form between the two filters. These are excluded by touching the second filter to the first in the middle and then allowing the edges to fall.
11. Lay three sheets of 20 × 20–cm 3MM paper on the two filters, followed by a 20 × 20 cm glass plate. Using the palms of your hands, press with all your weight down on the glass plate, thus transferring the bacterial colonies from the library filter to the replica filter. 12. Remove the glass plate and the filter paper and, using a 20-G needle, punch holes 2 to 4 cm apart through both of the filters. These holes will allow the orientation of the film produced from the replica filter down on the library filter for the isolation of the correct clones.
Plating and Transferring Cosmid and Plasmid Libraries
13. Carefully peel the two filters apart, placing them both bacteria up, on their respective agar plates. Grow the replica colonies at 37°C overnight, leaving the library filters at 25°C overnight. After overnight growth, store the library filters on the agar plates at 4°C, while screening the replica filters. Multiple replica filters can be made from the same library filter. Incubate library filters 2 to 4 hr at 37°C or overnight at 25°C to allow regrowth of the colonies.
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5. Label a nitrocellulose filter with a ballpoint pen on the side opposite that where the bacteria will be plated. Place the filter on the surface of the LB/antibiotic plate to wet it. The antibiotic plate must be permissive for cosmid- or plasmid-bearing bacterial cells and usually is ampicillin or tetracycline. Most ballpoint pen inks do not smudge during the hybridization reaction. If the one you choose runs, try another type.
6. Remove the wet filter from an antibiotic plate to the filtration apparatus. The suction should be off.
Carefully pipet the 5 to 10 ml of bacterial suspension onto the surface of the nitrocellulose filter, leaving the outer 4 to 5 mm of the filter free of solution. This outside bacteria-free ring leaves enough surface area to work with the filter without smearing or losing the colonies.
7. Slowly suction the solution down through the filter, taking care not to create any preferential suction pockets that would concentrate the bacteria. After suctioning all of the solution through the filter, transfer the filter back to the antibiotic plate on which it was wetted. In laying the filter down on the agar surface, take care to avoid trapping any air bubbles between the surface of the plate and the filter.
8. Plate the entire library in this way and incubate the plates upside down (agar side up) at 37°C until the colonies are ∼1 mm in diameter. Do not overgrow the filters, as smaller colonies can be lost beneath larger, faster-growing recombinant bacteria.
Preparing replica filters 9. Label and wet another set of nitrocellulose filters, as described in step 5. 10. Remove the initial library filter from its plate and place on several sheets of 20 × 20 cm 3MM paper, bacteria side up. While wearing gloves, carefully position the wetted replica filter above the bacterial lawn. Lay the second filter upon the first, leaving the two filters offset by 2 to 3 mm. This overlap will help in the separation of the two filters after the replica transfer. Do not allow air bubbles to form between the two filters. These are excluded by touching the second filter to the first in the middle and then allowing the edges to fall.
11. Lay three sheets of 20 × 20–cm 3MM paper on the two filters, followed by a 20 × 20 cm glass plate. Using the palms of your hands, press with all your weight down on the glass plate, thus transferring the bacterial colonies from the library filter to the replica filter. 12. Remove the glass plate and the filter paper and, using a 20-G needle, punch holes 2 to 4 cm apart through both of the filters. These holes will allow the orientation of the film produced from the replica filter down on the library filter for the isolation of the correct clones.
Plating and Transferring Cosmid and Plasmid Libraries
13. Carefully peel the two filters apart, placing them both bacteria up, on their respective agar plates. Grow the replica colonies at 37°C overnight, leaving the library filters at 25°C overnight. After overnight growth, store the library filters on the agar plates at 4°C, while screening the replica filters. Multiple replica filters can be made from the same library filter. Incubate library filters 2 to 4 hr at 37°C or overnight at 25°C to allow regrowth of the colonies.
6.2.2 Current Protocols in Molecular Biology
Then repeat steps 9 to 13. Normally, two copies of the cosmid are hybridized to each probe.
14. After the bacterial colonies have grown, the cosmids or plasmids on the replica filter are amplified by transferring them to an LB plate containing 50 µg/ml chloramphenicol and incubating at 37°C for 4 to 10 hr. This step will increase the signal produced by hybridization. Preparing filters for hybridization 15. Remove the replica filters from the LB/chloramphenicol plates, place filters bacteria side up on a sheet of 46 × 57–cm 3MM paper soaked with 0.5 M NaOH, and leave them for 5 min. 16. Carefully transfer to a sheet of 46 × 57–cm 3MM paper soaked with 1 M Tris⋅Cl, pH 7.5. Allow neutralization to occur for 5 min. 17. Transfer to a third 46 × 57–cm filter soaked in 0.5 M Tris⋅Cl, pH 7.5/1.25 M NaCl. Neutralize 5 min. 18. Transfer filter to a dry sheet of 3MM paper to allow filter to dry. After filters are completely dry, stack them on paper towels or other adsorbent paper. Each nitrocellulose filter should be separated by paper towels from other filters.
19. Transfer the stacked filters to a vacuum oven at 80°C for 90 min. Remove filters and hybridize with a nick-translated probe, as described in UNITS 6.3 and 6.4. COMMENTARY Background Information There are two commonly used protocols for the screening of recombinant bacteria with hybridization probes. The first method involves the spreading of bacteria on the surface of agar using a sterile spreader (UNIT 1.3). A nitrocellulose membrane filter is then placed on top of the colonies and most of each colony is transferred to the filter. The filter is then treated as described in steps 15 to 19. This method works well when relatively small numbers of positive colonies are being selected (up to several thousand). The second method employs a matrix of some type (here nitrocellulose filters are used) upon which bacteria can be plated and grown when the filter is placed on top of a nutrient agar surface. Once the plated bacteria have grown into visible colonies, the filters can be used for replica plating and in situ hybridization analysis.
Critical Parameters In order to provide a uniform lawn of recombinant bacteria for screening, it is critical to ensure that the suction applied to the filters is uniform and not spotty. The best way to accomplish this is to suction the suspension through
the filter slowly and to avoid any preferential suction sites in the filter. Make sure that the apparatus is level and that adequate layers of LB-soaked chromatography paper are used. Air bubbles will prevent bacterial growth, so be certain that air is not trapped between the filter and the agar surface.
Time Considerations
Once the apparatus is set up, it takes ∼5 min per filter to wet the filter, suction the bacteria, and transfer to an LB plate. The colonies take ∼15 hr to grow at 37°C, after which they can be transferred to 4°C until ready for the replica platings. Replica plating also requires 5 min per filter, and resulting filters will be ready for denaturation and hybridization after 15 hr at 37°C.
Key Reference Hanahan, D. and Meselson, M. 1983. Plasmid screening at high density. Meth. Enzymol. 100:333-342.
Contributed by John H. Weis Harvard Medical School Boston, Massachusetts Screening Recombinant DNA Libraries
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SECTION II
HYBRIDIZATION WITH RADIOACTIVE PROBES After plaques or colonies have been transferred to nitrocellulose filters, the desired clone can be detected by its ability to hybridize to a DNA probe. This is a rapid, effective screening procedure that allows the identification of a single clone within a population of millions of other clones. The filters are hybridized with a 32P-labeled nucleic acid probe, the excess and incorrectly matched probe is washed off the filter, and the filter is autoradiographed. Two features of the nucleic acid probe used for these experiments are critical to the successful screening of recombinant DNA libraries. First, the probe must hybridize only to the desired clones and not to any other clones. Thus, the nucleic acid sequence used for a probe must not contain any reiterated sequences or sequences that will hybridize to the vector. Second, the specific activity of the probe must be at least 107 cpm/µg. Most of the procedures for labeling DNA or copy RNA molecules are described in Chapter 3, and a support protocol is presented here that allows the 5′ end-labeling of a mixture of oligonucleotides. The two basic protocols presented in this section describe steps required to hybridize labeled probes to recombinant DNA clones on filters. Two protocols are presented because conditions for hybridizing short oligonucleotide probes and longer nucleic acid probes to filters are different.
UNIT 6.3 BASIC PROTOCOL
Using DNA Fragments as Probes HYBRIDIZATION IN FORMAMIDE Bacteriophage plaques or bacterial colonies bound to a filter membrane are detected by hybridization with a radioactive probe. Hybridization proceeds on prewet filters placed in a sealable plastic bag. After hybridization the filters are removed from the sealed bag, excess probe is washed off, and the filters are autoradiographed to identify the clones that have hybridized with the probe. Materials Nitrocellulose membrane filters bearing plaques, colonies, or DNA (UNITS 6.1 & 6.2) Hybridization solution I Radiolabeled probe, 1 to 15 ng/ml (UNIT 3.5) 2 mg/ml sonicated herring sperm DNA High-stringency wash buffer I Low-stringency wash buffer I Sealable bags 42°C incubator Water bath adjusted to washing temperature (see commentary) Glass baking dish Additional reagents and equipment for autoradiography (APPENDIX 3) Incubate filters with probe 1. Wet filters with hybridization solution I. Lay a filter membrane bearing plaques on top of 5 to 20 ml of hybridization solution I and allow solution to seep through filter. It is important to wet only one surface at a time to prevent trapping air in filter. Wet each filter in turn, producing a stack of wet filters.
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When multiple filters are to be hybridized to the same probe, no more than twenty 8.2-cm discs or ten 20 × 20 cm square filters should be placed in one stack. Contributed by William M. Strauss Current Protocols in Molecular Biology (1993) 6.3.1-6.3.6 Copyright © 2000 by John Wiley & Sons, Inc.
Estimate the volume of hybridization solution used to wet the filters; this is a significant fraction of the volume of the hybridization reaction.
2. Transfer the stack of wetted filters to an appropriately sized sealable bag. Add enough hybridization solution to generously cover filters and seal. Note the volume of hybridization solution used to cover the filters.
3. Prehybridize filters by placing the bag in a 42°C incubator for at least 1 hr. Some investigators omit this step.
4. While filters are prehybridizing, pipet the radioactive probe into a screw-cap tube, add 2 mg (1 ml) sonicated herring sperm DNA, and boil 10 min. Place boiled probes directly into ice to cool. The amount of probe used is important, and should be in the range of 1 to 15 ng/ml of hybridization reaction. The volume of the hybridization reaction can be assumed to be the amount of hybridization solution added to the filters.
5. Add 2 ml hybridization solution I to the boiled probe. 6. Remove bag containing filters from the 42°C incubator. Open bag, add probe mixture, exclude as many bubbles as possible, and reseal. A good way to add the radioactive probe is to take it up in a syringe with an 18-G needle and then inject it into the bag. Reseal the bag after adding probe.
7. Mix probe in the bag so that filter is evenly covered. Replace bag in the 42°C incubator and let hybridize overnight. Wash filters to remove nonhybridized probe 8. Warm 1 liter high-stringency wash buffer I to the “washing temperature” in a water bath. The stability of washing temperature and salt concentrations are critical features of this experiment. See discussion in commentary.
9. Remove bag containing hybridizing filters from the 42°C incubator. Cut bag open and squeeze hybridization solution out of the bag. CAUTION: Handle material carefully as it is extremely radioactive. This should be done on disposable paper bench covers.
10. Quickly immerse the filters in 500 ml low-stringency wash buffer I at room temperature in a glass baking dish. Separate all the filters, as they may stick together during hybridization. The volume of the low-stringency wash buffer is not important as long as the filters are completely covered. The filters must not be allowed to dry as the radioactive probe will irreversibly bind the filters if the filters dry in contact with probe. (The type of container used to hold the filters is not important as long as it transfers heat well. Thus glass, metal, or enamel containers are better than plastic.) The low-stringency wash only removes nonhybridized probe formamide and hybridization solution; it does not determine the stringency of the hybridization.
11. Rinse the filters three times with 500 ml low-stringency wash buffer. Let the filters sit 10 to 15 min at room temperature in low-stringency wash buffer with each rinse. 12. Pour off the low-stringency wash buffer and pour in 500 ml high-stringency wash buffer (prewarmed to washing temperature).
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6.3.2 Current Protocols in Molecular Biology
13. Replace the high-stringency wash buffer with another 500 ml of high-stringency wash buffer, then place the glass dish containing the filters in incubator at wash temperature. Make sure that the temperature in the glass dish reaches the desired washing temperature by placing a thermometer directly into the bath and measuring the temperature. Usually 15 to 20 min at the desired wash temperature is sufficient to remove most of the background radioactivity. Of course, if the glass dish is placed in a water bath, be careful that the water from the water bath does not get into the filters.
Autoradiographing filters 14. Remove filters and mount them either wet or dry on a plastic backing. If the filter(s) is to be exposed wet, then isolate it from the film by covering it with plastic wrap. Used X-ray film provides a good form of plastic backing for filters.
15. Mark the filters with radioactive ink to assist in alignment and autoradiograph. An easy way to apply radioactive ink is to mark adhesive-backed paper labels with radioactive ink and then attach the stickers to the plastic wrap cover. X-ray intensifying screens greatly decrease the amount of exposure time required. ALTERNATE PROTOCOL
HYBRIDIZATION IN AQUEOUS SOLUTION This method differs mainly in that formamide is not used in the hybridization solution. Follow the basic protocol except use the reagents and alternate parameters listed below. Additional Materials Hybridization solution II Low-stringency wash buffer II High-stringency wash buffer II 65°C incubator 1. Prehybridize as in basic protocol except that the filters are prehybridized at 65°C using hybridization solution II. Hybridization solution II may have to be prewarmed to solubilize the SDS.
2. Prepare probe as in step 4 of basic protocol and dilute with 2 ml of hybridization solution II. 3. Hybridize overnight as in steps 6 and 7 of basic protocol except use a hybridization temperature of 65°C. 4. Remove bag containing hybridization from the 65°C incubator. Squeeze out the hybridization solution, taking care to avoid contamination with the excess radioactive hybridization solution. 5. Immediately rinse filters twice with low-stringency wash buffer II. It is unnecessary to maintain a given temperature for this wash; just let the filters sit in wash buffer at room temperature until ready to proceed.
Using DNA Fragments as Probes
6. At 65°C, proceed to wash filters with high-stringency wash buffer II. Employ multiple quick washes (5 to 8) and immerse filter in a final wash for ∼20 min. Check the radioactivity of the filters with a Geiger counter and be certain that they produce a signal only a fewfold above background levels.
6.3.3 Current Protocols in Molecular Biology
REAGENTS AND SOLUTIONS High-stringency wash buffer I 0.2× SSC (APPENDIX 2) 0.1% sodium dodecyl sulfate (SDS) High-stringency wash buffer II 1 mM Na2EDTA 40 mM NaHPO4, pH 7.2 1% SDS Hybridization solution I Mix following ingredients for range of volumes indicated (in milliliters): Formamide 20× SSC 2 M Tris⋅Cl, pH 7.6 100× Denhardts solution Deionized H2O 50% dextran sulfate 10% SDSa Total volume aIn place
24 12 0.5 0.5
48 24 1.0 1.0
72 36 1.5 1.5
120 60 2.5 2.5
240 120 5.0 5.0
2.5 10 0.5 50
5.0 20 1 100
7.5 30 1.5 150
12.5 50 2.5 250
25 100 5 500
480 240 10 10 50 200 10 1000
of SDS, N-lauroylsarcosine (Sarkosyl) may be used.
Add the SDS last. The solution may be stored for prolonged periods at room temperature. The dextran sulfate should be of high quality. Pharmacia produces acceptable-grade dextran sulfate. Recipes for SSC and Denhardt’s solution are in APPENDIX 2.
Hybridization solution II 1% crystalline BSA (fraction V) 1 mM EDTA 0.5 M NaHPO4, pH 7.2 (134 g Na2HPO4⋅7H2O plus 4 ml 85% H3PO4/liter = 1 M NaHPO4) 7% SDS Low-stringency wash buffer I 2× SSC (APPENDIX 2) 0.1% SDS Low-stringency wash buffer II 0.5% BSA (fraction V) 1 mM Na2EDTA 40 mM NaHPO4, pH 7.2 5% SDS Sonicated herring sperm DNA, 2 mg/ml Resuspend 1 g herring sperm DNA (Boehringer Mannheim #223636) in a convenient volume (about 50 ml of water) by sonicating briefly. The DNA is now ready to be sheared into short molecules by sonication. Place the tube containing the herring sperm DNA solution in an ice bath (the tube must be stable even if the ice begins to melt). The sonicator probe is placed in the DNA solution (without touching the bottom of the vessel). The sonicator is turned on to 50% power 20 min, or until there is a uniform and obvious decrease in viscosity. At no time should the tube containing the DNA become hot to the touch. After sonication, the DNA is diluted to a final concentration of 2 mg/ml, frozen in 50-ml aliquots, and thawed as needed.
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COMMENTARY Background Information All hybridization methods depend upon the ability of denatured DNA to reanneal when complementary strands are present in an environment near but below their Tm (melting temperature). In a hybridization reaction involving double-stranded DNA on a filter and a singlestranded DNA probe there are three different annealing reactions occurring. First, there are the desired probe-DNA interactions which result in signal. Second, there are mismatch interactions that occur between related but nonhomologous sequences; these mismatch hybrids are the ones that must be eliminated during the washing of the filters. Non-sequence-specific interactions also occur and these result in noise. The ability to extract information from a particular hybridization experiment is a function of the signal-to-noise ratio. High background or poor specific signal can both result in uninterpretable results. Washing nitrocellulose filters is required to remove excess radioactive probe, as well as radioactive probe that has bound to the DNA on the filter as mismatch hybrids. Temperature and salt concentration dramatically affect the maintenance of specific hybrids. Detergents and other charged species can have a profound effect upon the nonspecific binding of species that contribute to background. In this protocol, hybridization is achieved in a solution containing 50% formamide. Excess probe is rinsed away under low-stringency conditions so that further hybridization will not occur. Once the hybridization solution is rinsed away, it is possible to proceed to a high-stringency wash without fear of further hybridization. When washing is complete, the filters should produce very little “noise” when monitored with a Geiger counter. Although single-copy sequence probe normally does not produce a signal that is detectable with a Geiger counter, a probe corresponding to more abundant sequences will produce a signal that can be “heard” with a Geiger counter.
Literature Review Hybridization to filter membranes forms a basis of recombinant DNA technology and is described in detail earlier in the manual (UNIT 2.9). The protocols described here vary from those used for Southern blot filter hybridization in that the volume of the hybridization is usually larger and the washing conditions are different. Dextran sulfate is an important component of the hybridiUsing DNA Fragments as Probes
zation solution as it increases the rate of reassociation of the nucleic acids. The protocols in this unit describe methods for hybridizing radioactive probes to membranebound plaques or colonies. These procedures for screening recombinant clones were first suggested by Grunstein and Hognes (1975) and by Benton and Davis (1977). The conditions of hybridization proposed in the basic protocol involving hybridization in formamide was originally described by Denhardt (1966) and Gillespie and Spiegelman (1965) while the alternate protocol using aqueous hybridization solution was introduced by Church and Gilbert (1984). The method of washing filters under stringent conditions to remove background was first proposed by Southern (1975). Botchan et al. (1976) described the benefit of adding SDS to the wash solution. Jeffreys and Flavell (1977) first employed the wash conditions described in the protocols presented here.
Critical Parameters Hybridization. Kinetically, the hybridization of DNA (or RNA) probes to filter-bound DNA is not significantly different from hybridization in solution. For single-stranded probes, the rate of hybridization follows first-order kinetics, since probe is available in excess. Under conditions of excess probe, the time for hybridization is inversely proportional to the probe concentration. For double-stranded probes the rate of hybridization displays a more complex relationship to the initial probe concentration. However, to a first approximation the initial probe concentration is inversely proportional to the rate of hybridization. To determine the actual time required for the successful hybridization of a given probe, either empirical data must be available or the following formula can be used to determine the length of time (in hours) required to achieve 50% hybridization (T50): 1⁄
x
× y⁄5 × z⁄10 × 2 = T50
where x is the weight of probe in micrograms; y is the complexity of probe in kilobases; and z is the volume of hybridization solution in milliliters. The length of time T is given in hours. Maximum hybridization signal will be obtained if the reaction is allowed to proceed to 5 × T50, although 1 to 2 × T50 is often used. It is also clear that nonspecific interactions
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occur and that in any hybridization, sources of noise will be present. Therefore, from a practical standpoint one conventionally utilizes concentrations of nick-translated probe on the order of 1 to 15 ng/ml of hybridization, where the specific activity of the probe is from 5 × 107 cpm/µg to >108 cpm/µg. Too much probe in a hybridization is as bad as too little. One important source of background hybridization to filters is due to the hybridization of the probe to vector sequences or to E. coli DNA. Be certain that there is no vector or E.coli DNA sequences in the probe. This can best be ensured by isolating the probe from one type of vector (e.g., plasmid) and screening a library made with a different type of vector (e.g., bacteriophage). Washing temperature. Washing at low stringency is a straightforward proposition. Buffer is added at room temperature and washing proceeds at room temperature. High-stringency wash is determined empirically. The relative homology between the probe and target sequence is a determining parameter. If the homology is 100%, a high temperature (65° to 75°C) can be used. As the homology drops, lower washing temperatures must be used. In general one starts at 37° to 40°C, raising the temperature by 3° to 5°C intervals until background is low enough not to be a major factor in the autoradiography. The length of the probe is also important. Very short probes (5 × 107 cpm/µg and an overnight hybridization reaction with a 1-kb unique sequence probe, hybridizing bacterial colonies or bacteriophage plaques can be visualized after a 1 to 18 hr exposure.
Time Considerations Generally hybridizations are carried on overnight for 12 to 16 hr. This is sufficient for most probes and blots. However, with probes of increasing complexity longer hybridization times are required. This is preferable to increasing the probe concentration from experiment to experiment. Autoradiography requires 1 to 18 hr.
Literature Cited
Benton, W.D. and Davis, R.W. 1977. Screening λgt recombinant clones by hybridization to single plaques in situ. Science 196:180. Botchan, M., Topp, W., and Sambrook, J. 1976. The arrangement of simian virus 40 sequences in the DNA of transformed cells. Cell 9:269-287. Church, G. and Gilbert, W. 1984. Genomic sequencing. Proc. Natl. Acad. Sci. U.S.A. 81:1991-1995. Denhardt, D. 1966. A membrane filter technique for the detection of complementary DNA. Biochem. Biophys. Res. Commun. 23:641-646. Gillespie, D. and Spiegelman, S. 1965. A quantitative assay for DNA–RNA hybrids with DNA im mobiliz ed on a m em b ra ne. J. M ol. Biol.12:829-842. Grunstein, M. and Hogness, D. 1975. Colony Hybridization: A method for the isolating of cloned DNA’s that contain a specific gene. Proc. Natl. Acad. Sci. U.S.A. 72:3961. Jeffreys, A.J. and Flavell, R.J. 1977. A physical map of the DNA region flanking the rabbit β globin gene. Cell 12:429-439. Southern, E.M. 1975. Detection of specific sequence among DNA fragments separated by gel electrophoresis. J. Mol. Biol. 98:503-517.
Contributed by William M. Strauss Harvard Medical School Boston, Massachusetts
Screening Recombinant DNA Libraries
6.3.6 Current Protocols in Molecular Biology
Supplement 2
UNIT 6.4
Using Synthetic Oligonucleotides as Probes The protocols in this unit describe procedures for using mixtures of 32P-labeled oligonucleotides to screen recombinant DNA clones bound to nitrocellulose filters. A partial amino acid sequence of a protein is used to predict the nucleotide sequence of the gene that would encode it. A mixture of oligonucleotides is chosen that includes all possible nucleotide sequences encoding that amino acid sequence. This mixture of oligonucleotides is then used to screen a recombinant DNA library for the corresponding clones. In some cases however, the exact nucleotide sequence of a desired clone is known and it is possible to use a unique oligonucleotide as a probe.
BASIC PROTOCOL
HYBRIDIZATION IN SODIUM CHLORIDE/SODIUM CITRATE (SSC) This procedure outlines the steps necessary to screen nitrocellulose filters bearing DNA from bacteriophage or plasmids with mixtures of synthetic oligonucleotide probes. Hybridization and washing steps are carried out in solutions containing SSC. The washing temperature that produces the lowest background is determined empirically. Materials Membrane filters bearing plasmid, bacteriophage, or cosmid libraries (UNITS 6.1 & 6.2) 3× SSC/0.1% SDS Prehybridization solution SSC hybridization solution 6× SSC/0.05% sodium pyrophosphate, prewarmed to wash temperature Filter forceps (e.g., American Scientific Products #2568-1) Sealable bags (or equivalent) Additional reagents and equipment for autoradiography (APPENDIX 3) Prehybridize the filters 1. Prepare duplicate nitrocellulose filters of bacterial colonies or bacteriophage plaques. These should be processed and baked as described in UNITS 6.1 and 6.2. Although some authors recommend wiping the wet filters prior to baking to remove bacterial debris, we do not advise this procedure because the hybridization signal may be reduced. Filter forceps (i.e., without serrated tips) should be used to handle membrane filters to prevent marring the surface.
2. Wash the filters 3 to 5 times in 3× SSC/0.1% SDS at room temperature; about 50 82-mm filters can be washed in 500 ml. Then wash them once in the same solution at 65°C for at least 1.5 hr or overnight. This step removes much of the bacterial debris from the filters.
3. Remove filters from 3× SSC/0.1% SDS and prehybridize them 1 hr at 37°C in prehybridization solution. Herring sperm DNA in the prehybridization solution blocks nonspecific binding of probe to the filters and thus decreases the background level of radioactivity.
Using Synthetic Oligonucleotides as Probes
6.4.1 Supplement 2
Hybridize oligonucleotides to the filters 4. Remove filters from the prehybridization solution and put them into sealable bags containing SSC hybridization solution. Place up to 20 filters and ≥20 ml SSC hybridization solution into each bag. Add 0.125 to 1.0 ng of each 32P-labeled oligonucleotide per ml of hybridization solution to each bag. The mixed oligonucleotide probe is end-labeled with 32P as described in the support protocol. For example, to 20 ml of hybridization solution that will contain a mixture of 128 Contributed by Allan Duby, Kenneth A. Jacobs, and Anthony Celeste Current Protocols in Molecular Biology (1993) 6.4.1-6.4.10 Copyright © 2000 by John Wiley & Sons, Inc.
17-base oligonucleotides, add 320 ng (0.125 ng/ml × 128 oligonucleotides × 20 ml) of labeled probe. Hybridize filters 14 to 48 hr at the temperature indicated below: 14-base oligonucleotide 17-base oligonucleotide 20-base oligonucleotide 23-base oligonucleotide
room temperature 37°C 42°C 48°C
For bacterial colonies, adding much more than 0.125 ng of each oligonucleotide probe per ml of hybridization solution significantly increases the background on the autoradiogram. For bacteriophage plaques, there is less DNA per plaque than in a bacterial colony; as high backgrounds are not a problem with filters bearing bacteriophage plaques, more probe should be added to the hybridization mixture.
5. Remove filters from the hybridization bag and wash filters for 5 to 15 min, 3 to 5 times, in 6× SSC/0.05% pyrophosphate at room temperature. It is important that the filters are well separated from each other and that the solution is occasionally or continuously gently agitated.
Wash the filters 6. Wash filters for 30 min in prewarmed 6× SSC/0.05% sodium pyrophosphate at the temperature indicated below: 14-base oligonucleotide 17-base oligonucleotide 20-base oligonucleotide 23-base oligonucleotide
37°C 48°C 55°C 60°C
Adjust the temperature of 6× SSC/0.05% pyrophosphate and filters. Measure the temperature of the filters and surrounding solution by putting the thermometer into the solution, not into the water bath. Make sure the filters are separated and are occasionally or continuously gently agitated.
7. Examine the filters with a Geiger counter; they should not exhibit above-background radioactivity. If the filters still show a significant degree of radioactivity above background, increase the temperature by 2° to 3°C for 15 to 30 min and reexamine the filters with the Geiger counter. Do not exceed the following temperatures: 14-base oligonucleotide 17-base oligonucleotide 20-base oligonucleotide 23-base oligonucleotide
41°C 53°C 63°C 70°C
The background level of bound radioactivity depends upon the amount of bacterial debris left on the filters, the amount of labeled oligonucleotides added to the hybridization mixture, and the guanosine-cytosine (G-C) content of the oligonucleotide mixture.
Perform autoradiography 8. When the filters exhibit a low level of radioactivity or the maximum temperatures referred to in step 7 have been reached, the filters should be removed from the wash solution and mounted wet on a solid support before exposure at −70°C to X-ray film, using an intensifying screen. Cover filters with plastic wrap. Do not allow the filters to dry out. Allow films to expose for 14 to 72 hr. Autoradiograms made from filters with a high background may still yield interpretable results.
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9. Develop the films; if a high background prevents proper interpretation of the films, rewash the filters at a higher temperature. 10. Number and mark the orientation of the films as described in UNIT 6.3. Spots that appear in precisely the same place on duplicate filters are “positives” (winners) and should be processed as described in UNIT 6.5. It is impossible to identify the characteristics of a true positive spot. Only colonies or plaques that produce evidence of hybridization on both filter copies should be processed as described below. Note that the intensity of the spot can vary dramatically between the duplicate filters. If a clear-cut spot appears on one filter and only a darkening of the background appears on the other, this should be considered positive and the plate should be processed as described in UNIT 6.5. Note that if two different oligonucleotide mixtures representing two different parts of the protein are available, either the positives obtained with one probe can then be hybridized with the other probe or four filter copies of the library can be made and hybridized to the two probes. Of course, depending on how far apart the sequences that hybridize to the two probes are, it is possible that neither will be present on a less than full-length cDNA clone. BASIC PROTOCOL
HYBRIDIZATION IN TETRAMETHYLAMMONIUM CHLORIDE (TMAC) This procedure is similar to the SSC protocol except that hybridization and washing are performed in solutions containing TMAC. In TMAC, the melting temperature of an oligonucleotide is a function of length and is independent of base composition; thus, spurious hybridization due to high G-C content of some of the oligonucleotides is reduced. Conditions are described for using 17-base oligonucleotides, but information is provided for determining the conditions when oligonucleotides of various lengths are employed. Materials Nitrocellulose or nylon membrane filters bearing plasmid, bacteriophage, or cosmid libraries (UNITS 6.1 and 6.2) 150-mm LB agarose plates (UNIT 1.1), prewarmed to 37°C 2× SSC/0.5% SDS/50 mM EDTA, pH 8.0, prewarmed to 50°C TMAC hybridization solution, prewarmed to hybridization temperature TMAC wash solution 2× SSC/0.1% SDS 15-cm glass crystallizing dishes Filter forceps (e.g., American Scientific Products #2568-1) Additional reagents and equipment for autoradiography (APPENDIX 3) Process and prehybridize the filters 1. Process filters bearing bacterial colonies as described in bearing amplified bacteriophage plaques as follows:
UNIT 6.2.
Produce filters
a. Plate the bacteriophage from the library on LB agarose plates and transfer to nitrocellulose filters as described in UNIT 6.1, steps 1 to 7. To obtain maximum sensitivity with oligonucleotide probes when the amplification procedure is used, plating density should be reduced to 8,000 to 10,000 plaques per 150-mm plate.
Using Synthetic Oligonucleotides as Probes
Either nitrocellulose or nylon (Colony/Plaque Screen Filters by New England Nuclear) filters can be used in this procedure. Nitrocellulose filters become fragile when hybridized in TMAC and must be handled very carefully. If this becomes a problem and nylon filters are substituted, the phage plaques must be amplified overnight. The rest of the protocol is unchanged.
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b. Amplify the bacteriophage by transferring the wet filter to a prewarmed (37°C) LB agarose plate so that the surface bearing the bacteriophage is faceup. Refrigerate the master plates upon which the recombinant phage library were plated to prevent any further plaque expansion.
c. Incubate the plates at 37°C until the bacterial lawn re-forms on the surface of the nitrocellulose and plaques are evident. Plaque size will be somewhat larger than those on the original plate. This usually requires a 5- to 12-hr incubation period. Longer periods of growth will produce a dense bacterial lawn without significantly increasing plaque size or affecting hybridization signal. Bacteriophage that produce small plaques (e.g., EMBL) are usually plated in the evening and allowed to grow overnight. The plaques are transferred to nitrocellulose filters the following morning and the phage are amplified on the filters by incubation for 5 to 7 hr during the day. Phage that produce large plaques (e.g., λgt10) are plated early in the morning, allowed to grow 5 to 7 hr, transferred to nitrocellulose filters (steps 6 and 7 of UNIT 6.1), transferred to fresh plates, and then incubated for amplification overnight.
d. Denature and bind the bacteriophage DNA to nitrocellulose filters as described in steps 8 to 11 of UNIT 6.1. 2. Wash filters bearing bacterial colonies as described in step 1 of the SSC protocol. Wet bacteriophage-bearing filters in a prewarmed (50°C) solution of 2× SSC/0.5% SDS/50 mM EDTA (pH 8.0). Float the filters on top of the solution (with the surface containing the dried bacteria and plaques faceup) to allow the nitrocellulose to wet completely. Submerge the filters and, with a gloved hand, gently rub the surface of the filters to remove the dried bacterial debris. Transfer the filters to a container of fresh solution of 2× SSC/0.5% SDS/50 mM EDTA to remove bacterial debris. Alternatively, the filters can be incubated in this solution at 65°C for one to several hours and then scrubbed. Inadequate scrubbing of the filters results in an increase of nonspecific background hybridization, obscuring positive hybridization signals in the subsequent screening procedure.
3. Transfer the filters to a 15-cm glass crystallizing dish containing 5 to 10 ml TMAC hybridization solution (per filter), which has been prewarmed to the appropriate hybridization temperature (48°C for 17-mer oligonucleotides; see Fig. 6.4.1 and commentary for other oligonucleotides) and seal the dishes with plastic wrap and rubberbands. Prehybridize 1 to 2 hr at the hybridization temperature, which is 5° to 10°C below the melting temperature. Prehybridization and hybridization can be performed in glass crystallizing dishes that are slightly larger in diameter than the nitrocellulose filters. Gentle agitation on an orbital platform shaker will allow the solution to pass freely between the stacked filters and prevent the filters from sticking together. Place no more than 25 to 30 filters in each dish. Alternatively, prehybridization and hybridization can be performed in a sealable bag (see SSC protocol) with 7000 Ci/mmol) 25 to 50 U T4 polynucleotide kinase (UNIT 3.10) and 10× kinase buffer (UNIT 3.4) Ice-cold 10% trichloroacetic acid (TCA) 1. Set up reaction mixture on ice in microcentrifuge tube as follows: 2.5 to 250 pmol mixed oligonucleotides 7.5 µl 10× T4 polynucleotide kinase buffer 66 pmol [γ-32P]ATP (200 µCi) 25 to 50 U T4 polynucleotide kinase H2O to 75 µl Incubate 30 min at 37°C. The reaction mixture should have either equimolar amounts of label and oligonucleotide ends, or the label should be in molar excess. 1 mol deoxyribonucleotide ≅ 330 g 1 OD260 ≅ 40 ìg/ml oligonucleotide 1 ìg 14-base oligonucleotide ≅ 0.24 nmol 1 ìg 17-base oligonucleotide ≅ 0.18 nmol 1 ìg 20-base oligonucleotide ≅ 0.15 nmol
2. At the end of the reaction, check for incorporation of label by precipitating 1 µl of a diluted aliquot with ice-cold 10% TCA (acid precipitation, UNIT 3.4) and counting the incorporated radioactivity. Using equimolar amounts of oligonucleotide and label, ∼30% to 90% of the counts are incorporated. The labeled oligonucleotide can be further purified by a combination of phenol extraction and/or ethanol precipitation (UNIT 2.1). To remove unincorporated label, oligonucleotides of 17 bases or longer can be quantitatively precipitated from a solution of 2.5 M ammonium acetate containing 25 ìg carrier DNA plus 9 vol of 100% ethanol. The resulting pellets are washed with 70% ethanol, followed by 95% ethanol, air dried, and resuspended in 100 ìl TE buffer.
3. Store mixture in appropriate container at −20°C. REAGENTS AND SOLUTIONS Prehybridization solution 6× SSC (APPENDIX 2) 5× Denhardts solution (APPENDIX 2) 0.05% sodium pyrophosphate 100 µg/ml boiled herring sperm DNA continued
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Supplement 9
0.5% sodium dodecyl sulfate (SDS) SSC hybridization solution 6× SSC (APPENDIX 2) 1× Denhardt’s solution (APPENDIX 2) 100 µg/ml yeast tRNA 0.05% sodium pyrophosphate TMAC hybridization solution 3 M tetramethylammonium chloride (see recipe below for stock solution) 0.1 M NaPO4, pH 6.8 1 mM EDTA, pH 8.0 5× Denhardt’s solution (APPENDIX 2) 0.6% SDS 100 µg/ml denatured salmon sperm DNA TMAC wash solution 3 M tetramethylammonium chloride (see recipe below for stock solution) 50 mM Tris⋅Cl, pH 8.0 0.2% SDS Tetramethylammonium chloride (TMAC), 6 M stock solution Dissolve 657.6 g TMAC (mol wt = 109.6) in H2O and bring to 1 liter. Filter the solution through Whatman No. 1 filter paper and determine the precise concentration of the solution by measuring the refractive index (n) of a 3-fold diluted solution. The molarity (M) of the diluted solution = 55.6(n − 1.331) and the molarity of the stock solution = 3 × M. TMAC can be stored at room temperature in brown bottles. CAUTION: TMAC can irritate eyes, skin, and mucous membranes. It should be used with adequate ventilation in a fume hood. Used TMAC solutions should be collected and discarded as hazardous and/or radioactive waste. Small amounts (1 Mb have been produced and these are routinely propagated with apparent stability, suggesting that the major limitation to the size of YAC inserts is the quality of the starting genomic DNA. Large “core” laboratories that generate human YAC libraries— such as the Center for Genetics in Medicine, Washington University School of Medicine, St. Louis; the Centre d’Etude du Polymorphisme Humain (CEPH), Paris; and the Genome Analysis Laboratory, Imperial Cancer Research Fund, London—prepare human YACs with average insert sizes ranging from 0.3 to 1.2 Mb. Additional high-quality YAC libraries have been constructed using inserts from Drosophila melanogaster, Caenorhabditis elegans, Schizosaccharomyces pombe, and mouse (Burke et al., 1991; Rossi et al., 1992). Anecdotal reports indicate YAC libraries may support the propagation of certain insert sequences that are poorly represented in Escherichia coli–based libraries. The YAC cloning system also offers the advantage that large genomic YAC inserts can be easily manipulated in yeast by homologous recombination. Thus, it is relatively simple to truncate a YAC insert or to introduce specific deletions, insertions, or point mutations with high efficiency using methods such as those described in UNIT 13.10. This unit provides an introduction to the use of yeast artificial chromosome–bearing yeast clones (hereafter referred to as YAC clones) in genome analysis. It describes criteria for de-
signing a polymerase chain reaction (PCR) assay to be used in screening a YAC core library and discusses the rationale for verification and characterization of YAC clones obtained from these core laboratories. Protocols for maintaining YAC clones, analyzing YAC insert structure, preparing YAC DNA, and subcloning YAC inserts into other vectors are presented in UNIT 6.10. These protocols are outlined in the flow chart in Figure 6.9.1.
GENERATING YAC LIBRARIES Although YAC cloning is the method of choice when insert sizes >100 kb are required, a number of features of the system have interfered with its rapid assimilation for routine cloning. Because the S. cerevisiae genome is at least an order of magnitude more complex than the E. coli genome and existing YACs are carried as only a single copy within yeast cells, the signal-to-noise ratio is less favorable for identifying a cognate clone in a YAC library than in a λ or cosmid library. Moreover, efforts to develop high-density screening methods for YACs have enjoyed only limited success. Most laboratories that maintain YAC libraries organize them as collections of individual clones in 96-well microtiter plates, which can be replicated faithfully and kept frozen for storage; in this form, a standard library representing 5 to 8 genome-equivalents comprises more than 500 microtiter plates. As a result, the effort and resources required to construct YAC libraries and prepare them for screening are enormous. Consequently, it is generally most practical for investigators wishing to obtain YACs carrying a specific DNA sequence to arrange for screening of a preexisting library maintained by a core laboratory. Initially, YAC libraries were constructed with total genomic DNA (Burke et al., 1987). More recently, there has been interest in generating libraries from targeted DNA using somatic cell hybrids carrying a specific chromosome or portion of a chromosome. The feasibility of this approach has been demonstrated with the construction of a library carrying a portion of the human X chromosome (Abidi et
Contributed by David D. Chaplin and Bernard H. Brownstein Current Protocols in Molecular Biology (1992) 6.9.1-6.9.7 Copyright © 2000 by John Wiley & Sons, Inc.
Screening of Recombinant DNA Libraries
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al., 1990). Additional targeted libraries are in the late stages of development and should reduce the cost and effort of screening for loci whose chromosomal location has been established.
YAC LIBRARY SCREENING BY A CORE LABORATORY Methods used by YAC core laboratories for library screening evolve rapidly. It is possible to screen a library by hybridizing a single-copy probe to nylon filters stamped with a replica of one or more microtiter arrays. However, because of the low signal-to-noise ratio for hybridization and the substantial cost required to produce all of the nylon filter replicas, most laboratories perform library screening using PCR (Green and Olson, 1990). At the time of this writing, most core facilities first extract DNA from pools of clones, usually representing 1 to 4 microtiter plates (96 to 384 YACs) per pool, and then combine this pooled DNA into more superpools of 1500 to 2000 YACs. The pools and superpools are screened by PCR to identify candidate microtiter plates containing at least one amplifying YAC clone. Final identification of the clone is most commonly performed either by colony hybridization using the PCR product as the probe or by screening pools of rows and col-
umns from the same microtiter plate using PCR. The time required for a YAC core laboratory to verify the specificity and parameters of the PCR assay and screen complex clone pools and subpools is usually 3 to 8 weeks. As an example, the screening strategy used by one major core laboratory is described in the accompanying box. It should be noted that this procedure may change as technology advances; for instance, the recent advent of techniques providing reliable DNA extraction from small quantities of thousands of individual clones has made it feasible to screen individual wells on a plate and eliminate the laborious filter-hybridization step.
DESIGNING A LOCUS-SPECIFIC PCR ASSAY FOR SCREENING An investigator arranging with a core laboratory for library screening is required to design a strategy for detecting the inserted genomic DNA and to provide the appropriate probe(s). It is worth investing considerable effort to create a convenient and reliable assay because the assay’s success depends on its ability to detect the target sequence with high sensitivity while being insensitive to the presence of large excesses of yeast and plasmid sequences. Because a core laboratory must adopt PCR assays that have been imported
EXAMPLE: SCREENING OF HUMAN-GENOME YAC LIBRARY AT THE WASHINGTON UNIVERSITY SCHOOL OF MEDICINE At this core facility, screening of the human-genome YAC library proceeds in three stages: (1) initial evaluation of the PCR assay; (2) screening of pools of YACs; and (3) identification of individual YACs from subpools to the single well by filter hybridization. To permit pretesting of assays before they are sent to a screening core, new PCR assays are evaluated using four control DNA samples as templates: (1) CGM-1 human genomic DNA (33 ng/µl) from a lymphoblastoid cell line established from the donor whose DNA was used in preparing the YAC library; (2) YY212 DNA from a yeast strain carrying a YAC whose insert is yeast chromosomal DNA; (3) “single-membrane-pool” DNA (33 ng/µl) prepared from a pool of 396 YAC isolates; and “spiked-pool” DNA, which is single-membrane-pool DNA augmented with 5 ng/µl of CGM-1 DNA.
Yeast Artificial Chromosome Libraries
CGM-1 DNA serves as a positive control to demonstrate that the sensitivity of the PCR assay is adequate. YY212 DNA serves as a negative control, demonstrating that no product is amplified from either yeast host genomic DNA or the YAC vector. DNA from the singlemembrane pool and the spiked pool provide additional negative/positive controls that more closely mimic library screening conditions. A negative signal from single-membrane-pool DNA demonstrates lack of cross-reactivity of the probe with the YAC vector, yeast genomic DNA, or common human repetitive sequences. A positive signal obtained from the spikedpool DNA (containing only 5 ng/µl of CGM-1 DNA) is a strong indication that the assay possesses sufficient sensitivity against a yeast DNA background for successful library screening.
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design a PCR assay for identifying YAC clone of interest from genomic YAC library (UNIT 6.9 )
obtain isolated YAC clone from core facility (UNIT 6.9 )
validate identify of YAC using PCR (UNITS 6.9 & 15.2 )
grow and store YAC clone (UNIT 6.10, first basic protocol)
prepare DNA from isolated YAC clone and analyze by Southern blotting (UNIT 6.10, second basic protocol, and UNIT 2.9 )
analyze isolated YAC clone for chimerism using PCR (UNIT 6.10, fourth basic protocol)
prepare DNA from isolated YAC clone using agarose plugs and analyze by PFGE (UNIT 6.10, third basic protocol, and UNIT 2.5B )
analyze isolated YAC clone for chimerism by subcloning in bacterial vector (UNIT 6.10, alternate and support protocols)
for high-resolution analysis (optional) prepare high-molecular-weight YAC-containing DNA (UNIT 6.10, fifth basic protocol)
subclone high-molecular-weight YAC-containing DNA into cosmid or λ vector (UNIT 6.10, sixth basic protocol)
Figure 6.9.1 Flow chart showing protocols used to obtain and analyze YAC clones.
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from outside laboratories, it is a good idea to inquire in advance about the protocols preferred by the specific core facility that will be performing the screening. In general, any highly specific, sensitive, and robust PCR assay is suitable for screening a YAC library (see Chapter 15). Typically, two 18- to 30-mer oligonucleotide primers for use in amplifying a single-copy 75- to 750-bp product are satisfactory. Such primers define a landmark for genome mapping called an STS (sequence-tagged site; see introduction to Chapter 7 and Olson et al., 1989). When designing a PCR assay from scratch, it is useful to consider the following:
Fragment size The STS should be 75 to 750 bp in length. Fragments in this range are most efficiently amplified by PCR and are easily detected by either polyacrylamide (UNIT 2.7) or standard agarose (UNIT 2.5A) gel electrophoresis.
assay, it does help eliminate some of the most trivial causes of assay failure.
ANALYZING INDIVIDUAL YAC CLONES Once library screening has been successfully completed and the isolated YAC clone has been furnished to the investigator, attention should be directed to analyzing its structure. Initial studies should focus on determining whether the genomic insert is chimeric, checking for evidence of rearrangement within the insert, and verifying that the YAC is propagated in stable fashion in the yeast cell (see below). Simply analyzing several isolates of the same YAC in parallel may provide a means of recognizing instability, as each isolate serves as a control for the others. The following sections give an overview of strategies for analyzing YAC clones; specific protocols are given in UNIT 6.10.
Chimerism of the YAC Insert Primer length Each primer should ideally be 18 to 30 nucleotides long, be composed of 50% to 55% G + C, and be contained within a single-copy human-genomic-DNA segment. This ensures efficient priming and decreases the probability of false priming, enhancing the sensitivity and specificity of the assay. This also permits the amplified fragment to be used as a hybridization probe in the final hybridization-dependent steps of library screening (see below). If it is not possible to amplify a single-copy fragment, then some other single-copy probe (e.g., a synthetic oligonucleotide 30 nucleotides long) should also be prepared. Oligonucleotide design strategies are discussed further in UNITS 2.11 & 15.1.
Primer affinity
Yeast Artificial Chromosome Libraries
Primers should show little affinity for selfannealing or for annealing with each other. This prevents the production of small, template-independent PCR products that compete for primers in the reaction. A number of academic and commercial DOS-based and Macintosh software programs permit rapid selection of non-self-annealing primers from within a known DNA sequence (e.g., Oligo 4.0, National Biosciences; Primer, S. Lincoln and M. Daly, Whitehead Institute for Biomedical Research, Cambridge, Mass., and OSP, Hillier and Green, 1991; see UNIT 7.7). Although the use of these programs cannot remove all the uncertainty associated with designing a new PCR
A consistent problem in YAC cloning is chimerism of the YAC insert—i.e., the insert is composed of two or more separate genomic fragments joined in a single YAC. The mechanism(s) giving rise to chimeric YAC clones are currently not fully understood (Green et al., 1991). In most existing total genomic YAC libraries, chimeric clones represent from 5% to 50% of the total clones. Preliminary data suggest that targeted, chromosome-specific libraries may contain only 5% to 15% chimeric clones. Although future generations of YAC libraries are likely to contain lower frequencies of chimeric clones, chimerism will probably remain a significant problem requiring assessment for every new YAC clone being analyzed. The most reliable way to determine if a YAC insert is chimeric is to isolate a small fragment from each end of the insert and determine its chromosome of origin and whether it shares sequences with overlapping YACs derived from the same chromosomal region. Many approaches have been suggested for isolating such YAC genomic insert end fragments, all of them relying upon the fact that end fragments are marked by their adjacent YAC vector sequences. Thus, it is possible to determine whether a YAC insert is chimeric by preparing probes from the two YAC vector arms and using these to demonstrate that both ends of the YAC map to the same general chromosomal region. This is generally done using hybridization or PCR analysis of a somatic hybrid cell line containing the appropriate human chromo-
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some (or preferably a fragment thereof) as its sole human DNA. The appropriate end-fragment probes may be produced in several ways, two of which are presented in UNIT 6.10. The most rapid and versatile approaches to producing end-fragment probes use PCR amplification (Riley et al., 1990; Green, 1992). The template is a restriction fragment produced by cutting the YAC DNA at sites near the two ends of the genomic insert. The YAC DNA is digested with a frequently cutting restriction endonuclease to produce a collection of small restriction fragments. One of the fragments contains the distal portion of the YAC insert still associated with a portion of the left vector arm, while another contains the other end of the insert associated with a portion of the right vector arm (the arm of the YAC vector containing the yeast centromere is arbitrarily designated the left vector arm and the arm containing the ura3 selection marker is arbitrarily designated the right vector arm). These fragments are prepared for PCR amplification by ligation of a synthetic double-stranded DNA tag to both ends. This tag contains a 29-nucleotide “bubble” of noncomplementary sequence flanked by two 12-nucleotide complementary sequences (Fig. 6.10.2). The two YAC-insert end fragments can then be selectively amplified using one PCR primer derived from the YAC vector and one primer with the sequence of the noncomplementary portion of the bubble. These methods are generally favored because of their speed, but they depend on the fortuitous placement of restriction sites close enough to the ends of the genomic insert that a fragment suitably sized for PCR amplification can be generated. Moreover, if highly repetitive sequences are present at the distal portions of the insert, the PCR method may fail to generate useful information. A reliable but more time-consuming method of generating probes for end-fragment analysis is conventional subcloning of larger YAC-derived restriction fragments into plasmid or λ vectors (Bronson et al., 1991). Subcloning an end fragment several kilobases in size is timeconsuming, but reliably assures identification of nonrepeated sequences for use as probes. The subcloning protocol given in UNIT 6.10 involves double-digesting the YAC DNA to enrich for end fragments in the course of subcloning the insert into a pUC19-based vector. One of two specific enzymes that cut rarely in yeast and human genomic DNA, ClaI or SalI, is included in the double digestion mixture. A ClaI recognition sequence lies in the left arm
of the YAC vector, while a SalI recognition site lies in the right arm (Fig. 6.10.1). When one of these rarely cutting restriction enzymes is used together with a frequently cutting enzyme, doubly-digested fragments constitute only a small fraction of the total digested product. Ligation to a doubly-digested plasmid vector eliminates all of the single-digested fragments, resulting in a substantial enrichment for the YAC end fragment.
Internal Rearrangement or Instability of the YAC Insert Internal rearrangement of a YAC insert is more difficult to identify than chimerism, and may become apparent only after high-resolution analysis of the clone. Existing reports of internal rearrangement of YAC inserts are anecdotal, infrequent, and usually identify only rather large-scale changes. It is likely, however, that subtle rearrangements will be recognized as more clones are analyzed. Nevertheless, the data suggest that important rearrangements will remain relatively infrequent and will not impede most YAC cloning efforts. Although YACs are usually stable in culture, deletion or other rearrangements of the insert may occur months after the initial isolation of a clone. Thus, it is wise to verify the size of a YAC following prolonged passage in culture or after it has been thawed from a frozen stock. Several different colonies of the same YAC strain should be analyzed in parallel, using the protocols in UNIT 6.10, to confirm that the artificial chromosome is the same size in each of the isolates. Because cytosine methylation, which is quite frequent in the DNA of higher eukaryotic species, does not occur in yeast (Proffitt et al., 1984), it is not possible to perform direct structural comparisons of the YAC inserts and the corresponding genomic DNA isolated from higher eukaryotic cells using infrequently cutting restriction enzymes to create large-scale restriction maps. Consequently, direct structural comparisons must be carried out using methylation-insensitive restriction enzymes and frequently spaced probes. Evidence of internal rearrangement within a YAC clone can be obtained by preparing chromosomes from the clone (UNIT 6.10) and analyzing them by pulsed-field gel electrophoresis (PFGE; UNIT 2.5B). The CHEF gel system (Vollrath and Davis, 1987) is particularly useful in that it permits excellent resolution in the size range most common for individual YAC clones. Following electrophoresis, the artificial chro-
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mosome can be visualized using ethidium bromide staining as an extra chromosome not present in the host yeast strain. Occasionally, the YAC is not immediately recognizable because it comigrates with one of the endogenous yeast chromosomes. As described in the third basic protocol in UNIT 6.10, the PFGE gel should be Southern blotted and analyzed by successive hybridization with probes specific for the locus used in the library screening and for one or both of the YAC vector arms [e.g., for pYAC4, the 351-bp ClaI/BamHI fragment of pBR322 (left YAC arm) and the 276-bp BamHI/SalI fragment of pBR322 (right YAC arm)]. These blots should show hybridization to a single chromosome of the same size in all isolates from the same YAC strain as well as no hybridization to the AB1380 host strain.
CONSTRUCTION AND ANALYSIS OF A YAC-INSERT SUBLIBRARY
Yeast Artificial Chromosome Libraries
Although the large genomic DNA fragments provided by the YAC cloning system are easy to manipulate, it is often convenient to reduce a YAC to smaller fragments by subcloning it into a cosmid or λ vector. In particular, such smaller fragments are more amenable to highresolution analysis; this is important because information concerning the specific content of the YAC insert is typically limited, and often the only internal probe that is available is the one used for YAC library screening. Protocols for preparing YAC insert DNA and constructing a cosmid sublibrary are provided in UNIT 6.10. Two general strategies are available for preparing YAC insert DNA in order to create a saturating collection of subclones. The more elegant strategy is to purify the artificial chromosome itself by preparative CHEF gel electrophoresis (UNIT 2.5B). This permits isolation and analysis of the resulting recombinant clones without further selection, assuming that only a small amount of contaminating yeast DNA is present in the purified YAC, and that essentially all subclones isolated are derived from the human YAC insert. In practice, however, it is difficult to recover sufficient quantities of purified YAC DNA to permit construction of a cosmid or λ library. An alternate approach is to prepare a library from the total DNA of the YAC-carrying yeast strain. YACspecific subclones must then be selected by hybridization. An initial round of screening is usually performed with total human genomic DNA (rich in repetitive sequences) as the probe.
This detects subclones that contain human repetitive elements and eliminates subclones consisting of yeast DNA. Additional analysis is performed to identify overlapping sequences and thereby establish an approximate map of the original YAC insert. Ultimately, one or more rounds of chromosome walking may be required to fill in gaps between contiguous groups of subclones.
Literature Cited Abidi, F.E., Wada, M., Little, R.D., and Schlessinger, D. 1990. Yeast artificial chromosomes containing human Xq24-Xq28 DNA: Library construction and representation of probe sequences. Genomics 7:363-376. Bronson, S.K., Pei, J., Taillon-Miller, P., Chorney, M.J., Geraghty, D.E., and Chaplin, D.D. 1991. Isolation and characterization of yeast artificial chromosome clones linking the HLA-B and HLA-C loci. Proc. Natl. Acad. Sci. U.S.A. 88:1671-1675. Burke, D.T., Carle, G.F., and Olson, M.V. 1987. Cloning of large segments of exogenous DNA into yeast by means of artificial chromosome vectors. Science 236:806-812. Burke, D.T., Rossi, J.M., Leung, J., Koos, D.S., and Tilghman, S.M. 1991. A mouse genomic library of yeast artificial chromosome clones. Mamm. Genome 1:65-69. Green, E.D. and Olson, M.V. 1990. Systematic screening of yeast artificial chromosome libraries by use of the polymerase chain reaction. Proc. Natl. Acad. Sci. U.S.A. 87:1213-1217. Green, E.D., Riethman, H.C., Dutchik, J.E., and Olson, M.V. 1991. Detection and characterization of chimeric yeast artificial-chromosome clones. Genomics 11:658-669. Green, E.D. 1992. Physical mapping of human chromosomes: Generation of chromosome-specific sequence-tagged sites (STS). Methods Mol. Genet. In press. Hillier, L. and Green, P. 1991. A computer program for choosing PCR and DNA sequencing primers. PCR Meth. Appl. 1:124-128. Olson, M., Hood, L., Cantor, C., and Botstein, D. 1989. A common language for physical mapping of the human genome. Science 245:1434-1435. Proffitt, J.H., Davie, J.R., Swinton, D., and Hattman, S. 1984. 5-Methylcytosine is not detectable in Saccharomyces cerevisiae DNA. Mol. Cell Biol. 4:985-988. Riley, J., Butler, R., Ogilvie, D., Finniear, R., Jenner, D., Powell, S., Anand, R., Smith, J.C., and Markham, A.F. 1990. A novel, rapid method for the isolation of terminal sequences from yeast artificial chromosome (YAC) clones. Nucl. Acids Res. 18:2887-2890.
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Rossi, J.M., Burke, D.T., Leung, J.C., Koos, D.S., Chen, H., and Tilghman, S.M. 1992. Genome analysis using a yeast artificial chromosome library with mouse DNA inserts. Proc. Natl. Acad. Sci. U.S.A. 89:2456-2460. Vollrath, D. and Davis, R.W. 1987. Resolution of DNA molecules greater than 5 megabases by contour-clamped homogeneous electric fields. Nucl. Acids Res. 15:7865-7876.
Contributed by David D. Chaplin and Bernard H. Brownstein Howard Hughes Medical Institute and Washington University School of Medicine St. Louis, Missouri
Key Reference Burke, et al., 1987. See above. Initial description of the YAC cloning system, covering general features of library construction.
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UNIT 6.10
Analysis of Isolated YAC Clones The preceding unit gives an overview of methods involved in screening a YAC library to isolate a particular clone of interest (UNIT 6.9), with the sequence of methods illustrated in a flow chart (Fig. 6.9.1). This unit provides a series of protocols describing the analysis and manipulation of an isolated YAC clone. The procedures are based upon the use of the YAC vector pYAC4. Once an isolated YAC clone has been obtained from a core laboratory (UNIT 6.9), the clone can be analyzed as described herein. As depicted in Figure 6.9.1, methods for analysis involve growing and storing YAC-containing yeast strains and purifying YAC DNA in a form suitable for assessing the size of the artificial chromosome and for conventional Southern blotting. Preparation of yeast chromosomes in agarose plugs for subsequent analysis by pulsed-field gel electrophoresis is also described. Additional protocols are provided for recovering DNA fragments from the ends of a YAC genomic insert to be used as probes for detecting chimerism and for chromosome walking. Finally, preparation of high-molecular-weight YAC DNA is described and a general method for subcloning YAC inserts into cosmid or λ vectors for higher-resolution analysis is provided. NOTE: All solutions, media, glassware, and plasticware coming into contact with yeast or bacterial cells must be sterile, and sterile techniques should be followed throughout.
BASIC PROTOCOL
PROPAGATION AND STORAGE OF YAC-CONTAINING YEAST STRAINS YACs prepared using the pYAC4 vector (Figs. 6.10.1 and 13.4.6; pYAC4 contains an EcoRI site within the SUP4 gene in addition to the SnaBl site found in pYAC3, but is otherwise identical to pYAC3, carrying selectable markers TRP1 and URA3) and the S. cerevisiae host strain AB1380 (trp1−, ura3−, ade2-1) are grown on AHC plates. They can be stored short-term on AHC plates or stored long-term (after growth in YPD medium) in YPD containing glycerol at −80°C.
EcoRI
Hinfl Clal
CEN4
SUP4
EcoRl Hin fl
genomic insert
Sal l
URA3
SUP4
1 2
3 4
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1
HYAC-C
2
LS-2
3
RA-2
4
HYAC-D
Figure 6.10.1 Structure of a representative pYAC4 clone at the vector/insert junction. Open boxes represent portions of the pYAC4 vector derived from yeast sequences (CEN4, the two halves of the SUP4 element, and URA3). Thin lines represent sequences derived from pBR322 and the bold line represents the YAC genomic insert fragment. Sites of annealing of the HYAC-C, LS-2, RA-2 and HYAC-D oligonucleotides are indicated by arrows 1, 2, 3, and 4, respectively. The EcoRI cloning site, the ClaI and Sal I sites (used for the end-fragment subcloning alternate protocol), and the HinfI sites that are utilized in the bubble linker end-fragment isolation protocol are indicated. Contributed by David D. Chaplin and Bernard H. Brownstein Current Protocols in Molecular Biology (1992) 6.10.1-6.10.19 Copyright © 2000 by John Wiley & Sons, Inc.
Materials S. cerevisiae strain AB1380 containing pYAC4 with insert (from core facility; UNIT 6.9) AHC plates (ura−, trp−) YPD medium (UNIT 13.1) 80% (v/v) glycerol in YPD medium 30°C orbital shaking incubator (e.g., New Brunswick Scientific #G-24) Cryovials Additional reagents for preparation of yeast media (UNIT 13.1) and growth and manipulation of yeast (UNIT 13.2) 1. Streak strain AB1380 containing pYAC4 with insert onto AHC plates. AHC medium selects for the presence of both arms of the YAC vector and thereby favors high stability of the YAC through successive passages.
2. Invert plate and incubate at 30°C until colonies are 1 to 3 mm in size. The AB1380 strain carries the ade2-1 ochre mutation, a block in the purine biosynthetic pathway that leads to accumulation of red-hued intermediates. Because a genomic insert in the YAC interrupts the SUP4 (ochre) gene in the YAC vector, colonies will have a red pigmentation.
3a. For short-term storage: Seal plates with Parafilm and store at 4°C for 4 to 6 weeks. 3b. For long-term storage: Inoculate an individual colony into 3.2 ml YPD medium and shake overnight at 30°C. Add 1 ml of 80% glycerol in YPD medium, mix thoroughly, and transfer in 0.2- to 1.0-ml aliquots to cryovials. Store at −80°C. YPD is a nonselective medium used to favor rapid growth and high cell viability. Strains stored in this fashion are stable for ≥5 years. Before strains are used in an experiment, they should first be grown on selective medium (e.g., AHC plates) to avoid recovery of a contaminant clone or one that has lost its YAC.
PREPARATION OF YAC-CONTAINING DNA FROM YEAST CLONES FOR ANALYSIS BY SOUTHERN BLOTTING
BASIC PROTOCOL
Procedures used by core laboratories for isolating an individual clone from a YAC library ensure that the purified YAC supports amplification of an appropriately sized PCR product using the screening primer pair. However, it is best to confirm the identity of the clone by hybridization analysis. Various methods can be used to prepare DNA suitable for Southern blot analysis using frequently cutting restriction enzymes. This protocol yields substantial quantities of DNA in the size range of 50 to 200 kb; it involves growing and lysing a single red colony containing pYAC4 with the insert DNA, then obtaining the DNA from the supernatant after centrifugation and analyzing by Southern blotting. Yeast chromosomes prepared in agarose plugs or very-high-molecular-weight DNA prepared in solution (third and fifth basic protocols) may also be used. Materials Single colony of S. cerevisiae AB1380 containing pYAC4 with insert (first basic protocol) AHC medium (ura−, trp−) SCE buffer SCEM buffer 50 mM Tris⋅Cl (pH 7.6)/20 mM EDTA (Tris/EDTA lysis buffer)
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10% (w/v) sodium dodecyl sulfate (SDS) 5 M potassium acetate, pH 4.8, ice-cold (UNIT 1.6) 95% ethanol, room temperature TE buffer, pH 8.0 (APPENDIX 2) 1 mg/ml DNase-free RNase A (UNIT 3.13) Isopropanol, room temperature 5 M NaCl Total genomic DNA of the species or individual from which the library was made (e.g., UNITS 2.2, 2.3 & 5.3) Appropriate single-copy probe designed to hybridize with the YAC insert (see UNITS 2.9 & 6.9) Orbital shaker (e.g., New Brunswick Scientific #G-24) 50-ml conical plastic centrifuge tubes Beckman JS-4.2 rotor or equivalent Additional reagents and equipment for digestion of DNA with restriction endonucleases (UNIT 3.1), Southern blotting and hybridization (UNIT 2.9), and pulsed-field gel electrophoresis (UNIT 2.5B) Culture and lyse cells from YAC clone 1. Inoculate a single red colony of a YAC-containing clone into 20 ml AHC medium in a 250-ml Erlenmeyer flask. Shake 24 hr at 250 rpm, 30°C, on an orbital shaker. The culture should begin to turn pink. If not, continue incubation an additional 24 hr. If culture is still not pink, discard and start over with a new red colony. Orbital shakers are preferred because they give much better aeration.
2. Inoculate 1 ml of culture from step 1 into 100 ml AHC medium in a 1-liter Erlenmeyer flask. Shake 24 hr at 250 rpm, 30°C. 3. Transfer culture to 50-ml plastic conical centrifuge tubes. Centrifuge 5 min at 2000 × g (2800 rpm in Beckman JS-4.2 rotor), 4°C. 4. Discard supernatants and resuspend cell pellets in a total of 5 ml SCE buffer. Pool into a single tube. 5. Add 1 ml SCEM buffer. Mix gently 1 to 2 hr at 100 rpm, 37°C, on an orbital shaker. SCEM buffer contains lyticase, which will digest the cell wall.
6. Centrifuge 5 min at 2000 × g, 4°C. Discard supernatant and resuspend cell pellet in 5 ml Tris/EDTA lysis buffer. 7. Add 0.5 ml of 10% SDS and invert several times to mix. Incubate 20 min at 65°C. Isolate nucleic acids 8. Add 2 ml of ice-cold 5 M potassium acetate, pH 4.8, and invert to mix. Keep 60 min on ice. 9. Centrifuge 10 min at 2000 × g, room temperature. Carefully pour nucleic acid–containing supernatant into a new tube. Add 2 vol room-temperature 95% ethanol and invert to mix.
Analysis of Isolated YAC Clones
10. Centrifuge 5 min at 2000 × g, room temperature. Discard supernatant and air-dry nucleic acid pellet 10 to 15 min. Add 3 ml TE buffer, pH 8.0, and dissolve overnight at 37°C.
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Recover and analyze DNA 11. Add 0.1 ml of 1 mg/ml DNase-free RNase A and incubate 1 hr at 37°C. 12. Add 6 ml room-temperature isopropanol with swirling, then invert to mix. 13. Spool DNA using a capillary pipet and dissolve in 0.5 ml TE buffer, pH 8.0. Add 50 µl of 5 M NaCl and 2 ml of room-temperature 95% ethanol. Mix by inverting. 14. Spool DNA again and dissolve in 0.5 ml TE buffer. Store at 4°C. A yield of 1 to 1.5 ìg DNA/108 yeast cells can be expected.
15. Analyze 2-µg aliquots of YAC DNA and 15-µg aliquots of total genomic DNA from the species or individual from which the YAC library was made by digesting with several frequently cutting restriction enzymes. Proceed with Southern blotting and hybridization using a single-copy probe. The product amplified by PCR screening (see UNIT 6.9) may be used as probe. Because the YAC donor may exhibit a restriction-fragment-length polymorphism for this probe, two restriction fragments may be observed in the donor DNA. One of these fragments should be identified in the isolated YAC DNA.
16. Once the YAC clone has been verified by Southern blotting, determine its size and obtain a preliminary assessment of its stability by preparing chromosomes in agarose plugs (third basic protocol) and analyzing by pulsed-field gel electrophoresis. PREPARATION OF YEAST CHROMOSOMES IN AGAROSE PLUGS FOR PULSED-FIELD GEL ELECTROPHORESIS
BASIC PROTOCOL
In order to assess size, stability, and possible rearrangements within YACs, and to identify overlapping YACs, it is useful to isolate the YACs by embedding them in agarose plugs for subsequent analysis by pulsed-field gel electrophoresis (PFGE). Most methods of pulsed-field gel electrophoresis can be used (UNIT 2.5B); the CHEF (contour-clamped homogeneous electric-field electrophoresis) gel system is particularly suitable in that it reliably permits excellent resolution in the size range most common for YACs. Materials AHC medium (ura−, trp−) Single colony of S. cerevisiae containing pYAC4 with insert (first basic protocol) 0.05 M EDTA, pH 8.0 (APPENDIX 2) SEM buffer 10 mg/ml Lyticase (Sigma #L-8137 or ICN Biomedicals #190123) 2% InCert or SeaPlaque agarose (FMC Bioproducts), dissolved in SEM buffer and equilibrated to 37°C SEMT buffer Lithium lysis solution 20% (v/v) NDS solution 0.5× TBE (APPENDIX 2) or GTBE buffer (UNIT 2.5B) 30°C rotary platform shaking incubator Beckman JS-4.2 rotor or equivalent Gel sample molds (e.g., CHEF gel molds, Bio-Rad #1703622) 60-mm tissue culture plate Additional reagents and equipment for pulsed-field gel electrophoresis (UNIT 2.5B) Screening of Recombinant DNA Libraries
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Prepare and lyse YAC clone 1. Inoculate 25 ml AHC medium with a single red colony of a YAC-containing clone. Shake 48 to 60 hr at 250 rpm, 30°C. The culture should be pink. If it is not, discard and start over with a new red colony. To assess the stability of an individual YAC and to facilitate distinguishing of the artificial chromosome from the native yeast chromosomes, it is useful to analyze 4 or 5 individual colonies from the same YAC strain as well as a colony of the untransformed yeast host.
2. Centrifuge 10 min at 2000 × g (2800 rpm in a Beckman JS-4.2 rotor), 4°C. Discard supernatant and resuspend cell pellet in 10 ml of 0.05 M EDTA, pH 8.0. 3. Centrifuge 10 min at 2000 × g, 4°C. Remove all liquid from pellet and resuspend in 150 µl SEM buffer. Prepare agarose molds 4. Warm YAC sample to 37°C and add 25 µl Lyticase. Add 250 µl of 2% InCert or SeaPlaque agarose that has been melted in SEM buffer and equilibrated to 37°C. 5. Mix quickly and pour into CHEF gel sample molds. Chill 10 min at 4°C. Transfer solidified plugs to a 60-mm tissue culture plate. 6. Cover each plug with 4 ml SEMT buffer. Incubate 2 hr with gentle shaking at 37°C. 7. With a pipet, remove SEMT buffer and replace with 4 ml lithium lysis solution. Incubate 1 hr with gentle shaking at 37°C. 8. Remove and replace lithium lysis solution two or three times, shaking ≥1 hr each time. Shake the last change overnight. 9. Remove lithium lysis solution, replace with 4 ml of 20% NDS solution, and shake 2 hr at room temperature. Repeat once. Electrophorese samples in individual agarose plugs 10. Cut into plugs of suitable size to fit into wells of a pulsed-field gel. Store plugs individually in 20% NDS solution at 4°C. Plugs prepared and stored in this manner are usually stable for 4 to 8 weeks.
11. Soak each plug 30 min in 1 ml of 0.5× TBE or GTBE buffer. Change three times. 12. Analyze by pulsed-field gel electrophoresis. Following electrophoresis, the artificial chromosome can be visualized in an ethidium bromide–stained gel as an extra chromosome not present in the host yeast strain. If desired, Southern blot hybridization (UNIT 2.9) with appropriate probes can be carried out.
Analysis of Isolated YAC Clones
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END-FRAGMENT ANALYSIS USING PCR AMPLIFICATION This protocol provides a means for recovering end fragments from the YAC insert using PCR amplification of end fragments. Digestion of YAC-containing DNA with a frequently cutting restriction enzyme produces a collection of small fragments: among these, one contains the distal portion of the YAC insert associated with part of the left vector arm, and another contains the other end of the insert associated with part of the right vector arm. Fragments encoding these vector sequences are prepared for PCR amplification by ligation of a double-stranded DNA tag containing a “bubble” of noncomplementary sequence flanked by short complementary sequences (Fig. 6.10.2). Selective amplification of these two end-fragment sequences is achieved using one PCR primer derived from the YAC vector (HYAC-C or LS-2 for the left arm or HYAC-D or RY-2 for the right arm) and one primer containing the sequence of the noncomplementary portion of the bubble (the 224 primer template, created by extension from the YAC-vector-specific primer; Fig. 6.10.3). Occasionally, nonspecific DNA fragments are amplified from the bubble PCR reaction. If this occurs, specificity may be restored using a hemi-nesting strategy. A small aliquot of the product of the initial PCR reaction (containing a mixture of the specific and nonspecific amplified fragments) is reamplified in a second round of PCR using an internal sequence from the vector arm as one of the primers. Because this sequence is not present in the nonspecific fragments, only the specific fragment will be amplified.
BASIC PROTOCOL
Materials “Bubble-top” and “bubble-bottom” oligonucleotide primers (Fig. 6.10.2) YAC-containing DNA (second basic protocol) RsaI and HinfI restriction endonucleases and appropriate buffers (UNIT 3.1) 10× T4 DNA ligase buffer and 1 U/µl T4 DNA ligase (UNITS 3.4 & 3.14) PCR reaction mix PCR amplification primers HYAC-C, HYAC-D, 224, and RA-2, 4 µM each (Fig. 6.10.2) Thermal cycling apparatus 65° and 68°C water baths Additional reagents and equipment for phosphorylating synthetic oligonucleotides (UNIT 3.10), restriction endonuclease digestion (UNIT 3.1), PCR (UNIT 15.1), nondenaturing PAGE (UNIT 2.7), preparing radiolabeled oligonucleotide probes (UNITS 3.10, 4.6 & 15.2), and blunt-end ligation (UNIT 3.16) Prepare bubble oligonucleotide tags 1. Phosphorylate the bubble-top oligonucleotide. This step can usually be eliminated, but may modestly increase efficiency.
2. Adjust bubble-top and bubble-bottom oligonucleotide concentrations to 4 nmol/ml with water. Mix together 1 nmol of each, then anneal by heating 15 min at 68°C in a water bath, followed by slow cooling to room temperature over 30 to 60 min. Digest YAC DNA and ligate to bubble oligonucleotides 3. Digest 2.5-µg aliquots of purified YAC-containing DNA to completion with RsaI or HinfI in 20 µl final volume, 37°C. Digestion of separate samples with RsaI and HinfI increases the chance of obtaining an end fragment of a size suitable for PCR amplification (75 bp).
4. Heat samples 15 min at 65°C to inactivate the restriction enzymes.
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A GCTGTCTGTCGAAGGTAAGGAACGGACGA
5 ′ - GAAGGAGAGGAC
Rsal bubble-top
GAGAAGGGAGAG - 3 ′
3 ′- CTTC C T C T CC TG CTCT T C C C TC TC - 5 ′ TCGCTAAGAGCATGCTTGCCAATGCTAAG
Universal bubble-bottom
* * * * * * * * * * * * * * * * * * * * * * * *
224 primer
3 ′- TCGCTAAGAGCATGCTTGCCAATGCTAAGC- 5 ′
B GCTGTCTGTCGAAGGTAAGGAACGGACGA
GAGAAGGGAGAG - 3 ′ Hin f l bubble-top
5 ′- A (N)TGAAGGAGAGGAC
3 ′ - C T T C C T C T CC T G
CTCT T C C C TC TC - 5 ′ Universal bubble-bottom TCGCTAAGAGCATGCTTGCCAATGCTAAG
C HYAC - C primer HYAC - D primer RA - 2 primer LS - 2 primer Bubble sequencing primer
5 ′- GCTACTTGGAGCCACTATCGACTACGCGAT- 3 ′ 5 ′- GGTGATGTCGGCGATATAGGCGCCAGCAAC- 3 ′ 5 ′- TCGAACGCCCGATCTCAAGATTAC- 3 ′ 5 ′- TCTCGGTAGCCAAGTTGGTTTAAGG- 3 ′ 5 ′- CGCTGTCCTCTCCTTC - 3 ′
Figure 6.10.2 Oligonucleotides for amplification and sequencing of YAC insert end-fragments. (A) Annealing of the 53-mer universal “bubble-bottom” oligonucleotide to the 53-mer RsaI bubble oligonucleotide yields a blunt-ended DNA duplex in which 12-bp complementary sequences flank a 29-nucleotide “bubble” of noncomplementary sequence. This bubble linker can be ligated to any blunt-ended fragment (e.g., one generated by digestion with RsaI). The 224 primer does not anneal to either strand of the bubble, but is fully complementary to any DNA strand that is generated during PCR using the universal bubble-bottom strand as a template (see Fig. 6.10.3). (B) Annealing of the 53-mer universal bubble-bottom oligonucleotide to the 56-mer HinfI bubble-top oligonucleotide yields a DNA duplex with one blunt end and one cohesive end with the degenerate HinfI site. A mixture of all four nucleotides at a specific position is indicated by (N). (C) The HYAC-C, HYAC-D, RA-2, and LS-2 primers anneal to sequences in the pYAC4 vector (see Fig. 6.10.1). The bubble sequencing primer anneals to the RsaI and HinfI bubble-top sequences near their 5′ ends, permitting DNA sequencing from the bubble linker back into the YAC insert end-fragment.
5. Prepare the following ligation mix (50 µl total): 2 µl (250 ng) digested DNA 1 µl (2 pmol) annealed bubble oligonucleotides (from step 2) 5 µl 10× ligase buffer 2 µl (2 U) T4 DNA ligase 40 µl H2O. Incubate 2 hr at 37°C or overnight at room temperature.
Analysis of Isolated YAC Clones
The blunt-ended bubble composed of the universal bubble-bottom oligonucleotide and the RsaI bubble-top oligonucleotide should be used with RsaI-digested YAC DNA. Likewise, the HinfI cohesive bubble composed of the universal bubble-bottom oligo-
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A random insert fragment
5′ 3′
B
3′ 5′
vector primer site insert end-fragment
5′ 3′
3′ 5′
YAC vector
1st cycle of PCR
3′
5′
insert end-fragment
3′ 5′
224 primer site
Figure 6.10.3 Selective PCR amplification from the YAC insert end-fragment. (A) Result of ligation of the bubble linker to a random fragment from the internal portion of the YAC insert. Because this fragment is not derived from the end of the YAC genomic insert, it contains no sequences from the YAC vector and has no site for annealing any of the HYAC-C, HYAC-D, RA-2, or LS-2 primers or for annealing of the 224 primer. Consequently, no fragment is amplified by PCR. (B) Result of ligation of the bubble linker to a fragment derived from the end of the YAC genomic insert and containing its associated YAC vector sequences. During the first cycle of PCR, extension from the YAC vector priming site produces sequences complementary to the universal bubble-bottom primer. This extended fragment provides a template for annealing of the 224 primer, thus permitting successful amplification of the insert end-fragment.
nucleotide and the HinfI bubble-top oligonucleotide should be used with HinfI-digested YAC DNA.
6. Add 200 µl water to bring the DNA concentration to 1 ng/µl final. Amplify fragments containing YAC-insert end sequences 7. Prepare the following PCR on ice (10 µl total): 8 µl PCR reaction mix 1 µl (2 µM each) PCR primer pair mix 1 µl (1 ng) digested, bubble-ligated YAC DNA. Carry out 35 cycles of amplification as follows: 1 min at 92°C, 2 min at 65°C, and 2 min at 72°C. To amplify left end of YAC insert, use primer pair mix made from equal amounts of primers 224 and HYAC-C (Fig. 6.10.1). To amplify right end of YAC insert, use primer pair mix made from equal amounts of primers 224 and RA-2 (specific for the SUP4 region of pYAC4; Fig. 6.10.1).
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These parameters were optimized using the Perkin-Elmer TC1 thermal cycler. If another instrument is used, some adjustment of parameters may be required.
8. Analyze a 1-µl aliquot of PCR product on a 5% polyacrylamide gel. A single, clearly visible amplified fragment should be observed after staining the gel with ethidium bromide. Produce end fragments using hemi-nested amplification To amplify the left end (either RsaI- or HinfI-digested DNA): 9a. Amplify 1 µl digested, bubble-ligated YAC DNA (from step 6) with primers 224 and HYAC-C, using 20 cycles of 1 min at 92°C, 2 min at 62°C, and 2 min at 72°C. 10a. Dilute the amplification product 1:100 with water and add 1 µl to a new PCR reaction containing primers 224 and LS-2 (specific for the SUP4 region of pYAC4; see Figs. 6.10.1 and 13.4.6). Carry out 30 cycles of 1 min at 92°C, 2 min at 65°C, and 2 min at 72°C. To amplify the right end using RsaI-digested DNA: 9b. Amplify 1 µl digested, bubble-ligated YAC DNA (from step 6) with primers 224 and HYAC-D, using 20 cycles of 1 min at 92°C, 2 min at 62°C, and 2 min at 72°C. 10b. Dilute amplification product 1:100 with water and add 1 µl to a new PCR reaction (see step 7) containing primers 224 and RA-2. Carry out 30 cycles of 1 min at 92°C, 2 min at 65°C, and 2 min at 72°C. Hemi-nesting of the right end cannot be performed with HinfI-digested DNA, because there is a HinfI site only 24 bp from the EcoRI YAC vector cloning site.
11. Analyze a 1-µl aliquot of each final PCR reaction on a 5% polyacrylamide gel. 12. End label amplified fragments with 32P and use as hybridization probes or for nucleotide sequencing to produce an end-specific STS. Alternatively, subclone by blunt-end ligation to a plasmid vector prior to further manipulation. Blunt-end subcloning of DNA fragments that have been amplified by PCR must be preceded by “polishing” of the ragged PCR ends with S1 nuclease or T4 DNA polymerase (see UNIT 15.7). ALTERNATE PROTOCOL
END-FRAGMENT ANALYSIS BY SUBCLONING INTO A BACTERIAL PLASMID VECTOR This method (an alternative to the previous protocol for recovering end fragments from the YAC insert) uses the strategy of double digesting the YAC-containing DNA to enhance the efficiency of subcloning into a pUC19-based vector; the SUP4 element containing the YAC EcoRI cloning site is located within a portion of the YAC vector derived from pBR322 (Figs. 1.5.2 and 6.10.5). The cloning strategy enriches for end-fragment-containing subclones because the restriction endonuclease used for digestion of YAC-containing DNA is either ClaI (for the left arm) or SalI (for the right arm). Both enzymes cut rarely in human or yeast genomic DNA; therefore, when one is combined with a more frequently cutting enzyme, the resulting doubly digested fragments will represent a minor portion of the total DNA pool. Size fractionation on an agarose gel prior to subcloning affords a still further enrichment for end fragments. Note that after the first step of the protocol, all steps are performed in duplicate to identify both the right and left end fragments of the YAC insert.
Analysis of Isolated YAC Clones
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Additional Materials ClaI, SalI, and other appropriate restriction endonucleases and digestion buffers (UNIT 3.1) Left- and right-vector-arm probes (Fig. 6.10.4) pUC19-ES and pUC19-HS plasmid vectors (support protocol and Fig. 6.10.5) Transformation-competent Rec− strain of E. coli (e.g., DH5; Table 1.4.5) 2× TY or LB agar plates (UNIT 1.1) containing 50 to 100 µg/ml ampicillin Additional reagents and equipment for agarose gel electrophoresis (UNIT 2.5A), subcloning of DNA fragments (UNIT 3.16), transformation of E. coli (UNIT 1.8), Southern blotting and hybridization (UNIT 2.9), labeling by random-primed synthesis (UNIT 3.5), isolation and purification of DNA fragments from agarose gels (UNIT 2.6), replica plating (UNIT 1.3), and purification of plasmid DNA (UNITS 1.6 & 1.7) Perform an appropriate double digest and analyze by hybridization 1a. For left arm: Digest 5-µg aliquots of YAC-containing DNA with ClaI and then with each possible second cloning enzyme—SacI, KpnI, SmaI, BamHI, XbaI, and SphI. In addition to SmaI, other blunt-cutters not represented within the ClaI-EcoRI interval of the YAC vector may be tested.
1b. For right arm: Digest 5-µg aliquots of YAC-containing DNA with SalI and then with each of the following possible second cloning enzymes: SacI, KpnI, SmaI, BamHI, XbaI, SphI, or HindIII. All of the above restriction endonucleases have compatible cleavage sites within the polylinker of the modified pUC19; AccI provides a cohesive site for ClaI.
Carry out all remaining steps in parallel for the left- and right-arm probes: 2. Electrophorese doubly digested DNA on an agarose gel and transfer to a filter for Southern hybridization. 3. Prepare left- and right-vector-arm probes by PCR as described in Fig. 6.10.4. Probes can also be obtained by digestion and fractionation of pBR322 DNA with subsequent labeling.
4. Hybridize each probe to the appropriate filter from step 2.
Left arm:
5′ ATCGATAAGCTTTAATGCGGTAGT 3′ (pBR322 bases 23-46) 5′ GATCCACAGGACGGGTGTGGTCGC 3′ (pBR322 bases 379-356)
Right arm: 5′ GATCCTCTACGCCGGACGCATCGT 3′ (pBR322 bases 375-399) 5′ GTCGACGCTCTCCCTTATGCGACT 3′ (pBR322 bases 656-632)
Figure 6.10.4 Generation of left- and right-vector-arm probes. The 351-bp ClaI-BamHI and 276-bp BamHI-SalI fragments of pBR322, which hybridize to sequences immediately flanking the sup4 sequences of the YAC vector, are appropriate probes for the YAC left and right vector arms. These probes can be obtained by restriction digestion and gel fractionation of pBR322 plasmid DNA or generated by PCR using 10 ng pBR322 as template for the primers illustrated here. Perform PCR using 25 cycles of 1 min at 92°C, 1 min at 50°C, and 2 min at 72°C. Extract the amplified material once with phenol and once with chloroform, then precipitate with ethanol (UNIT 2.1). Label directly by random priming (UNIT 3.5) without further purification.
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5. Examine autoradiogram and choose an enzyme combination that yields a hybridizing DNA fragment in the 2- to 7-kb size range. Digest a 50-µg aliquot of YAC-containing DNA with these two enzymes. This should yield ∼5 times more size-fractionated DNA than needed.
Isolate the DNA 6. Electrophorese doubly digested DNA on an agarose gel. Using a scalpel or razor blade, cut out the segment of gel that should contain the doubly digested DNA fragment. To avoid missing the critical portion of the gel, it may be useful to excise adjacent gel slices containing fragments larger than and smaller than the expected size, and to process them in parallel.
7. Purify size-fractionated DNA from gel slice and resuspend in a final volume of 20 µl TE buffer, pH 8.0. For purifying the DNA, the best results have been obtained by using the Geneclean II kit (BIO 101, La Jolla, CA).
Subclone the end fragments 8. Ligate 20% of the purified YAC-derived insert DNA with 0.2 µg of gel-purified, compatibly digested pUC19-HS or -ES vector DNA overnight in a total volume of 20 µl. Because the pUC19 plasmid from which they are derived has no homology with the portion of pBR322 detected by the ClaI/BamHI and BamHI/SalI probes (Fig. 6.10.4), these probes can be used to detect YAC-insert end-fragment-containing subclones in pUC19, and will not cross-hybridize to the pUC vector.
9. Transform the ligated DNA into a transformation-competent Rec− host strain of E. coli. Plate sufficient transformation mix on 2× TY/ampicillin or LB/ampicillin plates to obtain ∼200 colonies, a sufficiently low density that individual colonies may be recovered following hybridization. Invert plates and incubate overnight at 37°C. 10. Prepare colony-lift filters and hybridize overnight with ∼1–2 × 107 cpm of appropriate 32 P-labeled left- or right-arm probes. Wash and autoradiograph. Because of the enrichment afforded by double digestion, 1% to 4% of colonies will contain the end fragment.
11. Purify plasmid DNA from hybridizing colonies. 12. Verify the structure of the plasmid by comparing its restriction map to the data obtained during the initial analytical double digests of the YAC (steps 1 to 3). SUPPORT PROTOCOL
DESIGN AND PREPARATION OF pUC19-ES and pUC19-HS SUBCLONING VECTOR
Analysis of Isolated YAC Clones
This protocol describes the construction of two vectors for subcloning YACs (previous basic protocol). pUC19 is modified by insertion of a “stuffer” fragment in both possible orientations (see UNIT 3.16; Fig. 6.10.5). If a double digest is performed on the resulting construct (UNIT 3.1), using AccI (cohesive with ClaI) or SalI and any of the other enzymes in the pUC polylinker, the presence of the stuffer makes it possible to visualize whether the vector has been fully cut. Complete double digestion is critical to the success of the end-fragment subcloning described in the previous protocol. For example, digestion with AccI or SalI will linearize the pUC19-ES vector. Subsequent digestion with EcoRI, SacI, KpnI, SmaI, BamHI, or XbaI will result in a shift in vector size from 3161 bp to 2686 bp
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EcoRI Sacl Kpnl Smal BamHI Xbal
ampr
pUC19-ES 3161 bp
475-bp stuffer
SaII / AccI / HincII Ps tI SphI Hin dIII EcoRI Sacl Kpnl Smal BamHI Xbal SaI I / AccI / Hin cII
ori
ampr
pUC19-HS 3161 bp
ori
475-bp stuffer
Ps tI SphI Hin dIII
Figure 6.10.5 Structure of the pUC19-ES and pUC19-HS plasmids.
and free stuffer fragment will be generated. The doubly digested vector can then be isolated by fractionation in an agarose gel (UNIT 2.5A) and purified (UNIT 2.6). pUC19-ES: Modify the pUC19 (see Fig. 1.5.2) vector by inserting a stuffer consisting of 475-bp TaqI fragment of pBR322 (positions 653-1128) into the pUC19 polylinker AccI (HincII) site. In the resulting plasmid, the AccI (and SalI and HincII) site adjacent to the polylinker PstI site is preserved, but the AccI site previously found next to the polylinker XbaI site (which would now be at the other end of the stuffer) is lost (Fig. 6.10.5). pUC19-HS: Insert the 475-bp TaqI fragment stuffer described above into the same pUC19 AccI site but in the opposite orientation. In the resulting plasmid, the polylinker AccI site adjacent to the XbaI site is preserved, but the AccI site adjacent to the PstI site is lost (Fig. 6.10.5).
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BASIC PROTOCOL
PREPARATION OF HIGH-MOLECULAR-WEIGHT YAC-CONTAINING YEAST DNA IN SOLUTION This protocol describes the purification of YAC-containing DNA of sufficiently high molecular weight to provide a source of YAC insert material for subcloning in λ or cosmid vectors. This DNA is also suitable for restriction mapping or other genetic manipulations. A cell lysate is fractionated on a sucrose gradient; the DNA-containing fraction is subsequently dialyzed, concentrated, and examined by electrophoresis through a pulsedfield gel. Materials Single colony of S. cerevisiae containing pYAC4 with insert (first basic protocol) AHC medium (ura−, trp−) SCEM buffer Lysis buffer Step-gradient solutions: 50%, 20%, and 15% (w/v) sucrose TE buffer, pH 8.0 (APPENDIX 2) Dry granular sucrose 30°C orbital shaking incubator (e.g., New Brunswick Scientific #G-24) 250-ml conical centrifuge bottles (e.g., Corning #25350) 65°C water bath 25 × 89–mm tube (e.g., Beckman #344058) Beckman JS-4.2 and SW-27 rotors (or equivalents) Dialysis tubing (APPENDIX 3) Pyrex baking dish CHEF pulsed-field gel apparatus or equivalent (UNIT 2.5B) Additional reagents and equipment for size fractionation using a sucrose gradient (UNIT 5.3) and estimating DNA concentration (UNIT 2.6) Grow and prepare the cells 1. Inoculate a single red colony of a YAC-containing clone into 25 ml AHC medium in a 250-ml flask. Shake at 250 rpm, 30°C, until culture reaches saturation (∼3 days). 2. Transfer 1 ml of saturated culture to 100 ml AHC medium in a 1-liter flask. Shake 16 to 18 hr at 250 rpm, 30°C. 3. Harvest yeast cells by centrifuging 10 min at 2000 × g (2800 rpm in Beckman JS-4.2 rotor), room temperature, using a 250-ml conical centrifuge bottle. Discard supernatant. 4. Resuspend cells in 50 ml water. Centrifuge 5 min at 2000 × g, room temperature. Discard supernatant. A cell pellet of ∼4 g should be obtained.
5. Resuspend cells in 3.5 ml SCEM buffer. Lyse the cells 6. Incubate 2 hr at 37°C with occasional gentle mixing. The mixture will become highly viscous. 7. Gradually add cell mixture to 7 ml lysis buffer in a 250-ml Erlenmeyer flask by allowing viscous cell suspension to slide down side of flask. Analysis of Isolated YAC Clones
8. Gently mix by swirling flask until mixture is homogeneous and relatively clear. 9. Incubate 15 min at 65°C, then cool rapidly to room temperature in a water bath.
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Fractionate cell contents 10. Fractionate on a sucrose step gradient. In a 25 × 89–mm tube, prepare a step gradient consisting of: 3 ml 50% sucrose 12 ml 20% sucrose 12 ml 15% sucrose 11 ml lysed sample. Centrifuge 3 hr at 125,000 × g (26,000 rpm in a Beckman SW-27 rotor), room temperature. 11. Discard ∼25 ml from top of gradient using a 10-ml pipet. Dialyze and analyze DNA 12. Collect viscous DNA-containing solution at the 20% to 50% sucrose interface (∼5 ml total volume) and place in dialysis tubing, leaving room for volume to increase ≥2- to 3-fold. Dialyze overnight against 2 liters TE buffer, pH 8.0, at 4°C. 13. Reconcentrate dialyzed DNA by placing dialysis tubing in an autoclaved Pyrex baking dish and covering with granular sucrose. Recover dialysis tubing when volume of contents has been reduced to ∼2 ml. 14. Squeeze DNA solution to one end of dialysis tubing and tie an additional knot to keep DNA in a small volume. Dialyze overnight against 1 liter of TE buffer, pH 8.0, at 4°C. 15. Recover dialyzed DNA and check a small aliquot by electrophoresing in a CHEF pulsed-field gel. Stain with ethidium bromide and estimate DNA content by comparison to a known amount of λ DNA. The DNA sample will contain a substantial amount of yeast RNA but should also contain a population of YAC DNA fragments migrating at a size of >100 kb. The presence of the RNA may make it difficult to determine the DNA concentration accurately; the concentration may be estimated by comparison to known DNA standards in an ethidium bromide– stained gel. The RNA will not affect restriction digestion of the DNA.
PREPARATION AND ANALYSIS OF A YAC-INSERT SUBLIBRARY Construction of a sublibrary of fragments of the YAC insert facilitates high-resolution analysis of the insert sequence. This protocol details the steps required to produce a cosmid library, followed by a series of screenings to identify regions of interest and “walking” to establish a contiguous map of the insert.
BASIC PROTOCOL
Materials High-molecular-weight YAC-containing DNA (fifth basic protocol) Vector DNA (e.g., SuperCos 1, Stratagene #251301) 32 P-labeled (UNIT 3.10) probes: total genomic DNA of the individual or species from which the library was made (e.g., UNITS 2.2, 2.3 & 5.3), end-specific DNA (UNIT 3.10) or RNA (UNIT 3.8), and end fragment from YAC (fourth basic protocol or alternate protocol) Additional reagents and equipment for restriction endonuclease digestion (UNIT 3.1), genomic DNA library production (UNIT 5.7), plating and transferrin a cosmid library (UNIT 6.2), and hybridization with radioactive probes (UNITS 6.3 & 6.4)
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Construct the library 1. Partially digest 1 to 2 µg of YAC-containing DNA with restriction endonuclease(s) appropriate for cosmid vector to be used. For example, to clone into the BamHI site of SuperCos 1, digest the YAC DNA with either MboI or Sau3A. The quantity of restriction endonuclease should be adjusted to produce digested fragments with an average size of ∼40 kb. Although only a small fraction of the YAC-containing DNA used as starting material is actual YAC DNA (the rest being yeast genomic DNA), because of the low complexity of the yeast genome (e.g., compared to the human genome), only 3000 to 5000 cosmid clones are required to yield 3 yeast genome equivalents. Thus, only 1 to 2 ìg of yeast DNA are required to make an adequate library.
2. Perform a series of test ligations as described in UNIT 5.7. Using optimal conditions, ligate insert DNA to vector DNA. 3. Package cosmid recombinants; dilute packaged extract and determine the titer. 4. Plate and transfer the sublibrary as appropriate for the vector, and prepare resulting filters for hybridization. Screen the sublibrary 5. Perform a preliminary screen of the library using a 32P-labeled probe of total genomic DNA of the individual or species from which the library was made. This probe is a source of repetitive sequences. Because these repetitive sequences are spaced frequently throughout the source genome, and are absent from yeast, this probe will identify most of the source-DNA insert cosmids from the excess of yeast insert cosmids.
6. Organize this first set of cosmid clones into contigs by analyzing shared restriction fragments and by hybridizing with probes contained in the YAC insert or prepared from the ends of individual cosmid inserts. Cosmid end-fragment-specific probes can be generated by digesting cosmid DNA and end-labeling the purified restriction fragment(s) that contain(s) the cloning site. If the cloning vector is SuperCos 1 (or a comparable vector), end-specific RNA probes may also be transcribed from the ends of the cosmid clones using T3 and T7 polymerase (see critical parameters).
7. Establish a complete contiguous collection of cosmid clones of the original YAC insert by screening the library with specific YAC-derived probes and cosmid end-specific probes. Note that nitrocellulose filters may be reused for hybridization in subsequent steps without further washing or removal of probe. Repeated hybridization with sequential “walking probes” should reveal new hybridizing colonies at each step.
Analysis of Isolated YAC Clones
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REAGENTS AND SOLUTIONS AHC medium and plates (ura−, trp−) 1.7 g yeast nitrogen base without amino acids and without ammonium sulfate (Difco) 5 g ammonium sulfate 10 g casein hydrolysate-acid, salt-free and vitamin-free (U.S. Biochemical #12852) 50 ml (for medium) or 10 ml (for plates) of 2 mg/ml adenine hemisulfate (Sigma #A-9126) Dissolve in a final volume of 900 ml H2O Adjust pH to 5.8 Autoclave 30 min, then add 100 ml sterile 20% (w/v) glucose. For AHC plates, add 20 g agar prior to autoclaving. Store at 4°C for ≤6 weeks. Lithium lysis solution 1% lithium dodecyl sulfate (Sigma # L-4632) 100 mM EDTA 10 mM Tris⋅Cl, pH 8.0 (APPENDIX 2) Filter sterilize and store indefinitely at room temperature Lysis buffer 0.5 M Tris⋅Cl, pH 8.0 (APPENDIX 2) 3% (v/v) N-lauroylsarcosine (Sarkosyl) 0.2 M EDTA, pH 8.0 (APPENDIX 2) Store indefinitely at room temperature. Add 1 mg/ml proteinase K just before use. 100% NDS solution Mix 350 ml H2O, 93 g EDTA, and 0.6 g Tris base. Adjust pH to ∼8.0 with 100 to 200 pellets of solid NaOH. Add 5 g N-lauroylsarcosine (predissolved in 50 ml water) and adjust to pH 9.0 with concentrated NaOH. Bring volume to 500 ml with water. Filter sterilize and store indefinitely at 4°C. Dilute 1:5 with H2O (20% final) just before use. PCR reaction mix 1.5 mM MgCl2 50 mM KCl 10 mM Tris⋅Cl, pH 8.3 (APPENDIX 2) 0.2 mM each dATP, dCTP, dGTP, and dTTP 0.05 U AmpliTaq polymerase (Perkin-Elmer/Cetus)/µl reaction mixture 0.03 µl Perfect Match Enhancer (Stratagene)/µl reaction mixture Store all components at −20°C and mix just before use SCE buffer 0.9 M sorbitol (Fisher, molecular biology grade) 0.1 M sodium citrate 0.06 M EDTA, pH 8.0 Adjust pH to 7.0 Store at room temperature ≤3 months SCEM buffer 4.9 ml SCE buffer (see above) 0.1 ml 2-mercaptoethanol (2-ME) Add 1 to 2 mg Lyticase (Sigma #L-8137 or ICN Biomedicals #190123) just before use.
Screening of Recombinant DNA Libraries
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SEM buffer 1 M sorbitol 20 mM EDTA, pH 8.0 14 mM 2-ME Filter sterilize Store at 4°C for ≤6 weeks SEMT buffer 1 M sorbitol 20 mM EDTA 14 mM 2-ME 10 mM Tris⋅Cl, pH 8.0 (APPENDIX 2) Filter sterilize Add 1 mg/ml Lyticase (Sigma #L-8137 or ICN Biomedicals #190123) just before use. COMMENTARY Background Information An overview of strategies for screening YAC libraries and analyzing YAC clones is presented in UNIT 6.9.
Critical Parameters and Troubleshooting
Analysis of Isolated YAC Clones
The protocols provided in this unit are intended to describe the analysis and characterization of particular YAC clones of interest. It is initially desirable to assure the integrity of the clone; that is, to ensure that the YAC indeed carries the proper insert, that the insert is not chimeric or rearranged, and that it is stably maintained and propagated in the yeast host. Growth of YAC-containing strains. In YAC clones, genomic DNA is inserted into a cloning site carried within the SUP4 gene of the vector (see Figs. 6.10.1 and 13.4.6). In the parent vector, the SUP4 product complements the ade2-1 ochre mutation carried in the host AB1380. This mutation causes a block in purine biosynthesis, resulting in accumulation of red pigment in the culture. Thus, disruption of SUP4 by insertional inactivation prevents complementation of the ochre mutation in the host. Before a strain is used, it is important to check that upon growth the colonies or cultures exhibit a red pigmentation. If not, another isolate should be used. Additionally, growth on selective AHC medium requires the presence of both arms of the YAC vector and favors stability of the clone through passage. Analysis of YAC DNA. Restriction analysis of purified YAC DNA (second basic protocol) can be used to assess YAC structure. If fragments of unanticipated sizes are detected in the YAC, then it is likely that the YAC contains
sequences homologous to, but different from, the desired clone, or that the YAC insert has undergone some sort of rearrangement during cloning. Alternatively, lack of methylation of the YAC DNA at the restriction enzyme recognition site may give a digestion pattern not seen in uncloned genomic DNA. Determination of size and stability of the YAC clone is made by preparing chromosomes in agarose plugs and subsequent pulsed-field gel electrophoresis. The PFGE gel can be blotted and analyzed by hybridization with sequence-specific probes and end-fragment probes. The results should reveal hybridization to the same size chromosome in all isolates of a given YAC clone. Variation in the size of the artificial chromosome between yeast isolates derived from one clone indicates YAC instability. Hybridization of a YAC vector arm probe to more than one artificial chromosome may indicate multiple transformation of the strain at the time of library construction. Alternatively, it may represent strain instability, with the smaller chromosome(s) representing deletion products of the original YAC. When two or more artificial chromosomes are identified with a single-copy genomic insert-specific probe, instability of the YAC insert is the most likely cause. Assessing chimerism. In most existing YAC libraries, chimeric clones are observed to represent from 5% to 50% of the total clones. One of the most reliable ways to identify a chimeric YAC insert is to isolate a small fragment from each end of the YAC insert and define its chromosome of origin and whether it is contained in overlapping YACs from the same chromosomal region. Two procedures are given for analyzing the end fragments of
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the YAC clone, which can be used to identify chimerism in the YAC insert—one based upon subcloning (which for large-scale mapping projects can require prohibitive amounts of time and effort) and one based upon PCR. With the PCR strategy, the resulting amplified product should migrate as a single fragment in a polyacrylamide gel. If multiple bands are present, it may be possible to demonstrate that one is an appropriately amplified fragment because it should be digested by EcoRI to yield the vector-linker fragment plus the insert end fragment (see Fig. 6.10.3). If digestion by EcoRI cannot be confirmed or if no amplified band is observed, it is useful to try a “hemi-nested” PCR amplification in which the initially amplified product is reamplified using another primer that should be contained only in the properly amplified fragment. Each end-fragment probe should be shown to be single-copy by hybridization to a Southern blot of total DNA from the species used to prepare the YAC library. If a smear of hybridization is obtained, repetitive sequences are present within the probe. In the case of fragments obtained by subcloning, it is usually possible to identify a single-copy probe by digesting the fragment into several smaller pieces using selected restriction enzyme. The end fragments recovered by PCR are usually small, so that it is generally impossible to salvage a single-copy probe. It may be possible to suppress the repetitive DNA hybridization by including in the hybridization reaction an excess of unlabeled denatured repetitive DNA fragments from the species used to prepare the YAC library. If this is unsuccessful, the alternate protocol (subcloning into a bacterial vector) is usually necessary. Construction of a cosmid sublibrary. For further high-resolution analysis and mapping of the YAC insert, it is desirable to construct a sublibrary (final basic protocol) from the YACcontaining DNA (fifth basic protocol). A number of cosmid and phage vectors are available that are suitable for subcloning YACs into bacterial vectors (UNITS 3.16 & 5.7). One excellent candidate is the SuperCos-1 cosmid vector (Stratagene), which can accommodate inserts in the 35- to 42-kb range. It contains a neomycin-resistance cassette that permits selection of transfected clones in mammalian cells. It also contains T3 and T7 phage promoters flanking the genomic insert, which facilitate generation of RNA probes from the ends of the genomic inserts. This feature is useful for verifying overlaps of clones or to permit chromo-
some walking if a complete cosmid contig is not established in the first round of screening. Once a sublibrary has been constructed, it should be screened with a probe consisting of 32P-labeled total genomic DNA from the individual or species that was originally used to construct the library (a source of repetitive sequences). This will identify most of the cosmids containing DNA inserts from the source genome. Individual clones can be analyzed by Southern blotting with probes from the YAC insert or with genomic repetitive sequences. This data, together with results of Southern blots using probes derived from the ends of the cosmid clones using T3 or T7 polymerase, can be used to organize the cosmids into contigs. If a complete contig is not established, the cosmid library can be screened again with probes representing the ends of the YAC inserts, or derived from the ends of cosmid clones. Empirically, different portions of the YAC insert have been found to be nonrandomly represented in the cosmid library. Consequently, it is common for one or more rounds of chromosome walking to be required to fill in gaps between cosmid contigs.
Anticipated Results The basic protocol for preparation of DNA from YAC clones can be expected to yield ∼1 to 1.5 µg of DNA in the size range of 50 to 200 kb from 108 yeast cells. The yield of DNA obtained by purification using preparative CHEF gel electrophoresis is ≥10-fold lower. The basic protocol for preparation of yeast chromosomes in agarose plugs for PFGE should yield sufficient material for ∼40 lanes of a pulsed-field gel from a 25-ml yeast culture. The basic and alternate protocols for analysis of the YAC insert end fragments (by PCR and by subcloning into a plasmid vector) should each yield DNA fragments that identify single hybridizing bands in genomic DNA when used as probes for Southern blots. The basic protocol for preparation of high-molecular-weight YACcontaining yeast DNA in solution should yield ∼25 to 50 µg of DNA ≥100 kb in size from a 100-ml culture. Basic protocol for preparation of a YAC insert cosmid sublibrary should yield ∼1500 colonies/µg of starting yeast DNA. From these, ∼30 to 50 genomic DNA–containing cosmids should be recovered.
Time Considerations Purifying YAC DNA using the first basic protocol requires ∼3 days to grow the culture, 3.5 hr to isolate the DNA, and after an overnight
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resuspension, ∼2 hr to remove RNA and reprecipitate the DNA. Preparation of yeast chromosomes in agarose plugs takes ∼4 days to grow the yeast culture, ∼11⁄2 hr to form the yeast-containing plugs, ∼6 hr to lyse the yeast, an overnight incubation, and ∼6 hr to wash and prepare the plugs for electrophoresis. Preparation of YAC insert end fragments by PCR takes from 6 hr to overnight to anneal the bubble primers, digest the YAC DNA, and ligate it to the bubbles. Another 3 to 5 hr are needed to amplify the end fragment by PCR. Isolation of YAC end fragments by subcloning requires 2 to 3 days to perform the preliminary analytical Southern blot to identify the enzyme combination of choice for subcloning. Once this is identified, 1 day is required for preparative isolation of the doubly digested DNA fragments, followed by an overnight ligation to the modified pUC 19 vector. An additional day is needed to transform bacteria and grow colonies, and 1 to 2 days are required to identify the specific end-fragment
subclones by hybridization. Finally, 2 days are needed to purify the subcloned DNA and to verify its structure by restriction enzyme analysis. Preparation of high-molecular-weight YACcontaining DNA requires ∼4 days to grow the culture, 7 hr to lyse the cells and perform the sucrose-gradient fractionation, an overnight dialysis, and 2 to 3 hr to concentrate the DNA. Following an additional overnight dialysis, the DNA is ready for use. YAC insert cosmid sublibrary preparation and analysis takes 2 days to perform test digestions and ligations. Another 2 days are required to perform the preparative digestion, ligation, packaging, and plating of the library. Two more days are required for preparation of filters, hybridization, washing, and autoradiography. Contributed by David D. Chaplin and Bernard H. Brownstein Howard Hughes Medical Institute and Washington University School of Medicine St. Louis, Missouri
Analysis of Isolated YAC Clones
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SPECIALIZED STRATEGIES FOR SCREENING LIBRARIES
SECTION VI
Use of Monoclonal Antibodies for Expression Cloning
UNIT 6.11
This unit details the use of transient expression in mammalian cells to screen cDNA libraries with monoclonal antibodies (MAb) to isolate cDNA clones encoding cell-surface and intracellular proteins. The first basic protocol describes the cloning of cDNAs encoding cell-surface antigens. Several steps in this protocol involve transfection procedures that are described in greater detail in UNIT 16.12. The second basic protocol is a modification that facilitates isolation of cDNAs encoding antigens that are expressed intracellularly. Both protocols are designed for use with the expression vector CDM8, which contains a polylinker for subcloning double-stranded cDNA (Fig. 16.12.1). ISOLATION OF cDNA CLONES ENCODING CELL-SURFACE ANTIGENS This protocol is designed to isolate cDNAs encoding cell-surface proteins by screening cDNA libraries transiently expressed in mammalian cells. The procedure requires multiple rounds of transfection and immunoselection and is divided into four sections: (1) COS cell transfection by the DEAE-dextran method, (2) immunoselection by panning, (3) plasmid recovery and E. coli transformation, and (4) COS cell transfection by the spheroplast fusion method. A total of four rounds of transfection and immunoselection (one using DEAE-dextran, three using spheroplast fusions; Fig. 6.11.1) are used. After the final round of immunoselection, plasmid DNA is prepared from individual bacterial colonies. COS cells are then transfected with this DNA by the DEAE-dextran method and examined for their ability to express the foreign protein of interest by immunofluorescence microscopy (UNIT 14.6) or flow cytometry analysis (Holmes and Fowlkes, 1991).
BASIC PROTOCOL
DEAE-dextran transfection is a highly efficient means of introducing the cDNA library into COS cells to ensure that the transfected cells receive as complete a library representation as possible. Typically, ten 100-mm tissue culture plates of COS cells are transfected (Fig. 6.11.1). The subsequent panning steps allow rapid and efficient culling of cells expressing the protein of interest from the bulk of the transfected cells (each 60-mm, antibody-coated plate can be used to pan 1–2 × 107 transfectants). Plasmid DNA can be rescued from the panned cells by obtaining a Hirt supernatant (Hirt, 1967) and following amplification in E. coli, the plasmid DNA can be reintroduced into COS cells using spheroplast fusion. This transfection procedure ensures that a single plasmid type is delivered into each transfectant, allowing greater enrichment in subsequent rounds of transfection and immunoselection. Each round of screening usually requires a set of six fusions and each set of six fusions requires 100 ml of cells in broth. NOTE: All incubations are performed in a humidified 37°C, 5% CO2 incubator unless otherwise noted. See Chapter 9 introduction for critical parameters concerning media components and preparation. Materials Complete Dulbeccos minimum essential medium containing 10% (v/v) NuSerum or 10% (v/v) calf serum (complete DMEM-10 NS or complete DMEM-10 CS; APPENDIX 3F) 100-mm tissue culture plates seeded with COS cells (∼50% confluent) Contributed by Diane Hollenbaugh, Alejandro Aruffo, Bryan Jones, and Peter Linsley Current Protocols in Molecular Biology (1998) 6.11.1-6.11.16 Copyright © 2003 by John Wiley & Sons, Inc.
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cDNA library: plasmid expression vector DNA containing >106 of cDNA clones (UNIT 5.8; see background information), CsCl-purified (UNITS 1.7 & 9.1) Phosphate-buffered saline (PBS; APPENDIX 2) DEAE-dextran/chloroquine solution: PBS containing 10 mg/ml DEAE-dextran (Sigma) and 2.5 mM chloroquine (Sigma) 10% (v/v) DMSO in PBS Trypsin/EDTA solution: PBS containing 0.5 mg/ml trypsin + 0.2 mg/ml EDTA 0.5 mM EDTA/0.02% (v/v) azide in PBS 0.5 mM EDTA/0.02% (v/v) azide/5% (v/v) calf serum in PBS 1 µg/ml purified monoclonal antibody (MAb) or 1:100 dilution of ascites fluid (UNIT 11.1) 0.5 mM EDTA/0.02% (v/v) azide/2% (w/v) Ficoll 60-mm antibody-coated plates (first support protocol) 5% (v/v) calf serum in PBS 0.6% (w/v) SDS/10 mM EDTA 5 M NaCl (APPENDIX 2) Phenol (extracted twice with 1 M Tris⋅Cl, pH 7.5) 2 µg/µl linear polyacrylamide TE buffer, pH 7.5 (APPENDIX 2) Electroporation-competent E. coli cells (UNIT 1.8) LB medium (UNIT 1.1) 100 mg/ml spectinomycin or 35 mg/ml chloramphenicol in ethanol 20% (w/v) sucrose/50 mM Tris⋅Cl, pH 8.0, ice cold 5 mg/ml lysozyme (Sigma #L6876), freshly prepared in 250 mM Tris⋅Cl, pH 8.0 250 mM EDTA, ice cold (APPENDIX 2) 50 mM Tris⋅Cl, pH 8.0 (APPENDIX 2)
cDNA library
DEAE-dextran transfection (steps 1- 4)
panning (steps 6-12)
plasmid recovery (steps 13-18)
repeat 3 times
protoplast fusion (steps 20- 36)
plasmid amplification (step 19) isolation of DNA from single colonies
DEAE-dextran transfection
Use of Monoclonal Antibodies for Expression Cloning
immunofluorescence analysis
Figure 6.11.1 Isolation of a cDNA clone encoding a cell-surface antigen by transient expression in mammalian cells.
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10% (w/v) sucrose/10 mM MgCl2 in DMEM (GIBCO/BRL #320-1960AJ) without serum, filter sterilized 60-mm tissue culture plates seeded with COS cells (∼50% confluent) 50% (w/w) PEG 1000 or 1450 in DMEM (no serum), adjusted to pH 7 with 7.5% (w/v) sodium bicarbonate (Baker or Kodak) DMEM without serum Complete DMEM-10 CS (APPENDIX 3F) containing 15 µg/ml gentamycin sulfate Nylon mesh, 100-µm pore size (Tetco) Sorvall GSA rotor or equivalent Swinging-bucket centrifuge (e.g., Sorvall RT-6000B) Additional reagents and equipment for transformation of E. coli by electroporation (UNIT 1.8), phenol extraction and ethanol precipitation (UNIT 2.1), alkaline lysis miniprep (UNIT 1.7), and immunofluorescence (UNIT 14.6) Transfect COS cells using DEAE-dextran 1. Add 5 ml complete DMEM-10 NS to each 100-mm plate of COS cells to be transfected. Each 100-mm plate should be ∼50% confluent the day of transfection (∼5 × 106 cells). This protocol is designed to be used with COS cells and is too harsh for WOP or MOP cells (see background information). If these murine lines must be used, it is important to reduce both the concentration of DEAE-dextran used to 200 ìg/ml final and the time that the cells are exposed to the transfection medium to 2 hr, and to use IMDM (Iscoves modified Dulbeccos medium; GIBCO/BRL #430-2200) in place of DMEM (prepare complete IMDM media as for complete DMEM media, but omit amino acids).
2. To each dish, add 5 µg cDNA library and mix, then add 0.2 ml DEAE-dextran/chloroquine solution and mix. Incubate 4 hr. Typically, libraries of >106 clones are used to obtain plasmid DNA. It is important that the DNA and the DEAE-dextran form a fine, invisible precipitate. If the DNA is not diluted prior to addition of DEAE-dextran, a large DNA/DEAE-dextran precipitate forms (it is easily seen), which is not readily taken up by the cells. Check the cells after ∼3 hr exposure to the DEAE transfection mix, as their health can decline rapidly. This is particularly true of chloroquine transfections, and it is usually better to shorten the transfection than to allow too many cells to die.
3. Aspirate the medium and add 2 ml of 10% DMSO. Incubate ≥2 min at room temperature. The time that the cells are exposed to the DMSO is not critical.
4. Remove DMSO and replace with 10 ml complete DMEM-10 CS. Incubate overnight. 5. Aspirate the medium, add PBS, then aspirate the PBS. Add 2 ml trypsin/EDTA to each plate and incubate 5 to 15 min until cells have lifted from the plate. Replate the cells on two new 100-mm plates and incubate overnight. Replating the cells allows them to recover more effectively from the transfection. In addition, the DEAE-dextran transfection makes the cells sticky and replating allows them to be lifted from the plates with EDTA to initiate the panning step.
Immunoselect the cells by panning 6. Aspirate the medium, add 2 ml EDTA/azide solution, and incubate 10 to 20 min to detach cells from plates. 7. Pipet vigorously with a short Pasteur pipet to dislodge the cells, then transfer cells from each plate into a 15-ml centrifuge tube.
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8. Centrifuge 4 min at 200 × g (e.g., 1000 rpm in a Sorvall RT-6000B with GSA rotor or in a tabletop centrifuge) and discard supernatant. 9. Resuspend cells in 0.5 to 1.0 ml EDTA/azide/calf serum solution and add purified MAb to 1 µg/ml final or ascites at a 1:100 dilution final. Incubate 30 to 60 min on ice. 10. Add an equal volume of EDTA/azide solution and carefully layer on 3 ml EDTA/azide/Ficoll solution. Centrifuge 4 min at 200 × g. Aspirate supernatant in one smooth movement. 11. Add 3 ml EDTA/azide/calf serum solution to each antibody-coated plate. Resuspend cells in 0.5 ml EDTA/azide solution, then add aliquots of the cells to these plates by pipetting them through a nylon mesh. Leave 1 to 3 hr at room temperature. Four 6-mm antibody-coated plates are used in each round of panning. It is important to pass the cells through the nylon mesh to break up large clumps of cells which might contain both positive and negative cells. This ensures that individual antibody-coated cells bind to the panning plate.
12. Remove excess cells not adhering to the plate by gently washing two to three times with 3 ml of 5% calf serum (or complete DMEM-10 NS or complete DMEM-10 CS). Washing gently means swirling the plate with a smooth motion for ∼30 sec. The plate obtained after these washes is known as a panned plate.
Recover plasmid DNA and transform E. coli 13. Add 0.4 ml SDS/EDTA solution to the panned plate and leave 20 min at room temperature (to lyse the cells). This incubation period can be as little as 1 min if there are only a few cells on the plate.
14. Pipet the viscous mixture into a microcentrifuge tube. Add 0.1 ml of 5 M NaCl, mix, and place ≥3 hr on ice or leave overnight at 4°C. The viscosity is primarily due to the genomic DNA. It is important to avoid shearing the genomic DNA so that it will not contaminate the plasmid DNA. Keeping the mixture as cold as possible seems to improve the quality of the Hirt supernatant.
15. Microcentrifuge 4 min at top speed, 4°C, and remove supernatant carefully. 16. Extract with phenol (twice if the first interface is not clean) and add 5 µl (10 µg) of 2 µg/µl linear polyacrylamide (or other carrier). 17. Fill the tube to the top with 100% ethanol and precipitate. Resuspend the pellet in 0.1 ml TE buffer, pH 7.5. 18. Add 10 µl of 3 M sodium acetate and 300 µl of 100% ethanol, and repeat precipitation. Resuspend the pellet in 0.1 ml TE buffer, pH 7.5. 19. Transform electroporation-competent E. coli cells by electroporation using DNA obtained from step 18. Incubate overnight at 37°C. Approximately 105 bacterial colonies should be obtained. It is advisable to transform E. coli with an aliquot of DNA to determine the amount necessary to obtain 105 colonies. Generally 1⁄10 to 1⁄4 of the recovered DNA will be used. Use of Monoclonal Antibodies for Expression Cloning
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Prepare the spheroplasts 20. Rinse the plate from step 19 several times with LB medium while scraping with a spreader to dislodge the bacteria. Use 1⁄10 to 1⁄5 of the pooled scrapings to inoculate 200 ml of LB medium. Grow to OD600 = 0.5 at 37°C with shaking. 21. Add 100 mg/ml spectinomycin to 100 µg/ml or 35 mg/ml chloramphenicol to 150 µg/ml. Incubate with shaking 10 to 16 hr at 37°C. Do not let the cells grow >16 hr or they will begin to lyse. If the cells lyse, do not proceed.
22. Centrifuge 100 ml of the culture in a 250-ml bottle, 5 min at 4000 × g (e.g., 5000 rpm in a Sorvall with GSA rotor), room temperature or 4°C. 23. Drain well and resuspend pellet in 5 ml of ice-cold sucrose/Tris⋅Cl, pH 8.0. 24. Add 1 ml of 5 mg/ml lysozyme solution. Incubate 5 min on ice. 25. Add 2 ml of ice-cold 250 mM EDTA, pH 8.0, and incubate 5 min on ice. 26. Add 2 ml of 50 mM Tris⋅Cl, pH 8.0, and incubate 5 min in a 37°C water bath. 27. Place on ice. Check percent conversion to spheroplasts by microscopy. A good spheroplast preparation gives about 80% to 90% conversion; anything 106 cDNA clones (UNIT 5.8; see background information), CsCl purified (UNITS 1.7 & 9.11) Trypsin/EDTA solution: PBS containing 0.5 mg/ml trypsin + 0.2 mg/ml EDTA Phosphate-buffered saline (PBS; APPENDIX 2) Methanol 1% (w/v) nonfat dry milk in PBS with and without monoclonal antibody (MAb) 1% (w/v) nonfat dry milk in PBS containing 0.25 µCi/ml of 125I-labeled protein A 0.6% (w/v) SDS/10 mM EDTA buffer LB medium (UNIT 1.1) Polyvinylidene-wrapped plates (second support protocol) X-ray film Polyvinylidene wrap (e.g., Saran Wrap) Rubber cement Luminescent stickers Additional reagents and equipment for alkaline lysis miniprep (UNIT 1.7) and autoradiography (APPENDIX 3) Transfect cells and fix with methanol 1. Transfect ten 100-mm plates of COS cells with the cDNA library using the DEAEdextran method as described in steps 1 to 4 of the first basic protocol. 2. Trypsinize each plate of transfected COS cells using trypsin/EDTA as described in step 5 of the first basic protocol and replate onto polyvinylidene-wrapped plates. Incubate 1 to 2 days. Cells should be split at a ratio that will provide 50% to 75% confluency the following day. Complete DMEM containing penicillin and streptomycin should be used to avoid minor contamination that may occur because the plates are not sterile.
3. Remove medium from transfected COS cells. Wash plates by adding 5 ml PBS and aspirating at room temperature. Repeat wash one time. Generally, 15 to 20 plates are used.
4. Add ∼6 ml methanol and incubate 5 min at room temperature. 5. Wash plates three times with PBS, leaving the first addition of PBS on the plate for 2 to 3 min before aspirating. 6. Add 4 ml of 1% dry milk containing MAb to each plate. Incubate 30 to 60 min at room temperature. 7. Wash plates twice by adding 5 ml of 1% dry milk (without MAb), swirling gently, and removing solution. Radiolabel and locate positive cells 8. Add 4 ml of 1% dry milk in PBS containing Incubate 30 min at room temperature.
125
I-labeled protein A to each plate.
9. Rinse plates four times with 1% dry milk in PBS and one time with PBS at room temperature. Remove all excess liquid. Use of Monoclonal Antibodies for Expression Cloning
It is helpful to prop the plates on edge to remove the excess liquid.
10. Completely cover a piece of X-ray film with polyvinylidene wrap and tape the edges
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of the wrap to the film. Paint the wrap with a thin layer of rubber cement. Allow rubber cement to dry briefly. The X-ray film acts as a support and may be a used piece that would otherwise be discarded. The presence of rubber cement does not appear to affect DNA recovery. Five plates will fit on an 8 × 10–inch (20.3 × 25.4–cm) film or 15 plates will fit on a 14 × 17–inch (35 × 43–cm) film.
11. Paint the bottom surface of the polyvinylidene-wrapped plate with a thin layer of rubber cement and allow the rubber cement to dry briefly. 12. Place the plates on the support. Lift the plate slightly and cut the wrap away from the edge of the plate. The wrap with the cells will be left on the X-ray film. By slightly lifting the plate, a scalpel held nearly horizontal along the inside of the plate can be used to cut the wrap from the plate without cutting the wrap on the support. While this technique is not difficult, it is advisable to practice it on nonradioactive samples.
13. Affix luminescent stickers to the support. Cover the support and samples with polyvinylidene wrap. There are now three layers of wrap on the support. Without the luminescent stickers, the film cannot be aligned over the samples to recover positive cells. Careful alignment is necessary to recover positive cells (there will be insufficient background to use the location of the samples for alignment).
14. Autoradiograph with an intensifying screen for 1 to 2 days at −70°C. Develop the film. 15. Align the film over the support using the luminescent stickers. Mark the location of positive cells by piercing the film alongside the spot with a needle (this leaves a mark on the polyvinylidene wrap). A light box is helpful for locating the positive cells on the film.
16. Remove the film. Cut small squares of ∼3 mm in the polyvinylidene wrap at the places it is marked. Recover plasmid DNA and isolate the positive clone 17. Add 400 µl of SDS/EDTA buffer to a microcentrifuge tube. Place squares in tube (10 to 25 squares per tube) and incubate 30 min at room temperature. All three layers of wrap are placed in the tube.
18. Recover plasmid DNA and transform E. coli as in steps 14 to 19 of the first basic protocol, plating transformed bacteria on separate LB plates to form pools of appropriate numbers of clones. Incubate overnight at 37°C. 19. Collect the bacteria from the LB plates by rinsing several times with LB medium while scraping with a spreader to dislodge the bacteria. 20. Prepare plasmid DNA from the scraped colonies using an alkaline lysis miniprep. 21. Transfect COS cells with the pools of plasmid DNA by the DEAE-dextran method as described in steps 1 to 4 of the first basic protocol. In general, 1⁄10 to 1⁄5 of the DNA obtained from 1000 colonies is used to transfect one 100-mm plate of COS cells. Screening of Recombinant DNA Libraries
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22. Repeat steps 2 to 14 above to identify pools enriched with the gene of interest. Store the appropriate plasmid DNA in TE buffer at −20°C. When screening pools, it is possible to use 60-mm plates (reduce all volumes to 1⁄3).
23. Prepare DNA from a single bacterial colony, transfect COS cells, and analyze as in steps 38 and 39 of the first basic protocol. SUPPORT PROTOCOL
PREPARATION OF POLYVINYLIDENE-WRAPPED PLATES Plates are prepared in which the growth surface is a polyvinylidene wrap (to be used in the second basic protocol). The plates are quite sturdy and can be used in the same manner as standard tissue culture plates. Plates may be prepared a day or two in advance but the wrap will stretch and become floppy on prolonged storage. Additional Materials Chloroform 70% ethanol 0.1 mg/ml poly-L-lysine HCl (Sigma) in 50 mM Tris⋅Cl, pH 8.0, freshly prepared 100-mm or 60-mm tissue culture plates 1. Break the bottoms out of a 100- or 60-mm tissue culture plate with a blunt object. Safety glasses are advisable. Strike the plate near the sides of the plate rather than in the center. If too much force is used, the sides of the plate will break as well. Structural stability is increased if the outer edges of the bottoms are not removed.
2. In a fume hood at room temperature, dip the top rim of the plate in chloroform to a depth of ∼3 mm. 3. Shake off excess chloroform and place plate on a piece of polyvinylidene wrap laid flat. Place the plate and attached film into the lid of the plate to force the wrap into contact with the edges of the plate. 4. Remove the lid and gently but firmly pull the film tight to form a smooth surface. Adhesion of the film to the outside of the plate helps maintain the strength of the seal.
5. Cut excess wrap from the plate with a razor blade. The plate is now essentially inverted. The lid is placed over the opening that had been the bottom of the plate. When using 100-mm plates, a second lid is used to support the new wrap bottom.
6. Wash the plate two times with 70% ethanol. Allow the wrap to soak in the ethanol ∼30 min. 7. Wash the plate with water. 8. Add 0.1 mg/ml poly-L-lysine to the plate—11 ml for a 100-mm plate and 4 ml for a 60-mm plate. Incubate 2 hr to overnight at room temperature. 9. In a tissue culture hood, rinse the plates twice with PBS. Dishes should not be stored more than a few days because the wrap will stretch and become loose. Use of Monoclonal Antibodies for Expression Cloning
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COMMENTARY Background Information Transient expression in mammalian cells has emerged as a powerful method for isolating cDNA clones that encode secreted cell-surface and intracellular proteins. It was first used to isolate cDNA clones encoding the lymphokine granulocyte/macrophage colony stimulating factor (GM-CSF; Lee et al., 1985; Wong et al., 1985). This cloning strategy is well suited for isolating any secreted proteins for which a rapid and sensitive bioassay exists, and has since been applied to isolate cDNA clones encoding a number of different lymphokines. Transient expression cloning was combined with the simple but powerful immunoselection technique of panning (Wysocki and Sato, 1978) to isolate a cDNA encoding the T cell–surface proteins CD2 and CD28 (Seed and Aruffo, 1987; Aruffo and Seed, 1987). This procedure has since been used to isolate cDNA clones encoding a number of different cell-surface proteins when antibodies against them were available. When antibodies against a cell-surface receptor of interest are not available, but its ligand is, transient expression in mammalian cells has been combined with ligand-binding assays to isolate cDNA clones encoding the receptor. This strategy was first used to isolate a cDNA clone encoding the receptor for the lymphokine interleukin 1 (Sims et al., 1988). Modifications to this protocol (Gearing et al., 1989) have since allowed the use of this strategy to isolate a number of receptors. Recent improvements have allowed the use of transient expression in mammalian cells to isolate cDNA clones encoding intracellular proteins, including the major DNA-binding protein of the erythroid lineage (Tsai et al., 1989), the lysosomal membrane glycoprotein CD63 (Metzelaar, 1991), and fucosyltransferase, which adds fucose to N-acetylglucosamine with α(1,3) linkages, allowing the expression of the sialyl CD15 antigen (Goelz et al., 1990). Transient expression in mammalian cells. Mammalian cells are ideal hosts for screening cDNA libraries prepared using mRNA isolated from higher eukaryotes. These cells are able to synthesize transcripts correctly from the cDNA clones in the library and are likely to process the proteins that they encode appropriately, thus maximizing the likelihood that the foreign proteins will be present in their native state and will be detectable using functional or immunological assays.
Mammalian cells were initially used as the host for isolation of genomic DNA fragments encoding oncogenes by stably introducing genomic DNA fragments derived from human tumors into murine cells (Goldfarb et al., 1982; Shih and Weinberg, 1982). The gene encoding the oncogene was then rescued from the transfected cells that had acquired the transformed phenotype. Stable transfections of mammalian cells were subsequently combined with immunoselection procedures to isolate genomic DNA fragments encoding the human HLA and β2 microglobulin genes (Kavathas and Herzenberg, 1983) and cDNA clones encoding the T cell–surface proteins CD8 (Kavathas et al., 1984; Littman et al., 1985) and CD4 (Maddon et al., 1985). The time involved in obtaining stable transfectants expressing the gene of interest and the difficulties associated with recovering the transfected DNA from the host’s chromosomal DNA has limited the number of genes isolated using this cloning strategy. These difficulties prompted the development of transient expression systems for use in cloning with mammalian cells as the screening host. Unlike stable transfectants, these methods permit rapid preparation and detection of transfectants expressing the protein of interest and efficient recovery of the DNA encoding it. Many technical advances have permitted efficient and routine screening of cDNA libraries in transiently expressing mammalian cells. These include the following developments: shuttle vectors that contain the appropriate eukaryotic transcription elements for highlevel protein expression in transfected mammalian cells, mammalian cell lines that can act as effective heterologous expression hosts, and transfection protocols that allow efficient introduction of plasmid DNA into mammalian cells. Expression vectors. A number of mammalian expression vectors permit screening of cDNA libraries by transient expression in mammalian cells (Chapter 16; Kaufman, 1990). These plasmids contain at least four basic elements: an efficient eukaryotic transcription unit, a viral-derived origin of replication, a prokaryotic origin of replication, and a prokaryotic selectable marker. One particular expression vector, CDM8 (Seed, 1987; Fig. 16.12.1; available from Invitrogen #V308-20) is especially engineered for these purposes, and is described in detail in UNIT 16.12. CDM8 con-
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tains origins of replications derived from polyoma and SV40 viruses that allow for plasmid replication in cell lines expressing either the polyoma or SV40 large T antigens, respectively; usually, WOP (Dailey and Basilico, 1985) and COS (Gluzman, 1981) cells, respectively. Bacterial and M13 origins of replication (ori) are also present, allowing for plasmid amplification in bacteria and production of single-stranded DNA, respectively. The plasmid contains a supF gene as a prokaryotic selectable marker and a T7 RNA polymerase promoter for the preparation of mRNA in vitro from the subcloned cDNAs. Mammalian cell lines. A number of cell lines have been developed that are excellent hosts for screening cDNA libraries prepared in the vector described above. Perhaps the most popular is the COS cell line (Gluzman, 1981), which was derived from the African green monkey kidney cell line CV-1 by transformation with an origin-defective SV40 virus. This cell line produces wild-type SV40 large T antigen but no viral particles. When plasmids containing an SV40 virus–derived ori are transfected into COS cells, the plasmid is replicated to a high copy number 48 hr posttransfection (10,000 to 100,000 copies/cell). This high-level replication has two important consequences. First, it allows for amplification of all DNA templates available for transcription. Second, it allows for recovery of the plasmid encoding the protein of interest from the immunoselected cells. This last point is of importance in the two basic cloning protocols because each cycle of transfection and immunoselection is followed by a plasmid-rescue step. Other cell lines that have been used for expression cloning include the murine cell lines WOP (Dailey and Basilico, 1985) and MOP (Muller et al., 1984). These two cell lines express the polyoma large T antigen allowing the replication of plasmids containing a polyoma origin of replication in the transfected cells (1,000 to 10,000 copies/cell, 48 hr posttransfection). Another cell line, CV-1/EBNA, has been developed for screening cDNA libraries in conjunction with the expression vector pDC406, which contains an Epstein-Barr virus ori (McMahan et al., 1991). The ability of COS cells to endure the transfection protocols and their ability to replicate the transfected plasmid to a very high copy number make them the cells of choice when screening a cDNA library with the methods described here. However, when the antibodies to be used in the immunoselection step cross-
react with proteins expressed by COS cells, another cell line must be used. In these cases, WOP or MOP cells can be used successfully, in spite of their more delicate nature and lower copy numbers of transfected plasmid. Mammalian cell transfection. A number of transfection protocols have been developed for efficient introduction of foreign DNA into mammalian cells, including calcium phosphate, DEAE-dextran, spheroplast fusion, lipofection, and electroporation (UNITS 9.1-9.5). Two factors determine which transfection procedure should be used when screening a cDNA library by transient expression in mammalian cells: first, the efficiency of transfection, and second, the number of different plasmids that are introduced into each cell during transfection. Two transfection protocols, DEAE-dextran (McCutch and Pagano, 1968) and spheroplast fusion (Sandri-Goldin et al., 1981), are discussed below. The mechanism by which DEAE-dextran transfections allow for introduction of foreign DNA into cells is poorly understood. It is believed that the positive charge of the DEAEdextran polymer neutralizes the negative charge of the DNA polymer, forming a fine precipitate that can come into contact with the plasma membrane of the host cell. The DEAEdextran/DNA complex is then internalized by pinocytosis. Some of this DNA makes its way to the host-cell nucleus, where it is replicated and transcribed. Because the foreign DNA enters the cell via endosomes, DNA integrity is enhanced by the addition of chloroquine to the transfection medium to prevent endosome acidification. DEAE-dextran transfections are very efficient, allowing for transfection of up to 70% of the host cells and delivery of up to 200 different plasmids into each transfected cell. Introducing foreign DNA into mammalian cells by spheroplast fusion is very inefficient, allowing for transfection of only 1% to 2% of the host cells. Bacteria containing the foreign DNA are treated with lysozyme to remove their cell walls. The resulting spheroplasts are then fused with the host mammalian cell using polyethylene glycol (PEG), allowing introduction of the foreign DNA directly into the host cell cytoplasm. The DNA is then replicated and transcribed in the nucleus. Because of the inefficiency of the procedure, each host cell fuses with only one spheroplast, on average, introducing only a single plasmid type into each transfected cell. The immunoselection screening method de-
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scribed in the first basic protocol involves multiple rounds of mammalian cell transfection, immunoselection, and plasmid rescue steps. The rescued plasmids are amplified in E. coli and reintroduced into mammalian cells to initiate additional rounds of enrichment. To take full advantage of immunoselection, the two methods of transfection are used. When the cDNA library is first introduced into mammalian cells, it is important to obtain a complete representation of the library in the transfected host cells, ensuring that the protein of interest is expressed by the transfectants. For this reason, the first round of enrichment is initiated using DEAE-dextran transfection. In subsequent cycles of enrichment, it is important that a single plasmid type be delivered into each of the transfected cells to maximize the level of enrichment obtained in the subsequent immunoselection steps. This is accomplished using the spheroplast fusion transfection. Immunoselection procedures. Two immunoselection techniques designed to rapidly select and enrich for plasmids encoding proteins of interest from a cDNA library transfected into mammalian cells are described. The first strategy is designed to isolate cDNA clones encoding surface proteins. A cDNA library prepared in a mammalian expression vector is transfected into COS cells using DEAE-dextran transfection. Forty-eight hours posttransfection, the cells are lifted from the plate and incubated with antibodies directed against the protein of interest. Cells expressing the foreign protein on their cell surface are easily culled from the bulk of the transfected cells by panning on plastic plates coated with anti-antibody antibodies (Wysocki and Sato, 1978). Plasmid DNA is then recovered from the transfected cells by the method of Hirt (Hirt, 1967), amplified in E. coli, and reintroduced into COS cells by spheroplast fusion. Two additional rounds of spheroplast fusion and panning are usually required to enrich for the plasmid encoding the protein of interest. Panning has many advantages over other immunoselection procedures. It is fast, efficient (107 cells can easily be panned on two 60-mm plastic plates in 30 min), and very inexpensive. Other immunoselection techniques, such as sorting of fluorescence-labeled cells (Holmes and Fowlkes, 1991), may be used to screen cDNA libraries in transiently expressing mammalian cells (Yamasaki et al., 1988); but the greater demands on time, equipment, and technical expertise make these methods much less attractive.
The second strategy is designed to isolate cDNA clones encoding intracellular antigens. This method is a combination of the techniques described by Munro and Maniatis (1989) and Metzelaar et al. (1991). COS cells are transfected with a cDNA library by DEAE-dextran transfection. The day after transfection, they are replated onto poly-L-lysine-coated polyvinylidene wrap and allowed to grow for 1 to 2 additional days. They are then washed and fixed with methanol. The permeabilized cells are incubated with antibodies directed against the protein of interest, washed, and incubated with radiolabeled protein A (125I). After washing, they are exposed to film to identify radiolabeled cells, which are then recovered by cutting the polyvinylidene wrap. The plasmid DNA is recovered from these cells by the method of Hirt, amplified in E. coli, and subjected to additional rounds of transfection and immunoselection.
Critical Parameters The cloning strategy described in the first basic protocol is well suited for isolating cDNA clones encoding cell-surface proteins (see discussion above). If a cDNA library is thought to contain cDNA clones encoding a number of proteins of interest, it is possible to isolate all of them simultaneously by simply using a mixture of antibodies against all of the proteins of interest in the panning steps of the first three rounds of enrichment. The last cycle of immunoselection is carried out independently with antibodies against each of the different proteins. Many times antibodies directed against the protein(s) of interest are of multiple isotypes. It is important, in this case, to use panning plates that have been coated with anti-Ig antibodies that bind to each of the isotypes present in the initial antibody pool. Alternatively, individual panning plates can be prepared for each of the antibody isotypes present in the initial antibody pool, but no significant advantage is achieved. Although this cloning strategy has allowed isolation of a large number of cDNA clones encoding cell-surface proteins, it has some serious limitations. As with any expression cloning system, the gene of interest must initially be present in the library, the target protein must be functional or immunoreactive as a single chain, and the host system must posttranslationally modify the protein appropriately when these modifications are required for function or immunoreactivity. Compared with earlier bacterial expression cloning systems, the mam-
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malian expression systems presented here are more likely to appropriately modify the gene products of higher eukaryotes. However, these methods require the host mammalian cell to express the target protein on its surface as a single molecule. This may not occur in cases where the target protein is part of a heterocomplex that requires more than one member for surface expression. Several factors contribute to the successful application of this cloning strategy. The most critical parameter when screening a cDNA library by expression in mammalian cells is the quality of the cDNA library (for a more complete discussion, see UNIT 5.8). In addition, the quality of the COS cells and transfectability is of utmost importance. COS cells maintained in culture for prolonged periods tend to become refractory to transfection. For this reason, it is important to check the cells periodically for transfectability and replace them with cells from frozen stocks when necessary. The competency level of the bacterial cells used to amplify the DNA rescued from the immunoselection step is very important and should be determined prior to the start of the experiment. If only a few positive cells are immunoselected by panning, it is of utmost importance that this plasmid DNA work its way into the bacteria so that it can potentially be amplified and thus be available for subsequent rounds of transfection and immunoselection. Ideally, only cells whose competency level is ≥109 cfu/µg DNA should be used. For most bacterial strains, this can be achieved using electroporation. When using panning to immunoselect the transfected cells, it is important to check that the antibody directed against the protein of interest does not bind to COS cells. If it does, another cell line should be used for screening. The researcher must also be mindful that DEAE-dextran treatment of cells changes their phenotype and thus the antibodies to be used in the panning step should be tested for cross-reactivity with mock-transfected cells. The most difficult step, technically, is the spheroplast-fusion step. Careful timing of cell exposure to PEG is necessary to promote fusion while minimizing cell death. Although lowermolecular-weight PEG (PEG 1000) results in more efficient fusion, it is more toxic to the cells. The cloning strategy described in the second basic protocol will not be effective if the antibodies used recognize epitopes that are sensitive to methanol treatment. Testing of the target
cells may reveal this limitation. If methanol sensitivity cannot be assayed, multiple antibodies against a given protein should be used when available.
Anticipated Results In the first basic protocol, the authors typically use DNA prepared from a cDNA library of ≥106 clones for screening. After four rounds of transfection, immunoselection, and plasmid rescue, 12 individual colonies are picked and plasmid DNA prepared from them. If the cloning procedure has been successful, at least one DNA preparation directs the expression of the protein of interest. In general, if the screening is unsuccessful, this indicates that the clone of interest may not be present in the library. In this case, screening of a new cDNA library may be successful. Unsuccessful screening may indicate that the target gene is refractory to cloning using this strategy. Possible alternative methods are described in Chapter 5. In the second basic protocol, three to five spots are generally obtained on each 100-mm plate. It is possible to enrich a mixture that is 1:10,000 in the desired clone to 1:100 with a single immunoselection step.
Time Considerations In the first basic protocol, the cloning strategy involves multiple steps. Each cycle of transfection, immunoselection, plasmid rescue, and amplification can be comfortably accommodated in 1 week. On this schedule, it is possible to screen a cDNA library in 1 month. However, the more ambitious can screen a library in 3 weeks. In the second basic protocol, successful application allows identification of positive pools in 11⁄2 weeks. The screening of subsets of the pool can be accelerated by using 60-mm plates. When screening individual clones, transfectants may be assayed by immunofluorescence. The length of time required to obtain a single isolated positive clone will depend on the pool sizes used.
Literature Cited Aruffo, A. and Seed, B. 1987. Molecular cloning of a CD28 cDNA by a high-efficiency COS cell expression system. Proc. Natl. Acad. Sci. U.S.A. 84:8753-8577. Dailey, L. and Basilico, C. 1985. Sequence in the polyomavirus DNA regulatory region involved in viral DNA replication and early gene expression. J. Virol. 54:739-749.
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Gearing, D.P., King, J.A., Gough, N.M., and Nicola, N.A. 1989. Expression cloning of a receptor for human granulocyte-macrophage colony-stimulating factor. EMBO J. 8:3667-3676. Gluzman, Y. 1981. SV40-Transformed simian cells support the replication of early SV40 mutants. Cell 23:175-182. Goelz, S.E., Hession, C., Goof, D., Griffiths, B., Tizard, R., Newman, B., Chi-Rosso, G., and Lobb, R. 1990. ELFT: A gene that directs the expression of an ELAM-1 ligand. Cell 63:13491356. Goldfarb, M., Schimizu, K., Perucho, M. and Wigler, M. 1982. Isolation and preliminary characterization of a human transforming gene from T24 bladder carcinoma cells. Nature (Lond.) 296:404-409. Hirt, B. 1967. Selective extraction of polyoma DNA from infected mouse cell cultures. J. Mol. Biol. 26:365-369.
McMahan, C.J., Slack, J.L., Mosley, B., Cosman, D., Lupton, S.D., Brunton, L.L., Grubin, C.E., Wignall, J.M., Jenkins, N.A., Brannan, C.I., Copeland, N.G., Huebner, L., Croce, C.M., Cannizzarro, L.A., Benjamin, D., Dower, S.K., Spriggs, M.K., and Sims, J.E. 1991. A novel IL-1 receptor, cloned from B cell by mammalian expression, is expressed in many cell types. EMBO J. 10:2821-2832. Metzelaar, M.J., Wijngaard, P.L.J., Peters, P.J., Sixma, J.J., Nieuwenhuis, H.K., and Clevers, H.C. 1991. CD63 antigen. J. Biol. Chem. 266:3239-3245. Muller, W.J., Naujokas, M.A., and Hassell, J.A. 1984. Isolation of large T antigen-producing mouse cell lines capable of supporting replication of polyomavirus-plasmid recombinants. Mol. Cell. Biol. 4:2406-2412. Munro, S. and Maniatis, T. 1989. Expression cloning of the murine interferon γ receptor cDNA. Proc. Natl. Acad. Sci. U.S.A. 86:9248-9252.
Holmes, K. and Fowlkes, B.J. 1991. Preparation of cells and reagents for flow cytometry. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeek, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. 5.3.1-5.3.11. Greene Publishing and John Wiley & Sons, New York.
Sandri-Goldin, R.M., Goldin, A.L., Glorioso, J.C., and Levine, M. 1981. High-frequency transfer of cloned herpes simplex virus type I sequences to mammalian cells by protoplast fusion. Mol. Cell. Biol. 1:743-752.
Kaufman, R.J. 1990. Overview of vectors used for expression in mammalian cells. Methods Enzymol. 185:487-511.
Seed, B. 1987. An LFA-3 cDNA encodes a phospholipid-linked membrane protein homologous to its receptor CD2. Nature (Lond.) 329:840842.
Kavathas, P. and Herzenberg, L.A. 1983. Stable transformation of mouse L cells for human membrane T-cell differentiation antigens, HLA and 2-microglobulin: Selection by fluorescence-activated cell sorting. Proc. Natl. Acad. Sci. U.S.A. 80:524-528. Kavathas P., Sukhatme, V.P., Herzenberg, L.A., and Parnes, J.R. 1984. Isolation of the gene encoding the human T-lymphocyte differentiation antigen Leu-2 (T8) by gene transfer and cDNA subtraction. Proc. Natl. Acad. Sci. U.S.A. 81:7688-7692. Lee, F., Yokota, T., Otsuka, T., Gemmell, L., Larson, N., Luh, J., Arai, K.-I., and Rennick, D. 1985. Isolation of cDNA for a human granulocytemacrophage colony-stimulating factor by functional expression in mammalian cells. Proc. Natl. Acad. Sci. U.S.A. 82:4360-4364. Littman, D.R., Thomas Y., Maddon, P.J., Chess, L., and Axel, R. 1985. The isolation and sequence of the gene encoding T8: A molecule defining functional classes of T lymphocytes. Cell 40:237-246. Maddon, P.J. Littman, D.R., Godfrey, M., Maddon, D.E., Chess, L., and Axel, R. 1985. The isolation and nucleotide sequence of a cDNA encoding the T cell surface protein T4: A new member of the immunoglobulin gene family. Cell 42:93-104. McCutchan, J.H. and Pagano, J.S. 1968. Enhancement of the infectivity of simian virus 40 deoxyribonucleic acid with diethylaminoethyl-dextran. J. Natl. Cancer Inst. 40:351-357.
Seed, B. and Aruffo, A. 1987. Molecular cloning of the CD2 antigen, the T-cell erythrocyte receptor, by a rapid immunoselection procedure. Proc. Natl. Acad. Sci. U.S.A. 84:3365-3369. Shih, C. and Weinberg, R.A. 1982. Isolation of a transforming sequence from a human bladder carcinoma cell line. Cell 29:161-169. Sims, J.E., March, C.J., Cosman, D., Widmer, M.B., MacDonald, H.R., McMahan, C.J., Grubin, C.E., Wignall, J.M., Jackson, J.L., Call, S.M., Friend, D., Alpert, A.R., Gillis, S., Urdal, D.L., and Dower, S.K. 1988. cDNA expression cloning of the IL-1 receptor, a member of the immunoglobulin superfamily. Science 241:585-589. Tsai, S.F., Martin, D.I., Zon, L.I., D’Andrea, A.D., Wong, G.G., and Orkin, S.H. 1989. Cloning of cDNA for the major DNA-binding protein of the erythroid lineage through expression cloning. Nature (Lond.) 339:446-451. Wong, G.G., Witek, J.S., Tempel, P.A., Wilkens, K.M., Leary, A.C., Luxenberg, D.P., Jones, S.S., Brown, E.L., Kay, R.M., Orr, E.C., Shoemaker, C., Golde, D.W., Kaufman, R.J., Hewick, R.M., Wang, E.A., and Clark, S.C. 1985. Human GMCSF: Molecular cloning of the complementary DNA and purification of the natural and recombinant proteins. Science 228:810-815. Wysocki, L.J. and Sato, V.L. 1978. Panning for lymphocytes: A method for cell selection. Proc. Natl. Acad. Sci. U.S.A. 75:2844-2848. Screening of Recombinant DNA Libraries
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Yamasaki, K., Taga, T., Hirata, Y., Yawata, H., KaWanishi, Y., Seed, B., Taniguchi, T., Hirano, T., and Kishimoto, T. 1988. Cloning and expression of the human interleukin-6 (BSF-2/ IFNβ 2) receptor. Science 241:825-828.
Metzelaar et al., 1991; Munro and Maniatis, 1989. See above.
Key References
Contributed by Diane Hollenbaugh and Alejandro Aruffo (cell-surface and intracellular antigens)
Aruffo and Seed, 1987; Seed and Aruffo, 1987. See above. Contain original descriptions of cDNA library construction in CDM8 and isolation of cDNA clones encoding cell-surface antigens by expression cloning.
Contain descriptions of growth of COS cells on wrap and screening for extracellular ligands.
Bryan Jones and Peter Linsley (intracellular antigens) Bristol-Myers Squibb Seattle, Washington
Use of Monoclonal Antibodies for Expression Cloning
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Recombination-Based Assay (RBA) for Screening Bacteriophage Lambda Libraries The recombination-based assay represents a convenient way to screen a complex library constructed in bacteriophage λ for homology to a given sequence cloned into a specially designed plasmid. The technique serves to screen a bacteriophage library rapidly and efficiently with a sequence cloned into a plasmid; counterselection then yields the gene product of interest with its plasmid carrier deleted. Because 106 to 107 plaque-forming units (pfu) may be screened using several petri dishes, and the homology for crossing-over need only be >25 bp, the RBA represents an efficient way to screen complex λ libraries rapidly for homology to a given sequence.
UNIT 6.12
BASIC PROTOCOL
In this procedure (outlined in Fig. 6.12.1), a λ library is screened using a specially designed R6K supF plasmid, pAD1 (Fig. 6.12.2), carrying the desired target sequence. Recombinants arising from cross-over events between the plasmid and a bacteriophage carrying a corresponding region of homology are selected by their ability to grow on strain DM21 (Fig. 6.12.3). Growth of λ on DM21 requires the presence of the supF allele encoded on the plasmid to suppress an amber mutation in the host strain that prevents λ propagation. Recovery of the original phage carrying the target sequence requires a reversal of the homologous recombination event. This reversal occurs spontaneously, and is detected by PCR amplification using primers that flank the cloning site in the λ vector (Fig. 6.12.4).
supF
supF
Kmr
Kmr
ori ori DM21 blue plaque lacZam P1 ban dnaBam co
s
cos
λ-plasmid chimera colorless plaque plasmid cos cos
λ phage
human probe
cos cos
phage
Figure 6.12.1 The recombination-based assay (RBA). Homology between sequences in a plasmid and a bacteriophage >25 bp long (Watt et al., 1985; Shen and Huang, 1986, 1989; King and Richardson, 1986) mediates a recombination event between the two vectors. As a result supF is integrated into the bacteriophage, allowing it to plate on the dnaBam host DM21 (see Table 6.12.1). The cointegrate yields a blue plaque in the presence of IPTG and Xgal on the lacZam host DM21, as supF suppresses the amber mutations in both the dnaB and lacZ genes. Different shadings indicate origins of DNA regions. Contributed by David M. Kurnit Current Protocols in Molecular Biology (1994) 6.12.1-6.12.12 Copyright © 2000 by John Wiley & Sons, Inc.
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Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
DNA fragment encoding sequence of interest Plasmid pAD1 (Fig. 6.12.2; available from Dr. D. Kurnit) recA+ E. coli strain (Table 1.4.5 or commercial suppliers) L broth (see recipe) with 50 µg/ml kanamycin (Table 1.4.1) Bacteriophage λ library (UNIT 5.8) Lambda top agar (see recipe) Lambda plates (see recipe), some with 50 µg/ml kanamycin and some with 100 µg/ml streptomycin (Table 1.4.1) Suspension medium (SM; see recipe) Chloroform E. coli DM21, DM75, DM392, and DM1061 (Fig. 6.12.3 and Table 6.12.1), saturated overnight cultures freshly grown in LB medium (UNIT 1.1) with 100 µg/ml streptomycin 100 mM IPTG (isopropyl thiogalactoside; Table 1.4.2) 2% Xgal in DMF (see recipe) Additional reagents and equipment for subcloning DNA into plasmids (UNIT 3.16), culturing (UNIT 1.1) and transformation (UNIT 1.8) of bacteria, plating and titering λ phage (UNIT 1.11), β-galactosidase assay (UNIT 1.4), and PCR amplification (UNIT 15.1) NOTE: All incubations are at 37°C unless otherwise specified. Screen library and select recombinants 1. Clone the sequence of interest into a pAD1 plasmid and transform into recA+ E. coli strain yielding a kanamycin-resistant recA+ strain. Prepare a saturated overnight culture grown with aeration in L broth containing 50 µ/ml kanamycin.
EcoRI Sfi l
polylinker Notl Sacll Pstl Pvull
supF
BamHl
pAD1 4 kb Kmr R6K ori RBA for Screening Bacteriophage Lambda Libraries
Figure 6.12.2 Structure of pAD1. This plasmid incorporates the R6K replicon, Kmr, supF, and a polylinker. It is not homologous to ColE1 plasmids.
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2. Mix 3 ml lambda top agar, 200 µl of overnight culture, and 106 to 107 pfu of a bacteriophage λ library. Mix well and pour mixture onto a lambda/kanamycin plate. Incubate 7 hr to overnight until total lysis occurs. If more convenient, incubation overnight is perfectly acceptable, because there is no need to harvest the plates just as lysis occurs.
3. Add 3 ml SM and 0.5 ml chloroform to each plate. Swirl lightly. Incubate 2 hr to overnight at room temperature to allow the plates to elute. SM and chloroform are immiscible; swirling them together ensures that the SM is saturated with chloroform, killing any eluted bacteria and minimizing phage adsorption to bacterial debris. The easiest method is to rotate a stack of plates slowly by hand after adding the liquid. Care should be taken not to get chloroform on the petri dish cover, as this can cause fusion of the cover and the plate bottom. If fusion occurs, the cover can be pried from the bottom (e.g., with a screwdriver).
4. Using a nonsterile disposable transfer pipet, harvest the eluate from each plate into a 1.5-ml polypropylene microcentrifuge tube. Although the transfer pipets are polyethylene, they hold chloroform-saturated SM for too short a time-span to be damaged by the solvent. At this stage harvested eluates can be stored ≤1 week at 4°C before continuing the procedure.
5. Add 50 µl of eluate (5 × 108 to 1 × 109 pfu) to 200 µl DM21 culture. Add 3 ml lambda top agar and pour mixture onto a lambda/streptomycin plate. Incubate 7 hr to overnight until plaques form. DM21 is selective (dnaBam lacZam) and resistant to streptomycin. DM75, DM1061, and DM392 (used in later steps) are also streptomycin-resistant, with growth and plating conditions identical to those for DM21.
supF dnaBam
Kmr
ori
lacZam λ-plasmid chimera
tonA co
s
cos
bacterial chromosome
λ imm21 P1 ban
plasmid human probe
cos cos
phage
Figure 6.12.3 Bacterial strain DM21 (outer rectangle) containing λ plasmid chimera with supF integrated (inner circle). DM21 has the genotype lacZYA536(am), dnaB266(am), Smr, hsdR+, hsdM+, tonA− (λ imm21 b515 b519 nin5 att+P1 ban), supO lacZ(am) dnaB(am). The dnaB amber allele selects for λ phage that have supF integrated as shown. SupF also suppress the lacZ amber mutation, yielding blue plaques. Different shadings indicate origins of DNA regions.
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Titer eluates on permissive strain 6. Add 10 µl of each eluate to be titered to 990 µl SM to obtain a 1/100 dilution. Prepare a 100-fold dilution series (to 10−8) in SM. Several random eluates should be titered on the permissive (supF-bearing) strain DM392 to ensure that an appropriate number of phage have been added to the DM21 lawn.
7. Pour a lawn of DM392 (200 µl culture in 3 ml top agar) on a lambda/streptomycin plate. Drop 10-µl aliquots of each eluate dilution onto lawn. Dry 15 min in a forced-air hood (or for longer on bench or in incubator). Incubate 7 hr to overnight until total lysis occurs. This drop-titer procedure is the most convenient method of titering the eluates.
8. Count plaques in the lowest dilution that yields plaques. Convert the result to pfu/ml by multiplying it by the appropriate dilution factor and by a factor of 102. Titration ensures that sufficient phage have been added to the DM21 lawn. Should too many be added (more pfu than cells), the lawn will not materialize due to lysis from without. This phenomenon occurs because every cell that is infected with a bacteriophage will die, even though only cells infected by a phage carrying supF will yield a productive burst that then goes on to infect other cells. In rare cases of lysis from
supF
supF Kmr
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RBA for Screening Bacteriophage Lambda Libraries
cos
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Figure 6.12.4 Counterselection. Reversal of the recombination event (which is an equilibrium event) occurs spontaneously. PCR using primers abutting the cloning site of the bacteriophage is employed preparatively to obtain the cDNA without the genomic sequence in pAD1 that was used to retrieve it. The cDNA insert + pAD1 + genomic insert is too large to be amplified by PCR; in contrast, the cDNA insert alone can be amplified. Because there is an equilibrium between the selected and the counterselected phage, the counterselected insert can be amplified directly from the selected blue plaque, which contains a mixture of the two phages. Different shadings indicate origins of DNA regions.
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without, the plating should be repeated; either the eluate should be titered or less eluate used. Plaques on DM21 are very small, because suppression of the dnaBam mutation (which is not fully efficient) is required for growth. This makes it difficult to confirm that supF is present via simultaneous suppression of the lacZam mutation by supF; therefore, phage must be transferred to another strain as described in the following steps.
Confirm phages have integrated supF 9. Elute plaques on DM21 (from step 5) into 100 µl SM. Mix: 10 µl eluate 200 µl DM75 culture 3 ml lambda top agar 10 µl 100 mM IPTG 100 µl 2% Xgal in DMF. Plate on lambda/streptomycin plates. Incubate 7 hr to overnight until total lysis occurs. To mix water and DMF, the tubes of top agar must be inverted and righted several times, taking care not to create bubbles. It is best not to prepare more than several tubes at once, because cells do not tolerate the heating block for very long. Light blue plaques are the desired phage containing supF. A larger number of colorless plaques that have not integrated supF will also plate on this strain; these correspond to phage that were not adsorbed originally on DM21 and therefore remain viable. In addition, for a phage such as λgt11, in which interruption of an intact lacZ gene serves as evidence of successful cloning, blue color can result from an intact lacZ gene in the phage. To differentiate between the two, note that the desired supF suppression of the single-copy chromosomal lacZ locus results in a light blue color that extends only to the plaque margins, whereas the high-copy-number lacZ gene on λgt11 yields a dark blue halo that extends past the plaque margins.
10. Elute each plaque thought to contain an integrated supF (from step 9) into 100 µl SM. Pour lawns of DM75 and DM1061 (200 µl culture in 3 ml top agar/IPTG/Xgal, as in previous step) onto separate lambda/streptomycin plates. Drop 10-µl aliquots of each phage eluate onto a lawn of each strain. Incubate 7 hr to overnight until total lysis occurs. This serves to confirm that plaques result from phage with supF rather than lacZ. Phage with supF will be blue on DM75 (lacZam) but colorless on DM1061 (which contains a lacZ deletion), whereas phage carrying an intact lacZ gene will be blue on both strains.
Counterselect with PCR 11. Pour a lawn of 200 µl DM75 in 3 ml top agar onto lambda/streptomycin plate. Drop 10-µl aliquots of phage eluate onto lawn. Incubate 7 hr to overnight, until a single large plaque (“macroplaque”) appears. 12. PCR amplify the cloned product from the macroplaque using primers that abut the EcoRI cloning site of the λ phage vector used to construct the library. This reverses the selection process and accomplishes counterselection (see Fig. 6.12.4). Using the large macroplaque ensures that sufficient template is present. Because the recombination reaction is an equilibrium reaction, a small fraction of phage within a blue macroplaque represent colorless revertants that have excised the pAD1 plasmid and its insert. In contrast, the major product in the macroplaque carries the phage insert, the plasmid, and the insert. Because this is too large to be amplified efficiently by PCR, the technique preferentially yields the desired genic insert from the phage without the unwanted plasmid and its insert.
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13. If desired, sequence the isolated genic clone (UNITS 7.1-7.5) and compare it to a database of known expressed sequences (UNIT 7.7) to obtain information about its possible significance, if available. Repeatedly performing this protocol with different cDNA libraries allows determination of the timing of development and the tissue(s) in which the gene of interest is expressed. The latter can also be determined by using PCR primers specified by the sequence to see if amplification of different cDNA libraries occurs; given the sensitivity of this method, only cDNA library eluates, rather than DNA preparations, need be screened.
REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Lambda plates 10 g tryptone 5 g NaCl 13 g agar 3 ml 1 M MgCl2 H2O to 1 liter Sterilize by autoclaving. Allow to cool until comfortable to touch. Add antibiotics as needed, mix gently to avoid bubbles, and pour plates. Store up to several months at 4°C. Lambda top agar 10 g tryptone 5 g NaCl 8 g agar 3 ml 1 M MgCl2 H2O to 1 liter Sterilize by autoclaving. Maintain ≤1 month molten at 60°C. L broth 10 g tryptone 5 g NaCl 5 g yeast extract 5 g MgSO4⋅7H2O 1 g glucose 160 ml 12.5 M NaOH (to pH 7.2) H2O to 1 liter Sterilize by autoclaving. Allow to cool until comfortable to touch. Add antibiotics as needed and mix. Store up to several months at 4°C.
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Suspension medium (SM) 5.8 g NaCl 2 g MgSO4⋅7H2O 50 ml 1 M Tris⋅Cl, pH 7.5 (APPENDIX 2) 5 ml 2% (w/v) gelatin H2O to 1 liter Sterilize by autoclaving. Store up to several months at 4°C. Gelatin is prepared by adding 2 g gelatin to 100 ml H2O, then autoclaving to dissolve when needed.
Xgal, 2% (v/v) in DMF Dissolve 2% Xgal (5-bromo-4-chloro-3-indolyl-β-D-galactoside; see Table 1.4.2) in dimethylformamide (DMF). Place in polypropylene tube (not polystyrene, which will be dissolved by DMF), wrapped in aluminum foil. Store indefinitely at −20°C (solution will not freeze). COMMENTARY Background Information The recombination-based assay (RBA) permits screening of a complex library or group of libraries with a given probe using only two petri dishes. As a result, the RBA is unparalleled in its efficiency and speed. The crux of the RBA is the insertion of a DNA fragment into a plasmid containing supF, followed by screening of a complex λ library (106 to 107 recombinants) for homology to the fragment. If such homology exists, a recombination event ensues between the inserts in the plasmid and homologous phage at a frequency of 10−2 to 10−3 (see Fig. 6.12.1). As a result of this homology-mediated recombination event, the plasmid with supF is integrated into λ. Genetic selection for λ phage carrying the plasmid with supF results in the isolation of λ phage carrying an insert homologous to the insert in the plasmid. Given the high frequency of homologous recombination (10−2 to 10−3), and the fact that 5 × 108 to 109 pfu can be plated onto a single petri dish, it is feasible to screen rapidly a λ library with a complexity of 106 to 107. Bacterial host characteristics This assay employs a bacterial strain, DM21 (see Figs. 6.12.1 and 6.12.3), that has been constructed to require the presence of supF in λ for phage propagation. As a result, sequences from a λ library that are homologous to a sequence cloned into the supF-bearing plasmid can be isolated on this strain. By screening a λ library carrying human genomic DNA sequences (Lawn et al., 1978), the copy number of a given sequence can be determined analytically. Plasmids carrying repetitive sequences rescue more phage clones from a human
genomic library than do plasmids carrying nonrepetitive sequences (Neve and Kurnit, 1983). By screening a λ library corresponding to the genes encoded by a given tissue with singlecopy sequences, the tissue and time in which a single-copy sequence is transcribed can be determined analytically. Selection for the desired supF-bearing phage is done using the dnaB/P1 ban balanced lethal system. In constructing the host, the dnaB unwinding protein that is normally essential for λ phage growth was replaced by the related, but not identical, P1 ban gene for E. coli growth. The resulting streptomycin-resistant dnaBam P1 ban lacZam host, DK21 (Kurnit and Seed, 1990), was then protected against a contaminating large (?T1) phage infection by a ?tonA mutation to yield the strain DM21 that is used in the protocol (the question mark notes characteristics that are likely but not definite). Analogously, strains LE392, LG75, and MC1061 have each been altered to carry a ?tonA mutation and resistance to streptomycin for use in the protocol, and have been renamed DM392, DM75, and DM1061, respectively (Table 6.12.1). DM21 selects for the plasmid-borne supF by requiring the suppression of an amber mutation in the dnaB gene to permit λ propagation. Furthermore, supF also suppresses the amber mutation in the lacZ gene of DM21, yielding a blue plaque upon addition of the chromogenic substrate Xgal in the presence of IPTG. This makes it possible to discard rare (100 copies; Neve and Kurnit, 1983). 2. Determining tissue- and time-specific transcriptional activity of single-copy fragments and isolating genes. Gene libraries containing >106 independent recombinants are constructed: each corresponds to the totality of genes made in a given tissue at a given time in development. Screening a pool of 106 recombinants from a cDNA library requires only two petri dishes. The phage are first plated on a bacterial lawn carrying the sequence to be tested cloned in a supF-bearing plasmid. Following confluent lysis, 5 × 108 to 5 × 109 pfu are eluted and plated on DM21 to select for phage that have integrated supF. If no phage plaques are observed on DM21, this indicates that the sequence is not transcribed in the tissue
at the developmental stage present when the cDNA libraries were made. If plaques are observed on DM21, this indicates that the sequence is transcribed at that stage. The transcribed sequence is isolated free of the genomic sequence initially used to screen for it by reversing the recombination event (Fig. 6.12.4). In all the libraries used to date—λgt10 (Huynh et al., 1985), λgt11 (Young and Davis, 1983), and Sumo 15A (Kurachi et al., 1989)—the desired sequence is liberated as an EcoRI fragment that can be subcloned. As well as liberating the sequence, the reversal also makes it possible to discard rare nonhomologous (or imperfect) recombination events, which are identified by the fact that they reverse at the same low 10−9 frequency that they occur (for nonhomologous events) and at an intermediate frequency (for partially homologous events). In contrast, homologous recombination events, which can occur in a forward direction at a similar 10−8 frequency (assuming a worst case where a sequence is present only once per genome equivalent in a phage library of 106 recombinants, which is multiplied by a 10−2 chance of recombining if there is homology), reverse at a much higher 10−2 frequency. Thus, reversal of the recombination reaction will yield the cDNA free of the genomic sequence and will simultaneously allow rarer nonhomologous or partially homologous exchange events to be identified and discarded. The RBA can be employed to determine the tissue and time of transcription of candidate genes discovered by other technologies as well as to obtain the gene of interest (in the form of the larger gene sequence that is transcribed). The technique is useful either alone or in combination with other methods for defining single-copy transcribed sequences. If DNA sequencing (as part of the genome initiative) or techniques to define transcribed sequences are used to identify genes, the RBA is still useful for determining the tissue and developmental timing of transcription, as well as for isolating a larger gene of interest. Technologies for defining transcribed sequences include exon trapping/amplification (Nisson and Watkins, 1994; Duyk et al., 1990; Buckler et al., 1991), use of somatic cell hybrids (Liu et al., 1989), and the use of hybridization-based schemes (Hochgeschwender and Brennan, 1994; Hochgeschwender et al., 1989; Kao and Yu, 1991), including hybrid selection (Lovett, 1994; Lovett et al., 1991; Parimoo et al., 1991). The RBA will proceed cooperatively, rather than competitively, with these other methods be-
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cause it efficiently accomplishes two necessary tasks: identifying the timing and tissue of gene transcription and isolating a large transcribed sequence.
Critical Parameters Plaque size is a major issue in this assay because plaques on the dnaB am strain DM21 are so small. Fresh λ plates should be used to maximize plaque size, because plaques will be smaller on older (drier) plates; likewise, it is important to plate cells on lambda plates, because plaques will be smaller on richer (e.g., LB) plates. It is essential that there be no homology between the screening plasmid and sequences in the λ libraries (see Background Information). Therefore, screening should be performed solely with R6K supF plasmids, not with ColE1 supF plasmids. Although titering all eluates would be too time-consuming, a few eluates should be titered to ensure that lysis and elution are occurring as expected. This is especially important because a lysed plate may vary from clear to grainy, rendering it difficult to determine visually whether complete lysis has occurred. Eluates should be saved until the DM21 plates have been scored as a precaution in case too many phage have been added, resulting in lysis from without. If this happens, the eluate may be titered or a lesser amount plated on DM21.
Anticipated Results
The abundance of sequences in screened λ libraries should be reflected in the number of phage that plate on DM21. Assuming a recombination rate of 1/500 (the exact number that will depend on the extent of homology), a sequence abundance of 1/106 should yield one plaque on DM21 per 5 × 108 phage plated. A higher abundance should yield a correspondingly greater number of plaques on DM21. If mismatching occurs in an interspersed “saltand-pepper” manner (as for Alu sequences), recombination will be depressed (e.g., ∼1000fold for Alu sequences; Neve et al., 1983).
Time Considerations The major advantage of the RBA is its rapidity: selection can be completed in four days using the following schedule. Day 1, grow bacterial cultures; day 2, add λ library and perform lysis; day 3, elute and plate on DM21; and day 4, identify plaques on DM21. Counterselection takes an additional four days. One day is necessary for elution of
plaques from DM21 that are plated on DM75 with IPTG and Xgal in top agar. A second day is required for elution of putative light blue plaques and confirmatory macroplaque plating on DM75 and DM1061 with IPTG and Xgal. PCR counterselection of macroplaques that are blue on DM75 and colorless on DM1061 takes one day and a final day is necessary to isolate the counterselected PCR band from the gel.
Literature Cited Bolivar, F., Rodriguez, R., Green, P.J., Betlach, M., Heyneker, H.L., Boyer, H.W., Crosa, J., and Falkow, S. 1977. Construction and characterization of new cloning vehicles. Gene 2:95113. Buckler, A.J., Chang, D.D., Graw, S.L., Brook, J.D., Haber, D.A., Sharp, P.A., and Housman, D.E. 1991. Exon amplificaton: A strategy to isolate mammalian genes based on RNA splicing. Proc. Natl. Acad. Sci. U.S.A. 88:4005-4009. Casadaban, M.J. and Cohen, S.N. 1980. Analysis of gene control signals by DNA fusion and cloning in Escherichia coli. J. Mol. Biol. 138:179-207. Duyk, G.M., Kim, S., Myers, R.M., and Cox, D.R. 1990. Exon trapping: A genetic screen to identify candidate transcribed sequences in cloned mammalian genomic DNA. Proc. Natl. Acad. Sci. U.S.A. 87:8995-8999. Guarente, L., Lauer, G., Roberts, T.M., and Ptashne, M. 1980. Improved methods for maximizing expression of a cloned gene: A bacterium that synthesizes rabbit β-globin. Cell 20:543-553. Hanzlik, A.J., Hauser, M.A., Osemlak-Hanzlik, M.M., and Kurnit, D.M. 1993. The recombination-based assay demonstrates that the fragile X sequence is transcribed widely during development. Nature Genet. 3:44-48. Hochgeschwender, U. 1994. Identifying transcribed sequences in arrayed bacteriophage or cosmid libraries. In Current Protocols in Human Genetics (Dracopoli, N., Haines, J.L., Korf, B., Moir, D.T., Morton, C.M., Seidman, C.E., Seidman, J.G., and Smith, D.R., eds.) pp. 6.2.1-6.2.15. John Wiley & Sons, New York. Hochgeschwender, U., Sutcliffe, J.G., and Brennan, M.D. 1989. Construction and screening of a genomic library specific for mouse chromosome 16. Proc. Natl. Acad. Sci. U.S.A. 86:8482-8486. Huynh, T., Young, R.A., and Davis, R.W. 1985. Constructing and screening cDNA libraries in λgt10 and λgt11. In DNA cloning, Vol. II (D. Glover, ed.). IRL Press, Eynsham, U.K. Ikeda, H., Aoki, K., and Naito, A. 1982. Illegitimate recombination mediated in vitro by DNA gyrase of Escherichia coli: Structure of recombinant DNA molecules. Proc. Natl. Acad. Sci. U.S.A. 79:3724-3728. Jankowski, S., Stewart, G.D., Buraczynska, M., Galt, J., Van Keuren, M., and Kurnit, D.M. 1990. Molecular approaches to trisomy 21. Prog. Clin. Biol. Res. 360:79-88.
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Kao, F.-T. and Yu, J.-W. 1991. Chromsome microdissection and cloning in human genome and genetic disease analysis. Proc. Natl. Acad. Sci. U.S.A. 88:1844-1848. King, S.R. and Richardson, J.P. 1986. Role of homology and pathway specificity for recombination between plasmids and bacteriophage λ. Mol. Gen. Genet. 204:141-147. Kurachi, S., Baldori, N., and Kurnit, D.M. 1989. Sumo 15A: A lambda plasmid that permits easy selection for and against cloned inserts. Gene 85:35-43. Kurnit, D.M. and Seed, B. 1990. Improved genetic selection for screening bacteriophage libraries by homologous recombination in vivo. Proc. Natl. Acad. Sci. U.S.A. 87:3166-3169.
Parimoo, S., Patanjali, S.R., Shukla, H., Chaplin, D.D., and Weissman, S.M. 1991. cDNA selection: Efficient PCR approach for the selection of cDNAs encoded in large chromosomal DNA fragments. Proc. Natl. Acad. Sci. U.S.A. 88:9623-9627. Poustka, A., Rackwitz, H.-R., Frischauf, A., Hohn, B., and Lehrach, H. 1984. Selective isolation of cosmid clones by homologous recombination in Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 81:4129-4133. Rubin, C.M., Houck, C.M., Deininger, P.L., and Schmid, C.W. 1980. Partial nucleotide sequence of the 300 nucleotide interspersed repeated human DNA sequences. Nature 284:372-374.
Lawn, R.M., Fritsch, E.H., Parker, R.C., Blake, G., and Maniatis, T. 1978. The isolation and characterization of linked δ- and β-globin genes from a cloned library of human DNA. Cell 15:11571174.
Saiki, R.K., Scharf, S., Faloona, F., Mullis, K.B., Horn, G., Erlich, H.A., and Arnheim, N. 1985. Enzymatic amplification of β-globin genomic sequences and restriction site analysis for diagnosis of sickle cell anemia. Science 230:13501354.
Liu, P., Legerski, R., and Siciliano, M.J. 1989. Isolation of human transcribed sequences from human-rodent somatic cell hybrids. Science 246:813-815.
Seed, B. 1983. Purification of genomic sequences from bacteriophage libraries by recombination and selection in vivo. Nucl. Acids Res. 11:24272445.
Lovett, M. 1994. Direct selection of cDNAs using genomic contigs. In Current Protocols in Human Genetics (Dracopoli, N., Haines, J.L., Korf, B., Moir, D.T., Morton, C.M., Seidman, C.E., Seidman, J.G., and Smith, D.R., eds.) pp. 6.3.16.3.15. John Wiley & Sons, New York.
Shen, P. and Huang, H.V. 1986. Homologous recombination in Escherichia coli: Dependence on substrate length and homology. Genetics 112:441-457.
Lovett, M., Kere, J., and Hinton, L.M. 1991. Direct selection: A method for the isolation of cDNAs encoded by large genomic regions. Proc. Natl. Acad. Sci. U.S.A. 88:9628-9632. Lutz, C.T., Hollifield, W.C., Seed, B., Davie, J.M., and Huang, H.V. 1987. Syrinx 2A: An improved λ phage vector designed for screening DNA libraries by recombination in vivo. Proc. Natl. Acad. Sci. U.S.A. 84:4379-4383.
Shen, P. and Huang, H.V. 1989. Effect of base pair mismatches on recombination via the recBCD pathway. Mol. Gen. Genet. 218:358-360. Short, J.M., Fernandez, J.M., Sorge, J.A., and Huse, W.D. 1988. λ ZAP: A bacteriophage λ expression vector with in vivo excision properties. Nucl. Acids Res. 16:7583-7599. Stewart, G.D., Hauser, M.A., Kang, H., McCann, D.P., Osemlak, M.M., Kurnit, D.M., and Hanzlik, A.J. 1991. Plasmids for recombination-based screening. Gene 106:97-101.
Marvo, S.L., King, S.R., and Jaskunas, S.R. 1983. Role of short regions of homology in intermolecular illegitimate recombination events. Proc. Natl. Acad. Sci. U.S.A. 80:2452-2456.
Watt, V.M., Ingles, C.J., Urdea, M.S., and Rutter, W.J. 1985. Homology requirements for recombination in Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 82:4768-4772.
Neve, R.L. and Kurnit, D.M. 1983. Comparison of sequence repetitiveness of human cDNA and genomic DNA using the miniplasmid vector piVX. Gene 23:355-367.
Yanisch-Perron, C., Vieira, J., and Messing, J. 1985. Improved M13 phage cloning vectors and host strains: Nucleotide sequences of the M13mp18 and pUC19 vectors. Gene 33:103-119.
Neve, R.L., Bruns, G.A.P., Dryja, T.P., and Kurnit, D.M. 1983. Retrieval of human DNA from rodent-human genomic libraries by a recombination process. Gene 23:343-354.
Young, R.A. and Davis, R.W. 1983. Efficient isolation of genes by using antibody probes. Proc. Natl. Acad. Sci. U.S.A. 80:1194-1198.
Nisson, P.E. and Watkins, P.C. 1994. Isolation of exons from cloned DNA by exon trapping. In Current Protocols in Human Genetics (Dracopoli, N., Haines, J.L., Korf, B., Moir, D.T., Morton, C.M., Seidman, C.E., Seidman, J.G., and Smith, D.R., eds.) pp. 6.1.1-6.1.14. John Wiley & Sons, New York.
Contributed by David M. Kurnit University of Michigan Medical Center Ann Arbor, Michigan
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CHAPTER 7 DNA Sequencing INTRODUCTION For many recombinant DNA experiments, knowledge of a DNA sequence is a prerequisite for its further manipulation. DNA sequencing followed by computer-assisted searching for restriction endonuclease cleavage sites is often the fastest method of obtaining a detailed restriction map (UNITS 3.1-3.3). This information is particularly useful when vectors designed to overexpress proteins or to generate protein fusions are used to subclone a gene of interest (Chapter 16). Computer-assisted identification of protein-coding regions (open reading frames, or ORF) within the DNA sequence, followed by computer-assisted similarity searches of DNA and protein data bases, can lead to important insights about the function and structure of a cloned gene and its product (UNIT 7.7). In addition, the DNA sequence is a prerequisite for a detailed analysis of the 5′ and 3′ noncoding regulatory regions of a gene. DNA sequence information is essential for sitedirected mutagenesis (UNIT 8.1). Small amounts of DNA sequence information (sequence tagged sites, or STS; or expressed sequence tags, or ESTs) are the basis of methods for mapping and ordering large DNA segments cloned into yeast or bacterial artificial chromosomes (CYACs; BACs) or cosmids (Olson et al., 1989; Stephens et al., 1990; Green and Olson, 1990; Adams et al., 1991; Shizuya et al., 1992). EST databases are extremely valuable in gene discovery. DNA sequencing techniques are primarily based on electrophoretic procedures using high-resolution denaturing polyacrylamide gels. These so-called sequencing gels are capable of resolving single-stranded oligonucleotides up to 800 bases in length that differ in size by a single deoxynucleotide. In practice, for a given region to be sequenced, a set of labeled, single-stranded oligonucleotides is generated whose members have one fixed end, and which differ at the other end by each successive deoxynucleotide in the sequence. The key to determining the sequence of deoxynucleotides is to generate, in four separate enzymatic or chemical reactions, all oligonucleotides that terminate at the variable end in
A, T, G, or C. The oligonucleotide products of the four reactions are then resolved on adjacent lanes of a sequencing gel. Because all possible oligodeoxynucleotides are represented among the four lanes, the DNA sequence can be read directly from the four “ladders” of oligonucleotides as shown in Figure 7.0.1. In automated four-color fluorescent DNA sequencers, the oligonucleotide products terminated from each of the four bases (A, C, G, T) are run in a single lane and resolved on the basis that the DNA fragments ending with each of the four bases are labeled with four different fluorescent tags. The practical limit on the amount of information that can be obtained from a set of sequencing reactions is the resolution of the sequencing gel (see UNIT 7.6 for protocols on setting up and running sequencing gels). Current technology allows ∼500 nucleotides of sequence information to be reliably obtained in one set of sequencing reactions, although more information (up to 800 nucleotides) is often obtained using an automated sequencer. Thus, if the region of DNA to be sequenced is 30 kb), such as BAC clones, is very important for closing the gaps in large genomic sequencing and mapping projects. Such large templates are very difficult to
488 nm
Solid-Phase Sequencing Another recent innovation that is applicable to both manual and automated DNA sequencing is the use of solid-phase capture strategy to generate single-stranded DNA templates (Hultman et al., 1989, 1991; Jones et al., 1991; Kaneoka et al., 1991; Zimmerman et al., 1992).
energy transfer
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donor
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Introduction
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555 580 Wavelength (nm)
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Figure 7.0.5 In ET primers, a common donor with a high absorbance at the excitation wavelength harvests energy and transmits it efficiently to acceptor fluorophores that emit in distinct wavelength regions. To avoid fluorescence quenching, the donor and acceptor are separated by a spacer that can be an oligonucleotide or other chemical functionality. The fluorescence emission intensity of current ET primers is 2- to 24-fold higher than that of conventional single dye-labeled primers, leading to high-quality DNA sequencing data.
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In this approach, one strand of a doublestranded DNA molecule is biotinylated (e.g., by amplification using PCR in which one of the two primers is biotinylated; Chapter 15). The hemibiotinylated DNA molecule is then bound to streptavidin-ferromagnetic beads. The strands are denatured by treating the beads with alkali and the biotinylated strands are separated from the nonbiotinylated strands using a magnet that traps the bead complex to which the biotinylated strands are bound. Sequencing reactions can be performed using either the biotinylated strand-bead complex or the nonbiotinylated strand preparation as the template. Fluorescent sequencing procedures (both dye primer and dye terminator methods) have disadvantages, most notably false termination and background noise. In the dye primer method, all the extended DNA fragments—including false-terminated fragments—from the primer carry a fluorescent dye, and thus all are detected by the fluorescent sequencer. This causes background noise and results in inaccurate sequencing data. In the dye terminator method, the excess dye-labeled dideoxynucleotides need to be cleaned up completely. Furthermore, if RNAs and nicked DNAs are present in the DNA templates, they will act as primers to generate false termination or high background noise. Thus, a DNA sequencing method that overcomes these disadvantages is desirable. A sequencing chemistry using solid-phasecapturable dideoxynucleotides was recently developed that produces much cleaner sequencing data on both slab gel and capillary array sequencers, eliminating the disadvantages of current dye primer and dye terminator chemistries (Ju et al., 1997; Ju, 1999). The procedure involves coupling fluorescent ET primers that produce high fluorescent signals with solid phase–capturable terminators such as biotinylated dideoxynucleotides. After the sequencing reaction, the extension DNA fragments are captured with magnetic beads coated with streptavidin, while the other components in the sequencing reaction are washed away. Only the pure dideoxynucleotide terminated extension products are released from the magnetic beads and loaded on the sequencing gel, producing high-quality data.
Sequencing with Mass Spectrometry Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF MS; UNITS 10.21 & 10.22) has recently been explored for DNA sequencing (Koster et al.,
1996; Monforte et al., 1997; Fu et al., 1998). The Sanger dideoxy procedure is used to generate the DNA sequencing fragments and no labels are required. The fragments are mixed with a matrix compound, 3-hydroxypicolinic acid, forming microcrystals on a flat plate. The sample plate is then placed in the vacuum chamber of the mass spectrometer. Upon irradiation by a laser light, a microscopic portion of the matrix molecules desorbs off the plate, entraining the DNA fragments. In an electric field, the charged DNA molecules (ions) are accelerated toward the detector, which measures the mass of the fragments based on their time of flight, which is inversely proportional to their mass. Since the mass of each nucleotide (A, C, G, T) is different, the mass difference between adjacent peaks in the mass spectrum is used to establish the sequence of the DNA templates. For example, if the mass difference between the two adjacent peaks in a mass spectrum of an unknown DNA is the mass of “A” base, then the peak with the higher mass is identified as an “A”. Thus simple computation software is all that is needed to assemble the sequence of the DNA from its mass spectrum. Compared to gel-based sequencing systems, MS produces very high resolution of the sequencing fragments (sometimes as good as 1 Da) and extremely fast separation, on a time scale of microseconds. The high resolution allows accurate detection of mutations and heterozygosity. Another advantage of sequencing with mass spectrometry is that the compressions associated with gel-based systems are completely eliminated. However, in order for accurate measurements of the masses of the sequencing DNA fragments to be obtained, the sample must be free from alkaline and alkaline earth salts. The samples must therefore be desalted before the MS analysis. Solid-phase procedures using either biotinylated primer or immobilized templates are generally used for desalting (Koster et al., 1996; Monforte and Becker, 1997). Both approaches introduce false-terminated DNA fragments into the mass detector. An elegant method to obtain pure sequencing fragments is to use solid phase–capturable dideoxynucleotides—such as biotinylated terminators—in the Sanger reactions to generate sequencing fragments. In this procedure, only the correctly terminated DNA fragments are isolated by streptavidin-coated beads, which are subsequently released and loaded on the mass spectrometer, resulting in accurate sequencing data (Ju et al., 1997; Ju, 1999). The current limit of mass spectrometry for sequenc-
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ing is in the 100-bp range. Optimized solidphase sequencing chemistry plus improvement in detector sensitivity for large DNA fragments will further improve mass spectrometry for DNA sequencing.
Sequencing by Hybridization Sequencing by hybridization (SBH) makes use of an array of all possible short oligonucleotides to identify a segment of sequences present in an unknown DNA (Drmanac et al., 1989, 1993). This can be clearly explained by the following example. A pentanucleotide 5′ CAGTA-3′, with a complementary sequence of 5′-TACTG-3′ is the sequence that need to be determined from a pool of all possible trinucleotides (43 = 64). This pentanucleotide will specifically hybridize only with the complementary trinucleotides TAC, ACT, and CTG, revealing the presence of these blocs in the complementary sequence. From this the sequence 5′-TACTG-3′ can be reconstructed. Thus with a library of 8- to 10-mer oligonucleotides, much larger segments of DNA sequences can be established. Computational approaches are then used to assemble the complete sequence. In the current state of the art of this technology, SBH has been used for detecting mutations and for resequencing a genome as well as for detecting polymorphisms (Chee et al., 1996; Drmanac et al., 1998). Robust de novo sequencing has not yet been demonstrated. Potential applications of SBH include physical mapping (ordering) of overlapping DNA clones, sequence checking, DNA fingerprinting comparisons of normal and diseasecausing genes, and identification of DNA fragments with particular sequence motifs in complementary DNA and genomic libraries.
COMPUTER ANALYSIS
Introduction
Once the gels are run and autoradiograms are obtained, computer software is practically indispensable for analysis of the sequence information. Computer software can assist at three stages. First, DNA sequence data can be entered into a computer data base either by “reading” the sequencing gels manually with a digitizer system or by using an automated gel scanner. Second, several software packages are available for detecting overlaps in sequence data and then assembling contiguous DNA sequences (contigs) from individual templates. Third, computer assistance is indispensable for analyzing final sequence data, e.g., in finding open reading frames or finding homologies to other sequences present in the nucleotide (UNIT
19.2)
or protein databases (UNIT 19.3). UNIT 7.7 provides an overview of software and technology currently available.
LITERATURE CITED Adams, M.D., Keller, J.M., Gocayne, J.D., Dubnick, M., Polymeropoulos, M.H., Xaio, H., Merril, C.R., Wu, A., Olde, B., Moreno, R.F., Kerlavage, A.R., McCombie, W.R., and Venter, J.C. 1991. Complementary DNA sequencing: Expressed sequence tags and human genome project. Science 252:1651-1656. Adams, S. and Blakesley, R. 1991. Linear amplification sequencing. Focus (BRL) 13:56-57. Ambrose, B. and Pless, R. 1987. DNA sequencing: Chemical methods. Methods Enzymol. 52:522538. Ansorge, W., Sproat, B., Stegemann, J., Schwager, C., and Zenke, M. 1987. Automated DNA sequencing: Ultrasensitive detection of fluorescent bands during electrophoresis. Nucl. Acids Res. 15:4593-4602. Applied Biosystems. 1989 to present. Model 370 and 370A Automated Sequencer User Bulletin. Applied Biosystems, Foster City, Calif. Arnold, W. and Puhler, A. 1988. A family of high copy number plasmid vectors with single end-label sites in rapid nucleotide sequencing. Gene 70:171-179. Beck, S., O’Keefe, T.O., Coull, J.M., and Koster, H. 1989. Chemiluminescent detection of DNA: Application for DNA sequencing and hybridization. Nucl. Acids Res. 17:5115-5123. Brumbaugh, J., Middendorf, L., Grone, D., and Ruth, J. 1988. Continuous on-line DNA sequencing using oligonucleotide primers with multiple fluorophores. Proc. Natl. Acad. Sci. U.S.A. 85:5610-5614. Carothers, A.M., Urlab, G., Mucha, J., Grunburger, D., and Chasin, L.A. 1989. Point mutation analysis in a mammalian gene: Rapid preparation of total RNA, PCR amplification of cDNA and Taq sequencing by a novel method. BioTechniques 7:494-499. Chee, M., Yang, R., Hubbell, E., Berno, A., Huang, X.C., Stern, D., Winkler, J., Lockhart, D.J., Morris, M.S., and Fodor, S.P. 1996. Accessing genetic information with high-density DNA arrays. Science 274:610-614. Chen, E.Y. and Seeburg, P.H. 1985. Supercoil sequencing: A fast and simple method for sequencing plasmid DNA. DNA (N.Y.) 4:165-170. Church, G. and Gilbert, W. 1984. Genomic sequencing. Proc. Natl. Acad. Sci. U.S.A. 81:1991-1995. Church, G. and Kiefer-Higgins, S. 1988. Multiplex DNA sequencing. Science 240:185-188. Connel, C., Fung, S., Heiner, C., Bridgeham, J., Chakerian, V., Heron, E., Jones, B., Menchen, S., Mordan, W., Raff, M., Racknor, M., Smith, L., Springer, J., Woo, S., and Hunkapiller, M. 1987. Automated DNA sequence analysis. BioTechniques 5:342-348.
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Craxton, M. 1991. Linear amplification sequencing: A powerful method for sequencing DNA. Methods, a companion to Methods Enzymol. 3:20-26. Creasey, A., D’Angio, L.M., Dunne, T., Kissinger, C., O’Keefe, T., Perry-O’Keefe, H., Moran, L., Roskey, M., Shildkraut, I., Sears, L., and Slatko, B. 1991. Application of a novel chemiluminescent-based DNA detection method to single-vector and multiplex DNA sequencing. BioTechniques 11:102-109. D’Cunha, J., Berson, B.J., Brumly, R.L., Wagner, P.R., and Smith, L.M. 1990. An automated instrument for the performance of enzymatic DNA sequencing reactions. BioTechniques 9:80-90. Drmanac, R., Labat, I., Brukner, I., and Crkvenjakov, R. 1989. Sequencing of megabase-plus DNA by hybridization: Theory of the method. Genomics 4:114-128. Drmanac, R., Drmanac, S., Strezoska, Z., Paunesku, T., Labat, I., Zeremski, M., Snoddy, J., Funkhouser, W.K., Koop, B., Hood, L., et al. 1993. DNA sequence determination by hybridization: A strategy for efficient large-scale sequencing. Science 260:1649-1652. Drmanac, S., Kita, D., Labat, I., Hauser, B., Schmidt, C., Burczak, J.D., and Drmanac, R. 1998. Accurate sequencing by hybridization for DNA diagnostics and individual genomics. Nature Biotechnol. 16:54-58. Eckert, R. 1987. New vectors for rapid sequencing of DNA fragments by chemical degradation. Gene 51:245-252. Evans, S. 1991. Millipore’s system speeds up DNA sequencing and eliminates radioactivity. Genet. Eng. News 14:29-41. Frank, R., Bosserhoff, A., Boulin, C. Epstein, A. Gausepohl, H., and Ashman, K. 1988. Automation of DNA sequencing reactions and related techniques: A workstation for micromanipulation of liquids. Bio/Technology 6:1211-1213. Fu, D.J., Tang, K., Braun, A., Reuter, D., DarnhoferDemar, B., Little, D.P., O’Donnell, M.J., Cantor, C.R., and Koster, H. 1998. Sequencing exons 5 to 8 of the p53 gene by MALDI-TOF mass spectrometry. Nature Biotechnol. 16:381-384. Fujita, M., Usui, S., Kiyama, M., Kambara, H., Murakawa, K., Suzuki, S., Sambe, H., and Takachi, K. 1990. Chemical robot for enzymatic reactions and extraction processes of DNA in DNA sequence analysis. BioTechniques 9:584591. Green, E.D. and Olson, M.V. 1990. Chromosomal region of the cystic fibrosis gene in yeast artificial chromosomes: A model for human genome mapping. Science 250:92-98. Haltiner, M., Kempe, T., and Tjian, R. 1985. A novel strategy for constructing clustered point mutations. Nucl. Acids Res. 13:1015-1025. Hattori, M. and Sakaki, Y. 1986. Dideoxy DNA sequencing method using denatured plasmid templates. Anal. Biochem. 152:232-238.
Hawkins, T.L., McKernan, K.J., Jacotot, L.B., MacKenzie, J.B., Richardson, P.M., and Lander, E.S. 1997. A magnetic attraction to highthroughput genomics. Science 276:1887-1889 Heiner, C.R., Hunkapiller, K.L., Chen, S.M., Glass, J.I., and Chen, E.Y. 1998. Sequencing multimegabase-template DNA with BigDye terminator chemistry. Genome Res. 8:557-561. Huang, X.C., Quesada, M.A., and Mathies, R.A. 1992. DNA sequencing using capillary array electrophoresis. Anal. Chem. 64:2149-2154. Hultman, T., Stahl, S., Hornes, E., and Uhlen, M. 1989. Direct solid phase sequencing of genomic and plasmid DNA using magnetic beads as solid support. Nucl. Acids Res. 17:4937-4946. Hultman, T., Bergh, S., Moks, T., and Uhlen, M. 1991. Bidirectional solid phase sequencing of in vitro amplified plasmid DNA. BioTechniques 10:84-93. Johnston-Dow, E., Mardis, E., Heiner, C., and Roe, B.A. 1987. Optimized methods for fluorescent and radiolabeled DNA sequencing. BioTechniques 5:754-765. Jones, D. S., Schofield, J.P., and Vaudin, M. 1991. Fluorescent and radioactive solid phase dideoxy sequencing of PCR products in microtitre plates. J. DNA Seq. Map. 1:279-283. Ju, J. 1999. Nucleic Acid Sequencing with Solid Phase Capturable Terminators. U.S. Patent no. 5,876,936. Ju, J., Ruan, C., Fuller, C.W., Glazer, A.N., and Mathies, R.A. 1995. Energy transfer fluorescent dye-labeled primers for DNA sequencing and analysis. Proc. Natl. Acad. Sci. USA. 92:43474351. Ju, J., Glazer, A.N., and Mathies, R.A. 1996. Energy transfer primers: A new fluorescence labeling paradigm for DNA sequencing and analysis. Nature Med. 2:246-249. Ju, J., Yan, H., Zaro, M., Doctolero, M., Goralski, T., Konrad, K., Lachenmeier, E., and Cathcart., R. 1997. DNA sequencing with solid phase capturable terminators Microb. Comp. Genomics 2:223. Kambara, H., Nishikawa, T., Katayama, K., and Yamaguchi, T. 1988. Optimization of parameters in a DNA sequenator using fluorescence detection. Bio/Technology 6:816-821. Kaneoka, H., Lee, D.R., Hsu, K.-C., Sharp, G.C., and Hoffman, R.W. 1991. Solid phase DNA sequencing of allele specific polymerase chain reaction amplified HLA-DR genes. BioTechniques 10:30-40. Kheterpal, I. and Mathies, R.A. 1999. Capillary array electrophoresis DNA sequencing. Anal. Chem. 71:31A-37A. Koster, H., Tang. K., Fu, D.J., Braun, A., van den Boom, D., Smith, C.L., Cotter, R.J., and Cantor, C.R. 1996. A strategy for rapid and efficient DNA sequencing by mass spectrometry. Nature Biotechnol. 14:1123-1128. DNA Sequencing
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Krishnan, B.R., Blakesley, R.W., and Berg, D.E. 1991. Linear amplification DNA sequencing directly from single phage plaques and bacterial colonies. Nucl. Acids. Res. 19:1153. Lee, L.G., Spurgeon, S.L., Heiner, C.R., Benson, S.C., Rosenblum, B.B., Menchen, S.M., Graham, R.J., Constantinescu, A., Upadhya, K.G., and Cassel, J.M. 1997. New energy transfer dyes for DNA sequencing. Nucl. Acids Res. 25:28162822. Mardis, E.R. and Roe, B.A. 1989. Automated methods for single-stranded DNA isolation and dideoxynucleotide DNA sequencing reactions on a robotic workstation. BioTechniques 7:840-850. Marra, M., Weinstock, L.A., and Mardis, E.R. 1996. End sequence determination from large insert clones using energy transfer fluorescent primers. Genome Res. 6:1118-1122. Martin, W., Warmington, J., Galinski, B., Gallager, M., Davies, R., Beck, M., and Oliver, S. 1985. A system to perform the Sanger dideoxy sequencing reactions. Bio/Technology 3:911-915. Martin, C., Bresnick, L., Juo, R.-R., Voyta, J.C., and Bronstein, I. 1991. Improved chemiluminescence DNA sequencing. BioTechniques 11:110113. Maxam, A.M. and Gilbert, W. 1977. A new method for sequencing DNA. Proc. Natl. Acad. Sci. U.S.A. 74:560-564. Maxam, A.M. and Gilbert, W. 1980. Sequencing end-labeled DNA with base-specific chemical cleavages. Methods Enzymol. 65:499-559. Messing, J. 1983. New M13 vectors for cloning. Methods Enzymol. 101:20-78. Messing, J. 1988. M13, the universal primer and the polylinker. Focus (BRL) 10:21-26. Middendorf, L., Brumbaugh, J., Grone, D., Morgan, C., and Ruth, J. 1988. Large scale DNA sequencing. Am. Biol. Lab. (August) 14-22. Monforte, J.A. and Becker, C.H. 1997. Highthroughput DNA analysis by time-of-flight mass spectrometry. Nature Med. 3:360-362. Murray, V. 1989. Improved double-stranded DNA sequencing using the linear polymerase chain reaction. Nucl. Acids Res. 17:8889. Olson, M., Hood, L., Cantor, C., and Botstein, D. 1989. A common language for physical mapping of the human genome. Science 245:1434-1435. Prober, J., Trainor, G., Dam, R., Hobbs, F., Robertson, C., Zagursky, R., Cocuzza, R., Jensen, M., and Baumeister, K. 1987. A system for rapid DNA sequencing with fluorescent chain-terminating dideoxynucleotides. Science 238:336341. Rosenthal, A., Sproat, B., Voss, H., Stegemann, J., Schwager, C., Erfle, H., Zimmerman, J., Courelle, C., and Ansorge, W. 1990. Automated sequencing of fluorescently labeled DNA by chemical degradation. J. DNA Seq. Map. 1:6371.
Rubin, J. and Schmid, C. 1980. Pyrimidine-specific chemical reactions useful for DNA sequencing. Nucl. Acids Res. 8:4613-4619. Salas-Solano, O., Carrilho, E., Kotler, L., Miller, A.W., Goetzinger, W., Sosic, Z., and Karger, B.L. 1998. Routine DNA sequencing of 1000 bases in less than one hour by capillary electrophoresis with replaceable linear polyacrylamide solutions. Anal. Chem. 70:3996-4003. Sanger, F., Nicklen, S., and Coulson, A.R. 1977. DNA sequencing with chain-terminating inhibitors. Proc. Natl. Acad. Sci. U.S.A. 74:5463-5467. Sanger, F., Coulson, A.R., Barrell, B.G., Smith, A.J.M., and Roe, B.A. 1980. Cloning in singlestranded bacteriophage as an aid to rapid DNA sequencing. J. Mol. Biol. 143:161-178. Sears, L., Moran, L., Kissinger, C., Creasey, T., Perry-O’Keefe, H., Roskey, M., Sutherland, E., and Slatko, B. 1992. CircumVent thermal cycle sequencing and alternative manual and automated DNA sequencing protocols using the highly thermostable VentR (exo−) DNA polymerase. BioTechniques. 13:626-633. Shizuya, H., Birren, B., Kim, U.-J., Mancino, V., Slepak, T., Tachiiri, Y., and Simon, M. 1992. Cloning and stable maintenance of 300-kilobase-pair fragments of human DNA in Escherichia coli using an F-factor-based vector. Proc. Nat. Acad. Sci. U.S.A. 89:8794-8797. Smith, L., Fung, S., Hunkapiller, M., Hunkapiller, T., and Hood, L. 1985. The synthesis of oligonucleotides containing an aliphatic amino group at the 5′ terminus: Synthesis of fluorescent DNA primers for use in DNA sequencing analysis. Nucl. Acids Res. 13:2399-2412. Smith, L., Sanders, J., Kaiser, R., Hughes, P., Dodd, C., Heiner, C., Kent, S., and Hood, L. 1986. Fluorescence detection in automated DNA sequence analysis. Nature 321:674-679. Smith, V., Brown, C.M., Banker, A.T., and Barrell, B.G. 1990. Semiautomated preparation of DNA template for large scale sequencing projects. J. DNA Seq. Map. 1:73-78. Stephens, J.C., Cavanaugh, M.L., Gradie, M.I., Mador, M.L., and Kidd, K.K. 1990. Mapping the human genome: Current status. Science 250:237-244. Tabor, S. and Richardson, C.C. 1987a. Selective oxidation of the exonuclease domain of bacteriophage T7 DNA polymerase. J. Biol. Chem. 262:15330-15333. Tabor, S. and Richardson, C.C. 1987b. DNA sequence analysis with a modified bacteriophage T7 DNA polymerase. Proc. Natl. Acad. Sci. U.S.A. 84:4767-4771. Tabor, S. and Richardson, C.C. 1989a. Selective inactivation of the exonuclease activity of bacteriophage T7 DNA polymerase by in vitro mutagenesis. J. Biol. Chem. 264:6447-6458.
Introduction
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Tabor, S. and Richardson, C.C. 1989b. Effect of manganese ions on the incorporation of dideoxynucleotides by bacteriophage T7 DNA polymerase and Escherichia coli DNA polymerase I. Proc. Natl. Acad. Sci. U.S.A. 86:4076-4080. Tabor, S. and Richardson, C.C. 1990. DNA sequence analysis with a modified bacteriophage T7 DNA polymerase: Effect of pyrophosphorolysis and metal ions. J. Biol. Chem. 265:83228328. Tabor, S., Huber, H., and Richardson, C.C. 1987. Escherichia coli thioredoxin confers processivity of the DNA polymerase activity of the gene 5 protein of bacteriophage T7. J. Biol. Chem. 262:16212-16223. Tizard, R., Cate, R.L., Ramachandran, K.L., Wysek, M., Voyta, J.C., Murphy, O.J., and Bronstein, I. 1990. Imaging of DNA sequences with chemiluminescence. Proc. Natl. Acad. Sci. U.S.A. 87:4514-4518. Volckaert, G., de Vleeschouwer, E., Blocker, H., and Frank, R. 1984. A novel type of cloning vector for ultrarapid chemical degradation sequencing of DNA. Gene Anal. Tech. 1:52-59. Wilson, R., Yuen, A., Clark, S., Spence, C., Arakelian, P., and Hood, L. 1988. Automation of dideoxynucleotide DNA sequencing reactions using a robotic workstation. BioTechniques 6:776-787.
Zagursky, R.J., Conway, P.S., and Kashdan, M.A. 1991. Use of 33P for Sanger DNA sequencing. BioTechniques 11:36-38. Zimmerman, J., Voss, H., Schwager, C., Stegemann, J. Erfle, H., and Ansorge, W. 1989. Automated preparation and purification of M13 templates for DNA sequencing. Methods Mol. Cell Biol. 1:29-34. Zimmerman, J., Dietrich, T., Voss, H., Erfle, H., Schwager, C., Stegemann, J., Hewitt, N., and Ansorge, W. 1992. Fully automated Sanger sequencing protocol for double-stranded DNA. Methods Mol. Cell Biol. 3:39-42.
KEY REFERENCES Maxam and Gilbert, 1977. See above. Describes the chemical cleavage method. Sanger et al., 1977. See above. Describes the traditional Sanger procedure. Tabor and Richardson, 1987b. See above. Describes the labeling/termination method. Sears et al., 1992. See above. Describes the thermal cycle sequencing procedure.
Young, A. and Blakesley, R. 1991. Sequencing plasmids from single colonies with the dsDNA cycle sequencing system. Focus (BRL) 13:137.
Frederick M. Ausubel, Lisa M. Albright, and Jingyue Ju
Zagursky, R. and McCormick, R. 1990. DNA sequencing separations in capillary gels on a modified commercial DNA sequencing instrument. BioTechniques 9:74-79.
The chapter editors wish to acknowledge the substantial assistance of Richard L. Eckert of Case Western Reserve and Barton E. Slatko of New England Biolabs.
Zagursky, R., Baumeister, K., Lomax, N., and Berman, M. 1985. Rapid and easy sequencing of large double-stranded DNA and supercoiled plasmid DNA. Gene Anal.Tech. 2:89-94.
DNA Sequencing
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DNA Sequencing Strategies This unit contains a general discussion of factors that should be considered before embarking on a DNA sequencing project. In general, any sequencing strategy should include plans for sequencing both strands of the DNA fragment. Complementary strand confirmation leads to higher accuracy, especially when sequencing regions where artifacts such as “compressions” are a problem. Sequencing the opposite strand is often required to obtain accurate data for such regions. The most commonly used methods for generating appropriately sized DNA fragments for dideoxy and chemical sequencing are discussed below. The biochemistry underlying these procedures, as well as how to choose between these and alternative sequencing methods, are discussed in the introduction to this chapter.
DIDEOXY SEQUENCING Planning for Dideoxy Sequencing Sequencing determination of a fragment of 100 µg), the lipid can be toxic. For each particular liposome mixture tested, it is important to vary the amount as indicated in Table 9.1.3. Concentration of DNA. In many of the cell types tested, relatively small amounts of DNA are effectively taken up and expressed. In fact, higher levels of DNA can be inhibitory in some cell types with certain liposome preparations. In the optimization protocol outlined in Table 9.1.3, the standard reporter vector pSV2CAT is used; however, any plasmid DNA whose expression can be easily monitored would be suitable.
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Table 9.1.3
Dish (35-mm) 1 2 3 4 5 6 7 8 9 10
Optimization of Liposome-Mediated Transfection
pSV2CAT (µg) 0.1 0.1 0.1 0.1 0.1 0.5 0.5 0.5 0.5 0.5
Liposomes (µl) 1 2 4 8 12 1 2 4 8 12
Dish (35-mm)
pSV2CAT (µg)
11 12 13 14 15 16 17 18 19 20
5 5 5 5 5 10 10 10 10 10
Liposomes (µl) 5 10 15 20 30 5 10 15 20 30
Time of incubation. When the optimal amounts of lipid and DNA have been established, it is desirable to determine the length of time required for exposure of the liposome-DNA complex to the cells. In general, transfection efficiency increases with time of exposure to the liposome-DNA complex, although after 8 hr, toxic conditions can develop. HeLa or BHK-21 cells typically require ∼3 hr incubation with the liposome-DNA complex for optimal tranfection, while CV-1 and COS-7 cells require 5 hr of exposure.
UNIT 9.1
Calcium Phosphate Transfection This unit presents two methods of calcium phosphate–based eukaryotic cell transfection that can be used for both transient and stable (UNIT 9.5) transfections. In these protocols, plasmid DNA is introduced to monolayer cell cultures via a precipitate that adheres to the cell surface. A HEPES-buffered solution is used to form a calcium phosphate precipitate that is directly layered onto the cells (see Basic Protocol). For some cells, shocking the cells with glycerol or DMSO (see Support Protocol) improves transfection efficiency. In the alternate high-efficiency method, a BES-buffered system is used that allows the precipitate to form gradually in the medium and then drop onto the cells (see Alternate Protocol). The alternate method is particularly efficient for stable transformation of cells with circular plasmid DNA. For transformation with linear plasmid or genomic DNA, or for transient expression, however, the Alternate Protocol is comparable to the Basic Protocol. Both methods of transfection require very high-quality plasmid DNA. Factors that can be optimized for calcium phosphate transfections are presented in the introduction to Section I, and protein expression strategies are discussed in Chapter 16. Additional details of mammalian cell culture are given in APPENDIX 3F.
BASIC PROTOCOL
Calcium Phosphate Transfection
TRANSFECTION USING CALCIUM PHOSPHATE–DNA PRECIPITATE FORMED IN HEPES A precipitate containing calcium phosphate and DNA is formed by slowly mixing a HEPES-buffered saline solution with a solution containing calcium chloride and DNA. This precipitate adheres to the surface of cells and should be visible in the phase contrast microscope the day after transfection. Depending on the cell type, up to 10% of the cells on a dish will take up the DNA precipitate through an as yet undetermined mechanism. Glycerol or dimethyl sulfoxide shock increases the amount of DNA absorbed in some cell types (see Support Protocol).
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Materials Exponentially growing eukaryotic cells (e.g., HeLa, BALB/c 3T3, NIH 3T3, CHO, or rat embryo fibroblasts) Complete medium (depending on cell line used) CsCl-purified plasmid DNA (10 to 50 µg per transfection) 2.5 M CaCl2 (see recipe) 2× HEPES-buffered saline (HeBS; see recipe) PBS (APPENDIX 2) 10-cm tissue culture dishes 15-ml conical tube Additional reagents and equipment for ethanol precipitation (UNIT 2.1A) and mammalian cell tissue culture (APPENDIX 3F) NOTE: All solutions and equipment coming into contact with cells must be sterile, and proper sterile technique should be used accordingly. NOTE: All culture incubations are performed in a humidified 37°C, 5% CO2 incubator unless otherwise specified. 1. Split exponentially growing eukaryotic cells into 10-cm tissue culture dishes the day before transfection. Feed cells with 9.0 ml complete medium 2 to 4 hr prior to precipitation. When transfecting adherent cells that double every 18 to 24 hr, a 1:15 split from a confluent dish generally works well. On the day of the transfection, it is important that cells are thoroughly separated on the dish, as the ability to take up DNA is related to the surface area of the cell exposed to the medium. Cells should be split in a manner that accomplishes this. The desired density of cells on dishes to be transfected will vary with cell type and the reason for doing the transfection. The optimal density is that which produces a near confluent dish when the cells are harvested or split into selective medium.
2. Ethanol precipitate the DNA to be transfected and air dry the pellet by inverting the microcentrifuge tube on a fresh Kimwipe inside a tissue culture hood. Resuspend the pellet in 450 µl sterile water and add 50 µl of 2.5 M CaCl2. The amount of DNA that is optimal for transfection varies from 10 to 50 ìg per 10-cm plate, depending on the cell line to be transfected. DNA to be transfected should be purified twice by CsCl gradient centrifugation (UNIT 1.7). DNA can also be prepared using column procedures (UNIT 2.1B). Some column procedures produce DNA that does not transfect well, so column-purified DNA should be tested and compared to CsCl-purified DNA for transfection efficiency. Supercoiled DNA works well in transfections. Impurities in the DNA preparation can be deleterious to transfection efficiency. A description of how to optimize the amount of DNA to transfect and other parameters of calcium phosphate–mediated transfection is provided in the discussion of Optimization of Transfection. Ethanol precipitation sterilizes the DNA to be transfected. For transfections that will be harvested within 3 to 4 days (transient analysis), this is not necessary. For transient experiments, many researchers make a 450-ìl aqueous solution containing the DNA directly, without ethanol precipitation. If this is done, care should be taken to keep the amount of Tris in the solution to a minimum, as Tris may alter the pH of the precipitate and therefore reduce transfection efficiency.
3. Place 500 µl of 2× HeBS in a sterile 15-ml conical tube. Use a mechanical pipettor attached to a plugged 1- or 2-ml pipet to bubble the 2× HeBS and add the DNA/CaCl2
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Figure 9.1.1 Formation of calcium phosphate precipitate.
solution dropwise with a Pasteur pipet (see Fig. 9.1.1). Immediately vortex the solution for 5 sec. If no mechanical pipettor is available, the solution can be bubbled by blowing through rubber tubing that is attached to a pipet via a filter. The filter is necessary to maintain sterility. This does not give as reproducible results as the mechanical pipettor.
4. Allow precipitate to sit 20 min at room temperature. 5. Use a Pasteur pipet to distribute the precipitate evenly over a 10-cm plate of cells and gently agitate to mix precipitate and medium. 6. Incubate the cells 4 to 16 hr under standard growth conditions. Remove the medium. Wash cells twice with 5 ml PBS and feed cells with 10 ml complete medium. The amount of time that the precipitate should be left on the cells will vary with cell type. For hardy cells such as HeLa, NIH 3T3, and BALB/c 3T3, the precipitate can be left on for 16 hr. Other cell types will not survive this amount of exposure to the precipitate. See discussion of Optimization of Transfection for optimization of this step as well as for a discussion of how to determine whether glycerol shock is useful.
7. For transient analysis, harvest the cells at the desired time point (UNITS 9.6-9.8 & 14.6). For stable transformation, allow the cells to double twice before plating in selective medium (UNIT 9.5). Calcium Phosphate Transfection
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GLYCEROL/DMSO SHOCK OF MAMMALIAN CELLS The Basic Protocol works well for cell lines such as HeLa, BALB/c 3T3, NIH 3T3, and rat embryo fibroblasts. Transfection efficiency in some cell lines, such as CHO DUKX, is dramatically increased by “shocking” the cells with either glycerol or DMSO. Precipitates are left on the cell for only 4 to 6 hr, and the cells are shocked immediately after removal of the precipitate.
SUPPORT PROTOCOL
Additional Materials (also see Basic Protocol) 10% (v/v) glycerol solution or DMSO in complete medium, sterile PBS (APPENDIX 2), sterile Replace step 6 of the Basic Protocol with the following: 6a. Incubate the cells 4 to 6 hr and remove the medium. Add 2.0 ml of a sterile 10% glycerol solution. Let the cells sit 3 min at room temperature. Alternatively, 10% or 20% DMSO can be used. DMSO tends to be somewhat less harmful to the cells, but also may not work as well.
6b. Add 5 ml of PBS to the glycerol solution on the cells, agitate to mix, and remove the solution. Wash twice with 5 ml of PBS. Feed the cells with complete medium. It is important to dilute the glycerol solution on the cells with PBS before removing the glycerol solution so the cells do not stay in glycerol too long. Excessive exposure to glycerol will kill cells.
HIGH-EFFICIENCY TRANSFECTION USING CALCIUM PHOSPHATE–DNA PRECIPITATE FORMED IN BES
ALTERNATE PROTOCOL
A solution of calcium chloride, plasmid DNA, and N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) buffer, pH 6.95, is added to a plate of cells containing culture medium. The plates are incubated overnight while a calcium phosphate–DNA complex forms gradually in the medium under an atmosphere of 3% CO2. With this method, 10% to 50% of the cells on a plate stably integrate and express the transfected DNA. Transient expression under these conditions is comparable to that obtained with the Basic Protocol. Glycerol or DMSO shock does not increase the number of cells transformed. Materials Exponentially growing mammalian cells (see Critical Parameters) Complete medium: Dulbecco modified Eagle medium containing 10% (v/v) fetal bovine serum (FBS) CsCl-purified plasmid DNA TE buffer, pH 7.4 (APPENDIX 2) 2.5 M CaCl2 (see recipe) 2× BES-buffered solution (BBS; see recipe) PBS (APPENDIX 2) Selection medium (UNIT 9.5; optional) 10-cm tissue culture dishes 35°C, 3% CO2 humidified incubator 35° to 37°C, 5% CO2 humidified incubator Fyrite gas analyzer (optional; Fisher Scientific or Curtin Matheson) NOTE: All solutions and equipment coming into contact with cells must be sterile, and proper sterile technique should be used accordingly.
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1. Seed exponentially growing mammalian cells at 5 × 105 cells/10-cm tissue culture dish in 10 ml complete medium the day prior to transfection. There should be 10- to 100-fold higher when the BESbased protocol is used, with 10% to 50% of the cells on a plate stably transformed (Chen and Okayama, 1987). Transient expression is comparable in the Basic and Alternate Protocols (Chen and Okayama, 1988).
Time Considerations For the Basic Protocol, preparation of twelve DNA precipitates and addition of the precipitates to the cells takes 1 to 2 hr. Without the ethanol precipitation step, the procedure can be done in 1 hr. With practice, the actual mixing of the CaCl2 and 2× HeBS solutions will take ∼1 min. This means that up to eighteen precipitates can be made before the first precipitate is ready to apply to the cells.
For the Alternate Protocol, no ethanol precipitation step has been necessary for either transient or stable transfections. It takes slightly less time than the Basic Protocol because the 2× BBS does not need to be added dropwise to the calcium chloride–DNA solution.
Literature Cited Chen, C. and Okayama, H. 1987. High efficiency transformation of mammalian cells by plasmid DNA. Mol. Cell. Biol. 7:2745-2752. Chen, C. and Okayama, H. 1988. Calcium phosphate–mediated gene transfer: A highly efficient system for stably transforming cells with plasmid DNA. BioTechniques 6:632-638. Graham, F.L. and van der Eb, A.J. 1973. A new technique for the assay of infectivity of human adenovirus 5 DNA. Virology 52:456. Ishiura, M., Hirose, S., Uchida, T., Hamada, Y., Suzuki, Y., and Okada, Y. 1982. Phage particle– mediated gene transfer to cultured mammalian cells. Mol. Cell. Biol. 2:607-616. Wigler, M., Pellicer, A., Silverstein, S., and Axel, R. 1978. Biochemical transfer of single-copy eucaryotic genes using total cellular DNA as donor. Cell 14:725.
Key References Chen and Okayama, 1987. See above. Ishiura et al., 1982. See above. Provides the basis for BES-mediated transfection.
Contributed by Robert E. Kingston (HEPES method) Massachusetts General Hospital and Harvard Medical School Boston, Massachusetts Claudia A. Chen (BES method) National Institute of Mental Health Bethesda, Maryland Hiroto Okayama (BES method) Osaka University Osaka, Japan John K. Rose (optimization) Yale University School of Medicine New Haven, Connecticut
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Transfection Using DEAE-Dextran
UNIT 9.2
Transfection of cultured mammalian cells using diethylaminoethyl (DEAE)-dextran/DNA can be an attractive alternative to other transfection methods in many circumstances. The major advantages of the technique are its relative simplicity and speed, limited expense, and remarkably reproducible interexperimental and intraexperimental transfection efficiency. Disadvantages include inhibition of cell growth and induction of heterogeneous morphological changes in cells. Furthermore, the concentration of serum in the culture medium must be transiently reduced during the transfection. Any of these factors may adversely affect or be incompatible with some bioassays or experimental goals. In addition, for nonstandard cell types there may be a requirement for extensive preliminary investigation of optimal transfection conditions. Together, these factors influence the suitability of this technique to specific purposes. In general, DEAE-dextran DNA transfection is ideal for transient transfections with promoter/reporter plasmids in analyses of promoter and enhancer functions, and is suitable for overexpression of recombinant protein in transient transfections or for generation of stable cell lines using vectors designed to exist in the cell as episomes. The procedure may also be used for expression cloning (Aruffo and Seed, 1987; Kluxen and Lubbert, 1993; Levesque et al., 1991), although electroporation is usually preferred for this purpose (Puchalski and Fahl, 1992). This unit presents a general description of DEAE-dextran transfection (see Basic Protocol) as well as two more specific protocols for typical experimental applications (see Alternate Protocols 1 and 2). The Basic Protocol is suitable for transfection of anchorage-dependent (attached) cells. For cells that grow in suspension, electroporation (UNIT 9.3) or lipofection (UNIT 9.4) is usually preferred, although DEAE-dextran-mediated transfection can be used (Fregeau and Bleackley, 1991). For suspension cells, the transfection step should be performed on collected cells that have been resuspended at 107 cells/ml in transfection medium, using reagents and conditions that are otherwise similar to those of the Basic Protocol. NOTE: All reagents and equipment coming into contact with live cells must be sterile, and proper sterile technique should be followed accordingly. NOTE: All culture incubations are performed in a 37°C, 5% CO2 incubator unless otherwise specified. GENERAL PROCEDURE FOR DEAE-DEXTRAN TRANSFECTION Cultured cells are incubated in medium containing plasmid DNA and DEAE-dextran, which form complexes that are taken up by cells via endocytosis. Chloroquine can be included to inhibit degradation of plasmid DNA. Cells are exposed transiently to DMSO or another permeabilizing agent to increase DNA uptake (DMSO “shock”). Important variables include the concentration of DEAE-dextran, the ratio of DNA to DEAE-dextran, the duration of transfection, and the presence and timing of chloroquine exposure (see Critical Parameters). This protocol is suitable for transfection of COS and CV1 cells; Alternate Protocols 1 and 2 describe two examples of transfection experiments. Materials Cells to be transfected and appropriate culture medium (e.g., complete DMEM; APPENDIX 3F) with and without 10% FBS 100 mM (1000×) chloroquine diphosphate in PBS, filter-sterilized (store at 4°C) Plasmid DNA(s), prepared by CsCl density-gradient centrifugation or affinity chromatography (UNIT 1.7) Contributed by Tod Gulick Current Protocols in Molecular Biology (1997) 9.2.1-9.2.10 Copyright © 1997 by John Wiley & Sons, Inc.
BASIC PROTOCOL
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TE buffer (APPENDIX 2) 10 mg/ml DEAE-dextran stock solution (see recipe) 10% (v/v) dimethyl sulfoxide (DMSO) in PBS, filter-sterilized (store up to 1 month at room temperature) Phosphate-buffered saline (PBS; APPENDIX 2) Appropriate-sized tissue culture vessels (Table 9.2.1) Inverted microscope Additional reagents and equipment for mammalian cell culture (APPENDIX 3F) 1. Plate cells at a density to achieve 50% to 75% confluence on the target day for transfection. For COS or CV1 cells, perform a 1:10 split 2 days prior to transfection. The surface area of various cell culture vessels given in Table 9.2.1 can be used to determine how to split cells to the desired density. Some cell types including many primary cells show particular sensitivity to the toxicity of DEAE-dextran. These cells should be plated at higher density or transfected after reaching near-confluence.
2. Determine the total volume of medium to be used in the transfection based on the number of culture vessels containing cells to be transfected and the volume per vessel shown in Table 9.2.1. Make up this amount of medium (plus some excess) to contain 2.5% FBS by combining 1 part medium containing 10% FBS with 3 parts serum-free medium. DEAE-dextran can precipitate in the presence of high medium protein, necessitating use of a low FCS concentration. Alternatively, NuSerum (Collaborative Research), which contains only ∼30% serum, can be used at a final concentration of 10%.
3. Add 100 mM (1000×) chloroquine diphosphate stock solution to the 2.5%-FBS-containing transfection medium prepared in step 2 to achieve a final concentration of 100 µM. Warm transfection medium to 37°C.
Table 9.2.1 Surface Areas of Commonly Used Tissue Culture Vessels and Corresponding Appropriate DEAE-Dextran Transfection Medium Volumes
Vessel T175 flask T150 flask T75 flask T25 flask 150-mm dish 100-mm dish 60-mm dish 35-mm dish 6-well plate (35-mm wells) 12-well plate (22-mm wells) 24-well plate (15.5-mm wells)
Area (cm2) 175 150 75 25 148b 55b 21b 8b 9.4b 3.8b 1.9b
Appropriate vol. DEAE-dextran mediuma (ml)
10 4 2 1 1 0.5 0.25
aThese volumes are roughly a linear function of vessel surface area. To ensure that cells
Transfection Using DEAE-Dextran
are completely covered by medium during the transfection, small wells require proportionately larger volumes due to annular sequestration of medium because of surface tension at the periphery. bCostar; other manufacturer products may deviate slightly.
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Chloroquine is toxic to all cells, so exposure time should be limited to 15 min (up to an hour) work just as well.
3. Dilute cationic lipid reagent into dilution medium in a second tube and mix. Transfection of Cultured Eukaryotic Cells Using Cationic Lipid Reagents
4. Combine precomplexed DNA and diluted cationic lipid reagent, mix, and incubate for 15 min at room temperature. Incubation times >15 min (up to an hour) work just as well when LipofectAmine is the cationic lipid and Plus reagent is the enhancer.
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5. While complexes are forming, replace medium on the cells with the appropriate volume of fresh transfection medium. The medium can be the same as the dilution medium. It is possible to use serum in the transfection medium at this step. In some cells (e.g., HeLa and NIH 3T3), transfection in medium containing serum is as efficient or more efficient than in medium without serum.
6. Add DNA-enhancer-lipid complexes to each well containing cells. Mix complexes into the medium gently, holding the plate at a slant. Incubate at 37°C in 5% CO2 for several hours. The exposure time with LipofectAmine Plus may be as short as 3 hr or up to overnight. Be sure there is sufficient medium to prevent the cells from drying out (it is not necessary to increase other components if this is done).
7. After incubation, add cell culture medium to reach normal volume and add serum to bring the final concentration to that of normal cell culture medium. 8. Perform transient or stable expression analysis (see Basic Protocol 1, steps 8a and 8b). CATIONIC LIPID–MEDIATED TRANSFECTION OF SUSPENSION CELLS WITH DNA
BASIC PROTOCOL 2
This protocol is essentially the same as for adherent cells (see Basic Protocol 1) in that lipid and DNA are diluted separately into dilution medium, mixed, and allowed to form complexes before exposing to cells. However, complexes are formed in the wells of multiwell culture plates, and cells are then distributed into the wells containing complexes and allowed to transfect. Materials Dilution medium: cell culture medium without serum or specialized medium for transfection (e.g., Opti-MEM I, Life Technologies) Cationic liposome reagent (e.g., DMRIE-C or LipofectAmine 2000, Life Technologies; also see Table 9.4.1) Plasmid DNA, purified by anion-exchange chromatography (UNIT 2.14 or Goldsborough et al., 1998), cesium chloride density gradient (UNIT 1.7), or alkaline lysis (UNIT 1.6) Cell suspension: 1 × 107 cells/ml in normal cell culture medium without serum or antibiotics Cell culture medium (e.g., complete DMEM; APPENDIX 3F) Serum 6-well tissue culture plates 1. To each well of a 6-well tissue culture plate add 0.5 ml dilution medium. Commercial medium that is specialized for lipid-mediated transfection (e.g., Opti-MEM I), without serum or antibiotics, gives the best results. However, other serum-free media may also be used. When transfecting in different-sized culture plates, change the amounts of DNA, cationic lipid reagent, and medium in proportion to the difference in surface area (see Table 9.4.3).
2. Add 0, 2, 4, 6, 8, or 12 µl cationic lipid reagent to each well and mix gently by swirling the plate. DMRIE-C was found to give high efficiency transfection of DNA in Jurkat (human T cell lymphoma), K562 and KG-1 (human myeologenous leukemia), and MOLT-4 (human lymphoblastic leukemia) cell lines. It is a lipid suspension that may settle with time. To ensure that a homogenous sample is taken, mix thoroughly by inverting the tube 5 to 10 times before removing a sample for transfection.
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3. Add 0.5 ml dilution medium containing 4 µg plasmid DNA to each well. Mix by swirling plate. The amount of DNA should be optimized for each cell line.
4. Incubate at room temperature for 15 to 45 min to allow formation of lipid-DNA complexes. 5. Add 0.2 ml cell suspension (2 × 106 cells) to each well and mix gently. The single most important factor in reproducible, high-efficiency transfection is a consistent number of healthy, proliferating cells. Transfection is most efficient when the cells are maintained in mid-log growth.
6. Incubate several hours at 37°C in a 5% CO2 incubator. A 4-hr incubation is adequate for DMRIE-C transfections.
7. To each well add 2 ml cell culture medium containing 1.5× the usual amount of serum. For Jurkat and MOLT-4 cells, addition of 1 ìg/ml phytohemagglutinin (PHA) and 50 ng/ml phorbol myristate acetate (PMA) enhances promoter activity and gene expression. For K562 and KG-1 cells, PMA alone enhances promoter activity.
8. Assay the cells at 24 or 48 hr posttransfection for transient or stable expression (see Basic Protocol 1, step 8a or 8b). BASIC PROTOCOL 3
CATIONIC LIPID–MEDIATED TRANSFECTION OF ADHERENT CELLS WITH RNA In this protocol, lipid is first diluted into dilution medium and mixed. RNA is then mixed directly into the diluted lipid and immediately added to cells (which have been rinsed with serum-free medium), and cells are incubated for transfection. Materials Adherent cells Cell culture medium with serum (e.g., complete DMEM; APPENDIX 3F) Dilution medium: serum-free cell culture medium or specialized medium for transfection (e.g., Opti-MEM I, Life Technologies) Cationic lipid reagent (e.g., DMRIE-C, Life Technologies; also see Table 9.4.1) mRNA (UNIT 4.5) 6-well or 35-mm tissue culture plate 12 × 75–mm polystyrene tubes Additional reagents and equipment for trypsinizing, counting, and plating cells (APPENDIX 3F) 1. The day before transfection, trypsinize and count adherent cells (APPENDIX 3F). In each well of a 6-well tissue culture plate, or in six 35-mm tissue culture plates, seed ∼2–3 × 105 cells in 2 ml of the appropriate cell culture medium supplemented with serum. Transfection is most efficient when the cells are growing rapidly. Cultures should be maintained carefully and passaged frequently. As transfection efficiency may be sensitive to culture confluency, it is important to maintain a standard seeding protocol from experiment to experiment.
Transfection of Cultured Eukaryotic Cells Using Cationic Lipid Reagents
2. Incubate at 37°C in a 5% CO2 incubator until the cells are ∼80% confluent. This will usually take 18 to 24 hr, but the time will vary among cell types.
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3. On the day of transfection, wash the cells in each well with 2 ml dilution medium at room temperature. Commercial medium that is specialized for lipid-mediated transfection (e.g., Opti-MEM I), without serum or antibiotics, gives the best results. However, other serum-free media may also be used.
4. Add 1.0 ml dilution medium to each of six 12 × 75–mm polystyrene tubes. 5. Add 0, 2, 4, 6, 8, or 12 µl cationic lipid reagent to each tube and mix or vortex briefly. DMRIE-C was found to give high-efficiency transfection of RNA in adherent cell lines (Ciccarone et al., 1995). It is a lipid suspension that may settle with time. To ensure that a homogenous sample is taken, mix thoroughly by inverting the tube 5 to 10 times before removing a sample for transfection.
6. Add 2.5 to 5.0 µg RNA to each tube and vortex briefly. mRNA that is capped and polyadenylated is translated more efficiently and is more stable within the cell.
7. Immediately add lipid-RNA complexes to washed cells and incubate 4 hr at 37°C in a 5% CO2 incubator. The time of exposure of cells to lipid-RNA complexes, as well as the amount of RNA added to the cells, should be adjusted for each cell type.
8. Replace transfection medium with cell culture medium containing serum. 9. Allow cells to express the RNA for 16 to 24 hr and analyze them for expression of the transfected RNA as appropriate for the transgene used. CATIONIC LIPID–MEDIATED TRANSFECTION OF ADHERENT Sf9 AND Sf21 INSECT CELLS WITH BACULOVIRUS DNA
BASIC PROTOCOL 4
As for transfecting mammalian cells (see Basic Protocol 1), cationic lipid reagent and nucleic acid are diluted separately into serum-free medium and then mixed and allowed to form complexes. Complexes are then diluted with fresh transfection medium and added to the cells for transfection. After the cells are fed and incubated, budded virus can be isolated from the medium. Materials Insect cells: Sf 9 or Sf 21 cells (UNIT 16.9) Insect medium (UNIT 16.9; e.g., Sf-900 II SFM, Life Technologies) with and without serum and antibiotics Baculovirus DNA: purified DNA or bacmid DNA miniprep (UNITS 16.9 & 16.10; Anderson et al., 1995) Cationic lipid reagent (Table 9.4.1) 6-well tissue culture plate 27°C incubator 12 × 75–mm polystyrene tubes, sterile Additional reagents and equipment for culturing insect cells and harvesting baculovirus from cell supernatants (UNIT 16.10) 1. In each well of a 6-well tissue culture plate, seed ∼9 × 105 insect cells in 2 ml insect medium without serum or antibiotics (UNIT 16.10). Insect cells must be plated when they are in mid-log growth phase. Cells that have reached stationary phase transfect and infect at very low efficiency. Therefore, it is advisable to
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maintain a standard cell passage protocol that keeps the cells in log growth. For Sf9 or Sf21 cells adapted in Sf-900 II SFM, cells are passaged twice weekly to a density of 3 × 105 cells/ml in suspension, and plated for transfection on the third day postseeding, when they are in mid-log phase. For other cell culture media and growth conditions, adjust conditions to maintain similar growth characteristics. For culture of insect cells, use 50 units/ml penicillin and 50 ìg/ml streptomycin (half the usual final concentration). For transfection, it is preferable to omit antibiotics from the medium to avoid toxicity.
2. Allow cells to attach at 27°C for ≥1 hr. 3. For each transfection, dilute 1 to 2 µg baculovirus DNA into 100 µl insect medium without serum or antibiotics in a 12 ×75–mm polystyrene tube. 4. For each transfection, dilute 1.5 to 9 µl cationic lipid reagent into 100 µl insect medium without serum or antibiotics in a separate 12 ×75–mm polystyrene tube. The suggested amount is 6 ìl, but this should be optimized for each system. CellFectin gives high-efficiency transfection of DNA in insect cell lines (Anderson et al., 1995). It is a lipid suspension that may settle with time. To ensure that a homogenous sample is taken, mix thoroughly by inverting the tube 5 to 10 times before removing a sample for transfection.
5. Combine the two solutions, mix gently, and incubate at room temperature for 15 to 45 min to form lipid-DNA complexes. 6. For each transfection, add 0.8 ml insect medium without serum or antibiotics to each tube containing lipid-DNA complexes and mix gently. 7. Aspirate medium from cells and overlay diluted lipid-DNA complexes onto the washed cells. Alternatively, the medium on the cells can be replaced with 0.8 ml fresh insect medium and the undiluted complexes can be added directly to the fresh medium on the cells.
8. Incubate cells for 5 hr in a 27°C incubator. Protect plates from evaporation by putting them in a humidified container. 9. Remove transfection mixture and add 2 ml insect medium containing antibiotics and serum, if desired. Incubate cells in a 27°C incubator for 72 hr. 10. Harvest baculovirus from cell supernatants (UNIT 16.10). Gene expression may also be evaluated in the cells after removal of virus-containing medium. SUPPORT PROTOCOL
FINE TUNING OR OPTIMIZING CONDITIONS FOR CATIONIC LIPID REAGENT TRANSFECTIONS This protocol provides an example of a simple one-step procedure for determining conditions conducive to high-efficiency transfections using cationic lipid reagents in any target cell type. A matrix of DNA and lipid reagent concentrations is used on transfections performed in a multiwell plate (Fig. 9.4.3). Once the best conditions have been determined, the transfections may be scaled up to larger vessels using the relative surface area (see Table 9.4.3) to increase the amounts of all reagents proportionately. This protocol can be modified for use with any transfection protocol.
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Additional Materials (also see Basic Protocol 1 and Alternate Protocol) 24-well tissue culture plates 96-well round-bottom plates (sterile, with lid) 1. The day before transfection, trypsinize and count cells (APPENDIX 3F). Plate cells in each well of a 24-well tissue culture plate using normal cell culture medium with serum, so that they are 50% to 95% confluent on the day of transfection. Avoid antibiotics at the time of plating and during transfection. The single most important factor in high-efficiency transfection is healthy, proliferating cell cultures. Antibiotics may cause some toxicity if present during transfection. In a 24-well plate, seeding 4 × 104 to 2 × 105 cells per well will usually give good plating density. Any type of plate may be used by scaling the reagent and cell amounts in proportion to the relative surface area (see Table 9.4.3).
2. Dilute DNA into dilution medium (appropriate for the lipid being optimized) without serum or antibiotics in four microcentrifuge tubes. Use a range of DNA concentrations, and use a volume of dilution medium that is 7× the appropriate protocol volume (see Basic Protocol 1, step 2, and Table 9.4.1). Mix gently after each addition. This makes enough DNA per tube for seven wells on a 24-well plate. Good ranges include 0.2 to 1.6 ìg DNA per well. If the Plus enhancer is being used, include it in the diluted DNA tubes, using 10 ìl Plus reagent per ìg DNA. Add the Plus reagent to the diluted DNA after mixing well. If the Plus reagent is added first, precipitation may occur.
3. Dilute cationic lipid reagent into dilution medium without serum or antibiotics in six microcentrifuge tubes. Use a range of DNA concentrations, and use a volume of dilution medium that is 5× the appropriate protocol volume (see Basic Protocol 1, step 3, and Table 9.4.1). Mix gently after each addition. Be sure to observe timing that works best for the cationic lipid reagent being used. This makes enough diluted lipid per tube for five wells on a 24-well plate. Good ranges include 0.5 to 5 ìl lipid reagent per well.
Cationic lipid reagent (µl)
1.0
1.5
2.0
2.5
3.0
3.5
DNA (µg)
0.2
0.4
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Figure 9.4.3 A sample matrix for fine tuning (optimizing) transfection reagent efficiencies using cationic lipid reagents.
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4. Pipet equal per-well volumes of diluted DNA and diluted cationic lipid reagent into the wells of a 96-well plate in a matrix corresponding to the wells on the 24-well plate (Fig. 9.4.3). Mix the complexes with the pipet tip by triturating. Cover the plate and incubate for 15 min at room temperature. Incubation times >15 min (up to an hour) work just as well, but be sure the complexes do not dry by covering them well.
5. While complexes are forming, replace medium on the cells with fresh transfection medium. See Basic Protocol 1, steps 5 and 6, for alternate procedures for combining complexes, medium, and cells. The medium can be the same as the serum-free dilution medium. It is possible to use serum in the transfection medium at this step. It is also possible to omit this step when using LipofectAmine 2000.
6. Add aliquots of DNA-lipid complexes (total volume from wells in step 4) to each well containing cells with fresh transfection medium. Mix complexes into the medium gently, holding the plate at a slant. Incubate at 37°C in 5% CO2 for 5 hr. The exposure time may be >5 hr (up to overnight). Be sure there is sufficient medium to prevent the cells from drying out (it is not necessary to increase other components if this is done). If using the Plus enhancer, a 3-hr exposure is sufficient.
7. After 5 hr incubation, add cell culture medium to reach normal volume and add serum to bring the final concentration to that of normal cell culture medium. If necessary to maximize cell growth, replace the medium containing the complexes with fresh complete medium after 5 hr incubation. This step may be omitted entirely for some protocols.
8. Check expression as described (see Basic Protocol 1, steps 8a and 8b). If peak activity is found to occur on the edge of the matrix of concentrations tested, adjust the concentrations to include the observed peak at the center of a new matrix and repeat the transfection.
COMMENTARY Background Information
Transfection of Cultured Eukaryotic Cells Using Cationic Lipid Reagents
There are currently at least eight companies that market cationic lipid–based transfection reagents. A partial listing of companies and products may be seen in Table 9.4.1. Many companies offer more than one type of reagent. Among the more popular ones are LipofectAmine 2000 and LipofectAmine Plus (Life Technologies), DOTAP and FuGENE 6 (Roche), and Effectene (Qiagen). Some of the structures are proprietary. The structures that are published can be classified into two general categories based on the number of positive charges in the lipid headgroup. The first cationic lipid (DOTMA) has a single positive charge per molecule and is used in Lipofectin (Life Technologies; Felgner et al., 1987). Several other cationic lipid–based transfection reagents such as DOTAP liposomal transfection reagent (Roche) and DMRIE-C (Life Technologies) also make use of singly charged cationic lipid mole-
cules. Increasing the number of positive charges per cationic lipid molecule to as many as five improved transfection efficiency dramatically in most cell types. This can be seen in the examples of DOGS, the cationic lipid in Transfectam (Promega; Behr et al., 1989); DOSPA in LipofectAmine (Life Technologies; Hawley-Nelson et al., 1993); and TMTPS in CellFectin (Life Technologies; Anderson et al., 1995). Further increase in transfection efficiency can sometimes be achieved by precomplexing DNA with a proprietary enhancer. Two commercially available transfection kits with enhancers are LipofectAmine Plus (Life Technologies; Shih et al., 1997) and Effectene (Qiagen). Life Technologies has designed cationic lipid reagents with specialized applications such as high-efficiency transfection of insect cells (see Basic Protocol 4) or delivery of RNA (see Basic Protocol 3). Lipofectin has high
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activity for endothelial cell transfection (Tilkins et al., 1994). Basic Protocol 1 and the Alternate Protocol described in this unit are the procedures with the highest potential for efficient DNA transfection of adherent mammalian cells (Shih et al., 1997; Ciccarone et al., 1999). LipofectAmine 2000 has a simple protocol that yields the highest transfection efficiencies in many cell types. Using the enhancer reagent results in more reproducible transfections without extensive optimization because of the overall high activity. Prior to the availability of enhanced cationic liposome transfections (e.g., using LipofectAmine 2000 and Effectene), the most effective procedure for transfection of adherent mammalian cells with DNA was with other polycationic reagents (e.g., LipofectAmine; Hawley-Nelson et al., 1993) following Basic Protocol 1. In order to achieve high-efficiency transfections with Basic Protocol 1, it is necessary to optimize lipid and DNA concentrations with the target cells at the desired plating density using a procedure similar to that described in the Support Protocol. Many cationic lipid reagents, as well as transfection reagents based on other chemistries, are available that can be used in Basic Protocol 1 for adherent mammalian cell DNA transfection, but they may yield lower efficiencies than the Alternate Protocol with the enhancer. Optimization using the Support Protocol is highly recommended when not using the enhancer, and the protocol can be modified for use with any cationic lipid reagent.
Critical Parameters The most critical parameter for successful transfection is cell health. Cells should be proliferating as rapidly as possible at the time they are plated for transfection. On the day of transfection, mitoses should be abundant in healthy cultures. Fresh cultures with a finite life span should be used at the earliest possible passage. For reproducible transfection results, it is critical to plate the same number of healthy cells for each transfection. Cells should always be counted, preferably using a hemacytometer and trypan blue (APPENDIX 3F). Although optimization is not required for high-efficiency transfection when using an enhancer (see Alternate Protocol), it is essential for success without the enhancer, and may improve efficiency even with the enhanced method. The medium used to dilute and form complexes between the cationic liposomes and the DNA must not contain serum. Serum contains
sulfated proteoglycans and other proteins, which compete with the DNA for binding to the cationic lipids. The medium should also not contain antibiotics. There is toxicity to the cells when cationic lipid reagents are used in the presence of antibiotics. The dilution medium/plating medium for the cells may have some influence on transfection efficiency. Some proprietary serum-free media contain components that inhibit transfection and should be replaced with Opti-MEM I, DMEM, or other media without serum during transfection (Hawley-Nelson and Ciccarone, 1996). Serum may be present in the medium on the cells during transfection. For most cationic lipid reagents, on most cell types, transfection activity is not inhibited in the presence of serum provided the complexes were formed in serumfree medium (Brunette et al., 1992; Ciccarone et al., 1993, 1999; Shih et al., 1997). The specific serum-free medium used to dilute the lipid and DNA can have a slight effect on the efficiency of transfection. For the enhanced protocol (Alternate Protocol), normal culture medium such as DMEM is recommended. For the standard procedure (Basic Protocol 1, 2, and 3), Opti-MEM I medium works best. The improvements resulting from using the recommended media are less than two fold. When using Lipofectin, dilution in OptiMEM I followed by a 30- to 45-min incubation is recommended (Ciccarone and Hawley-Nelson, 1995). With LipofectAmine 2000, the reverse is true: extended incubation (>30 min) of LipofectAmine 2000 in Opti-MEM prior to addition of DNA results in lower transfection activity (Ciccarone et al., 1999). High-purity DNA will increase transfection efficiency. Miniprep DNA does work, however, when efficiency is not critical. A wide range of sizes of polynucleotides may be transfected, from 18-mer single-stranded oligonucleotides (Chiang et al., 1991; Bennett et al., 1992; Yeoman et al., 1992; Wagner et al., 1993) to 400-kb YAC DNAs (Lamb et al., 1993). Excess vortexing of complexes or DNA solutions may result in some shearing, especially with larger molecules. The concentration of EDTA in the diluted DNA should not exceed 0.3 mM. Transgene expression may be increased in some cell types by inducing the promoter. This is observed in Jurkat cells when phytohemagglutinin and phorbol myristate acetate are added following transfection to activate the cytomegalovirus promoter (Schifferli and Ciccarone, 1996).
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thing to try is to work with a freshly thawed culture or isolate (Hawley-Nelson and Shih, 1995). Be sure the same number of cells is plated in each experiment, since plating density affects efficiency and peak position (HawleyNelson et al., 1993). Low cell yield often results from the use of too much DNA or cationic lipid reagent. Use lower concentrations of these two components and examine the results for transfection efficiency as well as cell yield. Acceptable efficiency can usually be obtained with higher cell yield by using lower concentrations of lipid and DNA (Hawley-Nelson et al., 1993; Life Technologies, 1999).
Troubleshooting The most common complaints surrounding transfections include decreased transfection efficiency and low cell yield. Decreases in efficiency often result from changes in the target cell line. Cultured cell lines are usually aneuploid and often consist of a mixture of genotypes and phenotypes that can be subject to selection in the laboratory environment. Primary cultures, although usually genotypically uniform, often consist of a mixture of phenotypes from different tissues and can change their population characteristics in response to their environment. Whenever a decrease in transfection efficiency is observed, the first
DNA (µg/well)
A
Cationic lipid reagent (µl)
0.4
0
0.5
1
1.5
2
2.5
0.8
1
1.5
2
2.5
3
3.5
1.2
1
1.5
2
2.5
3
3.5
1.6
1
1.5
2
2.5
3
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B
Transfection of Cultured Eukaryotic Cells Using Cationic Lipid Reagents
Figure 9.4.4 Results of fine-tuning or optimizing conditions for transfection. Before transfection, 293 H cells were plated at 2 × 105/well in a 24-well plate precoated with poly-D-lysine. The following day, cells were transfected with pCMV⋅SPORTβgal DNA using LipofectAmine 2000 as described (see Support Protocol). One day posttransfection, cells were fixed and stained with Xgal. (A) Amounts of DNA and lipid reagent used. (B) Results of Xgal staining.
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Table 9.4.5
Plate 24-well 12-well 6-well 60-mm 100-mm
Activity for a Scaled-up Transfection Using LipofectAmine Plus in BHK-21 Cells
Surface area ratio to 24-well plate 1 2 5 10 28
Cells/well (× 104) 4 8 20 40 112
DNA/well Plus reagent (µg) (µl) 0.4 0.8 2 4 11.2
LipofectAmine reagent (µl)
2 4 10 20 560
2 4 10 20 56
β-gal (ng/cm2)a 188 ± 5 193 ± 12 179 ± 27 171 ± 16 157
aResults are the mean of three transfections ± the standard deviation.
Cell yield can also be improved in several other ways. (1) Increasing the plating density. This usually requires adjustment of lipid and DNA amounts, but often the transfection efficiency as well as the cell yield increases with higher plating input (Life Technologies, 1999). (2) Decreasing time of exposure of the cells to cationic lipid–DNA complexes. This can be done by increasing volume and adding back serum at earlier times or by removing the complexes from the cells at the end of transfection. (3) Performing the transfection in the presence of serum. Most cationic lipid reagents work well in transfection medium containing serum (Brunette et al., 1992; Ciccarone et al., 1993, 1999; Shih et al., 1997). One exception is LipofectAmine without the Plus enhancer. Some cationic lipid solutions are naturally cloudy. Sometimes cloudiness is observed when complexes are made with DNA. Usually this does not interfere with transfection efficiency. Most cationic liposome solutions (especially DMRIE-C and CellFectin) should be mixed gently by inversion just before use to produce a uniform suspension. With Plus reagent, it is possible to precipitate the DNA when the Plus reagent is diluted first into the DMEM and DNA is added second. Always dilute the DNA into DMEM and mix well before adding Plus reagent.
al., 1999). Efficiencies also vary for suspension cells. The authors note that while LipofectAmine Plus is relatively inefficient for transfecting Jurkat cells, DMRIE-C can yield up to 85% blue cells following pCMV⋅SPORTβgal plasmid DNA transfection, gene activation with phytohemagglutinin and phorbol myristate acetate, and Xgal staining (Ciccarone et al., 1995, Schifferli and Ciccarone, 1996). The result of a typical fine-tuning/optimization protocol is shown in Figure 9.4.4. The transfection reagent was LipofectAmine 2000, the DNA was pCMV⋅SPORTβgal, the cells were 293 H. Cells were stained the day following transfection and were allowed to stain overnight at 37°C. A selection of transfection conditions can be made. Conditions found to be advantageous for transfection in small wells may be scaled up, the results of a typical scale-up are given in Table 9.4.5. The results of a stable transfection of NIH 3T3 cells with pSV2neo DNA using an enhancer reagent (LipofectAmine Plus) are shown in Figure 9.4.5. The transfection was done on 24-well plates, the cells were passaged onto 6-well plates the following day, and selection with geneticin was done for 10 days. The figure shows optimization of conditions and the generally high efficiency that can be achieved using this method.
Anticipated Results Transfection should be observed for most adherent mammalian cells transfected with DNA using Basic Protocol 1. Efficiencies vary with cell type. For example, 293, COS-7, and CHO-K1 cells yield 95% or more blue cells following Xgal staining of cells transfected with pCMVSPORTβgal plasmid DNA using LipofectAmine 2000. The authors have noted efficiencies of other cell types as high as 49% for SK-BR3 breast cancer cell lines, 77% for BE(2)C human neuroblastoma cells, and 43% for MDCK canine kidney cells (Ciccarone et
Time Considerations Counting and plating the cells should be done the day before transfection and will usually require 106 for a wide variety of genes, and neither produces replication-competent retrovirus. Although retroviruses have been most widely used for infecting cells through the ecotropic or amphotropic receptors, methods have recently been described for expanding the host range of retroviral vectors. To date, this has been best exploited for vesicular stomatitis virus (VSV) G protein, which confers a polytropic host range capable of infecting mammalian, fish, and insect cells (Burns et al., 1993; reviewed in Yee et al., 1994). VSV G protein is thought to mediate cell entry by interacting with phosphatidyl serine and possibly other components of lipid bilayers present in the cell membranes of most mammalian and nonmammalian eukaryotic cells (Schlegel et al., 1983). In addition to their wide host range, retroviruses pseudotyped with VSV G protein can be concen-
Introduction of DNA into Mammalian Cells
Contributed by Warren Pear
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Current Protocols in Molecular Biology (1996) 9.11.1-9.11.18 Copyright © 2004 by John Wiley & Sons, Inc.
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trated to achieve titers >109 infectious U/ml. An important breakthrough for VSV G pseudotyping was the use of transient transfection in 293 cells for expressing VSV G protein. This is because VSV G protein is fusogenic and leads to syncytium formation and cell death. By expressing VSV G protein by transient transfection in 293 cells, it is possible to successfully pseudotype retroviral virions during a window between the onset of protein expression and massive cell death. This unit describes methods for optimizing retroviral production from the 293-based systems (see Basic Protocol 1) and for growing and freezing 293 cells (see Support Protocol). Methods are included for pseudotyping the virus with VSV G protein by sequential transfection (see Basic Protocol 2) or cotransfection (see Alternate Protocol 1). Virus produced by transiently transfected cells can be used to infect cells. Adherent cells can be infected directly with retroviral supernatant (see Basic Protocol 3) or by spin infection (see Alternate Protocol 2). Nonadherent cells can be infected by addition of retrovirus supernatant (see Basic Protocol 4), cocultivation with producer cells (see Alternate Protocol 3), or spin infection (see Alternate Protocol 4). These infection methods are also applicable to retrovirus produced by any of the stable producer cell lines (UNIT 9.10). CAUTION: The viral supernatants produced by these methods may contain potentially hazardous recombinant constructs. The user must exercise due caution in the production, use, and storage of recombinant retroviral virions, especially those with amphotropic and polytropic host ranges, for all genes expressed by the retroviral vectors. Appropriate NIH and regional guidelines should be followed in the use of these recombinant retrovirus production systems. NOTE: All solutions and equipment coming into contact with cells must be sterile, and proper sterile technique should be used accordingly. NOTE: All culture incubations are performed in a humidified 37°C, 5% CO2 incubator unless otherwise specified. BASIC PROTOCOL 1
TRANSIENT TRANSFECTION OF A RETROVIRUS VECTOR INTO 293 CELLS The first step in producing infectious virus from a bacterial plasmid involves introduction of the plasmid into the retroviral packaging cells. Transfection is the method of choice for this step because it causes many copies of the retroviral vector plasmid DNA to be taken up by each cell. This results in a high level of expression with consequent high retroviral titer. Although other equally efficacious methods exist (see Critical Parameters), the least expensive and simplest method is a modification of the calcium phosphate–mediated transfection protocol of Graham and van der Eb (1973; see UNIT 9.1 and discussion of Optimization of Transfection). Although it is possible to scale up, highest retroviral titers are obtained using 60-mm dishes. This protocol can be used to cotransfect retroviral structural genes together with the retroviral vector into 293 cells or to transfect retroviral vector into 293-derived packaging cell lines.
Transient Production of Retroviral Supernatants
Materials 293 cells or appropriate packaging cell line (see Table 9.9.1) 293 cell growth medium (see recipe) 25 mM chloroquine in PBS (1000×; store at −20°C; optional) Retroviral plasmid DNA 2 M CaCl2 2× HEPES-buffered saline (HeBS), pH 7.05 (see recipe)
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500 mM sodium butyrate, pH 7.0 (50×; optional) 60-mm tissue culture dishes 0.45-µm filter Sorvall RT-3000B centrifuge and rotor (or equivalent) Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and calcium phosphate transfection (UNIT 9.1) NOTE: All solutions should be filtered through a 0.2-µm filter or autoclaved. 1. Twenty-four hours prior to to transfection, plate 2.5 × 106 293 cells in 4 ml of 293 cell growth medium (6.25 × 105 cells/ml) in a 60-mm tissue culture dish (see Critical Parameters). The culture should be ∼80% confluent prior to transfection and cell clumping should be minimized.
2. Just prior to transfection, remove the medium and gently add fresh 293 cell growth medium. Addition of 25 ìM chloroquine to the medium before transfection may boost the retroviral titer by 2-fold. If chloroquine is used, it is important that it not be left on the cells for >12 hr (see Critical Parameters).
3. Prepare a transfection cocktail by adding 6 to 10 µg retroviral vector plasmid DNA to water and diluting to 438 µl final volume. Add 62 µl of 2 M CaCl2 to the DNA solution. Plasmid DNA prepared by cesium chloride gradients or by using a commercial kit appears to work equally well. It is unnecessary to perform additional phenol extraction or precipitation steps prior to using the DNA, and the DNA may be stored in either water or TE buffer. When cotransfecting retroviral vector with retroviral structural genes on multiple plasmids, the total amount of transfected DNA should be 10 to 15 ìg and the plasmids should be introduced in approximately equimolar ratios. These amounts and ratios may have to be altered to optimize retroviral titers (see Critical Parameters).
4. Add 500 µl of 2× HeBS and mix by shaking the tube or pipetting. Within 1 to 2 min, add this solution to the cells and gently swirl the plate to ensure uniform mixing. Incubate until transfected cells are close to 100% confluent (24 to 48 hr post transfection). Remove the medium and add fresh medium at ∼24 hr post transfection (see step 5). HeBS may also be added to the DNA/CaCl2 solution by bubbling. Transfection efficiency is highest if DNA/CaCl2/HeBS mix is added immediately to cells rather than waiting for a visible precipitate to form (30 to 60 min). Although a precipitate is usually visible on the plate within 1 to 2 hr after transfection, the quantity of precipitate appears to be related to the amount of input DNA (W. Pear, unpub. observ.), and the amount of precipitate is usually an unreliable indicator of transfection efficiency. If chloroquine is included in the medium, remove medium ∼10 hr after transfection and gently replace with fresh 293 cell growth medium. Addition of 10 mM sodium butyrate to the cells may enhance retroviral titers by several-fold (Soneoka et al., 1995). It should be added to the 293 growth medium 12 to 24 hr after transfection at a final concentration of 10 mM. Cells should be treated for 12 to 14 hr with sodium butyrate and washed with PBS prior to proceeding to step 5.
5. Approximately 16 to 24 hr prior to harvesting the retroviral supernatant, remove the medium and gently replace with 4 ml of 293 cell growth medium. The transfected cells should be close to 100% confluent at this time. The relative retroviral titer may be increased by adding 3 ml rather than 4 ml of medium. If the cells that will be infected require special medium (e.g., RPMI medium, growth factors), the special medium,
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rather than 293 cell growth medium, should be added to the cells. Except for the presence of fetal bovine serum, 293 cells appear to tolerate a wide variety of culture conditions.
6. Harvest retroviral supernatant ∼48 hr following transfection by gently removing the supernatant from the cells. Either filter the supernatant through a 0.45-µm filter or centrifuge it 5 min at 500 × g (1000 rpm in Sorvall RT-3000B rotor), 4°C, to remove live cells. At harvest, the supernatant will be yellow. Acidity does not appear to affect retroviral titer. Once retroviral supernatant is harvested, the transfected cells may be assayed for transfection efficiency by staining for a histochemical marker (such as lacZ, UNIT 9.10) or cell surface marker, by preparing DNA from the cells and assaying by Southern hybridization (UNIT 2.9A), or by lysing cells for protein immunoblot analysis (UNIT 10.8). Optimal transfection efficiencies should range from 30% to 60%. If the supernatant is to be tested for the presence of replication-competent (helper) virus, see UNIT 9.13. It is necessary to wait at least 36 hr post transfection to obtain high-titer retroviral supernatants. At this point, it may be possible to serially, harvest and replenish supernatant every 12 hr up to 72 hr post transfection without a significant drop in retroviral titer. If the cells are sparse at 24 hr post infection, it may be necessary to wait until 72 hr to obtain high-titer supernatant.
7. If the retroviral supernatant is to be used within 1 to 2 hr, store it on ice. For longer intervals, freeze the supernatant on dry ice and transfer it to −80°C. Retroviral half-life is 3 to 6 hr at 37°C (Sanes et al., 1986). To maintain high titers, supernatants should be left on ice or frozen following harvest. Freezing does not appear to cause >2-fold drop in titer, as long as the supernatant does not undergo more than one freeze/thaw cycle; exposure to more than one such cycle causes a marked drop in retroviral titer. For this reason, supernatant should be aliquoted in appropriate volumes for subsequent infection. To thaw frozen supernatants, warm for a minimal period at 37°C and use immediately. SUPPORT PROTOCOL
GROWTH AND STORAGE OF 293 CELLS The two most important variables in determining the success of retroviral production are the state of the packaging cells and the quality of the transfection reagents. This protocol discusses methods for optimizing growth of 293 cells, passaging the cells, and freezing stocks. Further details for the culture, freezing, and storage of mammalian cell lines can be found in APPENDIX 3F. Materials 293 cells or packaging cell lines (e.g., Bosc23; see Table 9.9.1) derived from 293 cells 293 cell growth medium (see recipe) Freezing medium: 90% (v/v) heat-inactivated FBS/10% (v/v) DMSO Trypsin solution: 0.05% (v/v) trypsin/0.53 mM EDTA PBS without Ca2+ or Mg2+ (APPENDIX 2) 10-cm tissue culture dishes Sorvall RT-3000B centrifuge and rotor (or equivalent) 2-ml cryogenic vials
Transient Production of Retroviral Supernatants
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Culture and passage 293 cells 1. Passage and grow 293 cells or the packaging cell lines derived from them in 10 ml of 293 cell growth medium in 10-cm tissue culture dishes. Cells are typically grown in 10-cm dishes; however, other sizes can be used and the protocols scaled appropriately. It is recommended that cells be maintained at relatively high density and passaged at a split ratio ≤1:5 to maintain uniformity of the cells in culture. This is especially true for packaging cell lines. It is also recommended that when cells are first received they be expanded to prepare 50 to 100 vials of frozen cells to provide a uniform source of cells for virus production. Cells should not be allowed to become overconfluent, as this leads to formation of cell clumps that can cause uneven cell distribution after replating and result in less efficient transfection. Cells should be split when they reach 90% confluence and the medium is acidic. If clumping occurs, growing cells for 1 to 2 passages at high density (1:3 split) should remove the clumps. 293 cells and packaging lines derived from those cells appear to tolerate a wide variety of growth conditions and addition of factors without drop in retroviral titer. One variable that is important for maximal retroviral titers is to use heat-inactivated fetal bovine serum (FBS) in the tissue culture medium, rather than horse serum (HS) or newborn calf serum (NCS). To date, both DMEM and RPMI have given good results, and the addition of IL-2, IL-3, IL-6, and stem cell factor (SCF) has not caused the retroviral titers to decrease (W. Pear, unpub. observ.).
2. When the cells are confluent, remove the medium and gently rinse once with PBS without Ca2+ and Mg2+. 293 cells are much less adherent than murine fibroblasts. As a result, washes must be performed very gently to avoid removing cells.
3. Add 1 ml trypsin solution and incubate 2 to 3 min at 37°C. Although cells may be detached by tapping the plate after 30 sec, a more efficacious method to obtain a single-cell suspensions is to return the cells to the incubator for 2 to 3 min. The cells should then detach by themselves.
4. Quench trypsin with 4 ml of 293 growth medium and transfer the cell suspension to a conical centrifuge tube. Centrifuge 3 min at 500 × g (1000 rpm in Sorval RT-3000B rotor), 4°C or room temperature. To obtain single-cell suspensions, it is recommended that cells be vigorously pipetted in a conical tube rather than on the tissue culture dish.
5. Resupend pellet vigorously in 5 ml of 293 cell growth medium. Let the suspension stand 1 min to allow the largest clumps to settle. Aliquot the cells without using the last 200 µl of cell suspension (clumps). Freeze 293 cells 6. Incubate a 10-cm dish of 293 cells in 293 growth medium until the cells are 80% confluent. Maximal viability is achieved if cells are frozen prior to confluence. One 10-cm dish that is 80% confluent will provide enough cells for 2 to 3 vials of frozen cells.
7. Using trypsin solution, remove the cells from the dish (see steps 2 to 4). Centrifuge the cells 3 min at 500 × g, 4°C or room temperature. 8. Remove the medium and resuspend the cells in 2 to 3 ml freezing medium per 10-cm dish.
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9. Add 1 ml cell suspension to each 2-ml cryogenic vial and store overnight at −80°C. On the following day, transfer the vials to liquid nitrogen. Thaw 293 cells 10. Remove a vial of cells from liquid nitrogen and thaw rapidly, warming by hand or at 37°C. 11. Add 1 ml 293 growth medium to the cryogenic vial and add the cell suspension to a tube containing 10 ml growth medium. Centrifuge 3 min at 500 × g, 4°C or room temperature. 12. Remove the supernatant and resuspend the cell pellet in 10 ml growth medium. Transfer the cells to a 10-cm tissue culture dish. Incubate. BASIC PROTOCOL 2
PSEUDOTYPING A STABLE CELL LINE SEQUENTIALLY WITH VSV G PROTEIN Pseudotyping retroviruses with the vesicular stomatitis virus (VSV) G protein creates virus with a polytropic host range and makes it possible to concentrate the virus by centrifugation. Calcium phosphate–mediated transfection yields retroviral titers in the range of 105 to 106 CFU/ml; however, concentration of the viruses can increase the titer by several log factors. Because concentration requires larger initial volumes of retroviral supernatants, best results are obtained if a stable cell line that expresses gag-pol and the gene of interest is prepared first. A VSV G protein expression vector is then transiently expressed in this cell line for production of retroviral supernatants. This method is adapted from Burns et al. (1993) and Matsubara et al. (1996). An alternative method is to introduce retroviral vector and VSV G protein–expressing plasmids by cotransfection (see Alternate Protocol 1); it should be possible to introduce gag-pol construct at the same time. Although the latter approach may be laborious for obtaining large quantities of retroviral supernatants for concentration, it may yield higher titers for proteins that are detrimental to the long-term growth of 293 cells. Materials Retroviral packaging cell line with amphoteric host range (e.g., PA317, ΨCRIP, GP+Am12, or Bing; see Table 9.9.1) and appropriate culture medium Retroviral vector DNA gag-pol-expressing cell line (e.g., Anjou65, ATCC CRL 11268; see Table 9.9.1) and appropriate culture medium 293 cell growth medium (see recipe) VSV G protein expression plasmid: e.g., pHCMV-VSV-G (Matsubara et al., 1996) TNE buffer (see recipe) or 1% (v/v) Hanks basic salt solution (HBSS; APPENDIX 2) 60-mm tissue culture dishes Sorvall RT-3000B centrifuge and rotor (or equivalent) Beckman L3-50 centrifuge and SW 41 rotor (or equivalent) Additional reagents and equipment for culturing of mammalian cells (APPENDIX 3F), transfecting cells and harvesting virus (see Basic Protocol 1), infecting cells (see Basic Protocols 3 or 4 or Alternate Protocols 2, 3, or 4), selecting clones with drugs (UNIT 9.5), assaying clones for the gene of interest (e.g., UNITS 9.6-9.8 & 9.10), and freezing cells (see Support Protocol)
Transient Production of Retroviral Supernatants
Create a stable cell line 1. Transfect a retroviral packaging cell line with an amphotropic host range with 6 to 20 µg retroviral vector DNA (see Basic Protocol 1). Incubate 48 hr.
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2. Twenty-four hours before harvesting retroviral supernatant, plate 2.5 × 106 gag-polexpressing cells into the appropriate culture medium in 60-mm tissue culture dish. 3. Collect retroviral supernatant at 48 hr post transfection. Filter through a 0.45-µm filter or centrifuge 5 min at 500 × g (1000 rpm in Sorvall RT-3000B rotor), 4°C, to remove cells. 4. Use 1 ml retroviral supernatant to infect the gag-pol-expressing cell line. For production of stable producer cell lines, infection is preferred to transfection because there is a reduced risk of rearrangement.
5. Select individual clones of stable producer cell lines—i.e., by drug selection (UNIT 9.5), FACS sorting, or other method. 6. Identify clones that express the gene of interest (e.g., UNITS 9.6-9.8 & 9.10). These are the stable producer cell line(s) that will be used for subsequent experiments.
7. Aliquot and prepare frozen stocks of the stable producer cell line (see Support Protocol). Pseudotype the producer cell line 8. Twenty-four hours prior to transfection, plate 2.5 × 106 stable producer cells in appropriate culture medium in a 60-mm dish. If desired, the procedure can be scaled up for larger volumes. It is even possible to transfect spinner cultures (J.C. Burns, pers. comm.).
9. Transfect stable producer cells with 3 to 6 µg VSV G protein expression plasmid by modified calcium phosphate–mediated transfection (see Basic Protocol 1). Incubate 24 hr. Adjust the volumes proportionally if larger volumes are used in step 8.
10. Replace the medium with fresh 293 cell growth medium and continue incubation 24 to 48 hr. 11. Harvest retroviral supernatants between 48 and 72 hr by collecting the medium from the culture. Concentrate supernatant immediately (proceed to step 12) or store at −80°C. Syncytium formation is evident ∼36 hr following transfection. Syncytia are identified by the formation of multinucleated giant cells which are the result of membrane fusion caused by VSV G protein.
Concentrate VSV G protein–pseudotyped viral supernatant 12. Concentrate retroviral supernatant by centrifuging 90 min at 50,000 × g (17,000 rpm in an SW 41 rotor), 4°C. 13. Resuspend the pellet overnight at 4°C in 20 µl of TNE buffer or 0.1% HBSS. Pellet is resuspended in 1/200 of viral culture volume.
14. Use concentrated retroviruses immediately for infection, store them frozen at −80°C, or concentrate the stock by another round of centrifugation (steps 12 and 13).
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ALTERNATE PROTOCOL 1
PSEUDOTYPING COTRANSFECTIONALLY WITH VSV G PROTEIN It is also possible to produce a retrovirus pseudotyped with vesicular stomatitis virus (VSV) G protein by transient cotransfection. This method is adapted from J. Burns (pers. comm.) and Pear et al. (1996). Materials 293 cells or 293-derived cells expressing retroviral gag-pol: e.g., Anjou65 (ATCC CRL 11269) 293 cell growth medium (see recipe) Retrovirus vector DNA VSV G protein expression plasmid: e.g., pHCMV-VSV-G (Matsubara et al., 1996) 60-mm tissue culture dishes Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F), modified calcium phosphate–mediated transient transfection of cells (see Basic Protocol 1), and concentrating pseudotyped viral supernatants (see Basic Protocol 2) 1. Twenty-four hours prior to transfection, plate 2.5 × 106 293 cells, or cells of a 293 derivative stably expressing the retroviral gag-pol genes, into 293 cell growth medium in a 60-mm dish. 2. Cotransfect 3 to 5 µg retroviral vector DNA and 3 to 5 µg of a VSV G protein expression plasmid by modified calcium phosphate–mediated transfection (see Basic Protocol 1). Incubate. If 293 cells were plated in step 1, also cotransfect a gag-pol expression plasmid.
3. On the following day, replace the medium with 4 ml fresh 293 growth medium. 4. Harvest retroviral supernatants between 48 and 72 hr by collecting the culture medium. Concentrate the virus immediately (see Basic Protocol 2) or store at −80°C. Syncytium formation is evident ∼36 hr following transfection. BASIC PROTOCOL 3
INFECTION OF ADHERENT CELLS WITH RETROVIRAL SUPERNATANT This protocol gives methods for infecting adherent cells with retroviral supernatant. This is the most common method for determining the retroviral titer (UNIT 9.10) and testing for the presence of helper virus (UNIT 9.13). Infectious titer can be quantitated based on marker staining—e.g., using lacZ (UNIT 9.10), or green fluorescent protein (GFP; UNIT 9.7C)—or drug resistance (UNIT 9.5). Recent reports suggest that viral transduction by centrifugation (spin infection; see Alternate Protocol 2) will increase infectious titers 2- to 4-fold (Kotani et al., 1994). These methods are written for cells of the commonly used murine fibroblast line NIH 3T3; however, they should be adaptable to other adherent cell lines with minor modifications.
Transient Production of Retroviral Supernatants
Materials Retroviral supernatant, fresh or frozen (see Basic Protocols 1 or 2) Target cells: e.g., NIH 3T3 cells 4 mg/ml polybrene in PBS (APPENDIX 2), filtered and stored at 4° or −20°C Fibroblast growth medium Antibiotic for drug selection (UNIT 9.5; optional) 10-cm tissue culture dishes
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Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and for assaying of reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or drug selection (UNIT 9.5). 1. Twelve to eighteen hours prior to transfection, plate 5 × 105 NIH 3T3 cells into appropriate tissue culture medium in a 10-cm tissue culture dish. Infection protocols can be scaled up or down. For a 60-mm plate, plate 2 × 105 cells and infect in a volume of 1 ml.
2. Prepare a 3-ml infection cocktail containing: Retroviral supernatant: ≤1.5 ml 4 µg/ml polybrene Fibroblast growth medium to a final volume of 3 ml. The retroviral supernatant should be ≤1⁄2 the final volume of the infection cocktail. 293 supernatants appear to contain a cytostatic factor that can be diluted out by changing the transfection medium 18 to 24 hr before retroviral harvest and by avoiding infectious stocks that contain viral supernatant at ratios >1:1 (Naviaux et. al., 1996).
3. Remove the medium from the dish of cells and add the infection cocktail. Incubate ≥3 hr. If infection continues >6 hr, a 5-ml infection cocktail is recommended to prevent dehydration.
4. After the infection incubation, add 7 ml fibroblast growth medium to the dish. Incubate 36 to 48 hr. Drug selection (e.g., G418, puromycin) can begin 24 hr post infection (UNIT 9.5). If the cells are not over-confluent (i.e., if they are 50% confluent, it is necessary to split the cells as described in UNIT 9.5.
5. Assay the cells for reporter gene expression (UNITS 9.6-9.8 & 9.10). INFECTION OF ADHERENT CELLS BY SPIN INFECTION Adherent cells can also be infected by centrifuging the cells in the presence of the infection cocktail (spin infection). This protocol is modified from the methods of Kotani et al. (1994).
ALTERNATE PROTOCOL 2
Materials Target cells: e.g., NIH 3T3 cells Fibroblast growth medium Retroviral supernatant 4 mg/ml polybrene in PBS (APPENDIX 2), filtered and stored at 4° or −20°C 6-well tissue culture plates Beckman GS-6KR or Sorvall RT-3000B centrifuge with microplate carriers (or equivalent) Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and for assaying of reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or drug selection (UNIT 9.5)
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1. Twelve to eighteen hours prior to infection, plate 1 × 105 target cells per well into fibroblast growth medium in a 6-well tissue culture plate. 2. For each well to be transfected, prepare a 4-ml infection cocktail containing: Retroviral supernatant: ≤2 ml 4 µg/ml polybrene Fibroblast growth medium up to 4 ml. An infection cocktail of ≥4 ml is necessary to prevent dehydration of the cells. The retroviral supernatant should be ≤1⁄2 the final volume of the infection cocktail. 293 supernatants appear to contain a cytostatic factor that can be diluted out by changing the transfection medium 18 to 24 hr before retroviral harvest and by avoiding infectious stocks that contain viral supernatant at ratios >1:1 (Naviaux et. al., 1996).
3. Remove the medium from the cells and add the infection cocktail. 4. Centrifuge the cells 1.5 to 2 hr at 1000 × g (2500 rpm in Beckman GS-6KR or Sorvall RT-3000B rotor), room temperature. 5. Return the cells to the incubator. 6. Change medium the next day and incubate until 48 hr post infection. 7. Assay for reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10), add antibiotic for drug selection (UNIT 9.5), or test for effect of the gene of interest. BASIC PROTOCOL 4
INFECTION OF NONADHERENT CELLS BY RETROVIRAL SUPERNATANT Nonadherent cells can be infected by direct addition of retroviral supernatant or by cocultivation of nonadherent cells with retroviral producer cells (see Alternate Protocol 3). The advantage of cocultivation is that there is continuous retroviral production; however, this must be weighed against the disadvantage of harvesting producer cells together with target cells. This effect may be minimized by irradiating producer cells prior to cocultivation. Alternatively, nonadherent cells can be infected by spin infection (see Alternate Protocol 4). In general, infection by cocultivation is equivalent to spin infection. Materials Retroviral supernatant 4 mg/ml polybrene in PBS (APPENDIX 2), filtered and stored at 4° or −20°C Target cell growth medium Exponentially growing nonadherent target cells 15-ml centrifuge tube 60-mm tissue culture dishes Sorvall RT-3000B centrifuge and rotor (or equivalent) Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and for assaying of reporter gene expression (UNITS 9.6-9.8 & 9.10) or drug selection (UNIT 9.5) 1. Prepare a 3-ml infection cocktail containing:
Transient Production of Retroviral Supernatants
Retroviral supernatant: ≤1.5 ml 2 µg/ml polybrene Target cell growth medium up to 3 ml. The retroviral supernatant should be ≤1⁄2 the final volume of the infection cocktail. 293 supernatants appear to contain a cytostatic factor that can be diluted out by changing
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the transfection medium 18 to 24 hr before retroviral harvest and by avoiding infectious stocks that contain viral supernatant at ratios >1:1 (Naviaux et. al., 1996). Many nonadherent cells are more stringent in their growth requirements than fibroblasts. For this reason, it may be necessary to use a medium other than 293 cell growth medium during the 24 hr prior to viral harvest and/or add growth factors or cytokines to the medium. It is essential, however, that heat-inactivated fetal bovine serum (FBS) be used as the serum source.
2. Centrifuge 3 × 105 to 3 × 106 exponentially growing target cells 5 min at 500 × g (1000 rpm in Sorvall RT-3000B rotor), 4°C or room temperature. 3. Remove supernatant and resuspend the cells in the infection cocktail at a final density of 105 to 106 cells/ml. 4. Add the cell suspension to a 60-mm dish and incubate 24 hr. 5. Twenty-four hours post transfection, centrifuge the cells 5 min at 500 × g, 4°C or room temperature, and resuspend the pellet in target cell growth medium. Incubate cells another 24 hr. 6. Assay for reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10), add antibiotic for drug selection (UNIT 9.5), or determine infectious titer (UNIT 9.10). INFECTION OF NONADHERENT CELLS BY COCULTIVATION Nonadherent cells can also be infected by cocultivating them with retroviral producer cells for 48 hr.
ALTERNATE PROTOCOL 3
Materials Transfected packaging cells (see Basic Protocol 1) in 60-mm tissue culture dishes Retroviral supernatant 4 mg/ml polybrene in PBS (APPENDIX 2), filtered and stored at 4° or −20°C 105 to 106 cells/ml nonadherent target cells Target cell growth medium 15-ml conical centrifuge tubes 60-mm tissue culture dishes Sorvall RT-3000B centrifuge (or equivalent) Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and for assaying of reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or drug selection (UNIT 9.5) 1. Twenty-four hours after packaging cells have been transfected, prepare a 3-ml infection cocktail containing: Retroviral supernatant: ≤1.5 ml 2 µg/ml polybrene 3 × 105 to 3 × 106 nonadherent target cells Target cell growth medium to 3 ml. Replication of transfected retroviral packaging cells without loss of retroviral production can be achieved by irradiating them with 1500 rads (C. Zent, Univ. of Chicago, unpub. observ.) prior to addition of infection cocktail.
2. Aspirate the medium from the transfected packaging cells and gently add the infection cocktail to the cells. Incubate 24 hr. To prevent detachment of adherent transfected packaging cells, add cocktail to the side of the dish rather than directly onto the cells.
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3. Twenty-four hours after the infection cocktail is added (48 hr post transfection), gently remove 2 ml medium (which will contain many nonadherent cells) and transfer to a conical tube. Centrifuge 5 min at 500 × g, 4°C or room temperature. It is not necessary to remove all of the nonadherent cells from the dish because the purpose of this step is to replenish the medium (which is acidic). Sufficient residual medium should remain on the dish to prevent dehydration during the short centrifugation step.
4. Prepare a fresh infection cocktail containing: Retroviral supernatant: ≤1.5 ml (optional) 2 µg/ml polybrene Target cell growth medium to 3 ml. Adding virus in this step is optional, but it may increase infection efficiency.
5. Remove the supernatant and gently resuspend the cell pellet in freshly prepared infection cocktail. 6. Using extreme care to avoid disruption of the adherent cell layer, add the fresh infection cocktail to the wall of the plate. Incubate 24 hr. 7. At 72 hr post transfection, collect nonadherent cells from the dish by gentle pipetting. It is necessary to sufficiently wash the plate with FBS or medium to remove most nonadherent cells while minimizing contamination with adherent cells. Typically, there is about 10% contamination of nonadherent cells. Because adherent cells detach in sheets, rather than as individual cells, many of the cells can be removed by allowing th large clumps of cells to settle out and removing the others (see Support Protocol, step 5). Alternatively, cells can be transferred to a new 60-mm dish and given sufficient time for the adherent cells to reattach (∼3 hr), after which the nonadherent cells can be removed.
8. Centrifuge the suspension of nonadherent cells 5 min at 500 × g, 4°C or room temperature. Resuspend the cells in target cell growth medium. Plate the target cells in 60-mm tissue culture dishes. Incubate an additional 24 to 48 hr. 9. Assay for reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or add antibiotics for drug selection (UNIT 9.5), or assay for the gene of interest. ALTERNATE PROTOCOL 4
INFECTION OF NONADHERENT CELLS BY SPIN INFECTION Alternatively, nonadherent target cells can be infected by centrifuging the cells in the presence of infection cocktail, a method modified from Kotani et al. (1994). Materials Retroviral supernatant 4 mg/ml polybrene in PBS (APPENDIX 2), filtered and stored at 4° or −20°C Target cell growth medium Exponentially growing nonadherent target cells 24-well tissue culture plates Sorvall RT-3000B centrifuge with microplate carrier (or equivalent) 10-cm tissue culture dishes Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F) and for assaying of reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or drug selection (UNIT 9.5)
Transient Production of Retroviral Supernatants
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1. For each well to be transfected, prepare a 2-ml infection cocktail containing: Retroviral supernatant: ≤1 ml 4 µg/ml polybrene Target cell growth medium to 2 ml. 2. Add 1–2 × 106 exponentially growing nonadherent target cells per well of a 24-well tissue culture plate. Centrifuge 5 min at 500 × g (1000 rpm in Sorvall RT-3000B rotor), 4°C or room temperature. The cell number should be adjusted so that cells completely cover the surface of the well and form a monolayer following centrifugation.
3. Remove the supernatant and add infection cocktail. Centrifuge the cells 1.5 to 2 hr at 1000 × g (2500 rpm in Sorvall RT-3000B rotor), room temperature. 4. Either immediately after centrifugation or after ≤6 hr of incubation, transfer the cells in the infection medium to a 10-cm dish. Dilute to 10 ml final volume with target cell growth medium. Incubate to 48 hr post infection. If the cells are left overnight in only 1 to 2 ml of infection cocktail, the medium will be very acidic by the following morning.
5. Assay cells for reporter gene expression (e.g., UNITS 9.6-9.8 & 9.10) or add antibiotic for drug selection (UNIT 9.5). REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
293 cell growth medium Dulbecco’s modified Eagle medium (high glucose) containing: 10% (v/v) heat-inactivated fetal bovine serum (FBS) 100 U/ml penicillin 100 U/ml streptomycin 2 mM L-glutamine Store at 4°C This medium should be stable for at least 3 months except that L-glutamine has a half-life of 1 month.
HEPES-buffered saline (HeBS), 2× 50 mM N-2-hydroxyethylpiperazine-N′-ethanesulfonic acid (HEPES) 10 mM KCl 12 mM dextrose 280 mM NaCl 1.5 mM Na2HPO4 Adjust pH to 7.05 ± 0.05 Filter through 0.2-µm filter Store in 50-µl aliquots at −20°C Avoid multiple freeze/thaw cycles. To thaw, warm to room temperature and invert or vortex the tube to achieve uniform mixing. Although it is unclear why, the ability of the 2× HeBS solution to produce working calcium phosphate precipitates deteriorates after 6 months to 1 year, even when the solution is stored at −20°C. Fresh 2× HeBS should therefore be prepared every 6 months.
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TNE buffer 50 mM Tris⋅Cl, pH 7.8 130 mM NaCl 1 mM EDTA Store at room temperature indefinately COMMENTARY Background Information
Transient Production of Retroviral Supernatants
Creation of high-titer retroviral supernatants by transient transfection offers several advantages over the creation of stable cell lines, especially for experimental purposes. The transient methods described in this unit take advantage of the high transfectability and high protein expression in 293 cells. When retroviral vectors (e.g., see Figs. 9.9.5 & 9.9.6) are expressed with the retroviral packaging constructs in these cells, the vector RNA (containing the Ψ site) is efficiently packaged and budded from the surface of the packaging cells. The infectious retrovirus vector may be harvested by removing the packaging cell supernatant. Because the transient methods are not dependent on stable integration of the introduced plasmid(s), high-titer viral supernatant may be harvested from the packaging cells from 36 to 96 hr following transfection. Two different approaches, both utilizing 293 cells, have successfully produced helper-free high-titer retroviral supernatants expressing a wide variety of genes. In the method of Pear et al. (1993), three cell lines (Anjou, Bosc23, and Bing) were made by stably transfecting plasmids expressing gag-pol (Anjou), gag-pol and ecotropic envelope (Bosc23), or gag-pol and amphotropic envelope (Bing) into 293T cells. 293T cells are a derivative of 293 cells that express the SV40 large T antigen (DuBridge et al., 1987). These cell lines were selected for high levels of expression of reverse transcriptase and the appropriate envelope protein. All of the retroviral structural genes are expressed from the Moloney murine leukemia virus (MoMuLV) long terminal repeat (LTR). High-titer retroviral supernatants may be made from these cell lines by introducing either a retroviral vector alone (Bosc and Bing) or a retroviral vector and a plasmid-expressing envelope (Anjou). Because the retroviral structural genes are expressed from two different stably integrated plasmids that contain multiple mutations (Danos and Mulligan, 1988), more than two recombination events are necessary for the creation of replication-competent retrovirus. To date, generation of helper virus by these cell lines has not been reported.
A second generation retroviral packaging cell line, termed φNX, has recently been developed for high-titer, transient production (P. Achacoso and G.P. Nolan, pers. comm.). The major addition is that the gag-pol construct has an IRES-CD8 surface marker, so it is possible to follow gag-pol expression in living cells on a cell-by-cell basis. This facilitates checking the line for continued high-level gag-pol expression using CD8 as proxy. Additionally, non-MoMuLV-based promoters are used to express gag-pol (and env) genes, further minimizing the potential for recombination with vector LTRs. Otherwise, the construction of these cell lines is similar to Bosc23. Amphotropic and ecotropic versions of these have been prepared, and they stably express gag-pol and env proteins over several months. These lines are helper-free and produce retroviral supernatants that have titers similar to Bosc23 and Bing. The methods described in this unit work well with the φNX cell line. In the strategies of Finer et al. (1994), Soneoka et al. (1995) and Naviaux et al. (1996), both the retroviral vector and the retroviral packaging constructs are cotransfected into 293 cells and retroviral supernatants are harvested 48 hr later. These groups found that titers were highest when the packaging constructs were expressed from a cytomegalovirus (CMV) promoter and a retroviral vector in which the 5′ LTR was a hybrid between CMV and MoMuLV sequences. Although previous reports suggested that cotransfection of retroviral vector and packaging constructs could lead to replication-competent retrovirus (Danos and Mulligan, 1988; Pear et al., 1993), these groups have engineered their constructs to minimize regions of homology that might lead to helper virus formation. In particular, deletion of 3′ LTR sequences appears important for preventing the formation of replication-competent retrovirus (Soneoka et al., 1995).
Critical Parameters Several parameters are important to optimize for high-titer retroviral production. Cells should be at optimal density (usually ∼80% confluent) with minimal clumping at the time
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of transfection. In addition to cell density, production of high-titer retroviral supernatants also requires that the cells continue to grow for ≥24 hr following transfection. A rough indicator is that the plate should be nearly 100% confluent ∼24 hr after transfection. Several factors that may account for failure to obtain this degree of confluence are the nature of the protein expressed by the retroviral vector, the quantity of input DNA, and the length of chloroquine and/or sodium butyrate treatment. Expression of some proteins (such as P210bcr/abl) appears detrimental to 293 cell growth. Hightiter retroviral supernatants can be obtained with vectors expressing such genes by increasing the number of cells plated prior to transfection and/or decreasing the quantity of transfected retroviral plasmid DNA. For expression of some genes, 293 cells may need to be nearly 100% confluent at the time of transfection. In addition to the protein product, it appears that the quantity of transfected DNA may have a toxic effect upon the cells. For up to 10 to 15 µg transfected DNA per 60-mm plate, increasing the amount of input DNA increases retroviral titer; however, as more DNA is added beyond that point, the quantity of DNA has a toxic effect on the cells. As discussed in the protocols, to prevent toxicity from chloroquine or sodium butyrate, cells should not be exposed to either of these reagents for ≥12 to 14 hr. When trying to optimize titers from a new retroviral construct, it is recommended that cell density, input DNA, and length of chloroquine and/or sodium butyrate treatment all be optimized. If the cells are not confluent at 24 hr post transfection, but reach confluence by 48 hr post transfection, it may be possible to obtain high-titer supernatants between 48 and 72 hr post transfection. Supernatants harvested >96 hr post transfection do not produce high retroviral titers. Another variable that must be optimized is the transfection reaction. Of particular importance is the pH of the 2× HEPES-buffered saline (HeBS), which should be 7.05 ± 0.05. For reasons that are not clear, 2× HeBS loses its effectiveness between 6 months and 1.5 years. Unfortunately, there is no way to detect this except for a bad experimental result. For this reason, it is recommended that fresh stocks of 2× HeBS be prepared every 6 months. Although the modified calcium phosphate– mediated transfection method is inexpensive and gives consistently good results, other methods of gene transfer, including lipid-mediated
transfection (UNIT 9.4), work equally well. A recent report suggests that receptor-mediated, adenovirus-augmented transfection is superior to calcium phosphate–mediated transfection for some cell types (von Ruden et al., 1995). It has not yet been determined if this method is superior for gene transfer into 293 cells. There are several techniques that may increase retroviral titer—adding factors to the transfection or cell growth medium, amplifying the plasmid DNA, decreasing the incubation temperature, and infecting by centrifugation. Addition of 25 µM chloroquine during transfection may increase titers 2-fold; presumably this is due to the lysosomal neutralizing activity of the chloroquine. Sodium butyrate has been reported to increase retroviral titers from several-fold to 1 log by acting as a transcriptional activator of the MoMuLV LTR, as well as other promoters including CMV (Soneoka et al., 1995). Increasing the extracellular concentration of dNTPs may increase retroviral titers up to 10-fold by enhancing endogenous reverse transcription (Zhang et al., 1995). 293T cells, a 293 cell derivative, contain a temperaturesensitive large T antigen, whose permissive temperature is 32°C (DuBridge et al., 1987). Thus, incubation at this temperature in conjunction with the use of constructs that contain the SV40 origin of replication may result in plasmid replication and higher retroviral titers. This may come at a cost, however, as large T–induced replication is associated with higher recombination rates (St.-Onge et al., 1993). Shifting the cells to lower incubation temperatures (32° to 34°C) following transfection has been reported to increase retroviral titers by 5to 15-fold (Kotani et al., 1994). This is hypothesized to result from both improved retroviral survival and increased production. Similarly, infecting by centrifugation may increase retroviral infection by 4- to 10-fold (Kotani et al., 1994). It is important to note that all of the above methods have given variable results in different investigators’ hands, and each should be optimized (see Troubleshooting). Unlike retroviruses expressing the amphotropic and ecotropic envelopes, VSV G protein–pseudotyped retroviruses can be efficiently concentrated 100- to 1000-fold because they can better withstand shearing forces during centrifugation. This attribute together with their polytropic host range makes them a very useful laboratory tool, especially for stable gene transfer into nonmammalian cells.
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Troubleshooting
Transient Production of Retroviral Supernatants
To maximize retroviral production, it is recommended that conditions be optimized using a nontoxic reporter protein such as lacZ (UNIT 9.10), green fluorescent protein (GFP; UNIT 9.7C), placental alkaline phosphatase (PLAP), or a cell-surface marker. During optimization, both transfection frequency of the producer cells and infection rate of the target cells should be measured. A number of variables that can affect retroviral production have been previously discussed (see Critical Parameters). To avoid drift of the packaging cell lines or highly transfectable 293 cells, it is recommended that they be maintained for ∼20 passages (2 months) before establishing a fresh stock by thawing a frozen vial of cells. It may be possible to carry the cells for longer periods without loss of efficacy, and it is recommended that the cells be assayed at various times to check the ability to produce high-titer supernatants. Bosc and Bing cells were made using 293T cells that contain selectable markers for neomycin (1 mg/ml), hygromycin (400 µg/ml), and mycophenolic acid (50 µg/ml). There does not appear to be any advantage in growing the cells in selection medium. If retroviral titers drop when using these cells, their growth may be tested by growing in hygromycin- and xanthine-guanine phosphoribosyltransferase-containing selection medium. If a majority of the cells are not resistant to these antibiotics, a fresh vial of earlier-passage cells should be thawed rather than attempting to reselect the cells. Similarly, when cells are initially received, aliquots should be tested for resistance to these antibiotics. 293 cells do not contain a selectable marker. It should be noted that the amphotropic envelope–expressing Bing cells release ∼100 units of infectious retrovirus per ml of retroviral supernatant expressing hygromycin resistance which results from the introduction of the gene for hygromycin resistance into these cells by a retroviral vector (W. Pear, unpub. observ.). Replication-competent retrovirus has not been detected in this cell line. The φNX-A cells do not produce a hygromycin-resistant retrovirus (G.P. Nolan, pers. comm.). For efficient infection, addition of a polycation increases infection efficiency by >100-fold (reviewed in Burns et al., 1993). Polybrene gives the best results at concentrations between 2 and 8 µg/ml. Polybrene may, however, be toxic to some cell types. For example, NIH 3T3 cells tolerate 4 µg/ml polybrene for at least 18
hr; however, treatment of these cells with 8 µg/ml for this period is detrimental. If high polybrene concentrations are chosen, the medium should be changed after 3 to 5 hr. For nonadherent cells, including primary bone marrow cultures, 1 to 2 µg/ml polybrene is well tolerated for ≤48 hr. It is possible to use other cations, such as protamine sulfate or poly-L-lysine; however, infectious titers are diminished by 2- to 4-fold with these agents (Burns et al., 1993). Efficient infection of nonadherent cells has been cumbersome due to the need to cocultivate the retroviral producer cells with the target cells. It is difficult to separate the producer cells from target cells. An additional difficulty is imposed by 293 cells which, because of their high density, tend to rapidly acidify medium. For this reason, it is important to replenish medium daily when performing cocultivations. An alternative to cocultivation is spin infection. This technique appears to produce equivalent results as cocultivation for several nonadherent cell lines (W. Pear, unpub. observ.). Although the original protocols of Kotani et al. (1994) indicate that infections should be performed at 32°C, many centrifuges are unable to maintain that temperature; equivalent results are obtained by spin infecting at room temperature (W. Pear, unpub. observ.). One major problem with retroviral gene transfer is that, despite producing high-titer retroviruses, it is difficult to maintain gene expression following infection. Although there are a number of potential mechanisms at the transcriptional and posttranscriptional levels, a teleologic explanation is that the retrovirally expressed protein is detrimental to cell growth resulting in outgrowth of clones that express lower levels of the protein. This is a particular problem with vectors that contain an internal promoter expressing a selectable marker, because even though the marker is selected there is no selective pressure to express the gene of interest. One method to increase the likelihood that the gene of interest will be expressed (although there may still be posttranscriptional mechanisms that affect the level of expression) is to use a polycistronic retroviral vector (see UNIT 9.10). Other methods of overcoming gene toxicity are being developed. Particularly promising are the tetracycline-inducible promoters originally described by Gossen and Bujard (1992); these promotors have shown some degree of success when incorporated into retro-
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viral vectors (Hoffman et al., 1996; Paulus et al., 1996).
Anticipated Results Transfection efficiencies of 293 cells should be in the range of 30% to 60%. Infectious titers of 106 to 107 infectious U/ml should be expected from most retroviral vectors when titered on murine NIH 3T3 cells. Titers may be slightly reduced with large cDNA inserts (>4 kb). Titers may be lower on other cell types, particularly cells expressing only the amphotropic envelope receptor. For VSV G protein pseudotyping followed by concentration by centrifugation, titers of 109 infectious U/ml should be expected.
Time Considerations Plating cells, transfections, infections, and medium changes may take from 200 ml), we recommend the longer spins and the use of a Beckman JA-14 rotor or its equivalent. With smaller volumes, we recommend the SW-27 or SW-41Ti swinging bucket rotors with shorter spins (2 hr at 20,000 rpm, 4°C). Swinging-bucket rotors make a firm pellet at the bottom of the tube, so that it is easy to pour off the supernatant without dislodging the pellet; in addition, it is easier to see the pellet in tubes spun in swinging-bucket rotors.
7. Discard supernatant while carefully avoiding discarding the pellet, which sometimes dislodges when using the large volumes.
Large-Scale Preparation and Concentration of Retrovirus Stocks
If using the JA-14 rotor, when removing the bottles from the rotor, mark the side of the bottle that should contain the pellet. The pellet will be smeared down the side of the bottle and will be difficult to see. Although normally the supernatant is discarded, a
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small volume of the supernatant may be saved to titer to determine whether the virus was indeed pelleted.
8. Gently resuspend the pellet in DMEM-10 using 0.1% to 1% of the original volume. Resuspension may take 2 hr if the pellet is fairly sticky. It is convenient to leave the centrifuge bottle in an ice bucket in the tissue culture hood and pipet the suspension approximately every 15 min. A 1- or 2-ml pipet can be used if large volumes were concentrated. If so, leave the original pipet in the bucket and use this pipet for the complete resuspension. This is because the virus pellet often sticks to the first pipet that touches it. Sometimes the pellet can be seen as an almost translucent, gooey yellow pellet. If the JA-14 rotor was used, be sure to wash down the sides of the bottle, as this is where much of the stock will be located. To resuspend pellets made in the SW-27 or SW-41 rotors, use a pipettor suitable for small volumes. In either case, avoid making bubbles, as this can denature the viral proteins. Alternatively, an easier way to resuspend pellets made in the swinging-bucket rotors is to pour off almost all of the supernatant, e.g., leaving only 0.10 to 0.15 ml in an SW-27 large bucket; this may require keeping a Pasteur pipet handy to remove the last few drips of supernatant from the mouth of the tube as one pours it off. Return the tube to the bucket and replace the bucket cap. Place the buckets on a shaker at low speed in a 4°C cold room, or place them in an ice bucket and slowly shake 1 to 2 hr on the benchtop. After shaking, gently pipet up and down several times and transfer to a smaller tube.
Titer and freeze concentrated virus 9. Store the virus at −70° or −80°C in small aliquots (typically 5 to 50 µl). Systematic tests in our laboratory have indicated that, in general, stocks can be frozen and thawed several times with no loss of titer. However, we have occasionally observed loss of titer over time in the freezer, or upon freeze-thaw. Thus, the most conservative approach is to make small aliquots and to minimize the number of times they are thawed and refrozen. We have also found that small volumes can desiccate over long-term storage. To combat this, we place a drop of the oil used to cover PCR reactions onto each small aliquot before freezing.
10. Titer an aliquot of concentrated and unconcentrated virus as described in UNIT 9.10. The titer can also be determined using an aliquot before freezing the stock.
CONCENTRATION BY PEG PRECIPITATION AND CHROMATOGRAPHY A method that is rapid and easy is to precipitate virus with polyethylene glycol, with an optional step of chromatography on Sepharose (Aboud et al., 1982).
ALTERNATE PROTOCOL 1
Additional Materials 5 M NaCl, filter sterilized Polyethylene glycol (PEG) 6000, filter sterilized NTE buffer (see recipe) Sepharose CL-4B or CL-2B (Pharmacia) Savant high-speed centrifuge or Beckman SW-41Ti rotor Additional equipment for preparation of Sepharose column (UNIT 5.6) Precipitate virus with PEG 1. Prepare a virus stock (see Basic Protocol, steps 1 to 4) 2. Add 5 M NaCl to the virus stock to 0.4 M final while stirring at 4°C. Introduction of DNA into Mammalian Cells
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3. Slowly add PEG 6000 to 8.5% final (w/v) and continue stirring 1 to 1.5 hr at 4°C. For reasons that are not clear, some batches of PEG do not work well as they lead to problems in resuspending the pellet.
4. Collect the precipitate by centrifuging 10 min at 7000 × g (e.g., at 7500 rpm in a SW-41Ti rotor or 10,000 rpm in a Savant high-speed centrifuge.) 5. Dissolve the pellet in NTE buffer in 1% of the original volume. If necessary, proceed to step 6 for column chromatography. The stock can be used directly after this step or stored at −70° or −80°C. It can also be further purified on a Sepharose CL-4B column to remove PEG.
Carry out Sepharose chromatography 6. Prepare a column of Sepharose CL-4B or CL-2B as described in Protocol), equilibrating in NTE buffer.
UNIT 5.6
(Support
Column size will depend upon the amount of virus pellet; 10-ml Econo-Columns from Bio-Rad work well for pellets prepared from 100 ml of virus stock.
7. Apply the virus to the top and chromatograph at 1 ml/min with NTE buffer. 8. Collect 0.3- to 0.5-ml fractions. 9. Assay fractions for virus by measuring absorbance at 280 or 260 nm (see Aboud et al., 1982) or by reverse transcriptase (RT) assay (UNIT 9.13). 10. Pool appropriate fractions and titer as in UNIT 9.10. For examples of virus recovery as determined by both Xgal CFU bioassay and RT assay, see Figure 9.12.1.
7000
100,000 kcpm/ml kCFU/ml
80,000
6000
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4000 3000
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kCFU/ml
kcpm/ml
5000
2000 20,000
1000 0
0 0
2
4
6
8
10
Elution volume (ml)
Figure 9.12.1 Ψ2 BAG supernatant (8 ml) was concentrated 100-fold by PEG precipitation. The resulting 80 µl was loaded onto 10 ml of Sepharose CL-2B in a Bio-Rad Econo-Column. Fractions of 0.5 ml were collected and tested for both reverse transcriptase activity and Xgal CFU. Large-Scale Preparation and Concentration of Retrovirus Stocks
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CONCENTRATION USING MOLECULAR-WEIGHT-CUTOFF FILTERS This method is the simplest means of concentrating small to medium volumes of retrovirus stocks. Supernatants prepared as in steps 1 to 4 of the Basic Protocol are centrifuged through filters, either the Centricon-30 microconcentrator from Amicon or the CentriCell 60 from Polysciences, essentially following the manufacturers’ instructions. These filters allow passage of molecules that are much smaller than the virus (e.g., Centricon-30 filters allow passage of molecules of 500 Ci/mmol. Because volume will vary with different lots, the volume of distilled H2O to add should be varied to compensate.
COMMENTARY Background Information
Detection of Helper Virus in Retrovirus Stocks
The production of wild-type, replicationcompetent helper virus by packaging cell lines can be an issue of concern when using replication-incompetent vectors. If the vectors are being used for lineage analysis, the virus stock must be free of helper virus. Similarly, for most infections of young animals, where the wildtype viruses are leukemogenic, helper viruses must not be present. The genome that supplies the gag, pol, and env genes in the Ψ2, Ψam, PA12, PA317, and Q2bn packaging cell lines (UNIT 9.9) does not encode the Ψ sequence, but can still become packaged—although at a low frequency. If the helper genome is coencapsidated with the vector genome, recombination in the next cycle of reverse transcription can occur. If recombination allows the helper genome to acquire the Ψ sequence from the vector genome, a recombinant that is capable of autonomous replication will result. This recombinant can spread through an entire culture (although slowly due to envelope
interference). Once this occurs, it is best to discard the producer clone, as there is no convenient way to eliminate the wild-type virus. As would be expected, helper virus contamination happens with a greater frequency in stocks with high titer. The PA317 packaging line has a safer design than Ψ2, Ψam, PA12, and Q2bn, while the ΨCRE, GP + E-86, ΩE, ΨCRIP, and Isolde lines probably have the best design for not producing helper virus. Recombination leading to helper virus creation has not yet been observed in ΨCRE, GP + E-86, ΩE, ΨCRIP, or Isolde, but one must be cautious and assay stocks produced by these lines (for a review of lines, see Table 9.9.1). These protocols can also be used to determine the host range of the helper virus. This can be tested by infection of cells of different species [e.g., dog (D17) or human (HeLa) cells cannot be infected with ecotropic virus, only with amphotropic virus]. Alternative methods to detect helper virus include the XC plaque assay (Rowe et al.,
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1970) and S+L− assay (Eckner and Hettrick, 1977; Miller and Buttimore, 1986). These assays require additional cell lines. The assay listed here for horizontal spread is quite sensitive but very time-consuming. The RT assay is sensitive, but does not always detect crippled helper (virus that can replicate, but is mutant and does not replicate as well as wild-type).
Time Considerations Virus supernatant generation (Basic Protocol) requires 10 days. To titer the stocks for presence of viruses carrying the selectable marker or lacZ, another 10 to 13 days are needed (see UNIT 9.11). To test supernatants for reverse transcriptase activity (Alternate Protocol 2), 50 samples can be processed in 2 to 3 hr.
Troubleshooting If supernatants are assayed by titration of selectable markers and many (e.g., greater than several thousand) marker-resistant colonies are observed, the original test stock contained wild-type helper virus. Once this occurs, it is best to discard the producer clone, as there is no convenient way to eliminate the helper virus. If a smaller number of colonies result than expected (see Anticipated Results), there are several possible explanations. There may be “crippled” helper in the original test stock generated by an imperfect recombination event. For many applications, crippled helper is not a confounding influence. Alternatively, some of the Ψ− genome (e.g., the genome carrying gag, pol, and env) can transfer to the first set of infected NIH 3T3 cells and lead to a low titer, without true helper virus contamination (e.g., see Danos and Mulligan, 1988).
Anticipated Results Virus stock that contains wild-type helper virus should produce more than several thousand colonies carrying the neo marker (G418resistant) colonies on the assay dishes generated by step 9 of the Basic Protocol or several thousand lacZ+ colonies generated by step 11 of Alternate Protocol 1. If the reverse transcriptase assay is used, the RT activity from the test supernatants should be compared to that in the control supernatants (cells infected with a known helper-free virus stock, and cells infected with a stock of known wild-type helper virus). Stocks contaminated with helper virus should show a >1000-fold enhancement of RT activity when compared to helper-free controls. A truly helper-free stock will be equal in RT activity to that generated by the helper-free control.
Literature Cited Danos, O. and Mulligan, R.C. 1988. Safe and efficient generation of recombinant retroviruses with amphotropic and ecotropic host ranges. Proc. Natl. Acad. Sci. U.S.A. 85:6460-6464. Eckner, R.J. and Hettrick, K.L. 1977. Defective Friend spleen focus-forming virus: Interfering properties and isolation free from standard leukemia-inducing helper virus. J. Virol. 24:383396. Goff, S., Trakman, P., and Baltimore, D. 1981. Isolation and properties of murine leukemia virus mutants: Use of a rapid assay for release of virion reverse transcriptase. J. Virol. 38:239-248. Miller, A.D. and Buttimore, C. 1986. Redesign of retrovirus packaging cell lines to avoid recombination leading to helper virus production. Mol. Cell. Biol. 6:2895-2902. Morgan, B.A. and Fekete, D.M. 1996. Manipulating gene expression with replication competent retroviruses. Methods Cell Biol. 51:185-218. Omer, C.A. and Faras, A.J., 1982. Mechanism of release of the avian retrovirus tRNATrp primer molecule from viral DNA by ribonuclease H during reverse transcription. Cell 30:797-805. Rowe, W.P., Pugh, W.E., and Hartley, J.W. 1970. Plaque assay techniques for murine leukemia viruses. Virology 42:1136-1139. Stoker, A.W. and Bissell, M.J. 1987. Quantitative immunocytochemical assay for infectious avian retroviruses. J. Gen. Virol. 68:2481-2485.
Contributed by Constance Cepko Harvard Medical School Boston, Massachusetts Warren Pear (proviral rescue) University of Pennsylvania Philadelphia, Pennsylvania
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Retrovirus Infection of Cells In Vitro and In Vivo There are many applications in which retrovirus vectors are used as transduction agents. In some cases, the vector carries a gene that one wishes to express in a target cell in order to study the function of that gene. In other cases, the virus is used to introduce a histochemical marker gene into cells in order to follow their fate. Retrovirus vectors can also be used in a variety of cells type to investigate regulatory sequences in which a reporter gene and regulatory sequences are carried by the vector and to immortalize or transform primary cells by transduction of oncogenes. For each application, the infection protocol may vary and must often be optimized. Guidelines for infection of cells in some typical in vivo and in vitro experiments are presented here. In addition to these guidelines, the cited literature contains protocols that have been used successfully to infect cells under specific circumstances and may provide a good starting point for similar cells. To optimize infection of a particular type of cell, it is often advantageous to use vectors that are easy to score. These include vectors carrying selectable or screenable markers (such as BAG; Figs. 9.10.1 and 9.10.2). CAUTION: When working with human blood, cells, or infecting agents, strict biosafety practices must be followed (see UNIT 16.15).
INFECTION OF CELLS IN VITRO Infection of target cells in vitro is accomplished by simply incubating the virus with the cells (e.g., UNIT 9.10, Basic Protocol 2). For most in vitro applications using a murine virus, a polycation such as polybrene is used to aid viral infection. These polycations apparently promote virus binding to the host cell surface by reducing electrostatic repulsion between the negatively charged surfaces of the cell and virion. When incubating target cells with virus in vitro, problems may occur if undiluted, hightiter virus is used directly on some cell types— i.e., fusion of target cells can occur with subsequent death of most fused cells within a few days. This is presumably due to the virus serving as a fusogen (i.e., binding to the surface of more than one cell and promoting membrane fusion). Alternatively, cells to be infected in vitro can be incubated with the packaging line that produces the desired vector (cocultivation Contributed by Constance Cepko and Warren Pear Current Protocols in Molecular Biology (1996) 9.14.1-9.14.6 Copyright © 1996 by John Wiley & Sons, Inc.
method). This method is used to infect hematopoetic cells and appears to greatly increase the infection efficiency (Williams et al., 1984; Lemischka et al., 1986). In some cases, it may be desirable to prevent cell division of the producer cells during or after cocultivation. This can be achieved by using protocols that allow virus production, but that prevent further cell division of the producer cells. For example, prior to cocultivation, confluent or nearly confluent producer cells can be killed by irradiation (2800 rad for NIH 3T3 cells, 1500 rad for 293 cells) or a 3-hr treatment with mitomycin C (10 µg/ml in medium) followed by several rinses with medium. After this treatment, the cells to be infected are plated onto producer cells. Target cells should continue to grow, but producer cells will die as they can no longer divide. If the target cells are nonadherent, they can be removed by simply washing the producer monolayer gently after ∼48 hr of cocultivation. An alternative to cocultivation is spin infection (UNIT 9.11; Kotani et al., 1994), which achieves similar infectious titers without the difficulties caused by the presence of producer cells. It is difficult to generalize about the efficiency of infection, although it can approach 100%. For the most part, the variables that influence infectability of a given cell are unknown. However, it is clear that for optimal results, the cells should be as mitotically active as possible. Although virus will enter cells and undergo partial reverse transcription in nonmitotic cells, there will be no integration or expression of the viral genes. If virus is applied to cells that are not dividing, it may remain competent to finish reverse transcription and integration for 1 to 2 days, although this has not been carefully determined for most cell types. One approach that may overcome the difficulties in transducing non-dividing cells is the use of retroviral packaging cell lines and vectors based on human immunodeficiency virus (HIV; Naldini et al., 1996). This approach has successfully produced high-titer, helper-free viruses that are able to infect cell lines arrested at various phases of the cell cycle at frequencies that far exceed those attainable using Moloneybased packaging systems. In addition, these vectors are able to infect adult rat neurons at low frequency. Expression of viral gene products is usually
UNIT 9.14
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assayed two or three cell cycles after infection. When assaying lacZ expression in NIH 3T3 cells via Xgal staining (UNIT 9.10), we have found that the number of clones peaks at ∼48 hr postinfection (three cell cycles). However, Xgal staining is visible within 24 hr in a subset of the infected cells. Infections of tissue explants or cultured embryos can also be performed essentially as described above. A tissue explant can be bathed in as much virus stock as is desired for a few hours (when using murine viruses or avian viruses of any subgroup other than A, include polybrene at 8 µg/ml), or it can be cultured over a monolayer of producer cells (as above in the cocultivation method).
Creation of Retroviral cDNA Libraries
Retrovirus Infection of Cells In Vitro and In Vivo
Expression cloning has been extremely useful for isolating a wide variety of genes, including cell surface markers, cytokines, and multiple receptors (see UNIT 6.11). This methodology relies on high levels of gene expression in the transfected cell line and the ability to recover the appropriate cDNA by plasmid rescue. To accomplish the former, COS cells are frequently used for expression cloning, as the expression of SV40 large T antigen causes high levels of protein expression from vectors that contain the SV40 origin of replication. Although expression cloning in COS cells has been extremely useful for the expression cloning of a number of cDNAs, this approach is limited to cell lines that are efficiently transfected and to phenotypic assays with a short readout period. These limitations have made it difficult to use expression cloning strategies in primary hematopoietic cells due to their poor transfectability and the long-term culture required for phenotypic selection. In contrast to expression cloning in COS cells, retroviral cDNA libraries offer the possibility of introducing cDNAs into a wide variety of cell types, including primary cells. As a result, it is possible to devise novel assays to select cDNAs that are based on specific properties of the cells themselves, such as transformation, growth factor dependence, or in vitro differentiation. Retroviral expression cloning has been successfully used to clone cDNAs using NIH 3T3– based packaging lines. These strategies have been useful for identifying genes involved in factor-dependent growth (Rayner and Gonda, 1994; Wong et al., 1994) and transformation (Whitehead et al., 1995), as well as for the selection of peptides and antisense messages that induce etoposide resistance (Gudkov et al.,
1993, 1994). Although overcoming some of the limitations of COS-based expression cloning, use of NIH 3T3–based packaging lines has two major disadvantages for expression cloning. These are that (1) it is difficult to obtain high titers by transient transfection, limiting the complexity of the library, and (2) the frequency of each cDNA may change during selection of the transfected population and the resulting retroviral stock may not represent the original cDNA library. An advance in the ability to prepare cDNA expression libraries has been the methodology to prepare high titer retroviral supernatants by transient transfection (UNIT 9.11; reviewed in Onishi et al., 1996). These approaches overcome the major disadvantages of the NIH 3T3–based packaging cells—i.e., it is possible to obtain high transient transfection efficiencies and it is unnecessary to select the cells prior to harvest of the retroviral stock. An application of the use of transient retroviral production for expression cloning has been described for cloning cDNAs from factor-dependent cell lines (Kitamura et al., 1995). In this report, the authors were able to develop expression libraries with complexities of >106 and detect cDNAs at a frequency of 1 in 107 (Kitamura et al., 1995). The technology for retroviral expression cloning is presently evolving, and the reader should consult the most recent literature for the best choice of packaging cell line and retroviral vector. With the methods for transient retroviral production, the basic strategy is to clone a cDNA library into a retroviral vector, transfect the library into the packaging cells, harvest the retroviral stock between 48 and 72 hr posttransfection, and infect the target cells. These methods are described in UNIT 9.11. The target cells are subsequently assayed for the phenotype of interest. Some considerations in the choice of retroviral vectors are the presence of multiple cloning sites for efficient cloning of the cDNAs into the vector, the presence of the extended packaging signal for improved titer (Bender et al., 1987), the use of IRES (internal ribosome entry site) elements rather than internal promoters to increase the likelihood that the cloned cDNA will be expressed, and the presence of primers that surround the multiple cloning site so that the cDNA can be amplified by PCR following phenotypic selection. Another consideration is the infection efficiency of the target cells. For example, in the studies of Onishi et al. (1996), the authors were able to infect Ba/F3 and BW5147 cells with >50% efficiency but infection of LG3 myeloid cells
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resulted in an infection efficiency of only 8.3%. To obtain adequate infection efficiencies, it m ay be necessar y to p seu do type the retroviruses (UNIT 9.11) or introduce a retroviral receptor into the cell line to be assayed (UNIT 9.9). One drawback to high infection efficiencies, however, is that with higher infection efficiency, the infected cells are more likely to contain multiple integrated proviruses (Onishi et al., 1996). This may increase the difficulty in identifying the cDNA that confers the phenotype of interest. Another consideration in the generation of retroviral cDNA libraries is whether the library should be normalized prior to screening. An advantage of normalization is that there will be an “enrichment” of rare messages; however, the cost is that the resulting cDNA library is no longer representative of initial population of cDNAs.
INFECTION OF RODENTS AND CHICKS IN VIVO Transduction of genes via retrovirus vectors can be performed in vivo. The following paragraphs describe in vivo infection of rodents, the laboratory animal most often used in experimental procedures; however, the techniques apply to other species (e.g., avian) as well. For more detailed information on infection of chick embryos, see Fekete and Cepko (1993a,b), Homburger and Fekete (1996), and Morgan and Fekete (1996). NOTE: Detailed protocols for the care and handling of laboratory animals are beyond the scope of this unit. The reader is referred to Current Protocols in Immunology, Chapter 1 (Donovan and Brown, 1995) for instructions on proper animal restraint, anesthesia, injections, and euthanasia techniques that are essential to the infection methods described.
Infection of Postnatal Animals In Situ Volume and titer of virus stock. The volume that can safely be delivered to a tissue in vivo is generally quite small—0.1 to 1 µl. It is therefore important to prepare high-titer (usually concentrated) virus stock (UNIT 9.12). In general, virus titer should range from 106 to 108 CFU/ml. Because of these limitations, it is quite important to deliver the virus directly to the area containing the highest percentage of mitotic target cells. It is unclear if there are factors in tissue fluids (e.g., ventricular fluids or cerebral spinal fluids) that inhibit or destroy viral infectivity. Delivery of virus. Virus delivery to postnatal animals is fairly straightforward. It is possible
to use a hand-held Hamilton syringe with a 33-G needle or use a drawn-out glass pipet. If glass pipets are used, the size of the needle tip should be determined empirically for the tissue under study. The skin, and even the skull, are soft enough on the first few days after birth for direct injection into the tissue. For tougher injection sites on older animals, it may be necessary to first make a hole using a stronger needle, such as one made of steel, prior to inserting a more delicate needle at a specific site. Coinjection with a dye such as 0.05% (w/v) trypan blue or 0.025% fast green aids in detecting the accuracy of injections and does not impair viral infectivity. Rodents ≤7 days old can be anesthetized by simply cooling on ice for a few minutes. Landmarks (e.g., sutures or blood vessels) near the area to be injected can be visualized using a fiber-optics light source. The injection is made directly into the desired area using a hand-held pipet. It is best to practice a series of injections with dye alone and then immediately dissect the animal for examination of the injection site. After the injection method is perfected, virus can be injected. Animals can then be examined for evidence of viral infection at any time, depending upon the experimental design. Setup of pilot experiments to optimize injection and expression efficiency. When performing infections of tissue for the first time, or when infection and expression efficiency of the target cells is unknown, it is useful to perform pilot experiments using a retrovirus vector carrying a histochemical marker gene such as lacZ. Infection with a lacZ-coding virus allows for examination of the injected animal a few days after infection to determine if the target cells have been infected and are capable of expressing the viral genes. For such experiments, it is necessary to optimize conditions of the Xgal staining reaction (UNIT 9.10) for the particular tissue under study. An excellent way to simultaneously determine the accuracy of injection, and whether the Xgal histochemistry is working, is to inject cells that contain the lacZ gene, such as the Ψ2 BAG producer cells (obtainable from ATCC, #CRL 9560; UNIT 9.10). The cells can be prelabeled with a fluorescent dye such as carboxyfluorescein diacetate succinimyl ester (CFSE, Molecular Probes; prepare 10 mM stock in DMSO and store in foil at 4°C). As detailed by Bronner-Fraser (1985), cells suspended in PBS are exposed to 0.3 mM CFSE solution at 37°C for 30 min just before trypsin treatment. After dissociation, the cells are pelleted and washed
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twice with medium or PBS, and resuspended to a concentration of 108 cells/ml. A few minutes after injection of the CFSE-labeled cells, the animal should be killed, the tissue fixed, then stained with Xgal (UNIT 9.10). The injected cells can then be monitored independently from the Xgal staining by viewing the CFSE fluorescence under UV illumination. If the Xgal histochemistry is working properly, no fluorescent cells should be visible because Xgal-stained cells are usually so full of indigo dye that all fluorescence is absorbed. Once the injection method and Xgal histochemistry have been optimized, BAG virus is injected into the site and the tissue is examined for viral infection several days later. If the Xgal histochemistry is not working well, fluorescent cells will be visible, indicating the site of the injection and the failure of the Xgal histochemistry. In this case, one should vary fixation and tissue preparation methods until good Xgal staining is achieved (Cepko, 1989). Alternatively, the DAP retrovirus vector (Fields-Berry et al., 1992) encoding human placental alkaline phosphatase can be used. A stable Ψ2 line that makes DAP at titers of 1–2 × 106 CFU/ml (more reliably than Ψ2 BAG) can be obtained from ATCC (CRL #1949). Histochemical detection of the alkaline phosphatase enzyme is as simple as Xgal histochemistry (Fields-Berry et al., 1992). For further discussion of both Xgal and alkaline phosphatase assays, see Cepko et al. (in press).
Infection of Prenatal Rodents In Utero
Retrovirus Infection of Cells In Vitro and In Vivo
Injections made in utero (through uterine wall) are performed with drawn-out glass pipets (e.g., see Austin and Cepko, 1990). The actual diameter and shape of the tip should be determined empirically (e.g., for injections of the lateral ventricle through the uterine wall of rats at embryonic day 15—E15—the outside diameter of the tip is ∼50 µm and the inside diameter is ∼15 µm). Animals are anesthetized with a mixture of ketamine (20 to 40 mg/kg) and xylazine (3 to 5 mg/kg) and are opened via an incision along the midline. The orientation of an embryo can be determined by visualizing the head with fiber optics. Injections of 0.1 to 1.0 µl are made through the uterine wall into the area of interest. When the target of the injection is the lateral ventricle, the inoculum can be seen filling the lateral ventricle when properly delivered to this area. Other areas may not be as visible and practice injections followed by immediate dissection and examination are again recommended. The mother is
then closed with suture. The injected animals can be delivered prenatally, or allowed to finish gestation and be born normally, to be sacrificed at a postnatal age, depending upon the design of the experiment.
Infection of Prenatal Rodents Exo Utero It is difficult to make precisely directed injections into many of the mitotic zones of prenatal mammals through the uterine wall. The exo utero surgical procedure developed by Muneoka et al. (1986) at least partially circumvents this problem. In this procedure, mouse embryos at embryonic days 11 to 19 (E11 to E19) are released from the uterus by cutting the uterine wall, but remain attached to the uterus via the placenta. The abdominal cavity of the mother is filled with a buffered saline solution to protect the embryos. An incision can be made in the extra-embryonic membranes that surround the embryo so that embryo can be directly manipulated or injected. Subsequently, the extra-embryonic membranes are closed with fine sutures. The embryos can be brought to term in the abdominal cavity and delivered by Caesarean section. Injections can be made through an incision in the extra-embryonic membranes that are subsequently closed with 10-0 suture. However, in our hands (Turner et al., 1990), embryonic and neonatal survival was improved by injecting directly through the extra-embryonic membranes without an incision or suture. Although injections made by this approach were more difficult to target, 25% of the embryos injected at E13 survived to adulthood and all injected animals contained clones of retrovirally infected cells. Additional factors that influence the success of this method are choice of mouse strain and health of the mouse colony. Outbred mouse strains such as CD-1 or Swiss Webster appear to be best, but even these strains may have different embryo survival rates when obtained from different suppliers or colonies. Apparently healthy mouse colonies can harbor subclinical infections that do not affect unoperated embryos but that can stress operated embryos beyond their ability to survive.
Ex Vivo Infection of Murine Bone Marrow Cells and Bone Marrow Transplant Ex vivo retroviral infection of murine bone marrow has been used for in vitro experiments as well as for subsequent transfer and rescue of
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lethally irradiated animals. The goal of many of these experiments is to transduce hematopoietic stem cells; however, the methods discussed below can be altered to optimize infection of a particular hematopoietic cell type. As discussed below, the conditions for infecting hematopoietic stem cells are still being developed, and the reader should consult the most recent literature to ascertain the best conditions. Optimal conditions for irradiating mice need to be established for each irradiator and are also dependent on weight, age, and mouse strain. Donor mice (age 5 to 8 weeks) are treated with 250 µg per gram (mouse weight) 5fluorouracil (5-FU) by tail vein injection in a volume of ≤200 µl. 5-FU treatment kills dividing cells, and in doing so, enriches for hematopoietic progenitor cells (van Zandt, 1984; Bodine et al., 1991). Five days later, donor mice are sacrificed and the tibia and fibias are removed. These bones are flushed with 2 to 3 ml per bone of PBS or DMEM using a 25- to 27-G needle attached to a syringe. The flushed marrow cells (including red blood cells) are pooled, centrifuged 10 min at 500 × g, 4°C, and resuspended in 106 cells/ml of a prestimulation cocktail consisting of DMEM, 15% fetal bovine serum, antibiotics (penicillin and streptomycin), 6 ng/ml IL-3, 10 ng/ml IL-6, and 100 ng/ml stem cell factor. The bone marrow cells are incubated in the prestimulation cocktail for 24 to 48 hr prior to infection. A number of other cytokines have been tried, including IL-1α, and the concentrations of the cytokines used vary widely from lab to lab (Bodine et al., 1989, 1994; Daley et al., 1990; Fraser et al., 1992; Pear et al., 1996). The cells are infected by either spin infection or cocultivation (UNIT 9.11) in the above infection cocktail with 4 µg/ml polybrene added. In addition, viral supernatant may replace one-third of the volume of DMEM. Following infection, nonadherent cells are removed and injected into the tail vein of lethally irradiated 4- to 8-weekold recipient mice in a volume of ≤200 µl/mouse. Approximately 2–5 × 105 unfractionated bone marrow cells per mouse are necessary to ensure long-term reconstitution of most mouse strains. The animals are subsequently monitored for bone marrow reconstitution.
LITERATURE CITED Austin, C.P. and Cepko, C.L. 1990. Cellular migration patterns in the developing mouse cerebral cortex. Development 110:713-732. Bender, M.A., Palmer, T.D., Gelinas, R.E., and Miller, A.D. 1987. Evidence that the packaging signal of Moloney murine leukemia virus extends into the gag region. J. Virol. 61:1639-1646. Bodine, D.M., Karlsson, S., and Nienhuis, A.W. 1989. Combination of interleukins 3 and 6 preserves stem cell function in culture and enhances retrovirus-mediated gene transfer into hematopoietic stem cells. Proc. Natl. Acad. Sci. U.S.A. 86:8897-8901. Bodine, D.M., McDonagh, K.T., Seidel, N.E., and Nienhuis, A.W. 1991. Survival and retrovirus infection of murine hematopoietic stem cells in vitro: effects of 5-FU and method of infection. Exp. Hematol. 19:206-212. Bodine, D.M., Seidel, N.E., Gale, M.S., Nienhuis, A.W., and Orlic, D. 1994. Efficient retrovirus transduction of mouse pluripotent hematopoietic stem cells mobilized into the peripheral blood by treatment with granulocyte colony-stimulating factor and stem cell factor. Blood 84:1482-1491. Bronner-Fraser, M. 1985. Alterations in neural crest migration by a monoclonal antibody that affects cell adhesion. J. Cell Biol. 101:610-617. Cepko, C.L. 1989. Lineage analysis and immortalization of neural cells via retrovirus vectors. In Neuromethods, Vol. 16: Molecular Neurobiological Techniques (A.A. Boulton, G.B. Baker, and A.T. Campagnoni, eds.) pp. 367-392. Humana Press, Clifton, N.J. Cepko, C.L., Ryder, E., Fekete, D.M., and Bruhn, S. In press. Detection of β-galactosidase and alkaline phosphatase activities in tissue. In Methods in Cell Biology (D. Spector, L. Leinwand, and R. Goldman, eds.). Cold Spring Harbor Laboratory Press. Cold Spring Harbor, N.Y. Daley, G.Q., Van Etten, R.A., and Baltimore, D. 1990. Induction of chronic myelogenous leukemia in mice by the P210 bcr/abl gene of the Philadelphia chromosome. Science 247:824830. Donovan, J. and Brown, P. 1995. Care and handling of laboratory animals. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeek, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. 1.0.1-1.8.4. John Wiley & Sons, New York. Fekete, D.M. and Cepko, C.L. 1993a. Retroviral infection coupled with tissue transplantation limits gene transfer in the chick embryo. Proc. Natl. Acad. Sci. U.S.A. 90:2350-2354. Fekete, D.M. and Cepko, C.L. 1993b. Replication competent retroviral vectors encoding alkaline phosphatase reveal spatial restriction of viral gene expression/transduction in the chick embryo. Mol. Cell. Biol. 13:2604-2613. Introduction of DNA into Mammalian Cells
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Fields-Berry, S.C., Halliday, A.L., and Cepko, C.L. 1992. Novel recombinant retrovirus encoding alkaline phosphatase confirms clonal boundary assignment in lineage analysis of murine retina. Proc. Natl. Acad. Sci. U.S.A. 89:693-697.
Naldini, L., Blomer, U., Gallay, P., Ory, D., Mulligan, R., Gage, F.H., Verma, I.M., and Trono, D. 1996. In vivo gene delivery and stable transduction of nondividing cells by a lentiviral vector. Science 272:263-267.
Fraser, C.C., Szilvassy, S.J., Eaves, C.J., and Humphries, R.K. 1992. Proliferation of totipotent hematopoietic stem cells in vitro with retention of long-term competitive in vivo reconstituting ability. Proc. Natl. Acad. Sci. U.S.A. 89:19681972.
Onishi, M., Kinoshita, S., Morikawa, Y., Shibuya, A., Phillips, J., Lanier, L.L., Gorman, D.M., Nolan, G.P., Miyajima, A., and Kitamura, T. 1996. Applications of retrovirus-mediated expression cloning. Exp. Hematol. 24:324-329.
Gudkov, A.V., Zelnick, C.R., Kazarov, A.R., Thimmapaya, R., Suttle, D.P., Beck, W.T., and Roninson, I.B. 1993. Isolation of genetic suppressor elements, inducing resistance to topoisomerase II-interactive cytotoxic drugs, from human topoisomerase II cDNA. Proc. Natl. Acad. Sci. U.S.A. 90:3231-3235. Gudkov, A.V., Kazarov, A.R., Thimmapaya, R., Axenovich, S.A., Mazo, I.A., and Roninson, I.B. 1994. Cloning mammalian genes by expression selection of genetic suppressor elements: association of kinesin with drug resistance and cell immortalization. Proc. Natl. Acad. Sci. U.S.A. 91:3744-3748. Homburger, S.A. and Fekete, D.M. 1996. High efficiency gene transfer into the embryonic chick CNS using B-subgroup retroviruses. Dev. Dyn. 206:112-120. Kitamura, T., Onishi, M., Kinoshita, S., Shibuya, A., Miyajima, A., and Nolan, G.P. 1995. Efficient screening of retroviral cDNA expression libraries. Proc. Natl. Acad. Sci. U.S.A. 92:9146-9150. Kotani, H., Newton, P.B.R., Zhang, S., Chiang, Y.L., Otto, E., Weaver, L., Blaese, R.M., Anderson, W.F., and McGarrity, G.J. 1994. Improved methods of retroviral vector transduction and production for gene therapy. Hum. Gene Ther. 5:19-28. Lemischka, I.R., Raulet, D.H., and Mulligan, R.C. 1986. Developmental potential and dynamic behavior of hematopoietic stem cells. Cell 45:917927. Morgan, B.A. and Fekete, D.M. 1996. Manipulating gene expression with replication competent retroviruses. Methods Cell Biol. 51:185-218. Muneoka, K., Wanek, N., and Bryant, S.V. 1986. Mouse embryos develop normally exo utero. J. Exp. Zool. 239:289-293.
Pear, W.S., Aster, J.C., Scott, M.L., Hasserjian, R.P., Soffer, B., Sklar, J., and Baltimore, D. 1996. Exclusive development of T cell neoplasms in mice transplanted with bone marrow expressing activated Notch alleles. J. Exp. Med. 183:22832291. Rayner, J.R. and Gonda, T.J. 1994. A simple and efficient procedure for generating stable expression libraries by cDNA cloning in a retroviral vector. Mol. Cell Biol. 14:880-887. Turner, D.L., Snyder, E.Y., and Cepko, C.L. 1990. Lineage-independent determination of cell type in the embryonic mouse retina. Neuron 4:833845. van Zandt, G. 1984. Studies of hematopoietic stem cells spared by 5-fluorouracil. J. Exp. Med. 159:679-690. Whitehead, I., Kirk, H., and Kay, R. 1995. Expression cloning of oncogenes by retroviral transfer of cDNA libraries. Mol. Cell Biol. 15:704-710. Williams, D.A., Lemischka, I.R., Nathan, D.G., and Mulligan, R.C. 1984. Introduction of a new genetic material into pluripotent haematopoietic stem cells of the mouse. Nature 310:476-480. Wong, B.Y., Chen, H., Chung, S.W., and Wong, P.M. 1994. High-efficiency identification of genes by functional analysis from a retroviral cDNA expression library. J. Virol. 68:5523-5531.
Contributed by Constance Cepko Harvard Medical School Boston, Massachusetts Warren Pear University of Pennsylvania Philadelphia, Pennsylvania
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INACTIVATION OF GENES IN MAMMALIAN CELLS
SECTION IV
Human Somatic Cell Gene Targeting
UNIT 9.15
Human somatic cell gene targeting provides a powerful tool to scientists studying gene function in cultured human cells. This technology allows scientists to knock out genes in human somatic cells in an fashion analogous to the creation of knockout mice. Human somatic cell gene targeting brings the power of genetics to the study of human genes in human cells by making it possible to compare cells or individuals that are genetically identical except for a single, well-defined mutation in an endogenous gene. A partial list of genes successfully targeted in human somatic cells is presented in Table 9.15.1. Once this “molecular scalpel” has been employed, a researcher has at his or her disposal a set of cultured human cells that are genetically identical except for a specific, targeted change—the presence (in the parental cells) or absence (in the knockout cells) of a functional human gene. Since the cells are otherwise genetically the same, any biological or biochemical difference between the cells sheds light on the function of the targeted gene. This type of classical genetic analysis—the study of gene function by comparing cells or organisms that are genetically identical except for a single, well-defined genetic change in an endogenous gene—has been a mainstay of genetics in model systems for decades and has yielded innumerable critical discoveries. Until relatively recently, however, this capability has been restricted to model organisms and was not applicable to human cells. The biological and biochemical characteristics of such isogenic sets of human cells can be analyzed in many different ways. For example, the growth properties of the cells can be studied in vitro to assess differences in their morphology and other growth characteristics (Shirasawa et al., 1993). The cells can be analyzed using flow cytometry to assess their cell-cycle profile during exponential growth or cell-cycle arrest, or after treatment with growth factors, small-molecule therapeutics, and lead compounds (Waldman et al., Table 9.15.1
Gene Targeting in Human Cells
Gene targeted
Reference
Smad4 p53
Zhou et al., 1998 Bunz et al., 1998
14-3-3σ DNMT1
Chan et al., 1999 Rhee et al., 2000
BAX PPARδ
Zhang et al., 2000a Park et al., 2001
Securin ORC2
Jallepalli et al., 2001 Dhar et al., 2001
Ferredoxin reductase ATR
Hwang et al., 2001 Cortez et al., 2001
DEC1 DNMT3b
Zawel et al., 2002 Rhee et al., 2002
β-Catenin
Kim et al., 2002a; Chan et al., 2002; Sekine et al., 2002
Contributed by Todd Waldman, Carolyn Lee, Tagvor G. Nishanian, and Jung-Sik Kim Current Protocols in Molecular Biology (2003) 9.15.1-9.15.20 Copyright © 2003 by John Wiley & Sons, Inc.
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1995). The propensity of the cells to undergo apoptosis in vitro can be assessed using a variety of assays (Waldman et al., 1996). Microarrays can be employed to identify novel putative effectors of the targeted gene (Kim et al., 2002a). The cells can also be studied in vivo. Since the cell lines commonly used for gene targeting are derived from human cancers, they will form tumors when injected subcutaneously into immunodeficient mice. By creating and comparing so-called “isogenic tumors,” it is possible to examine the role of the targeted gene in a wide variety of processes—e.g., one can study the effects of oncogenes and tumor-suppressor genes in tumor formation and on the sensitivity of tumors to anticancer agents (Shirasawa et al., 1993; Waldman et al., 1997). This unit presents protocols for human somatic cell gene targeting. The Strategic Planning section presents a number of considerations that must be made when planning such an experiment. Basic Protocol 1 provides details for vector construction. Basic Protocol 2 describes the transfection procedures. Basic Protocol 3 is a method for excising the antibiotic-resistance gene from a heterozygous knockout to make it possible to target the remaining allele; the Alternate Protocol describes how to switch selectable markers for the same purpose. Basic Protocol 4 describes how to target the remaining allele, converting a heterozygous knockout cell line to a homozygous knockout cell line. STRATEGIC PLANNING Selection of a Parental Cell Line To date, six different human cell lines have been successfully used for human somatic cell gene targeting (listed in Table 9.15.2). The list is short because suitable cell lines must possess each of three distinct characteristics. The first of these, which is very important, is that suitable cell lines should be diploid (or near-diploid), such that there are two (and only two) copies of the gene to be targeted. Second, the cells should transfect with high efficiency, since the promoterless enrichment built into human gene targeting vectors leads to the formation of relatively few colonies (since randomly integrated promoterless targeting vectors generally fail to express neor and therefore do not form drug-resistant colonies). Third, the cells should grow quickly and clonally, since one will need to grow individual clones in 96-well plates, preferably in 2 to 3 weeks. Of the cell lines listed in Table 9.15.1, HCT116 cells have proven to be the most robust and therefore have been used in the majority of human somatic cell gene targeting projects. As such, the authors of this unit tend to focus initial targeting efforts on HCT116 cells, since it is preferable to test new targeting vectors in a cell line that has been well established as suitable for human somatic cell gene targeting. Once the efficacy of targeting in HCT116 cells has been demonstrated, only then is the attempt made to target the gene in other cell lines as well.
Table 9.15.2
Human Somatic Cell Gene Targeting
Human Cell Lines for Gene Targeting
Cell line
Reference
HCT116 DLD1
Shirasawa et al., 1993 Shirasawa et al., 1993
HT1080 SW48
Porter and Itzhaki, 1993 Kim et al., 2002b
HaCaT Human fibroblasts
Zawel et al., 2002 Brown et al., 1997
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In addition to meeting these three requirements, it is important for both experimental and technical reasons that the chosen cell line express the gene to be targeted. Experimentally, there is not much point in deleting a gene in somatic cells that is not expressed. Technically, homologous integration of a promoterless targeting vector into a transcriptionally silent locus will lead to the absence of neor expression and the failure to form a drug-resistant colony. Thus, the authors recommend first confirming that the gene of interest is expressed in candidate cell lines prior to initiating a targeting project (via immunoblotting, northern blotting, or RT-PCR). Architecture of a Promoterless Targeting Vector Promoterless targeting vectors require that the selectable marker gene be inserted in-frame into the gene being targeted. Homologous integration of such a targeting vector into the human genome then leads to the expression of a functionally active drug-resistance protein. There are several general factors to consider when planning to construct such a targeting vector. One early concern was that targeting vectors for human somatic cell gene targeting would need to be composed of isogenic DNA—i.e., DNA derived from the actual cell line in which the gene would be targeted. These concerns were based on early gene-targeting studies performed in mouse embryonic stem cells, which indicated that the polymorphisms present in genomic DNAs derived from different strains of mice could affect the targeting frequency in a dramatically adverse fashion (Deng and Capecchi, 1992; van Deursen and Wieringa, 1992). If this were true for gene targeting in human cells, it would be difficult to target a gene in multiple cell lines using the same targeting vector. Furthermore, since humans (unlike mice) are not inbred, the targeting vector for each allele of a gene would need to be composed of different genomic DNA. As such, the need for isogenic DNA would substantially increase the technical complexity of human somatic cell gene targeting. Fortunately, experience has shown that targeting vectors composed of nonisogenic genomic DNA are able to create knockouts in a wide variety of genetically unrelated human cell lines (Sedivy et al., 1999). In the authors’ experience, the neor protein is frequently unable to tolerate the fusion of unrelated amino acids onto its amino terminus (C. Lee., J.S. Kim., and T. Waldman, unpub. observ.). This being the case, it is inadvisable to design a targeting strategy which leads to the formation of a neor fusion protein. There are two effective strategies that circumvent this limitation. First, one can design the targeting vector such that the initiating methionine of the gene being targeted is replaced precisely with the initiating methionine of neor gene. Second, one can replace internal exon(s) with an IRES-neor gene. Finally, it is not possible to design a true promoterless targeting vector for deleting the first exon of a gene, since the left arm of such a targeting vector would contain promoter elements of the gene being targeted. If the initiating methionine of a gene is, in fact, in the first exon, the authors recommend designing the targeting vector to delete internal exon(s) using an IRES-neor strategy. Building a Targeting Vector Building promoterless targeting vectors is a nontrivial exercise in recombinant DNA technology. There are two factors that complicate construction of these vectors. First, there is virtually no sequence flexibility in the junction between the left arm of the targeting vector and the selectable marker cassette, making the use of standard restriction enzyme–based recombinant DNA strategies problematic. Second, it is frequently necessary to add several sequence features between the end of the selectable marker gene and
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the right homology arm of the targeting vector, such as restriction sites and a PCR priming site. The authors have found that the most straightforward way to circumvent these difficulties is to build the targeting vectors in two steps, exploiting the high efficiency of homologous recombination in S. cerevisiae (Storck et al., 1996; Oldenburg et al., 1997). First, a 5- to 10-kb fragment containing the genomic region of interest is cloned (generally from a bacterial artificial chromosome, or BAC; UNIT 5.9) into a yeast shuttle vector, which contains selectable markers and origins of replication for propagation in both E. coli and S. cerevisiae. Next, the selectable marker gene (generally either neor or IRES-neor) is PCR-amplified from a plasmid using long primers that add both the needed sequence features, along with 30 to 50 nucleotides of homology to the cloned genomic region for directing the desired recombination event. This PCR product is then cotransformed into S. cerevisiae together with linearized recombinant shuttle vector, and recombinants are identified and used as the final targeting vector. This strategy makes it possible to design any junction and to add desired sequence features all in one step, and does not rely on the serendipitous presence of needed restriction sites. It should be noted that it is also theoretically possible to exploit homologous recombination in E. coli to accomplish the same goals (Zhang et al., 2000b). However, unlike in yeast, in E. coli it is generally necessary to select for homologous recombination between the vector and the PCR product. Therefore, the PCR-amplified neor cassette would need to contain a selectable marker for growth in bacteria as well. Screening for Knockouts in Human Cells The simplest approach for identifying gene-targeted cell lines is to conduct an initial PCR-based screen, then confirm the putative knockouts with a Southern blot. There are two general factors to consider when planning the details of such a screening strategy. Historically, PCR screening for knockouts has relied on one primer in the selectable marker gene (generally neor), and another primer located in the genomic region just outside one of the homology arms of the targeting vector. Homologous integration of the targeting vector creates a template for PCR, whereas random integration does not. As such, diagnostic PCR products are produced only from gene-targeted clones. The fundamental difficulty with this classic PCR-based screening strategy is the lack of a positive control for PCR—i.e.,by definition there exists no template for PCR unless a knockout is created. As such, it is virtually impossible to optimize the PCR reaction prior to screening for knockouts, and therefore difficult to definitively interpret a negative result. If none of the clones are PCR-positive, the question remains—were there truly no knockouts, or did the PCR not work?
Human Somatic Cell Gene Targeting
Recently, a strategy has been devised to overcome this limitation, providing an internal positive control for PCR in each reaction (Chan et al., 1999; for a schematic of this approach, see Fig. 9.15.1). This approach is very similar to the classic approach, except that the PCR primer generally located in the neor gene is substituted by a PCR primer located in the genomic region being deleted by the targeting vector. Importantly, this PCR priming site is built into the targeting vector such that homologous integration of the targeting vector simultaneously deletes the endogenous priming site, and reintroduces it in a slightly different location. As such, clones in which the targeting vector has integrated randomly create a PCR product of a certain size, and clones in which the PCR product has integrated via homologous recombination create a PCR product of a different size. Therefore, it is no longer the presence or absence of a PCR product that determines a knockout; instead, a PCR product is produced in all the PCR reactions and its size
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priming site A
A neomycin
targeting vector
priming site A genomic DNA deleted region priming site B
B
+/+
+/–
+/–
+/–
+/– wild-type allele knockout allele
Figure 9.15.1 PCR screening for knockouts.
determines whether a knockout is present. This elegant modification of the classic PCR screening approach has significant implications for interpreting PCR screening data. The other practical implication is that the diagnostic PCR must be optimized prior to completion of the targeting vector, since the PCR priming site must be built into the targeting vector itself. In addition to altering the size of a PCR product, homologous integration of the targeting vector invariably modifies the restriction map of the local genomic region. By using a Southern blot–based approach, it is possible to discriminate between wild-type alleles and targeted alleles. The details of choosing restriction sites and probes are described in Basic Protocol 1. Targeting the Second Allele One particularly challenging aspect of human somatic cell gene targeting is the creation of homozygous knockouts by disruption of the second, remaining allele. Since heterozygous knockout cells are already drug-resistant, it is necessary to modify the targeting vector to change the selectable marker gene. While swapping selectable markers is not technically demanding (especially if homologous recombination in S. cerevisiae was employed to create the targeting vector), experience has demonstrated that selectable markers other than neor (e.g., hygr and puror, among others) are suboptimal for somatic cell gene targeting. In particular, targeting vectors relying on these selectable markers are frequently either unable to generate drug-resistant colonies at all or require such low concentrations of drug that it is difficult to kill the background cells (Hanson and Sedivy, 1995; J.S. Kim, C. Lee, and T. Waldman, unpub. observ.). While the reasons for these difficulties are unknown, some have speculated that endogenous promoters are unable to generate a high enough level of gene expression to confer resistance to these specific drugs. The most straightforward way to circumvent difficulties with selectable markers other than neor is to build a Cre-loxP recyclable neor targeting vector and use it to target each allele sequentially. In such a targeting vector, the neor gene is flanked by loxP sites. After
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creation of heterozygous knockout cells, Cre is added to the cells (generally via infection with a recombinant Cre-containing adenovirus), and drug-sensitive derivatives are identified. These cells are then retransfected with the same targeting vector for deletion of the remaining allele. The second inherent difficulty in targeting the second allele is that the frequency of homologous integration is exactly half that of the first allele. This is because half the second allele homologous integration events will disrupt the already targeted allele. BASIC PROTOCOL 1
BUILDING A TARGETING VECTOR The first step in the creation of a human promoterless targeting vector is to obtain human genomic DNA that will form the basis of the homology arms of the targeting vector. To do this, the authors generally identify BAC clone(s) that contain the needed genomic region (UNIT 5.9). Once BACs have been identified and prepped, the next step is to subclone a smaller fragment (generally 5 to 10 kb) into a yeast shuttle vector. This subclone will be used as raw material for final assembly of the targeting vector via homologous recombination in S. cerevisiae (Storck et al., 1996; Oldenburg et al., 1997). Alternatively, it is possible to consecutively subclone each homology arm into a vector. However, subcloning a single larger piece that will ultimately form both homology arms saves a subcloning step. The final step in construction of the targeting vector is to use homologous recombination in S. cerevisiae to replace critical exon(s) in the cloned genomic region with an in-frame neor or IRES-neor gene. The decision whether to use neor or IRES-neor is made according to the guidelines in step 6, below, and Critical Parameters and Troubleshooting. There are two feasible alternative approaches. First, it is possible to add the promoterless neor gene via conventional restriction enzyme–based cloning. However, the authors find that such an approach often requires complex multistep strategies because of the lack of convenient restriction sites. Second, it is possible to conduct similar homologous recombination reactions in strains of E. coli that overexpress recombinase proteins (Zhang et al., 2000b). In this case the disadvantage is that such an approach generally requires that the insert include an E. coli selectable marker gene. Before beginning the homologous recombination, it is necessary to identify a unique restriction site in the cloned genomic region for linearization of the recombinant shuttle vector prior to transformation of S. cerevisiae. The recombination junctions will need to flank this restriction site. Once such a restriction site is identified, the neor gene (or IRES-neor gene) is PCR-amplified with long primers designed to add needed sequence features to the ends (e.g., a priming site for PCR-based diagnosis of knockouts, restriction site or sites for Southern blot-based diagnosis of knockouts, and sequence homologies for recombination in S. cerevisiae). This PCR product is then cotransformed into S. cerevisiae with the linearized recombinant yeast shuttle vector. After plating the transformation mix on the appropriate selective medium, individual yeast colonies are screened via whole-cell PCR to identify those harboring the desired recombinant plasmid. PCR products derived from recombinant plasmids are then sequenced to exclude plasmids harboring mutations in the recombination junctions. These plasmids are then transferred into E. coli and prepped. Finally, the integrity of the finished targeting vector is confirmed by restriction analysis and sequencing of critical junctions.
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Materials Genomic DNA (UNIT 2.2) derived from the parental cell line (e.g., HCT116; ATCC #CCL 247) BAC DNA purification kit: e.g., Nucleobond Plasmid Maxi kit (Clontech) or similar kit from Qiagen (also see UNIT 2.1B) Yeast shuttle vectors (e.g., YEp24, pRS423, pRS424, pRS425, or pRS426, available from ATCC) E. coli cells (DH10B) pMC1neopolyA (Stratagene) or an IRES-neor plasmid (available from John Sedivy at Brown University upon request) EXPAND High Fidelity PCR System (Roche) QIAX Gel Extraction Kit (Qiagen) or equivalent Appropriate selective yeast media (Qbiogene) Yeast PCR kit (Qbiogene) Yeast RPM kit (Qbiogene) Facility for BAC library screening (e.g., Research Genetics) 30°C incubator Additional reagents and equipment for primer design (UNIT 7.7), synthesis of oligonucleotides (UNIT 2.11), PCR (UNIT 15.1), gel purification of DNA fragments (UNIT 2.7), preparation and screening of a BAC library (http://www.chori.org/ bacpac/home.htm), plating bacteria (UNIT 1.3), single-cell PCR (UNIT 25A.3), restriction digestion (UNIT 3.1), agarose gel electrophoresis (UNIT 2.5A), restriction mapping (UNITS 3.2, 3.3, & 7.7), finding sequence information on the Web (UNITS 19.2 & 19.3), dephosphorylation of DNA by calf intestinal phosphatase (UNIT 3.10), DNA subcloning and ligation (UNIT 3.16), introduction of plasmid DNA into E. coli (UNIT 1.8), colony hybridizations (UNIT 6.3), preparation of plasmid DNA (UNIT 1.7), DNA sequencing (Chapter 7), phenol extraction and ethanol precipitation of DNA (UNIT 2.1A), gel purification of DNA fragments (UNITS 2.5A, 2.7, or 2.8), introduction of DNA into S. cerevisiae (UNIT 13.7), growth of S. cerevisiae (UNIT 13.2), and DNA preparation from S. cerevisiae (UNIT 13.11) Identify and isolate BAC clones 1. Design (UNIT 7.7) and synthesize (as described in UNIT 2.11; or order from synthesis facility) a PCR primer pair for amplification of the genomic region of interest from human genomic DNA derived from the parental cell line. Optimize and perform PCR amplification (UNIT 15.1; the PCR product should be 500 to 1000 nucleotides in length) to produce a probe for BAC library screening (step 2). Gel purify the DNA fragment obtained from PCR (UNIT 2.7). The primer pair will be used both to create a probe for library screening and to confirm that the BACs identified harbor the correct insert. Ideally, the probe will be derived from intronic sequence, since probes derived entirely from exons can hybridize to pseudogenes.
2. Perform a hybridization-based BAC library screening using the probe for the region of interest created in step 1. For protocol details, see the laboratory Web page of Pieter De Jong at the Children’s Hospital Oakland Research Institute (http://www.chori.org/bacpac/home.htm). Since the physical map and sequence of the human genome is by now largely completed, it is often possible to search GenBank (UNIT 19.2) and identify the needed BAC, then order it from a BAC repository. However, if such information is unavailable, or if the BAC or BACs ordered from the repository do not contain the correct insert, it is necessary to screen a library to identify the needed BACs. In fact, the authors often screen a library even if a single correct BAC is available, since it is generally advantageous to have several different BAC clones with the needed insert, each with different boundaries and junctions.
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Although the materials are available to screen BAC libraries in academic laboratories), the authors generally contract out the screening to Research Genetics, which performs this procedure routinely. Screening BAC libraries involves hybridization of radiolabeled probes to very large commercially available membranes that contain hundreds of thousands of gridded BAC clones. Performing the actual hybridizations is tricky because of the size of the membranes. Furthermore, identification of the correct clones on the huge filters takes practice. If sending the probe to a commercial facility (e.g., Research Genetics), it generally takes 2 to 6 weeks to receive the final product, which will be stabs inoculated with bacteria harboring the desired BACs. Alternatively, it is possible to skip this BAC step by generating the needed genomic DNA via PCR amplification of the needed homology arms from a genomic DNA template. However, the authors prefer to use BACs as the source of genomic DNA for building targeting vectors, since it can be difficult to create large homology arms using PCR. Also, PCR can introduce mutations into the homology arms and therefore reduce the targeting efficiency of the completed targeting vector.
3. Once the BACs are received, confirm that they contain the correct insert as follows. a. Streak out bacteria from the stabs onto the appropriate selective medium to generate single colonies (UNIT 1.3). b. Grid individual colonies (generally ten or so) to a grid plate (UNIT 1.3). c. Using the PCR primer pair optimized in step 1 above, test each of the grid positions with whole-cell PCR (UNIT 25A.3) to identify bacteria harboring the desired insert. Confirming the BAC clone is important since the bacterial stabs are created by directly inoculating bacteria stored as glycerol stocks in 384-well plates. It is possible (and in the authors’ experience not uncommon) for the bacteria in a particular well to be a mixed population caused by spillage from adjacent wells, or the wrong clone entirely.
4. Prepare BAC DNA from positive clones using the Nucleobond Plasmid Maxi or similar DNA purification kit. The authors generally use Nucleobond columns from Clontech, though kits available from other manufacturers (e.g., Qiagen; UNIT 2.1B) will undoubtedly also work well.
5. Digest the various BACs with several different restriction enzymes (UNIT 3.1) and compare the restriction digestion patterns by agarose gel electrophoresis (UNIT 2.5A). This step confirms the large size of the purified DNAs. Furthermore, it is generally expected that different BACs share some bands but differ in others, reflecting the fact that they are derived from the same genomic region but have different boundaries.
Human Somatic Cell Gene Targeting
Subclone genomic DNA from the BAC into the yeast shuttle vector To choose a restriction fragment to subclone, one must first decide what exon(s) the completed vector will be designed to target. There are several factors to consider when choosing an exon to target. If possible, target the exon containing the initiating methionine, since it is then possible to build a promoterless targeting vector in which the initiating methionine of neor precisely replaces the initiating methionine of the targeted gene. However, avoid targeting the first exon, since the left arm of such a targeting vector would contain promoter elements of the gene itself, reducing the efficiency of the promoterless selection. If the initiating methionine is in the first exon, choose an internal exon and use an IRES-neor based strategy (Sedivy and Dutriaux, 1999). There are two main issues to consider when targeting an internal exon with an IRES-neor. First, it is desirable to target an early exon and therefore create a protein that is as dramatically altered as possible. Second, it is desirable to target an exon which, if it were skipped by the splicing machinery, would change the reading frame and result in a premature stop codon.
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6. Using published or Web-based human genomic sequence information (UNITS 19.2 & 19.3), make a sequence-based restriction map of the desired genomic region (see UNITS 3.2, 3.3 & 7.7). Identify several suitable restriction fragments for subcloning into the yeast shuttle vector. When choosing potential restriction fragments, keep in mind that one arm of the final targeting vector will need to be short enough to PCR amplify across (i.e., less than ∼2 kb) when identifying gene-targeted cell lines.
7. Using the RepeatMasker Web site (http://repeatmasker.genome.washington.edu), analyze the sequence within and surrounding these restriction fragments to identify repeat elements such as Alu and LINE-1. It is important to avoid designing targeting vectors composed largely of repeat elements, since the arms of such targeting vectors will not provide the unique homologies required to guide homologous recombination in human cells.
8. To perform the actual subcloning, first digest both the BAC DNA and the yeast shuttle vector with the appropriate restriction enzyme or enzymes (see UNIT 3.1 and documentation for shuttle vector). There are numerous yeast shuttle vectors available for use in this subcloning step. The authors generally use one of five different yeast shuttle vectors available from ATCC: YEp24, pRS423, pRS424, pRS425, and pRS426 (Botstein et al., 1979; Christianson et al., 1992). YEp24 is a pBR322-based, low-copy-number vector, whereas the pRS series are pBluescript-based, high-copy-number vectors. All work well for creating targeting vectors via homologous recombination; however, in the authors’ experience it is slightly easier to clone larger fragments (>5 kb) into low-copy-number vectors such as YEp24.
9. Treat the linearized vector with calf intestinal alkaline phosphatase (CIP; UNIT 3.10) to limit recircularization of the vector during ligation. 10. Ligate the linearized, CIP-treated vector to the digested BAC DNA fragments (UNIT 3.16). Transform or electroporate appropriate E. coli cells with the ligated plasmid vector (UNIT 1.8). After plating on the appropriate selective medium, grid ∼500 colonies (UNIT 1.3). Also grid bacteria harboring various BACs to serve as positive and negative controls for the next step. 11. Identify bacteria harboring the desired recombinant plasmid by performing colony hybridizations with a radiolabeled oligonucleotide probe (UNIT 6.3). It is often possible to use one of the PCR primers from in step 1, above, as the probe. Note that the positive signals produced by bacteria harboring high-copy-number recombinant plasmids (e.g., pRS423-6) will be much stronger than the signals produced by bacteria harboring low-copy-number recombinant plasmids (e.g., BACs, YEp24).
12. Streak out the bacteria (UNIT 1.3) in positive grid positions to purify individual colonies. Prepare plasmid DNA using the Nucleobond Plasmid kit or equivalent (also see UNIT 1.7). 13. Confirm the integrity of the subclones by restriction analysis and sequencing of junctions (Chapter 7). It is important to identify and avoid subclones with multiple, concatamerized inserts.
Identify priming sites and restriction sites for PCR and Southern blot–based identification of gene-targeted cell lines It is important to identify these sequence features before completing the targeting vector, since the sequence comprising a PCR primer and possibly a restriction site will be built into the targeting vector during the final step of construction.
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14. To identify suitable PCR primers, design (UNIT 7.7) and synthesize (as described in UNIT 2.11; or order from synthesis facility) three sense and three antisense primers that meet the following criteria: a. By convention, one primer is located in the genomic DNA just adjacent to the shorter homology arm of the targeting vector. b. The other primer is not located in the neor gene (or IRES-neor gene) itself, but is instead located in the genomic region deleted by the targeting vector. Importantly, this priming site will be built into the targeting vector itself at the junction of the short homology arm and the neor gene. Homologous integration of the targeting vector deletes the endogenous site and moves it, altering the size of the PCR product (generally shortening it).
15. Test all nine permutations of these primers on genomic DNA derived from the parental cell line (e.g., HCT116) at several different annealing temperatures, as described in UNIT 15.1. Choose the most robust primer pair for eventual screening of putative gene-targeted clones. As described above (see Strategic Planning), the authors use a strategy for PCR-based diagnosis of knockouts that is slightly different from the conventional strategy used to identify knockouts in mouse embryonic stem cells. This strategy is preferable to the conventional PCR-based screen, since it includes a positive internal control with each PCR reaction.
16. To identify candidate restriction sites for Southern blot–based diagnosis of knockouts, make a computer-generated restriction map of the genomic region using published or Web-based genomic sequences (UNIT 7.7). Identify suitable sites based on the following criteria: a. The site should be present at a defined location in the genomic DNA at least 500 nucleotides outside one homology arm of the completed targeting vector (to allow for placement of a Southern blot probe; also see UNIT 10.8). b. The site should be absent in that homology arm itself. c. The site should be present at a defined location either in the other homology arm or in the genomic DNA adjacent to the other homology arm. d. Furthermore, the site should either be present in the neor gene itself, or should be added to the targeting vector (generally at a junction of a homology arm and neor) during the final step in vector construction. By following these guidelines, homologous integration of the targeting vector will alter the genomic restriction map in a way that should be easily measurable by Southern blot analysis (UNIT10.8).
Replace critical exon(s) with a promoterless neor or IRES-neor gene via homologous recombination in S. cerevisiae 17. Using sequence analysis software (UNIT 7.7), identify candidate restriction sites for linearization of the recombinant yeast shuttle vector created in steps 8 to 13. Confirm location and uniqueness of candidate site(s) via routine restriction analysis (UNIT 3.1). 18. Once an appropriate site has been chosen, perform a larger-scale preparative digest using approximately 2.5 µg of the recombinant shuttle vector plasmid DNA from step 12. Phenol extract and ethanol precipitate the linearized plasmid (UNIT 2.1A).
Human Somatic Cell Gene Targeting
19. Design (UNIT 7.7) and synthesize (as described in UNIT 2.11; or order from synthesis facility) PCR primers for amplification of the neor or IRES-neor gene with appropriate ends to add the PCR priming sites and restriction sites and to add 40 to 50 nucleotides of sequence homology to direct homologous recombination in S. cerevisiae.
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The authors order primers PAGE-purified from Integrated DNA Technologies. If using the neor gene as the template, the authors generally use the pMC1neoPolyA plasmid as the template and use the following PCR priming sites: sense 5′-ATGGGATCGGCCATTGAACAA-3′, antisense 5′-GTCGACGGATCCGAACAAACG-3′. Of note, the first three nucleotides of the sense primer comprise the initiating methionine of neor. If using IRES- neor (available from John Sedivy at Brown University) as the template, the authors use the following PCR priming sites: sense 5′-AACGTTACTGGCCGAAGCCGC-3′, antisense 5′-TCCCAACTCATCCCGGCCTC-3′.
20. Perform high-fidelity PCR with the pMC1neopolyA plasmid or IRES-neor as template, to create the neor PCR product, using the EXPAND High Fidelity PCR System or equivalent (also see UNIT 15.1). 21. Gel purify the PCR product using the QIAX Gel Extraction Kit or equivalent (also UNITS 2.5A, 2.7, or 2.8). 22. Cotransform (UNIT 13.7) 150 ng of linearized recombinant shuttle vector (from step 18) together with an estimated 500 ng of gel-purified PCR product (from step 21) into S. cerevisiae. Include negative controls consisting of vector alone and PCR product alone. The authors generally use either commercially available competent yeast (strain MaV203; Invitrogen) or prepare competent yeast from strain INVSc1 (Invitrogen) using the FrozenEZ Yeast Transformation II kit (Zymo Research). The choice of yeast strain depends on which auxotrophic marker is needed. Specific protocols for transformation are available in each kit. Also see UNIT 13.7 for additional information relating to yeast transformation.
23. Plate the transformation mixes on the appropriate selective medium (available from Qbiogene), and incubate at 30°C for 2 to 4 days. General considerations for the growth of S. cerevisiae are presented in UNIT 13.2. Expect to see ∼5-fold more colonies in the experimental reactions than in vector alone. There should be no colonies in insert alone.
24. Grid approximately 20 colonies from the vector + insert transformation mix (see UNIT 13.2). Also grid some vector-only colonies to serve as negative controls for the subsequent PCR reaction. 25. Design and synthesize (as described in UNIT 2.11; or order from synthesis facility) a set of PCR primers that flank the recombination event. Such a primer pair will create PCR products of different sizes in recombined and nonrecombined plasmids.
26. Test yeast from individual grid positions by PCR to identify those in which a recombination event has occurred. The authors generally use the yeast itself as a PCR template after pretreatment with zymolyase to destroy the cell wall. The authors use the Yeast PCR Kit (Qbiogene) for this purpose.
27. Once yeast-harboring recombinant plasmids have been identified, sequence the ends of the PCR products to demonstrate that the recombination junctions are intact and free of mutations. Design (UNIT 7.7) and synthesize (as described in UNIT 2.11; or order from synthesis facility) sequencing primers for the purpose of sequencing the junctions from “inside out,” making it possible to generate sequence from the very end of the PCR products. Sequence the PCR products (see Chapter 7). Introduction of DNA into Mammalian Cells
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Transfer completed targeting vectors into E. coli for large-scale DNA preparation 28. Prepare a miniprep of the recombinant yeast DNA using the Yeast RPM kit. 29. Electroporate the miniprep DNA into E. coli as described in UNIT 1.8. Plate on the appropriate selective medium (UNIT 1.3). Streak out individual colonies to purify. 30. Perform large-scale plasmid preps as described in UNIT 1.7. The authors generally use Nucleobond kits (Clontech).
31. Confirm the integrity of the resultant plasmid via restriction analysis and resequencing of the critical junctions (see Chapter 7) using the sequencing primers used in step 27 above. 32. Linearize the vector for transfection (Support Protocol). SUPPORT PROTOCOL
LINEARIZATION OF THE COMPLETED TARGETING VECTOR PRIOR TO TRANSFECTION INTO HUMAN CELLS The completed targeting vector must be linearized prior to transfection, since homologous recombination is more efficient with linear DNA. As such, it is necessary to identify a suitable restriction enzyme that cuts only in the vector backbone. There are generally sites for rare-cutting enzymes (e.g., SalI or NotI) in the polylinker of the yeast shuttle vector that are useful for this purpose Materials Completed targeting vectors (Basic Protocol 1) TE buffer (APPENDIX 2) Additional reagents and equipment for restriction digestion (UNIT 3.1), agarose gel electrophoresis (UNIT 2.5A), and phenol extraction and ethanol precipitation of DNA (UNIT 2.1A) 1. Identify an appropriate restriction enzyme (UNIT 3.1) for linearization of the completed targeting vector in the vector backbone. Confirm that the enzyme cuts the targeting vector singly and in the predicted location by performing small-scale restriction digests and analyzing an aliquot by agarose gel electrophoresis (UNIT 2.5A). 2. Perform a preparative digest of 50 to 100 µg of the targeting vector with the chosen enzyme (UNIT 3.1). Phenol extract and ethanol precipitate the digest (UNIT 2.1A), then resuspend in TE buffer to a final concentration of 0.5 µg/µl. 3. Confirm the efficiency of digestion by analyzing an aliquot of the digest by agarose gel electrophoresis (UNIT 2.5A). The authors generally prepare linearized DNA from three identical targeting vectors derived from different yeast colonies, in the unlikely event that one or several of the vectors has a PCR-generated inactivating mutation in the neor (or IRES-neor) gene.
BASIC PROTOCOL 2
Human Somatic Cell Gene Targeting
TRANSFECTION OF THE LINEARIZED TARGETING VECTOR INTO CULTURED HUMAN CELLS AND IDENTIFICATION OF HETEROZYGOUS KNOCKOUT CELLS The authors generally use HCT116 cells for the initial gene-targeting experiments, since these cells divide rapidly, grow well as individual clones in 96-well plates, and are generally well established as suitable for human somatic cell gene targeting. Once the targeting vector has been shown to work efficiently in HCT116 cells, attempts can be made to target the gene in other cell lines as well.
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Two sequential transfections are performed in this step. Initially, the cells are transfected, and colonies are selected directly in 25-cm2 tissue culture flasks and stained with crystal violet. This preliminary experiment demonstrates whether the new targeting vector(s) can create G418-resistant colonies, and identifies any targeting vectors that may harbor inactivating mutations in their neor (or IRES-neor) gene. Also, based on the number of colonies produced, it is possible to estimate the dilutions needed for creation of single colonies via limiting dilution in 96-well plates. Next, the HCT116 cells are retransfected and set up in a large number (50 to 100) of 96-well plates for obtaining individual drug-resistant clones by limiting dilution. Once single colonies have formed (first identifiable ∼10 days post-transfection), they are expanded (at ~20 days post-transfection) for cryopreservation and preparation of genomic DNA. The authors generally work up 200 to 500 colonies in this step. Finally, the genomic DNAs are tested by PCR to identify heterozygous knockout cell lines. Knockouts are then thawed, and are grown up to create more frozen vials and to reprepare genomic DNA. The genotype of the cell lines is then reconfirmed by PCR and Southern blotting. Materials Complete McCoy’s 5A medium (see recipe) with and without 0.6 µg/ml G418 (geneticin, Life Technologies; see UNIT 9.5) HCT116 cells (ATCC #CCL 247) Lipofectamine reagent (Invitrogen; see UNIT 9.4) Linearized targeting vector (Support Protocol) Plasmid unrelated to targeting vector and lacking neor gene (negative control for colony formation) PMC1neopolyA plasmid (Stratagene) CMV β-gal plasmid (Clontech) Hanks’ Balanced Salt Solution (HBSS, Life Technologies; also see APPENDIX 2) 0.05% (w/v) trypsin/0.53 mM tetrasodium EDTA (Life Technologies) 25-cm2 and 75-cm2 tissue culture flasks 8-channel pipettor (e.g., Easy Step with 8-port manifold from Continental Laboratory Products) with 50-ml reservoir (Brinkmann 22-49-614-0) 96-well tissue culture plates 24-well tissue culture plates 15-ml conical centrifuge tubes Additional reagents and equipment for culture of mammalian cells (APPENDIX 3F), transfection of mammalian cells (UNIT 9.4), Xgal staining of cultured cells (UNIT 9.10), crystal violet staining of cultured cells (as for plaque assay; see UNIT 16.16, Support Protocol), preparation of genomic DNA (UNIT 2.2), PCR (UNIT 15.1), and Southern blotting (UNIT 2.9A) NOTE: All culture incubations should be performed in a humidified 37°C, 5% CO2 incubator unless otherwise specified. NOTE: All reagents and equipment coming into contact with live cells must be sterile, and aseptic technique should be used accordingly. Determine if target vector is capable of creating neor clones 1. Grow HCT116 cells (see APPENDIX 3F for culture techniques) to 60% to 70% confluence in complete McCoy’s 5A medium (without selective agent). Transfect HCT116 cells with linearized targeting vectors using Lipofectamine reagent as described in
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and in the manufacturer’s instructions, using 3 µg DNA per 25-cm2 tissue culture flask of 60% to 70% confluent cells. Also transfect: UNIT 9.4
a. A plasmid harboring a neor gene driven by a heterologous promoter (e.g., pMC1neopolyA) as a positive control for colony formation. b. An unrelated plasmid lacking a neor gene as a negative control for colony formation. c. CMV β-Gal plasmid as a control for transfection efficiency. Do not add selective medium to the cells yet. The authors recommend transfecting ∼10 to 20 25-cm2 flasks in this step.
2. The day after transfection, stain the CMV β-Gal transfected cells with Xgal (UNIT 9.10; expect to see 20% to 40% blue cells). Change the medium on the remaining flasks to complete McCoy’s 5A medium containing 0.6 µg/ml G418. The authors generally use the β-Gal Staining Kit (Roche).
3. Replace the medium with fresh complete McCoy’s 5A medium containing 0.6 µg/ml G418 every 3 days (colonies should begin to appear at approximately day 10). Stain the colonies with crystal violet (see UNIT 16.16, Support Protocol) on approximately day 15. In the authors’ experience, high-efficiency promoterless targeting vectors generally create between 30 and 200 colonies per transfected 25-cm2 flask of HCT116 cells, whereas vectors harboring neor genes driven by heterologous promoters generally produce thousands of colonies per transfected 25-cm2 flask.
Perform selection on neor clones 4. Once the pilot experiment described in steps 1 to 3 above demonstrates that targeting vector is capable of creating neor clones, repeat to generate hundreds of individual neor clones. To do this, repeat the transfection strategy described above, including the positive and negative controls. At 18 to 24 hr after transfection, stain the CMV β-Gal–transfected control cells and feed the other control cells with complete McCoy’s 5A medium containing 0.6 µg/ml G418. The remainder of the flasks will be trypsinized and transferred to 96-well plates, as described is step 5.
5. Wash the cell monolayers in the 25-cm2 flasks with 5 ml HBSS, then add 1 ml of 0.05% trypsin/0.53 mM EDTA and swirl gently to detach the cells. In a sterile, disposable beaker, mix the trypsinized cells with complete McCoy’s 5A medium containing 0.6 µg/ml G418. The volumes to be used depend on the dilution and the number of 96-well plates to be created from the cell suspension (25 ml of medium per 96-well plate). Initially, try three different “dilutions” of cells to increase the likelihood of obtaining wells with single colonies: 1, 1/5, and 1/20th of a 25-cm2 flask in a 96-well plate.
6. Place an aliquot of the cell suspension in each well of the appropriate number of 96-well plates using an 8-channel pipettor with a sterile 50-ml reservoir. Stack plates, cover with plastic wrap to prevent evaporation, and begin incubating. 7. Identify wells containing single colonies as early as possible (after 10 days to 2 weeks). Human Somatic Cell Gene Targeting
As colonies become larger, cells tend to detach and form “satellite” colonies. Also, over time, two initially distinct colonies may merge together and appear to be a single colony.
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8. When colonies cover at least 50% of the surface area of the well, transfer to 24-well plates. To do this, wash the well once with 250 µl HBSS, then add 100 µl of 0.05% trypsin/0.53 mM EDTA. Transfer each colony to a separate well of a 24-well plate containing 1 ml of complete McCoy’s 5A medium containing 0.6 µg/ml G418. 9. As the wells become confluent, expand the cultures to 25-cm2 flasks. To do this, wash the well once with 1 ml HBSS, then add 250 µl 0.05% trypsin/0.53 mM EDTA. Transfer the trypsinized cells to a 25-cm2 flask containing 5 ml of complete McCoy’s 5A medium containing 0.6 µg/ml G418. Harvest cells and identify knockouts 10. As the 25-cm2 flasks become confluent, harvest the cells: use 1/3 to prepare genomic DNA; cryopreserve the remaining 2/3. To harvest: a. Wash the monolayer once with 2 ml HBSS, then add 1 ml of 0.05% trypsin/0.53 mM EDTA. b. Transfer 1/3 and 2/3 of the cells to separate 15-ml conical tubes, each containing 5 ml of of complete McCoy’s 5A medium (without selective agent). c. Centrifuge 7 min at 250 × g, room temperature, to create a cell pellet. 11. Prepare genomic DNA from the tube containing 1/3 of the cells (UNIT cryopreserve the cells in the tube containing 2/3 of the cells (APPENDIX 3F).
2.2)
and
12. Test genomic DNAs by PCR as described in Basic Protocol 1 and UNIT 15.1 to identify knockouts. 13. Thaw PCR-positive clones into a 75-cm2 tissue culture flask containing 20 ml of complete McCoy’s 5A medium with 0.6 µg/ml G418. Expand into a 225-cm2 flask for cryopreservation of nine cryovials (APPENDIX 3F) and repreparation of genomic DNA (UNIT 2.2). 14. Retest by PCR (UNIT 15.1) and confirm via Southern blotting (UNIT 2.9A). RESTORATION OF G418 SENSITIVITY VIA Cre-MEDIATED EXCISION OF THE neor GENE
BASIC PROTOCOL 3
In order to recycle the targeting vector to disrupt the remaining allele, it is necessary to excise the neor gene (or IRES- neor gene) from heterozygous knockouts via expression of Cre recombinase. Generally, a recombinant Cre-expressing adenovirus is used for this purpose. Materials High-titer Cre-expressing adenovirus (e.g., Microbix Biosystems) Heterozygous knockout cell lines (see Basic Protocol 2) growing at 50% confluence in 25-cm2 tissue culture flasks Complete McCoy’s 5A medium (see recipe) with and without 0.6 µg/ml G418 (geneticin, Life Technologies; see UNIT 9.5) Hanks’ Balanced Salt Solution (HBSS, Life Technologies; also see APPENDIX 2) 0.05% (w/v) trypsin/0.53 mM tetrasodium EDTA (Life Technologies) 25-cm2 tissue culture flasks 96-well tissue culture plates 24-well tissue culture plates Additional reagents and equipment for culture of mammalian cells (including counting and cryopreservation; APPENDIX 3F), plating cells at limiting dilution (as for hybridoma cells; UNIT 11.8), and crystal violet staining of cultured cells (as for plaque assay; see UNIT 16.16, Support Protocol)
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NOTE: All culture incubations should be performed in a humidified 37°C, 5% CO2 incubator unless otherwise specified. NOTE: All reagents and equipment coming into contact with live cells must be sterile, and aseptic technique should be used accordingly. 1. Add 1 µl recombinant adenovirus to heterozygous knockout cells at 50% confluence in a 25-cm2 tissue culture flasks containing 5 ml complete McCoy’s 5A medium without selective agent (G418). Incubate 24 hr. 2. Wash cells twice with HBSS, then trypsinize (see Basic Protocol 2) and count cells (APPENDIX 3F). 3. Plate out at limiting dilution (UNIT 11.8) in 96-well plates in complete McCoy’s 5A medium without selective agent, to obtain single colonies. To be sure to obtain single colonies, set up a variety of 96-well plates, e.g., with 1 cell/well, 5 cells/well, and 10 cells/well. 4. Identify single colonies after 10 days, and trypsinize at ∼2 weeks as described in Basic Protocol 2. Expand each clone to a well in a 24-well plate, and then to two 25-cm2 tissue culture flasks. One of the 25-cm2 flasks will be used to test the G418 sensitivity, and the other will be used for cryopreservation.
5. Add complete McCoy’s 5A medium containing 0.6 µg/ml G418 to one of the 25-cm2 flasks when it reaches 50% confluence. Continue incubating for ∼10 to 14 days, feeding every 3 days. Cryopreserve the cells in the other 25-cm2 flask when it reaches confluence, as described in APPENDIX 3F. 6. After ∼10 to 14 days, stain the 25-cm2 flask containing the cells exposed to selective agent with crystal violet as described in UNIT 16.16. Identify those cell lines that are now completely G418-sensitive, and use them to disrupt the remaining allele (see Basic Protocol 4). ALTERNATE PROTOCOL
SWAPPING SELECTABLE MARKERS IN THE TARGETING VECTOR An alternative to recycling the neor targeting vector (see Basic Protocol 3) is to modify the targeting vector to switch selectable markers, replacing neor with hygr. It should be noted that the authors have been consistently unable to generate drug-resistant colonies when using puror, IRES-hygr, or IRES-puror (C. Lee, J. S. Kim, T. Waldman, unpub. observ.). To do this, repeat Basic Protocol 1 with PCR amplification of a hygr gene. Perform Basic Protocol 2 as written, except substitute 0.1 µg/ml hygromycin for G418.
BASIC PROTOCOL 4
Human Somatic Cell Gene Targeting
TARGETING OF THE REMAINING ALLELE The principles and protocols needed for targeting the remaining allele are virtually identical to those already described for targeting the first allele. If recycling the neor targeting vector, one should repeat Basic Protocol 2, using G418-sensitive heterozygous knockout cell lines (see Basic Protocol 3) as the “parental” cells. If swapping selectable markers in the targeting vector, use neor heterozygous knockout cell lines as the “parental” cells. Of the homologous integration events obtained, approximately half should represent homozygous knockouts, and the other half should have targeted the already targeted allele. After confirmation of the homozygous knockout with PCR (UNIT 15.1) and Southern blotting (UNIT 2.5A), the authors generally perform an immunoblot (UNIT 10.8) to confirm the absence of protein.
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REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Complete McCoy’s 5A medium McCoy’s 5A medium (Life Technologies cat. no. 16600-082) 10% fetal bovine serum (FBS, Life Technologies; also see APPENDIX 3F) 1× penicillin/streptomycin (Life Technologies cat. no. 15140-122) Store up to 4 months at 4°C COMMENTARY Background Information History of human somatic cell gene targeting The first descriptions of successful gene targeting in cultured human cells appeared in 1991, when two groups reported heterozygous targeting of different genes. Heterozygous targeting of the 6-16 gene, an interferon-inducible transcript, was reported by Itzhaki and Porter (1991). The frequency of homologous integration was extremely low (estimated at 1 per 1550 colonies screened), leading these authors to employ an unusual screen involving integration of a human growth hormone gene (hGH) and the screening of conditioned media from putative clones via hGH ELISA. That same year, heterozygous targeting of the HPRT gene was reported (Zheng et al., 1991). That publication was a proof-of-principle study in which the authors were able to directly select for heterozygous knockout cell lines. In 1993, the first homozygous (or complete) knockout of a gene in human cells was reported (Porter and Itzhaki, 1993). Using a promoterless targeting vector, Porter and Itzhaki were able to disrupt the remaining allele of 6-16 in the heterozygous gene-targeted cells described in their 1991 paper. These three early studies demonstrated the feasibility of gene targeting in human cells; they also demonstrated that it was a very laborintensive process. In 1993, a landmark paper in the history of human somatic cell gene targeting was published, reporting the heterozygous gene targeting of the K-ras oncogene in two human colon cancer cell lines (Shirasawa et al., 1993). Although this was not the first manuscript to describe human somatic cell gene targeting, it pointed out the potential utility of somatic cell gene targeting for studying human cancer. In particular, these authors created derivatives of both HCT116 and DLD1 cells in which either the wild-type or the oncogenic allele of K-ras
had been deleted. They went on to show that the K-ras oncogene was absolutely required for the tumorigenic properties of both cell lines. They reported an extremely high frequency of knockouts (8/93 in HCT116; 7/24 in DLD1), attributing it to the use of a promoterless neor gene. Furthermore, they introduced the use of HCT116 and DLD1 cells for gene targeting. In 1995, a careful study was published that examined the merits of promoterless versus conventional targeting vectors for somatic cell gene targeting (Hanson and Sedivy, 1995). Their conclusion bolstered the work done by Shirasawa et al. (1993) showing that promoterless targeting vectors were virtually essential to achieve high frequencies of homologous recombination. They also reported that negative selectable markers commonly used for gene targeting in embryonic stem cells were of limited utility for somatic cell gene targeting. Furthermore, they pointed out that while the neor gene was extremely useful for gene targeting, the hygr gene seemed to work poorly. Subsequent studies, mostly unpublished, confirmed these seminal observations. Also in 1995, homozygous deletion of the p21 cdk inhibitor in human HCT116 cells was reported (Waldman et al., 1995). This study brought together several of the most recent technical advances, such as the use of promoterless targeting vectors, the lack of a need for negative selectable markers, and the value of HCT116 cells. Importantly, this study demonstrated the feasibility and utility of human somatic cell gene targeting for the study of tumor suppressor gene pathways. Other important incremental technical gains have been made as well. Advances have been made that simplify the construction of human promoterless targeting vectors, such as the use of homologous recombination in Saccharomyces cerevisiae (see UNIT 13.10). An important modification to the standard PCR screen for knockouts has made identification of gene tar-
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geted cell lines simpler and more robust (see UNIT 13.10). Cre-loxP recombination (see UNITS 9.5 & 13.3) has been successfully employed to overcome difficulties associated with creation of homozygous knockouts by making it possible to recycle the same targeting vector for both alleles of a gene. Today, through the implementation of these advances, human somatic cell gene targeting has emerged as an important tool for the study of gene function in human cells.
Critical Parameters and Troubleshooting
Human Somatic Cell Gene Targeting
The frequency of homologous recombination in human cells is 3 to 5 orders of magnitude lower than the frequency of nonhomologous recombination. As such, the vast majority of drug-resistant clones formed after transfection of a conventional targeting vector represent random integration events. Identification of the tiny fraction of gene-targeted clones can be extremely challenging. It should be noted that the frequency of homologous integration in human cells is thought to be significantly lower than the frequency of homologous integration in murine embryonic stem (ES) cells. This fact, coupled with the fact that creation of homozygous knockouts in human cells requires sequential targeting of both alleles (unlike in mice, in which heterozygous knockouts are mated to homozygosity), renders promoter-containing targeting vectors unsuitable for human somatic cell gene targeting. Instead, promoterless targeting vectors are required. In such targeting vectors, the selectable marker gene (generally neor) lacks its own promoter and is instead fused in-frame with the gene being targeted. The most important critical parameter is the efficiency with which the targeting vector creates gene-targeted cell lines. Vectors vary in their efficiency from 8 weeks at room temperature. CTC working solution: Mix equal volumes of CTC, 0.8 M NaOH solution, 10% (w/v) SDS solution, and water. The reagent is stable 1 to 2 weeks at room temperature. Spectrophotometric standard protein solution, 3 mg/ml Weigh out dry protein and prepare a stock solution at a concentration of 3 mg/ml in the same solvent as used for the sample protein. Store up to 3 months at −20°C. To prepare calibration standard solutions, dilute the stock solution in solvent to give the desired final concentrations for the standard curve. Bovine serum albumin (BSA, fraction V; Sigma) is frequently used as a protein standard solution. A 3 mg/ml solution of BSA should have an A280 of 1.98, based on an A280 of 6.61 for a 1% (w/v) solution. For quantitation of a purified or partially purified protein, the protein standard should, if possible, have an aromatic amino acid content similar to that of the sample protein.
COMMENTARY Background Information
Spectrophotometric and Colorimetric Determination of Protein Concentration
Measuring absorbance at 280 nm (A280) is one of the oldest methods for determining protein concentration (Warburg and Christian, 1942; Layne, 1957). This method is still widely used, because it is simple and does not require incubating the sample with exogenous chromophores. However, the detection limit is higher than with colorimetric methods and therefore higher concentrations of protein are necessary. The A280 method requires that the protein being quantitated contain aromatic amino acids, primarily tryptophan and tyrosine. Because of the variability in aromatic amino acid content among different proteins, their absorptivity at 280 nm (a280) also varies. Therefore, if calibration standards are used for quantitation, the aromatic amino acid content of the standard must be similar to that of the sample protein for
accurate results. Accordingly, the quantitation of proteins by peptide bond absorption at 205 nm (A205) is more universally applicable. Furthermore, the absorptivity for a given protein at 205 nm is several-fold greater than that at 280 nm (Scopes, 1974; Stoscheck, 1990). Thus, lower concentrations of protein can be quantitated with the A205 method. The disadvantage of this method is that some buffers and other components absorb at 205 nm (Stoscheck, 1990). In addition to the aromatic amino acids, several others have absorption maxima in the UV range. Table 10.1A.1 shows the wavelengths of absorption maxima and corresponding molar absorptivity (ε) for the amino acids with appreciable absorbance in the UV range. Only tryptophan has an absorption maximum near 280 nm, although tyrosine will absorb
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Table 10.1A.1 Absorption Maxima and Molar Absorptivity (ε) of Amino Acidsa
Amino acid
Wavelength maxima (nm)
ε × 10−3 (l/mol cm)
250 211 188 206 257 219 279 193 222 275
0.3 5.9 60.0 9.3 0.2 47.0 5.6 48.0 8.0 1.4
Cysteine Histidine Phenylalanine
Tryptophan Tyrosine
aValues are for aqueous solutions at pH 7.1 (Freifelder, 1982;
Fasman, 1989).
Table 10.1A.2 Molar Absorptivity (ε) of Aromatic Amino Acids at 280 nma
Amino acid Phenylalanine Tryptophan Tyrosine
ε × 10−3 (l/mol cm) 0.0007 5.559 1.197
aValues are for aqueous solutions at pH 7.1 (Fasman, 1989).
somewhat at that wavelength. The ε280 for tryptophan is nearly 5-fold greater than that for tyrosine (Table 10.1A.2). Several amino acids other than those in Table 10.1A.1 absorb light below 205 nm (Fasman, 1989), but either the molar absorptivities are too low to be significant or the wavelengths are too short for practical absorbance measurements. The aromatic amino acids also exhibit fluorescence emissions when excited by light in the UV range. Table 10.1A.3 gives the excitation wavelength, fluorescence emission wavelength, and quantum yield (Q) for tryptophan, tyrosine, and phenylalanine. The quantum yield is the ratio of photons emitted to photons absorbed. Typically, phenylalanine fluorescence is not detected in the presence of tyrosine and tryptophan due to low Q. Furthermore, tyrosine fluorescence is nearly completely quenched if the tyrosine residue is ionized or near an amino group, a carboxyl group, or a tryptophan residue (Teale, 1960; Freifelder, 1982). Therefore, tryptophan fluorescence is what is customarily measured. Measurement of intrinsic fluorescence by aromatic amino acids is primarily used to ob-
tain qualitative information (Freifelder, 1982). However, with a protein standard whose aromatic amino acid content is similar to that of the sample, intrinsic fluorescence can be used for quantitation (Hawkins and Honigs, 1987). An additional consideration is that the tertiary structure of a protein will influence the fluorescence, e.g., adjacent protonated acidic groups in a protein molecule will quench tryptophan fluorescence (Freifelder, 1982). The most frequently employed colorimetric methods for determining protein concentration are those of Bradford (1976) and Lowry et al. (1951). The Bradford method, which is faster than the Lowry method, is the method of choice for determining protein concentration. It is possible to miniaturize the Bradford assay by using a 96-well microtiter plate assay as described by Brogdon and Dickinson (1983). The Lowry method (as modified by Peterson, 1977) is dependent on the unknown and the standard protein having a similar content of tyrosine per microgram protein. If the protein has fewer (or more) tyrosines/µg than BSA, then the protein concentration determined will be too low (or too high). This method is not
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Table 10.1A.3
Fluorescence Properties of Aromatic Amino Acidsa
Amino acid
Excitation wavelength
Emission wavelength
Quantum yield
Phenylalanine Tryptophan Tyrosine
260 nm 285 nm 275 nm
283 nm 360 nm 310 nm
0.04 0.20 0.21
aValues are for aqueous solutions at pH 7 and 25°C (Hawkins and Honigs, 1987; Fasman,
1989).
appropriate for proteins without tyrosine residues, since the assay relies on the reaction of these residues with the reagent.
Critical Parameters and Troubleshooting A 1-cm-path-length quartz cuvette is most often used for spectrophotometric detection. However, quartz cuvettes with shorter path lengths (0.01 to 0.5 cm) are available (e.g., from Hellma Cells or Beckman); these allow higher concentrations of protein solutions to be measured. The equations relating absorbance and protein concentration (see Basic Protocol 1 and Alternate Protocol 1) assume the cuvette has a path length of 1 cm; when cuvettes of shorter path length are used, the correct value for b must be substituted in the equation. Glass cuvettes can be used for colorimetric detection. Quartz cuvettes are not recommended for the Bradford assay because the dye adheres more readily to quartz than to glass.
Spectrophotometric and Colorimetric Determination of Protein Concentration
Spectrophotometric detection The solvent pH and polarity will affect the absorbance and fluorescence properties of a protein. A notable example of pH effects on absorbance is seen with tyrosine residues, where a change in pH from neutral to alkaline results in a shift of the absorbance maximum to a longer wavelength and an increase in absorptivity due to dissociation of the tyrosine phenolic hydroxyl group (Freifelder, 1982; Fasman, 1989). An example of solvent polarity effects on fluorescence is that observed with tryptophan, where a decrease in solvent polarity results in a shift in fluorescence emission to shorter wavelengths and an increase in intensity (Freifelder, 1982). Because of these effects, the following precautions should be taken for accurate results: (1) when calibration curves are used for quantitation by absorbance or fluorescence, standards must be in the same solvent as the samples; and (2) when a published absorptivity at a given wavelength is used for quanti-
tation, the solvent composition of the sample must be the same as that used in obtaining the published data. Many buffers and other reagents can interfere with A280 and A205 spectrophotometric measurements. Stoscheck (1990) lists the concentration limits for many such reagents used in these spectrophotometric methods. The more commonly used reagents that absorb at 280 and 205 nm are listed in Table 10.1A.4. In addition, reagents that contain carbon-carbon or carbonoxygen double bonds can interfere with the A205 method. Because stray light can affect the linearity of absorbance versus concentration, absorbance values >2.0 should not be used for sample proteins measured by the A280 or A205 method. Samples with absorbance >2.0 should be diluted further in the appropriate buffer to obtain absorbances 20 kDa with a pI between 3 and 5 or 8 and 10 with an amount 800 Ci/mmol) can be obtained from several suppliers of radiolabeled amino acids. Protein hydrolyzates of Escherichia coli grown in the presence of [35SO4]2− (e.g., Tran35S-label from ICN Biomedicals or Expre35S35S from NEN Life Sciences) can be used as substitutes for [35S]methionine in metabolic labeling techniques. These preparations contain 35S distributed among several different compounds. In a typical batch, ∼70% of the radioactivity will be present as [35S]methionine and ∼15% as [35S]cysteine; the remainder are other 35S-labeled compounds. Labeling with these radioactive protein hydrolyzates in methionine-free medium will result in incorporation of label only in methionine residues; use of methionineand cysteine-free medium will result in labeling of both methionine and cysteine residues.
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Purified [35S]methionine should be used whenever certainty of labeling with only [35S]methionine is required. Another reason for using purified [35S]methionine is that it tends to be more stable and emit less volatile decomposition products than 35S-labeled protein hydrolyzates. In most cases, however, the relatively inexpensive radioactive protein hydrolyzates (∼1/3 the cost of [35S]methionine) can be used instead of purified [35S]methionine. [35S]methionine preparations should be stored frozen at −80°C. Under these conditions, they are stable for at least 1 month; the half-life of 35S is 88 days. For proteins with one or no methionine residues but several cysteine residues, labeling with [35S]cysteine (available with specific activities >800 Ci/mmol) is a good option. 35Slabeled protein hydrolyzates are not good sources of [35S]cysteine for radiolabeling because only ∼15% of the 35S-labeled compounds are [35S]cysteine. Purified [3H]leucine (available with specific activities of up to 190 Ci/mmol) is a good alternative to 35S-labeled amino acids. The halflife of 3H is ∼12 years. The specific activities of other 3H-labeled amino acids range between 5 and 140 Ci/mmol (Table 10.18.1). Several problems can arise when using certain 3H-labeled amino acids due to their participation in metabolic pathways. The following problems must be considered when using tritiated amino acids for metabolic labeling of proteins. Nonessential versus essential amino acids. Cells are able to synthesize nonessential amino acids from other compounds. If the radiolabeled amino acids used are nonessential, their specific activity will be reduced by dilution with the endogenously synthesized amino acids. Therefore, essential amino acids (E in Table 10.18.1) should be favored for metabolic labeling. Transamination. The α-amino groups of many amino acids can be removed in reactions catalyzed by transaminases (T in Table 10.18.1). Deamination of the radiolabeled amino acids can be prevented by addition of the transaminase inhibitor (aminooxy)acetic acid during starvation and labeling of the cells (Coligan et al., 1983). Interconversion. Certain radiolabeled amino acids can be converted by cells into other amino acids (I in Table 10.18.1). This problem is of particular importance in methods used to determine the amino acid composition or sequence of radiolabeled proteins.
Labeling time The experimental purpose, the turnover rate of the protein of interest, and the viability of the cells should all be considered when determining the length of time for labeling. If the purpose of the experiment is to identify or characterize a protein precursor, pulse-labeling (10 to 30 min) must be used (see Basic Protocol and Alternate Protocol 1). If the protein of interest has a low turnover rate or if it is necessary to accumulate a labeled mature product, a protocol for long-term labeling (6 to 32 hr) is more appropriate (see Alternate Protocol 3). If the biosynthesis, posttranslational modification, intracellular transport, or fate of newly synthesized proteins is being analyzed, a pulsechase protocol should be used (see Alternate Protocol 2). The turnover rate of the protein of interest is an important parameter to be considered when determining the labeling time. Proteins with a high turnover rate are optimally labeled for short times. Longer times will result in increased labeling of other cellular proteins, causing a decrease in the relative abundance of the labeled protein of interest in the cell lysate. This could result in increased detection of nonspecific proteins in immunoprecipitates (see UNIT 10.16). Conversely, proteins that turn over slowly should be labeled for longer times. Incorporation of radiolabeled amino acids into proteins is directly proportional to their length of labeling time for a certain period, after which it tends to plateau. When all the limiting amino acid is consumed, protein synthesis ceases. The length of the initial phase of linear incorporation will vary with the concentration of the labeling amino acid and the density and metabolic activity of the cells. Concentration and specific activity of the radiolabeled amino acid Because radiolabeled amino acids are used in limiting amounts, their incorporation into proteins is directly proportional to their concentration in the labeling medium. If very short pulses are required, the concentration of labeled amino acid can be increased to compensate for the shorter labeling time. The incorporation of radiolabeled amino acids is also directly proportional to their specific activity. Addition of unlabeled amino acids, as is required in long-term labeling protocols, will result in reduced incorporation over short periods. Analysis of Proteins
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Cell density At low cell densities, the amount of labeled protein synthesized is directly proportional to the cell concentration. At high densities, however, incorporation will increase nonlinearly. Very concentrated cell samples (>5 × 107 cells/ml) can only be labeled for short periods (≤5 min) as rapid loss of metabolic activity and cell viability will occur due to acidification of the medium and accumulation of toxic metabolites. Conditions for very short pulses Very short pulses (2 to 10 min) are required to study posttranslational modifications that occur shortly after synthesis. This includes folding, disulfide bond formation, and early carbohydrate modifications of newly synthesized proteins. Proteins that are rapidly degraded after synthesis are also best studied with very short pulses. The protocol for very short pulses is similar to the Basic Protocol for pulselabeling, with the following modifications: (1) the pulse-labeling medium contains higher concentration of radiolabeled amino acid (up to 1 mCi/ml), (2) labeling is stopped by addition of ice-cold PBS containing 20 mM freshly prepared N-ethylmaleimide (NEM) to prevent oxidation of free sulfhydryls, and (3) if a chase is necessary, 1 mM cycloheximide is added to the chase medium to quickly stop the elongation of nascent polypeptide chains. For additional details on very short pulses, see Braakman et al. (1991). Temperature and pH Unless otherwise required for special conditions (e.g., heat shock, temperature-sensitive mutants), metabolic labeling should be performed at 37°C. For this reason, it is important that the labeling medium be warmed to 37°C before adding it to the cells. A dramatic reduction in incorporation occurs at room temperature. It is also important that the pH of the labeling medium be ∼7.4.
Anticipated Results A typical incorporation of labeled amino acid precursor after a 30-min pulse under the conditions described in this unit (see Basic Protocol and Alternate Protocols 1 and 2) is 5% to 20%. In the case of long-term labeling, the incorporation typically reaches 30% to 60%. If the labeled cells are used for immunoprecipitation (UNIT 10.16), followed by SDS-PAGE (UNIT 10.2) and autoradiography (APPENDIX 3A), specific bands should be visible within two hours to two months of exposure.
Time Considerations It takes 1 to 2 hr to prepare cells and materials for labeling. The actual labeling time depends upon the protocol chosen—pulse-labeling takes 10 to 30 min and long-term labeling takes 6 to 32 hr. Washing and processing cells may take an additional 1.5 hr.
Literature Cited Braakman, I., Hoover-Litty, H., Wagner, K.R., and Helenius, A. 1991. Folding of influenza hemagglutinin in the endoplasmic reticulum. J. Cell Biol. 114:401-411. Coligan, J.E., Gates, F.T. III, Kimball, E.S., and Maloy, W.L. 1983. Radiochemical sequence analysis of metabolically labeled proteins. Methods Enzymol. 91:413-434. Lathe, R. 1985. Synthetic oligonucleotide probes deduced from amino acid sequence data. Theoretical and practical considerations. J. Mol. Biol. 183:1-12. Meisenhelder, J. and Hunter, T. 1988. Radioactive protein labelling techniques. Nature 335:120.
Key Reference Coligan et al., 1983. See above. Contains a detailed description of conditions used to metabolically label proteins with different radiolabeled amino acids.
Contributed by Juan S. Bonifacino National Institute of Child Health and Human Development Bethesda, Maryland
Metabolic Labeling with Amino Acids
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Isolation of Proteins for Microsequence Analysis
UNIT 10.19
The first basic protocol can be used to determine the amino-terminal sequence of polypeptides or proteins. It is particularly appropriate for large fragments of insoluble or hydrophobic proteins or proteins that cannot be purified to >90% molar purity without electrophoresis. Although the efficacy of this technique varies with the protein, it is possible to obtain useful sequence information starting with ≤50 pmol of the protein of interest. If the protein is blocked at the amino terminus, chemical cleavage or partial enzymatic digestion must be performed prior to electrophoresis. Upon isolation, the internal amino acid sequence is analyzed as described in the second basic protocol. This method requires ∼200 pmol of protein for analysis. It is understood that both of the analyses will be done in association with an expert operator of an automated protein sequencer. In addition, the second basic protocol requires expertise with reversed-phase HPLC to separate peptides. DETERMINATION OF AMINO ACID SEQUENCE BY SDS-PAGE AND TRANSFER TO PVDF MEMBRANES
BASIC PROTOCOL
A minigel is prepared and preelectrophoresed. A sample containing the desired protein is loaded onto the minigel and fractionated by SDS-PAGE at neutral pH. The proteins are then electrophoretically transferred to a sheet of polyvinylidene difluoride (PVDF) and stained with Coomassie blue. The separated bands are excised, then analyzed in an automated protein sequencer. Materials Separating and stacking gel solutions (Table 10.19.1) 4× gel buffer Glutathione, reduced powder (Sigma #G4251) 10× lower reservoir buffer 10× upper reservoir buffer Mercaptoacetic acid, sodium salt Protein sample in sample buffer (see support protocol) Methanol Transfer buffer 0.1% Coomassie blue in 50% methanol (v/v) 10% acetic acid in 50% methanol (v/v) Vertical minigel unit (e.g., Bio-Rad Mini-Protean II; Hoefer Mighty Small SE 250/280 is not recommended for this procedure) Power supply (constant voltage and constant current) Microvolume syringe or gel-loading pipet tip Powder-free plastic gloves PVDF membranes (e.g., Immobilon-P or -PSQ, Millipore; ProBlott, Applied Biosystems) Small-format transfer apparatus (Midget MultiBlot, Hoefer or Pharmacia LKB; Mini Trans-Blot, Bio-Rad) Automated protein sequencer (Applied Biosystems) Additional reagents and equipment for minigel preparation (UNIT 10.2) Analysis of Proteins Contributed by Malcolm Moos, Jr. Current Protocols in Molecular Biology (2000) 10.19.1-10.19.12 Copyright © 2000 by John Wiley & Sons, Inc.
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Table 10.19.1
Recipes for Polyacrylamide Separating and Stacking Gels
SEPARATING GEL Final acrylamide concentration in the separating gels (%)
Stock solutions 5 30% acrylamide monomera H2O TEMEDb
6
7
8
9
10
11
12
13
14
15
3.33 4.00 4.67 5.33 6.00 6.67 7.33 8.00 8.67 9.33 10.00 11.49 10.83 10.16 9.50 8.84 8.17 7.51 6.84 6.18 5.51 4.85 0.040 0.033 0.029 0.025 0.025 0.020 0.018 0.017 0.015 0.014 0.013
Mix the above ingredients (listed in milliliters of stock solution) with 5 ml of 4× gel buffer and 0.14 ml of 5% potassium persulfate or 70 µl of 10% ammonium persulfate. STACKING GEL 0.666 ml 30% acrylamide monomera 3.033 ml H2O 0.025 ml 10% ammonium persulfate or 0.05 ml 5% potassium persulfate 0.025 ml TEMEDb aGas-stabilized monomer solution (containing 37.5:1 acrylamide/N,N′-methylene-bisacrylamide) from which acrylic acid and carbonyl-containing compounds have been removed (Protogel, National Diagnostics; or PAGE1 protein gel mix, Boehringer Mannheim) bTEMED may have to be altered to facilitate proper polymerization. Values given are reasonable approximations.
Pour and preelectrophorese the minigels 1. Pour denaturing minigels as in UNIT 10.2, substituting the separating and stacking gel solutions listed in Table 10.19.1. Deaerate the gel polymerization mixtures prior to adding persulfate and TEMED and be careful not to draw in air through the polymerization mixtures when transferring or pipetting the solutions. After removal of combs, the wells should be rectangular and firm. If they are not, prepare a fresh gel; poorly polymerized stacking gels are the most common cause of low sequencing yields.
2. Assemble vertical minigel unit. 3. Dilute 80 ml of 4× gel buffer to 320 ml (to 1× gel buffer). Pour 200 ml of 1× gel buffer into lower buffer reservoir. Add reduced glutathione (powder) to remaining 1× gel buffer to 1.0 mM final. Pour this into upper buffer reservoir. Attach gel to a constant voltage/constant current power supply (see UNIT 10.2 introduction for a discussion of electricity and electrophoresis). Glutathione acts as a scavenger to eliminate the by-products of acrylamide polymerization.
4. Preelectrophorese by applying 10 mA per minigel for 45 min. Voltage applied for >45 min may impair resolution.
5. Turn off power supply. Allow gel to stand overnight. Gels may be stored this way for several days.
Load and electrophorese the sample 6. Pour off gel buffer and blot wells with tissue or filter paper. Isolation of Proteins for Microsequence Analysis
7. Dilute 10× lower and upper reservoir buffers to 1×. Pour 200 ml of 1× lower reservoir buffer into lower reservoir. Add 0.1 g mercaptoacetic acid (sodium salt) to 150 ml of 1× upper reservoir buffer and pour into upper reservoir.
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Mercaptoacetic acid is used as a substitute for glutathione at this step because it is less likely to interfere with sequence analysis.
8. Load protein sample dissolved in sample buffer with a microvolume syringe or gel-loading pipet tip. When using 5-well combs and 0.75-mm gels, results are best with sample volumes 60 kDa, reduce the amount of methanol in transfer buffer to 1%.
Stain the blots and excise the bands 15. Disassemble transfer apparatus. Immerse PVDF membrane blots in 0.1% Coomassie blue in 50% methanol and agitate 5 min. 16. Destain blots in 10% acetic acid prepared in 50% methanol by agitating until bands become clearly visible (5 to 10 min). 17. Transfer to water and photograph (optional; UNIT 10.6). 18. Excise band of interest with a razor blade (use a new razor blade for each band). Place each band in a microcentrifuge tube and allow to air dry at room temperature (do not heat). Store excised bands at −20°C. Sequence the proteins 19. Insert excised band into sequencer reaction cartridge, protein side facing the solvent delivery (see manufacturer’s instructions). All pieces of PVDF must fit in a single layer in the reaction cartridge; they may be cut or trimmed to fit as required. When using conventional chemistry (“NORMAL” program on Applied Biosystems instruments), two alternate configurations have proven optimal, depending on the sequencer. For many gas-phase instruments, placing the PVDF pieces between the upper-cartridge block and the TFA-etched glass-fiber filter gives the best results. For some gas-phase and most pulsed-liquid instruments, placement between the glass-fiber disk and the cartridge seal is better. The results of the cartridge leak test should be normal. The Blott cartridge should be used on Applied Biosystems instruments, if available.
20. Sequence the sample on an automated protein sequencer. The “Blott-2 reaction cycle” should be used if a Blott reaction cartridge is installed. The conversion cycle described by Tempst and Riviere (1989) is recommended. SUPPORT PROTOCOL
PREPARATION OF PROTEIN SAMPLES FOR SDS-PAGE Samples are concentrated and freed of contaminants that interfere with electrophoresis by precipitation. They are then solubilized in a sample buffer containing SDS. Additional Materials Protein samples 1 M NaHCO3 (optional) 100% ethanol, ice-cold (containing no denaturants; USP grade) Sample buffer 0.1% (w/v) pyronin Y Ultrafiltration concentrator (Amicon) or Speedvac evaporator (Savant) Drawn-out Pasteur pipet or gel-loading pipet tip Boiling water bath 1. Adjust the salt concentration of the protein sample to >100 mmol/liter with 1 M NaHCO3, if necessary.
Isolation of Proteins for Microsequence Analysis
2. Concentrate samples to 50 to 100 µl by ultrafiltration (follow manufacturer’s instructions explicitly) or vacuum centrifugation (it is crucial to avoid introduction of airborne debris when vacuum is released). Transfer to 1.5-ml microcentrifuge tubes.
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3. Add 9 vol ice-cold 100% ethanol to the samples. Incubate 1 hr on dry ice or overnight at −20°C. Most samples may be kept indefinitely at this stage.
4. Microcentrifuge 15 min at maximum speed. Aspirate the supernatant with a drawnout Pasteur pipet or a gel-loading pipet tip and save the pellet. A drum rotor, which holds the microcentrifuge tubes at a 90° angle, is more likely to give compact, easily visualized pellets than the commonly used fixed-angle rotor.
5. Dissolve the pellet in 10 µl sample buffer by drawing the sample buffer up and down with a pipettor. Boil 3 min in a boiling water bath. 6. Add 1 µl of 0.1% pyronin Y (tracking dye) to the sample. Load on the minigel in the basic protocol. DETERMINATION OF INTERNAL AMINO ACID SEQUENCE FROM ELECTROPHORETICALLY-SEPARATED PROTEINS
BASIC PROTOCOL
As many as 50% of all eukaryotic proteins are blocked at the amino terminus, making sequence determination of the intact protein impossible. The method presented here allows multiple stretches of internal amino acid sequence to be obtained from most proteins that can be isolated by any one- or two-dimensional gel electrophoresis and transfer method. About 200 pmol of protein is an adequate sample size; smaller amounts have sufficed in several cases. The protein(s) of interest is separated by electrophoresis and transferred to nitrocellulose. Protein bands or spots are visualized with Ponceau S and excised. After destaining, the nitrocellulose is treated with polyvinylpyrrolidone (PVP) to prevent adsorption and inactivation of proteolytic enzyme, which is added to cleave the protein. The resulting peptides, which elute from the membrane, are separated by reversed-phase HPLC. Each peptide is analyzed individually in an automated microsequencer. Materials Protein sample 0.1% (w/v) Ponceau S (Sigma) in 1% (v/v) acetic acid 1% acetic acid (v/v) 0.2 mM NaOH 0.5% (w/v) polyvinylpyrrolidone (Sigma # PVP-40) in 0.1 M acetic acid Digestion buffer 1 mg/ml sequencing-grade trypsin (Promega #V511A) Chromatography solvent A: 5% (v/v) acetonitrile in 0.1% (v/v) trifluoracetic acid (TFA) Chromatography solvent B: 70% (v/v) acetonitrile in 0.085% (v/v) TFA 0.22–µm nitrocellulose membrane (e.g., Schleicher & Schuell #BA83) Acid-washed glass plate or petri dish Powder-free gloves Fine-tipped forceps 0.5-ml microcentrifuge tube Bath sonicator (e.g., Bransonic 12) Centrifugal filter device, 0.22-µm membrane, low-protein-binding (e.g., Millipore #UFC3-OGV-00) Reversed-phase HPLC column (e.g., Vydac #214TP52), UV column monitor, and chart recorder Column oven (optional)
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Electrophorese and transfer the proteins 1. Resolve protein(s) of interest by electrophoresis and transfer to nitrocellulose membrane as described in UNIT 10.8. It is important to end up with as much protein as possible bound to the smallest possible area of nitrocellulose. Apart from this constraint, there are few limitations on the design of the separation, as chemical modification of the amino terminus will not interfere with subsequent analyses. Although PVDF membranes may be used, nitrocellulose seems to give better yields (see commentary). The membrane must never be allowed to dry from this point on.
Stain and excise the separated proteins 2. Place nitrocellulose membrane in an acid-washed glass petri dish (or similar vessel) containing 50 ml (for an 8 × 10–cm minigel) of 0.1% Ponceau S prepared in 1% aqueous acetic acid. Agitate gently 1 min. For larger blots, increase the volume of stain in proportion to the area.
3. Transfer to 1% acetic acid for 1 min, changing the solution as necessary to allow easy visualization of band(s). Use enough 1% acetic acid to cover the membrane generously.
4. Using a new razor blade and wearing powder-free gloves, cut out the band(s) of interest. Place in 1.5-ml microcentrifuge tube(s). It is important to remove all areas of blank nitrocellulose to minimize subsequent adsorption of the protease used for digestion. Meticulous technique is crucial at this step to eliminate contamination of the sample by adventitious proteins. A clean work area, scrupulously clean glassware and instruments, and powder-free gloves are minimum precautions; tryptic fragments of human keratin have been identified in some samples! To distinguish UV-absorbing peaks corresponding to the protein from those derived from the proteolytic enzyme or other reaction constituents, cut a piece of nitrocellulose from a blank area of the blot to use as a control. The bands may be transferred to microcentrifuge tubes containing 0.5 ml water and stored at −20°C at this step.
Destain the membrane pieces 5. Transfer membrane pieces to 1 ml of 0.2 mM NaOH and vortex 1 min. Aspirate the NaOH and immediately proceed to step 6. Proteins are more apt to be lost from the nitrocellulose under alkaline than acidic conditions. Because release of the peptides is desired at a later stage, the length of time for destaining should be minimized. Residual stain on the blot will not adversely affect the final results.
Block the membrane 6. Add 1 ml of 0.5% PVP-40 in 0.1 M acetic acid and agitate tube gently 30 min at room temperature. 7. Aspirate the PVP-40 and wash membrane five times with 1 ml water. Be sure to remove any liquid droplets caught under tube cap. Isolation of Proteins for Microsequence Analysis
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8. Using clean fine-tipped forceps, transfer excised band to a clean glass surface (e.g., acid-washed glass plate or petri dish) and cut it into 1- to 2-mm pieces. Use forceps to collect the pieces and squeeze out excess liquid. Immediately proceed to step 9. Digest the protein with protease 9. Transfer pieces to a 0.5-ml microcentrifuge containing 25 µl digestion buffer. Add 1 µl 1 mg/ml trypsin. Mix so that membrane pieces are evenly coated with solution. 10. Incubate overnight at room temperature. Elute the peptides 11. Microcentrifuge sample 1 sec at high speed, room temperature, to recover liquid that may have condensed on tube walls and cap. 12. Sonicate sample 5 min at room temperature. 13. Microcentrifuge 1 min at top speed, room temperature. 14. Transfer supernatant to a centrifugal filter device. Rinse membrane pieces with 100 µl digestion buffer and add to supernatant. Microcentrifuge sample 20 to 30 sec at top speed. Filtration removes nitrocellulose particles that might clog the HPLC column. Samples should be stored frozen unless they will be fractionated immediately.
Chromatograph and process the samples 15. Equilibrate a 2.1-mm-i.d. × 250-mm reversed-phase HPLC column with 95% chromatography solvent A/5% solvent B at a flow rate of 0.15 ml/min. For optimal resolution, perform the separation at 60°C if a column oven is available. 16. Inject the sample. Wash column 10 to 15 min with 95% solvent A/5% solvent B. 17. Elute the peptides with a gradient between chromatography solvents A and B as follows: 5% to 40% solvent B over 1 hr; 40% to 75% solvent B over 30 min; and 75% to 100% solvent B over 15 min. Monitor elution of the peptides at 215 nm. Elution gradient guidelines are described in Stone and Williams (1986). The TFA concentration should be adjusted to equalize the UV absorbance (215 nm) of the eluants. Alternatives to TFA, such as phosphoric acid or hydrochloric acid, are compatible with the procedure. Buffers containing ammonia or UV-containing impurities should not be used. Trypsin elutes at 60% solvent B and can be used as an internal standard. Characteristically, it elutes in a broader peak than the majority of peptides.
18. Monitor the appearance of peptide peaks with a chart recorder adjusted so that 0.05 to 0.1 AUFS corresponds to a full-scale deflection. Minimize the length and capacity of the capillary tubing between the column and the point of collection as much as possible. Polyethylether ketone (PEEK) tubing (0.005-in. i.d.) is helpful for this purpose.
19. When chart-recorder pen begins a deflection indicative of a peak, wipe tip of capillary tubing against a clean surface (e.g., a Kimwipe or the previous collection tube) and collect this fraction in a microcentrifuge tube. Clean gloves should be worn at this point. Commercial microcentrifuge tubes are sufficiently clean as supplied by the manufacturer if they are stored in the original container and handled with clean gloves.
Analysis of Proteins
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20. Immediately cap tube and place it on dry ice. Rapid freezing prevents adsorption of separated peptides to the walls of the microcentrifuge tube. The samples can be stored at −80°C indefinitely.
21. Sequence samples using glass support disks precycled with polybrene as prescribed by Applied Biosystems. Improved reaction (Speicher, 1989) and conversion (Tempst and Riviere, 1989) cycles will reduce background and improve sensitivity. It is very useful to compare chromatograms obtained from the sample and from the blank nitrocellulose to identify peaks corresponding to peptides derived from the protein of interest. UV-absorbing impurities may originate from several sources, including residual PVP-40, contaminated glassware, and the proteolytic enzyme (autolysis of the protease). These can be eliminated easily by inspecting the control chromatogram. Peaks as small as 0.002 AUFS have often been sequenced successfully.
REAGENTS AND SOLUTIONS Digestion buffer 0.1 M Tris⋅Cl, pH 8 1 mM CaCl2 10% (v/v) acetonitrile Store frozen 4× gel buffer 41.24 g bis-Tris (0.493 M) 7.08 ml 37.5% HCl (reagent grade) H2O to 400 ml Final pH should be 6.61; do not adjust the pH with acid or base. This buffer may be stored indefinitely at −20°C or ≤2 months at 4°C. 10× lower reservoir buffer 52.4 g bis-Tris (0.626M) 12.2 ml 37.5% HCl H2O to 400 ml Final pH should be 5.90; do not adjust the pH with acid or base. This buffer may be stored indefinitely at −20°C or ≤2 months at 4°C. 10× upper reservoir buffer 40.28 g N-tris-(hydroxymethyl)-methyl-2-aminoethanesulfonic acid (TES; 0.439 M) 94.64 g bis-Tris (1.131 M) 4.0 g sodium dodecyl sulfate (SDS) H2O to 400 ml Final pH should be 7.25; do not adjust the pH with acid or base. This buffer may be stored indefinitely at −20°C or ≤2 months at 4°C.
Isolation of Proteins for Microsequence Analysis
Sample buffer 50 mM Tris⋅Cl, pH 6.8 5% (v/v) 2-mercaptoethanol 10% (v/v) glycerol 1% (w/v) SDS Store frozen in small aliquots for ≤2 months (continued)
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Transfer buffer 2.21 g cyclohexylaminopropane sulfonic acid (CAPS), free acid 0.5 g dithiothreitol 150 ml methanol H2O to 1 liter Adjust pH to 10.5 with NaOH and chill to 4°C Prepare just before use For proteins >60 kDa, reduce amount of methanol to 1%.
COMMENTARY Background Information The most difficult part of any protein sequencing project is to obtain a sample that is sufficiently free of both protein and nonprotein contaminants. SDS-PAGE is used because of its resolving power and its ability to maintain solubility of almost any protein. Many proteins of interest are identified only by migration on an analytical gel or by immunoreactivity in immunoblot experiments. The sequencing procedures outlined here allow the direct comparison of primary structures. Following the demonstration that blotting procedures could be used to prepare samples for microsequencing (Aebersold et al., 1986; Vandekerckhove et al., 1986; Matsudaira, 1987), acceptance of these techniques was slow due to the inability of many investigators to obtain adequate sequencing signals. Evaluation of several parameters has indicated that preelectrophoresis with a scavenger such as glutathione or mercaptoacetic acid to eliminate reactive by-products of acrylamide polymerization, and lowering the separation pH to reduce the reactivity of the N-terminal amine, can dramatically improve the signals (Moos et al., 1988). Recently, Tris-Tricine gels have become popular, especially for low-molecular-weight proteins and peptides (Schägger and von Jagow, 1987). These gels address the concerns of pH-dependent N-terminal blockage and give good results in sequencing experiments. Their commercial availability in precast format makes them an attractive choice for the novice. The ability to sequence proteins blocked at the amino terminus following electrophoretic separation on gels (second basic protocol; Aebersold et al., 1987; Tempst et al., 1990) is an important improvement in protein analysis. Although meticulous cleanliness and exacting technique are required, the protocol is straightforward and highly reliable. Its particular strength is that it can be used following any type of gel electrophoresis or electrophoretic trans-
fer. It is even possible to combine small spots obtained from several gels (e.g., minor components detected by two-dimensional analysis). Its principal limitation is the relatively low yield of material for sequencing from a given amount of blotted protein; a dark band of Ponceau S–staining material corresponding to 100 to 250 pmol (or more for some proteins) is often required. This problem is obviated somewhat by the extremely high quality of sequence data that can be obtained from minute (e.g., 2 mAUFS) HPLC peaks. Modifications to enhance the sensitivity of the microsequencer 5- to 8-fold have been developed (Tempst and Riviere, 1989); these have proven extremely useful for all microsequencing procedures. A new automated chemistry cycle specifically optimized for PVDF significantly improves results (Speicher, 1989). Applied Biosystems has developed a reaction cartridge designed for PVDF blots that simplifies sample placement and substantially improves sequencing results (Sheer et al., 1990). Finally, a procedure for covalently entrapping blotted proteins in a polymer network may prove beneficial (information available from MilliGen/Biosearch; see APPENDIX 4).
Critical Parameters and Troubleshooting Determination of amino acid sequence by SDS-PAGE and transfer to PVDF membranes In practice, the electrophoresis conditions described in the first basic protocol improve the initial sequencing yield by 2-to 3-fold over those obtained with earlier procedures. Because many proteins are available in very small quantities, this consideration is often crucial. If the sample is not in short supply, it may be reasonable to attempt sequencing with electrophoresis conditions commonly in use (UNIT 10.2; Matsudaira, 1987), especially if the reagents are available in the laboratory. Samples already
Analysis of Proteins
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Isolation of Proteins for Microsequence Analysis
transferred to PVDF by other procedures may also be tried; these should be rewet with methanol and washed with water (3 times, 5 min each) before sequencing to remove dust and materials such as glycine that may have been used in the transfer. Efficient acrylamide polymerization is crucial to obtain good sequencing signals. Even the most efficiently polymerized gels contain 50 to 60 mM acrylamide monomer and possibly other compounds that react in a pH-dependent manner with the N terminus during electrophoresis. Therefore, fresh, high-quality reagents, ultrapure water, and thorough deaeration of the polymerization mixtures are necessary. TEMED is the most likely reagent to deteriorate and a fresh bottle should be opened each month. The buffer system used in this protocol operates at a lower ionic strength than others in common use and therefore is more sensitive to the presence of ionic impurities in the sample. The most efficient way to remove these is by precipitation with organic solvent. Ethanol was chosen for this application (support protocol) because it is least likely to contain impurities that could react with amino termini of proteins (Yuan et al., 1987). Precipitation is most efficient with concentrated samples, so centrifugal ultrafiltration can be useful. Too much SDS in the sample will impair stacking. This is why only 1% SDS, which is sufficient to saturate almost all proteins, is used in the sample buffer. Pyronin Y is used as a tracking dye because it binds to PVDF tightly and unambiguously locates the dye front. The gel buffers and polymerization mixtures should be prepared exactly as specified. No adjustments in pH should be necessary. SDS is not included in the gel polymerization mixtures as this could impair stacking. The same buffer is used in both the stacking and resolving gel. If efficient stacking and good resolution are not observed, the buffers are incorrect, the sample contains too much salt (see support protocol), or the gel is overloaded (10 to 20 µg/band is about right). If poor sequencing yields are obtained with a standard protein known to be unblocked, there are problems with gel polymerization, transfer, or placement of the blots in the reaction cartridge of the sequencer. Optimization of transfer is straightforward, and samples that have been applied directly to PVDF pieces without prior electrophoresis can be used to assess the transfer. It is strongly recommended that β-lac-
toglobulin, which has worked well in many laboratories and is widely available, be used on a test run before using valuable experimental samples. In addition, it is a good idea to use β-lactoglobulin as a positive control in every experiment. This will distinguish between existing N-terminal blockage and blockage that may occur during the procedure. Exposure of the blots to high pH and high temperature should be minimized. When preparing the sample for electrophoresis, the supernatant should be aspirated very carefully—a magnifying lens may be helpful to avoid aspirating the pellet. A 10-µg pellet is usually visible to the naked eye. Determination of internal amino acid sequence from electrophoreticallyseparated proteins Once a sufficiently pure protein has been obtained, the most troublesome aspect of sequencing is chemical modification of the N terminus of the protein, either in vivo or in vitro. Approximately 50% of eukaryotic cellular proteins are thought to have naturally occurring N-terminal blockage. For these proteins, enzymatic or chemical cleavage procedures are required in order to obtain sequence information by automated Edman degradation (Hewick et al., 1981; Tempst and Riviere, 1989). In addition, many reagents commonly encountered in protein isolation procedures can irreversibly modify unblocked N-terminal amino groups and prevent Edman degradation. Because of this, a standard protein, such as β-lactoglobulin, should be analyzed as a control. Failure to obtain any sequence or a low-yield sequence for the standard protein suggests problems with the reagents. The success of this method depends on a number of factors. (1) A reasonably homogeneous band or spot must be obtained. If two or more proteins are overlapping, it will be difficult to resolve individual peptides and impossible to assign sequence data to specific proteins. (2) It is important to maintain meticulous cleanliness throughout the procedure. Keratins from the skin are particularly problematic. (3) UV-absorbing contaminants in polyvinylpyrrolidone (PVP) can obscure the peptide peaks, which are often very small. Therefore, careful washing after the PVP blocking step is necessary. (4) The digestion should be performed in a minimum volume, as proteases are less active when dilute. The high concentrations used (compared to traditional
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solution digestion procedures) are not a problem because proteases are generally separated from most of the peptides by HPLC. If an autolysis product should be isolated, this will be apparent when its sequence is compared with databases. (5) Careful manual collection of the HPLC fractions can improve the results; a bit of practice, and paying special attention to changes in the rate of change of the recorder pen’s movement will be helpful. In general, quite closely spaced peaks will give unique amino acid sequences. In the author’s experience, trypsin is the most reliable enzyme for this procedure. Other enzymes—including Lys-C, Asp-N, and GluC—may also be tried to provide overlapping sequences or in case the tryptic pattern does not provide the desired information. In this event, the digestion buffer recommended by the supplier of the protease, supplemented as above with 10% acetonitrile, should be substituted. The most common problems are spurious peaks, no peaks, and only a trypsin peak in the HPLC analysis. Spurious peaks arise from UVabsorbing contaminants, as discussed above. If a trypsin peak is absent, a control reaction containing the buffer and protease but no protein or nitrocellulose should be injected. If there is still nothing, the trypsin concentration should be checked. If a trypsin peak is present in the control samples, blocking was inadequate or the amount of nitrocellulose without adsorbed protein was too high. If only a trypsin peak is detected, either not enough protein was present or the trypsin was inactive. The latter possibility can be assessed by digesting some standard protein with the same lot of protease.
have yielded high-quality sequence data, allowing determination of as many as two hundred residues from a single experiment. Though the efficiency of the precipitation procedure (support protocol) varies with the sample, it is usual to recover 70% to 100% of samples >1 µg from a volume of 50 µl.
Time Considerations Time required for the SDS-PAGE and transfer procedure: casting the gels—20 min handson time and ∼2 hr total, including deaeration and polymerization; preelectrophoresis—just a few minutes hands-on time and 45 min total; electrophoreses—60 to 80 min; setting up the transfer—15 min; transfer—30 to 90 min; staining—15 min; band excision—5 to 30 min; sample preparation—15 min hands-on time and 1 hr for incubation on dry ice. The entire procedure can be completed in a single day, though it is preferable to cast the gels a day in advance. Time required for determination of internal amino acid sequences following electrophoretic separation and transfer of proteins: destaining and PVP blocking—30 min; washing—5 min/sample; mincing the nitrocellulose—2 to 3 min/sample; digestions—overnight 12 to 24 hr; sample recovery and filtratio n—1 0 min ; r ever sed -p hase HPLC separations—2 hr/sample. A set of samples may be separated on a gel, transferred, and processed for digestion in ∼5 hr. If several proteins from a given gel are of interest, a few days of HPLC will be required; at 2 hr per run, only 3 samples per day can be analyzed.
Literature Cited Anticipated Results
In the first basic protocol, if 100 pmol β-lactoglobulin are loaded onto a 12% gel prepared as described, the initial sequencing signal should indicate a yield of 50 to 80 pmol with gas-phase instruments (e.g., Applied Biosystems 470A). In some cases, usable sequence may be obtained from as little as 10 pmol. In general, if a band is easy to see with Coomassie blue staining, there is enough to sequence, but if it is so faint that excision is difficult, it is doubtful that there is enough protein. However, very faint bands, particularly of low-molecular-weight proteins, have yielded sequence. For N-terminally blocked proteins, if enough material is present in a band or spot, usable sequence from 2 to 3 peptides can be obtained for most proteins. In several cases, two dozen or more HPLC peaks from a single band
Aebersold, R.H., Teplow, D.B., Hood, L.E., and Kent, S.B.H. 1986. Electroblotting onto activated glass. J. Biol. Chem. 261:4229-4238. Aebersold, R.H., Leavitt, J., Saavedra, R.A., Hood, L.E., and Kent, S.B.H. 1987. Internal amino acid sequence analysis of proteins separated by oneor two-dimensional electrophoresis after in situ protease digestion on nitrocellulose. Proc. Natl. Acad. Sci. U.S.A. 84:6970-6974. Hewick, R.M., Hunkapiller, M.W., Hood, L.E., and Dreyer, W.J. 1981. A gas-liquid solid phase peptide and protein sequenator. J. Biol. Chem. 256:7990-7997. Matsudaira, P. 1987. Sequence from picomole quantities of proteins electroblotted onto polyvinylidene difluoride membranes. J. Biol. Chem. 262:10035-10038. Moos, M., Nguyen, N.Y., and Liu, T-Y. 1988. Reproducible, high-yield sequencing of proteins electrophoretically separated and transferred to an inert support. J. Biol. Chem. 263:6005-6008.
Analysis of Proteins
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Schägger, H. and von Jagow, G. 1987. Tricine-sodium dodecylsulfate-polyacrylamide gel electrophoresis for the separation of proteins in the range from 1 to 100 KDa. Anal. Biochem. 166:368-379. Sheer, D.G., Yuen, S.W., Yamone, D.K., Mattaliano, R.J., and Yuan, P-M. 1990. A modified reaction cartridge for sequencing samples on polymeric sample matrices. Abstract T139, Fourth Symposium of the Protein Society, San Diego. Speicher, D.W. 1989. Microsequencing with PVDF membranes: Efficient electroblot-ting, direct protein adsorption and sequencer program modifications. In Techniques in Protein Chemistry (T.E. Hugli, ed.) pp. 24-35. Academic Press, San Diego. Stone, K.L. and Williams, K.R. 1986. High-performance liquid chromatographic peptide mapping and amino acid analysis in the subnanomole range. J. Chromatogr. 359:203-212. Tempst, P. and Riviere, L. 1989. Examination of automated polypeptide sequencing using standard phenylisothiocyanate reagent and subpicomole high-performance liquid chromatographic analysis. Anal. Biochem. 183:290-300. Tempst, P., Link, A.J., Riviere, L.R., Fleming, M., and Elicone, C. 1990. Internal sequence analysis of proteins separated on polyacrylamide gels at the picomole level: Improved methods, applica-
tions and gene cloning strategies. Electrophoresis 11:537-553. Vandekerckhove, J., Bauw, G., Puype, M., van Damme, J., and van Montagu, M. 1986. Protein blotting from polyacrylamide gels onto glass microfiber filters. In Advanced Methods in Protein Microsequence Analysis (B. Wittmann-Liebold, J. Salnikow, and V.A. Erdman, eds.) pp. 179-193. Springer-Verlag, Berlin. Yuan, P., Hawke, D., Blacher, R., Hunkapiller, M., and Wilson, K. 1987. Ethanol precipitation of electroeluted-electrodialyzed samples. Applied Biosystems User Bulletin #27.
Key References Moos et al., 1988. See above. Tempst, et al. See above. Address common problems encountered in procedures of this type.
Contributed by Malcolm Moos, Jr. Center for Biologics Evaluation & Research Food and Drug Administration Bethesda, Maryland
Isolation of Proteins for Microsequence Analysis
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Capillary Electrophoresis of Proteins and Peptides
UNIT 10.20
Capillary electrophoresis (CE) is a high-resolution technique for the separation of a wide variety of molecules of biological interest such as metabolites, drugs, amino acids, nucleic acids, and carbohydrates. This unit focuses on the use of CE to separate proteins and peptides. As with polyacrylamide gel electrophoresis (PAGE; UNIT 10.2), CE separations of proteins and peptides are based on charge-to-mass ratios. While PAGE separations are restricted to polyacrylamide matrices and a relatively small number of buffer systems, CE separations can be achieved in a variety of different matrices using a wide range of electrophoresis buffers. Consequently, there is a much greater flexibility in the design of optimal separation protocols. CE employs a fused-silica capillary column, which may or may not be derivatized, either in free solution or in the presence of a fluid matrix. In contrast to PAGE, the resulting protein or peptide bands cannot be fixed or stained. The CE separation is, in essence, a dynamic one and is more analogous to high-performance liquid chromatography (HPLC) than PAGE. However, CE separation is based on electrophoretic parameters, so it can be viewed as orthogonal to HPLC, which is based predominantly on solvent partitioning parameters. The combination of these two techniques is very powerful for analyzing and characterizing proteins and peptides. The initial expectation that CE would play a major role in protein separations has not been realized. However, CE is particularly useful for determining the purity of a sample and for rapid, efficient, and quantitative evaluation of protein purification. For best separation results, it is necessary to optimize the procedure. If some properties of the protein are known—e.g., its isoelectric point (pI)—the selection of a suitable running buffer is relatively straightforward. In fact, CE itself can be used to determine the isoelectric point of a protein, either in purified form or in a mixture, by focusing the sample in a pH gradient that is generated within the capillary during electrophoresis (Basic Protocol 1). After refocusing, the sample is mobilized either by changing the anode buffer, or by applying a hydrodynamic force to the column to move the separated components past the detector. However, there is little information available that will predict the potential of a protein to interact with the capillary wall. Underivatized silica capillaries have a strong tendency to adsorb proteins, thereby immobilizing them and making protein separations impractical. If such interactions occur, then an additive or a coated capillary column is required. A certain amount of trial and error is involved, and it may be necessary to perform several CE runs to optimize the separation conditions for a given protein. One approach to optimization is described in Basic Protocol 2. CE is most useful for separations of peptides (Basic Protocol 3), because it offers great flexibility in separation parameters. It can be used to monitor proteolytic digestions and optimize digestion conditions for the production of a representative peptide fingerprint of a protein. This profile can subsequently be used to provide structural information about the protein, especially when used in conjunction with reversed-phase HPLC (RP-HPLC; UNIT 10.14). It can be used to screen peptide fractions that are obtained from a preparative RP-HPLC separation of a protease digestion. Fractions that contain single or major components are suitable candidates for protein sequence analysis. Similarly, CE can be used to assess the purity of synthetic peptides. In the presence of an internal standard, it can provide quantitative information about the various components that are present in a peptide mixture. CE can also be used as a micropreparative technique—with either multiple separations that are pooled (Basic Protocol 4) or a single, larger-scale separation Contributed by Dean Burgi and Alan J. Smith Current Protocols in Molecular Biology (2000) 10.20.1-10.20.13 Copyright © 2000 by John Wiley & Sons, Inc.
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to data acquisition
detector
temperature control region capillary column
high voltage
buffer/sample
buffer
Figure 10.20.1 Schematic of a CE instrument.
(Alternate Protocol)—for the isolation of peptides from a protease digestion (in much the same way that RP-HPLC is currently used). In most of these examples the same capillary column can be used for all the separations. Only changes in buffer composition, ionic strength, and the presence or absence of additives are required for each specific application. INSTRUMENTATION CE separation in its simplest form can be achieved by passing a high voltage between two buffer reservoirs that are joined by a liquid-filled fused-silica capillary (Fig. 10.20.1). This results in the generation of electroosmotic flow (EOF; see Separation Theory), which allows the molecules of interest to be carried from one end of the capillary to the other. The capillaries are generally 30 to 50 cm long with 50 to 75 µm i.d. The net total volume of these capillaries is in the low microliter range. For comparison, the volume of a slab gel lane is ∼1000 µl. The capillaries are thin-walled; this allows rapid and efficient exchange of the Joule heating that results from the high voltages (20 to 25 kV) that are necessary for electrophoretic separations. This heat dissipation minimizes the negative convective effects that could result in band-broadening during electrophoresis. The fused-silica capillary is coated on the outside with a polymide layer that confers excellent tensile strength to the otherwise fragile capillary. The polyimide sheathing is carefully burned from a small portion of the capillary to expose a section of the silica. This clear section of the capillary is inserted into the light path of a UV detector and becomes the flow cell. As the protein and peptide molecules are swept through the capillary by EOF, they pass through the detector light path and are registered on the UV monitor. In effect the capillary becomes a very-low-volume flow cell. IMPORTANT NOTE: The removal of the polyimide coating makes the capillaries susceptible to breakage. Capillaries that are not provided in cartridges by the manufacturer should be handled with care to avoid breakage. Capillary Electrophoresis of Proteins and Peptides
The combination of high field strength and large surface-to-volume ratios results in rapid and very efficient separations of both proteins and peptides. Sample loading volumes are routinely in the nanoliter range, with starting sample concentrations of ∼0.1 µg/µl for UV detection. On-capillary preconcentration protocols are available when starting concentra-
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Table 10.20.1
Commercially Available CE Instruments
Manufacturer
Model
Applied Biosystems Beckman
270-HT
Detection method
Reversal of polarity
UV: 190-700 nm Controlled by software Manual P/ACE 5000 UV: filter; P/ACE 5510 Diode-array: 190-600 nm; P/ACE 5510 Fluorescence: argon laser; MS interface Bio-Rad Biofocus 3000 UV: 190-800 nm Controlled by software Hewlett-Packard HP 3D CE Diode-array: Controlled 190-600 nm by software Waters Quantra 4000E UV: filter Manual
Capillary Cooling of heating sample/buffer
Capillary format
Software available
FAC
Sample
Liquid (Peltier)
Sample/buffer
Free-hanginDA g Cartridge C, DA
Liquid
Sample/buffer
Cartridge
C, DA
FAC (Peltier) FAC (Peltier)
Sample/buffer
Cartridge
C, DA
Sample/buffer
Freehanging
DA
aAll instruments have hydrostatic/electrokinetic sample loading and fraction collecting. bAbbreviations: C, CE system control software; DA, data acquisition; FAC, forced-air convection; MS, mass spectrometry.
tions are below these levels. Sample loading can be achieved either hydrodynamically or electrokinetically. Hydrodynamic loading can be performed by pressure injection, vacuum injection, or gravity injection, e.g., pressure loading at 0.5 lb/in2 for 5 sec. Any of these methods will deliver a small aliquot of sample into the capillary column. Hydrodynamic sample loading has limitations where the capillary contains a viscous matrix or the sample itself has a high viscosity. Electrokinetic loading is performed by applying a low voltage across the capillary column for a fixed time, e.g., 7.5 kV for 4 sec. Sample buffer conductivity, run buffer conductivity, and EOF all play a part in determining the amount of material loaded into the capillary column. In general, samples in high salt buffers should be avoided because buffer ions will carry the charge rather than the sample ions. There are several methods for concentrating the sample onto the capillary column (Burgi and Chien, 1992). All instruments listed in Table 10.20.1 have both sample loading capabilities. Clearly, with respect to sensitivity, speed, and versatility, CE can offer some significant advantages over gel electrophoresis for the separation of proteins and peptides. As shown in Figure 10.20.1, CE can require minimal instrumentation. However, in reality, due to the high voltages that are utilized, safety issues require that the capillary column be incorporated into a dedicated CE instrument. These instruments can provide efficient capillary cooling and online detection of analytes. Table 10.20.1 lists a number of commercially available instruments and some of their characteristics. For the purpose of this discussion, it is assumed that a generic CE system possessing the following capabilities is used: an active capillary cooling system, a UV detector capable of monitoring at 200 nm, a thermostatted autosampler, and a chromatographic data package. Given this basic unit a variety of separations can be performed, depending upon the nature of the proteins or peptides that are being separated. SEPARATION THEORY CE is part of the family of electrophoretic techniques that separate species based upon their sizes and ionic properties. An ion (i) placed into an electric field will move in the direction parallel to the field with a velocity (vi) as follows: vi = µi E
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Here µi is the electrophoretic mobility of species i, and E is the electric field, as defined by the following equation, where V is the voltage and L is the total length from electrode to electrode: E=
V L
The electrophoretic mobility of an ion (µi) is: µi =
q 6 π η ai
where q is the charge on the ion, η is the viscosity of the solution, and ai is the radius of the ion. As seen from these three equations, the movement of the molecule in the column is dependent on the applied voltage, the length of the column, the charge on the molecule, and the size of the molecule. In CE, buffer flow is generated inside the column when the electric field is applied. This flow is from the cathode electrode to the anode electrode in a fused-silica capillary column (from left to right in Fig. 10.20.1); this movement of the buffer is called the electroosmotic flow (EOF). In addition, normal electrophoretic separation occurs whereby a positively charged molecule moves in the same direction as the EOF, a negatively charged molecule moves in a direction opposite to the flow, and a neutral molecule is carried along by the flow. Thus, the total velocity of the molecule (µt) is given by the following equation, where µeo is the EOF in the column. µt = ( µeo + µi ) E
The magnitude of the EOF will vary as a function of the pH of the carrier buffer. In uncoated fused-silica columns, a low pH generates a slow EOF, whereas a high pH generates a fast flow. The flow can be suppressed or completely reversed depending on the coating on the column or organic modifier added to the carrier buffer (Landers et al., 1992; Tsuiji and Little, 1992). Thus, EOF is another parameter that can be optimized to aid in difficult separations. Neutral molecules, because they are all carried along by the flow, can be difficult to separate. Addition of a micelle to the carrier buffer results in the partitioning of molecules between the micelle and carrier buffer and can effectively resolve neutral molecules (Khaledi, 1994). STRATEGIC PLANNING Proteins can be separated on coated or uncoated capillary columns, and the choice of separation protocol depends on the specific properties of the target protein. The most important property is the pI, which can be determined by either conventional gel electrophoresis or CE. The use of a buffer ∼2 pH units above the pI is optimal for CE separations. Table 10.20.2 lists a number of buffers and their pH ranges. The presence of high salt concentrations in the sample can interfere with the separation process, and dialysis or dilution may be required. Protein concentrations of microgram per microliter are optimal for CE separations; however, on-capillary concentration protocols exist that allow the separation of proteins with concentrations 1 to 2 orders of magnitude lower.
Capillary Electrophoresis of Proteins and Peptides
Peptides can be effectively separated in open-tube fused-silica capillary columns. The EOF generated within the capillary causes separation of both charged and neutral peptides. The respective migration times are dependent upon both the pH of the electrophoretic separation and the charge-to-mass ratios of the peptides. For example, at low pH, peptides with net positive charge migrate towards the anode faster than neutral or negatively charged peptides. Even negatively charged peptides are swept towards the anode by EOF.
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Table 10.20.2
Useful Electrolytes for CE of Peptides
Electrolyte
Useful pH range
Phosphate Citrate Acetate MESa Phosphate Tris Borate
1.14-3.14 3.06-5.40 3.76-5.76 5.15-7.15 6.20-8.20 7.30-9.30 8.14-10.14
Minimum useful wavelength (nm) 195 260 220 230 195 220 180
aAbbreviation: MES, 2-(N-morpholino)ethanesulfonic acid.
In practice this migration order is further modified by the mass of the peptides, with small positively charged peptides having faster migration rates. The choice of buffer for CE is of primary importance. The use of buffer systems that separate at high pH significantly alters the migration positions of peptides relative to those of low-pH separations. The selection of a specific buffer is dictated both by its buffering capacity at a selected pH and by its minimum usable UV absorbance wavelength. Characteristics of some useful buffer systems are shown in Table 10.20.2. Small differences in the pH or composition of the buffer can have a significant impact on the absolute mobilities of the peptides, so it is advisable to adjust the pH of buffers accurately and reproducibly. Alternatively, a suitable internal standard can be incorporated into the separation in order to obtain relative mobilities. The protein can be solubilized in water or 40 mM buffer in the presence or absence of urea. The sample buffer does not need to be the same composition as the running buffer, however, use of ionic detergents is not recommended. For the separation of peptides from a tryptic digest of a protein, where almost every peptide carries at least two positive charges, a phosphate buffer of pH 2.0 to 3.0 is commonly used. Likewise, for V8 protease digestions (which cleaves C-terminal to glutamic acid residues), the preferred separation conditions use a borate buffer with a pH range of 8.0 to 9.0. SEPARATION OF PROTEINS BY ISOELECTRIC FOCUSING Isoelectric focusing (IEF) separations can be performed readily by CE; this can be a useful first step in selecting subsequent buffer systems for protein separations. Proteins and peptides are amphoteric in nature, so their charge is dictated by the surrounding carrier buffer. When a protein or peptide is placed in an electric field, it moves to a region where the surrounding pH equals its isoelectric point (pI); at this point the molecule stops moving. A pH gradient is generated in a coated capillary column by filling the column with a sample solution that contains ampholytes. A high-pH solution (sodium hydroxide) is placed in the cathode reservoir and a low-pH solution (phosphoric acid) is placed in the anode reservoir. An electric field is then applied across the column, and the ampholytes and the sample move to a location in the column corresponding to their respective pIs. If the molecule drifts out of the isoelectric region, a charge is induced by the surrounding pH solution, and the molecule moves back to the region of zero charge. Narrow regions are formed with pI resolutions on the order of 0.01 pH units. To determine the pI of a particular protein in the column, markers of known pIs are added to the sample mixture, and extrapolation between markers yields the pI of the protein of interest. After separation of the sample in the absence of EOF, mobilization of the proteins past the detector is necessary. A simple replacement of the anode reservoir with the buffer in
BASIC PROTOCOL 1
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the cathode reservoir establishes EOF and causes the sample zone to migrate pass the detector. The separation discussed here assumes there is no EOF in the column during focusing. However, separations may also be done in the presence of EOF (Mazzeo and Krull, 1992). Materials Ampholyte mixture, pH 3 to 10 (Bio-Rad) Sample containing 0.5 to 1.0 mg protein/ml in water IEF markers (Bio-Rad; optional) 20 mM sodium hydroxide (NaOH; store at 4°C) 10 mM phosphoric acid (store at 4°C) 50-µm-i.d. coated silica capillary column CE instrument (see Table 10.20.1) 1. Add ampholyte mixture to 0.5 ml protein sample to give a final concentration of 2.5% (v/v) ampholytes. If desired, add IEF markers to a final concentration of 0.1 mg/ml to calibrate the column.
2. Add the solution to the sample reservoir. Fill the capillary by pressurizing the reservoir (0.5 lb/in2). 3. Place 10 mM phosphoric acid in the anode reservoir (negative end). Place 20 mM NaOH in the cathode reservoir (positive end). 4. Focus the sample 4 to 6 min at 8 to 10 kV constant voltage. Monitor the current until it reaches steady state. The instrument should be operated in accordance with the manufacturer’s instructions.
5. Mobilize the sample by placing 20 mM NaOH in the anode reservoir and setting the voltage to 10 kV. Monitor the mobilization of the proteins past the detector 15 to 20 min. Another method for mobilizing the proteins past the detector is to apply a hydrodynamic force to one end of the column and push the zones past the detector. Commercially available CE instruments have this capacity as an option. To minimize distortion of the pI regions, a constant electric field (10 kV) must be applied at the same time. The pressure applied is small (0.5 lb/in2) and is maintained for the entire time of mobilization.
6. Wash the column after each run with 10 mM phosphoric acid for 1 min at 0.5 lb/in2. Store the column at room temperature in running buffer (short-term) or water (long-term). BASIC PROTOCOL 2
Capillary Electrophoresis of Proteins and Peptides
SEPARATION OF PROTEINS To use CE for protein analyses, the more that is known about the particular protein, the higher the success rate for developing a separation method. Knowing the pI of a protein permits selection of the appropriate run buffer to maximize differences in the charge-tosize ratio. IEF separation performed using CE (Basic Protocol 1) can be a useful first step in selecting subsequent buffer systems for protein separations on an underivatized capillary column. If no protein is detected, it has probably adsorbed to the silica surface of the column. Repeating the separation in the presence of an ionic detergent such as SDS will coat the protein with negative charge and prevent adsorption. However, this means that proteins will separate on the basis of size alone. Alternatively, if the pI of the protein is unknown, the separation can often be achieved by using a coated capillary column in the presence of high-pH, high-ionic-strength buffer.
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Materials Sample containing 10 mg protein/ml in water 50 mM and 500 mM sodium borate, pH 8.0 to 9.5 (for separation of proteins with unknown pI) 5 mM and 50 mM buffer at pH >pI (for separation of proteins with known pI; see Table 10.20.2) 50-µm-i.d. coated (unknown pI) or uncoated (known pI) fused-silica capillary column CE instrument (see Table 10.20.1) For separations of proteins with unknown pI 1a. Dilute protein sample 1/10 (v/v) with 50 mM sodium borate buffer to give a final concentration of 1.0 mg/ml. Alternatively, dialyze sample (at a concentration of 1 mg/ml) in 50 mM sodium borate buffer.
2a. Fill a coated column with 500 mM sodium borate buffer. 3a. Inject the sample hydrodynamically, e.g., 3 sec at 1 lb/in2. Consult the manufacturer’s instruction manual for the proper procedure.
4a. Separate using the following conditions: Detector wavelength: 200 nm Temperature: 25°C Run voltage: 10 kV Run time: 30 min. For separations of proteins with known pI 1b. Dilute protein sample 1/10 (v/v) with a 5 mM buffer that has a pH above the pI of the protein to induce a charge on the protein (final protein concentration = 1.0 mg/ml). Alternatively, dialyze sample (at a concentration of 1 mg/ml) in 5 mM buffer.
2b. Fill an uncoated capillary column with the same buffer at 50 mM. 3b. Inject the sample hydrodynamically, e.g., 3 sec at 1 lb/in2. Consult the manufacturer’s instruction manual for the proper procedure.
4b. Separate using the following conditions: Detector wavelength: 200 nm Temperature: 25°C Run voltage: 25 kV Run time: 30 min. If the protein of interest is not detected, add 10 mM SDS to the run buffer to induce a charge on the molecule. If the peaks are too broad, add 0.1% (w/v) methylcellulose to coat the column and reduce adsorption of the molecule onto the wall of the column. The addition of SDS and/or methylcellulose will increase the run time of the separation.
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BASIC PROTOCOL 3
ANALYTICAL PEPTIDE SEPARATIONS CE is particularly useful for separating specific peptides from complex mixtures such as the products of proteolytic digestion. In this procedure, the peptide mixture to be analyzed is placed in a sample vial at the cathode end of the capillary column. The anode reservoir contains running buffer. The sample is loaded into the capillary either electrokinetically or hydrodynamically by pressure or vacuum, then the sample vial is replaced by a vial containing running buffer and electrophoresis begins. The following protocol describes the use of a P/ACE 5000 CE instrument (Beckman) using low-pressure sample injection. However, other instruments (see Table 10.20.1) are capable of similar separations when operated according to the manufacturer’s instructions. Materials Peptide mixture: e.g., tryptic digest of β-lactoglobulin (Applied Biosystems) 0.05 M and 0.25 M sodium phosphate buffer, pH 2.30 (store at 4°C) 0.1 M sodium hydroxide (NaOH) 75-µm-i.d. fused-silica capillary column (Beckman) CE instrument (e.g., P/ACE 5000, Beckman, or equivalent; see Table 10.20.1) 1. Precondition the capillary by flushing with the following solutions: 10 column volumes of 0.1 M NaOH at low pressure (0.5 lb/in2) 10 column volumes water 4 column volumes of 0.25 M sodium phosphate buffer, pH 2.30. Store the column in 0.25 M sodium phosphate buffer, pH 2.30, at 25°C. When changing to a different separation buffer, equilibrate the column 4 hr with the new buffer (Strickland and Strickland, 1990). If a new capillary is being used, it is essential that this preconditioning step be performed prior to attempting separations.
2. Prepare the peptide mixture by dissolving 10 nmol (∼300 µg) in 10 ml of 0.05 M sodium phosphate buffer, pH 2.30. Freeze unused mixture in 100-µl aliquots. 3. Apply 10 to 20 nl sample using low-pressure injection for 10 sec at 0.5 lb/in2. Consult the manufacturer’s manual for the proper procedure.
4. Separate the peptide mixture using the following conditions: Electrolyte: 0.05 M sodium phosphate buffer, pH 2.3 Detector wavelength: 200 nm Temperature: 25°C Voltage: 25 kV. Consult the manufacturer’s manual for proper operating conditions.
5. Rinse the column with the following solutions: 0.5 min with water 1.0 min with 0.1 N NaOH 1.5 min with water 1 min with 0.25 M sodium phosphate buffer, pH 2.30. Store the column in running buffer or water at room temperature. Capillary Electrophoresis of Proteins and Peptides
An example of a separation protocol for a Beckman P/ACE 5000 instrument with a 37-min run time is described in Table 10.20.3.
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Table 10.20.3
Operating Program for Peptide Analysisa
Step
Process
Duration
Inletb
Outletb
Control summaryc
1 2 3 4 5 6 7 8 9 10 11
Set Temp Set Detector Rinse Rinse Rinse Inject Separate Rinse Rinse Rinse Rinse
— — 1.0 min 1.5 min 0.5 min 10.0 sec 30.0 min 0.5 min 1.0 min 1.5 min 1.0 min
— — 33 29 29 11 29 33 32 33 34
— — 7 7 9 9 9 7 7 7 9
Temp 25°C UV 200nm Forward: HP Forward: HP Forward: HP Low pressure Voltage 25.00 kV Forward: HP Forward: HP Forward: HP Forward: HP
aFor analysis of proteolytic digests, HPLC fractions, and synthetic peptides. bVial contents: 7, waste vial (water level just to contact capillary effluent); 9, electrolyte (50 mM sodium phosphate buffer,
pH 2.30); 11, sample; 29, electrolyte (50 mM sodium phosphate buffer, pH 2.30); 32, regeneration solution (0.1 N NaOH); 33, water; and 34, 0.25 M phosphate buffer, pH 2.30. cAbbreviation: HP, high pressure.
MICROPREPARATIVE CAPILLARY ELECTROPHORESIS: MULTIPLE SEPARATIONS
BASIC PROTOCOL 4
Although CE has been most commonly used for analytical separations, considerable interest has developed in using its high resolving power in micropreparative applications. Two basic approaches have evolved, one utilizing multiple separations and collections from an analytical capillary (Bergman and Jörnvall, 1992) and the other utilizing a single separation and collection on a much larger-diameter (150-µm-i.d.) capillary (see Alternate Protocol). For the multiple collection approach to be effective, the elution times of the peptides must be reproducible. The following method uses the P/ACE 5000 capillary electrophoresis instrument to separate a mixture of peptides. Materials Peptides: ACTH 4-10, angiotensin I, and angiotensin II (Sigma) 0.05 mM and 0.25 mM sodium phosphate buffer, pH 2.30 (store at 4°C) 0.1 M sodium hydroxide (NaOH) 75-µm-i.d. fused-silica capillary column (Beckman) CE instrument (e.g., P/ACE 5000, Beckman, or equivalent; see Table 10.20.1) Conical microvials (Beckman) 1. Precondition the capillary column by flushing with the following solutions at low pressure (0.5 lb/in2): 10 column volumes of 0.1 M NaOH 10 column volumes of water 4 column volumes of 0.25 M sodium phosphate buffer, pH 2.30. Store in 0.25 M sodium phosphate, pH 2.30. When changing to a different separation buffer, equilibrate the column 4 hr with the new buffer (Strickland and Strickland, 1990).
2. Prepare a peptide mixture by dissolving 0.2 mg each of ACTH 4-10, angiotensin I, and angiotensin II in 1 ml of 0.05 M sodium phosphate buffer, pH 2.30 (final concentration = 0.6 mg peptide/ml). Store 100-µl aliquots at −20°C. 3. Load 10 to 20 nl peptide mixture by low-pressure injection for 10 sec at 0.5 lb/in2.
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Consult the manufacturer’s manual for the proper procedure.
4. Separate the mixture using the following conditions: Electrolyte: 0.05 M sodium phosphate, pH 2.30 Detector wavelength: 200 nm Temperature: 25°C Run voltage: 25 kV Fraction size: 3-min per collection vial. Consult the manufacturer’s manual for proper operating conditions.
5. Replace the standard outlet reservoir (anode) with a series of conical microvials, each containing 10 µl of 0.05 M sodium phosphate buffer, pH 2.30. Collect 3-min fractions into each of the vials for the length of the separation. 6. Repeat the injection and separation (steps 3 to 5) four times and combine the contents from the corresponding fraction numbers. It will take 2 to 3 hr to complete the fractionation.
7. Screen fractions for peptide content using the analytical separation previously described (see Basic Protocol 3). ALTERNATE PROTOCOL
MICROPREPARATIVE CAPILLARY ELECTROPHORESIS: SINGLE SEPARATION In this micropreparative protocol, a larger quantity of the peptide mixture is loaded onto a 150-µm-i.d. capillary column and the fractions are collected from a single separation (Kenny et al., 1993). For the single-collection approach the CE instrument must be able to effectively control the capillary temperature at the elevated power levels that are required for electrophoresis. Forced air cooling is generally inadequate for this purpose. Additional Materials (also see Basic Protocol 4) 4:1 (v/v) 0.5 M sodium phosphate buffer (pH 2.50)/ethylene glycol 150-µm-i.d. fused-silica capillary column (Polymicro) 1. Prepare peptide mixture and precondition the column (see Basic Protocol 4, steps 1 and 2). 2. Load 0.1 µl peptide mixture by low-pressure injection for 10 sec at 0.5 lb/in2. Consult the manufacturer’s manual for the proper procedure.
3. Separate the peptide mixture using the following conditions: Electrolyte: 0.05 M sodium phosphate buffer, pH 2.30 Detector wavelength: 200 nm Temperature: 25°C Run voltage: 7.5 kV Fraction size: 3-min per collection vial. Consult the manufacturer’s manual for proper operating conditions.
4. Replace the standard outlet reservoir (anode) by a series of conical microvials each containing 10 µl of 4:1 (v/v) 0.5 M sodium phosphate buffer/ethylene glycol. Collect 3-min fractions into each vial for the length of the separation. It will take ∼2 hr to fractionate the sample. Capillary Electrophoresis of Proteins and Peptides
5. Screen fractions for peptide content using the analytical separation protocol (see Basic Protocol 3).
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COMMENTARY Background Information Protein separation is one of the more difficult types of separation to perform by capillary electrophoresis (CE; Novotny et al., 1990). Each protein has it own set of optimal separation conditions, and one optimized protocol will not necessarily transfer well to the separation of a different protein. The more that is known about the properties of the molecule, the easier it will be to optimize the separation conditions. The most important property is probably the pI, because it will dictate the selection of pH and ionic strength conditions for the initial separation attempt. However, a variety of other properties must also be taken into account when optimizing a separation protocol: shape (globular or fibrous), aggregation tendencies, solubility in dilute solutions, salt requirements, thermal stability, and hydrophobic nature. Several parameters can be varied within CE separations in order to utilize this knowledge—e.g., temperature, buffer/salt selection, ionic and nonionic detergents, and matrix additives (polyacrylamide, methylcellulose). Several protocols have been developed that can serve as useful starting points for most protein separations. Table 10.20.4 lists references for some groups of proteins. Furthermore, many of the manufacturers listed in Table 10.20.1 have developed additional protocols for protein separations. Ultimately, the selection of an appropriate separation protocol for a protein depends on the specific properties of that protein. However, there are separation approaches that utilize specific properties of proteins. IEF-CE separates on the basis of charge alone; CE in the presence of ionic detergents such as SDS separates on the basis of size alone; and CE using underivatized capillaries at low pH or derivatized capillaries at high salt and high pH separates on the basis of both size and charge. Successful CE separations also depend on the behavior of
Table 10.20.4
other contaminants that are present in the mixture. It may also be possible to take advantage of the knowledge gained from previous purification steps, e.g., ion-exchange or size-exclusion chromatography, to select CE separation conditions. Analytical-scale separation of peptides by CE has found application in a variety of areas. Proteolytic digestions can be monitored by placing the digestion mixture in a sample vial on the sample table that is equilibrated to 37°C. The instrument can then be programmed to analyze an aliquot at desired intervals, such as every 4 hr. Digestion is complete when a stable CE profile is obtained. Also, fractions collected from an HPLC separation of a protease digest can readily be screened orthogonally by CE to determine the homogeneity of any given peak. Fractions with a single major component can then be subjected to protein sequence analysis. The sensitivity of UV detection at 200 nm is such that digest levels of ≥100 pmol can be effectively screened by this method. Below these load levels the fractions have to be reduced in volume or concentrated on the capillary to achieve a sufficiently high concentration for visualization with UV detection (Dolnik et al., 1990; Burgi and Chien, 1992). Selection of an appropriate micropreparative protocol will depend on the type of instrumentation that is available. The single-separation micropreparative protocol will only work with a CE instrument that has adequate cooling capabilities. The multiple-separation micropreparative protocol will work on any CE instrument that has reproducible and stable electrophoretic mobilities.
Critical Parameters For protein analysis, the charge on the protein is the most important parameter for separation. However, the charge-to-shape ratio also plays a role in effective separation of proteins.
References for Separations of Different Protein Types
Protein type
Reference
Basic proteins
Wiktorowicz and Colburn (1990); Bullock and Yuan (1991) Tran et al. (1991); Tsuiji and Little (1992) Nielsen et al. (1991) Chen et al. (1992)
Glycoproteins Antibody-antigen complexes Milk proteins
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The charge on the protein is controlled by the buffer and the additives used for the run. The shape is determined by the denaturing conditions of the solutions. The surface charge on a protein may ultimately dictate the extent of wall interactions in addition to electrophoretic mobility, whereas shape has its greatest effect upon mobility alone. The recovery of peptides from underivatized silica capillary columns appears to be very good—even at subpicomole levels. However, proteins are frequently irreversibly bound to these same capillaries. Clearly, at some point a “peptide” must become a “protein” in the context of CE. There is little published data available to clarify this issue, although good recoveries of peptides with 30-40 amino acid residues in length have been obtained from underivatized capillary columns. If the recoveries of higher molecular weight peptides are poor, then the separation should be repeated using a coated capillary column. During peptide synthesis, single-amino-acid-deletion products are frequently produced. The properties of these deletion products are very similar to those of the expected species, so the deletion products may not be readily resolved by reversed-phase HPLC. In this case, CE has proven to be a very useful orthogonal technique because of its high resolution power and separation based on charge-to-mass ratios. However, it must be emphasized that the effective separation and detection of proteins and peptides requires relatively high solute starting concentrations. When using uncoated columns, a sodium hydroxide rinse is required to maintain the capillary surface. However, this rinse is never used with coated capillaries because it will remove the coating. Commercially available coated columns are satisfactory, but linear polymers (e.g., methylcellulose) have been used with more success (Palmieri and Nolan, 1994). If a protocol can be worked out for the molecule of interest, rapid CE analysis with little loss of starting material can augment the tools used now for protein analysis (e.g., HPLC).
Troubleshooting
Capillary Electrophoresis of Proteins and Peptides
Mechanical and electrical problems that might be encountered during instrument use are addressed in the troubleshooting sections of the manufacturers’ manuals. One of the common difficulties is the loss of electrical contact during a run. This loss of current can be caused if a small bubble forms in the column during
injection or if the power generated by the run is so great that the solution boils or outgasses. Purging the column after a failed run removes any bubbles in the column. The sample vial must contain enough sample to cover the end of the column during injection to prevent injection of air into the column. The sample vial can dry out during a long run, so sample temperature control or the use of the proper cap on the vial is required to slow evaporation. Reducing the buffer concentration or the run voltage can eliminate bubble formation during a run. Degassing the run buffer is also useful if the outgassing problems continue. The instrument sometimes arcs during a run; this is caused by salt deposits or moisture on the high-voltage electrode stations. Particular attention should be given to keeping the electrode stations salt free. Moisture buildup can be minimized by reducing the separation voltage by 5 kV. Frequent visual inspection is also advised. The column can plug up because of salt deposition if the column dries out. Washing the column with water dissolves the plug; storing the column in water, if it is not going to be used for a long period of time, prevents plugging. Proteins can precipitate during a run and plug the column. Using lower protein concentrations prevents this type of plugging.
Anticipated Results Electrophoretic profiles for both protein and peptide separations should be highly reproducible. However, absolute migration times may vary from run to run, especially if the separation temperature is not adequately controlled. For this reason, the use of an appropriate internal standard is highly recommended (see the manufacturer’s instructions). At load levels 100 kDa were being routinely analyzed with subpicomole sensitivity (Hillenkamp et al., 1991) and molecular weights could be assigned with an accuracy of ∼0.1%. Beavis and Chait (1990) demonstrated that for small proteins (30 kDa that differ only by an initiator Met residue
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Overview of Mass Spectrometry
(e.g., see Fig. 10.21.8). These issues are considered in more detail in the discussion of Fundamentals of Mass Measurement Accuracy and Mass Resolution. Several important considerations relating to the production of ions in the MALDI source limit the resolving power of linear TOF instruments. The most important of these is the initial kinetic energy spread of individual ion populations: as a result of this spread, ions of any particular mass will, after reaching final accelerating voltage, exhibit a spread in velocities. This means that various members of the same ion population will arrive at the detector at slightly different times, and that in turn gives rise to a broader peak for each ion than would be observed if there were no initial kinetic energy spread. Because the initial energy spread is mass dependent, peaks for highermass ions will be disproportionally broader than those for lower-mass ions. Resolution in a linear TOF instrument can be improved by increasing the length of the flight tube. This enhances the time dispersion of ions of different m/z; unfortunately, it also increases the spread of arrival times for ions of the same m/z due to the initial kinetic energy distribution. This energy spread is typically reduced in magnitude by increasing the final accelerating voltage. A more effective way to correct for energy distributions is through the use of an ion mirror, or reflectron. A reflectron TOF instrument (Fig. 10.21.2, panel B) corrects for the initial energy spread by acting as an energy-focusing device. The reflector works by slowing an ion down until it stops (the back of the reflector is at a voltage slightly higher than the source-accelerating voltage), turning it around, and then reaccelerating it back out to a second detector. Ions with an initial kinetic energy (and velocity) slightly lower than full accelerating potential will not penetrate the reflector as deeply, and will therefore turn around sooner, catching up to those ions with full kinetic energy. Ions with slightly greater energies (and thus higher velocities) will penetrate more deeply into the reflector, be turned around later, and have their flight times retarded, allowing the other ions to catch up. All of this causes ions of a given mass-to-charge ratio to be spatially focused into packets having flight times that are closer together, thus improving the resolution. This improvement in resolution is illustrated in Figure 10.21.3, which shows the molecular ion region of the linear (panel A) and reflector (panel B) spectra of a synthetic tyrosine phosphorylated peptide TRDIYETDpYYRK. The
isotopically resolved ion cluster in the reflector spectrum shows the contributions of all of the isotopes of C, H, N, O, and P that make up the elemental composition of this peptide. The improvement in resolution is most noticeable at masses of ∼3000 and lower. For larger molecules, where the resolution is no longer sufficient to provide separation of the isotopes, the natural width of the unresolved isotopic envelope has a large influence on the apparent resolution of the instrument, as explained in the discussion of Fundamentals of Mass Measurement Accuracy and Mass Resolution. In addition to resolution, mass accuracy is also improved when spectra are recorded in the reflector mode (Vorm and Mann, 1994). Tremendous improvement in the resolution achievable on MALDI-TOF instruments has recently been demonstrated using a focusing technique developed many years ago by Wiley and McLaren (1953). This method, referred to as time-lag focusing or “delayed extraction,” has been shown to provide a resolution of ∼2000 to 4000 (full-width, half-maximum; or FWHM) in linear mode for peptides and resolution of ∼3000 to 6000 FWHM in reflecting mode, on a 2-meter-long instrument (Brown and Lennon, 1995; King et al., 1995; Vestal et al., 1995; Whittal and Li, 1995). In a delayed extraction source, ions are created in a field-free region and allowed to spread out before an extraction voltage is applied and the ions are accelerated into the drift tube. The delayed extraction also probably limits the number of collisions the ions undergo on their way out of the source, thereby reducing additional peak broadening caused by metastable decomposition. Mass accuracy with the use of an internal reference standard may also be improved with the use of the delayed extraction source. Assigning an accurate m/z value to an ion whose flight time has been measured requires that the mass spectrometer be properly calibrated. This is usually accomplished by measuring the flight times of two compounds whose molecular weights are accurately known. In practice, it is usually best to select two compounds with molecular weights that bracket the mass range over which samples are to be examined. For molecules >10,000 Da, the [M+H]+ and the [M+2H]2+ ions of a well-characterized protein standard work very well for calibration. Application of an internal calibration, where the calibrant peaks are present in the same spectrum as the sample, can provide accuracies of 0.01% to 0.05% (e.g., 0.1 to 0.5 Da at m/z = 1000). Internal calibration is sometimes experi-
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A
1690 1695 1700 1705 1710 1715
Figure 10.21.3 Molecular ion region from MALDI spectra of the peptide TRDIYETDpYYRK (monoisotopic Mr = 1701.8) recorded in the (+) ion mode. Spectra were recorded in (A) the linear mode, where the average mass is measured at the centroid of the unresolved isotopic cluster, and (B) the reflector mode, where the monoisotopic mass is measured at the centroid of the first peak in the cluster. This peak contains the lightest isotopes of the elements present (see discussion of Fundamentals of Mass Measurement Accuracy and Mass Resolution).
B
1690 1695 1700 1705 1710 1715 m /z
mentally difficult owing to the need to adjust the amount of calibrant compounds added so that the calibrant peaks and the analyte peaks are simultaneously observed with reasonable intensity. MALDI mass spectra may also be calibrated externally; this approach is experimentally simpler. In this method calibrant compounds are measured using the same matrix and laser irradiance as were used for sample analysis. A mass accuracy of ∼0.1 to 0.2% (e.g., ±1 to 2 Da at m/z = 1000) can be achieved in linear mode using external calibration (Karas et al., 1989). In reflector mode, one can routinely expect mass accuracies on the order of 0.01% to 0.05% using external calibrations. In reflector mode it is also possible to use either the [M+H]+ ion or the [2M+H]+ ion (the protonated gas-phase dimer) of the matrix as one of the two calibrant peaks, which means that only one calibrant compound needs to be added.
MALDI Post-Source Decay MS for Peptide Sequencing A peptide molecular ion that is sufficiently stable to be transported out of the ion source
but insufficiently stable to survive the flight to the detector will decompose in the flight tube, giving rise to a series of fragment ions that are characteristic of the molecule’s original structure. This process is referred to as metastable decay or post-source decay. It is believed that molecular ions acquire excess internal energy necessary for fragmentation via multiple collisions with matrix ions in the source (Kaufmann et al., 1994). When a precursor ion decomposes in the flight tube, all of the fragment ions will have the same velocity as the precursor, but they will have only a fraction of its kinetic energy. This means that the precursor and metastable fragments will all strike the linear detector at the same time and will be detected at the same apparent mass. Thus metastable or post-sourcedecay fragment ions are never observed in a linear spectrum. In contrast, a reflector TOF is capable of discriminating fragment ions by flight-time dispersion. Although all ions will have essentially the same velocity, the kinetic energy of a post-source-decay fragment ion will be determined by the ratio of its mass to the mass of the precursor. Thus the fragments, which have less
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Relative abundance (%)
A 100
2066
1284
50
804
1300
634
1705 1776 1550
1002 1250 1082
530
2168 2384
2705 2937
0
Relative abundance (%)
B
600
800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
100 2066 y3 50
15 14 13 12 11 10 9
y4
8
7 6
5
4 3 2 1
I–F–Y–P–E–I–E–E–V–Q–A–L–D–D–T–E–R
yn
PEIE
y1 PEI y2
PEIEE y6 y5
y7
600
800
y8
y9 y10
y11
y12
y14
y15
0 200
400
1000 1200 m /z
1400
1600
1800
2000
Figure 10.21.4 MALDI-MS analysis of a tryptic digest of an 18-kDa protein (Ladner et al., 1996). A total of 30 pmol of protein was separated on an SDS-PAGE gel, the band of interest excised from the gel, and the protein digested in situ with trypsin; then 1⁄20 of the sample was analyzed by MALDI-MS. (A) MALDI mass spectrum of the tryptic peptides recorded in the linear mode; (B) MALDI-PSD spectrum of the [M+H]+ ion at m/z 2066. The peak was selected using an ion gate (R = 100). A nearly complete series of yn ions (see Fig. 10.21.1) makes it possible to deduce the sequence of residues 3 to 17.
Overview of Mass Spectrometry
kinetic energy than the precursor, will not penetrate as deeply into the ion mirror, and so will have their flight times decreased relative to the precursor. The fragments are therefore detected at a lower mass. By successively lowering the reflector voltage, thereby bringing lower- and lower-mass fragment ions into focus, a complete post-source decay fragment ion spectrum can be acquired in segments. The individual segments are calibrated and assembled afterward automatically by the data system. The energy-focusing features of reflectrontype mass spectrometers make it possible to use post-source decay for peptide sequencing (Kaufmann et al., 1994). Post-source decay spectra resemble low-energy collision-induced dissociation (CID) spectra, similar to those commonly recorded on triple-quadrupole mass spectrometers (Hunt et al., 1986). The spectra contain mainly bn and yn ions, often with very abundant internal fragments and immonium ions (see Fig. 10.21.1 and Table 10.21.2). The
use of an ion gate makes it possible to perform a low-resolution (R = 100) selection of a precursor ion of interest from an unfractionated mixture, such as a protein digest. This approach is very powerful for providing internal sequence information on in situ–digested proteins that have been purified by SDS-PAGE (see Fig. 10.21.4 for an example).
ESI-MS In electrospray ionization (ESI)-MS, ions are formed from peptides and proteins by spraying a dilute solution of these analytes at atmospheric pressure from the tip of a fine metal capillary (Fig. 10.21.5). The spray process, often assisted by pneumatic nebulization, creates a fine mist of droplets (Fenn et al., 1990; Kebarle and Tang, 1993). The droplets are formed in a very high electric field created by applying a high voltage (∼4 kV) to either the spray tip (Fig. 10.21.5) or the counter electrode; in this process, the droplets become highly
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possible sample inlets: syringe pump sample injection loop HPLC
triple-quadrupole mass spectrometer
atmosphere (760 Torr)
vacuum (10 – 6 Torr)
liquid 1st quadrupole collision 2nd quadrupole mass analyzer cell mass analyzer
capillary electrophoresis
data system expansion of the ion formation and sampling regions
electrospray needle
nitrogen drying gas vacuum
atmosphere
3–5 kV
liquid nebulizing gas
droplets containing solvated ions
ions sampling orifice counter electrode
Figure 10.21.5 Schematic diagram of a triple-quadrupole mass spectrometer (top) and the electrospray interface (bottom).
charged. Solvent evaporation is rapid from these small droplets. As the droplets evaporate, the peptide and protein molecules in the droplets pick up one, two, or more protons from the solvent to form singly or, more frequently, multiply charged ions (e.g., [M+H]1+, [M+2H]2+, etc.). The number of charges acquired by a molecule is roughly equivalent to the number of possible sites of proton attachment. As the droplets continue to shrink, the charge density on the surface of each one increases to the point where charge repulsion overcomes the forces holding together the droplet and the solvated ions contained within it. Ions are then “emitted” or “evaporated” from the droplet surface. The ions are sampled into the high-vacuum region of the mass spectrometer for mass analysis and detection, most often using a quadrupole (or triple-quadrupole) mass analyzer (Fig. 10.21.5). Usually little or no fragmentation is observed in the normal ESI mass spectra of peptides. The typical solvent for peptides and proteins is a mixture of water, an organic modifier such as CH3CN, and up to a few percent by volume of acetic, trifluoroacetic, or another volatile acid (the latter included to enhance ionization of sample constituents). Because the ions are produced at atmospheric pressure from flowing liquid
streams, ESI is ideally suited for on-line coupling to high-performance liquid chromatography, making it possible to analyze mixtures of peptides and proteins rapidly (see below). As mentioned above, a key feature of the electrospray process is the formation of multiply charged molecular species from analytes that contain more than one possible site of proton attachment (Fenn et al., 1990; Smith et al., 1991). Proteins usually exhibit a characteristic series of multiply charged ions (e.g., see Fig. 10.21.8). As one proton (i.e., one charge) is typically attached for each 1000 Da of molecular mass, the ion series for a protein, even a large protein, will fall in the m/z range of 800 to 3000. This makes it possible for simple instruments like quadrupole mass spectrometers to be used for mass analysis of ions produced by ESI. The molecular mass of the protein can easily be calculated using the observed masses of any two adjacent ions in the series (Covey et al., 1988; Mann et al., 1989; Reinhold and Reinhold, 1992). Normally these calculations are performed automatically by the data system. Because each multiply charged peak provides an independent measure of the molecular mass of the protein, an ion series from a single experiment provides a measure of the mass measurement precision. Most data sys-
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Relative abundance (%)
A
100 75
2+ 673.8
2+ (– H, +Na) 2+ (– H, +K)
50
1+ 805.4
25
1+ 1346.6 800
600
Relative abundance (%)
B
1000
b3 y2
100
y4 75 50
b2
y6
1200 2+
10 9 8 7 6 5 4 3 2
D G D G pY I S A A E L R
y7
b5
bn
2 3 4 5 6 7 8 9 10
y3 b7 b4
200
yn
y5
y1
25
1400
400
b9
b6
600
y8 b 10 y9
b8 800
y10 1000
1200
1400
m /z
Figure 10.21.6 Electrospray analysis of an HPLC fraction from the tryptic digest of a 17-kDa phosphoprotein. The protein was purified by SDS-PAGE, electroblotted onto PVDF membrane, eluted, digested with trypsin, and the peptides analyzed by on-line LC-ESI-MS using a phosphopeptide-selective scanning method (Huddleston et al., 1993b). The column flow was split after UV detection, with 10% going to the mass spectrometer and 90% to a fraction collector. (A) Nanoelectrospray mass spectrum of a putative phosphopeptide-containing fraction. Peaks labeled (+Na) and (+K) correspond to [M + H + Na]2+ and [M + H + K]2+, respectively. (B) Collision-induced decomposition (CID) spectrum of the doubly charged ion ([M+2H]2+) at m/z = 673.8. This spectrum confirms the sequence of the peptide and identifies the site of phosphorylation as Tyr5.
Overview of Mass Spectrometry
tems also provide algorithms for transforming the mass-to-charge axis into a molecular mass axis for simplifying interpretation and presentation of the data. Proteins with molecular masses of up to 150,000 Da have been successfully analyzed by ESI using commercially available quadrupole instrumentation. Mass assignment accuracies of 0.01% are routine (see Table 10.21.1 and discussion of Fundamentals of Mass Measurement Accuracy and Mass Resolution). ESI has also been successfully interfaced with ion-trap mass spectrometers (Korner et al., 1996), Fourier-transform ion-cyclotron resonance mass spectrometers (Winger et al., 1993; O’Conner et al., 1995; Wu et al., 1995; Smith et al., 1996), and TOF mass spectrometers (Mirgorodskaya et al., 1994; Verentchikov et al., 1994).
In the case of peptides, the maximum number of charges observed often correlates reasonably well with the number of amino acid side chains that can readily accept a proton at the low pH of the analyte stream (i.e., Arg, Lys, and His, plus the free amino terminus). In the case of tryptic peptides, at least two charge sites are normally present, and tryptic peptides with molecular weights of 800 to 3000 typically exhibit a major, if not dominant, [M+2H]2+ ion. For example, the ESI mass spectrum of an HPLC fraction from a tryptic digest of a phosphoprotein shows a simple mixture of peptides, in which doubly charged forms of the major peptide predominate (Fig. 10.21.6). In this spectrum, in addition to proton attachment, doubly charged ions due to Na and K adduction are also abundant. In general, the smaller the pep-
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tide, the more likely it is that the singly charged [M+H]+ ion will also be observed. As the size of the peptide increases, triply and highercharged states of the peptide will increase in abundance in the spectra while the abundance of ions with lower charge states will decrease (Fig. 10.21.7). Electrospray mass spectra are keen reminders that mass spectrometers measure the ratio of mass to charge (m/z) and not molecular mass directly. The peak at m/z = 1346.6 in Figure 10.21.6 (panel A) is the monoisotopic [M+H]+ of a peptide of molecular weight 1345.6 (see section on Fundamentals of Mass Measurement Accuracy and Mass Resolution for a discussion of monoisotopic mass). Evidence that this ion carries a single charge (1+) as opposed to a higher number of charges is obtained from the spacing of the isotope peaks, which are one m/z unit apart. The doubly charged form of this same peptide is observed on the m/z scale at [1345.6 +2]/2 = 673.8. The isotope peaks in the [M+2H]2+ isotope cluster are spaced 0.5 m/z units apart, indicating that these ions are doubly charged. Quadrupole mass analyzers can routinely achieve resolution sufficient to distinguish singly from doubly charged ions up to m/z = 3000 (if sufficient sample is available and the instrument is scanned slowly). This capability is particularly relevant to tryptic digests of proteins where, as noted above, the doubly charged ion may be the only ion observed for a given peptide. In the absence of the peak spacing information, the analyst could have difficulty determining if such an ion is singly or doubly charged. For ions with three or more charges, a charge series (e.g., 2+ and 3+) is usually present that permits the analyst to determine the charge state and mass of any given ion in the spectrum without the need to determine the isotopic peak spacing.
LC-ESI-MS Electrospray ionization has made on-line liquid chromatography (LC)-MS a practical and robust method for sample introduction because gas-phase ions, as opposed to liquid droplets, are what is mainly being sampled by the instrument. The ideal milieu for presenting a sample to any mass spectrometer, regardless of the ionization mechanism, is in mixtures of highly pure water and volatile organic solvents containing low levels of volatile acid or base. Fortunately, such mixtures (e.g., water, acetonitrile, and trifluoroacetic acid at 0.05% to 0.1% by volume) are also those that provide optimal separation of peptides and proteins by reversed-
phase liquid chromatography. From a practical standpoint, LC-MS reduces or eliminates the need to preparatively fractionate complex mixtures prior to MS, thereby saving valuable instrument time and preventing possible sample losses. LC-MS data may also be searched retrospectively for components present at very minor levels that may not have been considered worthy of collection based on a weak UV response. In an LC-MS experiment, mass spectra are recorded continuously as the components elute from the HPLC column. If the mass spectrometer is made to scan once every 4 sec, the LC-MS data file will contain ∼900 mass spectra. To identify regions of data that are likely to contain useful spectra, the mass spectrometer’s data system produces a plot of the total number of ions detected during each mass spectrum scan versus time (the scan number). This plot, which is called a mass chromatogram or a total ion current (TIC) trace, looks very much like the UV chromatogram (Fig. 10.21.7). If it is of interest to know whether a particular peptide is present in the digest and where it elutes, the analyst simply requests the data system to display a selected ion-current trace for the specific masses of the most likely charge states (e.g., 1+ to 4+). Figure 10.21.7 illustrates the type of data typically acquired during the LC-ESI-MS analysis of a protein digest. In this example, the protein is heterogeneously glycosylated at a single site, and the multiplicity of peaks evident is due to differing carbohydrates attached to a single tryptic peptide (Fig. 10.21.7, panel C). Because an ESI mass spectrometer behaves as a concentration-dependent detector, much like a UV detector, only a few percent of the total column effluent (∼1 to 5 µl/min) needs to be directed into the mass spectrometer. The remainder can be diverted, via a flow splitter, through an in-line UV detector to a fraction collector. This arrangement makes it possible to perform straightforward correlation of the UV trace with the total ion current trace and then analyze individual fractions further by MS/MS (see discussion of ESI MS/MS for Peptide Sequencing) or some other method. The UV response is also useful for estimating the amount of a peptide that has been injected on the column. This is generally not possible to do by MS without appropriate internal standards (see section entitled Is MS Data Quantitative?). LC-ESI-MS has become a key analytical method for the analysis of natural and recombinant proteins, and data from these experiments is now often included in the regulatory
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Relative intensity (%)
A
125 54.6
100 75 10.8
50
19.8
25 0 1 0.0
101 7.5
201 14.9
301 22.4
401 29.8
501 37.3
601 44.9
701 52.4
801 59.8
scan/time (min)
Relative intensity (%)
B
125
10.8 min (scans 317– 319)
1+
439.2
100
= Mr 438.2 75 50 [2M+H]+ 877.6
25 0
Relative intensity (%)
C
[M+2H]2+
125 100 75 50 25
19.8 min (scans 261– 271)
2+
E292
1317.8 1398.8 x, y = 4 (Hex)x (HexNAc)y ( dHex) – N296 x = 3 y=4 204.0 366.2 R300 [M+3H]3+ x=5 933.0 y=4 1480.0
= Mr 2795.6
0
D
3+
54.6 min (scans 731–74 4)
Relative intensity (%)
950.0 2+
= Mr 2344.4
1173.2
= Mr 2846.8
4+
712.8
200
400
600
2+
1424.4 1564.4
1+
2345.2
800 1000 1200 1400 1600 1800 2000 2200 m/z
Overview of Mass Spectrometry
Figure 10.21.7 On-line LC-ESI-MS analysis of a tryptic digest of reduced and alkylated heavy chain (51 kDa) from a recombinant monoclonal antibody. (A) Total ion current trace showing the distribution of tryptic peptides. (B) ESI spectrum of the peak at 10.8 min showing a small singly charged peptide and its gas-phase dimer. (C) ESI spectrum of the peak at 19.8 min corresponding to a tryptic glycopeptide with heterogenous carbohydrate. At least five different carbohydrate structures are indicated. Abundant low-m/z ions at m/z = 204 and m/z = 366 are characteristic fragment ions produced from the carbohydrate (Roberts et al., 1995). (D) ESI spectrum of the peak at 54.6 min showing two large tryptic peptides, each represented by several different charge states.
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filings for new biopharmaceutical products (Carr et al., 1989; Palczewski et al., 1994; Stults et al., 1994; Watts et al., 1994; Resing et al., 1995; Roberts et al., 1995; Rush et al., 1995; Schindler et al., 1995; Hemling et al., 1996).
ESI MS/MS for Peptide Sequencing The principles of MS/MS using triple-quadrupole mass spectrometers have been reviewed (Yost and Boyd, 1990). Briefly, the first quadrupole (Fig. 10.21.5) acts as a mass filter to select specifically the [M+nH]n+ ion (where n is typically 1, 2, 3, or 4) of a peptide of interest and isolate it from other ions produced in the source. The selected precursor ion is then fragmented with a neutral gas such as nitrogen or argon in a quadrupole collision cell. This process is known as collision-induced decomposition. The fragment or product ions produced are then transmitted into the quadrupole second mass analyzer, which is being used as a scanning mass analyzer. The product ions are separated based on m/z and detected. The result is a product-ion mass spectrum of the selected precursor that is free of interferences from other components present in the mixture. An example of this process is shown in Figure 10.21.6. The fragmentation of this peptide reveals its sequence and the location of the phosphorylated residue. Fragmentation can also be induced in the ESI source by increasing the energetics of the ion sampling process. This is done by adjusting the lens potentials in the high-pressure region upstream from the first quadrupole mass filter. Ions with increased kinetic energy undergo decomposition via collisions with residual gas molecules (Katta et al., 1991). Structurally useful fragmentation can thus be obtained on a single-quadrupole instrument, with the caveat that because this method has no ability to preferentially select precursor ions, it requires a highly purified sample. Recently strategies have been developed that take advantage of this ability of the ESI interface to generate fragment ions prior to mass analysis. Selective detection of glycosylated peptides (Carr et al., 1993; Huddleston et al., 1993a), phosphorylated peptides (Huddleston et al., 1993b; Till et al., 1994), and sulfated peptides (Bean et al., 1995) during on-line LC-ESI-MS analysis of protein digests can be accomplished by monitoring the total ion current trace for highly diagnostic low-m/z fragment ions produced in the ESI interface. These procedures are equally applicable to the detection of these modifications in unseparated mixtures.
IS MS DATA QUANTITATIVE? A question that frequently arises in the analysis of peptide or protein mixtures is whether the relative peak heights (or areas) are quantitatively representative of the relative solution concentrations of the sample components. Unfortunately, the answer to this question is no, unless appropriate internal reference standards are used. Due to differing ionization and desorption efficiencies, the absolute yields of different ions from one peptide in a mass spectrometer can vary by perhaps as much as 2 orders of magnitude, depending on the sequence, composition, and size of the peptides (Dunayevskiy et al., 1995; Jesperson et al., 1995; Cohen and Chait, 1996). Furthermore, strongly ionizing components can frequently suppress the ionization of more weakly ionizing components present in the same sample or chromatographic fraction. In MALDI, the matrix itself can strongly influence the detectability of specific sample constituents. Quantitation by mass spectrometry requires that the response of a specific sample component be determined relative to that of a reference compound added to the sample and analyzed under identical conditions. In certain cases, semiquantitative estimates of relative amounts of components may be made if the components are of nearly identical structure and their chemical differences do not significantly affect the overall charge or hydrophobic/hydrophilic character of the molecules. For example, the relative ratios of the variants of β-casein observed in the ESI mass spectrum (Fig. 10.21.8) are probably a good reflection of the relative solution concentrations of these variants, as they differ from one another by only one or two amino acid substitutions out of more than 200 amino acids. The smaller the molecule, the more likely it is that such estimates will be in error.
SAMPLE PREPARATION The ideal method for preparing samples for any type of MS analysis is reversed-phase highperformance liquid chromatography (RPHPLC). If purification by RP-HPLC is not an option because of known problems with sample loss, then the cleanup protocol employed should produce a sample composition that resembles as closely as possible that of a sample which has been purified by RP-HPLC. That is, the sample should contain the least amount possible of buffers, salts, detergents, and anything else besides water, organic modifier, and volatile acid or base.
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A
B 23,984 2
14+ 13+
Relative abundance (%)
[M +H]1+ 24,010 30
24,024 2
+
18+ 15 16+
24,093 2 12+
1400
1420
11+ 10+ 9+
20,000 22,000 24,000 26,000
800
1200
1600
8+
2000
2400
m/z
Figure 10.21.8 Molecular weight determination of β-casein, a 24-kDa phosphoprotein with known sequence heterogeneity. (A) Linear MALDI-TOF spectrum. Molecular weight was determined from the centroid of the smoothed peak using an external calibration. (B) ESI spectrum. The inset, an expansion of the region around the 14+ charge state, reveals the presence of at least three variants. The molecular weights shown are the averages of determinations made from ten individual charge states.
Overview of Mass Spectrometry
The quality of the data obtained using ESIMS is very dependent on the type and concentration of excipients present in the sample. If buffers are necessary for protein solubility or enzymatic digestion, it is best to use a volatile buffer such as ammonium bicarbonate or ammonium acetate at a concentration of ≤30 mM. In general it is best to avoid using any ionic detergents (surfactants), even though it is possible to obtain spectra for some proteins when these are included as long as the surfactant concentration is kept 10%, repeat the assay for that sample.
18. Subtract the average nonspecific binding counts (average of tubes #3 and #4) from each average count of all tubes except the total counts (tubes #1 and #2). In this assay, nonspecific binding refers to tracer protein that remains in the tube without addition of primary or secondary antibody.
19. Calculate the percent of tracer bound to primary antibody according to the equation: % tracer bound = {(total tracer bound)/(total tracer added)} × 100 See Table 10.24.2 for sample assay results. Total tracer specifically bound to primary antibody (average of tubes #5 and #6 minus average of tubes #3 and #4) equals ∼2,200 cpm. The total counts of tracer added to each tube (average of tubes #1 and #2) equals ∼11,500 cpm/tube. Ideally, the amount of tracer bound would equal 15% to 50%. In this case, it is ∼19%. If the amount of tracer bound does not fall within this ideal range, a primary antibody with higher affinity for the protein or peptide of interest must be prepared or obtained.
20. Calculate the % B/B0 for each standard and unknown sample, where B is the net average counts (the average counts minus the average nonspecific binding, as determined in step 18) and B0 is the total tracer bound (determined in step 19). See sample calculations in Table 10.24.2.
21. Using log/log graph paper, plot % B/B0 for each standard on the y axis and the known concentration of each standard on the x axis (see Fig. 10.24.2). 22. Construct a standard curve by joining the points in a smooth curve. A standard curve should be generated for each protein assay.
23. Calculate the concentration (ng/ml) of antigen in the unknown samples by comparison with the standard curve. If the unknown sample volumes used are less than 100 µl and a dilution was performed (see step 8), it will be necessary to account for the dilution in the calculations (e.g., if 25 µl was used, the calculated quantity of protein must be multiplied by 4). The % B/Bo values for unknown samples #1 and #2 were 39.4 and 61.3, respectively, which correspond to 7.6 and 3.0 ng/ml (see Table 10.24.2 and Fig. 10.24.2).
24. Calculate the concentration (ng/ml) of antigen in the quality control samples (tubes #23 and #24 and tubes #25 and #26) and compare to the known values. The quality control samples (also see Reagents and Solutions) are other complex samples with known quantities of the protein of interest, and the values obtained in this assay should agree with the previously determined quantities. Such controls are useful for inter-assay comparisons.
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Table 10.24.2 Calculations of B/B0 and Protein Concentration for a Sample RIAa
Tube #
Identification
cpm
1
Total tracer counts
11574
2
Total tracer counts
11449
3
Nonspecific binding
745
4
Nonspecific binding
796
5
B0 counts
2854
6
B0 counts
3022
Avg. cpm
Avg. net cpm
% B/B0
Protein conc. (ng/ml)b
11512 771 2938
2167 (B0 )
2862
2091 (B)
96.5
2615
1844 (B)
85.1
2283
1512 (B)
69.8
1793
1022 (B)
47.2
1527
756 (B)
34.9
1261
490 (B)
22.6
1097
326 (B)
15.1
1008
237 (B)
10.9
2144
1373 (B)
63.4
2.8
1305
534 (B)
24.7
18.5
1625
854 (B)
39.4
7.6
2100
1329 (B)
61.3
3.0
Standards 7
0.5 ng/ml
2902
8
0.5 ng/ml
2821
9
1 ng/ml
2576
10
1 ng/ml
2654
11
2 ng/ml
2273
12
2 ng/ml
2293
13
5 ng/ml
1859
14
5 ng/ml
1727
15
10 ng/ml
1536
16
10 ng/ml
1518
17
20 ng/ml
1238
18
20 ng/ml
1283
19
50 ng/ml
1112
20
50 ng/ml
1082
21
100 ng/ml
1001
22
100 ng/ml
1014
23
QC #1
2147
24
QC #1
2141
25
QC #2
1314
26
QC #2
1296
Controls
Unknowns 27
Unknown #1
1600
28
Unknown #1
1650
29
Unknown #2
2000
30
Unknown #2
2200
a Adapted from data derived from a primate leptin RIA kit available from LINCO Research. Abbreviations: B, cpm of
bound protein tracer in presence of varying amounts of unlabeled protein; B0 , cpm of bound protein tracer in absence of competing unlabeled protein; QC, protein quality control. b Calculated from standard curve illustrated in Figure 10.24.2.
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Figure 10.24.2 Log-log plot of protein concentration versus % B/B0 . This graph is derived from the data in Table 10.24.2. The protein concentration range is between 0.5 ng/ml and 100 ng/ml. This range was determined by using protein standards at the concentrations of 0.5, 1, 2, 5, 10, 20, 50, and 100 ng/ml. The B/B0 values are determined by dividing the B values (cpm of bound tracer in the presence of variable amounts of unlabeled protein standard) by B0 (cpm of bound tracer without the presence of competing, unlabeled protein standard).
REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Anti-protein antiserum (primary antibody solution) This solution contains the primary (first) antibody used to complex the [125 I]tracer or the unlabeled antigen in the standards, quality controls, and unknown samples. If the unknown samples are of human origin, an example of an appropriate primary antibody might be a rabbit monoclonal or polyclonal antiserum against the human protein to be quantitated (see Chapter 11 for production of monoclonal and polyclonal antisera). Dilute the antiserum 1:20 to 1:200 in assay buffer (see recipe), depending on the titer of specific immunoglobulin in the antiserum. Prepare dilution on day of use and keep at 2◦ to 8◦ C. The primary antibody is either a polyclonal antibody (derived from serum; therefore referred to as an antiserum) or a monoclonal antibody (derived from ascites fluid or hybridoma supernatant). The serum, ascites fluid, or hybridoma supernatants may be used without purifying the primary antibody from these sources. In each case, a dilution with the diluting buffer is required. A dilution of 1:20 to 1:200 is specified above; however, the titer of a primary antibody directed at a protein (or peptide) of interest will differ depending on the immunogenicity of the antigen. Hence, the dilution of the anti-protein (or anti-peptide) antiserum, ascites fluid, or hybridoma supernatant may be increased (e.g., to 1:10,000) if the titer is high. The amount of primary antibody must be sufficient to bind 15% to 50% of the radiolabeled protein (or peptide) tracer used in the assay.
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Assay buffer Phosphate-buffered saline (PBS), pH 7.4 (see recipe) 0.025 M EDTA (APPENDIX 2) 0.1% (v/v) Triton X-100 0.08% (w/v) sodium azide 1% (w/v) BSA (RIA grade; Sigma-Aldrich) Store short term (up to 24 hr) at 2◦ to 8◦ C or long term at −20◦ C CAUTION: Sodium azide may react with lead or copper plumbing to form explosive metal azides; dispose of azide solutions according to local regulations and institutional guidelines; also see APPENDIX 1H.
Phosphate-buffered saline (PBS) 1.15 g NaH2 PO4 (anhydrous; 9.5 mM) 5.75 g Na2 HPO4 (anhydrous; 40.5 mM) 9.00 g NaCl (154 mM) Add H2 O to 900 ml Adjust to pH 7.4 using 1 M NaOH or 1 M HCl Add H2 O to 1 liter Precipitating secondary antibody/PEG solution This solution contains an antibody against the immunoglobulin of the species used to raise the primary antibody. For example, if the primary antibody is a rabbit antihuman immunoglobulin, an example of an appropriate secondary antibody might be a goat anti–rabbit IgG. The composition of the solution is as follows: Phosphate-buffered saline (PBS), pH 7.4 (see recipe) containing: 1% to 2% (v/v) antiserum 3% (w/v) polyethylene glycol (PEG; mol. wt. 8000) 0.1% (v/v) Triton X-100 0.025 M EDTA (APPENDIX 2) 0.08% (w/v) sodium azide Store short term (up to 24 hr) at 2◦ to 8◦ C or long term at −20◦ C CAUTION: Sodium azide may react with lead or copper plumbing to form explosive metal azides; dispose of azide solutions according to local regulations and institutional guidelines; also see APPENDIX 1H.
Protein quality controls Select complex samples (e.g., sera, tissue culture supernatants) from the same species as the unknown samples (e.g., human), but in which the concentration of the desired antigen is already known. Before the assay, if needed, dilute in assay buffer (see recipe) to bring the final antigen concentration within the assay range (e.g., 3 and 18 ng/ml, as in Table 10.24.2). Store short term (up to 24 hr) at 2◦ to 8◦ C or long term at −20◦ C.
Solution Radioimmunoassay of Proteins and Peptides
Protein standards Dilute the purified protein of interest (antigen) from the same species as the unknown samples (e.g., human) in assay buffer (see recipe) at 0.5, 1, 2, 5, 10, 20, 50, and 100 ng/ml. Prepare dilutions on the day of use and keep at 2◦ to 8◦ C.
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Protein tracer buffer Prepare assay buffer (see recipe) containing 2% (w/v) serum from the species of origin of the unknown sample (e.g., human) as carrier. Store short term (up to 24 hr) at 2◦ to 8◦ C or long term at −20◦ C. This buffer is used to dissolve iodinated protein.
Protein tracer, iodinated with 125 I Radiolabel the purified protein of interest (antigen) from the same species as the unknown samples (e.g., human) with 125 I (see, e.g., UNIT 11.17) to a specific activity of 130 to 140 µCi/µg, and purify by HPLC (UNITS 10.12 & 10.13). To maximize stability of the batch of iodinated protein, lyophilize in aliquots that are sufficient to complete a whole RIA in which each assay tube contains 5,000 to 20,000 cpm. Store lyophilized tracer up to 1 month at −20◦ C. Before use, dilute in protein tracer buffer (see recipe) such that each 100 µl contains 5,000 to 20,000 cpm, and keep at 2◦ to 8◦ C. COMMENTARY Background Information The first radioimmunoassay was developed to measure insulin in human plasma (Yalow and Berson, 1960). RIAs are most commonly used in clinical laboratories. They are used to measure, e.g., peptides and proteins with endocrine function, antibiotics, cocaine, steroids, antigens associated with autoimmunity, bacteria, viruses, parasites, neuropeptides, and expressed proteins. Radioimmunoassay is a simple, rapid and precise method for measuring picomolar amounts of an antigen if the antigen is radiolabeled to high specific activity. An RIA, as described by Re and Haber (1980), requires: (1) a substance that binds with high affinity to the protein (or peptide) being measured; (2) a radiolabeled protein (or peptide)— a so-called “tracer”—that competes specifically for binding with the protein (or peptide) being measured; (3) a simple method for separating free and bound forms of the radiolabeled tracer; and (4) the availability of sufficient and known amounts of the protein (or peptide) being measured—so-called calibrators or standards—free from any impurities that could also displace the radiolabeled tracer protein (or peptide) from its binding site. The quantification of a specific antigen (protein or peptide) in a complex biological sample results from allowing differing amounts of the antigen of interest contained in a complex mixture (e.g., cell lysates, serum, saliva, urine, cerebrospinal fluid) to combine with a fixed quantity of an antigen-specific antibody in the presence of a fixed quantity of radiolabeled tracer antigen. The radiolabeled
tracer and the antigen being measured compete for binding to the antigen-specific antibody. Labeled and unlabeled antigens bind with equal affinity and in indirect proportion to each other. The smaller the amount of sample antigen, the larger the amount of labeled antigen that will bind. Hence, the higher the number of counts detected on a γ counter, the lower the amount of sample antigen present. A plot (a so-called standard curve) of the percent of bound tracer in the presence of differing amounts of unlabeled standard antigen— referred to as % B/B0 —versus the concentration of standard antigen can be made, and the concentration of antigen in the unknown samples can be readily determined by comparing the amount of bound radiolabeled tracer in each unknown sample with the standard curve. RIA reagents and supplies are available from numerous commercial sources (e.g., Sigma Chemical Co., ICN Biochemicals, Perkin-Elmer, Pierce Chemicals, LINCO Research). These are supplied as reagents or kits, which, in the case of double-antibody assays (used in this unit), include labeled antigen, antigen-specific primary antibody, and a secondary antibody (a so-called precipitating antibody) recognizing the primary antibody, which is a component of the bound antigen-antibody complex. Under some circumstances, it is also possible to purchase unlabeled antigen and to iodinate with one of the five available methods: chloramine-T (Hunter and Greenwood, 1962; UNIT 11.17), IODOGEN (Salacinski et al., 1981; UNIT 11.17), PIB (Wilbur et al., 1989), Bolton-Hunter
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(Bolton and Hunter, 1973), or lactoperoxidase (Marchalonis, 1969). As described by Hunter (1973), radioiodine (either 125 I or 131 I) is the label of choice because of the significant increase in radioactive disintegrations per unit time (in comparison to 3 H or 14 C). Unless the radiolabeled antigen is unstable (due to radiochemical damage), 125 I, not 131 I, is the radiolabel of choice, because of its longer half-life (60 days versus 8 days), the higher isotope abundance available in the carrier-free, commercially available 125 I label, its higher counting efficiency (80% versus 40%), and the higher specific activity of labeled protein (or peptide) with one molecule of 125 I (∼100 µCi/µg at a molecular mass of 20,000 and ∼1,000 µCi/µg at a molecular mass of 2,000) in comparison to 131 I. Chloramine-T (the sodium salt of the Nmonochloro derivative of p-toluene sulfonate) in the presence of NaI (labeled with 125 I or 131 I) enables proteins (or peptides) containing tyrosyl residues to be specifically labeled in aqueous solutions under mildly alkaline conditions. The method was developed by Hunter and Greenwood (1962). The pH optimum is 7.5. At pH > 8.5, iodination of other amino acids (histidine or tryptophan) is possible. The efficiency of the reaction is dependent on the concentration of the protein (the lower the amount of protein, the less efficient the labeling). However, if only trace labeling is needed, only very low concentrations of chloramine-T are required. IODO-GEN (1,3,4,6-tetrachloro-3α,6αdiphenylglycouril; a trademark of Pierce developed by Salacinski, 1981), is a more gentle iodination method than chloramine-T, because the reaction occurs on a solid-phase surface of the oxidant, thereby minimizing the exposure of the protein or peptide to oxidative damage. The specific activity of the labeled proteins (or peptides) is less than that achievable by chloramine-T. The PIB (N-succinimidyl 4-iodobenzoate) method, developed by Wilbur et al. (1989), iodinates tyrosine residues. Because these iodinated proteins (or peptides) do not contain an ortho-phenol, they are not susceptible to delabeling by natural deiodinases selective for ortho-iodophenols. This leads to increased stability of the iodinated proteins (or peptides). The Bolton-Hunter (1973) method is a two-step labeling approach for introducing an iodinated p-hydroxyphenyl moiety (i.e., a tyrosine mimic) into proteins (or peptides) lacking tyrosine residues or under circum-
stances where an active tyrosine is involved in a binding or an antigenic site and therefore should not be iodinated. The first step is the iodination by chloramine-T or IODOGEN of the N-hydroxysuccinimide ester of 3(p-hydroxyphenyl)propionic acid. The second step is the active ester coupling of the iodinated 3-(p-hydroxyphenyl)propionic acid to lysyl residues in the protein (or peptide). Addition of a protein solution to the dried reagent residue results in a covalent attachment of the radiolabeled moiety to lysyl residues in the protein. The advantage of this acylating method is that the protein is not exposed to the oxidizing conditions associated with the chloramine-T or IODO-GEN. In addition, the method allows the introduction of a radiolabeled iodine into proteins lacking a tyrosyl residue but containing a lysyl residue. When the antibodies used in the RIA are directed at an antigenic site containing a tyrosyl residue, the Bolton-Hunter method does not affect the antigenic site unless a modified lysyl residue is also in the antigenic site. The lactoperoxidase method is an enzymic reaction method developed by Marchalonis (1969). Lactoperoxidase catalyzes the oxidation of iodine in the presence of hydrogen peroxide. In turn, selective iodination of tyrosyl residues in proteins occurs. The specific activity of the labeled proteins is less than that achievable by chloramine-T.
Critical Parameters and Troubleshooting In order to develop an RIA for a given protein (or peptide), the following questions should be asked and answered (http:// www.cyfc.psu.edu/funding/L2 dr2.html). 1. Is it feasible to develop a radioimmunoassay for a specific protein (or peptide)? Begin by searching the literature to determine the following: a. Information about the protein’s availability: i.e., has it been isolated and, if so, how easy will it be to obtain the amount required for developing and running subsequent RIAs? b. What is the molecular mass (and amino acid composition) of the protein? c. What is its half-life? d. What is its intracellular, organ, or body distribution? Has a previous assay been reported (i.e., are sensitivity, sample volumes, and assay protocols defined) for measuring the protein of interest? If so, was the assay validated? Such validation requires determination of:
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i. Minimal detection limit, defined as the minimum concentration of the protein that may be distinguished from zero at a 95% confidence limit, by extrapolating the mean minus two standard deviations for multiple replicates (e.g., 5 to 10 assays) of the zero calibrator (i.e., not containing standard protein). ii. Reproducibility of the standard curve using replicate runs (e.g., 5 to 10 assays) by comparing averages, standard deviations, and confidence values for the slope and correlation coefficient. iii. Precision, by measuring the intraassay variation (confidence value) for replicate samples at various concentrations (i.e., low, medium, and high). iv. Accuracy, by adding known amounts of unlabeled protein (or peptide) to samples containing various endogenous concentrations. v. Drift, due to delays in adding reagents due to distribution of samples in a given assay configuration (i.e., arrangement of tubes or placement of samples with presumed low, medium, or high concentrations). vi. Sensitivity sufficient to capture the full range of developmental, individual, gender, and diurnal variations. 2. Does an immunodiagnostic vendor provide a source of polyclonal (or monoclonal) antibodies specific to the protein (or peptide) of interest, chemically or enzymatically labeled standard protein (or peptide), and calibrators (i.e., standard protein or peptide at various defined concentrations required to construct a standard curve)? If so, is the laboratory prepared to develop an RIA? 3. Is an RIA commercially available for the protein (or peptide) of interest? If the answer to any of the above questions is yes, one is ready to develop or purchase a solution RIA. If not, a large amount of work may be required to prepare the required assay reagents, which may involve expressing and/or purifying a sufficient quantity of a standard protein or synthesizing and/or purifying a standard peptide (Chapter 10), preparing polyclonal or monoclonal antibodies (Chapter 11), and radiolabeling the standard protein (or peptide). A simple method to determine the minimal concentration of iodinating reagent required to label a protein (or peptide) is to carry out the labeling procedure and then measure, in a small aliquot, the amount of labeled protein (or peptide) that can be precipitated using 10% trichloroacetic acid (TCA). Addition of a carrier protein (e.g., 2% to 3% serum) will be required to precipitate most peptides,
since they are not readily precipitated by 10% TCA. The most common diluent is sodium phosphate–buffered saline (e.g., 0.01 to 0.05 M, pH 7.5). However, for membrane proteins, more complex buffer mixtures (containing nonionic or ionic detergents) are required to solubilize the proteins for use in a soluble RIA format. Futhermore, RIAs require the addition of a carrier protein at very large molar excess (e.g., 2% to 3% serum) over antigen and labeled tracer when working at low concentrations, to minimize losses due to adsorption to glass or plastic surfaces. In addition, the following should be checked: (1) the carrier protein concentration should prevent adsorption of radiolabeled antigen at the lowest concentration being assayed; (2) the carrier protein should be free of the antigen being assayed or any cross-reacting antigens; (3) the stability of the radiolabeled antigen in the presence of the carrier protein may be reduced, especially under prolonged incubation times at room temperature; and (4) a bacteriostatic agent (e.g., sodium azide at 0.1 g/liter or thimerosal at 0.01%, w/v) increases the stability of the assay by reducing inadvertent degradation of antigen or carrier protein by contaminating proteases. Separation of bound from free tracer can be achieved physicochemically (via electrophoresis, chromatography, or gel diffusion; see Chapter 10) or by immunoprecipitation, as in (the so-called double-antibody method in which the antibody-tracer complex is immunoprecipitated with a secondary anti-immunoglobulin raised in another species; UNIT 10.16). In the doubleantibody/PEG method described here, PEG is added to enhance the precipitation reaction. A perfect double-antibody separation system would show no tracer in the bound fraction in the absence of capture antibody and 100% of the tracer in the bound fraction after incubation with excess capture antibody. As described by Hunter (1973), any separation system may affect the system in two discrete ways. First, antigen-antibody interactions are reversible, even if the experimental system is at equilibrium. Hence, some bound antigen may dissociate if the separation removes free antigen from the system. Such dissociation would only occur if the association and dissociation constants for the antigenantibody interaction were equal. With antibodies of high avidity, this would not occur. Secondly, the separation system may disrupt the antigen-antibody complex. This could occur when nonpolar compounds are being assayed,
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since the organic solvents used to extract unbound label could also extract free and bound compounds equally. This is not a problem with the RIAs of proteins (or peptides) using the double-antibody/PEG method, since organic solvents are not used.
Anticipated Results As described by Hunter (1973), radioiodinated proteins (or peptides) are usually stable at 2◦ to 8◦ C for 30 days. High-specificactivity labeling of proteins (or peptides) can lead to a decrease in immunoreactivity due to radiochemical damage. Such damage can be differentiated from chemical damage by the fact that it is proportional to the amount of radioactivity reacted and should not vary with different batches of iodine. Chemical damage results from iodine’s propensity to oxidize sulfhydryl groups and/or from denaturation by the organic solvents, if used to extract the unreacted iodine. Iodination may also alter the immunoreactivity of proteins (or peptides) if the iodinated tyrosine is contained within the antigenic site recognized by the antibody. Hence, to minimize this possibility, trace labeling should not exceed one atom of iodine per mole of protein (or peptide). Interestingly, such a decrease in the apparent immunoreactivity may be specific for a single antiserum and not others, so it may be beneficial to use several antisera during the development of an RIA.
Time Considerations The solution RIA can be completed in 2 days or less. The incubation time for the assay using the double-antibody method is complete by 16 to 24 hr at 4◦ to 25◦ C. The separation of immunoprecipitated antigen requires different lengths of time depending on the number of samples being assayed, due to the time required to centrifuge the precipitate and to decant the supernatants (usually < 1 to 3 hr). The amount of time needed to count the samples with a γ counter depends on the specific activity of the labeled tracer, but is usually 10,000 combinations. Subsequent assemblage of heavy and light chains could yield >10 million specific antigen-binding sites. (For an excellent review of the molecular biology of the immune system, see Tonegawa, 1985.) This chapter presents the methodologies for the preparation of both monoclonal and polyclonal antibodies. Section I describes the enzyme-linked immunosorbent assay (ELISA), a highly sensitive, versatile, and quantitative technique that requires little equipment and for which critical reagents are readily available. The preparation of enzyme-antibody conjugates, which forms the basis of this assay, is described in UNIT 11.1. The versatility of ELISAs is demonstrated by the six distinct ELISA protocols presented in UNIT 11.2. These provide general methods for the detection of specific antibodies, soluble antigens, or cell-surface antigens. Protocols for determining the isotype (i.e., serological class) of antibodies are described in UNIT 11.3. The pioneering studies of Kohler and Milstein (1975) enable investigators to obtain milligram quantities of specific monoclonal antibodies after immunizing mice with relatively impure antigen. The spleen is removed from a previously immunized mouse that has a sufficient antibody titer. After separation into individual cells, B cells from the spleen are fused with myeloma cells of B cell origin to produce immortal antibody-secreting hybridoma cells of predetermined specificity. Each hybridoma cell is capable of producing an unlimited supply of a single, antigen-specific monoclonal antibody. Section II describes the preparation of these antigen-specific monoclonal antibodies in separate protocols that cover immunization of mice (UNIT 11.4), cell preparation and cell fusion for generating hybridoma cell lines (UNITS 11.5-11.7), cloning by limiting dilution to ensure the production of truly monoclonal antibodies derived from a single antibody-secreting cell (UNIT 11.8), freezing and recovery of hybridoma cell lines (UNIT 11.9), production of cell culture supernatants of monoclonal antibodies in ascites fluid (UNIT 11.10), and purification of these monoclonal antibodies by affinity chromatography (UNIT 11.11). Detection of antibody in serum, hybridoma supernatants (micrograms per milliliter), and ascites fluid (milligrams per milliliter) by ELISA is described in UNIT 11.2. Although monoclonal antibodies can be made available in unlimited quantities and without the need to purify the antigen to homogeneity, the reliance upon only monoclonal antibodies for detection and identification of antigen and cDNA clones can produce equivocal results. Because monoclonal antibodies may be specific for short peptide sequences, there is a possibility of obtaining false positives, since unrelated proteins can share small regions of homology. One way in which this uncertainty can be minimized is to utilize several different monoclonal antibodies specific for different sites on the antigen. Another disadvantage of using a monoclonal antibody is that it may have a relatively low affinity for a given antigenic site. Introduction
These problems caused by the use of monoclonal antibodies may be circumvented by generating polyclonal antibodies, which consist essentially of numerous monoclonal
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antibodies with different epitope specificities (Section III). When a purified antigen is available in sufficient amount for immunization, it is possible to obtain specific polyclonal antibodies with high affinity after repeated immunizations (UNIT 11.12; Klinman and Press, 1975). Choice of animal is determined by the amount of antiserum required for subsequent experiments. Although animals such as goats, sheep, or horses can provide larger volumes of antiserum, few institutions have adequate facilities for their care and maintenance. Mice, rats, and guinea pigs, on the other hand, may not yield sufficient volumes of antiserum. For these reasons, rabbits have become the animal of choice for the generation of polyclonal antibodies. UNIT 11.12 describes the proper preparation of antigen as well as various routes of immunization in rabbits to optimize the antibody response. Although a schedule for immunization and boosting is provided, this procedure is only a recommendation of what has worked for the author; optimal conditions should be determined empirically. UNIT 11.13 describes systems for in vitro antibody production, and subsequent measurement of secreted antibodies. UNIT 11.14 discusses the purification from serum, ascites fluid, or hybridoma supernatant of the immunoglobulin G fraction, which becomes the predominant antibody class after the booster injection. If purified antigen is in limited supply, polyclonal (as well as monoclonal) antibodies can still be raised by immunization with synthetic peptides whose sequences are based on that of the protein, which it is designed to mimic (Section IV). In this case, the selection of an immunogenic peptide is vital for obtaining a good antibody response. UNIT 11.15 discusses the necessary parameters to consider in the selection of a particular peptide sequence that will elicit an antibody that recognizes the native form of the protein. To enhance the immunogenicity of the peptide, it can be chemically cross-linked to a carrier molecule (UNIT 11.16). Such cross-linking of the peptide has been demonstrated to be helpful in generating an antibody response to peptides that might not otherwise elicit antibody production. The quantitation of specific antibody (as well as its isotypes) in polyclonal antisera, ascites fluid, or hybridoma supernatant by solid-phase radioimmunoassay (RIA) is described in UNIT 11.17. Although this method is more laborious than the nonradioactive ELISA described in UNIT 11.2, the disadvantages are offset by greater sensitivity and better reproducibility from assay to assay. Literature Cited Burnet, F.M. 1959. The Clonal Selection Theory of Acquired Immunity. Vanderbilt University Press, Nashville. Klinman, N.R. and Press, J. 1975. The B Cell specificity repertoire: Its relationship to definable subpopulations. Transplant. Rev. 24:41-83. Kohler, G. and Milstein, C. 1975. Continuous cultures of fused cells secreting antibody of predefined specificity. Nature (Lond.) 256:495-497. Tonegawa, S. 1985. The molecules of the immune system. Sci. Am. 253:122-131.
John A. Smith
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SECTION I
IMMUNOASSAYS Antigen can be detected and quantitated using an enzyme-linked immunosorbent assay (ELISA). The direct and sandwich ELISAs discussed in UNIT 11.2 are particularly useful for determining the presence or amount of antigen in samples ranging from crude bacterial lysates to highly purified protein antigen preparations. These assays require the preparation of enzyme-antibody conjugates. How these conjugates are prepared and what enzymes should be linked to the antigen-specific antibody in the conjugate are presented in UNIT 11.1.
UNIT 11.1
Conjugation of Enzymes to Antibodies Conjugation of enzymes to antibodies involves the formation of a stable, covalent linkage between an enzyme [e.g., horseradish peroxidase (HRPO), urease, or alkaline phosphatase] and an antigen-specific monoclonal or polyclonal antibody in which neither the antigen-combining site of the antibody nor the active site of the enzyme is functionally altered. The chemistry of cross-linking HRPO or urease to immunoaffinitypurified monoclonal or polyclonal antibodies (IgG) is presented in Figures 11.1.1 and
NO2
NO2 activated peroxidase NO2
NO2
NH HRPO
NH
CH2OH O
HO
Na IO4
HRPO
OH OH
CH2OH O
OH O
O
NH2 NO2
NO2
Na BH4
NO2
NH HRPO
lgG
NO2
NH
CH2OH O
HRPO
CH2OH O
Schiff base formation
stable conjugate
HO
NH lgG
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O
N lgG
Figure 11.1.1 Conjugation of horseradish peroxidase (HRPO) to antibody (IgG) using the periodate oxidation method. The method involves three chemical steps: (1) sodium periodate (NaIO4) oxidation of the carbohydrate side chains of HRPO, (2) Schiff base formation between activated peroxidase and amino groups of the antibody, and (3) sodium borohydride (NaBH4) reduction of the Schiff base to form a stable conjugate. Contributed by Scott E. Winston, Steven A. Fuller, Michael J. Evelegh, and John G.R. Hurrell Current Protocols in Molecular Biology (2000) 11.1.1-11.1.7 Copyright © 2000 by John Wiley & Sons, Inc.
O
MBS
O
N O
N O C
lgG
O O lgG
urease
NH2
O
SH
N
H
C O
N H lgG
N
H
N O
C
O O
S urease
N OH O
Figure 11.1.2 Conjugation of urease to antibody (IgG) with m-maleimidobenzoyl N-hydroxysuccinimide ester (MBS). The first step involves benzoylation of the amino groups of the antibody (IgG) oxidate oxidation method. The second step involves the thiolation of the maleimide moiety by the sulfhydryl groups of the urease enzyme.
11.1.2, respectively. The chemistry of cross-linking alkaline phosphatase to antibodies is presented in Figure 11.16.2. CONJUGATION OF HORSERADISH PEROXIDASE TO ANTIBODIES Horseradish peroxidase–antibody conjugates (Tijssen and Kurstak, 1984) can be used in ELISA (enzyme-linked immunosorbent assay; UNIT 11.2) and western blotting (UNIT 10.8).
BASIC PROTOCOL
Materials 1 mg/ml antibody solution (affinity-purified polyclonal or monoclonal antibodies; UNIT 11.11) 0.1 M phosphate buffer, pH 6.8 Horseradish peroxidase (HRPO; Sigma Type VI #P8375) 0.1 M carbonate buffer, pH 9.2 Sodium periodate (NaIO4) solution, freshly prepared Sodium borohydride (NaBH4) solution, freshly prepared Saturated ammonium sulfate [(NH4)2SO4] solution Tris/EDTA/NaCl (TEN) buffer, pH 7.2 Bovine serum albumin (BSA) Glycerol Dialysis membrane (see reagents and solutions and APPENDIX 3) Pasteur pipet fitted with glass wool Sephadex G-25, medium (size of gel matrix) 1. Dialyze 1 mg/ml antibody solution against 2 liters of 0.1 M phosphate buffer, pH 6.8, overnight at 4°C, stirring gently.
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Antibody may be polyclonal or monoclonal, purified as described in UNIT 11.11. A280/1.44 = mg IgG/ml. Concentration of antibody should be at least 1 mg/ml. Soak dialysis membrane 1 hr in 50% (v/v) aqueous ethanol, 1 hr in 10 mM NaHCO3, and 1 hr in 1 mM EDTA. Then rinse twice in distilled water and store at 4°C in phosphate buffer containing 0.01% (w/v) sodium azide.
2. Dissolve 10 mg HRPO in 1 ml 0.1 M carbonate buffer, pH 9.2. 3. Mix 0.25 ml freshly prepared NaIO4 solution with 0.25 ml of 10 mg/ml HRPO/ carbonate mixture from step 2, cap tightly, and incubate at room temperature for 2 hr in the dark (NaIO4 is light sensitive). 4. Into a Pasteur pipet fitted with glass wool and blocked at the tip with Parafilm, add 1 ml of the dialyzed 1 mg/ml antibody solution from step 1 to 0.5 ml of the 10 mg/ml HRPO solution from step 3. Add 0.25 g Sephadex G-25 to the antibody/HRPO mixture. Addition of Sephadex increases the concentration of antibody and HRPO by absorbing water. This enhances the conjugation of enzyme to antibody.
5. Incubate 3 hr at room temperature in the dark. 6. Wash column with 0.75 ml carbonate buffer to elute conjugate. 7. Add 38 µl freshly prepared NaBH4 solution to the eluate and incubate 30 min at room temperature in the dark. 8. Add 112 µl freshly prepared NaBH4 and incubate 60 min in the dark. 9. Add 0.9 ml of saturated (NH4)2SO4 solution and stir gently for 30 min at 4°C. Centrifuge 15 min at 10,000 × g, 4°C. 10. Decant, discard supernatant, and resuspend pellet in 0.75 ml TEN buffer. 11. Dialyze resuspended pellet overnight at 4°C against 2 liters TEN buffer. Change TEN solution in the morning and continue dialyzing for 4 hr. 12. Remove conjugate from dialysis membrane and add sufficient BSA to bring the conjugate solution to a final concentration of 20 mg BSA/ml. 13. Add an equal volume of glycerol and store at −20°C. ALTERNATE PROTOCOL
CONJUGATION OF UREASE TO ANTIBODIES Urease conjugates can be used in ELISA (UNIT 11.2) but not western blotting (UNIT 10.8). Additional Materials 20 mg/ml urease (Sigma Type VII #U0376; source is important) in 0.1 M phosphate buffer m-Maleimidobenzoyl N-hydroxysuccinimide ester in dimethylformamide (MBS/DMF solution) 0.143 M 2-mercaptoethanol (prepare from 14.3 M stock) Phosphate-buffered saline (PBS; APPENDIX 2)
Coagulation of Enzymes to Antibodies
12 × 75–mm glass tubes 1.5 × 5–cm PD-10 column (Pharmacia) Nitrogen tank
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1. Repeat step 1 of the basic protocol. 2. Dialyze 0.25 ml of 20 mg/ml urease in 0.1 M phosphate buffer against 2 liters 0.1 M phosphate buffer overnight at 4°C while stirring gently. 3. In the morning replace the phosphate buffer and continue to dialyze for 2 hr. 4. Remove antibody and urease solutions from dialysis membranes and place in separate glass tubes. Read A280 of antibody solution and dilute with phosphate buffer to 0.5 mg/ml. 5. Add 0.075 ml MBS/DMF solution to 1.5 ml of 0.5 mg/ml dialyzed antibody solution in a glass tube (MBS/antibody molar ratio, 120:1). Place a magnetic stir bar in the tube and stir gently at room temperature for 30 min. 6. Load on a PD-10 column (see support protocol, UNIT 10.9) preequilibrated with 100 ml phosphate buffer. Run column with phosphate buffer and collect 0.6-ml fractions. Read A280 of fractions and collect first peak that elutes. The first peak contains activated antibody and the second peak contains free MBS.
7. Pool first peak (generally about 2.5 to 3.0 ml) and add 0.15 ml of 20 mg/ml urease in 0.1 M phosphatase buffer (urease/antibody weight ratio, 4:1). 8. Stir at room temperature under N2 for 1.5 hr or until solution appears cloudy. 9. Add 0.143 M 2-mercaptoethanol to a final concentration of 2 mM (0.014 ml mercaptoethanol solution/ml urease-antibody conjugate) and stir at room temperature for 30 min. 10. Dialyze overnight at 4°C against 2 liters PBS. In the morning, replace PBS and dialyze 4 hr. 11. Add an equal volume of glycerol, divide into small aliquots, and store at −20°C (stable for 1 year). CONJUGATION OF ALKALINE PHOSPHATASE TO ANTIBODIES Alkaline phosphatase conjugates can be used in ELISA (UNIT 11.2) and western blotting (UNIT 10.8).
ALTERNATE PROTOCOL
Additional Materials 5 mg/ml antibody solution (affinity-purified polyclonal or monoclonal antibodies, UNIT 11.11) 10 mg/ml alkaline phosphatase (enzyme immunoassay grade; Boehringer Mannheim; source is important) 25% glutaraldehyde in H2O Tris/ovalbumin solution Sodium azide 1. Dialyze 5 mg/ml antibody solution as in step 1 of the basic protocol, except dialyze against PBS. 2. Remove antibody solution from dialysis membrane and place in a tube. Read A280 and dilute with PBS to 3 mg/ml. 3. Add 100 µl dialyzed antibody solution to 90 µl of 10 mg/ml alkaline phosphatase in a 1.5-ml microcentrifuge tube. Immunology
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4. Add 5 ml 25% glutaraldehyde and mix gently. Let stand at room temperature. 5. Remove 25-µl samples at time 0, 5, 10, 15, 30, 60, and 120 min and place in separate 1.5-ml microcentrifuge tubes. Add 125 µl PBS to each sample, then add 1.1 ml Tris/ovalbumin solution. Store each sample on ice until the time course is completed. 6. Dialyze the samples against PBS as described in step 1 of this protocol. Test each sample for alkaline phosphatase activity using a direct ELISA assay (UNIT 11.2) to determine which conjugation time yields the most active enzyme conjugate. 7. Repeat steps 1 to 4, but in step 4 allow the reaction to proceed for the optimal conjugation time, as determined in step 6. 8. Add sodium azide to 0.1% and store the conjugate protected from light at 4°C for up to 1 year. Alternatively, add an equal volume of glycerol and store the conjugates at −20°C for 1 year. REAGENTS AND SOLUTIONS 0.1 M carbonate buffer, pH 9.2 1.36 g sodium carbonate 7.35 g sodium bicarbonate 950 ml H2O Adjust pH to 9.2 with 1 M HCl or 1 M NaOH, if necessary Add H2O to 1 liter m-Maleimidobenzoyl N-hydroxysuccinimide ester in dimethylformamide (MBS/DMF solution) Prepare 0.25% (w/v) solution by adding 2.3 mg MBS (Pierce #22310) to 0.92 ml DMF. Use within 1 hr of preparation. MBS will deteriorate upon prolonged storage if repeatedly thawed, opened, and refrozen. Dispense into 10-mg aliquots and store desiccated at −20°C.
0.1 M phosphate buffer, pH 6.8 Stock A, 0.2 M: 31.2 g NaH2PO4 in 1 liter H20 Stock B, 0.2 M: 28.39 g Na2HPO4 in 1 liter H20 Mix 51 ml Stock A, 49 ml Stock B, and 100 ml H20 Saturated ammonium sulfate [(NH4)2SO4] solution Prepare 0.01 M Tris solution by adding 1.21 g Tris base to 990 ml water, adjust to pH 7.0, and bring to a final volume of 1 liter. Weigh 767 g (NH4)2SO4 and dissolve in 1 liter 0.01 M Tris by stirring and gently warming. Adjust the pH to 7.0 and store at 4°C. (NH4)2SO4 crystals should be seen at the bottom of the solution at 4°C. Sodium borohydride (NaBH4) solution 5 mg NaBH4/ml 0.1 mM NaOH (use immediately) Sodium periodate (NaIO4) solution 1.7l mg NaIO4/ml H20 (use immediately)
Coagulation of Enzymes to Antibodies
Tris/EDTA/NaCl (TEN) buffer, pH 7.2 To 930 ml H2O, add: 6.06 g Tris base 0.37 g Na2EDTA 8.77 g NaCl Adjust pH to 7.2 with HCl Add H2O to 1 liter
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Tris/ovalbumin buffer 0.05 M Tris⋅Cl, pH 8.0 5% ovalbumin 5 mM MgCl2 0.5% NaN3 0.5% mertiolate COMMENTARY Background Information Direct conjugation of enzymes to antibodies has greatly simplified the development and performance of many different types of immunoassays. The conjugation of HRPO (Nakane and Kawaoi, 1974) to antibody is dependent on the generation of aldehyde groups by periodate oxidation of the carbohydrate moieties on HRPO. Combination of these active aldehydes with amino groups on the antibody forms Schiff bases that, upon reduction by sodium borohydride, become stable. Horseradish peroxidase (HRPO) conjugates are useful in all types of immunological assays, but are generally less stable than urease conjugates. In addition, endogenous peroxidases may cause false positive reactions. For urease conjugation (Healey et al., 1983), cross-linking the enzyme and antibody with MBS is achieved through benzoylation of free amino groups on antibody. This is followed by thiolation of the maleimide moiety of MBS by the cysteine sulfhydryl groups of urease. The advantages of urease conjugates are their stability in solution at normal working dilutions, the rapid turnover rate of the enzyme, the easily discernible color change when substrate is added, and the fact that urease is not found in most mammalian or bacterial systems. The disadvantage is that since no precipitable substrate is available, urease conjugates cannot be used for immunohistology or western blotting. Alkaline phosphatase conjugates are useful for all types of immunological assays depending on the alkaline phosphatase substrate used (i.e., p-nitrophenyl phosphate in diethanolamine is the preferred substrate for ELISA with colorimetric detection, 4-methylumbelliferyl phosphate is useful for ELISA with fluorimetric detection, and nitroblue tetrazolium/5bromo-4-chloro-3-indolyl phosphate is the preferred substrate for western blotting). Alkaline phosphatase conjugates are as stable as urease conjugates and more stable than HRPO conjugates. Endogenous phosphatases can cause false positive reactions. However, levamisole will inhibit alkaline phosphatase in many mammalian tissues but not the alkaline phosphatase (i.e., bovine intestinal) used in the conjugates,
and for this reason levamisole may be added to the substrate solution. The one-step glutaraldehyde method (Voller et al., 1976) is the most simple available procedure for preparing alkaline phosphatase–antibody conjugates. Various alternative procedures for preparing alkaline phosphatase conjugates have been compared (Jeanson et al., 1988). The sensitivity that can be achieved with either HRPO, urease, or alkaline phosphatase conjugates is comparable and between 1 ng/ml and 10 ng/ml of antigen can be detected.
Critical Parameters The most critical parameters of both conjugation methods are the quality of enzyme and the cross-linking reagents. Several lots of these reagents should be tested as described in the protocol before conjugating to larger quantities of antibodies. It is imperative that the m-maleimidobenzoyl N-hydroxysuccinimide ester (MBS), sodium periodate (NaIO4), and sodium borohydride (NaBH4) be stored in a desiccator and that solutions containing these chemicals be prepared immediately prior to use. The method described is applicable to most antibodies and should produce conjugates that are useful for developing an ELISA for detecting sensitively and specifically a given antigen. However, not all antibodies conjugate in an identical manner. It may be necessary to vary the ratio of MBS/antibody or urease/antibody for the urease conjugation and the NaIO4/HRPO and HRPO/antibody ratios for a given HRPO conjugation. The quality and grade of alkaline phosphatase is crucial to the generation of effective conjugates. Immunoassay grade material is recommended over lower grades, and the enzyme should not be conjugated beyond its expiration date. In the case of polyclonal antisera, the specificity and titer of the antiserum will be reflected in the conjugate and any purification procedures that increase these values, such as immunoaffinity chromatography (UNIT 10.11) will enhance conjugate performance. The selection of an optimal conjugation time
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for preparing alkaline phosphatase–antibody conjugates varies for different antibodies, in particular when monoclonal antibodies are used. In contrast, polyclonal antibodies may be reliably conjugated in 120 min.
Troubleshooting There are several factors that may contribute to the production of poor enzyme-antibody conjugates. It is important to determine first whether a poor conjugate is the result of inactivation of either the antibody or the enzyme (or both) or the result of insufficient or excessive cross-linking. The affinity of the antibody for substrate can be measured by determining the presence of bound antibody with another immunoassay employing anti-antibody conjugated to a different enzyme. Enzyme activity can be measured by cleavage of substrate at different enzyme concentrations. Precipitation of material in the conjugate solution or opaque solutions are indicative of excessive cross-linking. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (UNIT 10.2) is useful for monitoring the extent of cross-linking by determining the Mr of the cross-linked species. Insufficient cross-linking usually results from the use of inactive or poor-quality crosslinking agents. Try using fresh reagents or different lots of reagent. Excessive cross-linking and inactivation of antibody or enzyme can be eliminated by either reducing the concentration of antibody and enzyme or by reducing the time of reaction. It may not be possible to generate effective alkaline phosphatase conjugates with all antibodies using the one-step glutaraldehyde method. An alternative is to try a different conjugation technique (see Jeanson et al., 1988). Another alternative is to use an anti-species antibody– alkaline phosphatase conjugate to detect the antibody in question. These reagents may be purchased or prepared using the above technique.
Anticipated Results Most monoclonal antibodies will couple to alkaline phosphatase very quickly, and a 5-min conjugation time will often be optimal. In general, polyclonal antibodies will take longer to conjugate, usually between 1 and 2 hr. The yield and titer of the resultant conjugate will depend on the original antibody’s properties and spe-
cific application. It is difficult to estimate the yield or working dilution of the conjugates, as it is dependent on numerous factors such as antibody affinity, type of ELISA, and quality of antigen. In general, the working dilutions range from 1:100 to 1:10,000.
Time Considerations The total time for conjugation is 1 to 3 days, with working times of several hours per conjugation. The majority of time is spent dialyzing or stirring. Once started, a protocol should be completed as described because the reactant products and solutions are unstable.
Literature Cited Healey, K., Chandler, H.M., Cox, J.C., and Hurrell, J.G.R. 1983. A rapid semi-quantitative capillary enzyme immunoassay for digoxin. Clin. Chim. Acta 134:51-58. Jeanson, A., Cloes, J.-M., Bouchet, M., and Rentier, B. 1988. Comparison of conjugation procedures for the preparation of monoclonal antibody-enzyme conjugates. J. Immunol. Methods 111:261270. Nakane, P.K. and Kawaoi, A. 1974. Peroxidase-labeled antibody. A new method of conjugation. J. Histochem. Cytochem. 22:1084-1091. Tijssen, P. and Kurstak, E. 1984. Highly efficient and simple methods for the preparation of peroxidase and active peroxidase-antibody conjugates for enzyme immunoassay. Anal. Biochem. 136:451457. Voller, A., Bidwell, D.E., and Barlett, A. Enzyme immunoassays in diagnostic medicine. Bull. W.H.O. 53:55-65.
Key Reference Van Vunakis, H. and Langone, J.J., eds. 1980. Immunochemical techniques. Methods Enzymol. 70:1-525. An excellent collection of articles on immunoassay techniques, including several on enzyme-antibody conjugation techniques.
Contributed by Scott E. Winston, Steven A. Fuller, and Michael J. Evelegh ADI Diagnostics Rexdale, Ontario John G.R. Hurrell Boehringer Mannheim Indianapolis, Indiana
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Enzyme-Linked Immunosorbent Assays (ELISA)
UNIT 11.2
This unit describes six different ELISA systems for the detection of specific antibodies, soluble antigens, or cell-surface antigens. In all six systems, soluble reactants are removed from solution after specifically binding to solid-phase reactants. Table 11.2.1 summarizes the different ELISA protocols, which are illustrated in Figures 11.2.1-11.2.6. In the first four protocols, solid-phase reactants are prepared by adsorbing an antigen or antibody onto plastic microtiter plates; in the next two protocols, the solid-phase reactants are cell-associated molecules. In all protocols, the solid-phase reagents are incubated with secondary or tertiary reactants covalently coupled to an enzyme. Table 11.2.1
Summary of ELISA Protocols
ELISA protocol
Uses
Required reagents
Comments
Indirect
Antibody screening; epitope mapping
Direct competitive
Antigen screening; detect soluble antigen
Does not require the use of preexisting specific antibodies; requires relatively large amounts of antigen Rapid assay with only two steps; excellent for measuring antigenic cross-reactivity
Antibody-sandwich
Antigen screening; detect soluble antigen
Double antibody– sandwich
Antibody-screening; epitope mapping
Direct cellular
Screen cells for expression of antigen; measure cellular antigen expression
Antigen, pure or semipure: test solution containing antibody; enzyme conjugate that binds Ig of immunized species Antigen, pure or semipure; test solution containing antigen; enzyme-antibody conjugate specific for antigen Capture antibody (purified or semi-purified specific antibody); test solution containing antigen; enzymeantibody conjugate specific for antigen Capture antibody: (specific for Ig of immunized species); test solution containing antigen; enzymeantibody conjugate specific for antigen Cells that express antigen of interest; enzyme-antibody conjugate specific for cellular antigen
Indirect cellular
Screen for antibodies against cellular antigens
Cells used for immunizing; test solution containing antibodies; enzyme conjugate that binds Ig of immunized species
Most sensitive antigen assay; requires relatively large amounts of pure or semi-pure specific antibody (capture antibody) Does not require purified antigen; relatively long assay with five steps
Sensitive assay for bulk screening; insensitive to heterogeneity of expression in mixed population of cells May not detect antibodies specific for cellular antigens expressed at a low density Immunology
Contributed by Peter Hornbeck, Scott E. Winston, and Steven A. Fuller Current Protocols in Molecular Biology (1991) 11.2.1-11.2.22 Copyright © 2000 by John Wiley & Sons, Inc.
11.2.1 Supplement 15
Unbound conjugates are washed out and a chromogenic or fluorogenic substrate is added. As the substrate is hydrolyzed by the bound enzyme conjugate, a colored or fluorescent product is generated. Finally, the product is detected visually or with a microtiter plate reader. The amount of product generated is proportional to the amount of analysate in the test mixture. The first support protocol can be used to optimize the different ELISAs. The second support protocol provides a method for preparing lysates for use as test antigen from bacterial cultures containing expressed protein. BASIC PROTOCOL
INDIRECT ELISA TO DETECT SPECIFIC ANTIBODIES This assay is useful for screening antisera or hybridoma supernatants for specific antibodies when milligram quantities of purified or semipurified antigen are available (1 mg of purified antigen will permit screening of 80 to 800 microtiter plates; Fig. 11.2.1). Antibodies are detected by coating the wells of microtiter plates with antigen, incubating the coated plates with test solutions containing specific antibodies, and washing away unbound antibodies. A solution containing a developing reagent, (e.g., alkaline phosphatase conjugated to protein A, protein G, or antibodies against the test solution antibodies) is then added to the plate. After incubation, unbound conjugate is washed away and substrate solution is added. After a second incubation, the amount of substrate hydrolyzed is assessed with a spectrophotometer or spectrofluorometer. The measured amount is proportional to the amount of specific antibody in the test solution. Visual inspection can also be used to detect hydrolysis. Materials Developing reagent: protein A–alkaline phosphatase conjugate (Sigma #P9650), protein G–alkaline phosphatase conjugate (Calbiochem #539304), or anti-Ig-alkaline phosphatase conjugate (UNIT 11.1) Antigen solution PBS (APPENDIX 2) containing 0.05% NaN3 (PBSN) Water, deionized or distilled Blocking buffer Test antibody samples 4-methylumbelliferyl phosphate (MUP) or p-nitrophenyl phosphate (NPP) substrate solution 0.5 M NaOH (optional) Multichannel pipet and disposable pipet tips Immulon 2 (Dynatech #011-010-3450), Immulon 4 (Dynatech #011-010-3850), or equivalent microtiter plates Plastic squirt bottles Microtiter plate reader (optional)—spectrophotometer with 405-nm filter or spectrofluorometer (Dynatech #011-970-1900) with 365-nm excitation filter and 450-nm emission filter Determine developing reagent and antigen concentrations 1. Determine the optimal concentration of the developing reagent (conjugate) by criss-cross serial dilution analysis (see first support protocol).
Enzyme-Linked Immunosorbent Assays
Good conjugates of many specificities are available commercially. Choice of developing reagent (i.e., conjugate) is determined by the goals of the assay. If it is necessary to detect all antibodies that bind to antigen, conjugates prepared with antibodies specific for Ig κ and λ light chains should be used. Alternatively, protein A– or protein G–enzyme conjugates may be preferable when screening monoclonal antibodies. Specific monoclonal antibodies that bind protein A or protein G are easy to purify and characterize.
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2. Determine the final concentration of antigen coating reagent by criss-cross serial dilution analysis (see first support protocol). Prepare an antigen solution in PBSN at this final concentration. The final concentration of antigen is usually 0.2 to 10.0 µg/ml. Prepare ∼6 ml antigen solution for each plate. Pure antigen solution concentrations are usually ≤2 ìg/ml. Although pure antigen preparations are not essential, >3% of the protein in the antigen solution should be the antigen. The total concentration of protein in the antigen solution should be increased for semipurified antigen preparations. Do not raise the total protein concentration in the antigen solution to >10 ìg/ml, since this concentration usually saturates >85% of the available sites on Immulon microtiter plates. For some antigens, coating may occur more efficiently at different pHs.
Coat plate with antigen 3. Using a multichannel pipet and tips, dispense 50 µl antigen solution into each well of an Immulon microtiter plate. Tap or shake the plate to ensure that the antigen solution is evenly distributed over the bottom of each well.
Ab
= detected Ab
coat well with anitgen Ag
Ag
block
incubate with antibody Ab
Ab
Ag
Ag
wash E
E
Ab
Ab
Ab
Ab
Ag
Ag
incubate with antibodyenzyme conjugate
wash E
E
Ab
Ab
Ab
Ab
Ag
Ag
add substrate and observe color change or fluorescence
Figure 11.2.1 Indirect ELISA to detect specific antibodies. Ag = antigen; Ab = antibody; E = enzyme.
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4. Wrap coated plates in plastic wrap to seal and incubate overnight at room temperature or 2 hr at 37°C. Individual adhesive plate sealers are sold commercially but plastic wrap is easier to use and works as well. Sealed plates can be stored at 4°C with antigen solution for months.
5. Rinse coated plate over a sink by filling wells with deionized or distilled water dispensed either from a plastic squirt bottle or from the tap. Flick the water into the sink and rinse with water two more times, flicking the water into the sink after each rinse. Block residual binding capacity of plate 6. Fill each well with blocking buffer dispensed as a stream from a squirt bottle and incubate 30 min at room temperature. Residual binding capacity of the plate is blocked in this step. Tween 20 (0.05%) by itself is more effective at blocking than any protein tested, but because the combination of protein and Tween 20 may be more effective than Tween 20 alone in some cases, bovine serum albumin (BSA; 0.25%) is included in the blocking buffer.
7. Rinse plate three times in water as in step 5. After the last rinse, remove residual liquid by wrapping each plate in a large paper tissue and gently flicking it face down onto several paper towels laying on the benchtop. Rinsing with water is cheaper and easier than rinsing with buffered solutions and is as effective.
Add antibody to plate 8. Add 50 µl antibody samples diluted in blocking buffer to each of the coated wells, wrap plate in plastic wrap, and incubate ≥2 hr at room temperature. While enough antibody may be bound after 1 to 2 hr to generate a strong signal, equilibrium binding is generally achieved after 5 to 10 hr. Thus, the specific signal may be significantly increased by longer incubations. For this and all steps involving the delivery of aliquots of many different solutions to microtiter plates with multichannel pipets, such as the primary screening of hybridoma supernatants, the same pipet tips can be reused for hundreds of separate transfers. Wash tips between transfers by expelling any liquid remaining in the tips onto an absorbent surface of paper tissues, rinsing tips five times in blocking buffer, and carefully expelling any residual liquid from tips onto the tissues. Avoid bubbles in the tips; any tip with intractable bubbles should be replaced.
Wash the plate 9. Rinse plate three times in water as in step 5. 10. Fill each well with blocking buffer, vortex, and incubate 10 min at room temperature. Plates are vortexed to remove any reagent remaining in the corners of the wells.
11. Rinse three times in water as in step 5. After the final rinse, remove residual liquid as in step 7. Add developing reagent to plate 12. Add 50 µl developing reagent in blocking buffer (at optimal concentration determined in step 1) to each well, wrap in plastic wrap, and incubate ≥2 hr at room temperature. The strength of the signal may be increased by longer incubations (see annotation to step 8). Enzyme-Linked Immunosorbent Assays
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13. Wash plates as in steps 9 to 11. After final rinsing, plates may be wrapped in plastic wrap and stored for months at 4°C prior to adding substrate.
Add substrate and measure hydrolysis 14. Add 75 µl MUP or NPP substrate solution to each well and incubate 1 hr at room temperature. 15. Monitor hydrolysis qualitatively by visual inspection or quantitatively with a microtiter plate reader (see below). Hydrolysis can be stopped by adding 25 µl of 0.5 M NaOH. a. Visually, hydrolysis of NPP can be detected by the appearance of a yellow color. If using a microtiter plate reader to measure NPP hydrolysis, use a 405-nm filter. b. Visually, hydrolysis of MUP can be monitored in a darkened room by illumination with a long-wavelength UV lamp. If using a microtiter plate spectrofluorometer to measure MUP hydrolysis, use a 365-nm excitation filter and a 450-nm emission filter. The fluorogenic system using the MUP substrate is 10 to 100 times faster than the chromogenic system using NPP. Furthermore, the rate of spontaneous hydrolysis of MUP is much lower than that of NPP. To detect bound antibodies that are present at low concentration, measure hydrolysis at a later time. To calculate when to measure hydrolysis the second time, remember that the amount of hydrolysis is almost directly proportional to the time of hydrolysis. For example, if the hydrolysis in the wells of interest reads 200 at 1 hr and a reading of 2000 is desired, incubate the plate ∼10 hr before taking the second reading.
DIRECT COMPETITIVE ELISA TO DETECT SOLUBLE ANTIGENS This assay is used to detect or quantitate soluble antigens and is most useful when both a specific antibody and milligram quantities of purified or semipurified antigen are available (Fig. 11.2.2). To detect soluble antigens, plates are coated with antigen and the binding of specific antibody-enzyme conjugates to antigen-coated plates is inhibited by test solutions containing soluble antigen. After incubation with mixtures of the conjugate and inhibitor in antigen-coated wells, unbound conjugate is washed away and substrate is added. The amount of antigen in the test solutions is proportional to the inhibition of substrate hydrolysis and can be quantitated by interpolation onto an inhibition curve generated with serial dilutions of a standard antigen solution.
ALTERNATE PROTOCOL
The direct assay may also be adapted as an indirect assay by substituting specific antibody for specific antibody-enzyme conjugate. The amount of specific antibody bound is then detected using a species-specific or isotype-specific conjugate as a tertiary reactant. Additional Materials Specific antibody–alkaline phosphatase conjugate (UNIT 11.1) Standard antigen solution Test antigen solutions Round- or cone-bottom microtiter plates 1. Determine the optimal concentration of coating reagent and antibody–alkaline phosphatase conjugate by criss-cross serial dilution analysis in which the concentrations of both the antigen (coating reagent) and the conjugate (developing reagent) are varied (see first support protocol). Prepare a 2× conjugate solution by diluting the Immunology
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specific antibody–alkaline phosphatase conjugate in blocking buffer to twice the optimal concentration. The final concentration is usually 25 to 500 ng antibody/ml. Prepare 3 ml antibody–alkaline phosphatase conjugate for each plate.
2. Coat and block wells of an Immulon microtiter plate with 50 µl antigen solution as in steps 2 to 7 of the basic protocol. 3. Prepare six 1:3 serial dilutions of standard antigen solution in blocking buffer (see first support protocol for preparation of serial dilutions)—these antigen concentrations will be used in preparing a standard inhibition curve (see step 10). Antigen concentrations should span the dynamic range of inhibition. The dynamic range of inhibition is defined as that range of inhibitor concentrations wherein changes of inhibitor concentration produce detectable changes in the amount of inhibition. The dynamic range of inhibition is empirically determined in an initial assay in which antigen concentration is typically varied from the micromolar (10−6 M) to the picomolar (10−12 M) range. For most protein antigens, initial concentration should be ∼10 ìg/ml, followed by nine 1:4 serial dilutions in blocking buffer. These antigen dilutions are assayed for their ability to inhibit the binding of conjugate to antigen-coated plates under standard assay conditions. From this initial assay, six 1:3 antigen dilutions spanning the dynamic range of inhibition are selected for further use as standard antigen-inhibitor dilutions. Prepare ≥75 ìl of each dilution for each plate to be assayed. Inhibitor curves are most sensitive in the region of the curve where small changes in inhibitor concentrations produce maximal changes in the amount of inhibition. This
with inhibitor antigen
without inhibitor antigen Ag
= detected AG
coat well with antigen
Ag
Ag
Ag
Ag
block E Ab Ag Ag
E
E
E
Ab
Ab
Ab
Ag
Ag
Ag
incubate with antibodyenzyme conjugate with or without inhibitor antigen
wash
Ag
Enzyme-Linked Immunosorbent Assays
E
E
E
Ab
Ab
Ab
Ag
Ag
Ag
add substrate and measure inhibition of color change or fluorescence
Figure 11.2.2 Direct competitive ELISA to detect soluble antigens. Ag = antigen; Ab = antibody; E = enzyme.
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region of the curve normally spans 15% to 85% inhibition. In most systems, this range of inhibition is produced by concentrations of inhibitor between 1 and 250 ng/ml.
4. Mix and incubate conjugate and inhibitor by adding 75 µl of 2× conjugate solution (from step 1) to each well of a round- or cone-bottom microtiter plate, followed by 75 µl inhibitor—either test antigen solution or standard antigen solution (from step 3). Mix the conjugate and inhibitor solutions by pipetting up and down in the pipet tip three times (see annotation to step 8 in the basic protocol) and incubate ≥30 min at room temperature. For accurate quantitation of the amount of antigen in the test solutions, test antigen solutions should inhibit conjugate binding between 15% to 85%. It may be necessary to assay two or three different dilutions of the test solutions to produce inhibitions within this range.
5. Prepare uninhibited control samples by mixing equal volumes of 2× conjugate solution and blocking buffer. 6. Transfer 50 µl of the mixture of conjugate plus inhibitor (from step 4) or conjugate plus blocking buffer (from step 5) to an antigen-coated plate (from step 2) and incubate 2 hr at room temperature. If samples are to be assayed in duplicate, the duplicates should be in adjacent columns on the same plate. Reserve column 11 for uninhibited control samples (step 5) and column 12 for substrate alone without any conjugate. If the concentration of antigen in the test samples is to be accurately quantitated, dilutions of homologous antigen solutions (step 3) should be included on each plate.
7. Wash plate as in steps 9 to 11 of the basic protocol. 8. Add 75 µl of MUP or NPP substrate solution to each well and incubate 1 hr at room temperature. 9. Read plates on the microtiter plate reader after ≥1 hr, at which time enough substrate has been hydrolyzed in the uninhibited reactions to permit accurate measurement of the inhibition. 10. Prepare a standard antigen-inhibition curve constructed from the inhibitions produced by the dilutions of the standard antigen solutions from step 3. Plot antigen concentration on the x axis, which is a log scale, and fluorescence or absorbance on the y axis, which is a linear scale. 11. Interpolate the concentration of antigen in the test solutions from the standard antigen-inhibition curve. The dynamic range of the inhibition curve may deviate from linearity if the specific antibodies are heterogeneous and possess significantly different affinities or if the standard antigen preparation contains heterogeneous forms of the antigen. Antigen concentration in test samples can be accurately interpolated from the inhibition curve as long as the test antigen is antigenically identical to the standard antigen and the concentration of test antigen falls within the dynamic range of inhibition.
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ALTERNATE PROTOCOL
ANTIBODY-SANDWICH ELISA TO DETECT SOLUBLE ANTIGENS Antibody-sandwich ELISAs may be the most useful of the immunosorbent assays for detecting antigen because they are frequently between 2 and 5 times more sensitive than those in which antigen is directly bound to the solid phase (Fig. 11.2.3). To detect antigen, the wells of microtiter plates are coated with specific (capture) antibody followed by incubation with test solutions containing antigen. Unbound antigen is washed out and a different antigen-specific antibody conjugated to enzyme (i.e., developing reagent) is added, followed by another incubation. Unbound conjugate is washed out and substrate is added. After another incubation, the degree of substrate hydrolysis is measured. The amount of substrate hydrolyzed is proportional to the amount of antigen in the test solution. Additional Materials Specific antibody or immunoglobulin fraction from antiserum or ascites fluid, or hybridoma supernatant (UNIT 11.10), or bacterial lysate (second support protocol)
Ag
= detected Ag
coat well with antibody
Ab
Ab
block
incubate with antigen Ag
Ag
Ab
Ab
wash
E
E
Ab
Ab
Ag
Ag
Ab
Ab
incubate with antibody-enzyme conjugate
wash
Enzyme-Linked Immunosorbent Assays
E
E
Ab
Ab
Ag
Ag
Ab
Ab
add substrate and observe color change or fluorescence
Figure 11.2.3 Antibody-sandwich ELISA to detect antigen. Ag = antigen; Ab = antibody; E = enzyme.
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1. Prepare the capture antibody by diluting specific antibody or immunoglobulin fraction in PBSN to a final concentration of 0.2 to 10 µg/ml. The capture antibodies can be monoclonal or polyclonal. If the immunoglobulin fraction from an antiserum or ascites fluid is used, the concentration of total protein may need to be increased to compensate for the lower content of specific antibody. Little advantage is gained by increasing the total protein concentration in the capture antibody solution beyond 10 ìg/ml.
2. Determine the concentration of capture antibody and conjugate necessary to detect the desired concentration of antigen by criss-cross serial dilution analysis (see first support protocol). Prepare a capture antibody solution in PBSN at this concentration. 3. Coat wells of an Immulon plate with capture-antibody solution as in steps 3 to 5 of the basic protocol. 4. Block wells as in steps 6 and 7 of the basic protocol. 5. Prepare a standard antigen-dilution series by successive 1:3 dilutions of the homologous antigen stock in blocking buffer (see first support protocol). In order to measure the amount of antigen in a test sample, the standard antigen-dilution series needs to span most of the dynamic range of binding. This range typically spans from 0.1 to 1000 ng antigen/ml. The dynamic range of binding is defined as that range of antigen concentrations wherein small, incremental changes in antigen concentration produce detectable differences in the amount of antigen bound (see annotation to step 3, in the preceding alternate protocol). In most assay systems, the amount of antigen in a test solution is most accurately interpolated from the standard curve if it produces between 15% to 85% of maximal binding. NOTE: While standard curves are necessary to accurately measure the amount of antigen in test samples, they are unnecessary for qualitative “yes/no” answers.
6. Prepare dilutions of test antigen solutions in blocking buffer. It may be necessary to assay one or two serial dilutions of the initial antigen test solution to ensure that at least one of the dilutions can be accurately measured. For most assay systems, test solutions containing 1 to 100 ng/ml of antigen can be accurately measured.
7. Add 50-µl aliquots of the antigen test solutions and the standard antigen dilutions (from step 5) to the antibody-coated wells and incubate ≥2 hr at room temperature. For accurate quantitation, samples should be run in duplicate or triplicate, and the standard antigen-dilution series should be included on each plate (see step 5). Pipetting should be performed rapidly to minimize differences in time of incubation between samples.
8. Wash plate as in steps 9 to 11 of the basic protocol. 9. Add 50 µl specific antibody–alkaline phosphatase conjugate and incubate 2 hr at room temperature. The conjugate concentration is typically 25 to 400 ng specific antibody/ml. When the capture antibody is specific for a single determinant, the conjugate must be prepared from antibodies which recognize different determinants that remain available after the antigen is bound to the plate by the capture antibody.
10. Wash plate as in steps 9 to 11 of the basic protocol. 11. Add 75 µl of MUP or NPP substrate solution to each well and incubate 1 hr at room temperature. Immunology
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12. Read the plate on a microtiter plate reader. To quantitate low-level reactions, the plate can be read again after several hours of hydrolysis.
13. Prepare a standard curve constructed from the data produced by serial dilutions of the standard antigen (step 5). Plot antigen concentration on the x axis which is a log scale, and fluorescence or absorbance on the y axis which is a linear scale. 14. Interpolate the concentration of antigen in the test solutions from the standard curve. ALTERNATE PROTOCOL
DOUBLE ANTIBODY–SANDWICH ELISA TO DETECT SPECIFIC ANTIBODIES This assay is especially useful when screening for specific antibodies in cases when a small amount of specific antibody is available and purified antigen is unavailable (Fig. 11.2.4). Additionally, this method can be used for epitope mapping of different monoclonal antibodies that are directed against the same antigen. Plates are coated with capture antibodies specific for immunoglobulin from the immunized species. The test antibody solution is incubated on the plates coated with the capture antibodies. Plates are then washed, incubated with antigen, washed again, and incubated with specific antibody conjugated to an enzyme. After incubation, unbound conjugate is washed out and substrate is added. Wells that are positive for hydrolysis may contain antibodies specific for the antigen. Additional Materials Capture antibodies specific for immunoglobulin from the immunized species Specific antibody–alkaline phosphatase conjugate 1. Coat wells of an Immulon microtiter plate with 50 µl of 2 to 10 µg/ml capture antibodies as in steps 2 to 5 of the basic protocol. NOTE: Capture antibodies must not bind the antigen or conjugate antibodies. When analyzing hybridoma supernatants or ascites fluid, coat plates with 2 ìg/ml capture antibody. When analyzing antisera, coat plates with 10 ìg/ml capture antibody.
2. Block wells as in steps 6 and 7 of the basic protocol. 3. Prepare dilutions of test antibody solutions in blocking buffer. Add 50 µl to coated wells and incubate ≥2 hr at room temperature. Hybridoma supernatants, antisera, or ascites fluid can be used as the test samples. Dilute hybridoma supernatants 1:5 and antisera or ascites fluid 1:200.
4. Wash plate as in steps 9 to 11 of the basic protocol. 5. Prepare an antigen solution in blocking buffer containing 20 to 200 ng/ml antigen. Although purified antigen preparations are not essential, the limit of detectability for most protein antigens in this type of system is 2 to 20 ng/ml. A concentration of 20 to 200 ng antigen/ml is recommended.
6. Add 50-µl aliquots of the antigen solution to antibody-coated wells and incubate ≥2 hr at room temperature. 7. Wash plate as in steps 9 to 11 of the basic protocol. Enzyme-Linked Immunosorbent Assays
8. Add 50 µl specific antibody–alkaline phosphatase conjugate to the wells and incubate 2 hr at room temperature.
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The conjugate antibodies must not react with the capture antibody or the test antibody. The conjugate concentration is typically between 25 to 500 ng specific antibody/ml, and should be high enough to result in ∼0.50 absorbance units/hr at 405 nm when using NPP as a substrate or a signal of 1000 to 1500 fluorescence units/hr when using MUP as a substrate. If no specific antibodies from the appropriate species are available to serve as a positive control, then a positive control system should be constructed out of available reagents. Such reagents can be found in Linscott’s Directory of Immunological and Biological Reagents.
Ab
= detected Ab
coat well with capture antibody
Ab
Ab
block
incubate with antibody Ab Ab
Ab Ab
wash
incubate with antigen Ag
Ag
Ab Ab
Ab Ab
wash E
E
Ab
Ab
Ag
Ag
Ab Ab
Ab Ab
incubate with antibody-enzyme conjugate
wash E
E
Ab
Ab
Ag
Ag
Ab Ab
Ab Ab
add substrate and observe color change or fluorescence
Figure 11.2.4 Double antibody–sandwich ELISA to detect specific antibodies. Ag = antigen; Ab = antibody; E = enzyme.
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9. Wash plate as in steps 9 to 11 of the basic protocol. 10. Add 75 µl of MUP or NPP substrate solution to each well and incubate 1 hr at room temperature. After 1 hr, examine hydrolysis visually or spectrophotometrically (see step 15 of the basic protocol). In order to detect low-level reactions, the plate can be read again after several hours or days of hydrolysis.
11. Check for false positives by rescreening samples that test positive for antigen-specific antibody. For each positive sample, coat four wells with capture antibody and arm the capture antibody with test antibody (steps 1 to 4). Incubate two of the wells with antigen (steps 5 to 7) and two of the wells with blocking buffer. Add conjugate and substrate to all four wells (steps 8 to 10) and measure hydrolysis after 1 hr. This procedure will eliminate false positives resulting from test antibodies that react with the enzyme-antibody complex. ALTERNATE PROTOCOL
DIRECT CELLULAR ELISA TO DETECT CELL-SURFACE ANTIGENS The expression of cell-surface antigens or receptors is measured using existing antibodies or other ligands specific for cell-surface molecules (Fig. 11.2.5). Cells are incubated with enzyme conjugated to antibodies that are specific for a cell-surface molecule. Unbound conjugate is washed away and substrate is added. The level of antigen expression is proportional to the amount of substrate hydrolysis. This procedure can be as sensitive as flow cytometry analysis in quantitating the level of antigen expression on a population of cells (Coligan et al., 1991). Unlike the flow cytometry analysis, however, this method is not sensitive for mixed populations. This assay can be converted to an indirect assay by substituting biotinylated antibody for the enzyme-antibody conjugate, followed by a second incubation with avidin–alkaline phosphatase.
E
E
Ab
Ab
Ag
Ag
C
C
Ag = detected Ag
incubate cells with antibody-enzyme conjugate
wash and centrifuge
Enzyme-Linked Immunosorbent Assays
E
E
Ab
Ab
Ag
Ag
C
C
add substrate, resuspend cells, and observe color change or fluorescence
Figure 11.2.5 Direct cellular ELISA to detect cell-surface antigens. Ab = antibody; E = enzyme; C = cell.
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Additional Materials Cell samples Specific antibody–alkaline phosphatase conjugate (see second support protocol) Wash buffer, ice-cold Cone- or round-bottom microtiter plates Sorvall H-1000B rotor (or equivalent) 1. Determine the optimal number of cells per well and the antibody-conjugate concentration by criss-cross serial dilution analysis (see first support protocol) using variable numbers of positive- and negative-control cell samples and varying concentrations of antibody-biotin conjugate. Titrate cells initially at 1-5 × 105/well and conjugate at 0.5 to 10 ìg/ml. For preparation and handling of cells, consult steps 2 to 5. Because eukaryotic cells express variable amounts of alkaline phosphatase, test cells must be assayed in a preliminary experiment for alkaline phosphatase by incubation with substrate alone. If the test cells express unacceptable levels of alkaline phosphatase, another enzyme conjugate such as β-galactosidase should be used. Both chromogenic and fluorogenic substrates are available for β-galactosidase.
2. Centrifuge cell samples in a table-top centrifuge 5 min in Sorvall H-1000B rotor at 1500 rpm (450 × g), 4°C, in a 15- to 50-ml centrifuge tube. Count cells (APPENDIX 3) and resuspend in ice-cold wash buffer at 1-5 × 106 cells/ml. If the surface antigen retains its antigenicity after fixation, cells may be fixed at the beginning of the experiment—but do not fix cells unless it can be demonstrated that the antigenicity is retained after fixation. Fix cells by suspending in glutaraldehyde (0.5% final; from a 25% stock, EM grade Sigma #G5882), and incubating 30 min at room temperature. Pellet cells, resuspend in PBSLE (see second support protocol), and incubate for 30 min at 37°C. Wash twice in PBSLE and resuspend in wash buffer. Cells can be kept for months at 4°C after fixation.
3. Dispense 100 µl of cell suspension (1-5 × 105 cells) into wells of cone- or round-bottom microtiter plates, and centrifuge 1 min at 450 × g, 4°C. Remove supernatant by vacuum aspiration, and disrupt pellet by briefly shaking microtiter plate on a vortex mixer or microtiter plate shaker. 4. Resuspend pellet in 100 µl of conjugate in ice-cold wash buffer at the optimal concentration (see step 1). Incubate 1.5 hr at 4°C, resuspending cells by gently shaking at 15-min intervals. Be careful not to splash cell suspensions out of wells.
5. Centrifuge cells 1 min at 450 × g, 4°C, remove supernatant by vacuum aspiration, briefly vortex pellet, and resuspend in 200 µl ice-cold wash buffer. Repeat three times. 6. Add 100 µl MUP or NPP substrate solution. Incubate 1 hr at room temperature, resuspending cells by gently shaking at 15-min intervals during hydrolysis. 7. Determine extent of hydrolysis by visual inspection or using a microtiter plate reader.
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ALTERNATE PROTOCOL
INDIRECT CELLULAR ELISA TO DETECT ANTIBODIES SPECIFIC FOR SURFACE ANTIGENS This assay is designed to screen for antibodies specific for cell-surface antigens (Fig. 11.2.6). Antibodies against surface antigens are detected by incubating whole cells with a test solution containing the primary antibody. The unbound antibody is washed away and the cells are then incubated with an enzyme conjugated to antibodies specific for the primary antibody. Unbound enzyme conjugate is washed away and substrate solution added. The level of bound primary antibody is proportional to the amount of substrate hydrolysis. Additional Materials Positive-control antibodies (i.e., those that react with the experimental cells and are from the immunized species) Negative-control antibodies (i.e., those that do not react with the experimental cells) Test antibody solution Antibody or F(ab′)2 (against immunoglobulin from the immunized species) conjugated to alkaline phosphatase Cone- or round-bottom microtiter plates
Ab
= detected Ab
incubate cells with antibody Ab
Ab
Ag
Ag
C
C
wash and centrifuge E
E
Ab
Ab
Ab
Ab
Ag
Ag
C
C
incubate with antibody-enzyme conjugate
wash and centrifuge
Enzyme-Linked Immunosorbent Assays
E
E
Ab
Ab
Ab
Ab
Ag
Ag
C
C
add substrate, resuspend cells, and observe color change or fluorescence
Figure 11.2.6 Indirect cellular ELISA to detect antibodies specific for surface antigens. Ab = antibody; E = enzyme; C = cell.
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1. Centrifuge and resuspend cell samples as in step 2 of the previous alternate protocol at 1-5 × 106 cells/ml. Because this technique detects antibodies against uncharacterized epitopes, fixation prior to analysis is not recommended. Fixation may destroy the antigenicity of the epitope. All steps must be performed at 4°C in physiological buffers containing NaN3. Because eukaryotic cells express variable amounts of alkaline phosphatase, test cells must be assayed for alkaline phosphatase activity. If the endogenous alkaline phosphatase level is too high, another enzyme should be substituted for alkaline phosphatase in the antibodyenzyme conjugate (see annotation to step 1 of the previous alternate protocol).
2. In preliminary assays, determine the optimal number of cells per well and conjugate concentration by criss-cross serial dilution analysis using positive- and negative-control antibodies instead of test antibodies (see first support protocol). In adapting the criss-cross serial dilution analysis, whole cells replace the solid-phase coating reagent; see techniques for handling cells are outlined in steps 3 to 8. Set up titrations by varying the number of cells between 1 × 105 and 5 × 105/well, the concentration of positive- and negative-control antibodies between 0.1 and 10 µg/ml, and the concentration of antibody-enzyme conjugate between 0.1 and 10 µg/ml. 3. Dispense 100 µl of cell suspension (1-5 × 105 cells) into wells of round- or cone-bottom microtiter plates. Centrifuge 1 min at 1500 rpm, 4°C, remove supernatant by vacuum aspiration, and disrupt pellet by briefly shaking microtiter plate on the vortex mixer. 4. Resuspend cells in 100 µl solutions containing 1 to 10 µg/ml test antibody or control antibodies in ice-cold wash buffer. Incubate 1.5 hr at 4°C, resuspending cells by gently shaking at 15-min intervals. Be careful not to splash cell suspensions out of wells.
5. Centrifuge cells 1 min at 1500 rpm, 4°C, remove supernatant by vacuum aspiration, briefly vortex pellet, and resuspend in 200 µl ice-cold wash buffer. Repeat twice. 6. Resuspend pellet in 100 µl enzyme-antibody conjugate or F(ab′)2-enzyme conjugate diluted in ice-cold wash buffer. The optimal concentration of antibody, determined in step 2, is usually 100 to 500 ng/ml. Incubate 1.5 hr at 4°C, resuspending cells by gently shaking at 15-min intervals. When working with cells that may express Fc receptors, it is best to use enzyme conjugated to F(ab′)2 fragments. F(ab′)2 fragments have had the Fc portion of the antibody enzymatically removed and no longer bind to Fc receptors.
7. Wash cells as in step 5. Repeat three times. 8. Add 100 µl MUP or NPP substrate solution. Allow hydrolysis to proceed until the signal has reached the desired levels; resuspend cells by gently shaking at 15 min intervals during hydrolysis. If desired, stop hydrolysis by adding 25 µl of 0.5 M NaOH. 9. Determine extent of hydrolysis by visual inspection or spectrophotometrically using a microtiter plate reader.
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CRISS-CROSS SERIAL DILUTION ANALYSIS TO DETERMINE OPTIMAL REAGENT CONCENTRATIONS
SUPPORT PROTOCOL
Serial dilution titration analyses are performed to determine optimal concentrations of reagents to be used in ELISAs. In this protocol, all three reactants in a three-step ELISA—a primary solid-phase coating reagent, a secondary reagent that binds the primary reagent, and an enzyme-conjugated tertiary developing reagent that binds to the secondary reagent—are serially diluted and analyzed by a criss-cross matrix analysis (Fig 11.2.7). Once the optimal concentrations of reagents to be used under particular assay conditions are determined, these variables are kept constant from experiment to experiment. The coating (primary), secondary, and developing (tertiary) reagents will vary depending upon which of the previous protocols needs to be optimized. Additional Materials Coating reagent Secondary reagent Developing reagent 17 × 100–mm and 12 × 74–mm test tubes
Secondary reactant
heterologous (antigen)
(ng/ml)
200
50
12.5
3.12
0.78
0
200
50
12.5
3.12
0.78
0 A
500
over
over
over
3200
1000
0
500
120
40
20
10
B
250
over
over
over
2060
560
0
300
80
20
0
0
C
125
over
over
3650
1370
360
0
195
40
10
10
0
D
62.5
3600
4000
2270
790
240
0
120
30
10
10
10
E
31.25
2700
2100
1200
410
120
0
60
10
10
10
0
F
0
0
0
0
0
0
0
0
0
0
0
0
G
1
2
3
4
5
6
7
8
9
10
11
Rows
Tertiary reactant (antibody-alkaline phosphatase)
homologous (antigen)
H 12
Columns
Figure 11.2.7 Results of a criss-cross serial dilution analysis (for optimization of secondary and tertiary reactant concentrations) of an antibody-sandwich ELISA to detect antigen. The numbers in columns 1 to 11 and rows B to G represent relative fluorescence units observed for each well on a 96-well microtiter plate. Plates were coated overnight with the capture antibody at 2 µg/ml. The secondary reactants, 4-fold serial dilutions of the homologous antigen and a non-cross-reactive heterologous antigen, were incubated on the plate 2 hr. The tertiary reactant, 2-fold serial dilutions of specific antibody–alkaline phosphatase conjugates, were incubated on the plate 2 hr. After 1 hr of incubation with the substrate MUP, the fluorescence was read in a microtiter plate spectrofluorometer.
Reagent concentrations depend upon individual assay variables that are set by the investigator. If the time of hydrolysis is set at 1 hr, the relative fluorescence at ∼1000 relative fluorescence units, and the sensitivity at 780 pg/ml of homologous antigen, then 500 ng/ml of enzyme-antibody conjugate must be used in the ELISA. If, however, the assay has to detect only 3.12 ng/ml of homologous antigen, then the concentration of conjugate can be reduced to 125 ng/ml. It should be noted by comparing the homologous with the heterologous reactions (wells B5 versus B11 and D4 versus D10) that both the specificity and the signal-to-noise ratio for this assay are excellent.
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Current Protocols in Molecular Biology
Prepare coating-reagent dilutions 1. Place four 17 × 100–mm test tubes in a rack and add 6 ml PBSN to the last three tubes. In tube 1, prepare a 12-ml solution of coating reagent at 10 µg/ml in PBSN. Transfer 6 ml of tube 1 solution to tube 2. Mix by pipetting up and down five times. Repeat this transfer and mix for tubes 3 and 4; the tubes now contain the coating reagent at 10, 5, 2.5, and 1.25 µg/ml. 2. Using a multichannel pipet, dispense 50 µl of the coating reagent solutions into wells of four Immulon microtiter plates (i.e., each plate is filled with one of the four dilutions). Incubate overnight at room temperature or 2 hr at 37°C. 3. Rinse and block plates with blocking buffer as in steps 5 to 7 of the basic protocol. Prepare secondary-reagent dilutions 4. Place five 12 × 75–mm test tubes in a rack and add 3 ml blocking buffer to the last four tubes. In tube 1, prepare a 4-ml solution of secondary reagent at 200 ng/ml in PBSN. Transfer 1 ml of tube 1 solution to tube 2. Pipet up and down five times. Repeat this transfer and mix for tubes 3 to 5; the tubes now contain the secondary reactant at 200, 50, 12.5, 3.125, and 0.78 ng/ml. If possible, prepare and test serial dilutions of a nonreactive heterologous form of the secondary reactant in parallel (Fig. 11.2.7). If the assay is especially insensitive, it may be necessary to increase the secondary reactant concentrations so the tube-1 solution is 1000 ng/ml.
5. Dispense 50 µl of the secondary reagent solutions into the first five columns of all four coated plates. The most dilute solution is dispensed into column 5, while solutions of increasing concentration are added successively into columns 4, 3, 2, and 1. Thus, the fifth column contains 0.78 ng/ml and the first column 200 ng/ml. Incubate 2 hr at room temperature. 6. Wash plates as in steps 9 to 11 of the basic protocol. Prepare developing-reagent dilutions 7. Place five 17 × 100–mm test tubes in a rack and add 3 ml blocking buffer to the last four tubes. In tube 1, prepare a 6-ml solution of developing reagent at 500 ng/ml in blocking buffer. Transfer 3 ml of tube 1 solution into tube 2 and mix. Repeat this transfer and mixing for tubes 3 and 4—the tubes now contain the developing reagent at 500, 250, 125, 62.5, and 31.25 ng/ml. 8. Dispense 50 µl of the developing reagent solutions into the wells of rows 2 to 6 of each plate, dispensing the most dilute solution into row 6 and solutions of increasing concentration successively into rows 5, 4, 3, and 2. Incubate 2 hr at room temperature. 9. Wash plates as in steps 9 to 11 of the basic protocol. Measure hydrolysis 10. Add 75 µl MUP or NPP substrate solution to each well, incubate 1 hr at room temperature, and measure the degree of hydrolysis visually or with a microtiter plate reader. An appropriate assay configuration results in 0.50 absorbance units/hr at 405 nm when using NPP as a substrate or 1000 to 1500 fluorescence units/hr when using MUP as a substrate. These results can be used to adjust optimal concentrations in the basic and alternate protocols.
Immunology
11.2.17 Current Protocols in Molecular Biology
Supplement 34
SUPPORT PROTOCOL
PREPARATION OF BACTERIAL CELL LYSATE ANTIGENS A culture of E. coli containing proteins expressed from cloned genes is lysed for use as test antigen in any of the first three protocols of this unit. For more extensive discussion on protein expression for antigen production, see UNITS 16.4-16.7 (expression by fusion protein vectors). Materials Escherichia coli culture in broth or agar (UNITS 1.2 & 1.3) Cell resuspension buffer Lysozyme solution Tris/EDTA/NaCl (TEN) buffer (UNIT 11.1) 10% sodium dodecyl sulfate (SDS) 8 M urea (optional) Nylon-tipped applicator (Falcon #2069, Becton Dickinson) 1. For liquid culture, centrifuge 5 ml of cells at 2500 rpm in a tabletop centrifuge for 10 min. Decant supernatant and resuspend pellet in 5 ml cell resuspension buffer by vortexing gently. For agar culture, remove about 10 colonies from the plate using a nylon-tipped applicator and resuspend in 2 ml cell resuspension buffer. Press swab against side of tube to remove as much liquid as possible. Yield of expressed protein may vary with growth phase. Samples should be taken for analysis at various periods of growth (e.g., mid-log and stationary phases). If samples are taken from agar plates, the culture should be grown overnight at 37°C.
2. Place 1 ml of resuspended cells in a microcentrifuge tube on ice. 3. Add 0.2 ml lysozyme solution to the tube and leave 5 min on ice. 4. Microcentrifuge 5 min. Decant supernatant and save. Resuspend pellet in 1.2 ml TEN buffer. Since many expressed proteins are insoluble, it is worthwhile to assay both the pellet and supernatant for activity.
5. Add 0.065 ml of 10% SDS solution to each sample. Incubate 10 min at 37°C. Samples are ready for ELISA at this point. Store frozen if not used within several hours. Alternatively, add urea to a final concentration of 8 M (4.8 g to a final volume of 10 ml) to denature and solubilize proteins.
REAGENTS AND SOLUTIONS Borate-buffered saline (BBS) 0.017 M Na2B4O7⋅10H2O 0.12 M NaCl Adjust to pH 8.5 with NaOH Blocking buffer BBS (see above) containing: 0.05% Tween 20 1 mM EDTA 0.25% bovine serum albumin (BSA) 0.05% NaN3 Store at 4°C Enzyme-Linked Immunosorbent Assays
Gelatin may be substituted for BSA; 5% instant milk has been successfully used but may interfere nonspecifically with antibody binding.
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Current Protocols in Molecular Biology
Cell resuspension buffer (10 mM HEPES) 2.38 g HEPES Add H2O to 1 liter Lysozyme solution 5 mg chicken egg white lysozyme (Sigma Grade VI #L2879) 1 ml TEN buffer (UNIT 11.1) Make fresh immediately before use MUP substrate solution 0.2 mM 4-methylumbelliferyl phosphate (MUP; Sigma #M8883) 0.05 M Na2CO3 0.05 mM MgCl2 Store at room temperature NPP substrate solution 3 mM p-nitrophenyl phosphate (NPP; Sigma #104-0) 0.05 M Na2CO3 0.05 mM MgCl2 Store at 4°C Test antibody solution Hybridoma supernatants (UNIT 11.10) can usually be diluted 1:5 and ascites fluid and antisera (UNIT 11.12) diluted 1:500 in blocking buffer and still generate a strong positive signal. Dilutions of nonimmune ascites or sera should be assayed as a negative control. Prepare antibody dilutions in cone- or round-bottom microtiter plates before adding them to antigen-coated plates. Sources of appropriate antibodies and conjugates can be found in Linscott’s Directory of Immunological and Biological Reagents.
Test antigen solution 0.2 to 10 µg/ml antigen, purified or partially purified in PBSN; store at 4°C Wash buffer Hanks balanced salt solution (HBSS; APPENDIX 2) 1% fetal calf serum (FCS; heat-inactivated 60 min, 56°C) 0.05% NaN3 Store at 4°C COMMENTARY Background Information Since their first description in 1971 (Engvall and Perlman), ELISAs have become the system of choice when assaying soluble antigens and antibodies. Factors that have contributed to their success include their sensitivity, the long shelf-life of the reagents (alkaline phosphatase conjugates typically lose only 5% to 10% of their activity per year), the lack of radiation hazards, the ease of preparation of the reagents, the speed and reproducibility of the assays, and the variety of ELISA formats that can be generated with a few well-chosen reagents. Additionally, no sophisticated equipment is necessary for many ELISA applications, including
screening hybridoma supernatants for specific antibodies and screening biological fluids for antigen content. The ELISAs described here combine the special properties of antigen-antibody interactions with simple phase separations to produce powerful assays for detecting biological molecules. The multivalency of antibodies can result in the formation of long-lived antigen-antibody complexes, thus allowing long periods of time during which such complexes can be measured. By designing an assay so that a capture reagent initiates the binding of antigen-antibody complexes and enzyme conjugates onto a solid phase, the unbound reagents can be easily and Immunology
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Enzyme-Linked Immunosorbent Assays
rapidly separated from the solid phase. The solid phase is washed and the amount of bound conjugate is visualized by incubating the solid phase with a substrate that forms a detectable product when hydrolyzed by the bound enzyme. ELISAs are similar in principle to radioimmunoassays, except that the radioactive label is replaced by an enzyme conjugate. A number of different enzymes have been successfully used in ELISAs, including alkaline phosphatase, horseradish peroxidase, βgalactosidase, glucoamylase, and urease. Alkaline phosphatase—perhaps the most widely used conjugated enzyme—is recommended because of its rapid catalytic rate, excellent intrinsic stability, availability, ease of conjugation, and resistance to inactivation by common laboratory reagents. Additionally, the substrates of alkaline phosphatase are nontoxic and relatively stable. Solutions of p-nitrophenyl phosphate (NPP) are stable for months at 4°C, while solutions of 4-methylumbelliferyl phosphate (MUP) can be kept for months at room temperature without any significant spontaneous hydrolysis. The biggest disadvantage of alkaline phosphatase is that if NPP is used as a substrate, the yellow color of the nitrophenyl product is relatively difficult to detect visually. Using the substrate MUP instead of NPP can greatly enhance the sensitivity of the assay. The fluorogenic system using MUP is 10 to 100 times faster than the chromogenic system using NPP, and appears to be as sensitive as an enhanced chromogenic assay in which alkaline phosphatase generates NAD+ from NADP (Macy et al., 1988). The disadvantage of using fluorogenic substrates is that they require a microplate fluorometer costing twice as much as a high-quality microtiter plate spectrophotometer. Cellular ELISAs have been shown to be as sensitive as flow cytometry analysis in detecting some cell-surface antigens (Bartlett and Noelle, 1987) and are potentially of great value in rapidly screening hybridoma supernatants for antibodies against surface molecules (Feit et al., 1983). Using ELISAs for screening large numbers of hybridoma supernatants has been hindered by the large number of cells required and high background signal. The increased sensitivity of the fluorogenic system should reduce the number of cells needed by a factor of 5, making the system more useful as a screening assay. In addition to the methods described here, hundreds of other ELISA applications have been described, including the determination of
antibody affinities (Beatty et al., 1987; Schots et al., 1988), the detection of antibodies specific for hormone receptors (Quinn et al., 1988; Wang and Leung, 1985), expression cloning of lymphokine receptors (Harada et al., 1990), and homogeneous assays in which a solid phase is not needed because the antigen-antibody interaction itself modifies the enzymatic activity (Rubenstein et al., 1972). A number of books are devoted to ELISAs and can be consulted for further discussion (Maggio, 1981; Kurstak, 1986).
Critical Parameters Sensitive ELISAs require antibodies of high affinity and high specificity. Since the sensitivity of an ELISA depends upon the affinity of the antibodies involved, antibodies with the highest affinities should be used when setting up ELISAs. Antibodies should be screened for unwanted cross-reactions. For instance, capture antibodies must not bind conjugate antibodies and vice versa. There are many commercial sources of reliable reagents. Linscott’s Directory of Immunological and Biological Reagents is an excellent source book for locating reagents used in ELISAs. If reagents from one source are inadequate, try another. When screening for expressed proteins in E. coli, it is important to utilize conjugates with antibodies that recognize nonnative and native molecules. Many foreign proteins expressed in E. coli will not assume their native conformation, and expression of such proteins will not be detected if antibody specific for the native form is used. It is also important to test enzymeantibody conjugates for cross-reactivity or nonspecific binding to host cell molecules. This potential problem can be eliminated by incorporating these as control antigens in the screening procedures used to select the original antibodies in the basic protocol. When coating plates with antigen, the antigen preparation need not be pure, but should generally comprise >3% of the protein in the coating solution. In some situations, dilution of the antigen solution with BSA has greatly improved the sensitivity of the ELISA (Jitsukawa et al., 1989). All steps after coating the microtiter plates should be carried out in solutions containing 0.05% Tween 20 and a carrier protein (0.25% BSA or gelatin). When using ELISAs for quantitative determinations of antigen or antibody concentrations, four guidelines should be followed.
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Current Protocols in Molecular Biology
First, it is essential that all experimental conditions up to the final wash after incubation with conjugate—including incubation times, wash times, reagent concentrations, and temperature—be kept constant between experiments. This is especially important in assays using polyvalent antibodies and complex mixtures of antigens. The optimal concentrations of all reagents for each system should be determined in an initial criss-cross serial dilution experiment (see first support protocol). Second, because the efficiency of binding and other micro-environmental conditions can vary from plate to plate, a standard curve should be included on each plate. Third, all samples must be analyzed at least in duplicate. Fourth, the concentration of the reagent being quantitated must lie within the dynamic range of the standard curve.
Anticipated Results Antibody-sandwich assays are generally the most sensitive ELISA configuration and can detect concentrations of protein antigens between 100 pg/ml and 1 ng/ml. ELISAs in which antigen is directly bound to plates are usually an order of magnitude less sensitive than sandwich techniques. Either the direct or sandwich ELISA may be used to detect and quantitate a bacterially expressed antigen or a purified or partially purified antigen in the range of 1 ng/ml to 1 µg/ml. A major disadvantage of the direct ELISA is that when an impure antigen preparation like a bacterial lysate is coated directly onto the surface of the microtiter well, the antigen must compete with all the other macro-molecules in the lysate for binding to the plastic and very little of the desired antigen may be bound. The sandwich ELISA bypasses this problem by relying on selective adsorption of an antigen to an antigen-specific antibody-coated surface.
Time Considerations
These assays are designed to take ∼6 hr, but the incubation times may be abbreviated or expanded as needed. Since equilibrium binding between the soluble and solid phases frequently takes 5 to 10 hr, stronger specific signals can usually be obtained by longer incubations. Fluorogenic ELISAs are generally 10 to 100 times faster than assays using chromogenic substrates.
Literature Cited Bartlett, W.C. and Noelle, R.J. 1987. A cell-surface ELISA to detect interleukin 4–induced class II MHC expression on murine B cells. J. Immunol. Methods 105:79-85. Beatty, J.D., Beatty, B.G., and Vlahos, W.G. 1987. Measurement of monoclonal affinity by noncompetitive immunoassay. J. Immunol. Methods 100:173-179. Coligan, J.E., Kruisbeek, A.M., Margulies, D.H., Shevach, E.M., and Strober, W., eds. 1991. Current Protocols in Immunology, Chapter 5: Immunofluorescence and cell sorting. Greene Publishing and Wiley-Interscience, New York. Engvall, E. and Perlman, P. 1971. Enzyme-linked immunosorbent assay (ELISA): Quantitative assay of immunoglobulin G. Immunochemistry 8:871-879. Feit, C., Bartal, A.H., Tauber, G., Dymbort, G., and Hirshaut, Y. 1983. An enzyme-linked immunosorbent assay (ELISA) for the detection of monoclonal antibodies recognizing antigens expressed on viable cells. J. Immunol. Methods 58:301-308. Harada, N., Castle, B.E., Gorman, D.M., Itoh, N., Schreurs, J., Barrett, R.L., Howard, M., and Miyajima, A. 1990. Expression cloning of a cDNA encoding the murine interleukin 4 receptor based on ligand binding. Proc. Natl. Acad. Sci. U.S.A. 87:857-861. Jitsukawa, T., Nakajima, S., Sugawara, I., and Watanabe, H. 1989. Increased coating efficiency of antigens and preservation of original antigenic structure after coating in ELISA. J. Immunol. Methods 116:251-257. Kurstak, E. 1986. Enzyme Immunodiagnosis. Academic Press, San Diego. Linscott’s Directory of Immunological and Biological Reagents, Santa Rosa, Calif. Macy, E., Kemeny, M., and Saxon, A. 1988. Enhanced ELISA: How to measure less than 10 picograms of a specific protein (immunoglobulin) in less than 8 hours. FASEB J. 2:3003-3009. Maggio, E.T. 1981. Enzyme Immunoassay. CRC Press, Boca Raton, Fla. Quinn, A., Harrison, R., Jehanli, A.M.T., Lunt, G.G., and Walsh, S.S. 1988. An ELISA for the detection of anti-acetylcholine receptor antibodies using biotinylated α-bungarotoxin. J. Immunol. Methods 107:197-203. Rubenstein, K.E., Schneider, R.S., and Ulmann, E.L. 1972. Homogeneous enzyme immunoassay: A new immunochemical technique. Biochem. Biophys. Res. Commun. 47:846. Schots, A., Van der Leede, B.J., De Jongh, E., and Egberts, E. 1988. A method for the determination of antibody affinity using a direct ELISA. J. Immunol. Methods 109:225-233.
Immunology
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Supplement 15
Wang, K.C. and Leung, B.S. 1985. Fluorometric ELISA methods for rapid screening of anti-estrogen receptor antibody production in hybridoma cultures. J. Immunol. Methods 84:279.
Key Reference Linscott’s Directory. See above. Highly recommended publication listing sources of immunological reagents, kits, and cells/organisms, including addresses and phone numbers of commercial suppliers (updated quarterly).
Contributed by Peter Hornbeck University of Maryland Baltimore, Maryland Scott E. Winston (bacterial cell lysate antigens) Univax Biologics Rockville, Maryland Steven A. Fuller (bacterial cell lysate antigens) Allelix Inc. Mississauga, Ontario
Enzyme-Linked Immunosorbent Assays
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Current Protocols in Molecular Biology
Isotype Determination of Antibodies
UNIT 11.3
Frequently it is necessary to know the amount and serological class of antibodies made by an immunized animal, produced by hybridomas, or present in the serum of patients with inflammatory or neoplastic conditions. The immunologist’s approach to such a problem is to consider the antibody or immunoglobulin molecules themselves as antigens and to use anti-immunoglobulin antibodies as the specific and sensitive agents of detection. This unit describes two methods for measurement and classification of supernatant or serum immunoglobulins—an ELISA (first basic protocol) and a method employing electrophoresis and immunofixation (second basic protocol). SANDWICH ELISA FOR ISOTYPE DETECTION The speed and sensitivity of sandwich ELISAs make them the assays of choice for isotype determination. The antibody-sandwich ELISA to measure soluble antigen (UNIT 11.2; Fig. 11.2.3) is adapted in this unit for isotype detection. Microtiter wells are coated with isotype-specific capture antibodies followed by incubation with test solutions containing the antibodies to be isotyped. After test antibodies have been bound to the plate by reacting with capture antibodies, unbound test antibodies are washed out. Developing reagent is added to the wells, followed by another incubation. Unbound conjugate is washed out and substrate is added. Substrate hydrolysis indicates that the test solution contained the appropriate isotype. A lack of hydrolysis indicates that the test solution did not contain the appropriate isotype. UNIT 11.2 should be consulted for additional information.
BASIC PROTOCOL
Materials Capture anti-isotype antibodies: heavy-chain class-specific antibodies (anti-µ, -α, -γ, -δ, -ε), heavy-chain subclass-specific antibodies (anti-γ1, -γ2a, -γ2b, -γ3, -γ4), or light-chain isotype-specific antibodies (anti-κ, -λ) PBS (APPENDIX 2) containing 0.05% NaN3 (PBSN) Test antibodies: hybridoma supernatants, ascites fluid, or antisera Blocking buffer (UNIT 11.2) Standard isotype antibodies (i.e., purified antibodies of known isotypes) Developing reagent: anti-Ig antibody (specific for all heavy-chain classes)–alkaline phosphatase conjugate (see UNIT 11.1; Southern Biotechnology or Linscott’s Directory) MUP or NPP substrate solution (UNIT 11.2) Immulon 2 or 4 microtiter plates (or equivalent; UNIT 11.2) Additional reagents and equipment for ELISA (UNIT 11.2) 1. Prepare the capture anti-isotype antibodies by diluting in PBSN to 2 µg/ml final. Capture antibodies can be monoclonal or polyclonal. When screening hybridoma supernatants for IgG (anti-γ), it is advisable to initially use a capture-antibody preparation that recognizes all IgG isotypes. Subsequent analysis using subclass-specific capture antibodies can determine which IgG subclass is expressed. It is crucial that the capture antibodies do not bind to the antibodies in the developing reagent.
2. Coat each well of an Immulon microtiter plate with 50 µl capture antibody solution as in steps 3 to 5 of the basic protocol in UNIT 11.2. 3. Block wells as in steps 6 and 7 of the basic protocol in UNIT 11.2. 4. Prepare dilutions of the test antibodies in blocking buffer—typically, hybridoma supernatants are diluted 1:5 while sera are diluted 1:500. Immunology Contributed by Peter Hornbeck, Thomas A. Fleisher, and Nicholas M. Papadopoulos Current Protocols in Molecular Biology (1992) 11.3.1-11.3.6 Copyright © 2000 by John Wiley & Sons, Inc.
11.3.1 Supplement 18
5. Prepare the standard isotype antibodies at ∼500 ng/ml to serve as positive controls. 6. Transfer 50-µl aliquots of test or standard isotype antibodies into the antibody-coated wells and incubate >2 hr at room temperature. Specificity controls consisting of wells coated with capture antibodies and a series of standard antibodies of different isotypes should be included on each plate. Negative controls should be included on each plate and consist of wells coated with capture antibodies but receive no test or control antibodies.
7. Wash plate as in steps 9 to 11 of the basic protocol in UNIT 11.2. 8. Prepare the developing reagent so that the final conjugate solution contains ∼200 ng anti-Ig/ml. Crucial that antibodies in developing reagent do not bind to capture antibodies.
9. Add 50 µl developing reagent to each well and incubate 2 hr at room temperature. All test wells, positive-control wells, and negative-control wells should receive the developing reagent.
10. Wash as in steps 9 to 11 of the basic protocol in UNIT 11.2. 11. Add 75 µl MUP or NPP substrate solution to each well and incubate at room temperature. Periodically check the plate for substrate hydrolysis. Hydrolysis of NPP results in liberation of a yellow product that can be detected by visual inspection in ambient light. Hydrolysis of MUP results in the liberation of a fluorescent product that can be detected by visual inspection under a long-wavelength UV lamp in a darkened room. For more quantitative estimates of isotype concentrations, plates can be read with microtiter plate reader. Positive-control wells give strong signals by 1 hr. Weaker reactions can be detected by incubation for many hours or overnight. BASIC PROTOCOL
DETECTING AND ISOTYPING ANTIBODIES BY ELECTROPHORESIS AND IMMUNOFIXATION Qualitative identification and quantitative determination of serum (and other biological fluid) proteins provide useful information concerning pathologic conditions of the lymphoid system. High-resolution zone electrophoresis is a simple method to separate serum proteins based on their classification defined by five electrophoretic zones: albumin, α1-globulin, α2-globulin, β-globulin, and γ-globulin. Following electrophoresis, the proteins can be detected by staining with amido black, or immunofixation can be performed first to achieve more precise identification. In immunofixation procedure described below, immunoglobulins within separated protein bands are identified, and clonality is established, using antisera to the α, γ, µ, ε, and δ heavy chains and κ and λ light chains of immunoglobulins. Other serum proteins—including glycoproteins, transferrin, and C3—can be identified in electrophoresed sample by same technique. The serum (or other fluid) is loaded on an agarose gel–covered microscope slide and electrophoresed. Proteins are detected after zone electrophoresis and can be identified by immunofixation.
Isotype Determination of Antibodies
Materials Serum (or other biological fluid) Normal saline 95% methanol/5% acetic acid 1% amido black (1 g in 100 ml of 2.5% acetic acid) 2.5% (v/v) acetic acid
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Current Protocols in Molecular Biology
γ
β
α
albumin
globulins
–
+
Figure 11.3.1 Normal serum protein electrophoretic pattern obtained by agarose gel zone electrophoresis demonstrating the five protein zones.
2 × 2–cm cellulose acetate strip Monospecific anti-Ig, heavy-chain-specific (α, γ, µ, ε, or δ) or light-chain-specific (κ or λ) 0.85% (w/v) NaCl Agarose gel–covered microscope slides Plexiglas electrophoresis cell with two agarose bridges Electrophoresis power supply (e.g., Pharmacia EPS 500/400) Electrophorese the sample 1. Cut a narrow slit with a razor blade in the middle of an agarose gel–covered microscope slide. For standard fixation and staining (steps 3 and 4), place 1.5 µl undiluted serum in the slit. For immunofixation (steps 5 to 8), dilute serum in normal saline to generate 1 to 2 mg/ml of the specific protein being evaluated (e.g., IgG, IgA, or IgM) and place in the slit. 2. Place the slide in an electrophoresis cell with two agarose bridges having the same composition as the gel on the slide. Electrophorese 12 min at 140 V. Proceed to steps 3 and 4 for standard fixation and staining. Proceed to steps 5 to 8 for immunofixation. Standard fixation and staining 3. Fix slides by submerging in 95% methanol/5% acetic acid 15 min. Air dry with filter paper placed on the gel. 4. Stain agarose gel by submerging in 1% amido black 10 min. Destain in 2.5% acetic acid until the background clears, then rinse in water. Dry 5 min at 60°C in a drying oven for visual inspection (Fig. 11.3.1). Immunofixation 5. Transfer the unstained and unfixed gel (from step 2) to a petri dish containing damp filter paper. Overlay the γ-globulin zone of the gel with a cellulose acetate strip impregnated with a monospecific anti-Ig. The γ-globulin zone is that nearest the cathode (Fig. 11.3.1). The cellulose acetate strip is cut from any commercial membrane and dipped into the appropriate antibody solution (see critical parameters). The concentration of the antibody solution provided by supplier (usually 0.5 to 1 mg/ml) is suitable.
6. Allow the antibody to diffuse into the gel 15 min at room temperature. 7. Remove the the slides and immerse in 0.85% NaCl for 2 hr to wash out unfixed proteins. Air dry with filter paper placed on the gel. 8. Stain as described in step 4 and visually inspect for immunoprecipitated protein bands (Fig. 11.3.2).
Immunology
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REAGENTS AND SOLUTIONS Agarose gel–covered microscope slides Add 0.5 g agarose (Seakem agarose HE, FMC Bioproducts) to 100 ml of 0.05 M barbital buffer (Sigma #B0500); store at room temperature and heat to 100°C. Stir until agarose has dissolved and allow to cool to 70°C. Pour 2.5 ml of the agarose solution on each microscope slide and allow to gel 2 to 3 min at room temperature. Store the prepared slides at 4°C in a humidified chamber. Set up a chamber by half-filling a petri dish with the above agarose solution. Allow to cool and place in a refrigerator. This humidified chamber can be used to store agarose slides up to 2 weeks. COMMENTARY Background Information Sandwich ELISA Antibodies are heteromeric molecules consisting of heavy and light chains, each of which contains a variable and a constant region. Heavy-chain constant regions include µ, α, γ1, γ2a, γ2b, γ3, γ4, δ, or ε, depending upon the species; light-chain constant regions include κ and λ. Immunoglobulin constant regions, commonly referred to as isotypes, determine many of the biological and immunochemical properties of the antibody molecule including complement fixation, binding to Fc receptors, and binding to proteins A and G. Because the iso-
type may influence the method of purification (Andrew and Titus, 1991), it is routine to determine the isotypes of monoclonal antibodies or other specific antibody preparations as part of their initial characterization. The identification of antibody isotypes can easily be performed with an ELISA employing commercially available anti-isotype reagents. Alternatively, isotypes can be determined using electrophoresis/immunofixation (second basic protocol) or a double-immunodiffusion assay (Hornbeck, 1991). See UNIT 11.2 for a full discussion of the ELISA technique.
A
+
+
γ
β
α
– albumin
globulins
–
monoclonal band –
B
anti-µ
+
C
Isotype Determination of Antibodies
patient
– anti-κ
Figure 11.3.2 Agarose gel zone electrophresis of patient serum demonstrating a monoclonal band in the γ-globuling zone (A; arrow). Immunofixation electrophoresis with anti-µ (B) and anti-κ (C) demonstrate that the band is a µ-κ monoclonal immunoglobulin. There is no reactivity with the antisera to the other heavy (α, γ, δ, ε) and light (λ) chains (data not shown).
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Electrophoresis and immunofixation Zone electrophoresis is based on the principle that charged particles migrate at different rates in an electric field based on the net charge of the particle. The application of this method to the evaluation of serum proteins was first described in a seminal paper by Tiselius (1937). In this paper it was shown that serum proteins were separable into defined zones (albumin, α-globulins, β-globulins, and γ-globulins). While protein separation by zone electrophoresis is excellent, protein quantitation using this method is poor. To overcome these shortcomings, various support media have been employed, including cellulose acetate, agar gel, and, more recently, agarose gel. Agarose gel provides improved stability and clarity, as well as greater sensitivity than other materials (Papadopoulos et al., 1982). Immunofixation electrophoresis combines the high resolution of agarose gel zone electrophoresis and the unique specificity of an antigen-antibody reaction (Johnson, 1982). After electrophoretic separation, the antigen of interest is reacted with an overlayed, monospecific antibody to form an immunoprecipitate, which can be easily detected. The location of the immunoprecipitate depends on the electrophoretic migration of the specific protein antigen. The unreacted proteins and antibody reagents are washed out of the agarose gel and the precipitin band is stained for visualization. Using this method, polyclonality, oligoclonality, or monoclonality can be ascertained (Fig. 11.3.2). Commercial sources for complete immunofixation electrophoresis setups are available. Immunoelectrophoresis, a classic method in which diffusion of antibody into the gel is combined with electrophoresis, is an alternative method for evaluation of protein clonality. However, this approach is less sensitive and more difficult to interpret as compared with immunofixation (Johnson, 1986).
Critical Parameters and Troubleshooting Sandwich ELISA Sources of isotype-specific antibodies can be found in Linscott’s Directory of Immunological and Biological Reagents (see key references, UNIT 11.2). Isotype-specific antibodies should have no detectable cross-reactivity against other isotypes and should not cross-react with other antibodies that might be used in the assay. Check isotype-specific reagents against standard isotype proteins to confirm
their specificity. Sources of purified isotypes from the species of interest, to be used as experimental standards, can also be found in Linscott’s Directory. Alternatively, standard isotype proteins can be prepared from myeloma and hybridoma lines of known isotypes using standard techniques (UNIT 11.8). Many myelomas and hybridomas of defined isotypes and specificities are available from ATCC and other sources; see full listing in Knapp et al. (1991). The concentration of the developing reagent should be adjusted so that the positive control gives a strong signal by 1 hr. Since the hydrolysis of MUP is at least 10-fold easier to detect than the hydrolysis of NPP, assays using MUP can be significantly faster than those using NPP. Test solutions should be scored as positive only when they give 3-fold higher signals than the negative controls. While this assay is designed to qualitatively determine the presence or absence of a given isotype in the test solution, it can be easily modified to quantitate the concentration of isotype by including serial dilutions of standard isotype proteins (for details of quantitation using a standard curve, see UNIT 11.2 protocol for antibody-sandwich ELISA to measure soluble antigen). Electrophoresis and immunofixation Antigen and antibody interaction at or near the point of equivalence results in the formation of immune complexes that produce an insoluble precipitate. In the case of a monoclonal (homogeneous) protein, this precipitate is found in a very narrow band, while a polyclonal protein will generate a broad band. Immunoprecipitation is optimal at antigen/antibody equivalence. It is often useful to quantitate total immunoglobulin levels in the sample to allow for the dilution of the serum (see step 1 of basic protocol). Antigen excess will result in clear spots (lack of detectable immunoprecipitate) in the location where the band(s) are anticipated. If this occurs, dilute the sample and repeat immunofixation electrophoresis. Failure to see a clearly discernible electrophoretic protein pattern after step 4 suggests a technical problem at some point during steps 1 through 4. Immunofixation should not be performed and the initial electrophoresis procedure should be repeated. Absence of a detectable immunoprecipitate after step 8 (in the presence of a gamma globulin band after step 4) suggests antigen excess. Quantitate immunoglobulins, dilute serum accordingly, and repeat entire experiment.
Immunology
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Anticipated Results Sandwich ELISAs are typically sensitive to 0.5 to 2.0 ng/ml of antibody isotype. During electrophoresis and immunofixation, the specific bands observed in immunofixation electrophoresis are dependent on both the presence of particular proteins and the appropriate antisera being used in the detection process. In the case of normal immunoglobulins, all three major isotypes should be observed. Monoclonal or oligoclonal immunoglobulins produce single or multiple darker (clonal) bands (Fig. 11.3.2). The detection limit for a specific protein using this immunofixation technique is 5 to 10 µg/ml.
Time Considerations The sandwich ELISA requires 6 to 8 hr. However, the times allotted for the various incubation steps can usually be reduced by half, so results can be obtained in 3 to 4 hr. For electrophoresis and immunofixation, running and developing the gel takes 2 to 3 hr. Additional time (10 to 15 min) is needed to prepare and titrate the serum sample if immunoglobulin levels are increased.
Literature Cited Andrew, S.M. and Titus, J.A. 1991. Purification of immunoglobulin G. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeek, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. 2.7.1-2.7.12. Greene Publishing and WileyInterscience, New York. Hornbeck, P. 1991. Double-immunodiffusion assay for detecting specific antibodies. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeeck, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. 2.3.1-2.3.4. Greene Publishing and Wiley-Interscience, New York.
Johnson, M.A. 1982. Immunofixation electrophoresis. Clin. Chem. 28:1797-1800. Johnson, M.A. 1986. Immunoprecipitation in gels. In Manual of Clinical Laboratory Immunology. (N.R. Rose, H. Friedman, and J.L. Fahey, eds.) pp. 14-24. Am. Soc. Microbiol., Washington, D.C. Knapp, W., Stockinger, H., Majdic, O., and Shevach, E.M. 1991. The CD system of leukocyte surface molecules. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeeck, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. A.4.1A.4.28. Greene Publishing and Wiley-Interscience, New York. Papadopoulos, N.M., Elin, R.J., and Wilson, D.M. 1982. Incidence of γ banding in a healthy population by high-resolution immunofixation electrophoresis. Clin. Chem. 28:707-708. Tiselius, A. 1937. A new apparatus for electrophoretic analysis of colloidal mixtures. Trans. Faraday Soc. 33:524-526.
Key References Johnson, 1986. See above. A concise discussion of the principles of immunoprecipitation with specific reference to immunofixation electrophoresis. Maggio, E.T. 1981. Enzyme Immunoassay. CRC Press, Boca Raton, Fla. A valuable reference describing parameters of ELISA technology.
Contributed by Peter Hornbeck (sandwich ELISA) University of Maryland Baltimore, Maryland Thomas A. Fleisher and Nicholas M. Papadopoulos (electrophoresis and immunofixation) Warren Grant Magnuson Clinical Center Bethesda, Maryland
Isotype Determination of Antibodies
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PREPARATION OF MONOCLONAL ANTIBODIES
SECTION II
The preparation of monoclonal antibodies should be undertaken carefully, since the production of monoclonal antibodies is expensive and time-consuming. Figure 11.4.1 summarizes the experimental procedures that must be carried out to prepare monoclonal antibodies. The various procedures are presented in individual protocols and described in sufficient detail to allow an individual with no prior experience to carry out a cell fusion, to produce monoclonal antibodies in ascites fluid, and to purify antigen-specific monoclonal antibodies.
immunization of mice ELISA screening of sera
isolation of spleen cells
preparation of myeloma cells
feeder cells cell fusion
ELISA screening of hybridoma supernatants
expansion and selection of cultures to be cloned
freeze cells
feeder cells
cloning by limiting dilution
isolation and expansion of clones
freeze cells
recovery of frozen cells production of ascites fluids
purification of monoclonal antibodies
Figure 11.4.1 Flow chart for preparation of monoclonal antibodies.
Immunology
Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell
11.4.1
Current Protocols in Molecular Biology (1992) 11.4.1-11.4.6 Copyright © 2000 by John Wiley & Sons, Inc.
Supplement 18
UNIT 11.4
Immunization of Mice Antigen is prepared for injection either by emulsifying an antigen solution with Freunds adjuvant or by homogenizing a polyacrylamide gel slice containing the protein antigen. Mice are immunized at 2- to 3-week intervals. Test bleeds are collected 7 days after each booster immunization to monitor serum antibody levels. Mice are chosen for hybridoma fusions when a sufficient antibody titer is reached.
BASIC PROTOCOL
PRODUCTION OF IMMUNE SPLEEN CELLS: IMMUNIZATION WITH SOLUBLE ANTIGEN Materials Phosphate-buffered saline (PBS; APPENDIX 2) Antigen Complete Freunds adjuvant Any strain mice, 6 to 8 weeks old Incomplete Freunds adjuvant 22-G needles 3-ml syringes with locking hubs (Luer-Lok, Becton Dickinson) Double-ended locking hub connector (Luer-Lok, Becton Dickinson) Sterile sharp scissors Sterile razor blades or scalpel blades Wooden applicator sticks 200-µl pipettor Additional reagents and equipment for ELISA (UNITS 11.2 & 11.3) and western blotting (optional; UNIT 10.8) 1. Prepare an emulsion (200 to 400 µl/mouse) of equal volumes PBS containing 25 to 100 µg antigen and complete Freunds adjuvant. Using a 22-G needle, inject mice intraperitoneally. For each antigen, 3 to 5 mice are immunized. Complete Freunds adjuvant contains mycobacteria—incomplete Freunds does not. An emulsion is most readily prepared by linking two locking syringes, one loaded with antigen and the other loaded with adjuvant, using a double-ended locking connector (see Fig. 11.4.2). Press syringe barrels back and forth, transferring contents from one syringe to the other, for 5 to 10 min until a stable emulsion is produced. For best antibody production, inject antigen in as small an emulsion volume as practicable. A stable emulsion is an oil-in-water emulsion which will not disperse when dropped into water. This is a useful check for the emulsification endpoint. Further, at the endpoint the emulsion will thicken noticeably. Mice may be restrained for immunization in the following manner: Place mouse on grilled cage top. Lift mouse by the tail (generally, when mice are lifted by the tail they will grab the bars of the cage top with their front feet, thus stabilizing themselves for restraint). Immobilize the mouse’s head by pinching together the skin at the base of the skull between thumb and forefinger. Turn hand over so that mouse is lying with its back against the palm. Wrap fourth finger around tail and stretch mouse over arched palm for intraperitoneal injection. CAUTION: Handle Freunds adjuvant carefully, since self-injection can cause a positive TB test and lead to a granulomatous reaction.
Immunization of Mice
2. Boost mice 3 weeks later by intraperitoneally injecting an emulsion (200 to 400 µl) of equal volumes PBS containing 10 to 50 µg antigen and incomplete Freunds adjuvant. The emulsion is prepared and injected as in step 1.
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Current Protocols in Molecular Biology
3-ml glass syringes double-ended Luer-Lok connector or 3-way stopcock
antigen and adjuvant
Figure 11.4.2 Double-syringe device for preparation of antigen-adjuvant emulsions.
3. Bleed mice 7 days after second immunization by cutting off 0.5 cm of the tail with sterile sharp scissors or a razor blade. Collect 100 to 200 µl blood into a 1.5-ml microcentrifuge tube. After clot formation, rim the clot with a wooden applicator stick to dislodge the clot from the surface of the tube, but do not break up the clot. After clot retraction, transfer the serum into another microcentrifuge tube with a 200-µl pipettor. If test bleeds are collected more than three times, it will be necessary to cut the tail vein to obtain further samples rather than cutting off additional lengths of the tail itself. This is done by nicking one of the lateral tail veins with a razor blade. The collection of blood may be facilitated by using a heat lamp to warm the mouse for 30 sec to 1 min prior to cutting of the tail. Additionally, if blood flow from the cut tail is slow, the tail may be “milked” from base to the cut tip with thumb and forefinger.
4. Determine the antibody titer in the serum by ELISA (UNITS 11.2 & 11.3). If desired, further characterize the antibody specificity by western blotting (UNIT 10.8). Antibody titer is operationally defined as that dilution of serum that results in 0.2 absorbance units above background in the ELISA procedure.
5. If the antibody titer is considered too low (≥1⁄1000) for cell fusion, mice can be boosted every 2 weeks until an adequate response is achieved. Bleed the mice and test the serum with an ELISA. 6. When the antibody titer is sufficient (>1⁄1000), boost mice by injecting 10 to 50 µg antigen in PBS intraperitoneally (200 to 400 µl), or intravenously (50 to 100 µl) via the tail veins, 3 days before fusion but >2 weeks after previous immunization. In general, the higher the serum antibody titer, the more antigen-specific antibody-producing hybridomas are obtained per fusion. If an antibody against a nonimmunodominant epitope is desired, the cell fusion may be done at an earlier or later time, since the percentage of antibody-producing cells in the spleen directed at these less immunogenic regions of the antigen may vary with time in an unpredictable fashion.
7. Perform cell fusion (UNIT 11.7) 3 days after the immunization (step 6). Immunology
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ALTERNATE PROTOCOL
IMMUNIZATION WITH COMPLEX ANTIGENS (MEMBRANES, WHOLE CELLS, AND MICROORGANISMS) 1. Prime the mice and boost intraperitoneally with adjuvant (i.e., complete Freunds for priming and incomplete Freunds for booster immunizations) as described for soluble antigen (see basic protocol, steps 1 and 2) or suspend antigen in PBS and inject. Use 1 to 2 × 107 cells for mammalian species or 108 to 109 bacterial or yeast cells. 2. Bleed the mice and determine the antibody titer of the serum as described for soluble antigen (see basic protocol, steps 3 to 6). 3. Perform cell fusion (UNIT 11.7) 3 days after final immunization.
ALTERNATE PROTOCOL
IMMUNIZATION WITH ANTIGEN ISOLATED BY ELECTROPHORESIS In some instances the antigen under investigation can be purified most conveniently by gel electrophoresis (UNIT 10.2). Mice can be immunized with protein antigens still contained in a polyacrylamide gel slice, as described in this protocol. Additional Materials 0.1 M KCl, cold Tissue grinder Additional reagents and equipment for denaturing (SDS) discontinuous gel electrophoresis (UNIT 10.2) 1. Apply a protein mixture containing 10 to 50 µg of the desired protein antigen to an appropriate denaturing (SDS) discontinuous gel electrophoresis system (e.g., the Laemmli gel system) and complete the electrophoresis as described in UNIT 10.2. 2. Soak gel 5 to 15 min in cold 0.1 M KCl. Protein bands will appear as white precipitates against a clear gel background. 3. Cut out the appropriate bands from the gel with a razor blade or scalpel blade. 4. Prepare gel suspension by homogenizing the gel slice in a minimum volume of PBS using a tissue grinder. Minimum volume is defined by adding successive 100-µl volumes of PBS until the homogenized gel is liquid. Alternatively, the gel may be air dried for 1 to 2 hr, smashed with a glass rod, and suspended in a minimum volume of PBS.
5. Immunize each mouse with 200 to 400 µl gel suspension containing 10 to 50 µg antigen via an intraperitoneal injection. Amount of antigen is estimated from prior observation of the proportion of desired protein antigen to other antigens in the sample as determined by the relative intensity of stained bands on the polyacrylamide gel (see UNIT 10.6 for staining procedures).
6. Boost mice after 3 weeks with 200 to 400 µl gel suspension containing 10 to 25 µg antigen. 7. Bleed the mice and determine the antibody titer of the serum as described for soluble antigen (see basic protocol). Mice immunized repeatedly with polyacrylamide tend to form adhesions that can make aseptic removal of the spleen difficult.
8. Perform cell fusion (UNIT 11.7) 3 days after final immunization. Immunization of Mice
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COMMENTARY Background Information The stimulation of an effective humoral immune response in mice is critical to the production of monoclonal antibodies directed at a particular antigen. The variety and quality of the monoclonal antibodies prepared is generally directly proportional to the serum antibody titer in the particular mouse used for cell fusion. Any means of antigen preparation, antigen delivery, or immunization schedule that increases antibody titer in the serum of the immunized mouse will potentiate the isolation of hybridomas secreting monoclonal antibodies of interest. We have described two methods of antigen preparation: (1) antigen emulsified in Freunds adjuvants (probably the most common technique used) and (2) antigen isolated in a polyacrylamide gel slice and homogenized. Other preparation methods (e.g., adsorption of antigen to supports such as aluminum hydroxide or aluminum phosphate, polystyrene beads, or nitrocellulose paper, and alternate sites of injection such as footpads) are discussed in the key references.
polyacrylamide gel slice), site of immunization (intraperitoneal versus footpad or tail vein), antigen dose, and frequency of immunization. Alternate immunization protocols are presented in the key references below.
Anticipated Results Isolation of high-quality monoclonal antibodies correlates with high-serum antibody titers. A serum ELISA titer of 1⁄1000 is the minimum level before attempting a cell fusion. Titers for most antigens (particularly from animals injected with highly purified antigen) will range from 1⁄1000 to 1⁄100,000 after 3 to 4 immunizations. Occasional serum samples will titer at greater than 106. The proportion of monoclonal antibodies of IgG class rather than IgM class generally increases proportionally to the duration of the immunization schedule, although this can vary dramatically among different antigens. [In general, IgG class antibodies are more suitable for immunoassays, western blotting (U NIT 10 .8), immunoaffinity chromatography (UNIT 10.11), and immunoprecipitation (UNIT 10.16)].
Critical Parameters It is desirable to use antigen of the highest available purity for immunizations, particularly for primary immunizations. Contaminants may be more immunogenic than the antigen of interest and as such may result in a low specificity antibody. Mice given primary immunizations of highly pure antigen may be boosted with less pure material (containing as little as one-third specific antigen in a complex protein mixture).
Troubleshooting Poor success in raising an adequate antibody titer to an antigen of interest can be attributed to several factors. Improperly prepared emulsion when using Freunds adjuvant (i.e., the aqueous and oil phases separate upon standing) is ineffective in stimulation of an immune response. Contaminants in an antigen preparation may be more immunogenic, necessitating a more homogeneous preparation of the desired antigen. Other parameters that can be varied in an effort to produce a higher antibody titer and increased specificity include presentation of antigen (Freunds adjuvant emulsion versus
Time Considerations A primary immunization followed by two booster immunizations and test bleeds will occupy 6 weeks. For many antigens, however, an adequate antibody response in the mice is achieved only after several months and multiple immunizations.
Key References Hurrell, J.G.R., ed. 1982. Monoclonal Hybridoma Antibodies: Techniques and Applications. CRC Press, Boca Raton, Fla. Langone, J.J. and Van Vunakis, H., eds. 1986. Immunological techniques, Part I: Hybridoma technology and monoclonal antibodies. Methods Enzymol. 121:1-947.
Contributed by Steven A. Fuller and Miyoko Takahashi ADI Diagnostics Rexdale, Ontario John G.R. Hurrell Boehringer Mannheim Diagnostics Indianapolis, Indiana
Immunology
11.4.5 Current Protocols in Molecular Biology
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UNIT 11.5 BASIC PROTOCOL
Preparation of Myeloma Cells Myeloma cells are cultured with 8-azaguanine to ensure their sensitivity to the HAT selection medium (see UNIT 11.6) used after cell fusion (UNIT 11.7). One week prior to cell fusion, myeloma cells are grown in medium without 8-azaguanine. Cell culture conditions are adjusted such that the Sp2/0 cells are in the log phase of growth and exhibit high viability at the time of collection for fusion (UNIT 11.7). Materials Sp2/0 murine myeloma cell line (ATCC #CRL 1581) Complete culture medium 20 µg/ml 8-azaguanine Tissue culture flasks, 25 cm2 or 75 cm2 8% CO2-in-air gas mixture Humidified 37°C, 8% CO2 incubator Inverted microscope 1. Recover frozen cells from liquid N2 storage, as described in UNIT 11.9. 2. Grow Sp2/0 cells overnight in complete medium in tissue culture flasks at 37°C in a CO2 incubator in 8% CO2-in-air atmosphere with 98% relative humidity. 3. Determine that the cells are growing by examining the cell cultures in the flasks with an inverted microscope and return culture flask to CO2 incubator for continuation of cell growth. 4. To ensure that the Sp2/0 cells remain aminopterin sensitive for the selection process following fusion, supplement the complete culture medium with 8-azaguanine at 20 µg/ml during maintenance. One week prior to fusion, culture cells in medium without 8-azaguanine. A seeding cell density of 2.5 to 5 × 104 cells/ml works well with Sp2/0 cells. Sp2/0 cells will grow to a maximum density of 6 to 9 × 105 cells/ml, with a doubling time of 10 to 15 hr. When this density is reached, there is a rapid decline in cell viability. The Sp2/0 cultures are split every 2 to 3 days either by discarding an appropriate volume from the old flask and replacing with fresh medium or by transferring an appropriate volume of cells to a new flask and adding fresh medium. A 1-in-10 or 1-in-20 split is recommended.
5. A total of 1 × 107 Sp2/0 cells (i.e., 1:10 ratio to immune spleen cells) is used for fusion. Cell viability at the time of collection should be greater than 95%. To ensure that cells are collected in log phase of growth, adjust the cell density to 2 × 105 cells/ml the day before the fusion by adding fresh medium. Determine cell viability using the trypan blue exclusion method (see support protocol, below) on cells suspended in serum-free medium or PBS. SUPPORT PROTOCOL
CELL VIABILITY TEST BY TRYPAN BLUE EXCLUSION This procedure is used to determine the number of viable cells present in the cell culture. A non-viable cell will have a blue cytoplasm; a viable cell will have a clear cytoplasm. Additional Materials
Preparation of Myeloma Cells
11.5.1 Supplement 18
Phosphate-buffered saline (PBS; APPENDIX 2) or serum-free complete culture medium 0.4% trypan blue solution Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Current Protocols in Molecular Biology (1988) 11.5.1-11.5.3 Copyright © 2000 by John Wiley & Sons, Inc.
Binocular microscope Hemacytometer 1. Centrifuge 1 ml cell suspension at 100 × g for 5 min. 2. Resuspend the cell pellet in 1 ml PBS or serum-free complete culture medium. Serum proteins stain with trypan blue and can produce misleading results. Determinations must be made in serum-free solution.
3. Mix 1 part of trypan blue solution and 1 part cell suspension (1⁄2 dilution). 4. Using a binocular microscope, count the unstained (viable) and stained (dead) cells separately in a hemacytometer. Each of the four corner squares (composed themselves of 16 smaller squares) have 1 mm sides and are 0.1 mm deep (0.1 mm3). Count all cells within each of the four corner squares, including those that lie on the bottom and left-hand perimeters, but not those that lie on the top and right-hand perimeters. Count any clumps of cells as one cell. Calculate the mean number of cells per 0.1-mm3 volume. Multiply by 104 to obtain the number of cells/ml (i.e., cells/cm3). Apply dilution factor for trypan blue (2×) to obtain the number of cells per milliliter of culture. 5. Calculate the percentage of viable cells as follows: Number of viable cells Viable cells (%) = × 100 Total number of cells (dead and viable) REAGENTS AND SOLUTIONS Complete culture medium Dulbecco modified Eagle medium (DMEM), high-glucose formula (4.5 g glucose/ liter; GIBCO/BRL #430-2100) supplemented to the indicated concentrations with the following additives: 2.8 g/liter sodium bicarbonate (33.3 mM) 4.8 g/liter HEPES (20 mM) 10% fetal calf serum (v/v) 10 ml/liter L-glutamine (2 mM) 10 ml/liter sodium pyruvate (1 mM) 10 ml/liter penicillin (50 IU/ml) and streptomycin (50 µg/ml) The last four additives are available as 100× solutions from GIBCO/BRL and other major suppliers of cell culture media. Penicillin and streptomycin are combined in one solution. Samples of fetal calf serum lots should be tested for ability to support efficient cell growth and cloning before a large purchase because there is much variability between lots of a given supplier. The fetal calf serum must be mycoplasma free. If low volume usage of fetal calf serum precludes testing of serum lots, purchase of mycoplasma-free, virus-free, low endotoxin sera from suppliers such as GIBCO/BRL, Flow Laboratories, or Sigma will generally provide satisfactory results. Horse or bovine serum is not an adequate substitute!
Immunology
11.5.2 Current Protocols in Molecular Biology
Supplement 1
COMMENTARY Background Information The Sp2/0 cell line was chosen as the fusion partner for immune spleen cells because of its good rate of growth, the efficiency with which hybridomas are obtained after fusion, and, most importantly, because it does not synthesize or secrete any immunoglobulin heavy or light chains itself. The Sp2/0 myeloma cell line was developed by Schulman et al. (1978). Other commonly used cell lines are P3X63-Ag8.653 (Kearney et al., 1979), which does not secrete immunoglobulins, and NS-1 (Kohler and Milstein, 1976), which produces only κ light chains.
Critical Parameters Optimal growth of myeloma cells is density dependent. Cultures should be split at regular intervals to maintain >95% viability. Do not culture Sp2/0 cells longer than 1 month to avoid genetic drift and development of antibiotic-resistant contaminants. Maintain several aliquots of Sp2/0 cells in liquid nitrogen storage.
Anticipated Results
myeloma cell culture able to sustain good production of hybridomas upon fusion.
Time Considerations Depending on culture conditions, 105 cells can be expanded to the 107 cells required for fusion in 4 to 6 days.
Literature Cited Kearney, J.F., Radbruch, A., Liesegang, B., and Rajewsky, K. 1979. A new mouse myeloma cell line that has lost immunoglobulin expression but permits the construction of antibody-secreting hybrid cell lines. J. Immunol. 123:1548-1550. Kohler, G. and Milstein, C. 1976. Fusion between immunoglobulin-secreting and nonsecreting myeloma cell lines. Eur. J. Immunol. 6:511-519. Schulman, M., Wilde, C.D., and Kohler, G. 1978. A better cell line for making hybridomas secreting specific antibodies. Nature 276:269-270.
Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Allelix Inc. Mississauga, Ontario
Proper care yields a healthy log phase
Preparation of Myeloma Cells
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Current Protocols in Molecular Biology
Preparation of Mouse Feeder Cells for Fusion and Cloning Chilled sucrose solution is injected intraperitoneally into mice. When withdrawn, the solution contains feeder cells (macrophages and other cells) that are placed in the wells of microtiter plates 1 day prior to seeding of hybridomas from cell fusion (UNIT 11.7) or cloning (UNIT 11.8) procedures.
UNIT 11.6
BASIC PROTOCOL
Materials 0.34 M sucrose solution, sterile and chilled Mice (any strain) 70% ethanol HAT medium, chilled Sterile phosphate-buffered saline (PBS; APPENDIX 2) 10-ml syringe, sterile 18-G needle, sterile 50-ml conical centrifuge tube, sterile Dissecting board Forceps, sterile Scissors, sterile 96-well microtiter plates 8% CO2-in-air gas mixture Humidified CO2 incubator Additional reagents and equipment for estimating cell viability by trypan blue exclusion (support protocol, UNIT 11.5) 1. Just prior to sacrificing a mouse, fill a 10-ml syringe with 8 ml chilled sucrose solution and attach 18-G needle. To avoid macrophages adhering to plastic surfaces, it is important to use chilled solutions to optimize cell harvest.
2. Chill the 50-ml conical centrifuge tube in ice. 3. Kill mouse by cervical dislocation. This is accomplished by firmly holding a thick pencil or similar rod-shaped object to the neck of the mouse just behind the skull and quickly and firmly pulling the tail.
4. Immerse the mouse in a 100-ml beaker containing 70% ethanol. 5. Lay out mouse on dissecting board. 6. Snip skin at diaphragm level and pull skin back, exposing the lower part of the rib cage and abdomen. With forceps pull skin from underlying tissue at the diaphragm level and snip with a scissors. With forceps or sterile gloved hands, pull skin back at both sides of the incision to expose the lower part of the rib cage and abdomen. Care must be taken not to tear or cut the peritoneal membrane.
7. Insert the needle into the peritoneal cavity at the base of the sternum and rest the tip of the needle over the liver. Inject sucrose solution. Gently squeeze the abdomen two or three times. 8. Harvest the peritoneal feeder cells by withdrawing as much solution as possible into the syringe. Care must be taken not to puncture the digestive organs, which may lead to fecal contamination of the feeder cells. Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Current Protocols in Molecular Biology (1988) 11.6.1-11.6.3 Copyright © 2000 by John Wiley & Sons, Inc.
Immunology
11.6.1 Supplement 1
Enough peritoneal feeder cells can usually be isolated from one mouse to seed ∼100 to 300 wells. However, some mice do not yield effective feeder cells. Depending on the total number of wells that must be seeded with mouse feeder cells, an appropriate number of mice must be killed. Peritoneal exudate feeder cells can be prepared up to 3 days prior to use.
9. Transfer the feeder cell–containing sucrose solution into the 50-ml centrifuge tube. 10. In a sterile fume hood, add 20 ml chilled HAT medium. 11. Centrifuge at 100 × g for 5 min at room temperature. 12. Resuspend the pellet in 1 ml chilled HAT medium and perform cell viability test by trypan blue exclusion as described in the support protocol, UNIT 11.5. 13. Suspend the cell pellet in chilled HAT medium at 1 × 105 cells/ml. 14. Add 100 µl cell suspension to each of the 60 inner wells of the 96-well plates. The peripheral 36 wells are filled with sterile PBS. Plates having 24 wells may be used. If this is the case, add 1 ml cell suspension/well.
15. Incubate plates overnight at 37°C in a CO2 incubator in 8% CO2-in-air with 98% relative humidity. REAGENTS AND SOLUTIONS The following solutions are sterilized by filtration through a 0.22-µm membrane. A suitable sterilization system is a disposable filter unit (e.g., Nalgene #120-0020). Glassdistilled water should be used for all preparations. 0.34 M sucrose solution 58.2 g sucrose H2O to 500 ml Filter sterilize and store at 4°C in 100-ml aliquots HAT (hypoxanthine/aminopterin/thymidine) medium Complete culture medium (see reagents and solutions, UNIT 11.5) supplemented to the indicated concentrations with the following additives: 20% (v/v) fetal calf serum 0.1 mM nonessential amino acids 100 µM hypoxanthine 0.4 µM aminopterin 16 µM thymidine These additives may be purchased in concentrated and sterile solutions from the major suppliers of cell culture media and reagents. Concentrated solutions of hypoxanthine and thymidine (HT) and aminopterin may also be prepared in the laboratory (see following recipes). 100× HT solution Weigh 340.3 mg hypoxanthine and 96.9 mg thymidine; add water to 250 ml. Heat to 70°C to dissolve. Filter sterilize and store in 20-ml aliquots at −20°C. Thaw at 70°C for 10 to 15 min.
Preparation of Mouse Feeder Cells for Fusion and Cloning
1000× aminopterin solution Weigh 17.6 mg aminopterin. Add 60 ml water and dissolve by adding 0.1 M NaOH dropwise. Titrate with HCl to pH ∼8.5. Adjust volume to 100 ml and filter sterilize. Make 100× working solution by diluting stock in complete culture medium. Store in 5-ml aliquots at −20°C. Aminopterin precipitates at low pH and is light sensitive.
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Current Protocols in Molecular Biology
COMMENTARY Background Information To maximize the yield of hybrids from the fusion and cloning procedures, feeder cells are required to be cocultured with the hybrids, while hybrid cell density is low. Mouse peritoneal cells, most of which are macrophages, have been found to be convenient and effective feeder cells which are a source of soluble growth factors for hybridoma cells.
Anticipated Results
From 1 to 3 × 106 peritoneal feeder cells are harvested from one mouse. The number of feeder cells will be enough to seed 100 to 300 wells.
Time Considerations Peritoneal feeder cells from one mouse can be processed in 1 hr or less.
Critical Parameters Feeder cells such as peritoneal cells provide best support of hybridoma growth when used 1 to 3 days after harvest. Use of chilled solutions is necessary for optimum cell harvest, to prevent macrophages adhering to plastic surfaces.
Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Allelix Inc. Mississauga, Ontario
Immunology
11.6.3 Current Protocols in Molecular Biology
Supplement 1
UNIT 11.7
BASIC PROTOCOL
Fusion of Myeloma Cells with Immune Spleen Cells Freshly harvested spleen cells and myeloma cells are copelleted by centrifugation and fused by addition of polyethylene glycol solution to the pellet. Cells are centrifuged again and the PEG solution diluted by slow addition of medium. Fused cells are centrifuged, resuspended in selection medium, and aliquoted into 96-well microtiter plates. Hybridomas are grown to 10 to 50% confluence and then assayed for production of antigen-specific antibody. Materials Any strain immunized mouse (UNIT 11.3) Sp2/0 murine myeloma cells in active log phase (Am. Type Culture Collection #CRL 1581; UNIT 11.5) Diethyl ether 70% ethanol Dulbecco modified Eagle medium (DMEM) with supplements Sterile polyethylene glycol (PEG) solution HAT medium (UNIT 11.6) HT medium Phosphate-buffered saline (PBS; APPENDIX 2) 15- and 50-ml centrifuge tubes Glass desiccator or metal can with lid Dissecting board 10.5-cm scissors (Irex #IR-105), sterile 10.5-cm forceps (Irex #IR-1393), sterile 60- and 100-mm petri dishes Stainless-steel strainer (Cellector; GIBCO #1985-8500), sterile 3-cc glass syringes with 26-G needle 5-ml serological pipets 37° C water bath Stopwatch 8% CO2-in-air gas mixture Humidified CO2 incubator Polyvinyl or polystyrene 96-well microtiter plates Inverted microscope Additional reagents and equipment for estimating cell viability by trypan blue exclusion (UNIT 11.5) and for detection of antibodies (UNIT 11.4) Preparation of myeloma and spleen cells 1. Just prior to sacrificing the mouse, transfer 1 × 107 Sp2/0 murine myeloma cells (prepared as described in UNIT 11.5) to a 50-ml centrifuge tube. Check the percentage of viable cells using the trypan blue exclusion method (support protocol, UNIT 11.5). 2. Sacrifice the mouse by anesthetizing with diethyl ether in a closed container (e.g., a glass desiccator or a metal can with a lid). At this point, a blood sample may be collected from the mouse by severing the blood vessels of one forelimb. Collect the blood with a Pasteur pipet and place the blood in a microcentrifuge tube.
Fusion of Myeloma Cells with Immune Spleen Cells
11.7.1 Supplement 1
3. Immerse the mouse in a beaker containing 70% ethanol and lay out on a dissecting board. Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Current Protocols in Molecular Biology (1988) 11.7.1-11.7.4 Copyright © 2000 by John Wiley & Sons, Inc.
4. Using sterile forceps, lift skin over the thorax area and snip with sterile scissors. Peel skin over both sides to expose left side of the rib cage. 5. Using another set of sterile forceps and scissors, remove the spleen from the left upper abdomen of the mouse. The spleen is a small dark red organ. 6. Place the spleen in a 60-mm petri dish containing 3 ml supplemented DMEM. 7. Take spleen in petri dish to sterile hood and carefully dissect away surface fat and other adhering tissue by using a sterile forceps and scissors. 8. Transfer spleen to sterilized, stainless-steel strainer in 100-mm petri dish with 10 ml DMEM. 9. Fill the 3-cc syringe with 2 ml supplemented DMEM. Using a 26-G needle, fill the spleen with DMEM by injecting at several sites. 10. With sterile scissors, cut the supplemented DMEM–filled spleen in 3 or 4 places. 11. Using circular movements, press the spleen against the screen of the stainless-steel strainer with the glass syringe plunger of the 3-cc syringe until only fibrous tissue remains on top of the strainer screen. The tissue that is forced through the strainer is collected in a sterile petri dish underneath. 12. Rinse the screen with 2 ml supplemented DMEM. 13. Transfer the suspension of spleen cells to a 15-ml centrifuge tube. Using a 5-ml serological pipet, disperse the clumps by drawing up and expelling several times. 14. Let suspension stand for 3 min at room temperature. 15. Transfer the top 95% of the cell suspension to a 15-ml centrifuge tube. 16. Perform a viable cell count using the trypan blue exclusion procedure (support protocol, UNIT 11.5); record the percentage of viable cells. Ignore red blood cells that are substantially smaller than the nucleated cells.
17. Transfer 1 × 108 viable spleen cells into a 15-ml centrifuge tube. 18. Wash myeloma cells (from step 1) twice with supplemented DMEM followed by centrifugation at 200 × g for 5 min and resuspend the cells in 5 ml supplemented DMEM. 19. Add spleen cells to myeloma cells in the 50-ml tube and fill the tube with DMEM. 20. Centrifuge the suspension at 200 × g for 5 min at room temperature. 21. Resuspend the cell pellet in 50 ml supplemented DMEM and centrifuge as above. 22. Warm the cell pellet by placing the tube in a 37°C water bath in a beaker for 2 min. 23. Loosen the pellet by flicking the tip of the tube gently. Cell fusion 24. Fuse spleen and Sp2/0 cells with sterile PEG solution: Over the first 1 min—Add 1 ml PEG solution at 37°C. Mix gently. Over the next 2 min—Spin at 100 × g (2 min, total time). Over the next 3 min—Add 4.5 ml supplemented DMEM. Over the next 2 min—Add 5 ml supplemented DMEM. Fill the tube with supplemented DMEM.
Immunology
11.7.2 Current Protocols in Molecular Biology
Supplement 1
Timing is critical and should be monitored with a stopwatch.
25. Centrifuge at 100 × g for 5 min at room temperature. 26. Aspirate supernatant. 27. Resuspend the cell pellet in 35 ml HAT medium. Do not force the dispersion of small cell clumps.
28. Incubate cell suspension at 37°C in a CO2 incubator in 8% CO2-in-air with 98% relative humidity for a minimum of 30 min. Plating and culture of fused cells 29. Add 100 µl cell suspension to each of the 60 inner wells of six 96-well plates. Peripheral 36 wells are filled with sterile PBS. Twenty-four (24) hr prior to use, the 60 inner wells are conditioned with 1 × 104 mouse peritoneal macrophages per well in 100 ìl HAT medium.
30. Incubate plates at 37°C in CO2 incubator in 8% CO2-in-air with 98% relative humidity. (This is day 1 of the culture). 31. On day 5 of the culture, add 100 µl HAT medium to each well. 32. On day 7 of the culture, remove 100 µl from each well and add 100 µl fresh HAT medium. 33. Repeat step 32 every other day until hybrid cell growth covers 10% to 50% of the surface area of the wells. This is monitored by examining the bottom of the wells with an inverted microscope. At this time, the wells should be screened for antibody (UNIT 11.4). 34. Grow the hybrids in HAT medium for 2 weeks after fusion. 35. After 2 weeks, change the medium to HT medium. 36. The hybrids are grown in the HT medium until the completion of two cloning procedures (as described in the protocol for cloning of hybridoma cells by limiting dilution, UNIT 11.8). REAGENTS AND SOLUTIONS Dulbecco modified Eagle medium (DMEM), high-glucose formula (GIBCO/BRL #430- 2100), supplemented to the indicated concentrations with the following additives: 2.8 g/liter sodium bicarbonate (33.3 mM) 4.8 g/liter HEPES (20 mM) HT medium Prepare as described in UNIT 11.6 for HAT medium but without aminopterin solution. PEG (polyethylene glycol) solution Weigh 10 g PEG 4000 (Merck) into a 100-ml glass bottle. Autoclave PEG for 10 min at 121°C. Cool the molten PEG to 50°C. Mix with 10 ml prewarmed supplemented DMEM (50°C) that contains 5% (v/v) dimethylsulfoxide (DMSO). Aliquot the mixture into small glass bottles (3 ml/bottle). Store at 4°C in the dark. Fusion of Myeloma Cells with Immune Spleen Cells
PEG 4000, Merck’s gas chromatography grade, appears to be the best PEG for cell fusions using Sp2/0 myeloma cells regardless of lot number. Remove PEG from the autoclave as soon as the pressure is down in order to avoid prolonged heating of PEG. Incubate PEG solution at 37°C for 24 hr prior to use to test its sterility.
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Current Protocols in Molecular Biology
Alternatively, if the temperature control of the autoclave is uncertain, melt the PEG at 65°C on the day of fusion. Mix the PEG with supplemented DMEM to make a 45% PEG solution and sterilize by filtration through a 0.22-ìm membrane filter.
COMMENTARY Background Information
Troubleshooting
Murine spleen cells, some of which are involved in production of the desired antibodies, are fused with a murine myeloma cell line to form a stable antibody-producing hybridoma cell line. The myeloma cells (Sp2/0) are hypoxanthine–guanine phosphoribosyltransferase deficient (HGPRT−) and therefore are unable to use the purine salvage pathway when de novo purine synthesis is blocked by aminopterin, which is included in the HAT selection medium. See discussion of selectable markers in UNIT 9.5. Hypoxanthine and thymidine in the HAT selection medium allow HGPRT+ cells, the spleen cell–myeloma hybrids, to survive and grow. Unfused spleen cells eventually die.
When poor fusion results (i.e., poor hybridoma growth) are obtained despite close adherence to the protocol, the likely causes are the PEG, the myeloma cells used as fusion partner, or the culture conditions. Most critical is the PEG (see critical parameters). The quality of the fusion partner (i.e., the myeloma culture; see UNIT 11.5 for details) is also very important. The CO2 incubator must provide a stable temperature, pH, and humidity for optimal hybridoma growth.
Literature Review Kohler and Milstein (1975) first demonstrated that somatic cell fusion could be used to generate a hybridoma cell line producing a monoclonal antibody of predetermined specificity. Cell fusion was initially accomplished by addition of Sendai virus. The fusion procedure used in this protocol is a modification of the method of Gefter et al. (1977), which uses polyethylene glycol (PEG) as the fusogen. A similar procedure is presented by Oi and Herzenberg (1980). Each protocol encompasses careful timing of the PEG addition to the cell pellet and its subsequent dilution after fusion.
Critical Parameters The choice and use of PEG in the fusion protocol is the most critical factor. PEG can vary dramatically in efficiency between manufacturers and among lots of a particular manufacturer. The suggested source (Merck) has provided the most consistent lots of PEG. Care must be taken to autoclave the PEG at 121°C for only 10 min in order to minimize the production of toxic aldehydes. In the fusion procedure itself, it is important to adhere carefully to the time schedule established. Extended incubation of the cells with PEG results in decreased cell viability. Dilution of the PEG by medium must be done carefully to avoid lysis of the cells.
Anticipated Results Hybridoma growth should be observed with the aid of an inverted microscope in nearly all wells after a few days in culture. For wells with viable cells, generally 10 to 50% will contain antigen-specific antibody. Results can vary widely, of course, and it has been observed that 0 to 99% of the supernatants in the wells will contain specific antibody.
Time Considerations The fusion procedure requires 3 to 4 hr to complete. For best results, it should be accomplished without interruption. Wells with hybridoma growth can be assayed for specific antibody 7 to 12 days after fusion.
Literature Cited Gefter, M.L., Margulies, D.H., and Scharff, M.D. 1977. A simple method for polyethylene glycolpromoted hybridization of mouse myeloma cells. Somat. Cell Genet. 3:231-236. Kohler, G. and Milstein, C. 1975. Continuous cultures of fused cells secreting antibody of predefined specificity. Nature 256:495-497. Oi, V.T. and Herzenberg, L.A. 1980. Immunoglobulin-producing hybrid cell lines. In Selected Methods in Cellular Immunology (B.B. Mishell and S.M. Shiigi, eds.) pp. 351-372. W.H. Freeman, San Francisco.
Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Allelix Inc. Mississauga, Ontario
Immunology
11.7.4 Current Protocols in Molecular Biology
Supplement 1
UNIT 11.8
BASIC PROTOCOL
Cloning of Hybridoma Cell Lines by Limiting Dilution Hybridomas to be cloned are diluted to 0.8 cells/well. This dilution provides 36% of wells with 1 cell/well by Poisson statistics. When cultures are 10 to 50% confluent, antibody is assayed by ELISA. Two or more cloning procedures are carried out until >90% of the wells containing single clones are positive for antibody production. Materials HAT medium (UNIT 11.6) HT medium (UNIT 11.7) Complete culture medium (UNIT 11.5) Polyvinyl or polystyrene 96-well microtiter plates 6- and 24-well culture plates 8% CO2-in-air gas mixture Humidified CO2 incubator Cryotubes (Nunc #3-63401) 2-, 5-, and 10-ml serological pipets Multichannel pipet and tips Inverted microscope Additional reagents and equipment for preparing mouse feeder cells for fusion and cloning (UNIT 11.6), for ELISA screening (UNIT 11.4), and for estimating cell viability by trypan blue exclusion (UNIT 11.5) 1. The day before cloning, isolate mouse feeder cells and prepare 96-well plates with feeder cells in appropriate medium, as described in UNIT 11.6. Choice of medium depends on the current stage of the cloning process. The first cloning uses HAT medium, and the second cloning uses HT medium. Any other clonings use complete culture medium.
2. Transfer all of the cells from each well containing antigen-specific antibody in its hybridoma supernatant (as determined by an ELISA, UNIT 11.4) into a separate well of a 24-well plate that has been preincubated with 0.5 ml of an appropriate culture medium (see note in step 1) and culture overnight at 37°C, 8% CO2-in-air, and 98% humidity in a CO2 incubator. If there are greater than 40 to 50 positive wells, it is difficult to manage conveniently the cloning procedure. The ELISA assays (UNIT 11.4) or western blotting (UNIT 10.8) can be performed using the 24-well culture supernatants in order to select the most promising samples to be cloned. The remaining positive samples can be transferred to cryotubes and frozen, as described in UNIT 11.9. Cloning efficiency is always improved when hybrid cells are grown to log phase in 24-well plates. The efficiency is still better if the 24 wells are preconditioned with feeder cells prior to transferring the hybrids.
3. Perform cell viability count using trypan blue exclusion method (support protocol, UNIT 11.5) on the overnight cultures in 24-well plates. 4. Using a 6-well plate, make dilutions of cells from overnight cultures in HT or complete medium. In the first well, make a 1:100 dilution in a total of 3 ml; in the second well, dilute an aliquot of the first dilution to 80 cells/ml in 5 ml; in the third well, prepare 8 cells/ml in 10 ml (i.e., a 1:10 dilution from the second well). Cloning of Hybridoma Cell Lines by Limiting Dilution
11.8.1 Supplement 1
Choice of medium is discussed in step 1. Using 6-well plates is far easier for preparing cell dilutions than using tubes. However, using 6-well plates is expensive.
5. With a multichannel pipet, fill the upper 50 wells of the inner 60 wells of the 96-well plate from step 1 with 100 µl of 8 cells/ml dilution (i.e., 0.8 cells/well) Contributed by Steven A. Fuller, Miyoko Takahashi, and John G.R. Hurrell Current Protocols in Molecular Biology (1988) 11.8.1-11.8.2 Copyright © 2000 by John Wiley & Sons, Inc.
and the 10 wells of the bottom row with 100 µl of 80 cells/ml dilution (i.e., 8 cells/well). 6. Incubate at 37°C in a CO2 incubator in 8% CO2-in-air with 98% relative humidity (day 1). As a precaution, the hybridoma cells remaining in the 24-well plate can be transferred to cryotubes and frozen, as described in UNIT 11.9.
7. On day 6, feed the culture with the addition of 100 µl/well of fresh medium, using a multichannel pipet. Thereafter, if necessary, refeed the culture every other day by removing 100 µl media from each well and adding 100 µl fresh media. 8. When cell growth in the bottom of the wells is 10 to 50% confluent (as monitored using an inverted microscope), assay for specific antibody in the hybridoma supernatants using an ELISA (UNIT 11.4). 9. Transfer 2 to 3 selected positive subclones from each plate into a 24-well plate (as for step 2) and incubate overnight. Expand the subclones and freeze one aliquot for each subclone in a cryotube. This is done as a precaution in case one fails to recover positive clones.
10. Repeat cloning procedure from the beginning until a stable and single hybridoma cell line is established. Hybridomas that yield >90% antibody-positive cultures upon recloning are considered to be stable. Those that yield 40 ml) of ascites can be collected from each mouse.
9. Centrifuge the ascites 10 min in TH-4 rotor at 2700 rpm (1500 × g), room temperature. Harvest supernatants and discard pellet. Store ascites fluid at 4°C until all collection is completed (2) in vivo serial passes of the hybridomas.
13. Dilute >1:10 and filter sterilize through a 0.45-µm filter. Aliquot and freeze at −70°C, avoiding repeated freezing and thawing (Yokoyama, 1991a). Shelf life should be several years. Add sodium azide to 0.02% final if the ascites will not be used for bioassay. It is possible that ascites fluid will not form. If the mice die without any ascites forming, particularly within 2 weeks of inoculation, try fewer cells. If the mice do not form ascites after 2 weeks and they appear healthy, inject those mice—as well as naive, Pristane-primed mice—with more cells. If solid tumors form, tease cells into suspension and inject tumor cells into another Pristane-primed mouse. Even if a little ascites forms, the fluid can be transferred to another mouse (∼0.5 ml/mouse), and large amounts of ascites should accumulate. Once the ascites is formed, the mouse-adapted cells can be frozen and used to reinoculate mice in the future.
Immunology
11.10.5 Current Protocols in Molecular Biology
Supplement 18
COMMENTARY Background Information Production of MAb supernatant There are three basic preparations that contain monoclonal antibodies: supernatant from a MAb-producing hybridoma, ascites from a mouse inoculated with the hybridoma, or purified MAb. In the first protocol, a MAb-containing supernatant is produced. The hybridoma continues to secrete MAb into the culture fluid until cell death occurs. Because the MAb is not metabolized, it accumulates in the culture supernatant. Hybridoma supernatants are advantageous because small amounts (0.5% may also have significant nonspecific effects. In addition to the appropriate
isotype control MAb ascites, ascites harvested from mice inoculated with the nonsecretory partner cell line (i.e., SP2/0) also provides a useful nonspecific control. The general procedure involves the elicitation of nonspecific inflammation in the peritoneal cavity of an appropriate host mouse, usually with Pristane, and injection of the hybridoma cells. The tumor cells then grow as an ascites tumor and should continue to secrete the MAb. Eventually, the mouse develops a monoclonal gammopathy similar to the human disease, multiple myeloma, in that a monoclonal protein reaches high titer in the serum. Since the combination of the ascites tumor and Pristane results in an inflammatory exudate in the peritoneal cavity, the ascites should contain concentrations of the MAb similar to that in serum.
Critical Parameters Monoclonal antibody production by hybridomas is an unstable phenotype. Hybridoma cells always should be grown under log-phase growth conditions. Prolonged in vitro culture and in vivo passage should be avoided. Thus, the most critical parameter is whether the hybridoma of interest is secreting a high titer of the MAb. This should be checked before a major effort is made to grow large amounts of supernatant or to produce ascites fluid. The MAb titer can be determined by serial dilution of the culture supernatant in the assay appropriate for that MAb, such as ELISA (UNIT 11.2) or flow cytometry (Holmes and Fowlkes, 1991; Yokoyama, 1991b). Titers of ≥1:10 should be saturating if spent culture supernatants are examined. If necessary, the hybridoma can be recloned by limiting dilution (UNIT 11.8) to find high-producing clones. If cells are known to produce MAb at high titers, aliquots frozen immediately after cloning (Yokoyama, 1991a) will retain this phenotype. The most critical parameter in the largescale production of cell lines and hybridomas is the adaptation of the cells to roller flasks. Most hybridomas and other nonadherent cells that grow in suspension can be easily adapted. If the cells (particularly adherent cells) cannot be adapted, other methods should be tried. For example, large-scale production of cells for use in isolating cellular components can be performed using multiple 175-cm2 flasks instead of roller flasks. Unfortunately, the
Isotype Determination of Antibodies
11.10.6 Supplement 18
Current Protocols in Molecular Biology
relative surface area is small, and therefore the number of flasks required can become prohibitive. Adherent cell lines are less easily adaptable to growth in roller flasks. The surface area for growing adherent cells can be increased by the use of dextran beads (e.g., Cytodex beads from Pharmacia). These beads can increase the surface area of a culture flask severalfold. If there is any suspicion that the cells may be mycoplasma contaminated, diagnosis and treatment are indicated (Fitch et al., 1991). Mycoplasma-contaminated lines will produce a much poorer yield of final cell numbers because they do not grow to as high a cell density as normal cells. Supernatants frequently contain 1 to 10 µg/ml of MAb, but the concentration is cell-line dependent. The supernatants could be concentrated by salt precipitation, but this is not generally recommended because large volumes of culture supernatants have to be concentrated to derive the amount of MAb in small amounts of ascites. Moreover, the FCS in the supernatant will also be concentrated. While the hybridoma could be adapted to culture in serum-free medium, this requires additional testing and yields may decrease. Affinity purification of the culture supernatants would take a similar amount of effort and produce purified MAb at high concentrations. Thus, instead of concentrating supernatants (if high concentrations of MAb are desired), it is recommended to produce purified MAb by first growing hybridomas at a larger scale (liters) or to produce ascites. For ascites production, it is important to consider the appropriate host for the hybridoma since an injection of allogeneic or xenogeneic cells may result in rejection. For most mousemouse hybridomas, an F1 hybrid—between the BALB/c strain (origin of the commonly used SP2/0 fusion partner) and the strain from which the normal cells were obtained—could be used. For xenogeneic hybridomas, nude mice or lowdose irradiated normal mice are potential hosts. Outbred nude mice are somewhat more expensive than normal mice but do not require irradiation. It is not necessary to use the prohibitively expensive inbred nude mouse strains. Because the level of normal immunoglobulin in mouse serum is in the same range as ascites fluid (mg/ml), ascites fluid can be only partially purified by salt fractionation or antimouse-Ig- or protein A–affinity chromatography (UNIT 2.7). However, it is a convenient source of raw material from which to affinity purify rat MAb with a mouse anti-rat κ MAb (e.g., MAR 18.5) column (Andrew and Titus, 1991).
Troubleshooting It is possible that ascites fluid will not form. The reasons for this are unclear but are probably related to a property of the individual hybridoma. If the mice die without any ascites forming, particularly within 2 weeks of inoculation, try fewer cells. If the mice do not form detectable ascites after 2 weeks and they appear healthy, inject those mice—as well as naive, Pristane-primed mice—with more cells. If solid tumors form, tease cells into suspension and inject the tumor cells into another Pristaneprimed mouse. Even if a little ascites forms, the fluid can be transferred to another mouse (∼0.5 ml/mouse), and large amounts of ascites should accumulate. Once the ascites is formed, the mouse-adapted cells can be frozen and used to reinoculate mice in the future.
Anticipated Results Most culture supernatants will have saturating MAb titers of ≥1:10 when tested at 100 µl for 106 cells. If the spent culture supernatant is used for MAb purification by affinity chromatography (UNITS 10.9 & 10.10; Andrew and Titus, 1991), 1 to 10 mg of purified MAb/liter can be anticipated. If a much lower titer or yield is achieved, recloning of the hybridoma line may be indicated. Hamster-mouse hybridomas are particularly notorious for instability. Most hybridomas can be grown as ascites tumors. The saturating concentration of the MAb in such fluids should be detected at dilutions of 1:500. If MAb titers are significantly lower, the hybridoma may be a poor producer. If ascites do not form, see troubleshooting above. Most tumor cells that grow in suspension should be amenable to growth in roller flasks and densities of >106 cells/ml should be attained. Careful work should result in no contamination.
Time Considerations For high-titer and large-scale production of MAb supernatants, a flask split 1:10 will be overgrown, with cell viability definitely decreasing by day 5 to 6, at which time the supernatants can be harvested. If several liters of supernatant are required, ∼10 days are required to expand a 25-cm2 flask (10 ml) to 2.4 liters. For production of ascites fluid containing MAb, 4 to 6 weeks are necessary for growth of the cells for inoculation, ascites accumulation, tapping the fluid, centrifugation, and determination of the MAb titer.
Immunology
11.10.7 Current Protocols in Molecular Biology
Supplement 18
Once the 175-cm2 flasks are seeded for large-scale production of hybridomas and cell lines, 10%, resulting in 108 clones) are still acceptable. Usually, the original phage library will consist of in the order of 106 independent recombinants. A 100-fold excess of clones in the excised plasmid pool will ensure that the complexity of the original library is maintained.
7. Analyze the plasmid content of a number (20 to 40) of random colonies using a miniprep procedure (UNIT 1.6), digestions with restriction enzymes (UNIT 3.1), and gel electrophoresis (UNIT 2.5A). Determine the percentage of clones with insertions and the cDNA size distribution. Process phage-infected E. coli and plate library 8. If the excision frequency was sufficient, divide the previously stored phage-infected E. coli BNN132 culture (step 6) over eight 1.5-ml microcentrifuge tubes and microcentrifuge for 1 min to pellet the bacteria. 9. Decant the supernatant, resuspend each bacterial pellet in 1 ml liquid LB medium, and spread the content of each tube on a 24 × 24–cm LB-Cb plate. Grow overnight at 37°C. Yeast One-Hybrid Screening for DNA-Protein Interactions
A dense field of small colonies will result. The initial growth of the plasmid library on plates avoids underrepresentation of toxic cDNA clones. If the culture is directly grown in liquid medium, bacteria containing such clones could be competed out.
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Current Protocols in Molecular Biology
10. Scrape the colonies from the plates using sterile cell scrapers. Resuspend in 1 liter prewarmed LB medium (with 200 mg/liter carbenicillin) and grow for another 4 hr, shaking at 37°C. 11. Harvest the bacteria by centrifuging in a centrifuge with large buckets (e.g., 250- or 500-ml buckets) and isolate the DNA using maxiprep procedures (UNIT 1.7) or commercially available maxiprep kits. Determine the DNA concentration by UV spectrometry (APPENDIX 3D). 12. Proceed with the library transformations, following the instructions for library screening (see Basic Protocol 2). REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
3-Amino-1,2,4,-triazole (3-AT), 1 M Prepare a 1 M stock solution of 3-AT (e.g., Sigma). Do not autoclave, but filter sterilize the solution. Store at 4°C. Add the 3-AT to the CM medium after the latter has cooled down to ∼60°C. Store up to 1 year at 4°C. LB-Cb plates Solidify LB medium (UNIT 1.1) with 1.5% agar and supplement with 200 mg/liter carbenicillin (add from 1000-fold stock). Sterilize LB medium with 1.5% agar by autoclaving 20 min. Add carbenicillin after LB medium has cooled to 60°C. Pour into 24 × 24–cm square dishes (Nunc) and standard (9-cm) petri dishes. Store up to 3 months at 4°C. Lithium acetate stock, 10× Prepare 1 M lithium acetate, pH 7.5, adjust with acetic acid; sterilize by autoclaving 20 min. Store indefinitely at room temperature. PEG stock, 50% (w/v) Prepare 50% (w/v) polyethylene glycol (PEG 3,350; e.g., Sigma); sterilize by autoclaving 20 min Store indefinitely at room temperature, in small portions in well-closed bottles, because transformation efficiencies are critically dependent on the correct PEG concentration.
TE buffer stock, 10× 100 mM Tris⋅Cl, pH 7.5 (APPENDIX 2) 10 mM EDTA Sterilize by autoclaving 20 min Store indefinitely at room temperature COMMENTARY Background Information To gain insight into the regulation of a gene, it is necessary to identify the transcription factors binding to regulatory sequence elements. Such cis-acting elements mostly have been identified in the 5′ promoter regions of genes, but can also be located in introns, exons, or 3′ untranslated regions. Binding of factors can result in activation or silencing of gene repression, depending on the developmental stage of
the cell or on internal or external signals. Using techniques like gel-shift assays, DNA methylation interference, or DNase footprinting, binding sites for transcription factors can be mapped. Often, these are short sequences of only 5 to 10 bp. Almost all promoters contain a TATA box close to the transcription start site. The TATA box is recognized by a complex of factors, the basal transcription machinery. In turn, this complex interacts with the transcrip-
DNA-Protein Interactions
12.12.15 Current Protocols in Molecular Biology
Supplement 55
Yeast One-Hybrid Screening for DNA-Protein Interactions
tion factors that specifically bind to the cis-acting elements. Traditionally, the cloning of transcription factors relied on biochemical strategies such as affinity chromatography. Furthermore, molecular biological methods have been developed that involve the screening of cDNA expression libraries using labeled oligonucleotides. In the past decade, newly developed yeast genetic selection methods have provided particularly useful alternatives for cloning of transcription factors. One of the possibilities is to complement yeast transcription factor mutants. An illustrative example is the cDNA cloning of a human CCAAT-binding protein via complementation of the yeast hap2 mutant (Becker et al., 1991). With the availability of the complete yeast genome sequence, it has become relatively easy to construct yeast transcription factor mutants and to assess the feasibility of a complementation strategy to clone a heterologous factor. Obviously, the possibilities are limited to cloning of functionally conserved transcription factor genes. More generally applicable is the use of a yeast genetic selection strategy that has become known as the one-hybrid system. In fact, the one-hybrid system is a simplification of the two-hybrid approach for detection of protein-protein interactions (Fields and Song, 1989; UNIT 20.1). The one-hybrid concept (Fig. 12.12.1), relies on the detection of the interaction between a protein expressed from a cDNA library (prey) with a DNA sequence of interest (bait) fused to a reporter gene. Activation of this reporter gene is dependent on the interaction and can be selected for. The required cDNA libraries are designed such that the expressed proteins carry a strong transcriptional activation domain from a known factor (hence the term “one-hybrid”). Therefore, an interaction with the bait sequence will usually guarantee reporter gene activation. This makes the selection method suitable to clone all types of DNA-binding proteins, independent of their normal functional properties. It is also possible to apply a “non-hybrid” selection strategy (Grueneberg et al., 1992; Chan et al., 1993), but this limits cloning possibilities to transcription factors with an intrinsic activation domain that functions properly in yeast. One-hybrid libraries are normally constructed with shuttle vectors containing the replication sequences and marker genes necessary for maintenance in both yeast and E. coli. The activation domain (AD) in the expressed hybrid proteins is usually from the
yeast Gal4p transcription factor, but there are also vectors where the Herpes simplex VP16 AD or the acidic B42 domain (UNIT 20.1; Clontech) is used. To facilitate nuclear import of the hybrid proteins, the ADs used in all vectors are equipped with an SV40 nuclear localization signal. As a frequent additional feature, an epitope domain, such as HA (hemagglutinin), is included to facilitate detection on western blots or other applications. Constitutive promoters, such as the ADH1 promoter, are often used to drive cDNA expression, but alternatives are conditional expression systems, e.g., based on the galactose-inducible GAL1 promoter. All AD vectors designed for two-hybrid screenings are exchangeable with one-hybrid systems. Some AD vectors are available as lambda vector systems (Clontech, Stratagene), with which cDNA cloning is more efficient and it is easier to achieve and maintain high complexity. Described lambda phage vectors for AD-library construction can easily be converted into E. coli–yeast shuttle vectors by automatic subcloning. A selection of useful vectors is shown in Table 12.12.3. The authors and others have had good experiences with use of the λACTII vector (Memelink, 1997) for cloning of a wide variety of transcription factors from plants (Menke et al., 1999; Meijer et al., 2000; van der Fits et al., 2000) and human cell lines (P.B.F. Ouwerkerk, unpub. observ.). The most frequently used reporter for genetic selection procedures is the HIS3 gene, operative in the biosynthesis of histidine and encoding the enzyme imidazoleglycerol-phosphate dehydratase. Detection of its activation is through complementation of the auxotrophic his3 mutation. Usually, HIS3 reporters contain the minimal HIS3 promoter, including the TATA box and transcription start site (Grueneberg et al., 1992). There are several examples of successful application of the HIS3 reporter in one-hybrid screenings (e.g., Wilson et al., 1991; Grueneberg et al., 1992; Wang and Reed, 1993). Alternative strategies make use of the lacZ (Li and Herskowitz, 1993) or green fluorescence (GFP) reporter genes (Display Systems Biotech; UNIT 9.7C). Furthermore, the use of MEL1 as an alternative color marker was recently described (Melcher et al., 2000). Although detection of these reporters is easy, the use of HIS3 as a reporter has important advantages in a genetic selection procedure. Because HIS3 is a growth selection marker, detection of positives is very simple since these appear as clear colonies in a field of nongrowing or slowly growing cells. Screening of lacZ, MEL1,
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or GFP activity requires that all the transformed cells grow into colonies. Therefore, the plating density must be far lower than when HIS3 is used as reporter, which complicates the setup of saturating library screens. Another advantage is that the choice of HIS3 offers a solution to possible problems of leaky reporter expression, also termed background activity. Leaky expression can occur when bait sequences are recognized by endogenous yeast transcription factors. Since the activity of the His3p enzyme can be competitively inhibited by 3-amino1,2,4-triazole (3-AT), leaky HIS3 expression can often be eliminated by a 3-AT titration, so that it remains possible to detect increased HIS3 expression resulting from the specific binding of a library protein during the screening. With lacZ, MEL1, or GFP reporters, the distinction between specific and background expression is difficult, and other auxotrophic markers that could be used as reporters (LEU2, ADE2) are not titratable. A good alternative for the HIS3 reporter is a dominant antibiotic marker such as G418, hygromycin, or chloramphenicol. Here, leaky expression can be eliminated by increasing the concentration of the selective compound. An example is the cloning of the transcription factor Nrf1 using a neomycin-resistance gene in combination with G418 selection (Chan et al., 1993). With some bait sequences, the occurrence of excessively high leaky expression, which needs to be eliminated, may be observed, which is the most important constraint on the application of the one-hybrid screening system. Initially, one-hybrid screenings involved the use of two replicating vector systems, one for the AD library and the other for the reporter (e.g., Wilson et al., 1991; Li and Herskowitz, 1993; Wang and Reed, 1993). Afterwards, a switch was made to chromosomally integrated reporters, which has the advantage that the reporter gene copy number is constant and that DNA-protein interactions are studied within the context of the nucleosome (e.g., Clontech). The pHIS3/pINT1 vectors described by the authors (Meijer et al., 1998) provide a system for the integration of reporter genes in the yeast genome. In this system, a bait sequence is first cloned upstream of the HIS3 gene in one of the reporter vectors pHIS3NB or pHIS3NX (Table 12.12.1 and Fig. 12.12.3). In the second step, the reporter construct is cloned in the integrative vector pINT1. Finally, the pINT1-HIS3 reporter construct is integrated via double cross-over in the nonessential PDC6 gene (YGR087c, Chr. VII) using selection for the
dominant APT1 antibiotic marker. Advantages of the pHIS3/pINT1 system are discussed in the introduction to this unit, and examples of its application have been published (Menke et al., 1999; Meijer et al., 2000; van der Fits et al., 2000). A further refinement of one-hybrid screenings lies in the use of dual or triple reporter strains to facilitate discrimination between true and false positives. The Y187 strain that the authors recommend be used in conjunction with the pHIS3/pINT1 reporter system provides the opportunity to use a dual reporter system. It contains an additional GAL1UASGAL1TATA-lacZ construct, thus, a reporter gene with an upstream sequence that is unrelated to the bait sequence. Based on the unwanted activation of this second reporter, it is possible to discard a subset of false positives, probably representing nonspecific DNA-binding proteins. The lacZ reporter in Y187 was integrated with the URA3 marker and can be counterselected with 0.1% (v/v) 5-fluoro-orotic acid. If desired, it can therefore be replaced to make alternative dual-reporter strains, or another construct can be added to make a triple reporter. One possibility is addition of a construct containing the bait sequence upstream of another reporter gene that also has a different minimal promoter than that of the HIS3 reporter (see Table 12.12.4). Such a reporter should be activated by true positives, and a negative interaction would select against false positives that activated the bait-HIS3 reporter through sequences outside the bait itself. Another elegant alternative is to add a reporter with a mutant bait sequence, but this is only applicable if it is already known which nucleotides in the bait sequence are critical for its cis-acting function. A good control is also to retransform isolated library clones to a strain containing a mutant bait-HIS3 repo rter integrated via the pHIS3/pINT1 system, so that it is present in an exactly identical chromosomal context as the bait reporter. Furthermore, control transformations can be performed to strains containing pHIS3/pINT1-based reporters lacking the upstream bait sequence or containing an unrelated upstream sequence. The nature of false positives is diverse (see Anticipated Results) and it should be taken into account that even with various different control reporters, not necessarily all the false positives can be eliminated. Some may follow specificity criteria in yeast exactly as expected for true positives (Schouten et al., 2000). Therefore, it remains essential that
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Table 12.12.4 Procedures
A Selection of Useful Cloning, Expression and Reporter Vectors for Modified Screening Applications or
Vectorsa
Features
Reference
pRS300 seriesb,c
Yeast markers URA3, TRP1, LEU2, HIS3, and LYS2. Replication via ARS-CEN
Sikorski and Hieter, 1989; Sikorski and Boeke, 1991
pRS400 seriesb,c
Yeast markers URA3, TRP1, LEU2, HIS3, MET15, ADE2, and LYS2. Integrating, ARS-CEN, and 2-µm versions
Sikorski and Hieter, 1989; Christianson et al., 1992; Brachmann et al., 1998
pYC seriesd
Yeast markers URA3, MET2-CA, G418. ARS-CEN and 2-µm versions. Counterselectable marker PKA3 regulatable by conditional CHAI or MET25 promoters
Olesen et al., 2000
YcpIF seriesc
Yeast markers URA3, TRP1, LEU2, HIS3. Replication via ARS-CEN. Expression cassettes for translational fusions, equipped with inducible GAL1 promoter
Foreman and Davis, 1994
p4XX seriesc
Yeast markers URA3, TRP1, LEU2, HIS3. ARS-CEN and 2-µm versions. Expression cassettes for transcriptional fusions, equipped with the CYC1 terminator and CYC1, ADH1, GPD, TEF, MET25, or GAL1 promoters
Mumberg et al., 1994, 1995
pMELα/β2, YIpMELα/β2e
MEL1 or lacZ reporter vectors. Integrating versions and ARS-CEN-replicating versions with URA3 marker
Melcher et al., 2000
pSLF187K
lacZ reporter vector, URA3 marker, replication via 2-µm
Forsburg and Guarente, 1988
LacZib
lacZ reporter, URA3 marker. Integrative vector
Clontech
aAll vectors replicate high-copy in E. coli and confer carbenicillin resistance. bComplete sequences are deposited in the NCBI database. cAvailable from the American Type Culture Collection (ATCC). dVectors and sequences available from http://www.crc.dk/phys/pyc/index.htm. eVectors and sequences are available from http://www.rz.uni-frankfurt.de/FB/fb16/mikro/euroscarf/data/km_rep.html.
Yeast One-Hybrid Screening for DNA-Protein Interactions
a yeast result be followed up by an independent confirmation of the specific interaction. Finally, several interesting variations of the one-hybrid concept have been developed. One system, which the authors call “bridge one-hybrid,” can, for example, be applied for cloning coactivators of transcription factors (Sieweke et al., 1996; Yu et al., 1997). Here, a transcription factor, specifically interacting with a target sequence upstream of the reporter gene, is used to attract library proteins that increase the reporter gene expression. This type of “interactor hunt” differs from the conventional two-hybrid approach in that there is a specific selection for proteins that interact when the transcription factor is bound to its own target sequence. Therefore, this would more likely reflect authentic interactions. Another system is a reverse one-hybrid approach that can detect library proteins capable of dissociating the interaction between a known factor and its target sequence (Vidal et al., 1996). Both the one-hybrid and reverse one-hybrid systems can also be modified to mutation screens for identifica-
tion of critical amino acid residues in functional domains of the transcription factor (Bush et al., 1996; Vidal et al., 1996). A last and particularly challenging variation is to express an AD hybrid of a known transcription factor and screen for its target sites. This requires a reporter gene library with genomic DNA fragments or random oligonucleotides upstream of the reporter gene, and a procedure to discriminate between true positive target sequences and false-positive sequences that are activated by endogenous yeast factors (Wilson et al., 1991; Liu et al., 1993; Meijer et al., 1998).
Critical Parameters and Troubleshooting A critical step in setting up yeast one-hybrid screenings is how to deal with the leaky HIS3 expression that can occur due to recognition of the bait sequence by endogenous yeast transcription factors. Whether or not a particular reporter construct will be activated by a yeast transcription factor is difficult to predict in advance, but it is known that several cis-acting
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sequences of plant or animal genes can also be recognized by yeast factors. Nevertheless, it may still be possible to screen in yeast for the corresponding plant or animal factors. One-hybrid screenings have even been used to clone factors from yeast itself (Li and Herskowitz, 1993; Kunoh et al., 2000). In recent years, a wide variety of transcription factors from plants (e.g., Kim et al., 1997; Menke et al., 1999; Meijer et al., 2000; van der Fits et al., 2000) or animals (e.g., Wang and Reed, 1993; Wei et al., 1999) have been cloned with a range of promoters and binding sites. Therefore, it does not seem that the one-hybrid approach is limited to cloning of particular transcription factor classes. With the lack of knowledge to predict in advance whether a certain promoter will be activated in yeast, the best and fastest strategy is still trial and error. The authors have used promoters over 1 kb in length that needed to be titrated with only 5 mM 3-AT. On the other hand the authors have experienced the situation whereby occasionally even a short sequence could confer so much HIS3 expression that this could not be inhibited with 3-AT up to 50 mM or higher, making its use in a screening impossible. Obviously, with longer promoters, the risk that such a strong enhancer is present becomes increased. Therefore, it is advised that several lengths or subfragments be tested. Screening conditions will be optimal if concentrations of 3-AT up to 25 mM suffice to reduce the HIS3 expression to an acceptable minimum. Higher concentrations of 50 up to 100 mM can still be tried, but the growth of the positives will also be slowed considerably, and it will be more difficult to distinguish these from the background. If the background is unworkably high, it may be useful to test the reporter in other yeast strains (Table 12.12.2), because the level of leaky expression can depend considerably on the genotype. Furthermore, the use of a yeast TF mutant as host strain could be considered, if clues exist about the identity of the endogenous yeast factor that might cause the background problem. If there are no leaky expression problems, it can be useful to make multimers of the bait sequence. This can increase the sensitivity of the detection of positive interactions in the library screening. The authors have designed a stepwise unidirectional multimerization procedure (Ouwerkerk and Memelink, 1997) to tetramerize sequences ranging from 9 to 250 bp, and have used these successfully in one-hybrid screenings (e.g., Meijer et al., 2000).
It is important to keep track of the number of clones that have been screened to be able to decide at what stage the library screening has been saturating. This is dependent on the complexity of the primary cDNA library. Furthermore, discrimination between true and false positives is an important factor. Isolated library clones should be retransformed to the baitHIS3 strain, and preferably also to different strains with negative control reporters (see Background Information). The nature of common false-positive cDNA clones is discussed under Anticipated Results. Finally, it is important to be aware of possible bacterial contamination in a yeast screening. Occasionally, bacterial contaminants can be very misleading because they can appear in close association with the yeast cells. Therefore, seemingly normal yeast colonies can occur on selective medium, while their capacity to grow actually results from bacterial crossfeeding rather than from expression of the auxotrophic marker gene. The presence of bacteria in suspected yeast colonies can easily be spotted with phase-contrast microscopy. Elimination is possible by streaking the colonies on CM −Leu −His plates supplemented with carbenicillin (50 to 200 mg/liter) and kanamycin (100 mg/liter)—antibiotics which do not affect the growth of yeast.
Anticipated Results It is difficult to predict how many positive clones are to be expected from a saturating library screening. This is very much dependent on the bait sequence as well as on the cDNA library source. First of all, the frequency of false positives is extremely variable. With some bait sequences, these can represent the majority of the clones obtained, whereas nearly all positives in other screenings may be specific transcription factors. Furthermore, the frequency of isolation of a certain clone is no indication for its validity. This frequency is determined by factors such as the abundance in the mRNA population from which the library was derived and the stability in E. coli and yeast. If no positives are obtained at all, it is useful to change the cDNA library. However, it is also possible that a library protein that could potentially interact with the bait sequence was present, but is degraded in yeast or sequestered due to interaction with a certain yeast protein. Another cause for an unsuccessful result may be that the bait sequence is masked or repressed due to binding of a yeast transcription factor. For this reason, it may be worthwhile to try
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different lengths or subfragments of the bait sequence. In the case where a very large number of positives are isolated, it is useful to attempt to sort them into classes, e.g., based on restriction or cross-hybridization patterns. Several groups of possible false positives can be discriminated. The first group includes, among others, nucleoside transporters, ribosomal proteins, RNA binding proteins, thymine glycosylases, amino-acyltransferases, and histones (P.B.F. Ouwerkerk, unpub. observ.; van der Fits, 2000). These factors all have in common the fact that their normal function is related to interactions with nucleic acids. As AD hybrids, such proteins are likely to turn into artifactual transcriptional activators. A second group of artifacts are components of signal transduction pathways such as phosphatases, kinases (Schouten, 1999), and GTPases (van der Fits, 2000), or of proteolysis pathways. Strikingly, their possible occurrence is very much dependent on the bait sequence used. Therefore, it is most likely that such proteins interfere with signal transduction pathways in yeast in such a way that specific activation occurs of endogenous yeast transcription factors that recognize the bait sequence. An exemplifying case is the search for plant factors regulating the T-cyt gene of the tumorinducing soil bacterium Agrobacterium tumefaciens (Schouten et al., 2000). Screenings in yeast resulted in several library clones that were able to activate HIS3 expression via a wild-type bait sequence but not via a mutant sequence, suggesting that the interaction was specific. However, instead of DNA-binding proteins, the discovered clones turned out to represent plant proteins homologous with the yeast Skp1 protein that is involved in ubiquitin-mediated protein degradation. The following mechanism was proposed for the interference of the Skp1 homologs with reporter gene activation. The Skp1-like proteins would interact with the yeast F-box protein, Grr1p. This contact would result in the destabilization of Grr1p, which in turn would inactivate the yeast repressor protein Mig1p. The Mig1p repressor interacted with the bait sequence and consequently its inactivation could make this sequence accessible to another yeast factor, resulting in activation of the reporter gene. Because both the Mig1p repressor and the yet unidentified yeast activator appeared to have the same binding specificity as the searched plant T-cyt binding factor, the identification of this plant factor via yeast screening was impossible.
The third group of false positives are His3p homologs, which can complement the mutant his3 locus in the yeast genome. These exist in plants and fungi, but not in animals. In the authors’ experience, their frequency of isolation in one-hybrid screenings is low. The fourth and last identified category of artifacts is miscellaneous and includes proteins involved in metabolic pathways, such as L-ascorbate peroxidase, pectinesterase, and others (van der Fits, 2000). Their occurrence is enigmatic and it is not very likely that these proteins directly activate reporter gene expression. The fact is that somehow they are beneficial for the yeast cells to survive under the selection conditions. It might be that they are involved in sequestering or degrading the His3p inhibitor 3-AT, due to which titration of leaky HIS3 reporter expression would fail and a false positive colony would grow. The protein-coding sequences of isolated cDNAs can be either partial or full-length. Preceding leader sequences can be translated to form a spacer between the AD and the library protein. Unexpectedly, sometimes positive cDNA clones are found for which the encoded protein sequence is not in frame with the AD sequence. Such clones can still represent true positives, as demonstrated by the following example. In a one-hybrid screening in yeast strains that contained reporters with upstream recognition sites for homeodomain proteins, the authors identified cDNAs from rice encoding homeodomain-leucine zipper (HD-Zip) proteins of two different families, I and II (Meijer et al., 2000). Strikingly all the family II members were in frame with the AD, but all the family I members were not. The authors concluded that the reporters must have been activated due to the fact that truncated HD-Zip I proteins were expressed, which possessed an intrinsic activation function. Subsequently, in plant experiments, it was indeed confirmed that HD-Zip I proteins could act as activators, whereas the HD-Zip II proteins turned out to be repressors. Since none of the isolated HD-ZIP I clones were in-frame fusions, in this case AD-hybrids might have been toxic, or the combination of Gal4p AD and intrinsic AD might have been incompatible.
Time Considerations When the required pHIS3NB/NX and pINT1 reporter constructs have been cloned, it will take ∼1 week to transform yeast and to select for G418-resistant pINT1-HIS3 reporter strains. The next week, the strains should be
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tested on a range of 3-AT concentrations in order to determine the selective conditions for the actual library screening. Subsequently, it is useful to invest 1 to 2 weeks to fine tune the 3-AT concentration during pilot library transformations. Unless the transformation efficiency is suboptimal, all the transformations necessary to perform a saturating library screening, including the media preparation, can be finished within 1 week. The screening plates are incubated for 1 to 2 weeks during which positives can already be restreaked. As described in the protocol, from this point onward several strategies can be followed to discriminate between true and false positives, and to confirm the specificity of putative positives. Dependent on the numbers of positives and their nature, these analyses can take several weeks to many months.
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Gietz, D., St. Jean., A., Woods, R.A., and Schiestl, R.H. 1992. Improved method for high efficiency transformation of intact yeast cells. Nucleic Acids Res. 20:1425. Grueneberg, D.A., Natesan, S., Alexandre, C., and Gilman, M.Z. 1992. Human and Drosophila homeodomains proteins that enhance the DNAbinding activity of serum response factor. Science 257:1089-1095. Hadfield, C., Fordan, B.E., Mount, R.C., Pretorius, G.H.J., and Burak, E. 1990. G418 resistance as a dominant marker and reporter for gene expression in Saccharomyces cerevisiae. Curr. Genet. 18:303-313. Harper, J.W., Adami, G.R., Wei, N., Keyomarsi, K., and Elledge, S.J. 1993. The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclindependent kinases. Cell 75:805-16. Kim, S.Y., Chung, H.-W., and Thomas, T. 1997. Isolation of a novel class of bZIP transcription factors that interact with ABA-responsive and embryo-specification elements in the Dc3 promoter using a modified yeast one-hybrid system. Plant J. 11:1237-1251. Kunoh, T., Kaneko, Y., and Harashima, S. 2000. YHP1 encodes a new homeoprotein that binds to the IME1 promoter in Saccharomyces cerevisiae. Yeast 16:439-449. Leuther, K.K. and Johnston, S.A. 1992. Nondissociation of GAL4 and GAL80 in vivo after galactose induction. Science 256:1333-1335. Li, J.J. and Herskowitz, I. 1993. Isolation of ORC6, a component of the yeast origin recognition complex by a one-hybrid system. Science 262:18701874.
Chan, J.Y., Han, X.-L., and Kan, Y.W. 1993. Cloning of Nrf1, an NF-E2-related transcription factor, by genetic selection in yeast. Proc. Natl. Acad. Sci. U.S.A. 90:11371-11375.
Liu, J., Wilson, T.E., Milbrandt, J., and Johnston, M. 1993. Identifying DNA-binding sites and analyzing DNA-binding domains using a yeast selection system. In Methods: A Companion to Methods in Enzymology. Vol. 5 pp. 125-137. Academic Press, New York.
Christianson, T.W., Sikorski, R.S., Dante, M., Shero, J.H., and Hieter, P. 1992. Multifunctional yeast high-copy-number shuttle vectors. Gene 110:119-122.
Meijer, A.H., Ouwerkerk, P.B.F., and Hoge, J.H.C. 1998. Vectors for transcription factor cloning and target site identification by means of genetic selection in yeast. Yeast 14:1407-1415.
Elledge, S.J., Mulligan, J.T., Ramer, S.W., Spottswoo, M., and Davis, R.W. 1991. λYES: A multifunctional cDNA expression vector for the isolation of genes by complementation of yeast and Escherichia coli mutations. Proc. Natl. Acad. Sci. U.S.A. 88:1731-1735.
Meijer, A.H., de Kam, R.J., d’Erfurth I., Shen W., and Hoge, J.H.C. 2000. HD-Zip proteins of families I and II from rice: Interactions and functional properties. Mol. Gen. Genet. 263:12-21.
Fields, S. and Song, O. 1989. A novel genetic selection system to detect protein-protein interaction. Nature 340:245-246.
Melcher, K., Sharma B., Ding W.V., and Nolden, M. 2000. Zero background yeast reporter plasmids. Gene 247:53-61.
Foreman, P.K. and Davis, R.W. 1994. Cloning vectors for the synthesis of epitope-tagged, truncated and chimeric proteins in Saccharomyces cerevisiae. Gene 144:63-68.
Memelink, J. 1997. Two yeast/Escherichia coli lambda/plasmid vectors designed for yeast one- and two-hybrid screens that allow directional cDNA cloning. Technical Tips Online TT01111 (http://research.bmn.com/). Trends Genet. 13:376.
Forsburg, S.L. and Guarente, L. 1988. Mutational analysis of upstream activation sequence 2 of the CYC1 gene of Saccharomyces cerevisiae: A HAP2-HAP3-responsive site. Mol. Cell. Biol. 8:647-654.
Menke, F.L., Champion, A., Kijne, J.W., and Memelink, J. 1999. A novel jasmonate- and elicitor-responsive element in the periwinkle secondary metabolite biosynthetic gene Str interacts with a jasmonate- and elicitor-inducible AP2-domain
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transcription factor, ORCA2. EMBO J. 18:44554463. Mumberg, D., Müller, R., and Funk, M. 1994. Regulatable promoters of Saccharomyces cerevisiae: Comparison of transcriptional activity and their use for heterologous expression. Nucleic Acids Res. 14:7861-7871. Mumberg, D., Müller, R., and Funk, M. 1995. Yeast vectors for the controlled expression of heterologous proteins in different genetic backgrounds. Gene 156:119-122. Olesen, K., Johannesen, P.F., Hoffmann, L., Sørensen, S.B., Gjermansen, C., and Hansen, J. 2000. The pYC plasmids, a series of cassette-based yeast plasmid vectors providing means of counter-selection. Yeast 16:1035-1043.
van der Fits, L., Zhang, H., Menke, F.H.M., Deneka, M., and Memelink, J. 2000. A Catharanthus roseus BPF-1 homologue interacts with an elicitor-responsive region of the secondary metabolite biosynthetic gene Str and is induced by elicitor via a JA-independent signal transduction pathway. Plant Mol. Biol. 44:675-685. Vidal, M., Brachmann, R.K., Fattaey, A., Harlow, E., and Boeke, J.D. 1996. Reverse two-hybrid and one-hybrid systems to detect dissociation of protein-protein and DNA-protein interactions. Proc. Natl. Acad. Sci. U.S.A. 93:10315-10320. Wang, M. and Reed, R.R. 1993. Molecular cloning of the olfactory neuronal transcription factor Olf-1 by genetic selection in yeast. Nature 364:121-126.
Ouwerkerk, P.B.F. and Memelink, J. 1997. A simple method for directional multimerization of DNA s eq u e n ce s. Tec h n i cal Tips On line T01051(http://research.bmn.com/)/Trends Genet. 13:207.
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Schouten, J. 1999. Identification of Arabidopsis thaliana cDNAs coding for proteins that can activate gene expression in yeast via the cyt-1 element of the Agrobacterium tumefaciens T-cyt gene promoter. Ph.D. Thesis, Leiden University, The Netherlands.
Wilson, T.E., Fahrner, T.J., Johnston, M., and Milbrandt, J. 1991. Identification of the DNA binding site for NGFI-B by genetic selection in yeast. Science 252:1296-1300.
Schouten, J.S., de Kam, R.J., Fetter, K., and Hoge, J.H.C. 2000. Overexpression of Arabidopsis thaliana SKP1 homologues in yeast inactivates the Mig1 repressor by destabilising the F-box protein Grr1. Mol. Gen. Genet. 263:309-319.
Yu, Y., Li, W., Su, K., Yussa, M., Han, W., Perrimon, N., and Pick, L. 1997. The nuclear hormone receptor Ftz-F1 is a cofactor for the Drososphila homeodomain protein Ftz. Nature 385:552-555.
Key References Meijer et al., 1998. See above.
Sieweke, M.H., Tekotte, H., Frampton, J., and Graf, T. 1996. MafB is an interaction partner and repressor of Ets-1 that inhibits erythroid differentiation. Cell 85:49-60.
First description of the pHIS3/pINT1 vector system for yeast reporter strain construction.
Sikorski, R.S. and Boeke, J.D. 1991. In vitro mutagenesis and plasmid shuffling: From cloned gene to mutant yeast. In Methods of Enzymology, Vol. 194: Guide to Yeast Genetics and Molecular Biology (C. Guthrie and G.R. Fink, eds.) pp. 302-318. Academic Press, San Diego.
http://www.atcc.org/
Sikorski, R.S. and Hieter, P. 1989. A system of shuttle vectors and yeast host strains designed for efficient manipulation of DNA in Saccharomyces cerevisiae. Genetics 122:19-27. Singh, H., LeBowitz, J.H., Baldwin, A.S., Jr., and Sharp, P.A. 1988. Molecular cloning of an enhancer binding protein: Isolation by screening of an expression library with a recognition site DNA. Cell 52:415-423. van der Fits, L. 2000. Transcriptional regulation of stress-induced plant secondary metabolism. Ph.D. Thesis, Leiden University, The Netherlands.
Internet Resources American Type Culture Collection(ATCC): source for yeast vectors and strains. http://www.ncbi.nlm.nih.gov/ National Center for Biotechnology Information (NCBI, Genbank): public database of genes and vectors sequences, computational tools for sequence analysis http://www.clontech.com http://www.stratagene.com/ Commercial sources for vectors, AD-hybrid libraries, yeast strains, and supplies.
Contributed by Pieter B.F. Ouwerkerk and Annemarie H. Meijer Leiden University Leiden, The Netherlands
Yeast One-Hybrid Screening for DNA-Protein Interactions
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CHAPTER 13 Yeast INTRODUCTION Saccharomyces cerevisiae (baker’s yeast) and Schizosaccharomyces pombe (fission yeast) are often considered to be model eukaryotic organisms, in a manner analogous to Escherichia coli as a model prokaryotic organism. Both yeasts have been extensively characterized and their genomes completely sequenced. They are as easy to grow as other microorganisms, and they have a haploid nuclear DNA content only 3.5 times that of E. coli. However, despite the small genomes sizes, these yeasts display most of the features of higher eukaryotes. The fact that many cellular processes are conserved among different eukaryotic species- combined with the powerful genetic and molecular tools that are available- has made these yeasts important experimental organisms for a variety of basic problems in eukaryotic molecular biology. Primarily for historical reasons, most studies on yeast have involved Saccharomyces cerevisiae (hereafter termed yeast). Culturing yeast is simple, economical, and rapid, characterized by a doubling time of ∼90 min on rich medium. In addition, yeast has been well adapted to both aerobic and anaerobic large-scale culture. Cells divide mitotically by forming a bud, which pinches off to form a daughter cell. The progression through the cell cycle can be monitored by the size of the bud; this has been used to isolate a large collection of mutants (called cdc mutants) that are blocked at various stages of the cell cycle. Since yeast can be grown on a completely defined medium (see UNIT 13.1), many nutritional auxotrophs have been isolated. This has not only permitted the analysis of complex metabolic pathways but has also provided a large number of mutations useful for genetic analysis. Yeast can exist stably in either haploid or diploid states. A haploid cell can be either of two mating types, called a and α. Diploid a/α cells—formed by fusion of an α cell and an a cell (UNIT 13.2)—can grow mitotically indefinitely, but under conditions of carbon and nitrogen starvation will undergo meiosis. The meiotic products, called spores, are contained in a structure called an ascus. After gentle enzymatic digestion of the thick cell wall of the ascus, the haploid spore products can be individually isolated and analyzed (UNIT 13.2). This ability to recover all four products of meiosis has allowed detailed genetic studies of recombination and gene conversion that are not possible in most other eukaryotic organisms. The existence of stable haploid and diploid states also facilitates classical mutational analysis, such as complementation tests and identification of both dominant and recessive mutations. The haploid yeast cell has a genome size of about 15 megabases and contains 16 linear chromosomes, ranging in size from 200 to 2200 kb. Thus, the largest yeast chromosome is still 100 times smaller than the average mammalian chromosome. This small chromosome size, combined with the advent of techniques for cloning yeast genes as well as manipulating yeast chromosomes, has allowed detailed studies of chromosome structure. Three types of structural elements required for yeast chromosome function have been identified and cloned: origins of replication (ARS elements), centromeres (CEN elements), and telomeres. The cloning of these elements has led to the construction of artificial chromosomes that can be used to study various aspects of chromosome behavior, such as how chromosomes pair and segregate from each other during mitosis and meiosis. In addition, systems using artificial chromosomes have been designed that allow cloning of Contributed by Victoria Lundblad and Kevin Struhl Current Protocols in Molecular Biology (2003) 13.0.1-13.0.3 Copyright © 2003 by John Wiley & Sons, Inc.
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larger contiguous segments of DNA (up to 400 kb) than are obtainable in other cloning systems. These structural elements, as well as cloned selectable yeast genes, have permitted the construction of yeast/E. coli shuttle vectors that can be maintained in yeast as well as in E. coli (UNITS 13.4 & 13.6). Procedures for high-efficiency transformation of yeast (UNIT 13.7) have been available for nearly two decades, allowing cloning of genes by genetic complementation (UNITS 13.8 & 13.9). Because yeast has a highly efficient recombination system, DNAs with alterations in cloned genes can be reintroduced into the chromosome at the corresponding homologous sites (UNIT 13.10). This has permitted the rapid identification of the phenotypic consequences of a mutation in any cloned gene, a technique generally unavailable in higher eukaryotes. In addition, homologous recombination permits a wide variety of genetic techniques that have greatly facilitated the analysis of biological processes. Despite its small genome size, yeast is a characteristic eukaryote, containing all the major membrane-bound subcellular organelles found in higher eukaryotes, as well as a cytoskeleton. Yeast DNA is found within a nucleus and nucleosome organization of chromosomal DNA is similar to that of higher eukaryotes, although no histone H1 is present. Three different RNA polymerases transcribe yeast DNA, and yeast mRNAs (transcribed by polymerase II) show characteristic modifications of eukaryotic mRNAs [such as a 5′ methyl-G cap and a 3′ poly(A) tail], although only a few S. cerevisiae genes contain introns. Transcriptional regulation has been extensively studied and at least one yeast transcriptional activator has been shown to function in higher eukaryotes as well. High-molecular-weight yeast DNA and RNA can be prepared fairly quickly (UNITS 13.11 & 13.12). Another characteristic of eukaryotes is the proteolytic processing of precursor proteins to yield functional products, which is often coupled to secretion. Yeast has several well-studied examples of secreted proteins and pheromones, and the large number of genes that have been identified as involved in protease processing and secretion suggests a highly complex pathway. Yeast protein extracts can be prepared using three different protocols (UNIT 13.13); the best choice will depend on the particular application. The ease and power of genetic manipulation in yeast facilitate the use of this organism to detect novel interacting proteins using the two-hybrid system or interaction trap (UNIT 20.1). Although Saccharomyces cerevisiae is the most commonly studied yeast, S. pombe is also an important experimental organism (UNIT 13.14). Although both yeasts are unicellular microorganisms that grow in similar medium, they are evolutionarily quite distant. It has become increasingly clear that, in terms of molecular mechanisms, S. pombe is more similar to higher eukaryotic organisms than S. cerevisiae. Experimental manipulations in S. pombe are broadly similar to those in S. cerevisiae, although the technical details often differ. The chapter includes units on S. pombe relating to strain maintenance and media (UNIT 13.15), growth and genetic manipulation (UNIT 13.16), and introduction of DNA into cells (UNIT 13.17). This chapter is written for the molecular biologist who has not previously worked with yeast. The glossary below introduces the terms of yeast molecular biology. aerobic growth growth in the presence of oxygen, utilizing the Krebs cycle. α and a factor mating type–specific polypeptides secreted by either α or a haploid cells, respectively, which interact with haploid cells of the opposite mating type to stimulate mating. anaerobic growth growth in the absence of oxygen, utilizing fermentation (via glycolysis).
ARS elements DNA sequences present throughout the yeast genome that confer autonomous replication on plasmids in yeast; most of these sequences also function as chromosomal origins of replication as well. ascus thick-walled sac containing the four haploid products, called spores, resulting from meiosis.
Introduction
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cdc mutants strains of yeast that exhibit stagespecific blocks of the cell cycle; these mutations define genes important in DNA replication, meiosis, and sporulation. CEN element DNA sequences present at the centromere that ensure proper segregation of chromosomes during mitosis and meiosis, presumably by promoting interaction with the mitotic spindle. cir+ strains of yeast that contain the naturally occurring 2µm plasmid. ciro yeast strains that have lost this endogenous 2µm plasmid. δ element ∼330-bp sequence, present as direct repeats at the ends of the transposable element Ty1, and also found dispersed throughout the genome. gene disruption a mutation constructed in vitro in a cloned gene which, upon reintroduction into the genome at the homologous chromosomal site, results in inactivation of the gene function. glusulase a digestive enzyme isolated from snails that breaks down thick cell walls of either an ascus to allow isolation of spore products, or a yeast cell to produce spheroplasts. heterothallic common laboratory strains of yeast which—due to a mutation in the HO gene—stably maintain a given allele at the MAT locus. homothallic strains of yeast (typically found in the wild) that, in a haploid state, rapidly interconvert the MAT locus, resulting in rapid switching between the α and a mating types; cultures of such strains rapidly diploidize. killer strains strains of yeast that harbor a double-stranded RNA virus; such strains kill sensitive yeast strains via secretion of a protein toxin, to which killer strains are immune. MAT the mating-type locus which is expressed and therefore determines the mating type of a haploid cell; this locus has two alleles—the MATa allele confers the a mating type, while MATα specifies the α mating type. meiosis the process by which the number of chromosomes present in a diploid cell is halved to yield haploid products. mitosis vegetative cell division (of either haploid or diploid cells) in which the chromosome
number stays the same. petites mutants of yeast (either nuclear or mitochondrial) with impaired mitochondria function; they grow as small colonies on fermentable carbon sources and are unable to grow on nonfermentable carbon sources. schmoo a distinctive shape of a haploid cell (pear-shaped), induced by exposure to mating pheromone. sporulation the end product of meiosis, induced by carbon and nitrogen starvation of a diploid cell, which results in four haploid progeny contained as spores within an ascus; this complicated developmental process requires over 200 genes. telomeres DNA sequences found at the end of linear chromosomes that are essential for chromosome stability and complete replication; in S. cerevisiae, telomeres consist of tandem repeats of the sequence 5′dG1-3dT3′. Ty1 elements the primary transposable element found in yeast, which functions as a retrotransposon (via reverse transcription of its RNA and subsequent reinsertion into the genome). UAS element upstream activating sequences in yeast promoters, to which regulatory proteins bind in order to enhance the rate of transcription. zygote a morphologically distinct cellular structure formed by the fusion of two haploid cells of opposite mating type, which results in formation of a diploid. Zymolyase β-glucanase, isolated from Arthrobacter luteus, that hydrolyzes the yeast cell wall and is used to prepare spheroplasts for a variety of purposes.
Key References Watson, J.D., Hopkins, N.H., Roberts, J.W., Steitz, J.A., and Weiner, A.M. 1987. Yeasts as the E. coli of eukaryotic cells. In Molecular Biology of the Gene, Vol. 1, pp. 550-594. Benjamin/Cummings, Menlo Park, Calif. Strathern, J.N., Jones, E.W., and Broach, J.R. (eds.) 1981. The Molecular Biology of the Yeast Saccharomyces: Life Cycle and Inheritance. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. Strathern, J.N., Jones, E.W. and Broach, J.R. (eds.) 1982. The Molecular Biology of the Yeast Saccharomyces: Metabolism and Gene Expression. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.
Victoria Lundblad and Kevin Struhl Yeast
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BASIC TECHNIQUES OF YEAST GENETICS
SECTION I
Like Escherichia coli, yeast can be grown in either liquid media or on the surface of (or embedded in) solid agar plates. Yeast cells grow well on a minimal medium containing dextrose (glucose) as a carbon source and salts which supply nitrogen, phosphorus, and trace metals. Yeast cells grow much more rapidly, however, in the presence of protein and yeast cell extract hydrolysates, which provide amino acids, nucleotide precursors, vitamins, and other metabolites which the cells would normally synthesize de novo. During exponential or log-phase growth, yeast cells divide every 90 min when grown in such media. Log phase can be divided into three stages based on the rate of cell division (or the proportion of budded cells within a culture), which is in turn a function of the cell density of the culture. As cell density increases, nutrient supplies drop and the rate of cell division slows (the measurement of cell density, as well as techniques for the propagation and genetic manipulation of yeast, are described in UNIT 13.2). Early-log phase is the period when cell densities are 5 years), or at 4°C on slants consisting of rich medium supplemented with potato starch (viable for 1 to 2 years). Both methods are described below. BASIC PROTOCOL
Preparation and Inoculation of Frozen Stocks Make a solution of 30% (w/v) glycerol. Pipet 1 ml into 15 × 45-mm, 4-ml screwcap vials. Loosely cap the vials and autoclave 15 min. To inoculate vials for storage, add 1 ml of a late-log or early-stationary phase culture, mix, and set on dry ice. Store at −70°C. Revive by scraping some of the cells off the frozen surface and streak onto plates. Do not thaw the entire vial. Cells can also be stored in the same way by adding 80 µl dimethyl sulfoxide (DMSO) to 1 ml cells (8% v/v) and storing at −70°C.
ALTERNATE PROTOCOL
Preparation and Inoculation of Slants 1. For 250 slants, add the following ingredients to a 1-liter flask: 70 g YPD plate premix 20 mg adenine (hemisulfate salt) 20 g potato starch H2O to 500 ml Excess adenine prevents ade− mutations from being lost.
2. Set the flask in a 4-liter beaker filled with 1 liter of water. Place the beaker on a heat-controlled, magnetic stirring apparatus and stir with the heat setting on high. With smooth and continuous stirring, the contents should not burn and should be molten after ∼1 hr.
3. With the entire setup in place, begin pipetting 2-ml aliquots into 15 × 45-mm, 4-ml screwcap vials. When all vials have been filled, put the caps on loosely, pack in the original boxes, and autoclave 15 min. 4. Lean the boxes against a support at an angle of ∼70°. Allow the slants to dry 2 days and screw the caps on tightly. Preparation of Yeast Media
Slants can be stored at room temperature for at least 6 months.
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5. To inoculate a slant, smear cells from the flat end of a sterile toothpick onto the agar surface of the slant. Cap loosely and incubate 1 or 2 days at 30°C. After growth, screw the cap on as tightly as possible and store at 4°C. Slants are a convenient way to store and mail strains. They can be mailed immediately after inoculating since sufficient growth will occur in transit. See another method for mailing strains below.
Mailing and Reviving Strains Yeast strains can be conveniently mailed as slants. Alternatively, transfer cells to a piece of sterile Whatman 3MM paper by pressing the paper onto the desired yeast colony using forceps that have been dipped in ethanol and flamed. Wrap the paper in sterile aluminum foil and mail to recipient.
BASIC PROTOCOL
Revive the strain by placing the paper (yeast side down) on the surface of an agar plate. Incubate the plate at 30°C. A thick patch of yeast should be visible after lifting the paper. LITERATURE CITED Boeke, J.D., LaCroute, F., and Fink, G.R. 1984. A positive selection for mutants lacking orotidine-5′-phosphate decarboxylase activity in yeast: 5-fluoroorotic acid resistance. Mol. Gen. Genet. 197:345-346. Brown, P.A. and Szostak, J.W. 1983. Yeast vectors with negative selection. Meth. Enzymol. 101:278-290. Sherman, F., Fink, G.R., and Lawrence, C.W. 1979. Methods in Yeast Genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.
Contributed by Douglas A. Treco Massachusetts General Hospital Boston, Massachusetts Victoria Lundblad Baylor College of Medicine Houston, Texas
Saccharomyces cerevisiae
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UNIT 13.2
Growth and Manipulation of Yeast Aside from different media requirements, yeast cells are physically manipulated essentially as described for bacterial cells—i.e., they are grown in liquid culture (in tubes or flasks) or on the surface of agar plates and are manipulated using the basic equipment described in UNITS 1.1-1.3. In addition, a well-equipped yeast laboratory requires static and shaking incubators dedicated to 30°C and a microscope with magnification up to 400×. A second microscope adapted for dissecting yeast tetrads is extremely valuable for the genetic analyses and strain constructions described in this unit. A small electric clothes dryer is indispensable when replica plating is done frequently and large numbers of velvets are regularly used. This unit first presents the necessary details for growing yeast cells. This is followed by a description of replica plating methods for assessing the nutritional requirements and mating types of strains. Yeast genetic experiments often require the construction of strains with specific genotypes, as well as an analysis of the meiotic segregation patterns of newly introduced mutations (see UNIT 13.8). These genetic manipulations are carried out using the protocols presented in the final sections of this unit, which describe the construction and selection of diploids, sporulation, and tetrad analysis.
BASIC PROTOCOL
GROWTH IN LIQUID MEDIA Wild-type S. cerevisiae grows well at 30°C with good aeration and with glucose as a carbon source. When using culture tubes, vortex the contents briefly after inoculation to disperse the cells. Erlenmeyer flasks work well for growing larger liquid cultures, and baffled-bottom flasks to increase aeration are especially good. It is important that all glassware be detergent-free. For good aeration, the medium should constitute no more than one-fifth of the total flask volume, and growth should be carried out in a shaking incubator at 300 rpm. For small-scale preparations of DNA and RNA, yeast can be grown in glass or plastic culture tubes filled one-third full with medium and shaken at 350 rpm in a rack firmly attached to a shaking incubator platform.
BASIC PROTOCOL
BASIC PROTOCOL
GROWTH ON SOLID MEDIA Yeast cells can be streaked or spread on plates as shown for bacteria in the sketches in UNIT 1.3. When a dilute suspension of wild-type haploid yeast cells is spread over the surface of a YPD plate and incubated at 30°C, single colonies may be seen after ∼24 hr but require ≥48 hr before they can be picked or replica plated (see below). Growth on dropout media (UNIT 13.1) is about 50% slower. DETERMINATION OF CELL DENSITY The density of cells in a culture can be determined spectrophotometrically by measuring its optical density (OD) at 600 nm. For reliable measurements, cultures should be diluted such that the OD600 is Brightness/Contrast Process >Subtract Background (this requires loading of a background image that is devoid of any structures of interest) Process > Filters > Gaussian Blur Analyze >Measure 3. Rotate digital sequences by selecting Rotate from the Image menu for export to QuickTime, which can then be played in Powerpoint presentations or uploaded onto the Web in the orientation that makes most sense in comparison to other image sequences or to a model/diagram. In ImageJ, if a processing routine is applied to a digital sequence, one will be stuck with the results (i.e., this is destructive editing). Therefore, one should always keep a file of the original data separate from the sequence being processed, and save often to preserve desired changes in case subsequent processing is not useful.
4. Save the new image sequences by selecting Save As from the File menu. For a TIFF series, select Image Sequence from the submenu that appears. For QuickTime, select options from the popup menu: Compressor: Video Color Quality: Medium to High Timing: Frames per Second 6-10 Keyframe every 10 Further processing and analysis can be performed in ever increasingly complex software applications.
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COMMENTARY Background Information
Image Analysis, Image Importing, and Image Processing Using Freeware
Image analysis to determine the structure of molecules and particles Prior to the advent of digital image processing, various manual manipulations were required to extract quantitative information from structural data. The term “structural data” in this context refers to photographic or digital images of biological specimens, ranging from whole animals to tiny single molecules. For example, diffraction patterns can be obtained by passing laser light through film containing images of subcellular structures displaying repetitive patterns resembling crystalline arrays, such as the catalase crystal in the peroxisome (Bearer and Orci, 1986), the acetylcholine receptor channel (Unwin, 2002), and the protein coat of viruses (DeRosier and Klug, 1972). Reconstruction of the “ideal” crystalline structure is obtained by solving the diffraction pattern in reciprocal space. Another approach superimposes multiple electron microscopic images of the same structure, which “averages” the image, decreasing the intensity of nonrepeating structures, including those of unrelated background particles or of the movable parts within a fluid region of the structure. This technique has commonly been used in the analysis of electron micrographs. Multiple exposures of the same photographic paper with different images or rotations of the same image superimposed upon itself identifies repeating structures and internal symmetries within a structure. This technique was used to discover the eight-fold symmetry of the endothelial fenestral diaphragm, responsible for the passage of nutrients from blood to tissue in human organs (Bearer and Orci, 1985). Statistical analysis of biological specimens, also known as “morphometrics,” involves compiling measurements of multiple specimens of the same structure and then mathematically deriving the average size and other statistical values such as standard deviation within the population studied. Such mathematical compilations rely on the acquisition of a large number of measurements from multiple specimens, which until very recently was performed by hand. However, only relatively small sample sizes are easily analyzed by hand, since this requires a large time investment. Recent advances in structural determination of complex cellular components not amenable to crystallization include tomographic recon-
struction of electron-microscopic images. In this approach, images are captured from thick sections of biological specimens embedded in either plastic or vitreous ice using an intermediate-voltage electron microscope. The section is photographed at various angles of tilt, and the 3-D structure assembled digitally. The structure of the neuronal synapse is yielding to this approach (Harlow et al., 1998), which was recently cited as one of the ten top scientific advances in 2002 by the journal Science (News and Editorial Staff, 2002). New technology exploiting digital image processing includes laser scanning and twophoton confocal microscopy, which allows elimination of the out-of-focus light from fluorescently labeled specimens. Confocal microscopy allows optical sectioning and thereby the acquisition of sharp images of objects deep within thick specimens (White et al., 1987). In confocal microscopy, the image is filtered before capture. Confocal microscopy relies on monochromatic laser excitation of fluorescent tags, and collects only in-focus emission using a pinhole to block out-of-focus emitted light. In two-photon confocal imaging, a recent advance, excitation and collection occurs only from the same optical section of interest, thus prolonging the length of time that a living biological specimen can be observed without laser damage (Kaech et al., 2001; Meyer et al., 2003). New computational approaches (Meta and Multi-Tracker options in the Zeiss 510 LSM package) narrow the bandwidth of detected emitted light, thus providing the possibility of imaging multiple fluorochromes with the same excitation and even only slightly differing emission spectra in the same specimen. An alternative way to eliminate out-of-focus light is deconvolution, which involves digital processing of a series of images obtained by focusing up and down in the same microscopic field. In this approach, out-of-focus light is eliminated digitally after image capture (Hiraoka et al., 1990). Deconvolution requires a motorized stage to collect a series of images in the z-plane (Kam et al., 2001). These images are then processed digitally to extract in-focus structure and detail and eliminate fuzz from out-of-focus or autofluorescent emissions. The equipment is less costly than that used for confocal, but collection times are longer, and thus deconvolution is not as suitable for living specimens undergoing rapid changes. Both of these approaches allow visualization of struc-
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tures by fluorescence microscopy that would not be resolved by conventional microscopy. Useful texts on microscopy include Matsumoto (1993) and Inoué and Spring (1997). Microscopy courses at the Marine Biological Laborato ry in Wo od s Hole, Massachusetts (http://www.mbl.edu/) are also excellent for viewing demonstrations of the latest equipment. Video microscopy, pioneered by Robert Allen in the late 1970s using Nomarski optics (differential interference contrast or DIC), is capable of detecting structures that are theoretically below the resolution of the light microscope if lenses with high numerical apertures are used (Allen et al., 1979; Hayden and Allen, 1984; Brady et al., 1985). Such lenses have narrow focal planes, and often have short working distances as well; thus, they have not yet proven useful for imaging objects deep within thick specimens. However, like confocal microscopy, video imaging with either DIC or fluorescent tags can be performed on live specimens. Analysis of such sequence data can provide information about structural changes over time, such as the rates of changes in cellular surface contours (Zhang et al., 2002), the velocities of movement of intracellular particles such as viruses (Bearer et al., 2000), and even the dynamic behavior of purified cytoskeletal filaments during interactions with motors or severing factors (Vale et al., 1985; Bearer, 1991). In this case, microscopy becomes an assay for enzymatic activity. An Open Microscopy Environment (OME) is being developed as an informatics solution for storage and analysis of optical microscope data (Swerdlow et al., 2003). This public resource hopes to provide image analysis automation, modeling tools, and large sets of data for mining biological material. The goal is for data to be stored as an XML-encoded standard file which can be uploaded and processed by any software. The URL for this project http://www.openmicroscopy.org. Image analysis to determine clues of function from subcellular location and behavior In addition to providing information about the exact measurements of the “ideal” for a particular structure, image analysis is also useful to discover expression patterns, location within cells, and dynamic behaviors of molecules both large (proteins and RNA) and small (ATP and calcium ions). The expression patterns and behaviors of proteins inside cells
detected by microscopy complement those detected by biochemical and molecular approaches, and readily provide functional information about a protein or gene product. Because the genome projects have identified a large number of novel genes whose function is not easily predicted from DNA sequence, functional clues derived from structural analysis have become increasingly fashionable. Morphological data are being collected in highthroughput screening assays to identify key proteins from large data sets. Such key proteins can then be selected for analysis by more timeconsuming approaches in subsequent experiments. Strategies using localization to derive functional information about individual proteins and thereby select interesting proteins from large data sets were originally developed using antibodies to localize the protein (Bearer and Alberts, 1988; Miller et al., 1989). Antibody staining provides evidence concerning the type of cell within a tissue that expresses the protein and the subcellular location of that protein in cells. In conjunction with other stains, including antibodies, against other cellular proteins whose function is known, the contribution of the unknown protein to a particular aspect of cellular behavior can be inferred. In addition to antibody staining, several other methods have been developed to detect gene expression. In situ hybridization (ISH) to detect mRNA expression speeds up the analysis because it uses the DNA as a probe and bypasses antibody production, which is time-consuming and not always reliable. However, results from ISH do not produce information about protein function, only about relative expression between cells in a tissue. Another method, transfection and overexpression of fusions of the target protein with tags such as green fluorescent protein (GFP), allows observation of the dynamic behavior of a protein in the living cell. This method has most commonly been applied to individual proteins once they are determined to be of interest and is not a commonly used screen, as construction of the GFP fusion takes time. Another powerful approach is to label a protein whose function is known with GFP and then observe the effect of overexpression, deletion, or mutation of the unknown protein on the GFP-labeled structures. Finally, labeling of a protein within a larger complex of proteins allows microscopic tracking of the complex within cells, a strategy used to study the movement of viruses within cells (Bearer et al., 2000).
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Questions that microscopic analyses usually address include: (1) the timing of expression changes after stimulation, (2) the types of cells within a multicellular tissue in which the gene product is expressed, and (3) the location of the gene product within the cell and its association with other cellular structures of known function. The function of an unknown protein or small molecule can be assessed using substrate cells that are transfected with some other protein whose function is already known, and monitoring the effect of the novel protein on the behavior of the known protein. This approach has proven very useful in identifying proteins whose function affects the secretory pathway, such as Golgi markers (LippincottSchwartz, 2001; Kreitzer et al., 2003) or the actin cytoskeleton (Westphal et al., 1997; Fischer et al., 1998). While all of these types of structural expression analysis have in common the use of the microscope to obtain images for further analysis, the exact method and interpretation of results differ depending on the question being addressed. Microscope assays can be divided into two subsets: those acquired from living specimens in real time or by time lapse, and those acquired from fixed specimens.These assays use a variety of different microscope technologies and experimental methods. This is a fast-moving area of research, and specific applications and equipment are likely to have evolved between the writing of this unit and its appearance in print.
tailored to the individual monitor being used. Printing of processed images requires that the printer be coordinated with the monitor as well. This is dependent on the manufacturer’s specifications for the particular printer and monitor being used. Monitor-based processing requires a low-light work area and a maximum-resolution screen.
Critical Parameters
Bearer, E.L. and Orci, L. 1985. Endothelial fenestral diaphragms: A quick-freeze, deep-etch study. J. Cell Biol. 100:418-428.
Image analysis is only as good as the images being analyzed. It is often a fruitless waste of time to use digital image processing to improve poor images. Therefore, before embarking on an analytical or processing project, images should be carefully scrutinized for: (1) focus; (2) good signal-to-noise ratios; (3) reproducibility; and (4) background contamination. Artifacts to be avoided are those from: (1) laser or other light damage; (2) aggregation of fluorochrome marker; (3) nonphysiologic localizations of fusion proteins; (4) movement of the stage during collection of video images; and (5) other effects of the experiment on the cell that might damage it or perturb its normal processes. Scientists engaged in monitor-based digital processing tasks should ensure that the monitor’s colors, brightness, and contrast are set appropriately. Instructions for this come with the Adobe Photoshop package, and must be
Time Considerations Digital image processing and analysis are usually more time-consuming than expected. The better the original image(s), the less time is required for processing. The type of data being extracted also define the length of time that must be invested. If software routines are available, then applying them can be fast once a sequence of steps is defined. It usually takes a graduate student a week to learn a routine, then a half-day to a day to make a processed QuickTime movie from a preselected, excellent series of images.
Literature Cited Allen, R.D., Zacharski, L.R., Widirstky, S.T., Rosenstein, R, Zaitlin, L.M., and Burgess, D.R. 1979. Transformation and motility of human platelets: Details of the shape change and release reaction observed by optical and electron microscopy. J. Cell Biol. 83:126-142. Bearer, E.L. 1991. Direct observation of actin filament severing by gelsolin and binding by gCap 39 and CapZ. J. Cell Biol. 115:1629-1638. Bearer, E.L. and Alberts, B.M. 1988. Novel actinbinding proteins associated with platelet activation. FASEB J. 2:A1538.
Bearer, E.L. and Orci, L. 1986. A simple method for quick-freezing. J. Electron Microsc. Tech. 3:233241. Bearer, E.L., Breakefield, X.O., Schuback, D., Reese, T.S., and LaVail, J.H. 2000. Retrograde axonal transport of Herpes Simplex Virus: Evidence for a single mechanism and a role for tegument. Proc. Natl. Acad. Sci. U.S.A. 97:81468150. Brady, S.T., Lasek, R.J., and Allen, R.D. 1985. Video microscopy of fast axonal transport in extruded axoplasm: A new model for study of molecular mechanisms. Cell Motil. 5:81-101. DeRosier, D.J. and Klug, A. 1972. Structure of the tubular variants of the head of bacteriophage T4 (polyheads). I. Arrangement of subunits in some classes of polyheads. J. Mol. Biol. 65:469-488. Fischer, M., Kaech, S., Knutti, D., and Matus, A. 1998. Rapid actin-based plasticity in dendritic spines. Neuron 20:847-854.
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Harlow, M., Ress, D., Koster, A., Marshall, R.M., Schwarz, M., and McMahan, U.J. 1998. Dissection of active zones at the neuromuscular junction by EM tomography. J. Physiol. Paris 92:75-78. Hayden, J.H. and Allen, R.D. 1984. Detection of single microtubules in living cells: Particle transport can occur in both directions along the same microtubule. J. Cell Biol. 99:1785-1793. Hiraoka, Y., Agard, D.A., and Sedat, J.W. 1990. Temporal and spatial coordination of chromosomal movements. J. Cell Biol. 111:2815-2828. Inoué, S. and Spring, K.R. 1997. Video Microscopy. Plenum, New York. Kaech, S., Parmar, H., Roelandse, M., Barnmann, C., and Matus, A. 2001. Cytoskeletal microdiffentiation: A mechanism of organizing morphological plasticity in dendrites. Proc. Natl. Acad. Sci. U.S.A. 98:7086-7092. Kam, Z., Hanser, B., Gustafsson, M.G., Agard, D.A., and Sedat, J.W. 2001. Computational adaptive optics for live three-dimensional biological imaging. Proc. Natl. Acad. Sci. U.S.A. 98:3790-3795. Kreitzer, G., Schmoranzer, J., Low, S.H., Li, X., Gan, Y., Weimbs, T., Simon, S.M., and Rodriguez-Boulan, E. 2003. Three-dimensional analysis of post-Golgi carrier exocytosis in epithelial cells. Nat. Cell Biol. 5:126-136. Lippincott-Schwartz, J. 2001. The secretory membrane system studied in real-time: Robert Feulgen Prize Lecture, 2001. Histochem. Cell Biol. 116: 97-107. Matsumoto, B. 1993. Cell Biological Applications of Confocal Microscopy. In Methods in Cell Biology vol. 38 (B. Matsumoto, ed.) Academic Press, San Diego. Meyer, M.P., Niell, C.M., and Smith, S.J. 2003. Brain imaging: How stable are synaptic connections? Curr. Biol. 13:R180-182.
Miller, K.G., Field, C.M., and Alberts, B.M. 1989. Actin-binding proteins from Drosophila embryos: A complex network of interacting proteins detected by F-actin affinity chromatography. J. Cell Biol. 109:2963-2975. News and Editorial Staff. 2002. Breakthrough of the year: The runners-up. Science 298:2297-2303. Swerdlow, J., Goldberg, I., Brauner, E., and Sorger, P.K. 2003. Informatics and quantitative analysis in biological imaging. Science 300:100-102. Unwin, N. 2002. Structure of the acetylcholinegated channel. Novartis Found Symp. 245:5-21. Vale, R.D., Schnapp, B.J., Reese, T.S., and Sheetz, M.P. 1985. Movement of organelles along filaments dissociated from the axoplasm of the squid giant axon. Cell 40:449-454. Westphal, M., Jungbluth, A., Heidecker, M., Muhlbauer, B., Heizer, C., Schwartz, J.M., Marriott, G., and Gerisch, G. 1997. Microfilament dynamics during cell movement and chemotaxis monitored using a GFP-actin fusion protein. Curr. Biol. 7:176-183. White, J.G., Amos, W.B., and Fordham, M. 1987. An evaluation of confocal versus conventional imaging of biological structure by fluorescence light microscopy. J. Cell Biol. 105:41-48. Zhang, H., Wessels, D., Fey, P., Daniels, K., Chisholm, R.L., and Soll, D.R. 2002. Phosphorylation of the myosin regulatory light chain plays a role in motility and polarity during Dictyostelium chemotaxis. J. Cell Sci. 115:17331747.
Contributed by E.L. Bearer Brown University Providence, Rhode Island
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Three-Dimensional Reconstruction of Tissues Three-dimensional (3-D) tissue reconstruction relies on the acquisition of threedimensional data followed by the assembly of the data into a 3-D data block from which information can then be extracted. This unit provides a protocol for performing serial microscopy for acquiring the data, reconstructing the data into data blocks, and extracting cross-sectional areas of some features of the tissue. Because of the wide range of tissues that can be processed by serial microscopy and the reconstruction of the data into data blocks, and because each investigator will focus on different parameters of interest, the last section of the protocol is limited to high-contrast areas.
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BASIC PROTOCOL
Three-dimensional imaging is advancing at a rapid rate. However, most 3-D histological techniques rely on expensive instrumentation, often with extensive custom assembly. As a result, this unit will focus on a protocol that requires standard histology and fluorescence microscopy equipment. The protocol outline starts with a brief guide to labeling and histological processing, followed by detailed steps for imaging and post-processing of the images to reconstruct 3-D data. The end result is a series of images that are merged from multiple images from each histological slide, aligned with each other for purposes of reconstruction. This data is useful because the user can display a much larger field of view than what the microscope camera is capable of acquiring, and because the user can follow features in three dimensions with alignment much more easily. This protocol was written with the assumption that the user is competent with anesthesia, harvesting and processing of tissue, and euthanasia in the desired organism, as well as with fluorescence microscopy, including knowledge of the objectives, filters, cameras, and image output. For post-processing, experience with Matlab or adapting Matlab coding to another programming language is highly encouraged. Although the code provided for this protocol can run without the user knowing Matlab, the most effective use of the coding will require user interaction. Thus, at least minimal knowledge of Matlab is highly recommended for this protocol. Labeling and histology. To make full use of the protocol and avoid multiple repetitions, it is essential to carefully plan the labels to be used ahead of time. Specifically, interference in wavelengths, the use of bright-field versus fluorescence labels, the expected number and distribution of the components being labeled, and the orientation of the tissue in the paraffin block should be carefully considered in advance of the experiment in order to make the study most fruitful. A few general guidelines that may be helpful include the following: 1. Fluorescence emission wavelengths tend to bleed into higher wavelengths, i.e., a blue dye will leak into green, but a green dye is unlikely to leak into blue. Thus, the smaller, dimmer, and more diffuse components should be labeled with the shorter (closer to blue) wavelengths and filtered appropriately. 2. Some light-based dyes will quench fluorescent labels. While both bright-field and fluorescent dyes can be used, and this can theoretically increase the amount of information acquired, interference and photobleaching must be considered. It is important to test combinations on sample tissues prior to experimentation on the real specimens. In Situ Hybridization and Immunohistochemistry Contributed by Francisco Cruz, Richard T. Lee, and Hayden Huang Current Protocols in Molecular Biology (2006) 14.16.1-14.16.13 C 2006 by John Wiley & Sons, Inc. Copyright
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3. The tissue should be oriented in the paraffin block (or other cutting block) in such a way that slices yield the information desired with minimal error. The errors in z (the direction perpendicular to each slice) are generally higher than in the plane of the slice (x-y), due both to cutting and to limitations of the optics. 4. With any 3-D reconstruction technique, it will be most difficult to identify the ends of features (cells, vessels) unambiguously in the direction with the least resolution. This will generally be the z direction. 5. The images being acquired should be the same size (the use of different binning for images to be stitched should be avoided), and should be arranged in a rectangular array (2 by 1, 5 by 3, and so on). One should avoid moving samples diagonally.
Materials Labeled tissue, or animal to be labeled and appropriate anesthesia Labeling compounds (e.g., Invitrogen): 10 mg/ml Hoechst stain; 5 mg/136 µl Texas red maleimide in DMEM; 500 µg/ml Alexa Fluor 488 in PBS with Ca2+ and Mg2+ ; or other label of interest Fixative (e.g., 4% paraformaldehyde) 70% ethanol Paraffin 1-ml or smaller syringes Small-gauge needle Microtome Glass slides and coverslips Fluorescence microscope with appropriate objectives, filters, cameras, and acquisition software Computer (minimum requirements are Pentium 4 with 1 GHz or faster processor, 512 MB or higher RAM, and hard drive of 100-GB capacity or greater, depending on size and number of image files, running MacOS X or Windows 2000) Mathematical processing package (e.g., MatLab, Mathworks) Additional reagents and equipment for fixation, embedding and sectioning of tissues (e.g., UNIT 14.1) NOTE: Using other programming languages, such as Visual C, requires having the appropriate compiler and knowledge of the programming syntax, which is not provided here. For the code presented in this protocol (available for download at http://vascular.bwh. harvard.edu/cpmb3d/cpmb3d.html), Matlab 7.0 or higher with Image Processing and Curve Fitting toolboxes is required.
Label tissue of interest If the tissue is already labeled ex vivo and embedded in some blocking medium, proceed to step 8. Steps 1 to 7 describe intravital fluorescence labeling of an entire mouse. 1. Anesthetize the animal. 2. Using a 1-ml or smaller syringe, inject the appropriate amounts of the labeling compounds into the tail vein. For example:
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50 µl Hoechst stain for nuclear labeling (stock 10 mg/ml) 20 µl Texas red maleimide for matrix labeling (5 mg/136 µl DMEM) 50 ml Alexa Fluor 488 isolectin stain for endothelial labeling (500 µg/ml PBS with Ca2+ and Mg2+ ). 3. Keep the animal under anesthesia for 15 min to 1 hr to allow intravital staining.
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4. Sacrifice the animal and harvest/fix the tissue by removing the organs and fixing in 4% paraformaldehyde overnight (UNIT 14.1). 5. Take the tissue from the 4% paraformaldehyde solution and place in 70% ethanol overnight (may be extended up to 1 week). 6. Embed the tissue in paraffin using standard histological protocols (UNIT 14.1). 7. Optional: In each block, puncture the paraffin around the tissue using a small-gauge needle (those used for insulin injections are usually adequate). Do not puncture the tissue itself, but place the punctures as close as possible to the tissue to avoid having to acquire extraneous data. These holes can be used to help perform gross alignment once the images have been registered. This is especially useful for homogeneous tissue.
Section tissue 8. Optional: Notch the corner of the block through the entire height of the block, so that the notch will be visible in each slice. This will help orient the tissue, and should be done if the tissue does not have gross features for orientation purposes.
9. Using a microtome (UNIT 14.1), slice 50 to 100 consecutive times and plate the samples on glass slides (coverslips are optional). Multiple sections may be placed on the same slide as long as the order is preserved. Each slice should be 5 to 20 µm thick, although this can be adjusted depending on the nature of the tissue and the features being examined. In general, the thickness of each slice should be selected to give ∼1/10 the resolution of the features of interest, and the number of slices should be chosen to span roughly two to three times the size of the features. For example, if the features of interest are individual cardiac myocytes (expected length of 100 to 150 µm, expected diameter 10 to 30 µm), and the myocytes are being cut in cross-section, slice thicknesses of 10 to 15 µm should be used, and at least 30 consecutive slices should be acquired. If the myocyte is being cut longitudinally (along its long axis) then at least 60 slices of 1 to 2 µm should be acquired.
10. Examine two to three random slides to ensure that the labels are intact and that the notches and holes from steps 7 and 8, if any, are visible.
Image tissue 11. Using a fluorescence microscope, place one slide on the stage and acquire images from the first section in the region of interest. If holes were made in step 7, also acquire adjacent images that include at least two holes in the paraffin. It is vital that all images have some overlap with each other. This can be done manually using visual landmarks, or by using a properly calibrated automated stage. The images should also form one large rectangular array to be assembled later. For ease of postprocessing, the images should be acquired top-to-bottom (the y direction in plane, not the z direction through the section) and then left-to-right. After completion of imaging from one section, the acquired images should fit together to form one large image without separations. Small gaps in the images where there is no useful information are acceptable, but stand-alone islands are unacceptable. For each section, it is recommended that the same number and arrangement of images be acquired.
12. Acquire images from the other sections in sequence, using the same template of images as in the first section from step 11. That is, acquire images from all sections in a similar fashion, so that they encompass the area of interest and the holes, and so that they form the same-sized rectangular array.
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Figure 14.16.1
Flowchart of the post-processing for extraction of 3-D data from serial slices.
It is useful to have the file names follow a standard convention so that post-processing will be easier. For example, a file from a heart sample can be labeled heart 3 s6.tif to indicate that it is the third image of section 6. For processing, the image files must be the same size and dimensions. Thus, binning or cropping should be avoided. If they are necessary, image processing should be performed using software to ensure the images are identical in size, resolution, and format before proceeding to the next step. The flowchart in Figure 14.16.1 depicts the outline of the rest of the protocol. Images will be registered in-plane, then assembled in the z direction, and then features will be isolated.
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Perform two-dimensional post-processing 13. Copy the images to the computer for image processing with the mathematical image analysis software. Make sure the images are in the Matlab root directory (for Matlab Version 7 Release 14 on a PC, this would default to C:\MATLAB701\work).
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Figure 14.16.2 Two-dimensional registration of images. (A) Two images to be registered. Notice that the images overlap horizontally but are shifted slightly vertically. (B) Registered image.
14. Load images from a single plane (multiple images from a single slice in the rectangular array) into Matlab for viewing purposes. To do this, type >> imshow filename.tif at the Matlab command prompt, where filename.tif is the image file to be displayed. Examine the images to ensure the overlap is visible by looking for common features on the edges of the images. For example, in the images shown in Figure 14.16.2A, the white arrows point to a section of strong labeling that is common to both images. After beginning this step, do not type ‘clear’ at the Matlab command prompt because some of the values are needed to execute subsequent commands. Any clearing that has to be done will be in the corresponding code.
15. Register the images in a single plane. Perform registration by cross-correlation using the Matlab codes named plane register.m, with functions TBreg.m and RLreg.m, if downloaded (for codes, go to http://vascular.bwh.harvard.edu/ cpmb3d/cpmb3d.html). If manually entering the code, type the contents of the code into separate notepad files and save as plane register.m, TBreg.m, and RLreg.m in the Matlab directory. If the files have .txt appended to them, highlight the files one at a time and press F2 to rename the files without the .txt extension. Launch Matlab and type >> plane register
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at the Matlab command prompt to run the script.
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a. A dialog box will open asking for the Overlay Pattern. Depending on how the images are designed to fit together, the input should consist of two numbers separated by a space. The first number is the number of rows (vertical number of images), and the second number is the number of columns (horizontal number of images). For the images in Figure 14.16.2A, one would enter 1 2 and then click OK. b. Enter the percent overlap of the images. For the images in Figure 14.16.2A, a good initial overlap would be 25%, although this can be adjusted if the program does not obtain a satisfactory result. For automated stages, once one overlap is figured, the rest should be nearly identical. Note that the percent should be entered as a true percentage (and not a fraction). Thus for Figure 14.16.2A, one would enter 25 and then click on OK. c. An open file dialog will now open for each image, up to the number of images based on the input in step a. Thus, if the input into step a is 2 3, a total of 6 files will be requested. These must be opened in the order of the image acquisition top to bottom, then left to right. For example: 1 3 5 2 4 6 would be the order of the images for a 2 by 3 array of images. d. The program will then run and register the images into one big image. At the end, a large image will appear of the registered image with some black space used for buffer space. It is recommended that the black space not be removed if z-registration is to be performed immediately. 16. Save the displayed registered images as a single large image. This can be done using the Matlab command imwrite.m. At the Matlab command prompt, type: >> imwrite(final image, ‘desired name.tif’, ‘tif’) where ‘desired name.tif’ can be custom (‘heart001.tif’ for example). This will save the registered image as a TIFF file. Do not save by using the menu option File/Save As. Make sure the file type is of a suitable format (TIFF is recommended). It is recommended that each registered slice be assigned a slice number for z-registration (heart001.tif being the first slice, heart002.tif being the second, and so on) for easier recall later. Figure 14.16.2B depicts the images from panel A, now in register.
17. Repeat steps 15 and 16 for all the slices (e.g., 50 to 100) that were imaged.
Perform three-dimensional reconstruction 18. Close all images in the workspace to clear memory. Close the figure window or type >> close all at the Matlab command prompt. 19. Open two consecutive (big) images of the slice (saved in step 17) by typing at the Matlab command prompt: >> cpselect(‘image1name.tif’, ‘image2name.tif’) where image1name.tif and image2name.tif are the filenames of the images. ThreeDimensional Reconstruction of Tissues
A new window will open with four windows (ignore the legend). The bottom two windows display the images in total, while the upper portion displays magnified images of the areas in the white-bordered rectangle in the bottom images. Using the scroll bars in the upper
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images, it is possible to move around the whole image. The rectangle in the lower image will move to show the region being magnified in the upper image.
20. Identify several (at least three) morphological features that appear in the same location in both images (e.g., tears, dark spots, a cell with particularly odd morphology, or, failing that, the holes punctured in the paraffin from step 7). Left-click on one feature in the upper left image, followed by the corresponding feature in the upper right image (lower images are for reference). Repeat for at least three pairs of points. 21. Under the menu option File, select Save control points to workspace. Use default checkboxes and click OK. 22. Optional: Save the coordinates to a file for future reference by typing at the Matlab command prompt: >> csvwrite(‘filename.csv’,[input points base points]) where filename.csv is a comma-separated value file with a filename of one’s choice. The output is four columns of x and y coordinates of the features in the left-hand image followed by the coordinates of the same features in the right-hand image. The number of rows should be the same as the number of features selected in step 20.
23. Perform least-squares fitting to determine the best orientation and translation of the second image with respect to the first. Use the code rotate second image.m from download or by typing/saving as a new M-file. Execute the code by typing at the Matlab command prompt: >> rotate second image When the open file dialog box appears, choose the name of the second image in the stack to be rotated and shifted in reference to the first, which was done in steps 19 and 20. Once the program has completed its run, save the image by entering the following at the Matlab command prompt: >> imwrite(transformed, ‘desired name.tif‘’, ‘tif’) where desired name.tif is the filename of the image to be saved. 24. Repeat steps 17 to 23 for the second and third images, then for the third and fourth images, and so on. 25. Once completed, display a data block of the corrected scan volume as individual images or as a movie if the images are stitched together.
Isolate features This section of the protocol calculates the cross-sectional area of a feature that is dark and bordered by a bright zone. If the image in question is reversed, use an image processing program to invert the image. 26. Optional: Open the two consecutive (big and corrected) images aligned from the three-dimensional reconstruction (i.e., ending at step 25). Open the images so that they are the same size and take up the same position in the window when they are active. This may help with identification of features or landmarks in successive sections. It is best to open images from the center of the stack (images 25 and 26 in a stack of 50 images or 50 and 51 in a stack of 100 images).
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27. Run the script feature extract.m, with the two functions image mask.m and bwcfind.m (again, type each script/function into separate files and save, or download). Open the image at the top of the z-stack when prompted by the dialog box. Set threshold at the desired value (25 is default; lower means dimmer signal gets included, higher means brighter signals get excluded) and click OK. Drag the mouse to select a region containing the cells or features of interest (click once and hold the mouse button to start dragging, once the area has been selected, release the button). Two images will display, one with the cropped original image and one entitled ‘water,’ which has a watershed and edge-detected segmentation of the cropped image. Save the water image if desired, otherwise continue to step 28 with the cropped original image and ignore the water image. 28. Identify a cell or feature of interest for which the cross-sectional area is desired by clicking in the middle of the cell/feature. Repeat for as many cells/features as desired, then click the middle mouse button/scroll wheel (if using a PC) to end. If the mouse does not have a center button, hit Enter, and type at the Matlab command prompt: >> Areas The cross-sectional areas (in pixels) will be displayed in the order the cells/features were selected.
29. Optional: Save the images for future reference. Use the imwrite command as previously described (File/Save As is possible but not recommended). Make sure the desired file format is selected (TIFF is recommended). 30. Repeat steps 27 to 29 for subsequent images and identify the same cell or feature. This can be accomplished using several landmarks: a. The cell is in the same location and overlaps significantly with the first. b. The cell is fluorescently labeled and the ones near it are unlabeled or labeled differently. c. The morphology of the cell is distinctive and the same distinctiveness shows up in the second image. In general, method b is best for clear identification, but is most difficult to attain.
31. To identify the ends of the cell, use one or more of the following landmarks: a. The cell is fluorescently labeled and the ones adjacent to it are not. When the fluorescence ends, so does the cell. b. The location of the cell is no longer taken up by a cell but by non-cell features. Alternatively, if a cell overlaps with previous cells by a large percentage, then suddenly no cells overlap by the same amount, that is an indication that the cell has ended. c. If a nuclear stain was used, and the number of nuclei in the cells is known, locations between the nuclei of a single cell where the cell visibly shrinks are likely to represent a partition. d. Use the image stack in 3-D, and extract a perpendicular section. Use the perpendicular section to identify the ends of the cells. In general, method a is best, although method d permits similar analysis without requiring additional staining. ThreeDimensional Reconstruction of Tissues
32. Once the complete cell is identified, calculate the volume by adding the crosssectional areas of each slice containing the cell and multiplying by the thickness of the slices. Convert from pixels to microns according to the microscope/camera setup,
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Figure 14.16.3 Example of extracted cell with various planes showing part of a cardiac myocyte. This processing was performed using Amira.
using a Ronchi ruling or similar, if desired. Calculate the surface area by adding the perimeter of each cross-sectional area multiplied by distance between slices. The image stack can also be imported for use in other 3-D packages, such as Amira (Fig. 14.16.3).
COMMENTARY Background Information Three-dimensional (3-D) image processing of tissue data is a powerful tool that can provide clear visualization of the tissue properties and enhance quantization of relevant morphology. This unit refers to any reconstruction and analysis requiring all three dimensions, regardless of the thickness of the specimen. For example, a globe of the Earth is threedimensional, despite the fact that the information on the globe has no theoretical thickness, because simple attempts to flatten the map will induce distortions or introduce discontinuities. Thus, all three dimensions are necessary. From a visualization perspective, being able to see and rotate a solid model in space has advantages over two-dimensional (2-D) representations. For example, consider explaining the structure of coronary arteries using a 3-D surface map versus one where a movie of serial slices is shown and the audience is expected to mentally construct the map. The 3-D
map is much less likely to create confusion. Additionally, once a 3-D data block has been acquired, the tissue can be sectioned in any desired plane. Having 3-D data also obviates problems that might arise when determining the length, breadth, and width of the same cell using classic 2-D techniques. Quantitatively, there are numerous advantages to using 3-D analysis compared to 2-D. Determination of volumes, surface areas, and spatial orientation is facilitated with 3-D data. Statistical advantages exist as well. Due to heterogeneity in some tissues, 2-D data reflect not only the inherent variability of the cells in the tissue but also the variability resulting from cutting. This weakens statistical analysis, resulting in changes of significance or increasing the number of specimens required to make a significant finding. To clarify this point, consider sampling a number of cylinders with varying cross-sectional areas. If one has the 3-D image of each cylinder, the cross-sectional
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area determined by a caliper-type program would reflect the variability in the cylinders themselves (along with measurement errors). However, if one were to cut the cylinders at various angles, the variances in cross-sectional areas will generally be increased, artificially inflating the values and the variance itself. Despite the advantages of 3-D analysis, few studies have focused on developing techniques and even fewer studies have involved 3-D analysis on tissues. Some reasons for this include the expense and difficulty in use of the necessary instrumentation, difficulty of postprocessing, large amounts of data storage and processing required, and increased time required for analysis. With the advent of commercial confocal and two-photon microscopes and improved computation, more studies are relying upon three-dimensional analysis. Confocal microscopy uses standard fluorescence illumination (laser or lamp based) and then filters out-of-plane signals using a pinhole (see, for example, Periasamy, 2001). Depending on the illumination source, the scanner/pinhole setup, and the objective, the expected z signal can be narrowed down to an ∼0.5-µm-thick plane. Studies using confocal microscopy show improved image quality in relatively thin planes as well as in deep scanning. Disadvantages of confocal microscopy include the cost of a laser-scanning system (typically $200,000 USD and above), limited penetration depths (especially for shorter wavelengths), and photobleaching when taking volumetric data over an extended time. Despite these disadvantages, the signal clarity and rejection of out-of-plane noise provide sufficient advantages that several studies in three-dimensional cell and tissue structure rely on confocal microscopy and post-imaging reconstruction (Roeder et al., 2004; Sun et al., 2004; Sugawara et al., 2005; Tobita et al., 2005). Two-photon microscopy typically uses a red-to-infrared (>700-nm) laser to generate an excitation volume confined to the area of maximal focus after the objective (Denk et al., 1990, 1991; Williams et al., 1994). Because the excitation relies on two photons exciting the same molecule nearly simultaneously, only the focal region has sufficient photon density to generate a fluorescent signal. The practical result is that the signal received by the camera or photomultiplier tube is emanated from a small volume, with a z-plane thickness of ∼1 µm (depending on the objective used). There is subsequently no need for a pinhole/scanner arrangement, although this can be added to fur-
ther clarify the signal. Two-photon microscopy has two main advantages: (1) the limitation of excitation to a small volume, which reduces overall photobleaching and out-of-plane noise, and (2) the use of long wavelengths that have greater penetration depth into tissues, allowing scanning of optically favorable tissues of up to several hundred microns. Disadvantages of two-photon microscopy include heat damage to the specimen, expense of laser and microscope maintenance, and, in some cases, loss of resolution and difficulty in locating the feature of interest due to the scanning nature of the majority of two-photon microscopes. Two-photon microscopy is well-suited for deep scanning of tissues and for performing three-dimensional cell and tissue scanning, in part because of its penetration depth. When using standard epifluorescence (sometimes called wide-field fluorescence), the out-of-plane noise is generally unavoidable. However, software programs can perform deconvolution on acquired data, assuming that information regarding the optical setup is known. There are several types of deconvolution, and the technique most suitable for a given application depends on the equipment, the tissue sample, and the desired outcome. In general, deconvolution can greatly improve epifluorescence images but not to the extent of confocal image acquisition. However, the combined cost of a standard fluorescence microscope and deconvolution software is much lower than the cost of a confocal system. Deconvolution has been used to clarify threedimensional images of cells mainly, but also of some tissues (see, for example, Maierhofer et al., 2003; Mudry et al., 2003; Segretain et al., 2003; Carreno et al., 2004; Bick et al., 2005). The technique described in this unit does not rely on deconvolution. The out-ofplane noise is eliminated simply by imaging only the thin section that is desired, and then the 3-D reconstruction is performed by using cross-correlation algorithms to find a match to splice the images together; finally, a leastsquares fit is applied to the spliced images to perform the alignment in z. While some commercial microscope and imaging processing packages can perform the in-plane registration automatically when coupled with an automated stage, the protocol described in this unit does not require an automated stage and can do alignment in the vertical direction based on serial slices. Three-dimensional tissue scanning has been applied to an increasing number of tissue types and sizes. Certain new techniques
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such as optical projection tomography permit the acquisition of larger and increasingly complex specimens (Sharpe et al., 2002; Sharpe, 2003). The addition of a lateral microtome to remove scanned sections on confocal or twophoton microscopes allows theoretically unlimited scanning (Young et al., 1998; Ewald et al., 2002; Ragan et al., 2003; LeGrice et al., 2004). Laser ablation (the burning away of scanned tissues using a second laser) has been used in a similar manner (Tsai et al., 2003). Certain other techniques—such as MRI, ultrasound, and, more recently, optical coherence tomography—allow fairly deep (millimeters to tens of centimeters), nondestructive scanning but are limited in resolution and feature extraction. Current challenges include increasing the speed of image acquisition, performing high-resolution non-invasive scans, labeling of specific tissue and cell components (at the organ level or above), and storage and postprocessing of data.
Critical Parameters In performing 3-D analysis using the method described here, it is vital that potential errors associated with the method be understood. The sections to be imaged must be cut with a microtome with care so that distortion is minimized and all or almost all slices can be saved. This is not a major issue if the reconstruction is meant to be a rough estimate or if the slices are extremely thin (1 to 3 µm), but for most purposes, dropping slices leads to missing volumes that might contain the end of a cell or other feature that is important. The programs offered in this protocol allow the user to register (in x-y) and assemble (in z) stacks of slices in order to obtain a three-dimensional tissue stack. However, it cannot be overemphasized that the user must check each step to make sure the alignments are realistic. Even with guides and crosscorrelations, it is possible for the program to settle on a fit that is rotated, shifted entirely out of view, or matched to incorrect features. The old adage of GIGO (garbage in, garbage out) is especially applicable in three-dimensions, where one needs to look at successive images to see if the construct makes sense. When locating features of interest, it is useful to check a few slices in z ahead of where the current slice is located to see whether the feature will be useful. Good features (1) persist for several slices instead of just one or two, (2) do not apparently change in shape or position much, if at all, (3) are unique in some
way, and are therefore unambiguously identifiable from slice to slice, and (4) are small or have small features that are uniform. Small size permits the coordinates of the features to have the smallest error from slice to slice. Finally, for statistical analyses or analyses involving differential determinations, it is important that the person performing the registration and extraction be blind to the experimental groups for the specimens being analyzed. The user of the programs can unintentionally distort the size and shape of the features. Use of more advanced technologies, such as the lateral microtome or laser ablation techniques, might alleviate user bias.
Troubleshooting Poor registration can be a result of several issues. If the image does not have sufficient resolution for feature alignment, then the imaging setup might need to be examined. In particular, the illumination, exposure time, objective, focus, and amount of overlap may need to be adjusted for each specimen. Incorrect image registration usually results from input error. First, make sure the images are the correct ones being registered. It may be helpful when acquiring the images to tag filenames consistently with a code so that the slice and position can be determined. Overlap should be visible on opening the images. The user should attempt to input as accurate an overlap percentage as possible. Input of a 60% overlap when the images only overlap 20% will increase the chances of registering incorrectly due to mathematical noise; in contrast, attempting a 10% overlap when the images have a 20% overlap will almost never yield correct results. The overlap percentage should be slightly greater than the actual overlap, but not by too much. During imaging, overlaps of 20% to 40% are optimal for most cases. Registering images may present the following error: Matrix index must be positive or logical. This occurs when Matlab is creating the final image and either an image below the first input image has an offset >200 pixels to the left, or an image to the right has an offset >200 pixels above of the first image. If this occurs, two options are accessible without editing the programs. The first one is to reexamine and retake the images of the section with lower (diagonal) shifts. The second option is to crop the images to a smaller size using Matlab or another image processing package, making certain that all the images in the layer are of the same dimensions.
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Tissue distortion may result in a poor 3-D stack. If this is the case, the sample preparation needs to be adjusted. For example, the thickness of the cut can be adjusted (keeping in mind not to increase the thickness far beyond 1/10 the useful resolution), the tissue fixation can be adjusted (i.e., by increasing or decreasing the length of fixation), and/or the embedding medium can be adjusted from the standard paraffin (e.g., with frozen sections in specialty media, or epoxy resins). If the ends of cells are difficult to identify (this is a common problem), then selective labeling should be used to highlight the cells of interest. Some examples include transfecting or infecting cells of the organ with a fluorescent reporter, using dyes such as CellTracker (Invitrogen), labeling the nuclei (using Hoechst, for example, which will help identify the number of expected cells if the cells are known to have exactly one nucleus), sectioning in a perpendicular direction, or using thinner slices, which may help expose the ends of the cells. If there is a lack of features for z alignment, for example, because the tissue is extremely homogeneous under a microscope, then one can introduce artificial features by puncturing the paraffin surrounding the tissue, puncturing the tissue itself, or cutting the tissue block into a rectangular piece so that the overall geometry is easily identified. Puncturing the tissue is not recommended, since this will likely cause tears in the tissue during sectioning and may introduce distortions into the tissue directly. During segmentation, too little or too much segmentation may occur (i.e., the area selected may be much larger or much smaller than the region that should be segmented). In this case, change the threshold value parameter depending on the contrast of the image to obtain appropriate segmentation. The default value is 25; a lower value will generally yield larger areas of segmentation while a higher value will yield a smaller area. If possible, increase the contrast of the image prior to segmentation by using an image processing package or the Matlab command: >> imadjust(‘filename.tif’)
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Occasionally, Matlab will not write an image using imwrite after many registrations have been performed and the images written to files using imwrite. This generally occurs because the computer memory is full. If this happens, type at the Matlab command prompt: >> pack; dumpmemex;
or alternatively, exit and restart Matlab. The last program may need to be rerun.
Anticipated Results By following this method, the user should obtain 3-D data of individual cells that can be quantitatively analyzed. Extraction of cell data and information includes cell volume, length, and surface visual representation. Location of inner features such as nuclei can be made. Depending on the quality of the slices, the entire image section may have usable data; however, due to shifts in overlap and expected distortions, not all of the cells will be usable. As an illustration, for a section that is ∼1400 × 1000 pixels from a single image, combining six images together with 40% overlap will yield one large image that is 3000 × 1600 pixels. At 20× magnification, with 1 µm per pixel, the field is therefore 3 × 1.6 mm. Given that a cardiac myocyte is ∼500 µm2 in cross-section, one might expect there to be nearly 10,000 usable cells. However, not all of the area will have cells, not all of the area will have correct overlap (and portions outside the overlap region cannot be used), not all of the cells will be complete in the z direction, and not all of the cells will have identifiable ends.
Time Considerations Registration in x-y should take 6 hr for a stack of 50 slices, each slice having six images with image dimensions of 1392 × 1040 with 40% overlap (using a 2.2 GHz Pentium 4, with 1 GB of RAM, running Matlab 7 Release 14 under Windows XP). Registration in z should take 1 to 2 hr for a stack of 50 images (same configuration as above). Identifying a single cell should take 6 to 10 min with practice. A single data set (50 slices, all aligned and pre-processing files deleted) should require 4 to 10 GB of HDD space, post-processing. While this information is not strictly a time consideration, it is presented here because copying, backing up, and/or moving the files can take substantial time even at high transfer rates. For example, data transfer of a 10 GB set of files at 100 KB/sec (Ethernet speed) would take over 24 hr.
Literature Cited Bick, R.J., Bagwell, S.H., Jones, C.E., Poindexter, B.J., Buja, L.M., Youker, K.A., Grigore, A., Clubb, F., Radovancevic, B., and Frazier, O.H. 2005. Fluorescence imaging microscopy of cellular markers in ischemic vs non-ischemic cardiomyopathy after left ventricular unloading. J. Heart Lung Transplant. 24:454-461. Current Protocols in Molecular Biology
Carreno, S., Engqvist-Goldstein, A.E., Zhang, C.X., McDonald, K.L., and Drubin, D.G. 2004. Actin dynamics coupled to clathrin-coated vesicle formation at the trans-Golgi network. J. Cell Biol. 165:781-788. Denk, W., Strickler, J.H., and Webb, W.W. 1990. Two-photon laser scanning fluorescence microscopy. Science 248:73-76. Denk, W.J., Strickler, J.P., and Webb, W.W. 1991. Two-photon laser microscopy. United States Patent 5,034,613. Ewald, A.J., McBride, H., Reddington, M., Fraser, S.E., and Kerschmann, R. 2002. Surface imaging microscopy, an automated method for visualizing whole embryo samples in three dimensions at high resolution. Dev. Dyn. 225: 369-375. LeGrice, I., Sands, G., Hooks, D., Gerneke, D., and Smaill, B. 2004. Microscopic imaging of extended tissue volumes. Clin. Exp. Pharmacol. Physiol. 31:902-905. Maierhofer, C., Gangnus, R., Diebold, J., and Speicher, M.R. 2003. Multicolor deconvolution microscopy of thick biological specimens. Am. J. Pathol. 162:373-379. Mudry, R.E., Perry, C.N., Richards, M., Fowler, V.M., and Gregorio, C.C. 2003. The interaction of tropomodulin with tropomyosin stabilizes thin filaments in cardiac myocytes. J. Cell Biol. 162:1057-1068. Periasamy, A. 2001. Methods in Cellular Imaging. Oxford University Press, Oxford. Ragan, T., Huang, H., and So, P.T. 2003. In vivo and ex vivo tissue applications of two-photon microscopy. Methods Enzymol. 361:481-504. Roeder, B.A., Kokini, K., Robinson, J.P., and Voytik-Harbin, S.L. 2004. Local, threedimensional strain measurements within largely deformed extracellular matrix constructs. J. Biomech. Eng. 126:699-708. Segretain, D., Decrouy, X., Dompierre, J., Escalier, D., Rahman, N., Fiorini, C., Mograbi, B., Siffroi, J.P., Huhtaniemi, I., Fenichel, P., and Pointis, G. 2003. Sequestration of connexin43 in the early endosomes: An early event of Leydig cell tumor progression. Mol. Carcinog. 38:179-187.
Sharpe, J. 2003. Optical projection tomography as a new tool for studying embryo anatomy. J. Anat. 202:175-181. Sharpe, J., Ahlgren, U., Perry, P., Hill, B., Ross, A., Hecksher-Sorensen, J., Baldock, R., and Davidson, D. 2002. Optical projection tomography as a tool for 3D microscopy and gene expression studies. Science 296:541-545. Sugawara, Y., Kamioka, H., Honjo, T., Tezuka, K., and Takano-Yamamoto, T. 2005. Threedimensional reconstruction of chick calvarial osteocytes and their cell processes using confocal microscopy. Bone 36:877-883. Sun, Y., Rajwa, B., and Robinson, J.P. 2004. Adaptive image-processing technique and effective visualization of confocal microscopy images. Microsc. Res. Tech. 64:156-163. Tobita, K., Garrison, J.B., Liu, L.J., Tinney, J.P., and Keller, B.B. 2005. Three-dimensional myofiber architecture of the embryonic left ventricle during normal development and altered mechanical loads. Anat. Rec. A Discov. Mol. Cell. Evol. Biol. 283:193-201. Tsai, P.S., Friedman, B., Ifarraguerri, A.I., Thompson, B.D., Lev-Ram, V., Schaffer, C.B., Xiong, Q., Tsien, R.Y., Squier, J.A., and Kleinfeld, D. 2003. All-optical histology using ultrashort laser pulses. Neuron 39:27-41. Williams, R.M., Piston, D.W., and Webb, W.W. 1994. Two-photon molecular excitation provides intrinsic 3-dimensional resolution for laser-based microscopy and microphotochemistry. FASEB J. 8:804-813. Young, A.A., Legrice, I.J., Young, M.A., and Smaill, B.H. 1998. Extended confocal microscopy of myocardial laminae and collagen network. J. Microsc. 192:139-150.
Contributed by Francisco Cruz, Richard T. Lee, and Hayden Huang Brigham and Women’s Hospital Boston, Massachusetts
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CHAPTER 15 The Polymerase Chain Reaction INTRODUCTION
T
he polymerase chain reaction (PCR) is a rapid procedure for in vitro enzymatic amplification of a specific segment of DNA. Like molecular cloning, PCR has spawned a multitude of experiments that were previously impossible. The number of applications of PCR seems infinite—and is still growing. They include direct cloning from genomic DNA or cDNA, in vitro mutagenesis and engineering of DNA, genetic fingerprinting of forensic samples, assays for the presence of infectious agents, prenatal diagnosis of genetic diseases, analysis of allelic sequence variations, analysis of RNA transcript structure, genomic footprinting, and direct nucleotide sequencing of genomic DNA and cDNA. The theoretical basis of PCR is outlined in Figure 15.0.1. There are three nucleic acid segments: the segment of double-stranded DNA to be amplified and two single-stranded oligonucleotide primers flanking it. Additionally, there is a protein component (a DNA polymerase), appropriate deoxyribonucleoside triphosphates (dNTPs), a buffer, and salts.
original DNA PCR primer new DNA
DNA + primers + dNTPs + DNA polymerase
denature and synthesize
denature and synthesize
denature and synthesize
etc.
etc.
etc.
Figure 15.0.1 The polymerase chain reaction. DNA to be amplified is denatured by heating the sample. In the presence of DNA polymerase and excess dNTPs, oligonucleotides that hybridize specifically to the target sequence can prime new DNA synthesis. The first cycle is characterized by a product of indeterminate length; however, the second cycle produces the discrete “short product” which accumulates exponentially with each successive round of amplification. This can lead to the many million-fold amplification of the discrete fragment over the course of 20 to 30 cycles.
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The primers are added in vast excess compared to the DNA to be amplified. They hybridize to opposite strands of the DNA and are oriented with their 3′ ends facing each other so that synthesis by DNA polymerase (which catalyzes growth of new strands 5′→3′) extends across the segment of DNA between them. One round of synthesis results in new strands of indeterminate length which, like the parental strands, can hybridize to the primers upon denaturation and annealing. These products accumulate only arithmetically with each subsequent cycle of denaturation, annealing to primers, and synthesis. However, the second cycle of denaturation, annealing, and synthesis produces two single-stranded products that together compose a discrete double-stranded product which is exactly the length between the primer ends. Each strand of this discrete product is complementary to one of the two primers and can therefore participate as a template in subsequent cycles. The amount of this product doubles with every subsequent cycle of synthesis, denaturation, and annealing, accumulating exponentially so that 30 cycles should result in a 228-fold (270 million–fold) amplification of the discrete product. This chapter consists of protocols that cover some of the more common applications of PCR. For many applications, the first step is simply to get PCR working with a known segment of DNA and a set of primers. Therefore, UNIT 15.1 presents a basic PCR protocol and ways to optimize it for the sequence of interest. PCR permits direct sequencing of nucleic acids without requiring cloning, thus avoiding cloning difficulties and artifacts. Several different protocols for preparing PCR products for sequencing using either dideoxy (Sanger) sequencing methods or chemical (MaxamGilbert) methods are presented in UNIT 15.2. This unit should permit the practitioner to choose a protocol best suited to the problem at hand and to his or her taste. Several PCR methods have been developed that require knowledge of only a small stretch of sequence (30-40 bases) and add sequence to the ends of amplified molecules to facilitate analyses. One of these, ligation-mediated PCR (UNIT 15.3) has broad applications including genomic footprinting and sequencing. PCR can be used to help clone and manipulate sequences. Various methods for generating suitable ends to facilitate the direct cloning of PCR products are detailed in UNIT 15.4. Other protocols for cloning and mutagenesis of DNA using PCR can be found in UNIT 3.7 and UNIT 8.5. An important application of PCR is to detect RNA transcripts, analyze their structure, and amplify their sequences to permit cloning and/or sequencing. UNIT 15.5 presents procedures that adapt PCR to RNA templates, via production of a cDNA copy of the RNA by reverse transcriptase (RT-PCR). Anchored PCR, which, like ligation-mediated PCR, requires little knowledge of sequence and makes use of the ends of nucleic acids, is applied in UNIT 15.6 to analysis of mRNAs. PCR is frequently used because it is the most sensitive assay for rare sequences. A protocol that not only detects rare DNAs but quantitates them as well is presented in UNIT 15.7. The downside of sensitivity is contamination by infinitesimal amounts of unwanted exogenous sequences. Procedures designed to avoid contamination with undesired DNA sequences are emphasized in this unit.
Introduction
In Supplement 56, we have rearranged the order of the units and have updated some of them. Further updates and new units, especially on quantitative approaches to PCR, are forthcoming. We have also moved one unit, formerly 15.8, which describes a powerful application of PCR called differential display to Chapter 25, “Discovery and analysis of differentially expressed genes in single cells and cell populations”, where it is now UNIT
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25B.3. Chapter 25 includes other PCR-based protocols as well. Of course, as PCR has become an invaluable tool for nearly every branch of molecular biology, applications of PCR can be found in many other chapters of Current Protocols in Molecular Biology including Chapters 3, 7, 8, 12-14, 16, 21, 22, and 24.
Donald M. Coen Contributing Editor (Chapter 15) Harvard Medical School
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Enzymatic Amplification of DNA by PCR: Standard Procedures and Optimization This unit describes a method for amplifying DNA enzymatically by the polymerase chain reaction (PCR), including procedures to quickly determine conditions for successful amplification of the sequence and primer sets of interest, and to optimize for specificity, sensitivity, and yield. The first step of PCR simply entails mixing template DNA, two appropriate oligonucleotide primers, Taq or other thermostable DNA polymerases, deoxyribonucleoside triphosphates (dNTPs), and a buffer. Once assembled, the mixture is cycled many times (usually 30) through temperatures that permit denaturation, annealing, and synthesis to exponentially amplify a product of specific size and sequence. The PCR products are then displayed on an appropriate gel and examined for yield and specificity.
UNIT 15.1
BASIC PROTOCOL
Many important variables can influence the outcome of PCR. Careful titration of the MgCl2 concentration is critical. Additives that promote polymerase stability and processivity or increase hybridization stringency, and strategies that reduce nonspecific primertemplate interactions, especially prior to the critical first cycle, generally improve amplification efficiency. This protocol, using Taq DNA polymerase (see UNIT 3.5), is designed to optimize the reaction components and conditions in one or two stages. The first stage (steps 1 to 7) determines the optimal MgCl2 concentration and screens several enhancing additives. Most suppliers (of which there are many) of Taq and other thermostable DNA polymerases provide a unique optimized MgCl2-free buffer with MgCl2 in a separate vial for user titration. The second stage (steps 8 to 13) compares methods for preventing pre-PCR low-stringency primer extension, which can generate nonspecific products. This has come to be known as “hot start,” whether one omits an essential reaction component prior to the first denaturing-temperature step or adds a reversible inhibitor of polymerase. Hot-start methods can greatly improve specificity, sensitivity, and yield. Use of any one of the hot-start approaches is strongly recommended if primer-dimers or other nonspecific products are generated, or if relatively rare template DNA is contained in a complex mixture, such as viral nucleic acids in cell or tissue preparations. This protocol suggests some relatively inexpensive methods to achieve hot start, and lists several commercial hot-start options which may be more convenient, but of course more expensive. NOTE: Use only molecular biology–grade water (i.e., DNase, RNase, and nucleic acid free) in all steps and solutions. Materials 10× MgCl2-free PCR buffer (see recipe) 50 µM oligonucleotide primer 1: 50 pmol/µl in sterile H2O (store at −20°C) 50 µM oligonucleotide primer 2: 50 pmol/µl in sterile H2O (store at −20°C) Template DNA: 1 µg mammalian genomic DNA or 1.0 to 100.0 pg of plasmid DNA (UNIT 2.1-2.4) 25 mM 4dNTP mix (see recipe) 5 U/µl Taq DNA polymerase (native or recombinant) Enhancer agents (optional; see recipe) 15 mM (L), 30 mM (M), and 45 mM (H) MgCl2 Mineral oil TaqStart Antibody (Clontech) Ficoll 400 (optional): prepare as 10× stock; store indefinitely at room temperature Tartrazine dye (optional): prepare as 10× stock; store indefinitely at room temperature Contributed by Martha F. Kramer and Donald M. Coen Current Protocols in Molecular Biology (2001) 15.1.1-15.1.14 Copyright © 2001 by John Wiley & Sons, Inc.
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Table 15.1.1
Master Mixes for Optimizing Reaction Components
Components
Final concentration
Master mixa (µl)
Per reaction I
II
III
IV
10× MgCl2-free PCR buffer Primer 1
1×
10 µl
40.0
40.0
40.0
40.0
0.5 µM
1 µl
4.0
4.0
4.0
4.0
Primer 2 Template DNAb
0.5 µM Undiluted
1 µl 1 vol
4.0 4 vol
4.0 4 vol
4.0 4 vol
4.0 4 vol
25 mM 4dNTP mixc Taq polymerase
0.2 mM 2.5 U
0.8 µl 0.5 µl
3.2 2.0
3.2 2.0
3.2 2.0
3.2 2.0
DMSOd (20×) Glycerold (10×)
5% 10%
5 µl 10 µl
— —
20.0 —
— 40.0
— —
PMPEd (100×) H 2O
1% —
1 µl To 90 µl
— To 360
— To 360
— To 360
4.0 To 360
aTotal volume = 360 µl (i.e., enough for n + 1 reactions). bTemplate DNA volume (“vol”) is generally 1 to 10 µl. cIf 2 mM 4dNTP mix is preferred, use 10 µl per reaction, or 40 µl for each master mix; adjust the volume of water
accordingly. dSubstitute with other enhancer agents (see recipe in Reagents and Solutions) as available.
0.5 ml thin-walled PCR tubes Automated thermal cycler Additional reagents and equipment for DNA preparation (UNIT 2.1-2.4, agarose gel electrophoresis (UNIT 2.5A), nondenaturing PAGE (UNIT 2.7), or sieving agarose gel electrophoresis (UNIT 2.8), restriction endonuclease digestion (UNIT 3.1), and Southern blotting and hybridization (UNITS 2.9 & 6.4) NOTE: Do not use DEPC to treat water, reagents, or glassware. NOTE: Reagents should be prepared in sterile, disposable labware, taken directly from its packaging, or in glassware that has been soaked in 10% bleach, thoroughly rinsed in tap water followed by distilled water, and if available, exposed to UV irradiation for ∼10 min. Multiple small volumes of each reagent should be stored in screw-cap tubes. This will then serve as the user’s own optimization “kit.” Thin-walled PCR tubes are recommended. Optimize reaction components 1. Prepare four reaction master mixes according to the recipes given in Table 15.1.1. Enhancing agents probably work by different mechanisms, such as protecting enzyme activity and decreasing nonspecific primer binding. However, their effects cannot be readily predicted—what improves amplification efficiency for one primer pair may decrease the amplification efficiency for another. Thus it is best to check a panel of enhancers during development of a new assay.
Enzymatic Amplification of DNA by PCR
2. Aliquot 90 µl master mix I into each of three 0.5-ml thin-walled PCR tubes labeled I-L, I-M, and I-H. Similarly, aliquot mixes II through IV into appropriately labeled tubes. Add 10 µl of 15 mM MgCl2 into one tube of each master mix (labeled L; 1.5 mM final). Similarly, aliquot 10 µl of 30 mM and 45 mM MgCl2 to separate tubes of each master mix (labeled M and H, respectively; 3.0 and 4.5 mM final concentrations respectively).
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It is helpful to set the tubes up in a three-by-four array to simplify aliquotting. Each of the three Mg2+ concentrations is combined with each of the four master mixes.
3. Overlay the reaction mixture with 50 to 100 µl mineral oil (2 to 3 drops). To include hot start in the first step, overlay reaction mixes with oil before adding the MgCl2, heat the samples to 95°C in the thermal cycler or other heating block, and add the MgCl2 once the elevated temperature is reached. Once the MgCl2 has been added, do not allow the samples to cool below the optimum annealing temperature prior to performing PCR. Alternatives to mineral oil include silicone oil and paraffin beads. Additionally, certain cyclers feature heated lids that are designed to obviate the need for an oil overlay.
Choose cycling parameters 4. Using the following guidelines, program the automated thermal cycler according to the manufacturer’s instructions. 30 cycles: 30 sec 30 sec
94°C (denaturation) 55° (GC content ≤50%) or 60°C (GC content >50%) (annealing)
∼60 sec/kb product sequence 72°C
(extension)
Cycling parameters are dependent upon the sequence and length of the template DNA, the sequence and percent complementarity of the primers, and the ramp times of the thermal cycler used. Thoughtful primer design will reduce potential problems (see Commentary). Denaturation, annealing, and extension are each quite rapid at the optimal temperatures. The time it takes to achieve the desired temperature inside the reaction tube (i.e., the ramp time) is usually longer than either denaturation or primer annealing. Thus, ramp time is a crucial cycling parameter. Manufacturers of the various thermal cyclers on the market provide ramp time specifications for their instruments. Ramp times are lower with thinwalled reaction tubes. The optimal extension time also depends on the length of the target sequence. Allow ∼1 min/kb for this step for target sequences >1 kb, and as little as a 2-sec pause for targets 40) can reduce the polymerase specific activity, increase nonspecific amplification, and deplete substrate (nucleotides). Many investigators lengthen the time for the last extension step—to 7 min, for example—to try to ensure that all the PCR products are full length. These guidelines are appropriate for most commercially available thermal cyclers. For rapid cyclers, consult the manufacturers’ protocols.
Analyze the product 5. Electrophorese 10 µl from each reaction on an agarose (UNIT 2.5A), nondenaturing polyacrylamide (UNIT 2.7), or sieving agarose gel (UNIT 2.8) appropriate for the PCR product size expected. Stain with ethidium bromide. For resolution of PCR products between 100 and 1000 bp, an alternative to nondenaturing polyacrylamide gels or sieving agarose is a composite 3% (w/v) NuSieve (FMC Bioproducts) agarose/1% (w/v) SeaKem (FMC Bioproducts) agarose gel. SeaKem increases the mechanical strength of the gel without decreasing resolution. An alternative to ethidium bromide, SYBR Gold Nucleic Acid Gel Stain (Molecular Probes), is 25 to 100 times more sensitive than ethidium bromide, is more convenient to use, and permits optimization of 10- to 100-fold lower starting template copy number.
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Table 15.1.2
Master Mixes for Optimizing First-Cycle Reactions
Components 10× PCR buffer MgCl2 (L, M, or H) Primer 1 Primer 2 Additive Template DNA 25 mM 4dNTP mixb Taq polymerase Taq pol + TaqStart H 2O Preparation temperature
Final concentration 1× Optimal 0.5 µM 0.5 µM Optimal —b 0.2 mM 2.5 U 2.5 U To 100 µl
Master mix (µl) A 10 10 1.0 1.0 Va Va 0.8 0.5 — Va Room temperature
B
C
10 10 1.0 1.0 Va Va 0.8 0.5 — Va Ice slurry
10 10 1.0 1.0 Va Va 0.8 — — Va Room temperature
D 10 10 1.0 1.0 Va Va 0.8 — 1.0 Va Room temperature
aV, variable amount (total volume should be 100 µl). bUse undiluted or diluted template DNA based on results obtained in step 6.
6. Examine the stained gel to determine which condition resulted in the greatest amount of product. Minor, nonspecific products may be present even under optimal conditions.
7. To ensure that the major product is the correct one, digest an aliquot of the reaction with a restriction endonuclease known to cut within the PCR product. Check buffer compatibility for the restriction endonuclease of choice. If necessary, add Na+ or precipitate in ethanol (UNIT 2.1A) and resuspend in the appropriate buffer. Electrophorese the digestion product on a gel to verify that the resulting fragments have the expected sizes. Alternatively, transfer the PCR products to a nitrocellulose or nylon filter and hybridize with an oligonucleotide derived from the sequence internal to the primers (UNITS 2.9 & 6.4). With appropriately stringent hybridization and washing conditions, only the correct product (and possibly some minor related products) should hybridize.
Optimize the first cycle These optional steps optimize initial hybridization and may improve efficiency and yield. They are used when primer-dimers and other nonspecific products are detected, when there is only a very small amount of starting template, or when a rare sequence is to be amplified from a complex mixture. For an optimal reaction, polymerization during the initial denaturation and annealing steps should be prevented. Taq DNA polymerase activity can be inhibited by temperature (reaction B), physical separation (reaction C), or reversible antibody binding (reaction D). PCR without hot start is performed for comparison (reaction A). 8. Prepare four reaction mixtures using the optimal MgCl2 concentration and additive requirement determined in step 6. Prepare the mixes according to the recipes in Table 15.1.2. Use the following variations for addition of Taq polymerase. Enzymatic Amplification of DNA by PCR
a. Prepare reactions A and C at room temperature. b. Chill all components of reaction B in an ice slurry before they are combined.
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c. For reaction D, combine 1.0 µl TaqStart antibody with 4.0 µl of the dilution buffer provided with the antibody, add 1.0 µl Taq DNA polymerase (for 1:4:1 mixture of these components), mix, and incubate 5 to 10 min at room temperature before adding to reaction mixture D (glycerol and PMPE are compatible with TaqStart antibody but DMSO will interfere with antibody binding). To ensure that the reaction does not plateau and thereby obfuscate the results, use the smallest amount of template DNA necessary for visualization of the PCR product by ethidium bromide staining. Use the results from step 6 to decide how much template to use. If the desired product stains intensely, dilute the starting material as much as 1/100. If only a faint signal is apparent, use undiluted sample.
9. Overlay each reaction mixture with 50 to 100 µl mineral oil. 10. Heat all reactions 5 min at 94°C. It is most convenient to use the automated thermal cycler for this step and then initiate the cycling program directly.
11. Cool the reactions to the appropriate annealing temperature as determined in step 4. Add 0.5 µl Taq DNA polymerase to reaction C, making sure the pipet tip is inserted through the layer of mineral oil into the reaction mix. Time is also an important factor in this step. If the temperature drops below the annealing temperature and is allowed to remain low, nonspecific annealing will occur. Taq DNA polymerase retains some activity even at room temperature.
12. Begin amplification of all four reactions at once, using the same cycling parameters as before. 13. Analyze the PCR products on an agarose gel and evaluate the results as in steps 5 and 6. 14. Prepare a batch of the optimized reaction mixture, but omit Taq DNA polymerase, TaqStart antibody, PMPE, and 4dNTP mix—these ingredients should be added fresh just prior to use. If desired, add Ficoll 400 to a final concentration of 0.5% to 1% (v/v) and tartrazine to a final concentration of 1 mM. Adding Ficoll 400 and tartrazine dye to the reaction mix precludes the need for a gel loading buffer and permits direct application of PCR products to agarose or acrylamide gels. At these concentrations, Ficoll 400 and tartrazine do not decrease PCR efficiency and do not interfere with PMPE or TaqStart antibodies. Other dyes, such as bromphenol blue and xylene cyanol, do inhibit PCR. Tartrazine is a yellow dye and is not as easily visualized as other dyes; this may make gel loading more difficult. Ficoll 400 and tartrazine dye may be prepared as 10× stocks and stored indefinitely at room temperature.
REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Enhancer agents For a discussion of how to select enhancer agents, see Commentary. 5× stocks: 25% acetamide (20 µl/reaction; 5% final) 5 M N,N,N-trimethylglycine (betaine; 20 µl/reaction; 1 M final) 40% polyethylene glycol (PEG) 8000 (20 µl/reaction; 8% final) The Polymerase Chain Reaction
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10× stocks: Glycerol (concentrated; 10 µl/reaction; 10% final) 20× stocks: Dimethylsulfoxide (DMSO; concentrated; 5 µl/reaction; 5% final) Formamide (concentrated; 5 µl/reaction; 5% final) 100× stocks: 1 U/µl Perfect Match Polymerase Enhancer (Strategene; 1 µl/reaction; 1 U final) 10 mg/ml acetylated bovine serum albumin (BSA) or gelatin (1 µl/reaction; 10 µg/ml final) 1 to 5 U/µl thermostable pyrophosphatase (PPase; Roche Diagnostics; ; 1 µl/reaction; 1 to 5 U final) 5 M tetramethylammonium chloride (TMAC; betaine hydrochloride; 1 µl/reaction; 50 mM final) 0.5 mg/ml E. coli single-stranded DNA-binding protein (SSB; Sigma; 1 µl/reaction; 5 µg/ml final) 0.5 mg/ml Gene 32 protein (Amersham Pharmacia Biotech; 1 µl/reaction; 5 µg/ml final) 10% Tween 20, Triton X-100, or Nonidet P-40 (1 µl/reaction; 0.1% final) 1 M (NH4)2SO4 (1 µl/reaction; 10 mM final; use with thermostable DNA polymerases other than Taq) MgCl2-free PCR buffer, 10× 500 mM KCl 100 mM Tris⋅Cl, pH 9.0 (at 25°C; see APPENDIX 2) 0.1% Triton X-100 Store indefinitely at −20°C This buffer can be obtained from Promega; it is supplied with Taq DNA polymerase.
4dNTP mix For 2 mM 4dNTP mix: Prepare 2 mM each dNTP in TE buffer, pH 7.5 (APPENDIX 2). Store up to 1 year at −20°C in 1-ml aliquots. For 25 mM 4dNTP mix: Combine equal volumes of 100 mM dNTPs (Promega). Store indefinitely at −20°C in 1-ml aliquots. COMMENTARY Background Information
Enzymatic Amplification of DNA by PCR
The theoretical basis of the polymerase chain reaction (PCR; see chapter introduction) was probably first described in a paper by Kleppe et al. (1971). However, this technique did not excite general interest until the mid-1980s, when Kary Mullis and co-workers at Cetus developed PCR into a technique that could be used to generate large amounts of single-copy genes from genomic DNA (Saiki et al., 1985, 1986; Mullis et al., 1986; Embury et al., 1987). The initial procedure entailed adding a fresh aliquot of the Klenow fragment of E. coli DNA polymerase I during each cycle because this enzyme was inactivated during the subsequent denaturation step. The introduction of thermostable Taq DNA polymerase from Thermus
aquaticus (Saiki et al., 1988) alleviated this tedium and facilitated automation of the thermal cycling portion of the procedure. Taq DNA polymerase also permitted the use of higher temperatures for annealing and extension, which improved the stringency of primer–template hybridization and thus the specificity of the products. This also served to increase the yield of the desired product. All applications of PCR depend upon an optimized PCR. The basic protocol in this unit optimizes PCR for several variables, including MgCl2 concentration, enhancing additives— dimethyl sulfoxide (DMSO), glycerol, or Perfect Match Polymerase Enchancer (PMPE)— and prevention of pre-PCR mispriming. These and other parameters can be extremely impor-
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tant, as every element of PCR can affect the outcome (see Critical Parameters and Troubleshooting for discussion of individual parameters). There are several PCR optimization kits and proprietary enhancers on the market (Table 15.1.3). Optimization kits generally provide a panel of buffers in which the pH, buffer, nonionic detergents, and addition of (NH4)2SO4 are varied, MgCl2 may be added at several concentrations, and enhancers (e.g., DMSO, glycerol, formamide, betaine, and/or proprietary compounds) may be chosen. The protocol presented here is aimed at keeping the costs low and the options broad.
Critical Parameters and Troubleshooting MgCl2 concentration Determining the optimum MgCl2 concentration, which can vary even for different primers from the same region of a given template (Saiki, 1989), can have an enormous influence on PCR success. In this protocol three concentrations are tested—1.5 mM (L), 3.0 mM (M), and 4.5 mM (H)—against three enhancers. Enhancers tend to broaden the MgCl2 optimal range, contributing to the success of the PCR at one of these concentrations. A 10× buffer optimized for a given enzyme and a separate vial of MgCl2 are typically provided with the polymerase, so that the user may titrate the MgCl2 concentration for their unique primertemplate set. Reagent purity For applications that amplify rare templates, reagent purity is the most important parameter, and avoiding contamination at every step is critical. To maintain purity, store multiple small volumes of each reagent in screw-cap tubes. For many applications, simply using highquality reagents and avoiding nuclease contamination is sufficient; however, avoid one common reagent used to inactivate nucleases, diethylpyrocarbonate (DEPC). Even tiny amounts of chemical left after treatment of water by autoclaving are enough to ruin a PCR. Primer selection This is the factor that is least predictable and most difficult to troubleshoot. Simply put, some primers just do not work. To maximize the probability that a given primer pair will
work, pay attention to the following parameters. General considerations. An optimal primer set should hybridize efficiently to the sequence of interest with negligible hybridization to other sequences present in the sample. If there are reasonable amounts of template available, hybridization specificity can be tested by performing oligonucleotide hybridization as described in UNIT 6.4. The distance between the primers is rather flexible, ranging up to 10 kb. There can be, however, a considerable drop-off in synthesis efficiency with distances >3 kb (Jeffreys et al., 1988). Small distances between primers, however, lessen the ability to obtain much sequence information or to reamplify with nested internal oligonucleotides, should that be necessary. Design primers to allow demonstration of the specificity of the PCR product. Be sure that there are diagnostic restriction endonuclease sites between the primers or that an oligonucleotide can detect the PCR product specifically by hybridization. Several computer programs can assist in primer design (see Internet Resources at end of unit). These are most useful for avoiding primer sets with intra- and intermolecular complementarity, which can dramatically raise the effective Tm. Given the abundance of primers relative to template, this can preclude template priming. Computer primer design is not foolproof. If possible, start with a primer or primer set known to efficiently prime extensions. In addition, manufacturers’ Web sites offer technical help with primer design. Complementarity to template. For many applications, primers are designed to be exactly complementary to the template. For others, however, such as engineering of mutations or new restriction endonuclease sites, or for efforts to clone or detect gene homologs where sequence information is lacking, base-pair mismatches will be intentionally or unavoidably created. It is best to have mismatches (e.g., in a restriction endonuclease linker) at the 5′ end of the primer. The closer a mismatch is to the 3′ end of the primer, the more likely it is to prevent extension. The use of degenerate oligonucleotide primers to clone genes where only protein sequence is available, or to fish out gene homologs in other species, has sometimes been successful, but it has also failed an untold (and unpublished) number of times. When the reaction works it can be extremely valuable, but it can
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Table 15.1.3
PCR Optimization Products
Optimization goal
Supplier
Product
Optimization support Optimization support
Perkin-Elmer Promega
Optimization kits
Boehringer-Mannheim, Invitrogen, Stratagene, Sigma, Epicentre Technologies, Life Technologies Amersham Pharmacia Biotech
Technical information in appendix to catalog PCR troubleshooting program on the Internet: http://www.promega.com/amplification/assistant Several buffers, Mg2+, and enhancers which may include DMSO, glycerol, formamide, (NH4)2SO4, and other unspecified or proprietary agents
Quick startup
Quick startup
Fisher
Quick startup
Life Technologies
Quick startup
Marsh Biomedical
Hot-start/physical barrier Fisher, Life Technologies
Hot-start/separate MgCl2 Invitrogen Hot-start/separate MgCl2 Stratagene Hot Start/separate polymerase
Promega
Hot-start/reversible inactivation of polymerase by antibody binding Hot-start/antibody binding Hot-start/antibody binding Hot-start/reversible chemical modification Hot-start/reversible chemical modification Enhancer
Clontech
Ready-To-Go Beads “optimized for standard PCR” and Ready-To-Go RAPD Analysis Beads (buffer, nucleotides, Taq DNA polymerase) EasyStart PCR Mix-in-a-Tube—tubes prepackaged with wax beads containing buffer, MgCl2, nucleotides, Taq DNA polymerase PCR SuperMix—1.1× conc.—premix containing buffer, MgCl2, nucleotides, Taq DNA polymerase Advanced Biochemicals Red Hot DNA Polymerase—a new rival for Taq polymerase with convenience features Molecular Bio-Products HotStart Storage and Reaction Tubes—preadhered wax bead in each tube; requires manual addition of one component at high temperature HotWax Mg2+ beads—wax beads contain preformulated MgCl2 which is released at first elevated-temperature step StrataSphere Magnesium Wax Beads—wax beads containing preformulated Mg2+ TaqBead Hot Start Polymerase—wax beads encapsulating Taq DNA polymerase which is released at first elevated-temperature step TaqStart Antibody, TthStart Antibody—reversibly inactivate Taq and Tth DNA polymerases until first denaturation at 95°C
Life Technologies
PlatinumTaq—contains PlatinumTaq antibody
Sigma
JumpStart Taq—contains TaqStart antibody
Perkin-Elmer
AmpliTaq Gold—activated at high temperature
Qiagen
HotStarTaq DNA Polymerase—activated at high temperature Tth pyrophosphatase, thermostable
Enhancer Enhancer
Boehringer Mannheim, New England Biolabs Clontech CPG
Enhancer
Fisher
Enhancer Enhancer Enhancer Enhancer Enhancer
Life Technologies Promega Qiagen Stratagene Stratagene
GC-Melt (in Advantage-GC Kits)—proprietary Taq-FORCE Amplification System and MIGHTY Buffer—proprietary Eppendorf MasterTaq Kit with TaqMaster Enhancer—proprietary PCRx Enhancer System—proprietary E.coli Single Stranded Binding Protein (SSB) Q-Solution—proprietary Perfect Match Polymerase Enhancer—proprietary TaqExtender PCR Additive—proprietary
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also generate seemingly specific products that require much labor to identify and yield no useful information. The less degenerate the oligonucleotides, especially at the 3′ end, the better. Caveat emptor. Primer length. A primer should be 20 to 30 bases in length. It is unlikely that longer primers will help increase specificity significantly. Primer sequence. Design primers with a GC content similar to that of the template. Avoid primers with unusual sequence distributions, such as stretches of polypurines or polypyrimidines, as their secondary structure can be disastrous. It is worthwhile to check for potential secondary structure using one of the appropriate computer programs that are available. “Primer-dimers.” Primer-dimers are a common artifact most frequently observed when small amounts of template are taken through many amplification cycles. They form when the 3′ end of one primer anneals to the 3′ end of the other primer, and polymerase then extends each primer to the end of the other. The ensuing product can compete very effectively against the PCR product of interest. Primer-dimers can best be avoided by using primers without complementarity, especially in their 3′ ends. Should they occur, optimizing the MgCl2 concentration may minimize their abundance relative to that of the product of interest. Template Aside from standard methods for preparing DNA (UNIT 2.1-2.4), a number of simple and rapid procedures have been developed for particular tissues (Higuchi, 1989). Even relatively degraded DNA preparations can serve as useful templates for generation of moderate-sized PCR products. The two main concerns regarding template are purity and amount. A number of contaminants found in DNA preparations can decrease the efficiency of PCR. These include urea, the detergent SDS (whose inhibitory action can be reversed by nonionic detergents), sodium acetate, and, sometimes, components carried over in purifying DNA from agarose gels (Gelfand, 1989; Gyllensten, 1989; K. Hicks and D. Coen, unpub. observ.). Additional organic extractions, ethanol precipitation from 2.5 M ammonium acetate, and/or gel purification on polyacrylamide rather than agarose, can all be beneficial in minimizing such contamination if the simplest method (precipitating the sample with ethanol and repeatedly washing the pellet with 70% ethanol) is not sufficient.
Clearly the amount of template must be sufficient to be able to visualize PCR products using ethidium bromide. Usually 100 ng of genomic DNA is sufficient to detect a PCR product from a single-copy mammalian gene. Using too much template is not advisable when optimizing for MgCl2 or other parameters, as it may obscure differences in amplification efficiency. Moreover, too much template may decrease efficiency due to contaminants in the DNA preparation. Amount of template, especially in terms of the amount of target sequence versus nonspecific sequences, can have a major effect on the yield of nonspecific products. With less target sequence, it is more likely that nonspecific products will be seen. For some applications, such as certain DNA sequencing protocols where it is important to have a single product, gel purification of the specific PCR product and reamplification are advisable. Taq and other thermostable DNA polymerases Among the advantages conferred by the thermostability of Taq DNA polymerase is its ability to withstand the repeated heating and cooling inherent in PCR and to synthesize DNA at high temperatures that melt out mismatched primers and regions of local secondary structure. The enzyme, however, is not infinitely resistant to heat, and for greatest efficiency it should not be put through unnecessary denaturation steps. Indeed, some protocols (e.g., UNIT 15.7 and the “hot start” method described here) recommend adding it after the first denaturation step. Increasing the amount of Taq DNA polymerase beyond 2.5 U/reaction can sometimes increase PCR efficiency, but only up to a point. Adding more enzyme can sometimes increase the yield of nonspecific PCR products at the expense of the product of interest. Moreover, Taq DNA polymerase is not inexpensive. A very important property of Taq DNA polymerase is its error rate, which was initially estimated at 2 × 10−4 nucleotides/cycle (Saiki et al., 1988). The purified enzyme supplied by manufacturers lacks a proofreading 3′→5′ exonuclease activity, which lowers error rates of other polymerases such as the Klenow fragment of E. coli DNA polymerase I. For many applications, this does not present any difficulties. However, for sequencing clones derived from PCR, or when starting with very few templates, this can lead to major problems. Direct sequencing of PCR products (UNIT 15.2),
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sequencing numerous PCR-generated clones, and/or the use of appropriate negative controls can help overcome these problems. Alternatively, changing reaction conditions (Eckert and Kunkel, 1990) or changing to a non–Taq DNA polymerase (with greater fidelity) may be useful. Another important property of Taq DNA polymerase is its propensity for adding nontemplated nucleotides to the 3′ ends of DNA chains. This can be especially problematic in cloning PCR products. It is frequently necessary to “polish” PCR products with enzymes such as other DNA polymerases before adding linkers or proceeding to blunt-end cloning. Conversely, addition of a nontemplated A by Taq DNA polymerase can be advantageous in cloning (UNIT 15.4). Certain PCR protocols may work better with one thermostable polymerase rather than another. Table 15.1.4 lists currently available thermostable DNA polymerases by generic and trade names, the original source of native and recombinant enzymes, the supplier, the end generated (3′A addition versus blunt), and associated exonuclease activities. A 3′ to 5′ exonuclease activity is proofreading. Removal of the 5′ to 3′ exonuclease activity of Taq DNA polymerase (N-terminal deletion) is reported to produce a higher yield. A 5′ to 3′ exonuclease activity may degrade the primers somewhat. Proofreading enzymes synthesize DNA with higher fidelity and can generate longer products than Taq, but tend to generate low yields. Enzyme blends (Table 15.1.5) have been optimized for increased fidelity and length along with sensitivity and yield.
Enzymatic Amplification of DNA by PCR
Hot start What happens prior to thermal cycling is critical to the success of PCR. Taq DNA polymerase retains some activity even at room temperature. Therefore, under nonstringent annealing conditions, such as at room temperature, products can be generated from annealing of primers to target DNA at locations of low complementarity or having complementarity of just a few nucleotides at the 3′ ends. The latter would in effect create new templates “tagged” with the primer sequences. Subsequent cycles amplify these tagged sequences in abundance, both generating nonspecific products and possibly reducing amplification efficiency of specific products by competition for substrates or polymerase. Thus conditions preventing polymerization prior to the first temperature-controlled steps are desirable. In this protocol, three
methods of inhibiting polymerization prior to the temperature-controlled step are compared. These include physical separation of an essential reaction component prior to the first denaturation step, cooling reagents to 0°C, and reversibly blocking enzymatic activity with an antibody. Denaturation of the template before Taq polymerase or MgCl2 is added to the reaction provides a dramatic improvement in specificity and sensitivity in many cases (Chou et al., 1992). The main drawback of this method is that it requires opening the reaction tubes a second time to add the essential missing component. This creates both an inconvenience and an increase in the risk of contamination, an important consideration when testing for the presence of a given sequence in experimental or clinical samples. Cooling all components of the reaction mixture to 0°C prior to mixing is more convenient and the least expensive method but is also the least reliable. Transferring the PCR reaction tubes from the ice slurry to a 95°C preheated thermocycler block may improve the chance of success. Reversible inhibition of Taq DNA polymerase by TaqStart antibody (Clontech) is the most convenient method and very effective (Kellogg et al., 1994). Complete reactions can be set up, overlaid with oil, and stored at 4°C for up to several hours prior to thermal cycling with no loss of sensitivity or specificity compared to the other hot start methods (M.F. Kramer and D.M. Coen, unpub. observ.). Cycling is initiated immediately following 5-min denaturation of the antibody at 94°C. DMSO inhibits antibody binding and should not be used with TaqStart. Several hot-start products are now commercially available (Table 15.1.3). Success with each may depend on strict adherence to the manufacturer’s protocols, even on a specific thermocycler. Wax barrier and reversible antibody binding methods are more forgiving, while chemical modifications have more stringent activation temperature requirements. Deoxyribonucleoside triphosphates In an effort to increase efficiency of PCR, it may be tempting to increase the concentration of dNTPs. Don’t! When each dNTP is 200 mM, there is enough to synthesize 12.5 mg of DNA when half the dNTPs are incorporated. dNTPs chelate magnesium and thereby change the effective optimal magnesium concentration. Moreover, dNTP concentrations >200
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Table 15.1.4
Thermostable DNA Polymerases
DNA polymerase Generic name
Trade name
Pfu Pfu (exo-) Psp Psp (exo-)
— — Deep Vent Deep Vent (exo-)
Pwo Taq (native and/or recombinant)
—
Taq, N-terminal Stoffel deletion fragment Klen-Taq Tbr DyNAzyme Tfl Tli
Vent
Tli (exo-)
Vent (exo-)
Tma Tth
UlTma —
Biological source
Supplier
Product ends
Exonuclease activity
Pyrococcus furiosus Pyrococcus furiosus Pyrococcus sp.GB-D Pyrococcus sp.GB-D
Stratagene, Promega Stratagene New England Biolabs New England Biolabs
Blunt Blunt Blunt Blunt
3′-5′ (proofreading) No 3′-5′ (proofreading) No
Pyrococcus woesei Thermus aquaticus
Boehringer Mannheim Blunt 3′A Ambion, Amersham Pharmacia Biotech, Boehringer Mannheim, Clontech, Fisher, Life Technologies, Marsh Biomedical, Perkin Elmer, Promega, Qiagen, Sigma, Stratagene Perkin-Elmer, Sigma 3′A
3′-5′ (proofreading) 5′-3′
Thermus aquaticus
Thermus brocianus Thermus flavus
MJ Research Promega, Epicentre Technologies New England Biolabs (Vent), Promega New England Biolabs
Thermococcus litoralis Thermococcus litoralis Thermotoga maritima Perkin-Elmer Thermus thermophilus Amersham Pharmacia Biotech, Boehringer Mannheim, Epicentre Technologies, Perkin Elmer, Promega
No
—a Blunt
5′-3′ —a
Blunt
3′-5′ (proofreading)
Blunt
No
Blunt 3′ A
3′-5′ (proofreading) 5′-3′
aNo information at this time.
mM each increase the error rate of the polymerase. Millimolar concentrations of dNTPs actually inhibit Taq DNA polymerase (Gelfand, 1989). The protocol in this unit calls for preparing 4dNTPs in 10 mM Tris⋅Cl/1 mM EDTA (TE buffer), pH 7.4 to 7.5. This is easier and less prone to disaster than neutralization with sodium hydroxide. However, EDTA also chelates magnesium, and this should be taken into account if stocks of dNTPs are changed. Alternatively, to lower the risk of contamination, a 4dNTP mix can be made by combining equal volumes of commercially prepared stocks.
Enhancers Enhancers are used to increase yield and specificity and to overcome difficulties encountered with high GC content or long templates. Nonionic detergents (Triton X-100, Tween 20, or Nonidet P-40) neutralize charges of ionic detergents often used in template preparation, and should be used in the basic reaction mixture, rather than as optional enhancers. Higher yields can be achieved by stabilizing/enhancing the polymerase activity with enzyme-stabilizing proteins (BSA or gelatin), enzyme-stabilizing solutes such as betaine or betaine⋅HCl (TMAC), enzyme-stabilizing solvents (glycerol), solubility-enhancing sol-
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Table 15.1.5
Thermostable DNA Polymerase Blends
Product (trade name)
Supplier
Thermostable DNA polymerases and other components
Expand High Fidelity, Expand Long Template, Boehringer Mannheim Taq + Pwo and Expand 20kb PCR Systems KlenTaq LA Polymerase Mix
Clontech, Sigma
Advantage-HF PCR Kit
Clontech
Advantage-cDNA and Advantage-GC cDNA Polymerase Mixes and Kits
Clontech
Advantage Genomic and Advantage-GC Genomic Polymerase Mixes and Kits
Clontech
Tth + unspecified proofreading polymerase + TthStart Antibody; GC Kit contains GC Melt
eLONGase Enzyme Mix
Life Technologies
Platinum Taq DNA Polymerase
Life Technologies
Taq + Psp + unspecified proofreading polymerase(s) + eLONGase Buffer Taq + Psp + Platinum Taq Antibody
Platinum High Fidelity DNA Polymerase DyNAzyme EXT Polymerase
Life Technologies MJ Research
Taq + Psp + Taq Antibody Tbr with unspecified enhancer
GeneAmp XL PCR and XL RNA PCR Kits OmniBase Sequencing Enzyme Mix
Perkin-Elmer Promega
Tth + Tli Unspecified proofreading polymerase(s) with thermostable pyrophosphatase
AccuTaq LA DNA Polymerase Mix Sigma TaqPlus Long and TaqPlus Precision PCR Stratagene Systems Accurase Fidelity PCR Enzyme Mix; Calypso Tetralink High Fidelity Single Tube RT-PCR System
Enzymatic Amplification of DNA by PCR
vents (DMSO or acetamide), molecular crowding solvents (PEG), and polymerase salt preferences [(NH4)SO4 is recommended for polymerases other than Taq]. Greater specificity can be achieved by lowering the TM of dsDNA (using formamide), destabilizing mismatchedprimer annealing (using PMPE or hot-start strategies), and stabilizing ssDNA (using E. coli SSB or T4 Gene 32 Protein). Amplification of high-GC-content templates can be improved by decreasing the base pair composition dependence of the TM of dsDNA (with betaine; Rees et al., 1993). Betaine is an osmolyte widely distributed in plants and animals and is nontoxic, a feature that recommends it for convenience in handling, storage, and disposal. Betaine may be the proprietary ingredient in various commercial formulations. For long templates, a higher pH is recommended (pH 9.0). The pH of Tris buffer decreases at high temperatures, long-template PCR requires
KlenTaq-1 (5′-exonuclease deficient Taq) + unspecified proofreading polymerase KlenTaq-1 + unspecified proofreading polymerase + TaqStart Antibody KlenTaq-1 + unspecified proofreading polymerase + TaqStart Antibody; GC Kit contains GC Melt
Taq + unspecified proofreading polymerase Pfu + Taq; TaqPlus Precision Reaction Buffer (proprietary) Thermus sp. + Thermococcus sp.; Calypso also contains AMV-RT
more time at high temperatures, and increased time at lower pH may cause some depurination of the template, resulting in reduced yield of specific product. Inorganic phosphate (PPi), a product of DNA synthesis, may accumulate with amplification of long products to levels that may favor reversal of polymerization. Accumulation of PPi may be prevented by addition of thermostable PPase. When large numbers of samples are being analyzed, the convenience of adding PCR products directly to a gel represents a significant time savings. Some companies combine their thermostable polymerase with a red dye and a high density component to facilitate loading of reaction products onto gels without further addition of loading buffer. Thermal cycling parameters Each step in the cycle requires a minimal amount of time to be effective, while too much
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time can be both wasteful and deleterious to the DNA polymerase. If the amount of time in each step can be reduced, so much the better. Denaturation. It is critical that complete strand separation occur during the denaturation step. This is a unimolecular reaction which, in itself, is very fast. The suggested 30-sec denaturation used in the protocol ensures that the tube contents reach 94°C. If PCR is not working, it is well worth checking the temperature inside a control tube containing 100 µl water. If GC content is extremely high, higher denaturation temperatures may be necessary; however, Taq DNA polymerase activity falls off quickly at higher temperatures (Gelfand, 1989). To amplify a long sequence (>3 kb), minimize the denaturation time to protect the target DNA from possible effects, such as depurination, of lowered pH of the Tris buffer at elevated temperatures. Annealing. It is critical that the primers anneal stably to the template. Primers with relatively low GC content (48 hr) are acceptable.
2. Prepare a microcentrifuge tube for PCR containing the following: 10 µl 10× amplification buffer (optimized as in UNIT 15.1) 10 µl 2 mM 4dNTP mix 20 to 40 pmol oligonucleotide primer 2 (not labeled in step 1) DNA template 1.5 to 2.5 U Taq DNA polymerase 10 µl kinase reaction (step 1) H2O to 100 µl. Overlay with 100 µl mineral oil. Carry out PCR under optimized conditions for 5 to 10 cycles (see UNIT 15.1, steps 6 to 8). Complete PCR with a final long extension step of 5 to 7 min. The final extension ensures that all PCR products are complete and blunt-ended. If the DNA template is a single band from a previous PCR reaction that has been cut out and extracted from a gel, >10 cycles of PCR may be necessary. Similarly, if only a small amount of initial template is present, 20 to 25 cycles of PCR may be necessary to generate sufficient labeled product.
3. Remove the mineral oil with a Pasteur pipet. 4. Separate the labeled PCR product from the unincorporated labeled primer and the unincorporated labeled nucleotides by electrophoresis on an agarose or nondenaturing polyacrylamide gel. Using the characteristic mobilities of the loading dyes (bromphenol blue and/or xylene cyanol) as guideposts (UNITS 2.5A & 2.7), run the gel until the unincorporated primers and nucleotides migrate off the gel and into the lower buffer chamber. Visualize the labeled PCR products by autoradiography for 10 to 60 min. Be conservative in running the gel. Avoid running the PCR products off the gel.
5. Cut and isolate the desired labeled PCR product from the gel, using one of the described protocols for fragment purification (UNITS 2.5A, 2.6, or 2.7).
Direct DNA Sequencing of PCR Products
A preparative gel confirms the presence of the desired amplified product. However, amplified DNA can be also be purified using disposable columns, which yield clean double-stranded DNA. Chemical sequencing can also be carried out without gel or column purification. In such cases, the PCR sample should be phenol extracted and ethanol precipitated in the presence of 0.3 M sodium acetate and 2.5 vol cold 100% ethanol (UNIT 2.1A). The resulting pellet should be washed with 70% ethanol.
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The columns and extraction procedures described above should not be used instead of a preparative gel unless it is known that only a single, homogeneous product has been amplified during PCR.
6. Resuspend the labeled PCR product in 30 to 50 µl water. 7. Carry out chemical sequencing using 8% to 10% of the product for each sequencing reaction (UNIT 7.5). 8. Resolve sequencing products on a sequencing gel (UNIT 7.6). Wrap gel in plastic wrap and autoradiograph at −20°C for 2 to 48 hr. Read sequence as described in UNIT 7.5. The gel may be dried, but it is generally not necessary to do so.
GENOMIC SEQUENCING OF PCR PRODUCTS This method combines the PCR amplification method with the genomic sequencing technique (Church and Gilbert, 1984). Following PCR, the amplified DNA is chemically sequenced, transferred by electroblotting, and covalently bound by UV crosslinking onto a nylon filter that can be repeatedly probed with short, sequence-specific oligonucleotides. This method is ideally suited to situations where large amounts of sequence are sought, or where more than one region is being amplified. Several amplified fragments can be mixed, simultaneously sequenced, run out on a single set of lanes, and the sequence of the different fragments successively visualized by the use of appropriate probes.
ALTERNATE PROTOCOL 5
Materials Filter paper (Schleicher & Schuell #410), precut to gel size Additional reagents and equipment for phenol extraction and ethanol precipitation (UNIT 2.1A); transfer by electroblotting, UV cross-linking, and hybridization (UNITS 2.9, 6.3, & 6.4); and labeling with terminal deoxynucleotidyltransferase (UNIT 3.6) 1. Carry out PCR under optimized conditions, ending with a final long extension step (UNIT 15.1). The template concentrations that yield optimum amplifications must be empirically determined. These concentrations range from 20 to 1000 ng for eukaryotic genomic DNA, 10 to 100 ng for bacterial DNA, and 1 to 20 ng for cloned DNA inserts.
2. After the final long extension step, remove the mineral oil with a Pasteur pipet. 3. Phenol extract the amplified product (UNIT 2.1A). Precipitate with 0.1 vol of 3 M sodium acetate, pH 5.2, and 2.5 vol of 100% ethanol for 20 min in a dry ice/ethanol bath. Microcentrifuge 5 min at top speed, 4°C. Wash the pellet twice with 70% ethanol and dry under vacuum (UNIT 2.1A). 4. Resuspend the pellet in 30 to 50 µl water. Electrophorese 5% to 10% of the PCR sample on an agarose or nondenaturing polyacrylamide gel to determine the yield and purity of the amplified product. Because every double-stranded product generated by PCR will give rise to a sequence ladder when chemically sequenced, the homogeneity of the amplified product is critical. If amplification yields at least 2 bands of similar intensity, a gel separation prior to sequencing is necessary, unless the different products can be separately visualized by using different oligonucleotide probes. The use of internal probes (3′ of the PCR primers) to visualize the sequence will prevent spurious amplification product sequences from appearing, but will not allow sequencing of the entire amplified fragment. The presence of short products (“primer-dimers”) will generally not interfere with the majority of the sequence, but will obscure the bottom section of the sequencing gel.
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5. Carry out chemical sequencing using 8% to 10% of the amplified product for each sequencing reaction (UNIT 7.5). 6. Electrophorese the sequenced samples on a denaturing polyacrylamide sequencing gel (UNIT 7.6). The number of resolvable base pairs can be increased by using a gradient or wedge gel, performing multiple loadings of the sequenced sample, and/or using longer gel plates (≤1 meter; UNIT 7.6).
7. Carefully remove one of the glass gel plates. Place the sheet of filter paper carefully on the gel, making sure not to trap air bubbles between the filter paper and gel. Lift the gel (adhered to the filter paper) onto the appropriate transfer apparatus. 8. Wet the gel with a thin layer of TBE electrophoresis buffer and place the nylon filter over the gel, again being careful not to trap air bubbles between the gel and membrane. Transfer the DNA onto the filter membrane by electroblotting (UNIT 2.9). It is necessary to use a large electroblotting apparatus for this purpose.
9. UV cross-link the DNA onto the filter (UNIT 2.9). 10. Hybridize the filter with the appropriate probe. The filter can now be probed, visualized, stripped, and reprobed as many as 40 times. Best results are obtained by probing with the PCR oligonucleotide primers, ensuring that only completed PCR products are visualized. Degenerate probes or probes internal to the amplified fragment can also be used. A high-specific-activity probe can be obtained by “tailing” 3 to 6 pmol of probe using terminal deoxynucleotidyltransferase.
REAGENTS AND SOLUTIONS Annealing buffer, 5× 200 mM Tris⋅Cl, pH 7.5 100 mM MgCl2 250 mM NaCl Store at −20°C PEG/NaCl solution 20%(w/v) polyethylene glycol (PEG) 6000 2.5 M NaCl Store at room temperature COMMENTARY Background Information
Direct DNA Sequencing of PCR Products
The methods presented in this unit permit rapid sequencing of PCR-amplified DNA. Although the cloning of amplified DNA is relatively straightforward, direct DNA sequencing of PCR products facilitates and speeds the acquisition of sequence information. As long as the PCR reaction produces a single amplified product (or a set of discrete products that can be conveniently separated), such products are amenable to direct sequencing. In contrast to methods where the PCR product is cloned and a single clone sequenced, the
direct sequencing of “bulk” PCR product is generally unaffected by the comparatively high error rate of Taq DNA polymerase. Unless a certain feature of the template causes consistent misincorporation at the same point in the sequence, errors are likely to be stochastically distributed throughout the molecule—thus, the overwhelming majority of the amplified product consists of the correct sequence. The only exceptions to this rule may be those cases where only a very small number of template molecules (1000 bp). In such cases, the sequences of interest can be amplified simultaneously in a single PCR reaction, or separately, as a set of discrete adjacent or overlapping fragments. The fragments can then be mixed, simultaneously sequenced, electrophoresed on gels, and transferred onto nylon filters. Each fragment can be successively visualized by probing with the appropriate labeled oligonucleotide. In addition, sequences from both strands can be derived from a single filter, with a consequent increase in sequence accuracy. This “multiplex” approach, however, requires the additional blotting and hybridization steps. The direct chemical sequencing of a labeled strand is a fast alternative if only a single product (and a single strand) is being sequenced. The utility of this direct approach is thus limited to those cases when the PCR yields a single DNA species or when the product of interest can be readily gel purified. Several excellent papers dealing with direct DNA sequencing of PCR products have been published. Detailed discussions of dideoxy se-
quencing approaches can be found in Wrischnik et al. (1987), Gyllensten and Erlich (1988), Innis et al. (1988), Kreitman and Landweber (1989), and Gyllensten (1989). Chemical sequencing of PCR products is described in DiMarzo et al. (1988) and Ohara et al. (1989). Purification methods commonly used to purify PCR products involve extractions and precipitations, the use of commercial filtration or chromatography columns, or enzymatic cleanup. The protocols used for column purification benefit from their simplicity. However, enzymatic purification (Alternate Protocol 4) is as fast as column purification. This method, developed by Hanke and Wink (1994) and Werle et al. (1994), requires relatively little hands-on time and results in a negligible loss of template, as all steps are performed in the very same PCR tube or well. In addition, the principal advantage of enzymatic purification is the relative ease with which it can be scaled up to highthroughput capacity at a mere fraction of the cost.
Critical Parameters and Troubleshooting The majority of problems that may hinder direct DNA sequencing of PCR products are described in the units on dideoxy and chemical sequencing (UNITS 7.4 & 7.5) and in the unit pertaining to optimization of PCR amplification (UNIT 15.1). Regardless of the source of DNA or the sequencing methods employed, the quality of a DNA sequence depends critically on the quality of the starting material. In the case of PCR products, it is imperative (before sequencing) to optimize the reaction conditions (UNIT 15.1) in order to obtain a homogeneous amplification product. For this reason, a final long extension step must be included in any PCR reaction. In certain cases, no amount of optimization will produce a single amplified product, and individual products must be purified by gel electrophoresis. In general, gel purification is sufficient to generate a homogeneous product. On occasion, even a single isolated band contains more than one amplified product (as revealed by the sequence), and more sophisticated methods, such as denaturing gel gradient electrophoresis (Myers et al., 1988), or reamplification with a set of nested internal primers (Ohara et al., 1989), must be used. Because PCR is normally carried out under conditions strongly favoring synthesis (high concentrations of enzyme, primers, and nucleotides), artifact sequences are easily generated.
The Polymerase Chain Reaction
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Direct DNA Sequencing of PCR Products
This is a particular risk when amplifying in the presence of only one primer. In such cases, DNA synthesis is not bounded by a second primer, and a heterogeneous collection of molecules, including “hybrid” or “artificial” molecules, can easily arise. In addition, snap-back synthesis can take place, again resulting in artificial amplified molecules. Whenever possible, the identity of amplified fragments should be confirmed prior to sequencing by probing of Southern blots, preferably with specific sequences internal to the amplified region (UNIT 2.9), or by restriction endonuclease digestion analyses of amplified products to confirm the expected restriction sites (UNIT 3.1). Conditions for obtaining single-stranded template must be optimized empirically before undertaking dideoxy sequencing. Alternatively, the alternate protocol described here for sequencing double-stranded PCR products can be employed. Many difficulties with the dideoxy method are caused by the secondary structure of the template strand. The inclusion of nucleotide analogs, coupled with the use of a thermostable polymerase (e.g., Taq DNA polymerase) for chain extension, can reduce or resolve many of these problems. In sequencing double-stranded PCR products, the crucial point is to prevent reannealing of the template strands that compete with binding of the sequencing primer. This can be overcome by two critical manipulations—heat denaturation of the template and quick addition of the primer to the template. For the one-step procedure (Alternate Protocol 4), titration of the amount of DNA template used for the cycle sequencing reaction is critical to sequence quality. Most polymerbased capillary sequencers require ∼50 to 500 ng DNA per sequencing reaction. Insufficient DNA will result in low-amplitude peaks on the sequencing chromatograms. Excessive DNA, which is more often the case, will result in the rapid depletion of the fluorophore-dNTPs in the cycle sequencing reaction. The sequencing chromatogram will appear to have extremely high-amplitude peaks that burn out relatively early, resulting in a very short region of usable sequence data. Often, the direct pipetting of undiluted PCR product will result in satisfactory sequences. However, optimal sequence lengths may require dilution of the PCR product prior to enzymatic purification. The shrimp alkaline phosphatase (ShrAP) should be screened for contaminating endonuclease or exonuclease activity. The ShrAP from USB has undetectable or negligible levels of
the above contaminating activities. Exonuclease I (Exo I) preferentially hydrolyzes single-stranded DNA. However, Exo I does have some detectable double-stranded DNA exonuclease activity. Therefore, prolonged incubation with this enzyme may result in minor hydrolysis at the ends of the double-stranded DNA template. For this reason, using a nested primer for sequencing may give improved sequence data over the use of one of the original PCR primers. However, using one of the original PCR primers as the sequencing primer often gives satisfactory results. Further, the enzyme incubation time should be kept to a minimum (i.e., 15 min) prior to heat inactivation. The amount of hydrolyzing enzyme used in this protocol has been titrated as low as possible without sacrificing sequencing quality. There are two reasons for using minimal enzyme quantities. First, large amounts of protein contaminants have been suspected to cause precipitation within the capillary arrays of ABI 3700 analyzers. The small amounts of enzyme used in this protocol have not caused any increased wear on capillary sequencers to date. Second, using less enzyme results in cost savings for high-throughput sequencing efforts.
Anticipated Results These protocols capitalize on the efficiency and specificity of PCR and streamline the process of DNA sequencing. From 150 to 300 bp of sequence can routinely be obtained from overnight exposures of either dideoxy or chemical (or genomic) sequencing gels, provided oligonucleotides or strands have been labeled with 32P. Longer exposures (24 to 72 hr) may be needed when using 35S as a label. With appropriate modifications, including multiple loadings, repeated probings, or a series of sequencing primers, the yield of legible sequence can be substantially increased. Rapid purification of PCR products can be achieved within one PCR tube or well, which can directly proceed to cycle sequencing using conventional dye-terminator reactions. The sequence quality should be comparable to or better than that achieved through column purification or ethanol precipitation. Further, this protocol can be easily scaled for high-throughput use.
Time Considerations Each protocol described here can be completed in a single day or less, with additional time required for sequencing reactions and analysis.
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Literature Cited Church, G.M. and Gilbert, W. 1984. Genomic sequencing. Proc. Natl. Acad. Sci. U.S.A. 81:19911995. Church, G.M. and Kieffer-Higgins, S. 1988. Multiplex DNA sequencing. Science 240:185-188. DiMarzo, R., Dowling, C.E., Wong, C., Maggio, A., and Kazazian, H.H. 1988. The spectrum of βthalassaemia mutations in Sicily. Br. J. Haematol. 69:393-397. Gyllensten, U.B. 1989. Direct sequencing of in vitro amplified DNA. In PCR Technology (H.A. Erlich, ed.) pp. 45-60. Stockton Press, New York. Gyllensten, U.B. and Erlich, H.A. 1988. Generation of single-stranded DNA by the polymerase chain reaction and its application to the direct sequencing of the HLA-DQA locus. Proc. Natl. Acad. Sci. U.S.A. 85:7652-7656. Hanke, M. and Wink, M. 1994. Direct DNA sequencing of PCR-amplified vector inserts following enzymatic degradation of primer and dNTPs. Biotechniques 17:858-860.
Werle, E., Schneider, C., Renner, M., Volker, M., and Fiehn, W. 1994. Convenient single-step, one tube purification of PCR products for direct sequencing. Nucl. Acids Res. 22:4354-4355. Wrischnik, L.A., Higuchi, R.G., Stoneking, M., Erlich, H.A., Arnheim, N., and Wilson, A.C. 1987. Length mutations in human mitochondrial DNA: Direct sequencing of enzymatically amplified DNA. Nucl. Acids Res. 15:529-542.
Key References Kusukawa, N., Uemori, T., Asada, K., and Kato, I. 1990. Rapid and reliable protocol for direct sequencing of material amplified by polymerase chain reaction. BioTechniques 9:66-72. Outlines the method for double-stranded sequencing of PCR products. Maxam, A.M. and Gilbert, W. 1980. Sequencing end-labeled DNA with base-specific chemical cleavages. Methods Enzymol. 65:499-559. Sanger, F., Nicklen, S., and Coulson, A.R. 1977. DNA sequencing with chain-terminating inhibitors. Proc. Natl. Acad. Sci. U.S.A. 74:5463-5467.
Higuchi, R.G. and Ochman, H. 1989. Production of single-stranded DNA templates by exonuclease digestion following the polymerase chain reaction. Nucl. Acids Res. 17:5865.
These two papers outline the basic principles and techniques of chemical and dideoxy sequencing.
Innis, T.M.A., Myambo, K.B., Gelfand, D.H., and Brow, M.A. 1988. DNA sequencing with Thermus aquaticus DNA polymerase and direct sequencing of polymerase chain reaction-amplified DNA. Proc. Natl. Acad. Sci. U.S.A. 85:94369440.
Contributed by Robert L. Dorit Yale University New Haven, Connecticut
Kreitman, M. and Landweber L. 1989. A strategy for producing single-stranded DNA in the polymerase chain reaction. Gene Anal. Technol. 6:84-88. Myers, R.M., Sheffield, V., and Cox, D.R. 1988. Detection of single-base changes in DNA: Ribonuclease cleavage and denaturing gel electrophoresis. In Genomic Analysis: A Practical Approach (K. Davies, ed.) pp. 95-139. IRL Press, Oxford. Ohara, O., Dorit, R.L., and Gilbert, W. 1989. Onesided polymerase chain reaction: The amplification of cDNA. Proc. Natl. Acad. Sci. U.S.A. 86:5673-5677.
Osamu Ohara Shionogi Research Laboratories Osaka, Japan Charles B-C. Hwang (double-stranded sequencing) Harvard Medical School Boston, Massachusetts Jae Bum Kim and Seth Blackshaw (one-step purification) Harvard Medical School Boston, Massachusetts
The Polymerase Chain Reaction
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Ligation-Mediated PCR for Genomic Sequencing and Footprinting
UNIT 15.3
The polymerase chain reaction (PCR) can be used to exponentially amplify segments of DNA located between two specified primer hybridization sites. This unit describes a single-sided PCR method that initially requires specification of only one primer hybridization site; the second is defined by the ligation-based addition of a unique DNA linker. This linker, together with the flanking gene-specific primer, allows exponential amplification of any fragment of DNA. Because a defined, discrete-length sequence is added to every fragment, complex populations of DNA such as sequence ladders can be amplified intact with retention of single-base resolution. Although it is also suitable for other applications, the ligation-based protocol (see Basic Protocol) was specifically designed for genomic footprinting and direct sequencing reactions, and is described in this context. The starting material is genomic DNA that has been cleaved so that it retains a 5′ phosphate at the cleavage site. In the schematic of the protocol (Fig. 15.3.1), only one cleavage product is shown for clarity; in practice, there would be a population of fragments resulting from the partial cleavage of DNA during footprinting or sequencing reactions. Preparation of DNA for footprinting analysis and genomic sequencing is described in the support protocols. Support Protocol 1 details in vivo and in vitro dimethyl sulfate (DMS) treatment and isolation of DNA from monolayer cells and its subsequent piperidine cleavage. Support Protocol 2 describes in vivo DMS treatment and harvesting of DNA from suspension cells. Support Protocol 3 describes how control DNA generated in the first two support protocols can be prepared for genomic sequencing. NOTE: It is essential that fresh, high-quality reagents be used throughout this unit. In addition, all solutions should be prepared with glass-distilled water. LIGATION-MEDIATED SINGLE-SIDED PCR In this protocol, cleaved DNA is denatured and a gene-specific primer (primer 1) is annealed to the region of interest. In the first-strand synthesis, this primer is extended with a processive polymerase (Vent DNA polymerase) to the cleavage site to create a blunt end. DNA ligase catalyzes the attachment of a unidirectional (staggered) linker to this blunt end. The 3′ end of the longer strand of the linker is ligated to the 5′ end of the genomic DNA. The shorter strand of the linker lacks a 5′ phosphate and therefore is not ligated to the extension product of the gene-specific primer. The DNA is denatured and a second gene-specific primer (primer 2) is annealed to the genomic DNA and extended by Vent DNA polymerase through the ligated linker region. (In theory, the first gene-specific primer could be used here, but in practice substituting a second primer greatly reduces background.) The extended product is now a suitable substrate for a PCR reaction; on one end (left in Fig. 15.3.1) there is a linker sequence to which a linker primer can anneal and on the other end (right in Fig. 15.3.1) there is a genomic sequence to which a gene-specific primer can anneal. Only molecules that have both sequences will be exponentially amplified during the subsequent PCR reaction; molecules with only one of the sequences will be linearly amplified. For the last extension, a third gene-specific primer (primer 3) that overlaps the second is used to label the DNA indirectly. These end-labeled extension products are visualized on a sequencing gel. High-quality, reproducible footprinting or sequencing reactions can be obtained starting from 6 × 105 genomes (3 × 105 diploid nuclei). This corresponds to 2 µg of mouse (mammalian) genomic DNA. For other species, the recommended minimum number of Contributed by Paul R. Mueller, Barbara Wold, and Paul A. Garrity Current Protocols in Molecular Biology (2001) 15.3.1-15.3.26 Copyright © 2001 by John Wiley & Sons, Inc.
BASIC PROTOCOL
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original DNA primer • • • • • new DNA linker • • • • • • • • PCR DNA * end-labeled primer
randomly cleave DNA
denature DNA, anneal primer 1
(steps 1-5)
primer 1
not part of rest of reactions extend primer 1 with Vent polymerase
(steps 1-5)
primer 1 ••••••••••••• (steps 6-9) ligate linker to blunt end linker ••••••••••••• denature newly made DNA, anneal (steps 10-16) primer 2
primer 2 not part of rest of reactions
••••••••••• extend primer 2 with Vent polymerase
(step 16)
primer 2 ••••••••••••••• denature newly made DNA, (step 16) anneal linker primer and primer 2 primer 2 linker primer •••••••••••••••• extend linker primer and primer 2 with Vent polymerase
(step 16)
primer 2 •••••••••••••••••••••••
linker primer
•••• •••••••••••••••••
••••••••••••••••• repeat for 16 (total of 18 rounds) cycles and exponentially amplify DNA by PCR 4 5 (10 - to 10 -fold) denature amplified DNA, anneal end-labeled (steps 17-19) primer 3, with Vent polymerase ••••••••••••••••••••••••••
••••••••••••••
primer 3 *
•••••••••••••••••••••••••••••
(steps 20-24) visualize extended, end-labeled products on sequencing gel
LigationMediated PCR
Figure 15.3.1 Flowchart of ligation-mediated PCR protocol (see text for details). The steps correspond to those listed in the Basic Protocol.
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haploid genomes remains 6 × 105, but different genome sizes lead to corresponding differences in the absolute mass of DNA. Materials 0.4 µg/µl cleaved genomic DNA in TE buffer, pH 7.5 (Support Protocol 1, 2, or 3) First-strand synthesis mix (see recipe), containing oligonucleotide primer 1 (Figs. 15.3.1 and 15.3.2; also see Critical Parameters) 20 µM unidirectional linker mix (see recipe) Ligase dilution solution (see recipe) Ligase mix (see recipe) 2000 to 3000 “Weiss” U/ml T4 DNA ligase (UNIT 3.14; Promega or Pharmacia Biotech) Precipitation salt mix (see recipe) 100% ethanol, ice-cold and room temperature 75% ethanol, room temperature Amplification mix (see recipe), containing linker primer and primer 2 2 U/µl Vent DNA polymerase mix (see recipe; also see UNIT 7.4) Mineral oil End-labeling mix (see recipe), containing end-labeled primer 3 (Fig. 15.3.3) Vent DNA polymerase stop solution (see recipe) 25:24:1 (v/v/v) phenol/chloroform/isoamyl alcohol (see recipe) Loading buffer (see recipe) 1.5-ml microcentrifuge tubes, silanized (APPENDIX 3) and with Lid-Loks (optional; Intermountain Scientific) 4° and 17°C water baths Automated thermal cycler or water baths at 95°, 60°, 76°, and 60° to 70°C Additional reagents and equipment for PCR (UNIT 15.1), denaturing gel electrophoresis for DNA sequencing (UNIT 7.6), and autoradiography (APPENDIX 3A) Carry out first-strand synthesis 1. Transfer 5 µl (2 µg) cleaved genomic DNA to a silanized 1.5-ml microcentrifuge tube and chill several minutes in an ice-water bath. The cleaved DNA sample must be clean; contaminants left in the DNA (e.g., piperidine) will interfere with the reaction.
2. Prepare first-strand synthesis mix containing primer 1 and chill several minutes in an ice-water bath. Add 25 µl to DNA sample, gently mix with a pipettor, and return sample to the ice-water bath. Place Lid-Loks on sample to prevent tube from popping open during denaturation.
A
B genomic DNA
genomic DNA
primer 1 primer 2 primer 3
primer 1 increasing Tm
primer 2 primer 3
Figure 15.3.2 Two possible arrangements of gene-specific primers (see Critical Parameters).
The Polymerase Chain Reaction
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HO 5′
OH
GCGGTGACCCGGGAGATCTGAATTC
3′
LMPCR.1
3′ CTAGACTTAAG
5′
LMPCR.2
HO
OH
Figure 15.3.3 Staggered linker used in ligation-mediated PCR (LMPCR). This staggered linker is made by annealing the oligonucleotides LMPCR.1 and LMPCR.2 to each other. LMPCR.1 has 25 bases and a GC content of 60%, whereas LMPCR.2 has 11 bases and a GC content of 36%. Other oligonucleotides could be used in place of these as long as they meet the criteria defined in text. Note the lack of 5′ phosphates on oligonucleotides (i.e., 5′ OH instead) and the unconventional orientation of LMPCR.2 (3′ to 5′) in the figure.
When mixing with the pipettor, three or four careful strokes are usually sufficient. Try to avoid splashing the sample while mixing; if this occurs, microcentrifuge tubes briefly at 4°C to collect droplets.
3. Denature DNA 5 min at 95°C, anneal primer 30 min at 60°C, and extend 10 min at 76°C. As first-strand synthesis proceeds, prepare solutions in step 4. First-strand synthesis is most easily performed in an automated thermal cycler, but can be performed manually by transferring the tubes to different temperature water baths. It is important that the DNA be completely denatured so the primer can anneal. For this reason it is a good idea to check the calibration of the thermal cycler. The authors have found that some machines must be set to 96° or 97°C to actually obtain 95°C. Furthermore, if a thermal cycler is used, put the samples in the machine only after it is close to the denaturation temperature. This will minimize polymerase activity prior to the initial denaturation step. Most thermal cyclers have a “hold” function that allows sample addition followed by continuation of the program for the full denaturation time. If this reaction is performed manually using water baths, covering the tubes with a styrofoam block during denaturation and annealing minimizes condensation. The extension step creates a blunt-end substrate for the subsequent ligation reaction (see Fig. 15.3.1); therefore, success at this step is absolutely crucial because it determines the molecules that will ultimately be amplified (see Critical Parameters).
4. Thaw 20 µM unidirectional linker mix in ice-water bath. Prepare ligase dilution solution and partially prepare ligase mix (minus unidirectional linker and T4 DNA ligase; see reagents and solutions). Chill both in ice-water bath. Do not add unidirectional linker mix and T4 DNA ligase to ligase mix until step 6.
5. When extension in step 3 is complete, immediately transfer sample to the ice-water bath. Microcentrifuge tube briefly at 4°C to collect condensation, then return to ice-water bath. The samples should be kept cold during steps 6, 7, and 8 to minimize Vent polymerase activity (see Critical Parameters).
Carry out ligation 6. Finish preparing ice-cold ligase mix from step 4 by adding unidirectional linker mix, mixing, adding T4 DNA ligase, mixing, and keeping in ice-water bath. This order of addition is important because the unidirectional linker is prepared in 250 mM Tris⋅Cl, pH 7.7, which is required by the ligase.
LigationMediated PCR
7. Add 20 µl ice-cold ligase dilution solution from step 4 to sample, gently mix with a pipettor, and return sample to the ice-water bath. Add 25 µl ice-cold ligase mix from step 6 to sample, gently mix with a pipettor, and return to the ice-water bath.
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Microcentrifuge tube briefly at 4°C and incubate overnight in a 17°C water bath (see critical parameters). Addition of ligase dilution solution makes the first-strand synthesis mix into a buffer compatible with T4 DNA ligase (the NaCl concentration and, more importantly, the pH, are reduced). Addition of ligase mix provides a convenient way to add ligase and linker to each sample.
8. Prepare sample for precipitation by placing in the ice-water bath for several minutes. Microcentrifuge tube briefly at 4°C, then return to ice-water bath. Placing samples in the ice-water bath is a convenient way of transferring them from the 17°C bath to the 4°C microcentrifuge without warming them.
9. Prepare and chill precipitation salt mix. Add 9.4 µl ice-cold precipitation salt mix and 220 µl of ice-cold 100% ethanol to the sample. Mix thoroughly by inversion and chill ≥2 hr at −20°C. If desired, the experiment may be stopped at this point. Samples under ethanol are stable for weeks at −20°C.
Carry out PCR 10. Microcentrifuge precipitated ligation reaction 15 min at 4°C and discard supernatant. 11. Add 500 µl of room-temperature 75% ethanol and invert several times to wash pellet and walls of tube. Microcentrifuge sample ∼5 min at room temperature and discard supernatant. Remove last traces of ethanol with a pipettor and allow any remaining ethanol to evaporate by air drying or by using a Speedvac evaporator. The precipitated pellet will be rather large and will spread up the side of the tube.
12. Add 70 µl water and leave sample at room temperature to dissolve pellet. Vortex tube occasionally to assist dissolution, and after each vortexing, collect droplets by microcentrifuging 2 to 3 sec. When pellet is dissolved (usually ≤30 min), chill sample in ice-water bath. While pellet is dissolving, prepare and chill amplification mix containing linker primer and primer 2. 13. Add 30 µl ice-cold amplification mix to sample, gently mix with a pipettor, and return sample to the ice-water bath. 14. Add 3 µl (1 U) Vent DNA polymerase mix to sample and mix carefully with pipettor. Return sample to the ice-water bath. The reaction is very sensitive to the amount of Vent DNA polymerase used; excess polymerase results in high background. Surprisingly, this effect has been observed when the amount of Vent polymerase is merely double the optimal level. The basis for the empirically observed threshold for this background is not known. Different lots of Vent polymerase may need to be titrated (see Critical Parameters).
15. Cover sample with 90 µl mineral oil, microcentrifuge briefly at 4°C, and return to the ice-water bath. If the PCR reaction is performed manually using water baths, a Lid-Lock should be used to prevent the tube from popping open during denaturation.
16. Carry out 18 cycles of PCR. Perform first denaturation for 3 to 4 min at 95°C, and subsequent ones for 1 min. Anneal primers 2 min at a temperature 0° to 2°C above the calculated Tms (if primer 2 and linker primer have different Tms, use the lower Tm; see critical parameters for Tm calculations). Extend 3 min at 76°C; for every cycle, add an extra 5 sec to the extension step. Allow final extension to proceed 10 min.
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Transfer sample to ice-water bath, remove Lid-Lock (if applicable), and microcentrifuge briefly at 4°C to collect any condensation. Keep sample in ice-water bath. In this procedure, 18 rounds of amplification yield highly reproducible sequence ladders that can be visualized (after the radiolabeling step) following an overnight exposure without an intensifying screen. Performing additional rounds of amplification could reduce the exposure time required, but this has not been tested exhaustively. A theoretical concern is that if too many amplification cycles are carried out, some reagents (e.g., polymerase, primers, or dNTPs) may become limiting, leading to undesirable lane-to-lane variability (18 rounds of amplification do not show variability). An important parameter in this step is the temperature of denaturation. If it is too low, no signal or shortened sequence ladders will be seen; if it is too high, the polymerase will be destroyed. This may be a may be a “hidden” problem because some automatic thermal cycling machines and thermometers do not accurately measure the denaturation temperature (see Critical Parameters).
Carry out end-labeling 17. Prepare end-labeling mix containing labeled primer 3 and chill several minutes in the ice-water bath. Add 5 µl to the sample. Mix aqueous phase by gently pipetting up and down, keeping the sample on ice as much as possible. Microcentrifuge briefly at 4°C and return to ice-water bath. CAUTION: The label mix contains a significant amount of 32P. Appropriate care should be taken in handling and disposal of mix and samples to which it is added.
18. Carry out two rounds of PCR to label the DNA. Perform first denaturation 3 to 4 min at 95°C; the second for 1 min. Anneal end-labeled primer 3 for 2 min at a temperature 0° to 2°C above its calculated Tm. Extend 10 min at 76°C. When second extension is complete, transfer sample to ice-water bath. Although it is rare, in a few experimental situations, having two cycles of PCR during the end-labeling step has led to increased background and/or unusual primer artifacts such as primer-dimer products. It is best to try two cycles of end-labeling, cutting back to one cycle if this problem is encountered. It is also possible to label the amplification products using a filter-blotting procedure (Pfeifer et al., 1989). If this reaction is performed manually using water baths, a Lid-Lock should be used to prevent the tube from popping open during denaturation.
19. Place sample at room temperature and immediately add 295 µl Vent DNA polymerase stop solution. Mix by vortexing. Microcentrifuge briefly to collect radioactive droplets from top and sides of tube. Add 500 µl phenol/chloroform/isoamyl alcohol and mix either by vigorous shaking or vortexing. Microcentrifuge 3 to 5 min at room temperature. Transfer upper aqueous layer (∼400 µl; avoid interface if any) to a clean, silanized 1.5-ml microcentrifuge tube, and thoroughly mix. Microcentrifuge briefly to collect droplets from sides of tube. EDTA in the Vent DNA polymerase stop solution chelates the magnesium required by Vent polymerase. The organic extraction removes proteins before precipitation and, more importantly, removes the mineral oil (there should be little or no interface). Transferring the aqueous layer to a new tube and mixing it ensures the reaction products will be equally aliquoted for multiple loadings on the sequencing gel (step 20).
20. Set up four clean, silanized 1.5-ml microcentrifuge tubes and add 235 µl of roomtemperature 100% ethanol to each tube. Transfer 94 µl of the aqueous layer into each tube, thoroughly mix by vortexing, and chill ≥2 hr at −20°C. Discard any remaining aqueous layer. LigationMediated PCR
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This yields enough material for four samples of each reaction to be run on a sequencing gel. This accommodates multiple loadings to maximize the number of bases that can be read and provides extra samples for use in case a gel problem is encountered. If desired, this precipitation step can be a stopping point. Samples under ethanol are stable for several weeks at −20°C; however, the specific activity of the product decreases with time.
21. Microcentrifuge precipitated samples 15 min at 4°C and discard supernatants. 22. Add 500 µl of room temperature 75% ethanol, vortex, microcentrifuge samples 5 min at room temperature, and discard supernatants. Remove last traces of ethanol by using a pipettor and allow any remaining ethanol to evaporate by air drying or by using a Speedvac evaporator. 23. Add 7 µl loading buffer to each tube and leave at room temperature while pellets are dissolving. Vortex occasionally to assist pellets in dissolving and to recover any of the DNA pellet that is on the side of the tube. After each vortexing, collect droplets by microcentrifuging 2 to 3 sec. Samples usually resuspend rapidly (within 5 min). Check resuspension by removing the sample from the tube with a pipet and using a Geiger counter to ensure that the radioactivity is in the sample and not left behind in the tube. Return sample to same tube. If >10% of the total radioactivity has remained in the tube, vortex, microcentrifuge, and repeat the resuspension check. Do not let the samples sit longer than necessary in loading buffer, as this may result in smeared background and poor band resolution.
Run and analyze sequencing gel 24. Denature samples 5 min at 85° to 90°C. Load entire content of each tube on a 6% sequencing gel. At the completion of the run, fix and dry gel, then autoradiograph 6 to 24 hr without an intensifying screen. The exposure time varies and is dependent upon several factors, including the gene being amplified, the number of amplification and labeling cycles, and the specific activity of primer 3. The ladder will appear 25 bases longer than the original footprinting or sequencing products due to the addition of the 25-base linker. Because a large amount of material is being applied to the gel, use of the usual 0.2-mm-thick sequencing gels may result in smeared bands; therefore, results are usually better if a relatively thick gel (0.35 to 0.56 mm) is used instead. The first readable sequence band will run at ∼50 nucleotides, depending on the length of the labeling primer used. Therefore, to read as much sequence as possible, multiple loadings on 80-cm gels are recommended (60-cm gels also work well if an electrolyte gradient is used; UNIT 7.6), and any fragments shorter than 50 nucleotides are run into the bottom buffer tank. This means that the bottom buffer tank will contain a significant amount of radioactive 32P in the form of unused primer 3; thus, disposal guidelines must be rigorously applied.
PREPARATION OF GENOMIC DNA FROM MONOLAYER CELLS FOR DMS FOOTPRINTING This protocol describes the preparation of in vivo and in vitro dimethyl sulfate (DMS)– treated genomic DNA from monolayer cells for use in conjunction with ligation-mediated PCR. Figure 15.3.4 outlines the parallel processing of duplicate sets of cultured cells: one set is left untreated (control) and the second is treated with DMS (in vivo treatment). Both cultures are lysed and their DNA harvested. The control-cell DNA is then treated with DMS (in vitro treatment). Both in vivo and in vitro DMS-treated DNA are cleaved with piperidine before use in the ligation-mediated PCR assay (Basic Protocol). The protocol can be modified to prepare DNA from suspension cells (Support Protocol 2). The deproteinized control DNA (not treated with DMS) is of high quality and thus can also be used for genomic sequencing as described in Support Protocol 3.
SUPPORT PROTOCOL 1
The Polymerase Chain Reaction
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obtain duplicate sets of monolayer or suspension cell cultures
treat one culture as control (steps 1-2 of Support Protocol 1 or steps 1-3 of Support Protocol 2)
use control DNA in genomic sequencing (Support Protocol 3) or genomic methyl cytosine analysis (Saluz and Jost, 1987)
treat one culture with DMS (in vivo treatment; steps 1 & 3-4 of Support Protocol 1 or steps 1, 2 & 4-7 of Support Protocol 2)
harvest DNA from both cultures (steps 5-16 of Support Protocol 1 and step 8 of Support Protocol 2)
treat control DNA with DMS (in vitro treatment; steps 17-20 of Support Protocol 1)
cleave both DNA samples with piperidine (steps 21-29 of Support Protocol 1)
use in ligation-mediated PCR (Basic Protocol)
Figure 15.3.4 Flow chart of preparation of DMS-footprinted genomic DNA for use in ligation-mediated PCR.
DMS methylates guanine residues at the N7 position, rendering them susceptible to subsequent cleavage with piperidine. DMS is commonly used for in vivo footprinting analysis because cellular membranes are freely and rapidly permeable to it. Treatment of both control (in vitro–treated) and experimental (in vivo treated) samples with DMS is limited so that only ∼1 in 150 guanines is methylated. On the other hand, piperidine treatment is quantitative so that all of the methylated guanines are cleaved. The extraction and purification of the DNA must be thorough so that all proteins and reagents are removed before the DNA is used in ligation-mediated PCR (Basic Protocol). This procedure works well with ∼5 × 107 cells/set (i.e., one 15-cm plate at ∼70% confluence for many fibroblasts), although more cells can be used by scaling up the extractions and A. The DNA will be quite viscous, so pipet and aliquot it carefully.
2. Treat with DMS as described in Support Protocol 1, steps 18 and 19. Proceed to step 9. Prepare G+A, C+T, and C reactions 3. Place 20 to 40 µg control DNA in each of three 1.5-ml microcentrifuge tubes. Add sufficient TE buffer to bring the total volume in each to 100 µl. Label one tube G+A, one tube C+T, and one tube C. The DNA will be quite viscous, so carefully pipet and aliquot it.
4. Add 11 µl of 3 M sodium acetate and 222 µl of room-temperature 100% ethanol to each tube. Mix thoroughly by gently inverting a few times. The DNA should precipitate immediately.
5. Microcentrifuge precipitated samples 5 min at maximum speed, room temperature, then remove and discard supernatants. Add 500 µl of room-temperature 75% ethanol to each pellet and mix by inversion until pellets are dislodged. Microcentrifuge 3 min at maximum speed, room temperature, then remove and discard supernatants. Try to remove as much of the 75% ethanol as possible with a pipettor, but do not allow the DNA pellets to dry out. Do not use a Speedvac evaporator, as it will be impossible to resuspend the DNA once it is dried.
6. Redissolve samples in water as follows: add 18 µl to G+A tube, 40 µl to C+T tube, and 10 µl to C tube. Incubate overnight at 4°C. Warm redissolved DNA to room temperature. Uncleaved genomic DNA redissolves slowly at these concentrations. Gentle mixing by flicking the tube with a finger a few times during the resuspension can help speed this process. Overnight incubation is usually sufficient to allow resuspension. A small amount of insoluble material is occasionally seen at this step. It does not interfere with subsequent
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reactions and can be ignored; it will be discarded after the piperidine cleavage reaction in step 9.
For the G+A sample: 7a. Add 54 µl of 88% formic acid, mix thoroughly by vortexing 25 sec, and microcentrifuge briefly to collect droplets. Incubate 7 min at room temperature. 8a. Add 164 µl ice-cold G+A stop solution and vortex briefly (3 sec). Add 750 µl of 100% ethanol prechilled on dry ice, mix by vigorously shaking, and plunge tube into powdered dry ice. Leave samples in dry ice for ∼30 min, then proceed to step 9. The 7-min incubation time used here is a good starting condition. The extent of reaction can be changed by varying the incubation time; the average size of the final product after piperidine cleavage will decrease as incubation time is increased.
For the C+T sample: 7b. Add 60 µl hydrazine, mix thoroughly by vortexing 25 sec, and microcentrifuge briefly to collect droplets. Incubate 3 min at room temperature. 8b. Add 150 µl ice-cold C+T/C stop solution and vortex briefly (3 sec). Add 750 µl of 100% ethanol prechilled on dry ice, mix by vigorously shaking, and plunge tube into powdered dry ice. Leave samples in dry ice for ∼30 min, then proceed to step 9. The 3-min incubation time used here is a good starting condition. The extent of reaction can be changed by varying the incubation time, as described in step 8a.
For the C sample: 7c. Add 30 µl of 5 M sodium chloride, mix thoroughly by vortexing 25 sec, and microcentrifuge briefly to collect droplets. Add 60 µl hydrazine, mix thoroughly by vortexing 25 sec, and microcentrifuge briefly to collect droplets. Incubate 3 min at room temperature. 8c. Add 150 µl ice-cold C+T/C stop solution and vortex briefly (3 sec). Add 750 µl of 100% ethanol prechilled on dry ice, mix by vigorously shaking, and plunge tube into powdered dry ice. Leave samples in dry ice for ∼30 min, then proceed to step 9. The 3-min incubation time used here is a good starting condition. The extent of reaction can be changed by varying the incubation time, as described in step 8a.
Carry out cleavage and sequencing 9. Cleave DNA from each of the four reactions with piperidine as described in Support Protocol 1, steps 21 to 29. Proceed to ligation-mediated PCR (Basic Protocol). REAGENTS AND SOLUTIONS 4dNTP mix, pH 7.0, 25mM Prepare as in UNIT 3.4. Alternatively, purchase as individual 100-mM stock solutions from Pharmacia Biotech and combine equal amounts of dATP, dCTP, dGTP, and dTTP to obtain the 25 mM 4dNTP mix. Store at −20°C.
LigationMediated PCR
Amplification buffer, 5× 200 mM NaCl 100 mM Tris⋅Cl, pH 8.9 (APPENDIX 2), room temperature 25 mM MgSO4 0.05% gelatin (from bovine skin; Sigma) 0.5% Triton X-100 Store at −20°C
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Amplification mix, per reaction (30 ìl): 20.0 µl 5× amplification buffer (see recipe) 1.0 µl 10 pmol/µl oligonucleotide LMPCR.1 (linker primer; Fig. 15.3.3) 1.0 µl 10 pmol/µl oligonucleotide primer 2 (Figs. 15.3.1 & 15.3.2) 0.8 µl 25 mM 4dNTP mix, pH 7.0 (see recipe) 7.2 µl H2O Prepare immediately before use and chill on ice Addition of LMPCR.1 may not be required, but this has not been tested.
C+T/C stop solution 400 mM sodium acetate, pH 7.5 0.14 mM EDTA, pH 8.0 (APPENDIX 2) Prepare fresh and chill on ice before use End-labeling mix, per reaction (5 ìl): 1.0 µl 5× amplification buffer (see recipe) 2.3 µl 1 pmol/µl end-labeled primer 3 (Fig. 15.3.3) 0.4 µl 25 mM 4dNTP mix, pH 7.0 (see recipe) 0.8 µl H2O 0.5 µl 2 U/µl Thermococcus litoralis DNA polymerase (Vent; New England Biolabs) Prepare immediately before use. Mix first four components and chill on ice before adding Vent DNA polymerase and chill on ice. Then add polymerase and keep on ice. 5′ end-labeled primer 3 can be prepared as in UNIT 3.10 (forward reaction) with the following modifications: end-label 20 to 100 pmol of primer 3 with labeling-grade (i.e., less expensive) [γ-32]P ATP (∼6000 Ci/mmol); incubate at 37°C for 30 min instead of 60 min. Remove unincorporated 32P by gel purification (UNIT 2.12) or by using a Nensorb-20 nucleic acid purification cartridge (Du Pont NEN)—elute primer with 50% ethanol instead of 50% methanol. Resuspend primer 3 in TE buffer, pH 7.5, so that its concentration is 1 pmol/µl; the specific activity should be 4–9 × 106 cpm/pmol.
First-strand buffer, 5× 200 mM NaCl 50 mM Tris⋅Cl, pH 8.9 (APPENDIX 2) room temperature 25 mM MgSO4 0.05% gelatin (from bovine skin; Sigma) Store at −20°C First-strand synthesis mix, per reaction (25 ìl): 6.0 µl 5× first-strand buffer (see recipe) 0.3 µl 1 pmol/µl oligonucleotide primer 1 0.24 µl 25 mM 4dNTP mix, pH 7.0 (see recipe) 18.21 µl H2O 0.25 µl 2 U/µl Thermococcus litoralis DNA polymerase (Vent; New England Biolabs #254L) Prepare immediately before use. Mix all components except Vent DNA polymerase and chill on ice. Then add polymerase and keep on ice. Primer 1 should be stored in a silanized tube at a concentration ≥10 pmol, then diluted in TE buffer, pH 7.5, immediately before use. This will minimize primer loss due to sticking to the tube walls.
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G+A stop solution 360 mM sodium acetate, pH 7.5 0.14 mM EDTA, pH 8.0 (APPENDIX 2) Prepare fresh and chill on ice before use Ligase dilution solution, per reaction (20 ìl): 2.2 µl 1 M Tris⋅Cl, pH 7.5, at room temperature (APPENDIX 2; 110 mM final) 0.35 µl 1 M MgCl2 (17.5 mM final) 1.0 µl 1 M DTT (50 mM final) 0.25 µl 10 mg/ml BSA (DNase-free; Pharmacia Biotech; 125 µg/ml final) 16.2 µl H2O Prepare immediately before use and chill on ice Ligase mix, per reaction (25.0 ìl): 0.25 µl 1 M MgCl2 (APPENDIX 2; 10 mM final) 0.50 µl 1 M DTT (APPENDIX 2; 20 mM final) 0.75 µl 100 mM ATP, pH 7.0 (Pharmacia Biotech; 3 mM final) 0.125 µl 10 mg/ml BSA (DNase-free; Pharmacia Biotech; 50 µg/ml final) 17.375 µl H2O 5.0 µl 20 µM unidirectional linker mix (see recipe; 4 µM linker and 50 mM Tris⋅Cl, pH 7.7, final) 1.0 µl 3 “Weiss” U/µl T4 DNA ligase (UNIT 3.14; 3 U final) Prepare immediately before use. First mix MgCl2, DTT, rATP, BSA, and H2O and chill on ice. Next add ice-cold unidirectional linker mix and T4 DNA ligase. Tris⋅Cl added in the linker mix is 50 mM, pH 7.7, at final concentration.
Loading buffer 80% (v/v) formamide, deionized (see UNIT 14.3) 45 mM Tris base 45 mM boric acid 1 mM EDTA 0.05% (w/v) bromphenol blue 0.05% (w/v) xylene cyanol Store in aliquots at −20°C and discard after 3 months Lysis solution 300 mM NaCl 50 mM Tris⋅Cl, pH 8.0 (APPENDIX 2), room temperature 25 mM EDTA, pH 8.0 (APPENDIX 2) 0.2% (v/v) SDS (prepare fresh; add just before use) 0.2 mg/ml proteinase K (prepare fresh; add just before use) Lysis solution without SDS and proteinase K can be stored indefinitely at room temperature. Immediately before use, add 10 µl of 20% SDS and 10 µl of 20 mg/ml proteinase K per milliliter lysis solution. Phenol/chloroform/isoamyl alcohol Mix 25 parts phenol (equilibrated in 150 mM NaCl/50 mM Tris⋅Cl (pH 7.5)/1 mM EDTA) with 24 parts chloroform and 1 part isoamyl alcohol. Add 8-hydroxyquinoline to 0.1%. Store in aliquots at −20°C and discard after 6 months.
LigationMediated PCR
Precipitation salt mix, per reaction (9.4 ìl): 8.4 µl 3 M sodium acetate, pH 7.0 (2.7 M final) 1.0 µl 10 mg/ml yeast tRNA (∼1 mg/ml final) Prepare immediately before use
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Unidirectional linker mix, 20 ìM 20 µM oligonucleotide LMPCR.1 (Fig. 15.3.3) 20 µM oligonucleotide LMPCR.2 (Fig. 15.3.3) 250 mM Tris⋅Cl, pH 7.7 (APPENDIX 2) Prepare this mix in advance as follows: (1) purify the oligonucleotides on denaturing polyacrylamide gels (UNIT 2.12), (2) combine the two oligonucleotides and Tris⋅Cl and heat 5 min at 95°C, (3) transfer to 70°C and gradually cool ∼1 hr to room temperature, (4) leave ∼1 hr at room temperature and then gradually cool ∼1 hr to 4°C, and (5) leave ∼12 hr at 4°C and store in aliquots at −20°C. Thaw linker on ice before use. Hybridization of the linker undoubtedly takes place more rapidly than allowed for in this procedure, but this has reproducibly worked well. Because the monovalent salts (e.g., NaCl) more traditionally used to raise ionic concentration inhibit T4 DNA ligase, and Tris⋅Cl is required by the ligase in any case, Tris⋅Cl is used as the salt in this hybridization reaction. Consequently, it is difficult to calculate the kinetics of oligonucleotide annealing, and so the question of timing has not been studied in detail.
Vent DNA polymerase mix, per reaction (3.0 ìl) Mix 0.6 µl of 5× amplification buffer (see recipe) with 1.9 µl H2O and chill several minutes in an ice-water bath. Add 0.5 µl (1 U) Thermococcus litoralis DNA polymerase (Vent; New England Biolabs), gently mix, and return to ice-water bath. Prepare just before use. Vent DNA polymerase stop solution, per reaction (295 ìl): 25 µl 3 M sodium acetate, pH 7.0 (260 mM final) 266 µl TE buffer, pH 7.5 (APPENDIX 2; 10 mM Tris⋅Cl, pH 7.5, final) 2 µl 0.5 M EDTA, pH 8.0 (APPENDIX 2; ∼4 mM final) 2 µl 10 mg/ml yeast tRNA (68 µg/ml final) Prepare immediately before use COMMENTARY Background Information Ligation-mediated PCR was originally developed to study in vivo protein-DNA interactions at regions of genes important for transcriptional control of expression. Several genomic sequencing strategies have been previously applied to in vivo footprinting. These relied on the principles of Southern blotting (Church and Gilbert, 1984), solution hybridization (Jackson and Felsenfeld, 1985), or primer extension (Huibregtse and Engelke, 1986). Unfortunately, with these strategies, in vivo footprinting of a single-copy regulatory region in large genomes (e.g., mammalian) is technically challenging, requires large cell numbers, and often produces results with unacceptable signal-to-noise ratios. The polymerase chain reaction (Saiki et al., 1988; White et al., 1989) is used to amplify specific fragments of single-copy genes (see Chapter 15 introduction and UNIT 15.1). It relies on two primers that flank the specific fragment of DNA to be amplified and uses repeated
cycles of template denaturation, primer annealing, and extension by a thermally stable DNA polymerase to exponentially amplify that fragment. PCR is not immediately applicable to sequencing or footprinting because it requires that every fragment of DNA have two defined ends (a sequence ladder is composed of a population of related DNA fragments that differ in length; they all have one end in common but the other end varies depending on the sequence). The Basic Protocol in this unit circumvents this problem by attaching a common sequence to the variable end of every fragment, thus creating substrates suitable for PCR and resulting in exponential amplification of each representative of the sequence ladder. One end of every fragment is primed with a gene-specific primer and the other is primed with the common attached sequence. Footprint and sequence information contained in the original sample of genomic DNA is amplified by an additional 104 to 105 copies. When visualized on a sequencing gel, each band in the ladder is
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shifted to a higher position that corresponds to the length of attached sequence. In practical terms, starting with as little as 2 µg of mammalian DNA (equivalent to 3 × 105 cells), a genomic sequence ladder of a single-copy gene can be seen following overnight exposure without an intensifying screen. Ligation-mediated PCR differs significantly from another single-sided PCR procedure that uses terminal transferase to add a homopolymeric tail of guanines or adenines to one end of the substrate DNA (Frohman et al., 1988; Loh et al., 1989; Ohara et al., 1989). This tail is added nonspecifically to all DNA and is expected to be somewhat variable in length. In contrast, ligation-mediated PCR results in the blunt-end ligation-based addition of a sequence of defined length and composition. The blunt end is generated by the extension of a gene-specific primer. The initial hybridization of this primer selects a small subset of specific gene fragments (to be modified in later steps) from the original complex DNA sample. This level of selection is unique to ligation-mediated single-sided PCR. These features, together with the use of multiple gene-specific primers, greatly reduce nonspecific background and provide the single-base resolution required for genomic footprinting, sequencing genomic DNA (Pfeifer et al., 1989; Garrity and Wold, 1992), studying in vivo methylation patterns of cytosine residues (Pfeifer et al., 1989, 1990; Rideout et al., 1990; Garrity and Wold, 1992), and cloning promoter elements from restriction-digested genomic DNA (Fors et al., 1990). This last adaptation uses gene-specific primers selected from known sequences located at the 5′ end of an mRNA to clone the corresponding unknown promoter region. Support Protocols 1, 2, and 3 describe methods for the preparation of genomic DNA for genomic DMS footprinting and genomic sequencing. Both of these methods are based on the Maxam-Gilbert chemistry described in UNIT 7.5. In addition, DNA cleaved randomly by a nuclease such as DNase I (UNIT 12.4; Tanguay et al., 1990; Rigaud et al., 1991), or site-specifically by a restriction endonuclease (UNIT 3.1) can also in principle be utilized in ligation-mediated PCR. In any case, the cleaved DNA must contain a 5′ phosphate to participate in the ligation reaction.
Critical Parameters LigationMediated PCR
Several important parameters are discussed in this section, but two underlying technical
points deserve emphasis at the outset. First, in order to keep background to a minimum, the hybridizations and extensions should be performed at the highest possible temperatures. Hybridizations are incubated at or 2°C above the calculated melting temperature (Tm) of the primers, and extensions are performed at 76°C with Vent DNA polymerase. (In specific cases, these temperatures may need to be adjusted, but they have worked with all combinations of primers tested to date). Second, background can be greatly reduced by using multiple primers, each primer having an end that extends 3′ to the previous one. This produces a level of specificity that cannot be obtained with a single 25-base primer. It is recommended that a test experiment be done before any footprinting or sequencing experiments are undertaken. This should include several identical samples that are processed in parallel. When visualized on a sequencing gel, these samples should appear identical. In addition, this is a convenient way to test for any problems in the operation of the automated thermal cycler or in the execution of ligation-mediated PCR. Whenever possible, prepare solutions as a reaction cocktail; this will minimize sample variation. Oligonucleotides should be gel-purified (UNIT 2.12) to remove contaminants and stored in TE buffer, pH 7.5, at ≥10 pmol/µl and at −20°C. Gene-specific primers Primer position. Figure 15.3.2 shows possible arrangements for gene-specific primers. Several different primer combinations have been tested and the following operating rules were derived (P.R.M. and B.W., unpub. observ.). Primers 1 and 2 may or may not overlap but the extending end of primer 2 must be 3′ to primer 1. If they do overlap, the overlap should be less than half their length. On the other hand, primer 3 can completely overlap primer 2 and extend a few extra bases 3′ to it (Fig. 15.3.2A), and it must be positioned with an overlap of at least half the length of primer 2 (Fig. 15.3.2B). If primers 2 and 3 do not overlap, the labeling extension will not be successful, because it is necessary for these primers to compete for the same binding site. The basis of this phenomenon is not known. It may be caused by the premature extension product of primer 2 excluding the hybridization of primer 3. The hybridization temperatures used during the annealing phase of the labeling cycle(s) are close to the optimal temperature for Vent DNA po-
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lymerase. This is done to minimize background, but probably results in the immediate extension of any primer following its binding. Primer composition and melting temperature. Elevated temperatures minimize the formation of imperfect duplexes between genomic DNA and primers. Thus, to minimize background, hybridizations and reactions should be carried out under temperature conditions that demand as much as much specificity as possible. For hybridizations, the optimal temperature is at or even above the calculated Tm of the primer (see below for calculation). This principle is also used to favor the binding of the labeling primer (primer 3) over the amplification primer (primer 2). In general, the Tm of the primers should increase in the order primer 1 < primer 2 < primer 3. Thus, the binding of the first-strand synthesis primer (primer 1) will be less stable under amplification conditions and the binding of the amplification primer (primer 2) will be less stable under labeling conditions. In practice, this doesn’t appear to be essential, but it would probably be unwise to have the relative Tm of the primers reversed. The Tm of the primers can be adjusted by choosing sequences with differing GC content and/or length (see below). The first-strand synthesis primer (primer 1) should have a Tm of ≥60°C; primers of 20 (∼60% GC content) to 25 bases (∼50% GC content) work well. If the Tm of the primer is below 60°C, it may be necessary to reduce the hybridization temperature below the suggested 60°C (see Basic Protocol, step 3). The amplification primer (primer 2) should have a Tm equal to or higher than that of primer 1, and should closely match the Tm of the linker primer; a primer of 25 bases with a GC content of ∼60% works well. The labeling primer (primer 3) should have an even higher Tm. During the labeling extension, the linker primer does not need to extend, and the labeling primer can better compete with the amplification primer for occupancy if it has a higher Tm (recall that the amplification primer and labeling primer must overlap or the labeling primer will be excluded). Overall, the exact GC content or length of the primers is probably not very important. What is important is their relationship to each other and to the temperature of the reactions. Primers must be long enough to be specific to the gene of interest, but they should not be excessively long or have a Tm greater than ∼76°C (this may lead to nonspecific binding). The following formula (Wahl et al., 1987) may be used to approximate the Tm of primers:
Tm = 81.5 + 16.6 (logM) + 0.41(%GC) − (500 ⁄ n )
where n = length of primer and M = molarity of the salt in the buffer. To determine the molarity, ignore the contribution of Mg2+ and multiply the Tris concentration by 0.67 (N. Davidson, pers. comm.). Therefore, for the amplification buffer, the molarity would be 0.040 (NaCl) + 0.67 × 0.020 (Tris) = 0.053 M salt. Hybridizations work best if done at or 2°C above this calculated Tm. As an example, the Tm calculation for a 25-base primer with a 60% GC content is as follows: 81.5 + 16.6(log 0.053) + .41(60) − (500/25) = 65°C. Hybridization with this primer should be carried out between 65° and 67°C. This is only an approximation, and base stacking and near-neighbor effects may be significant for particular primers. Therefore, it may be necessary to determine empirically the optimal hybridization temperatures for each primer. Unidirectional linker The staggered design of the oligonucleotide linker is important because it assures that the ligation event is directional (see Fig. 15.3.3). The 3′ hydroxyl of the long oligonucleotide (LMPCR.1) will be ligated to the 5′ phosphate of the genomic DNA. For cloning purposes, LMPCR.1 contains restriction sites for BstEII, SmaI, BglII, and EcoRI; however, these are not used in sequencing or footprinting experiments. This oligonucleotide will also function as the linker primer in the PCR amplification. On the other hand, the short oligonucleotide (LMPCR.2) functions only in a structural role. When it is hybridized to the long oligonucleotide, it creates a blunt-end duplex at the 3′ end of LMPCR.1. This enables T4 DNA ligase to attach LMPCR.1 to the blunt-end duplex of the genomic DNA. By making the short oligonucleotide small and low in GC content, it is possible to make it unable to serve as a primer in subsequent Vent DNA polymerase reactions that are carried out far above its Tm. To prevent linker self-ligation, neither oligonucleotide has a 5′ phosphate. By default, this prevents LMPCR.2 from ligating to the first-strand synthesis extension products, but this is not an essential design feature (see Fig. 15.3.1). The exact sequence, GC content, length, and type of restriction endonuclease sites in this linker are not thought to be important for the function of the linker. The crucial properties for the linker are that (1) it contain no 5′ phosphates and be staggered to eliminate self-ligation and assure unidirectionality, (2) it be a ligatable
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structure, (3) the duplex between the two olignucleotides be stable during ligation conditions but not during PCR conditions, and (4) the long oligonucleotide have a Tm in Vent DNA polymerase buffer comparable to that of primer 2. Ligation Most products of the first-strand synthesis reaction appear to be ligated after 45 min at 15°C. However, particular bands in some ladders require longer incubation times to reach full intensity (Mueller and Wold, 1991). For this reason, an overnight (6 to 15 hr) ligation at 15° to 17°C is suggested. Although a higher ligation temperature might decrease the time required for complete ligation, this could inadvertently permit undesirable Vent DNA polymerase activity. Prior to ligation, no attempt is made to destroy the Vent DNA polymerase used during the first-strand synthesis. A possible concern is that if the polymerase is active during the ligation, it may extend the short oligonucleotide of the staggered linker (this would compromise the unidirectionality of the linker addition). With ligation at 15° to 17°C, this does not appear to be a problem, but it may be unwise to attempt higher temperatures unless the polymerase is inactivated.
LigationMediated PCR
Vent DNA polymerase Earlier versions of ligation-mediated PCR used a combination of Sequenase and Taq DNA polymerase for the first-strand synthesis and amplification/labeling reactions, respectively. Both of these enzymes have been replaced by Thermococcus litoralis DNA polymerase (Vent DNA polymerase). Vent DNA polymerase virtually eliminates the unequal representation of some members of a sequence ladder and occasional spurious bands that were previously observed with Sequenase/Taq polymerase–based ligation-mediated PCR. It also provides an approximate three-fold increase in signal strength to all members of the sequence ladder (Garrity and Wold, 1992). A very important procedural point is that ligation-mediated PCR is very sensitive to the amount of Vent DNA polymerase activity. A small (e.g., two-fold) increase in polymerase can result in an unacceptable background level. The reason for this threshold response to the amount of Vent DNA polymerase activity is unknown, but it does mean that this polymerase must be used carefully. If high background is a problem (or suddenly becomes one), the polymerase should be titrated. Normally it is not necessary to titrate each lot of Vent DNA po-
lymerase, but a change in manufacturing at New England Biolabs occurred between early lots (i.e., prior to the fall of 1991), which were less active, and more recent ones. Thus, originally 1 U of Vent DNA polymerase was needed in the first-strand synthesis and 3 U were needed at the amplification step (Garrity and Wold, 1992), but now 0.5 U in the first-strand synthesis and 1.0 U at the amplification step appears to be the ideal amount (P.G. and B.W., unpub. observ.). The 1.0 U added at the labeling step does not need to be altered. The “exonuclease-minus” version of Vent DNA polymerase has not been tested in this procedure. The buffers recommended here for use with Vent DNA polymerase in ligation-mediated PCR have been developed through empirical testing and differ from the one recommended by the manufacturer (P.G. and B.W., unpub. observ.). KCl has been replaced by NaCl, which gives better extension through certain guaninerich regions. BSA has been replaced by gelatin, because this reduces the interface formed after the organic extraction. (NH4)2SO4 has been eliminated, because this reduces background and the amount of excess salt deposited in the ethanol-precipitation pellets. Finally, the concentration of MgSO4 has been increased, because this increases signal strength. These conditions may need to be adjusted in specific cases, but so far they have worked with all combinations of primers tested. The first-strand synthesis buffer differs slightly from the amplification/labeling buffer (i.e., there is less Tris and no Triton X-100). This change facilitates the conversion of first-strand buffer to ligation buffer without affecting polymerase activity for the single round of extension. Vent DNA polymerase is added last to each sample and then only after the sample has been chilled on ice. This is to minimize the singleand double-stranded exonuclease activities reported by the manufacturer that are inhibited by cold and by the presence of dNTPs. Efficiency and statistical limitations (founder effects) In this procedure, exponential amplification is so effective that the minimum amount of starting DNA needed is not dictated by signal strength; instead statistical considerations become important. Conventional PCR can be successfully performed using the DNA of a single genome (White et al., 1989). Obviously, footprinting or sequencing reactions require much more material than this, but determining exactly how much more is needed is not straight-
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forward (UNITS 7.5 & 12.4). It is useful to first estimate the number of starting genomes or founder molecules that would be required in an idealized case in which all steps in the procedure were 100% efficient. To generate a proper sequence ladder, all “rungs” must be represented at least once (i.e., by at least one starting chromosome). Limited cleavage is used to generate DNA fragments that are, on average, ∼600 bases long (UNITS 7.5 & 12.4), and each of these can be used to generate a rung on the ladder. In the specific case of a DMS/piperidine guanine ladder, there are ∼150 possible cleavage sites (guanine residues) in an average 600-nucleotide fragment. Therefore, ≥150 chromosomes are required to generate all of the rungs in a ladder. However, because cleavage is random, ≥10 representatives (on average) must be generated to ensure that each rung is derived at least once. Furthermore, in footprinting, quantitative differences in the intensity of individual bands is important, and the maximum acceptable variation in band intensity due to sampling considerations should be 17% CV). After removing the outlier point, recalculate avg, stdev, and CV. Only one point per replicate may be removed. A 17% CV correlates with the maximum allowable standard deviation that can distinguish a two-fold change with 99% confidence when samples are assayed in triplicate wells for both the endogenous reference and the GOI. If a 95% confidence interval is acceptable, a 21.8% CV may be used as the threshold for removing outliers. On the other hand, the Q-test (a test for rejection of discordant data) may be used to determine outlier points. Refer to Shoemaker et al. (1974) for a more in-depth description of this test.
14. For each sample, normalize the GOI quantity to that of the reference gene for the sample according to Equation 15.8.1. Use the recalculated values if outlier points were removed in step 13.
Equation 15.8.1
15. Calculate the standard deviation of the normalized value (SD) according to Equation 15.8.2.
Equation 15.8.2
16. Plot the resulting values as a bar graph of normalized value versus sample name or experimental treatment group, with the error bars equal to the SD. 17. If desired, calculate fold-changes between samples by choosing a calibrator sample (usually vehicle-treated or wild-type control) and dividing all of the normalized values from step 14 and the SD calculated in step 15 by the normalized value of this calibrator. The resulting values are then expressed as fold-changes relative to the calibrator sample, which should now be equal to 1.
EFFICIENCY-CORRECTED Ct METHOD The efficiency-corrected Ct method is used for determining the relative amounts of different GOIs that are normalized to an endogenous reference RNA. It may also be used to determine fold-changes of a specific RNA between samples, but may be excessive if the desired output is only fold-change (see Basic Protocol 1 or Alternate Protocol). Data obtained from the efficiency-corrected Ct method are expressed as "normalized RNA level" in arbitrary units, and the calculated levels may be compared to those of other GOIs when the same threshold setting and assay chemistry are used (i.e., SYBR Green or TaqMan chemistry). It should be noted that an assumption is made that the reverse transcription efficiency is equal for all RNA transcripts in a single sample and for the same transcript between samples. In some cases, this may not be true (see Pfaffl, 2004, for further discussion); therefore, it is recommended that the RNA extraction method
BASIC PROTOCOL 2
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remain the same for all samples, and that all samples under study be reverse transcribed at the same time with the same reaction buffer. The basis of quantitative PCR lies in the principle that for every additional thermocycle, a two-fold increase of template-specific product occurs. Several factors affect whether a change in one cycle truly represents a two-fold growth in product, in other words, whether the reaction is 100% efficient. To assess the reaction’s efficiency, linear regression analysis is applied to a standard cDNA dilution series, just as in Basic Protocol 1. The slope of the resulting standard curve is used as a measure of PCR efficiency (E) according to the equation E = 10(–1/slope) . Note that different GOIs may produce different E values. A slope of −3.3 produces an E value of 2, indicating that a perfect doubling of the template has occurred. Calculated E values of less than 2 imply that the template has not been perfectly doubled. Template, primer, and probe quality and quantity, in addition to pipet calibration and buffer conditions like MgCl2 , salt, additives, and deoxynucleotide concentrations, all contribute to this efficiency (Pfaffl, 2004; see UNITS 15.1 & 15.5 for further details on these parameters). Given optimal buffer conditions and adequate primer, probe, and sample qualities, slight fluctuations in efficiency may still be observed between primer sets run on the same plate or between assay plates, even if they have been assembled and run on the same day by the same user. Efficiency correction is a means to account for inter- or intra-assay variability that is attributable to the aforementioned parameters. The materials and setup of the assay are the same as in the standard curve method (see Basic Protocol 1), except that the linear regression formula produced by plotting the Ct versus the log ng of input standard total RNA is used only to determine PCR efficiency. This computed efficiency is then used to calculate the RNA levels (in arbitrary units) of the GOI and the endogenous control genes. The GOI RNA level in each sample is then expressed as a ratio relative to the endogenous control RNA level in that sample. Because the data are dependent upon Ct values and not an arbitrary standard curve, the resulting values may be compared to those of another RNA. 1. Set up and run assay as described in Basic Protocol 1 (steps 1 to 9). Refer to Figure 15.8.1 for an example of a typical plate setup and to Table 15.8.1 for master mix components. 2. Analyze and export raw data (see Basic Protocol 1, step 10), and then import into Microsoft Excel or equivalent spreadsheet program. The threshold values for all RNAs measured (including the endogenous reference) must be the same. It is important to determine a suitable threshold within the geometric phase of the amplification plots for all RNA transcripts to be compared.
3. Calculate PCR efficiency, E = 10(–1/slope) , for the endogenous control RNA and each GOI from the slopes of their corresponding standard curves. 4. Calculate the quantity of the endogenous control RNA and each GOI from their Ct values according to the formula quantity = E–Ct . When the efficiency is 100% (i.e., slope = –3.3 and E =2), the equation becomes quantity = 2–Ct . This serves as the basis for the calculation performed in Ct method (Alternate Protocol).
High-Throughput Real-Time Quantitative Reverse Transcription PCR
5. For each of the three replicates of a sample, calculate the average quantity (avg), the standard deviation of the average (stdev), and the coefficient of variation (CV), where CV = stdev/avg. 6. Remove outliers, normalize the GOI, calculate the SD, and plot the results (see Basic Protocol 1, steps 13 to 17).
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COMPARATIVE OR Ct METHOD The comparative Ct or Ct method is used for measuring the fold-changes in expression of a particular RNA transcript between experimental samples. Typically, this assay is used when investigating gene-expression differences between wild-type and knockout or transgenic animals, or between vehicle-control and drug-treated samples. The results are then expressed as "fold-changes" relative to a calibrator, such as an untreated or wild-type sample. The Ct method is only applicable when the primer sets for both the GOI and the endogenous reference gene have been shown to give perfect standard curve slopes (slopes = −3.3 ± 0.1 with R2 = 0.99) as assayed in the Support Protocol.
ALTERNATE PROTOCOL
1. Set up assay as described in Basic Protocol 1 (steps 1 to 9), except refer to Figure 15.8.2 for an example of a typical plate setup and to Table 15.8.2 for master mix components. Standard cDNA samples are not needed in this method.
2. Analyze and export raw data (see Basic Protocol 1, step 10) and then import into Microsoft Excel or equivalent spreadsheet program. 3. For each of the three replicates of a sample, calculate the average (avg) cycle time (Ct) and then calculate the standard deviation (stdev). 4. Remove any outlier wells from the averaged Ct values (>0.3 stdev). Only one point per replicate may be removed. %CV may not be used, due to the logarithmic nature of both the Ct avg and Ct stdev. Instead, stdev must be used, where a stdev of 0.3 correlates with the maximum allowable standard deviation that can distinguish a two-fold change with 99% confidence; 0.4 stdev may be used for a 95% confidence interval.
Figure 15.8.2 Typical plate setup for the Ct method. The plate arrangement shown represents an experiment in which 20 samples taken from experimental animals will be assayed for one endogenous control gene and four GOIs. The samples are plated in triplicate for each of the RNAs assayed. Abbreviations: GOI, gene of interest; NTC, no-template control; Ref, endogenous control gene.
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Table 15.8.2 Master Mixes for the Ct Assay
Final concentration
Volume per well
Volume for each Master mix (no. of cDNA/primer mix (per samples + 1 NTC + 1 sample in triplicate + 1 extra) extra)
2× SYBR Green mix
1×
5 µl
20 µl
1:1 primer mix (1.25 µM each)
150 nM each primer
1.2 µl
4.8 µl
Template cDNA
10-25 nga
1.25 µl
5 µl
H2 O
N/A
to 10 µl
to 40 µl
10 µl
40 µl
Components
SYBR Green assay
Total volume
— Aliquot 35 µl into each tube containing cDNA
TaqMan assay 2× TaqMan mix
1×
5 µl
20 µl
1:1 primer mix (1.25 µM each)
300 nM each primer
2.4 µl
9.6 µl
5 µM TaqMan probe
250 nM
0.5 µl
2.0 µl
a
Template cDNA
10-25 ng
1.25 µl
5 µl
H2 O
N/A
to 10 µl
to 40 µl
10 µl
40 µl
Total volume
— Aliquot 35 µl into each tube containing cDNA
a Recommended amount of template for detection of both high and low levels of GOIs. If necessary, significantly less template may be used (picograms).
Note that the cDNA template quantity is based upon the amount of total RNA input into the reverse transcription reaction.
5. For each sample, normalize the GOI Ct values to those of the reference gene for the same sample according to the equation:
Equation 15.8.3
Calculate the standard deviation of Ct (stdevCt ) as:
Equation 15.8.4
6. Choose a calibrator. This will be the sample, tissue, gene, or control group to which the others will be compared.
7. Find the Ct, or calibrated value, for each sample, according to the equation:
High-Throughput Real-Time Quantitative Reverse Transcription PCR
Equation 15.8.5
The stdevCt will be the same as stdevCt , since the calibrator is arbitrarily set to be a constant.
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8. Find the fold-change for each sample relative to the calibrator according to the equation:
Equation 15.8.6
For the sample that is chosen as the calibrator, the Ct = 0 and therefore the foldchange = 2(−Ct) = 1.
9. Plot the resulting fold-changes on a bar graph of fold-change versus sample name or experimental treatment group. Determine the measure of experimental error as:
Equation 15.8.7
DESIGN AND VALIDATION OF SYBR GREEN AND TaqMan PRIMER/PROBE SETS
SUPPORT PROTOCOL
Considering that QPCR relies on the quality and the fidelity of the primers and probes that are used, very strict parameters for their design and subsequent validation are required. A common misconception in performing QPCR assays is that if the primer works for traditional end-point PCR, it is suitable for QPCR. In some cases this is true. However, the primer set must be tested in a QPCR validation assay before it can be used for RNA expression analysis. In keeping with the high-throughput capacity of QPCR, the thermocycling conditions are kept constant for all assays: 10 min at 95◦ C activates the hot-start Taq polymerase, followed by 40 cycles of the two-step 95◦ C melting and 60◦ C annealing. An extension step in the thermocycler program is not required, since all of the PCR products are 50 to 150 bp, thus making the run last only 1.5 to 2 hr. The concentrations of PCR reagents such as Taq DNA polymerase, MgCl2 , other salts, and dNTPs remain constant within the same chemistry (i.e., SYBR Green or TaqMan), and these so-called universal cycling conditions make primer and probe sequences the only point of flexibility in performing the assays. The design of primer/probe sets requires the availability of reliable sequence information that may be obtained from databases like NCBI’s GenBank or Ensembl, or from data produced by direct sequencing. The assay does not tolerate base mismatches between primer and template, especially in the probe sequence, a feature that allows for the detection of single-nucleotide polymorphisms (SNPs). Several primer/probe design software packages are available either for purchase or online. Otherwise, the user may design the primer sets by directly examining the sequence and choosing primers with the correct characteristics, as outlined in this protocol. The probe is labeled at the 5 end with a fluorescent reporter such as 6-FAM or VIC, and at the 3 end with a fluorescent or nonfluorescent quencher. The user should consult with the vendor that will synthesize the probe for the availability of each type of label. The assay consists of a standard cDNA dilution series from which linear regression curves may be plotted. The slope of the resulting curve gives a measure of PCR efficiency, where −3.3 ± 0.1 with a coefficient of determination (R2 ) of 0.99 indicates a reaction efficiency of 100%. Part of the initial SYBR Green validation also includes a melting (or dissociation) curve analysis. At the end of the repetitive cycles of the PCR, an additional melt-anneal-melt cycle is performed. The final melt occurs very slowly and
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the changes in both temperature and fluorescent signal are monitored over time. This decrease in fluorescence correlates with the dissociation of the double-stranded PCR product releasing the bound SYBR Green I fluorophores. The instrument software uses an algorithm to transform and display the melting curve as the negative first derivative of the normalized fluorescence versus temperature (Applied Biosystems, 2001a). The presence of a single peak in the melting curve is indicative of a single PCR product, and occurs at the melting temperature of the product. Multiple peaks in this plot indicate that nonspecific products or primer dimers have been formed. Formation of a single product can be confirmed by running the PCR products on a 2% agarose gel following the QPCR run. Occasionally, when two products are observed, the second product may have been formed during the plateau phase, which would not affect quantitation. To confirm whether this has occurred, the QPCR run could be repeated and stopped during the exponential phase, and the reaction products run on an agarose gel. However, this is not feasible in practice because of the high-throughput nature of the assay. The best course of action when multiple products are observed in the dissociation curve is to redesign and validate a new primer set. Only primer sets that give a single peak in this curve should be used for experimental assays. Once a SYBR Green–based primer set has passed validation testing, the corresponding TaqMan probe is ordered and validated for PCR efficiency only. In rare cases, the SYBR Green assay conditions (e.g., primer concentration, Mg2+ concentration) will not be appropriate for TaqMan assays. This is observed as a decline in PCR efficiency. In this case, new primers may be designed to flank the probe sequence.
Additional Materials (also see Basic Protocol 1) Primer/probe design software (Primer Express, Applied Biosystems) Design primers 1. Retrieve the RNA sequence information from the appropriate source (e.g., Genbank or Ensembl). 2. Determine the locations of exon boundaries by aligning the mRNA sequence with its gene or by using NCBI’s Entrez Gene Evidence Viewer (http://www.ncbi. nlm.nih.gov) or Ensembl’s Genome Browser (http://www.ensembl.org). Some genes do not have introns, so this step may not be applicable.
3. Copy the sequence into the design software. Several design programs are available both commercially as stand-alone applications and as Web-based applications. Alternatively, primers and probes may be designed "by hand." Omit this step if designing by hand.
4. If using software other than Primer Express, use the following parameters:
High-Throughput Real-Time Quantitative Reverse Transcription PCR
a. QPCR primers: Should have 40% to 60% GC content and melting temperatures around 60◦ C. Should not contain runs of the same nucleotide, repetitive sequences, or more than two G’s and/or C’s on the 3 end (also called GC clamp). b. PCR product (amplicon): Should be 50 to 150 bases in length with an approximate melting temperature between 85◦ and 95◦ C. c. TaqMan probe (anneals to sequence between primers): Should have the same properties as the primers, except that the melting temperature should be around 70◦ C, and the sequence should not contain G’s within a few bases of the 5 end because of increased reporter quenching. In addition, the sequence must have more C’s than G’s, which can be accomplished by using the complementary strand sequence for the probe (Applied Biosystems, 2002b). Primer Express contains templates into which these parameters have been preloaded.
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Figure 15.8.3 Typical plate setup for primer and probe validation assays. The plate arrangement shows a standard cDNA template dilution series that is being used to test 15 primer sets along with an endogenous control primer set that has already been validated. The same format is used when testing new probe sets. If different standard cDNAs are used to test primers/probes on the same plate (i.e., standards derived from different tissues), the validated endogenous reference primers/probes must also be run for that standard.
5. Choose a primer/probe set for which the primers anneal in different exons, or which have less than 5-bp overhangs into the adjacent exon on the 3 end of the primer. This step is necessary to avoid amplification of contaminating genomic DNA. Although the RNA is DNase-treated prior to reverse transcription, complete removal of genomic DNA is never achieved. For intron-less transcripts or other primer sets that bind sequence within a single intron, the –RT controls are essential for each sample, to ensure that genomic DNA is not being amplified.
6. Perform a BLAST (or equivalent) search of both primers of the set together to verify that they will fully anneal to the correct sequence and only that sequence. 7. If the TaqMan probe will be used, run BLAST (UNIT 19.3) on the probe sequence to assess whether it binds to the correct sequence with 100% identity. 8. Order a small-scale synthesis of the primers from a suitable vendor. Standard desalting of the primers is sufficient, and no additional purification (e.g., HPLC) is required. Once a primer set has been validated, large-scale synthesis may be more cost effective, especially for frequently used primers like the reference genes.
9. Validate the primer set according to the remaining steps of this protocol. If performing TaqMan-based assays, validate the primer set before ordering the probe. Once the primer set is validated, order the dual-labeled probe from an appropriate vendor, and validate according to the following steps.
Validate primer set 10. Set up assay as described in Basic Protocol 1 (steps 1 to 9), except refer to Figure 15.8.3 for an example of a typical plate setup and to Table 15.8.3 for master mix components. 11. Test the new primer set using SYBR Green chemistry. If valid, test the TaqMan-based chemistry. Current Protocols in Molecular Biology
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Table 15.8.3 Master Mixes for Primer/Probe Validation Assays
Final concentration
Volume per well
Volume for each cDNA/primer mix (per Master mix (for 8 sample in triplicate + 1 standardsb + 1 extra) extra)
2× SYBR Green mix
1×
5 µl
20 µl
180 µl
1:1 primer mix (1.25 µM each)
150 nM each primer
1.2 µl
4.8 µl
43.2 µl
Template cDNA
0.016-50 nga
1.25 µl
5 µl
—
H2 O
N/A
to 10 µl
to 40 µl
91.8 µl
10 µl
40 µl
Aliquot 35 µl into each tube containing cDNA
Components
SYBR Green assay
Total volume TaqMan assay 2× TaqMan mix
1×
5 µl
20 µl
180 µl
1:1 primer mix (1.25 µM each)
300 nM each primer
2.4 µl
9.6 µl
86.4µl
5 µM TaqMan probe
250 nM
0.5 µl
2.0 µl
18 µl
a
Template cDNA
0.016-50 ng
1.25 µl
5 µl
—
H2 O
N/A
to 10 µl
to 40 µl
30.6 µl
10 µl
40 µl
Aliquot 35 µl into each tube containing cDNA
Total volume
a Recommended template dilution series. The user may modify the range of cDNA concentrations based on the experimental system. Note that the
cDNA template quantity is based upon the amount of total RNA input into the reverse transcription reaction. b For this assay, the NTC and –RT controls are included as part of the standard sample set.
12. Following the instrument run, first check the dissociation curve. If more than one peak is present, the primer set is invalid and no other parameters are checked. 13. If a single peak is found in the dissociation curve, assess the PCR efficiency by calculating the slope of the linear regression curve as follows. a. Plot Ct (or crossing point, CP) versus log ng of the standard cDNA input for each concentration of standard as an xy scatter plot. This may be performed directly in the instrument software on the Applied Biosystems instrument, or can be done in Excel (or equivalent).
b. Apply a best-fit curve and display the corresponding linear regression formula. If the slope of the curve is −3.3 ± 0.1 with R2 = 0.99, the primer set amplifies at 100% efficiency, and the set is considered valid.
High-Throughput Real-Time Quantitative Reverse Transcription PCR
Efficiency is dependent upon several factors including pipet calibration, primer quality and dilution, and even variability in the instrument run. The slope, therefore, may not be exactly −3.3 ± 0.1. In this case, the slope of the test primer set must match that of a previously validated endogenous reference gene run for the same standard cDNA within ±0.1. For example, if the test primer gives a slope of −3.6 and the endogenous reference for the standard cDNA used to test the set gives a slope of −3.5, then the test set is valid. However, if the endogenous reference primer set gives a slope of −3.3 and the test primer has a slope of −3.6, the test set is invalid.
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14. Repeat the linear regression analysis for the TaqMan probe/primer set to assess PCR efficiency. Melting curve analysis cannot be performed for TaqMan-based assays, since cleavage of the probe releases the reporter that continuously fluoresces.
REAGENTS AND SOLUTIONS Use molecular-biology-grade (nucleic acid– and nuclease-free) or sterile-filtered doubledeionized water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Primer mixes, 1.25 µM each forward and reverse primer Mix small aliquots of 2.5 µM forward and reverse primer stocks (see recipe) in equal volumes (1:1). Store up to 1 to 2 months at 4◦ C in screw-capped tubes to prevent evaporation. Do not freeze, as repeated freeze-thaw cycles will degrade the primers. Primer stocks, 100 and 2.5 µM Purchase lyophilized oligonucleotides from any commercial source (synthesized at a 25-nM scale; standard desalting is sufficient, no additional purification is needed). Briefly centrifuge the tubes of powdered oligonucleotide to get contents to the bottom. Resuspend in molecular-biology-grade water to 100 µM. Before preparing primer mixes, dilute each stock (forward and reverse) to 2.5 µM. Store at –20◦ C. When stored properly and subjected to minimal freeze-thaw cycles, primer stocks can last >2 years. Primer pairs produced at a 25-nM scale will yield enough reagent to test ∼4000 samples using SYBR Green, or ∼2000 samples using TaqMan assays. Sterile-filtered double-deionized water may be used instead of purchased molecular-grade water. Do not use DEPC-treated water, because the slightly acidic pH may promote primer degradation. Tris buffer may be used instead of water, but should not contain EDTA, which acts as a Mg2+ -chelating agent and can inhibit the PCR.
Standard cDNAs, dilution series DNAse-treat (UNIT 3.12) and reverse transcribe (UNIT 3.7) a suitable RNA using 0.08 µg/µl random hexamer primers such that the final concentration of the standard is 40 ng/µl (based on RNA quantity; see UNIT 15.5). Include both a no-template control (NTC) for which water is used instead of RNA, and a no-reverse-transcriptase control (−RT). Following the reverse transcription, make a 5-fold dilution series of the 40 ng/µl standard to obtain working concentration standards of 40, 8, 1.6, 0.32, 0.064, and 0.0128 ng/µl. Store up to 1 month at 4◦ C or >2 years at –20◦ C (with minimal freeze-thaw cycles). By using 1.25 µl/well of each of the standards in the final reaction plate, the resulting amount of starting template will be 50, 10, 2, 0.4, 0.08, and 0.016 ng. See Commentary for more information and suggestions about control and standard RNA samples.
SYBR Green mix, 2× Combine the following components as indicated in Table 15.8.4: 25 mM MgCl2 , molecular biology grade (store at –20◦ C) 10× Gold PCR buffer (supplied with PCR enzyme; Applied Biosystems; store at –20◦ C) 10× dNTP mix: equal volumes of 2 mM dATP, dTTP, dCTP, and dGTP (store at –20◦ C) Dimethylsulfoxide (DMSO), molecular biology grade (store at room temperature) continued
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Table 15.8.4 Preparation of 2× SYBR Green Mix
Component
Volume (µl)
25 mM MgCl2
48
288
600
720
1200
10× Gold PCR buffer
40
240
500
600
1000
10× dNTP mix
40
240
500
600
1000
DMSO
40
240
500
600
1000
1:1000 SYBR Green I
20
120
250
300
500
50× ROX
8
48
100
120
200
AmpliTaq Gold polymerase (5 U/µl)
2
12
25
30
50
H2 O
2
12
25
30
50
Total volume in 2× buffer
200
1200
2500
3000
5000
SYBR Green I dye (Molecular Probes; store at –20◦ C), diluted 1:1000 in water 50× ROX passive reference dye (Invitrogen; store at –20◦ C) AmpliTaq Gold polymerase (5 U/µl; Applied Biosystems; store at –20◦ C) Water, molecular biology grade Prepare fresh and keep on ice prior to use in primer master mixes. Protect dyes and all prepared mixes from prolonged exposure to light by wrapping tubes in foil. To maintain the high-throughput nature of the assay, all buffer conditions, including the concentrations of Mg2+ , dNTPs, and other additives, are kept constant. It is strongly recommended that a preformulated buffer be purchased from a reliable vendor, since these are stable at room temperature and have been quality-control tested to ensure optimal performance. The authors have obtained comparable results using mixes from Applied Biosystems, Bio-Rad, Invitrogen, and Sigma.
TaqMan probe, 100 and 5 µM Depending on the vendor, the probe may be supplied in a lyophilized form. In this case, resuspend to 100 µM in Tris·Cl, pH 8.0 (APPENDIX 2), prepared with molecularbiology-grade water and reagents. Before use, dilute a small amount of 100 µM stock to 5 µM with more Tris·Cl, pH 8.0. Store either concentration at –20◦ C. Avoid repetitive freeze-thaw cycles, and thaw on ice prior to use to preserve the integrity of the probe. Protect the probe from excessive exposure to light (e.g., using ambercolored screw-cap tubes) to prevent photobleaching of the fluorescent dyes and evaporation. See Support Protocol and Commentary for additional considerations regarding the TaqMan probe. COMMENTARY Background Information
High-Throughput Real-Time Quantitative Reverse Transcription PCR
Quantitative PCR is a rapid, robust, and highly sensitive polymerase chain reaction method used to quantify specific nucleic acid targets. Real-time quantitative PCR is different from end-point, or in-gel, analysis (see UNIT 15.7) in several ways. For real-time analysis, the increase in fluorescent signal resulting from PCR product synthesis is recorded during the course of the thermocycle. This allows the user to specify the point in the assay at which to "read" the data. Measurements are obtained from the geometric phase of the amplification
reaction. This is the phase during which all of the components required for the PCR (e.g., dNTPs, primers, polymerase) are in excess, and therefore the deficit of an essential reaction component will not quench the efficiency of product synthesis. Following geometric amplification, the fluorescence curve reaches a plateau (i.e., the saturation point) as the reaction components begin to become limited and the kinetics of the reaction become unpredictable. At this stage, an increase of one thermocycle no longer correlates with a twofold change in product (Applied Biosystems,
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2002a). In contrast, in end-point PCR, quantitation is often obtained by in-gel densitometry measurements at the end (or the saturation point) of the reaction. The unpredictable nature of the PCR at this stage makes this method semiquantitative at best. Real-time PCR depends both on a set of universal thermocycling and buffer conditions and on primer efficiency testing and correction where necessary. As a result, the accuracy and precision of the resolution (smallest detectable fold-change) is less than 2-fold, whereas resolution for end-point in-gel measurement is limited to about 10-fold (Applied Biosystems, 2002a). Semiquantitative PCR assays (in which samples are removed at measured cycle times and electrophoresed and possibly hybridized) have better resolving power than end-point analysis. However, both in-gel methods suffer from the lengthy processing steps, compared to real-time PCR. The two most widely used fluorescent detection methods, or chemistries, for QPCR are SYBR Green, a DNA-intercalating dye, and the fluorogenic probe. Several types of fluorogenic probes are currently available: the popular, dual-labeled hydrolysis probe (TaqMan probe) and the hybridization probes known as molecular beacons and scorpions. Both types of probes bind the sequence intervening the forward and reverse primer binding sites, and both rely on fluorescence resonance energy transfer (FRET) to silence the signal from the reporter dye while it is in proximity to the quencher dye. A hydrolysis probe is cleaved by the 5 nuclease activity of the polymerase during the primer-extension phase of the reaction, and the reporter is released and becomes free to fluoresce continuously. Hybridization probes rely on a stem-loop structure to keep the reporter and quencher in proximity. Upon hybridization to the specific sequence, the distance between the reporter and quencher becomes too large to silence the reporter, and signal is detected (Tyagi and Kramer, 1996; Whitcombe et al., 1999). There are advantages and disadvantages to each type of chemistry (SYBR Green versus TaqMan) in terms of sensitivity and specificity. SYBR Green I binds any double-stranded DNA and does not depend on a probe-cleavage event. Therefore, SYBR Green produces earlier Ct values, resulting in an apparent enhanced sensitivity (Whittwer et al., 1997; Morrison et al., 1998). TaqMan probes, on the other hand, supply another layer of sequence specificity in addition to the forward and reverse primers.
There are two methods of quantitation that may be performed using real-time PCR: absolute and relative. Absolute quantitation allows for the assessment of the copy number of a specific nucleic acid target in a sample, while relative quantitation results in the ratio of a specific target relative to an endogenous reference gene. The absolute method requires the availability of a standard sample in which the copy number of the particular target has been carefully and accurately measured. From this standard sample, a dilution series is made and assayed for the target at the same time as the unknown samples. From the values obtained from linear regression analysis of the standard dilution series, the GOI copy number values may be interpolated. This will allow the user to assess the sensitivity (i.e., the lowest detectable copy number) of the GOI primer set. As an example, this type of analysis is used in both clinical and food science for the assessment of pathogen load and gene copy number (Pfaffl, 2004). Relative quantitation, which is covered in detail in protocols in this unit, does not rely on the knowledge of a given transcript copy number in a standard sample. Instead, the changes in gene expression or the levels of a specified transcript may be measured and described as an arbitrary unit relative to some control sample (Ct or standard curve method), or to the level of some other control transcript in the same sample (standard curve or efficiencycorrected Ct method). For example, relative quantitation allows for the measurement of the fold-change in expression for gene A in a treated sample versus an untreated sample, or for the assessment of the level of gene A in a sample relative to some housekeeping gene in the same sample.
Critical Parameters and Troubleshooting Primer/probe design and validation For RNA analysis, it is important to differentiate message from genomic DNA. In this respect, the use of amplicons that span an exon junction allows this requirement to be met. This is achieved by designing primer sets with the forward and reverse primers sitting in different exons. However, when the intervening intron is small, genomic DNA may still be amplified. This can be avoided by allowing a few base pairs on the 3 end of either primer to overhang onto the neighboring exon. To distinguish knockout or mutant samples using QPCR, primers/probes should be
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designed in the knocked-out or mutated region of the transcript. In this way, no amplification of the transcript will occur for the knockout, since the region recognized by the primers has been deleted or altered. Primers for transcripts that are rapidly degraded should be placed near the 5 end of the RNA sequence, as degradation by ribonucleases generally occurs 3 to 5 (Brown, 2002). However, RNA transcripts that have undergone linear RNA amplification (for instance, RNA isolated from laser capture microdissected cells) are around 200 to 1000 bases in length and represent the 3 ends of the transcripts. In this case, primers must be designed in a region near the poly(A)+ tail. It is absolutely mandatory to validate both primers and probes on the same instrument used to perform the experimental assay, i.e., when the instrument used to validate the primer sets is a different brand than the one that will be used for the experimental assays (e.g., Roche iCycler versus ABI 7900HT). Published primer/probe sets for which the PCR product is ≤150 base pairs and the annealing temperature is around 60◦ C, which have been validated properly, should be transferable to any system, but this must be empirically determined to ensure the reliability of the data. In rare cases, the PCR efficiency of a validated primer set changes when switched from SYBR Green to TaqMan-based chemistry. This is due to the difference in primer concentration and/or MgCl2 concentrations between the two buffers, and may often be overcome by redesigning the primers to recognize a sequence around the already synthesized probe.
High-Throughput Real-Time Quantitative Reverse Transcription PCR
Choice of chemistry The selection between SYBR Green I and TaqMan-based assays depends upon the RNA sequence. If the RNA of interest has no polymorphisms or other variations in the region to which the primers bind, and the primers have been correctly designed and validated, then SYBR Green–based assays are adequate for gene-expression analyses. It is up to the user to decide if the addition of the often-costly TaqMan probe is worth the additional specificity it confers. In the case of polymorphisms or variants, differentiation between different RNA species may require the specificity of the probe, which can discriminate a single base difference. Template quality The quality of the cDNA template depends upon the integrity of the RNA. QPCR will tol-
erate some degradation of the RNA when random hexamers (or other -mers) are used to prime the reverse transcription reaction. However, it is not good laboratory practice to use degraded RNA, and the cause of the degradation should be addressed. Transcripts decay at different rates and have variable stability, so partial degradation of a sample at any point could lead to complete absence of detection of the desired target RNA in a subsequent QPCR assay, with little to no change in the housekeeping gene used as a control. Total RNA is used for QPCR to reduce the number of steps and potential sources of degradation during sample preparation. Purified messenger, or poly(A)+ , RNA can also be used. However, this subtractive purification could lead to the loss of transcripts that do not have a poly(A)+ tail, or to the preferential enrichment of RNAs that have internal A tracts. The added processing reduces the recovery of material for subsequent use, and can cause degradation. If poly(A)+ RNA is used, 50- to 100-fold less (∼100 pg) sample template is required in the reaction mixture because mRNA represents 3% of total cellular RNA (Alberts et al., 1994). Reverse transcription primers Reverse transcription (RT) of RNA into complementary DNA (cDNA; refer to UNIT 15.5) may be performed using several different types of oligonucleotide primers. For QPCR, the preferred primer is a random hexamer, nonamer, or dodecamer oligo with 6, 9, or 12-base stretches of random sequences, respectively. Random primers have a much higher probability of efficiently amplifying all RNA transcripts, due to their indiscriminate nature (ABI, pers. comm.). This property also enables RNA secondary structure to be overcome, since the priming occurs in random places along the length of the transcript. mRNA-specific priming by oligo(dT)s at the poly(A)+ tail and any internal poly(A) tract is another method of RT priming. This method will allow only polyadenylated RNAs to be converted to cDNA, thus limiting amplification of some GOIs or partially degraded samples. A mixture of a random oligo with an oligo(dT) primer may enhance detection of rare messages while still allowing for the detection of transcripts that lack polyadenylation. However, the use of this procedure may skew the measurement of relative RNA abundance towards intact, full-length mRNA over incomplete or rapidly degraded messages. Users should decide and test these parameters
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in their particular experimental systems. The third method of reverse transcription priming is the gene-specific reverse primer. The reverse PCR primer is used to specifically target the GOI for conversion to cDNA. This is often performed in the same QPCR plate as the PCR by adding RT enzyme to the PCR mix and adding an incubation step prior to the first step in the PCR cycling program (known as one-step RTPCR). While this may enhance detection of a specific RNA target, RNA secondary structure may not be overcome, depending on the priming site, and RT efficiency must be considered in addition to PCR efficiency. An important assumption that is made when performing RT-QPCR is that RT efficiency is similar for the GOI between samples and for different GOIs. However, different tissues/ sample types may contain variable levels of RT-inhibiting or -enhancing factors (Pfaffl, 2004). To control for these variables, it is recommended that all samples to be compared be prepared under the same conditions and at the same time (i.e., with regard to RNA extraction method and RT reaction). Endogenous reference gene Normalization of sample loading is essential in any quantitative comparative analysis to ensure that the measured differences between samples is not attributable to disproportionate amounts of starting material. For gene-expression assays, the normalizer must be an endogenous gene that is expressed at equal levels in all tissue or cell types and treatment conditions under study. Traditionally, one of the so-called housekeeping genes (e.g., GAPDH, cyclophilin, β-actin, HPRT, U36B4, 18S rRNA) is selected to serve this function. The choice is a point of controversy, since there are examples of fluctuations for most of the abovementioned genes under various treatment or physiological conditions (Schmittgen and Zakrajsek, 2000; Suzuki et al., 2000; Guo et al., 2001; Vandesompele et al., 2002; Dheda et al., 2004). The user should run a small pilot experiment to determine which endogenous reference is appropriate for the particular studies. A good method for doing this is to perform a Ct assay using several potential normalizer RNAs and a GOI that is not expected to show any fold-changes within a small set of samples. The GOI is then normalized to each of the reference RNAs individually for all of the samples. Finally, the housekeeping gene for which there is no detectable fold-change of the GOI between the test samples is chosen for use in experimental assays (Applied
Biosystems, 2001b; Guo et al., 2001; Roche Applied Science, 2002). Controls and standards Controls for the assay that are made alongside the cDNA standards and unknowns include a no-template control (NTC), made by substituting water for RNA, and a no-reversetranscriptase (–RT) control, made by omitting the reverse transcriptase. For primers that span an exon junction, a –RT control is not needed for every sample if, during the validation process, this control shows no amplification product. If the system under study is the result of introduction of an expression construct into cells, or if the GOI does not have introns or has a known processed pseudogene, then –RT controls should be made and assayed for every sample. An NTC should be run for every primer set of an assay to facilitate the detection of contaminants that contribute to fluorescent signal. If amplification of the NTC occurs, primerdimer or other nonspecific PCR products may have been formed, or contamination of a reagent or degradation of the primer mix may have taken place. If amplification occurs in the –RT control, this indicates the presence of genomic DNA if the primer/probe set does not span an exon junction, or the presence of primer-dimer or nonspecific PCR products, a contaminant, or degradation of the primer mix. Although all amplification products (including those that are nonspecific) contribute to the fluorescence signal, this fluorescence contribution can be considered negligible if the Ct values of the NTC and –RT are ≥7 cycles different from the experimental samples (Applied Biosystems, pers. comm.). If the unknowns are RNA samples, the standard of choice is an RNA that is reverse transcribed in the same manner as the unknowns. A suitable standard RNA is one in which the expression of the GOI is at a moderate to high level and that has a similar composition to the unknown samples. The use of a plasmid or linear DNA standard is not advised for measurement of an endogenous tissue transcript, because these types of nucleic acids have different background compositions than RNA, are extracted differently, and are not reverse transcribed, and therefore may not have the same RT or PCR amplification efficiency as the experimental samples (Applied Biosystems, 2003; Pfaffl, 2004). Several vendors (e.g., Ambion, Clontech, Stratagene) supply total RNA preparations of many cell and tissue types collected from many different
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species. Both Stratagene and Clontech make total RNA pools termed "universal reference RNA," composed of mixtures of either various cell lines or whole tissues, respectively. These RNA pools represent ≥90% gene coverage on microarrays (Novoradovskaya et al., 2000; Clontech, 2002), and are very useful as standards both for validation of primer/probe sets and for large-scale multigene studies.
Prepare QPCR plate(s) (∼30 min to 1 hr per plate). Run plate on instrument (1.5 to 2 hr). Collect and analyze data (1 to 3 hr).
Anticipated Results
Applied Biosytems. 2001a. ABI Prism 7900HT User Manual, rev. 4. Applied Biosystems, Foster City, Calif.
The setup of the Ct assay allows 64 samples (one of which should be the NTC) to be assayed for an endogenous reference gene and one gene of interest in triplicate; the standard curve and efficiency-corrected Ct assays will accommodate an endogenous reference gene and one gene of interest in triplicate for 56 unknowns, six standards, and two control samples. Assays performed on the ABI 7900HT are completed in 1.5 and 2 hr for TaqMan and SYBR Green-based assays, respectively. The results yield data that are highly reproducible and correlate well with traditional northern blotting and RNaseprotection assays.
Time Considerations
High-Throughput Real-Time Quantitative Reverse Transcription PCR
In preparation for performing a QPCR assay, RNA and subsequent cDNA preparation may be carried out in advance. Prior to an actual experimental assay, primers and probes must be designed and validated. In most cases, design and validation may take several days to a few weeks. This includes the time to design, order, and synthesize the primers (∼2 days, depending on the vendor), test the primers (a few hours), and, if desired, synthesize (∼7 to 14 days, depending on the vendor) and test (a few hours) the corresponding probes. Once the required primer/probe sets are validated, the experimental assays are performed. A single-plate assay may take 0.5 to 2 hr to prepare and 2 hr to run. The preliminary raw-data analyses on the instrument software may take less than half an hour. The time required for the following final analyses will depend upon the user’s familiarity with both the mathematical and software applications. The typical workflow for an experimental assay is as follows: Day 1: Prepare total RNA and determine concentration by UV or fluorescence spectroscopy (1 to 4 hr, depending on number of samples and method of preparation). DNAse-treat and reverse transcribe RNA to cDNA (∼3 hr for setup and incubations). Day 2: Prepare master mixes for the assay (∼30 min to 1 hr).
Literature Cited Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, J.D. 1994. The cell nucleus. In Molecular Biology of the Cell, 3rd ed., p. 370. Garland Publishing, New York.
Applied Biosystems. 2001b. TaqMan human endogenous control plate: Protocol: Revision C. http://docs.appliedbiosystems .com/pebiodocs/ 04308134.pdf. Applied Biosystems, Foster City, Calif. Applied Biosystems. 2002a. Real-time PCR vs. traditional PCR. https://www.appliedbiosystems. com/support/tutorials/pdf/rtpcr vs tradpcr.pdf. Applied Biosystems, Foster City, Calif. Applied Biosystems. 2002b. Designing TaqMan MGB probe and primer sets for gene expression using Primer Express software. http://www. appliedbiosystems . com / support /tutorials /pdf/ taqman mgb primersprobes for gene expression. pdf. Applied Biosystems, Foster City, Calif. Applied Biosystems. 2003. Creating standard curves with genomic DNA or plasmid DNA templates for use in quantitative PCR. https://www. appliedbiosystems.com/support/tutorials /pdf / quant pcr.pdf. Applied Biosystems, Foster City, Calif. Brown, T.A. 2002. How genomes function. In Genomes, 2nd ed. (S. Carlson, ed.) section 10.4. John Wiley & Sons, Hoboken, N.J. Clontech. 2002. Control RNA for microarray experiments. Clontechniques XVII:6. http://www. clontech.com/clontech/archive/APR02UPD/pdf/ ControlRNA.pdf. Clontech, Palo Alto, Calif. Dheda, K. Huggett, J.F. Bustin, S.A., Johnson, M.A., Rook, G., and Zumla, A. 2004. Validation of housekeeping genes for normalizing RNA expression in real-time PCR. BioTechniques 37:112-119. Guo, D., Henriksson, R., and Hedman, H. 2001. The iCycler iQ detection system for evaluating reference gene expression in normal human tissue, rev. A. Amplification 2804. http://www.bio-rad.com/LifeScience/pdf/ Bulletin 2804.pdf. Bio-Rad, Hercules, Calif. Morrison, T.B., Weis, J.J., and Wittwer, C.T. 1998. Quantification of low-copy transcripts by continuous SYBR Green I monitoring during amplification. BioTechniques 24:954-962. Novoradovskaya, N., Payette, T., Novoradovsky, A., Braman, J., Chin, N., Pergamenschikov, A., Fero, M., and Botstein, D. 2000. Pooled, highquality reference RNA for human microarrays. Strategies 13:121-122. http://www.stratagene. com / news / newsletter.aspx?iid=6. Strategene, La Jolla, Calif.
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Pfaffl, M.W. 2004. Quantification strategies in realtime PCR. In IUL Biotechnology Series, No. 5: A-Z of Quantitative PCR (S.A. Bustin, ed.) pp. 87-120. International University Line, La Jolla, Calif. Roche Applied Science. 2002. Selection of housekeeping genes. Technical Note No. LC 15/ 2002. http://www.roche-applied-science.com/ lightcycler-online /lc support/pdfs/lc 15.pdf. Roche Applied Science, Indianapolis, Ind. Schmittgen, T.D. and Zakrajsek, B.A. 2000. Effect of experimental treatment on housekeeping gene expression: Validation by real-time, quantitative RT-PCR. J. Biochem. Biophys. Methods 46:6981. Shoemaker, J.P., Garland, C.W., and Steinfeld, J.I. 1974. Experiments in Physical Chemistry, pp. 34-39. McGraw-Hill, New York. Suzuki, T., Higgins, P.J., and Crawford, D.R. 2000. Control selection for RNA quantitation. BioTechniques 29:332-337. Tyagi, S. and Kramer, F.R. 1996. Molecular beacons: Probes that fluoresce upon hybridization. Nat. Biotechnol. 14:303-308. Vandesompele, J., De Preter, K., Pattyn, F., Poppe, B., Van Roy, N., De Psepe, A., and Speleman, F. 2002. Accurate normalization of real-time quantitative RT-PCR data by geometric averaging of multiple internal control genes. Genome Biol. 3:1-12. Whitcombe, D., Theaker, J., Guy, S.P., Brown, T., and Little, S. 1999. Detection of PCR products using self-probing amplicons and fluorescence. Nat. Biotechnol. 17:804-807. Whittwer, C.T., Herrmann, M.G., Moss, A.A., and Rasmussen, R.P. 1997. Continuous fluorescence monitoring of rapid cycle DNA amplification. BioTechniques 22:130-138.
Key References Ambion. 2001. The top 10 most common quantitative RT-PCR pitfalls. Technotes Newsletter 8:8. Ambion, Houston, Tex. A short, but useful checklist of critical considerations for performing any type of reverse transcription PCR. Applied Biosystems. 1997. Relative quantitation of gene expression: ABI PRISM 7700 Sequence Detection System: User Bulletin #2: Rev B. Applied Biosystems, Foster City, Calif.
Applied Biosytems. 2001a. See above. This instrument manual contains explanations about the transformation of fluorescence signal into Ct data in addition to outlining the proper method of baseline and threshold settings for ABI machines. Users should consult their specific instrument manuals, since each type of instrumentation will require knowledge of slightly different terminology and parameters. Livak, K.J. and Schmittgen, T.D. 2001. Analysis of relative gene expression data using real-time quantitative PCR and the 2−Ct method. Methods 25:402-408. This article presents a detailed review of the derivation of the mathematical applications described in this unit.
Internet Resources http://www.ncbi.nlm.nih.gov NCBI Web site. http://www.ensembl.org Ensembl Web site. http://www.gene-quantification.info/ The Gene Quantification Web site contains a host of information concerning QPCR. http://pga.mgh.harvard.edu/primerbank/index.html The Primer Bank database, hosted by Harvard University, contains user-submitted primer sequences for several mouse and human genes. http://web.ncifcrf.gov/rtp/gel/primerdb/ The Quantitative PCR Primer Database (QPPD), maintained by the National Cancer Institute, contains primer and probe sequences for mouse and human genes collected from articles cited in PubMed. Commercial vendor-sponsored technical support Web sites are also a very good resource for tips about RNA and QPCR applications.
Contributed by Angie L. Bookout, Carolyn L. Cummins, and David J. Mangelsdorf Howard Hughes Medical Institute University of Texas Southwestern Medical Center Dallas, Texas
This bulletin outlines both the standard curve and Ct methods and shows, by comparing data obtained using both calculations, that the resulting values are very similar regardless of the assay.
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CHAPTER 16 Protein Expression INTRODUCTION rotein expression, as used in this chapter, refers to the directed synthesis of large amounts of desired proteins. In early applications, molecular biologists interested in obtaining great quantities of prokaryotic regulatory proteins arranged their synthesis in large amounts, a process that came to be called overproduction, expression, or overexpression. These early techniques used genetic manipulations to select in vivo recombination events that inserted the desired gene into bacteriophages. Later, as it was developed, recombinant DNA technology was used to create phages and plasmids in vitro, which directed the synthesis of large amounts of the products of cloned genes.
P
This chapter describes methods to express proteins. In all these methods, a gene whose product is to be expressed is introduced into a plasmid or other vector, and that vector is introduced into living cells. Typical expression vectors contain promoters that direct the synthesis of large amounts of mRNA corresponding to the gene. They may also include, for example, sequences that allow their autonomous replication within the host organism, sequences that encode genetic traits allowing cells containing the vectors to be selected, and sequences that increase the efficiency with which the mRNA is translated. There are three basic strategies used to express proteins, each of which has advantages. E. coli expression techniques are probably the most popular: the organism is already used by most investigators, the techniques necessary to express usable amounts of protein are relatively simple, the amount of time necessary to generate an overexpressing strain is very short, and a familiarity with standard recombinant DNA techniques is all that is necessary to begin pilot expression experiments. E. coli has other advantages that have made it widely used for expression of commercially important proteins: it is cheap to grow, and the vast body of knowledge about it has made it possible to tinker intelligently with its genetics and physiology, so that strains producing 30% of their total protein as the expressed gene product can often be obtained. However, expression in E. coli does have disadvantages. First, eukaryotic proteins expressed in E. coli are not properly modified. Second, proteins expressed in large amounts in E. coli often precipitate into insoluble aggregates called “inclusion bodies,” from which they can only be recovered in an active form by solubilization in denaturing agents followed by careful renaturation. Third, it is relatively difficult to arrange the secretion of large amounts of expressed proteins from E. coli, although it has often been possible to secrete small amounts into the periplasmic space and to recover them by osmotic shock. The baculoviral expression system has a number of advantages that have contributed to its recent popularity: proteins are almost always expressed at high levels; expressed proteins are usually expressed in the proper cellular compartment (that is, membrane proteins are usually localized to the membrane, nuclear proteins to the nucleus, and secreted proteins secreted into the medium); and the expressed protein is often properly modified. Expression using baculoviral vectors also has some drawbacks: the techniques to grow and work with the virus are more sophisticated than those used with E.coli, and thus the system is more prone to “crashing” due to cell growth difficulties; the expressed proteins Protein Expression Contributed by Bob Kingston and Roger Brent Current Protocols in Molecular Biology (2005) 16.0.1-16.0.4 C 2005 by John Wiley & Sons, Inc. Copyright
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are not always properly modified; and generation of a recombinant baculovirus to express a given protein takes several steps. Compared with the above systems, all mammalian expression techniques have certain advantages, particularly for the expression of higher eukaryotic proteins: expressed proteins are usually properly modified, and they almost always accumulate in the correct cellular compartment. Generally speaking, mammalian expression techniques are more difficult, time-consuming, and expensive than those used to express proteins in E. coli, and they are much more difficult to perform on a large scale; however, they are quite practical for small- and medium-scale work by investigators already familiar with mammalian cell culture techniques. The several procedures described here for mammalian expression can produce transient expression, stable expression, or inducible expression. Each technique has specific advantages, and considerable care should be taken in deciding which technique to pursue (see below) because of the amount of time required to establish these techniques.
GENERAL OVERVIEW Section I (UNITS 16.1-16.8) describes techniques for expressing proteins in E. coli. UNIT 16.1 contains an introduction to E. coli expression. UNIT 16.2 describes the use of T7 vectors, in which synthesis of large amounts of foreign gene products is directed by the phage T7 gene 10 promoter, which uses T7 RNA polymerase. This polymerase transcribes the gene 10 promoter so efficiently that it uses up most of the ribonucleotide triphosphates in the cell and drastically inhibits transcription of genes by the host polymerase. UNIT 16.3 describes the use of pL -derived vectors and their appropriate host strains. These vectors carry the powerful bacteriophage pL promoter and take advantage of a number of other useful aspects of phage lambda biology. The next units contain techniques for expression of fusion proteins in which the expressed protein carries an additional stretch of amino acids at its N terminus to aid its expression and purification. UNIT 16.4 introduces the concept of fusions and provides methods for cleavage of fusion proteins. UNITS 16.5, 16.6, 16.7 & 16.8 describe techniques for expressing lacZ protein (β-galactosidase), trpE protein, maltose-binding protein, glutathione-S-transferase (GST), and thioredoxin fusions. Section II (UNITS 16.9-16.11) describes the use of the baculovirus system. In this system, genes for proteins to be expressed are inserted into an insect virus in lieu of a highly expressed dispensable gene. The foreign protein is produced by growing the recombinant virus in cultured insect cells. UNIT 16.9 introduces the system. UNIT 16.10 describes how to grow the cultured insect cells and viral stocks and how to isolate recombinant baculoviruses and use them to produce the desired protein. Finally, UNIT 16.11 describes the optimization of protein expression, first on a small scale, then maximized for large-scale production, and then provides instructions for purifying the recombinant proteins using GST- and His-tag systems.
Introduction
Section III (UNITS 16.12-16.24) describe techniques for expressing proteins in mammalian cells. UNIT 16.12 describes expression using COS cell vectors. In this approach, vectors containing the gene to be expressed are transiently transfected into COS cells, which constitutively produce SV40 large T antigen. COS cell vectors contain an SV40 replication origin; when they are transfected into COS cells, they replicate, and protein is expressed from mRNA synthesized by hundreds of copies of the vectors. UNITS 16.13 & 16.14 describe expression from stably transfected cell lines. UNIT 16.13 describes the fusion of a protein of interest to an epitope tag, and the production of a stable cell line that expresses the tagged protein. Cell lines of this type are widely used to purify either individual proteins, or more frequently intact complexes that contain as one component the tagged protein. This strategy does not always work, as in certain cases even slight misexpression of a
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protein can impair cell growth. To get around this problem, inducible systems are used; one widely used system uses the bacterial Tet repressor protein to engineer an expression system that responds to tetracycline concentration in the medium (UNIT 16.14). Expression of proteins using viral vectors is presented in UNITS 16.15-16.22 and UNIT 16.24. UNITS 16.15-16.19 outline how to construct recombinant vaccinia viruses and characterize their products. UNIT 16.20 describes how to carry out expression using Semliki Forest Virus (SFV) vectors. Both vaccinia systems and SFV systems are able to produce large amount of protein following infection of cells with the appropriate virus. In both cases, the normal viral machinery is used to produce a single protein at very high levels, and thus these systems are useful for overexpression of protein in mammalian cells, but not useful if it is desirable to isolate the protein of interest in complex with a group of cellular proteins, as the expressed protein will be made at much higher levels than any endogenous protein. An important goal that cannot be achieved by any of the above methods is the stable expression of protein in nondividing cells. In both therapeutic and basic science settings it can be important to express a protein in a post-mitotic cell, a need that has led to the development of vectors based on HIV and other related lentiviruses. Use of these systems is described in UNITS 16.21 & 16.22. Another approach is to use vectors based on Adenovirus. Earlier generations of this strategy, based on replication-competent viruses, raised some concerns related to safety and duration of protein expression. Both issues have been addressed, at least in part, by a newer generation of helper-dependent (sometimes called “gutless”) adenoviral vectors (HDAds). The only Adenoviral cis elements present in these vectors are those required for vector genome replication and packaging into the capsid. Preparation of these defective viruses is dependent on a helper virus that provides the various necessary factors but lacks the packaging sequences and is not incorporated into viral particles. Advantages of these vectors include the large insert size of approximately 30 kb and the ability to transduce a variety of cell types, both in vitro and in vivo. addresses a somewhat different problem than is addressed in the previous units. While it is relatively easy to produce milligram amounts of any protein using viral expression systems, it is considerably more challenging to make stable cell lines that will continue to produce milligram quantities of a given protein over extended periods of time. Specially constructed CHO cell lines are used to solve this problem. These lines contain integrated constructions to direct the synthesis of the mammalian protein; the constructions carry either the dihydrofolate reductase or the glutamine synthetase gene, whose products confer drug resistance. Lines that carry increased numbers of the constructs are obtained by selecting cell lines that grow in increasing drug concentrations of methotrexate. Once selected, these lines are permanent reagents, which can be stored frozen and used to produce the protein whenever desired.
UNIT 16.23
STRATEGIC CONSIDERATIONS WHEN USING MAMMALIAN EXPRESSION SYSTEMS The importance of the initial planning stage cannot be overemphasized when discussing strategies for mammalian overexpression. With the exception of COS cell transfection, each of the strategies described in Section III requires a significant amount of set-up time. While a lot of this time is not hands-on time, as it consists of waiting for cells to grow, the total time between conception and completion of a specific experimental approach might easily be several months. Thus, it is more important than usual to devise an appropriate strategy, as strategic errors might not become apparent for several months.
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The first consideration in choosing a strategy is to consider the goals of the expression work. If expression is desired to demonstrate that a clone has some functional activity or to perform an initial characterization of this activity, then transfection of COS cells might prove sufficient. If large amounts of an individual protein are required, then viral systems, such as vaccinia or SFV, or stable cell lines, such as CHO, will be the method of choice. If expression of an ectopic protein at the level of endogenous proteins is the goal, then creation of stable cell lines using retroviral (UNIT 16.13, see also UNITS 9.9-9.14) or lentiviral vectors should work. The decision can come down to choosing between a viral system or the creation of stable cell lines. Creating stable cell lines offers the advantage that the engineered cells can be frozen and then thawed for use whenever the expressing line is needed. In addition, selection techniques can be used to isolate a clonal line that has desired expression levels of the protein. This clonal line should express similar amounts of protein in every cell, an important consideration if the goal is to purify a complex of proteins, or if the function of a given protein in an entire cell population is under study. Creating stable cell lines requires both the choice of a suitable host cell and a suitable means of introducing the gene to be expressed. Obviously, the appropriate cell type (e.g., a lymphoid cell or neuronal cell) must be used if a specific cellular process is under study. Other considerations can be important later in the process. If eventually it will be important to grow large amounts of cells, then a cell line that can be adapted easily to growth in spinner culture is needed. If cells must be infected at a later stage in the experimental plan, then a cell line that is easily infected is important. Thus, planning several steps ahead when creating stable cell lines can help create a cell line that is maximally useful. Since it can take several months to isolate a clonal cell line in reasonable amounts, this planning takes on added importance. Stable cell lines can be made using either transfection procedures or infection procedures. Transfection can be used to introduce several genes at once; however, the integrated genes might not prove to be completely stable. Retroviral vectors can be used, and can lead to more stable expression, but are considerably more cumbersome if more than one protein is to be introduced. If overproduction of a single protein is the goal, frequently viral systems are the best choice. It usually takes less time to create an overexpressing virus than it does to make a high production stable cell line. Viral overproduction can also offer flexibility in that coinfection can be used to express two, three, or even more proteins in the same cell at the same time. This approach is not useful when it is important to purify the expressed protein with interacting proteins within the cell (e.g., as described in UNIT 16.13). The overexpressed protein will be expressed at such high levels that there will be insufficient endogenous protein to copurify in anywhere near stoichiometric amounts. This necessarily brief discussion of mammalian expression systems points out just a few of the numerous specific issues that might be important in choosing an effective strategy. A thorough plan, with all future goals for the experimental system explicitly taken into account, will lead to the most efficient strategy for creating a maximally useful mammalian expression system. Bob Kingston and Roger Brent
Introduction
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EXPRESSION OF PROTEINS IN ESCHERICHIA COLI
SECTION I
Overview of Protein Expression in E. coli
UNIT 16.1
The study of Escherichia coli during the 1960s and 1970s made it the best understood organism in nature (Chapter 1). Today’s recombinant DNA technology is a direct extension of the genetic and biochemical analyses carried out at that time. Even before the advent of molecular cloning, genetically altered E. coli strains were used to produce quantities of proteins of scientific interest. When cloning techniques became available, most cloning vectors utilized E. coli as their host organism. Thus, it is not surprising that the first attempts to express large quantities of proteins encoded by cloned genes were carried out in E. coli. E. coli has two characteristics that make it ideally suited as an expression system for many kinds of proteins: it is easy to manipulate and it grows quickly in inexpensive media. These characteristics, coupled with more than 10 years’ experience with expression of foreign genes, have established E. coli as the leading host organism for most scientific applications of protein expression. Despite a growing literature describing successful protein expression from cloned genes, each new gene still presents its own unique expression problems. No one, and certainly no laboratory manual, can provide a set of methods that will guarantee successful production of every protein in a useful form. Nevertheless, the vast body of accumulated knowledge has led to a general approach that often helps to solve specific expression problems. This unit introduces general considerations and strategies, while subsequent units (16.2-16.7) describe procedures that can be applied to specific expression problems.
GENERAL STRATEGY FOR GENE EXPRESSION IN E. COLI The basic approach used to express all foreign genes in E. coli begins with insertion of the gene into an expression vector, usually a plasmid. This vector generally contains several elements: (1) sequences encoding a selectable marker that assure maintenance of the vector in the cell; (2) a controllable transcriptional promoter (e.g., lac, trp, or tac) which, upon induction, can produce large amounts of mRNA from Contributed by Paul F. Schendel Current Protocols in Molecular Biology (1998) 16.1.1-16.1.3 Copyright © 1998 by John Wiley & Sons, Inc.
the cloned gene; (3) translational control sequences, such as an appropriately positioned ribosome-binding site and initiator ATG; and (4) a polylinker to simplify the insertion of the gene in the correct orientation within the vector. Once constructed, the expression vector containing the gene to be expressed is introduced into an appropriate E. coli strain by transformation (UNIT 1.8).
SPECIFIC EXPRESSION SCENARIOS Although this general approach—insertion of the gene of interest into an expression vector followed by transformation in E. coli—is common to all expression systems, specific procedures differ greatly. When choosing a procedure, it is helpful to consider the final application of the expressed protein, as this often dictates which expression strategy to use (UNIT 16.4A).
Antigen Production If the goal is to use the expressed protein as an antigen to make antibodies, several approaches are available to make protein reliably and to allow for rapid purification of the antigen. The two best approaches are synthesis of fusion proteins with specific “tag” sequences that can be retrieved by affinity chromatography (UNITS 16.5, 16.6, 16.7 & 16.8; see also UNIT 10.11B) and synthesis of the native protein, or a fragment of it, under conditions that cause it to precipitate into insoluble inclusion bodies (UNITS 16.4A & 16.6). These inclusion bodies can be purified sufficiently by differential centrifugation so preparative denaturing polyacrylamide gel electrophoresis (UNIT 10.2) will yield an isolated band that can be cut out and crushed, or electroeluted, to provide antigenic material for injection into an animal.
Biochemical or Cell Biology Studies If the goal is to use the expressed protein as a reagent in a series of biochemical or cell biology experiments, other considerations are relevant. In this case, the authenticity of the protein’s function (e.g., high-specific-activity enzyme, binding protein, or growth factor) is
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very important, while the ease of preparing the protein matters less. For this application, it is possible to express the protein as a fusion protein containing a specific protease-sensitive cleavage site so the N-terminal peptide tail can be removed easily, leaving only the native amino acid sequence (UNITS 16.4, 16.6, 16.7 & 16.8). Alternatively, direct expression vectors of the type described in UNITS 16.2 & 16.3 may be used to produce the authentic primary sequence. When expressed, the protein may be soluble and active, as is the case with many intracellular enzymes. If it is insoluble, as is the case for many secreted growth factors when they are made cytoplasmically in E. coli, it may be necessary to isolate inclusion bodies, solubilize the protein using denaturing agents, and refold the protein. Refolding is usually not too difficult when the protein is of moderate size (Marston and Hartley, 1990). Whether the protein is expressed in a soluble form or whether it requires refolding, its integrity can usually be checked by specific enzyme assays or by bioassays.
Structural Studies If the goal is to do structural studies of the expressed protein, the greatest constraints are imposed on the expression system. Because it is nearly impossible to show that a protein of unknown structure has been precisely refolded after denaturing, the protein must generally be made in a soluble form so its purification does not require a denaturation/renaturation step. Usually, the soluble form of the protein—either intracellular or secreted—must be made in strains and by induction protocols that minimize proteolytic degradation. Soluble expression of most eukaryotic proteins is best achieved with systems that allow induction of synthesis without changing the temperature; for example, by inducing transcription from the trp (Edman et al., 1981; de Boer et al., 1983) or tac (de Boer et al., 1983) promoters. Maximum accumulation of soluble product is best achieved by testing expression in several strains and at several temperatures, and picking the combination that works best. This is an active area of research at present (Schein, 1989); the rules are not yet understood, so little more than trial and error can be recommended.
Overview of Protein Expression in E. coli
TROUBLESHOOTING GENE EXPRESSION Once an expression strategy has been chosen and the gene is introduced into an appropriate
expression vector, several strains of E. coli should be transformed with the vector and protein production should be monitored. Ideally, the protein of interest will be produced in an active form and in sufficient amounts to allow its isolation. Often, however, the protein will be made either in very small amounts or in an insoluble form, or both. If this happens, there are various approaches that may correct the problem. If not enough protein is produced: 1. Reconstruct the 5′-end of the gene, maximizing its A+T content while preserving the protein sequence it encodes. This may reduce secondary structure within the mRNA (DeLamarter et al., 1985), or it may alter an as yet undefined parameter of the reaction. Regardless of the underlying cause, this procedure usually increases translation efficiency. 2. Determine if a transcriptional terminator is present. If the vector does not have a transcriptional terminator downstream from the site at which the gene is inserted, put one in. This often aids expression, probably by increasing mRNA stability and by decreasing nucleotide drain on the cell. 3. Examine the sequence of the cloned gene for codons used infrequently in E. coli genes. These so-called rare codons are usually not a rate-limiting problem, but if four or more happen to occur contiguously, they can reduce expression significantly (Robinson et al., 1984), perhaps by causing ribosomes to pause. Ribosomal pausing can uncouple transcription from translation, leading to premature termination of the message. Even if transcription proceeds normally, the mRNA 3′ to the stalled ribosomes can be exposed to degradation by host ribonucleases, reducing its stability. Thus, if stretches of rare codons are found, they should be altered to codons more favorable to high expression in E. coli. If enough protein is produced, but it is insoluble when the application requires it to be active and soluble: 1. Vary the growth temperature. As mentioned above, many proteins are more soluble at lower than at higher temperatures (Schein and Noteborn, 1988). On the other hand, some enzymes have a higher specific activity when made at temperatures >37°C (J. McCoy and P. Schendel, unpub. observ.). E. coli can synthesize proteins at temperatures ranging from 10° to 43°C, so trying expression at different temperatures is often worthwhile.
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2. Change fermentation conditions. Many proteins contain metals as structural and catalytic cofactors. If the protein is being made faster than metals can be transported into the cell, the apoprotein without its metal cofactor will accumulate. This apoprotein will not fold correctly and will likely be insoluble. At the very least, the average specific activity of the expressed protein will be lower than expected. Different media and metal supplements can be tested and the best combination used. Clearly, if there is information about the metal content of the protein, these supplements can be designed more rationally. If no information is available, a more random approach must be tried. 3. Alter the rate of expression by using low-copy-number plasmids. This can be done by using the pACYC family (Chang and Cohen, 1978) or using single-copy chromosomal inserts of the cloned gene into a suitable target gene (Hamilton et al., 1989). Such reductions in gene dosage often reduce the final yield of protein, but the slower kinetics of synthesis they afford can sometimes result in production of soluble proteins. To restate the obvious, protein expression is an inexact science at present. However, most proteins can be made in E. coli in a form that is useful for a variety of functions. The procedures employed are relatively quick and uncomplicated, and the rewards for success are great.
Literature Cited
de Boer, H.A., Comstock, L.J., and Vasser, M. 1983. The tac promoter: A functional hybrid derived from the trp and lac promoters. Proc. Nat. Acad. Sci. U.S.A. 80:21-25. DeLamarter, J.F., Mermod, J.J., Liang, C.M., Eliason, J.F., and Thatcher, D.R. 1985. Recombinant murine GM-CSF from E. coli has biological activity and is neutralized by a specific antiserum. EMBO J. 4:2575-2581. Edman, J.C., Hallewell, R.A., Valenzuela, P., Goodman, H.M., and Rutter, W.J. 1981. Synthesis of Hepatitis B surface and core antigens in E. coli. Nature 291:503-506. Hamilton, C.M. Aldea, M., Washburn, B.K., Babitzke, P., and Kushner, S.R. 1989. New methods for generating deletions and gene replacem en ts in Escherichia coli. J. Bacteriol. 171:4617-4622. Marston, F.A.O. and Hartley, D.L. 1990. Solubilization of protein aggregates. Methods Enzymol. 182:264-276. Robinson, M., Lilley, R., Little, S., Emtage, J.S., Yarranton, G., Stephens, P., Millican, A., Eaton, M., and Humphreys, G. 1984. Codon usage can affect efficiency of translation of genes in Escherichia coli. Nucl. Acids Res. 12:6663-6671. Schein, C.H. 1989. Production of soluble recombinant proteins in bacteria. Bio/Technology 7:1141-1148. Schein, C.H. and Noteborn, M.H.M. 1988. Formation of soluble recombinant proteins in Escherichia coli is favored by lower growth temperatures. Bio/Technology 6:291-294.
Contributed by Paul F. Schendel Genetics Institute Cambridge, Massachusetts
Chang, A.C.Y. and Cohen, S.N. 1978. Construction and characterization of amplifiable multicopy DNA cloning vehicles derived from the P15A cryptic miniplasmid. J. Bacteriol. 134:11411156.
Protein Expression
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Supplement 28
Expression Using the T7 RNA Polymerase/Promoter System
UNIT 16.2
This unit describes the expression of genes by placing them under the control of the bacteriophage T7 RNA polymerase. This approach has a number of advantages compared to approaches that rely on E. coli RNA polymerase. First, T7 RNA polymerase is a very active enzyme: it synthesizes RNA at a rate several times that of E. coli RNA polymerase and it terminates transcription less frequently; in fact, its transcription can circumnavigate a plasmid, resulting in RNA several times the plasmid length in size. Second, T7 RNA polymerase is highly selective for initiation at its own promoter sequences and it does not initiate transcription from any sequences on E. coli DNA. Finally, T7 RNA polymerase is resistant to antibiotics such as rifampicin that inhibit E. coli RNA polymerase, and consequently, the addition of rifampicin to cells that are producing T7 RNA polymerase results in the exclusive expression of genes under the control of a T7 RNA polymerase promoter (hereafter referred to as pT7). To use the two-plasmid pT7 system, it is necessary to clone the gene to be expressed into a plasmid containing a promoter recognized by the T7 RNA polymerase. The gene is then expressed by induction of T7 RNA polymerase. The gene for T7 RNA polymerase is present on a second DNA construction. This second construction can either permanently reside within the E. coli cell (basic protocol), or can be introduced into the cell at the time of induction by infection with a specialized phage, such as an M13 vector (mGP1-2; Tabor and Richardson, 1987) or a λ vector (CE6; Studier et al., 1990) containing the T7 RNA polymerase gene (second alternate protocol). In the basic protocol, two plasmids are maintained within the same E. coli cell. One (the expression vector) contains pT7 upstream of the gene to be expressed. The second contains the T7 RNA polymerase gene under the control of a heat-inducible E. coli promoter. Upon heat induction, the T7 RNA polymerase is produced and initiates transcription on the expression vector, resulting in turn in the expression of the gene(s) under the control of pT7. If desired, the gene products can be uniquely labeled by carrying out the procedure in minimal medium, adding rifampicin to inhibit the E. coli RNA polymerase, and then labeling the proteins with [35S]methionine (first alternate protocol). EXPRESSION USING THE TWO-PLASMID SYSTEM The gene to be induced is subcloned into an expression vector containing pT7. Two series of vectors have been developed for this purpose—the pT7 series (Fig. 16.2.1) and the pET series (Studier et al., 1990); see commentary for discussion of choice of vector. The plasmid containing the introduced gene is then used to transform an E. coli strain already containing the plasmid pGP1-2 (Fig. 16.2.2). pGP1-2 contains the gene for T7 RNA polymerase under the control of the λ pL promoter that is repressed by a temperature-sensitive repressor (cI857). pGP1-2 contains a p15A origin of replication that is compatible with the ColE1 origin of replication on the expression vector. The two plasmids are maintained in the same cell by selection with kanamycin (pGP1-2) and ampicillin (the expression vector). Cells containing the two plasmids are grown for several hours at 30°C and then the gene for T7 RNA polymerase is induced by raising the temperature to 42°C. The production of T7 RNA polymerase in turn induces expression of the genes under the control of pT7. (Rifampicin can be subsequently added to inhibit transcription by E. coli RNA polymerase, although this is usually not necessary since T7 RNA polymerase becomes responsible for most of the transcription even in the absence of rifampicin.) After expression Contributed by Stanley Tabor Current Protocols in Molecular Biology (1990) 16.2.1-16.2.11 Copyright © 2000 by John Wiley & Sons, Inc.
BASIC PROTOCOL
Protein Expression
16.2.1 Supplement 11
Hin cll
Hincll
Ap
r
Ap
pT7-5 2400 bp Clal Hindlll Pstl Sall Xbal BamHl Smal Sacl EcoRl
ColE1 ori Pvu ll p T7
r
pT7-6 2210 bp
ColE1 ori Pvu ll p T7
EcoRl Sac l Smal BamHl Xbal Sal l Pst l Hindlll
Hin cll
Ap
r
pT7-7 2470 bp ColE1 ori Bgl ll p T7
ATG rbs Xba l
Clal Hindlll Pstl Sall Xbal BamHl Smal EcoRl Ndel p T7
T7 (22,857) CGATTCGAACTTCTCGATTCGAACTTCTGATAGACTTCGAAATTAATACGACTCACTATAGGGAGA
Met Ala Arg lle CCACAACGGTTTCCCTCTAGAAATAATTTTGTTTAACTTTAAGAAGGAGATATACAT ATG GCT AGA ATT rbs
Xbal
Ndel
EcoRl T7 (22,972)
Arg Ala Arg Gly Ser Ser Arg Val Asp Leu Gln Pro Lys Leu lle lle Asp... CGC GCC CGG GGA TCC TCT AGA GTC GAC CTG CAG CCC AAG CTT ATC ATC GAT... Smal
BamHl
Xbal
Sall
Pstl
Hindlll
Clal
Figure 16.2.1 pT7-5, pT7-6, and pT7-7. pT7-5, pT7-6, and pT7-7 are cloning vectors that contain a T7 promoter and are used to express genes using T7 RNA polymerase. All three vectors contain a T7 RNA polymerase promoter, the gene encoding resistance to the antibiotic ampicillin and the ColE1 origin of replication. pT7-7 has a strong ribosome-binding site (rbs) and start codon (ATG) upstream of the polylinker sequence; the sequence of this region is shown below the map of pT7-7. pT7-5 and pT7-6 lack any ribosome-binding site upstream of the polylinker sequence and consequently are only useful when expressing genes that already contain the proper control sequences. pT7-5, pT7-6, and pT7-7 were constructed by S. Tabor and are derivatives of pT7-1 described in Tabor and Richardson (1985).
Expression Using the T7 RNA Polymerase/ Promoter System
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of the genes at 37°C, the cells are harvested and the induced proteins are analyzed. An alternative approach is to induce T7 RNA polymerase with IPTG rather than by heat induction. In this method, the expression plasmid containing pT7 can be placed into E. coli BL21 (DE3), which contains the gene for T7 RNA polymerase on the E. coli chromosome under the control of the lac promoter (Studier and Moffatt, 1986; Studier et al., 1990). Materials pT7-5, pT7-6, or pT7-7 vectors (available from author) E. coli JM105, DH1, or equivalent (Table 1.4.5) LB plates and medium containing 60 µg/ml ampicillin (UNIT 1.1) E. coli K38 or equivalent (Table 1.4.5) pGP1-2 (available from author) LB plates and medium containing 60 µg/ml kanamycin (UNIT 1.1) LB plates and medium containing 60 µg/ml ampicillin plus 60 µg/ml kanamycin (UNIT 1.1) Cracking buffer Sorvall SS-34 or GS-3 rotor or equivalent Additional reagents and equipment for subcloning DNA fragments (UNITS 1.4 & 3.16), transformation of competent E. coli cells (UNIT 1.8), minipreps of plasmid DNA (UNIT 1.6), restriction mapping (UNITS 3.1-3.3), and SDS-PAGE (UNIT 10.2). 1. Subclone the fragment containing the gene to be expressed into pT7-5, pT7-6, or pT7-7. Transform a standard E. coli strain (e.g., JM105 or DH1); this strain should not carry a plasmid that directs synthesis of T7 RNA polymerase (i.e., pGP1-2). Plate the transformants on LB/ampicillin plates and grow overnight at 37°C. λ (35,259) T7 (3106)
pL
λ (35,711) Smal Sacl EcoRl
plac
T7 RNA polymerase
c1857 pGP1-2 7140 bp
T7 (5840) BamHI kanr
p15A ori
Pst1
Hindlll Smal
Xhol
Figure 16.2.2 pGP1-2. pGP1-2 enables T7 RNA polymerase to be produced by heat induction in any E. coli host. pGP1-2 contains the gene for T7 RNA polymerase under the control of the λ pL promoter. It also contains the gene for the λ repressor (cI857) that is expressed under the control of E. coli plac promoter. This repressor inhibits transcription from the λ pL promoter at low temperature (30°C); however, at high temperature (42°C) it is inactivated, resulting in induction of the pL promoter, that in turn results in induction of the T7 RNA polymerase. pGP1-2 also contains the gene encoding resistance to the antibiotic kanamycin, and the p15A origin of replication. pGP1-2 is described in Tabor and Richardson (1985).
Protein Expression
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It is important to first transform the plasmid into a strain that contains no T7 RNA polymerase, in case small amounts of the gene product are toxic to the cell (see critical parameters for discussion on toxic genes).
2. Grow individual transformants in LB/ampicillin medium at 37°C and obtain plasmid DNA by a miniprep procedure. Confirm that the gene has been correctly inserted by restriction mapping. 3. Transform E. coli K38 with pGP1-2, plate on LB/kanamycin plates, and grow overnight at 30°C. Grow an individual E. coli K38/pGP1-2 transformant in LB/ kanamycin medium at 30°C. Colonies take ∼24 hr to appear on plates at 30°C. E. coli K38/pGP1-2 can be stored in the absence of the plasmid containing pT7 as a glycerol stock at −80°C (see commentary).
4. Transform the vector containing the gene to be expressed under the control of pT7 into E. coli K38/pGP1-2 grown in LB/kanamycin medium. Plate the transformants (containing both plasmids) on LB/ampicillin/kanamycin plates and grow overnight at 30°C. Cells may be heat-shocked during transformation; the T7 RNA polymerase gene, under the control of a heat-inducible promoter, is not induced by this brief heating step. As a control, transform E. coli K38/pGP1-2 with the parent pT7 vector (without an insert). If the transformation efficiency of the vector containing the insert is significantly lower (by more than a factor of 50) than that of the parent vector, the gene product may be toxic to E. coli cells. This toxicity arises from background expression of the gene product by basal levels of T7 RNA polymerase. In this situation, the transformants that do arise invariably contain deletions or other mutations in one of the two plasmids, and the desired gene product will not be produced. If the expression of the inserted gene is toxic, it is necessary to use an alternative strategy for the repression and induction of the T7 RNA polymerase gene (see discussion on toxic genes in critical parameters).
5. Pick a single E. coli colony that contains the two plasmids with a sterile toothpick or pipet. Inoculate it into 5 ml LB/ampicillin/kanamycin medium and grow overnight at 30°C. 6. Dilute the overnight culture of cells 1:40 into fresh LB/ampicillin/kanamycin medium and grow several hours at 30°C to an OD590 ≅ 0.4. The size of the culture will depend on the amount of cells needed. For an analytical preparation, use ∼1 ml of cells.
7. Induce the gene for T7 RNA polymerase by raising the temperature to 42°C for 30 min, which in turn induces the genes under the control of pT7. To obtain consistent results, raise the temperature relatively quickly. If small cultures (∼1 ml) are being induced, place the cultures into a 42°C water bath. For larger cultures (∼500 ml), place the flask under hot tap water until the temperature of the media reaches 42°C (measured by inserting a thermometer wiped with ethanol into the flask). Once the cells reach 42°C, continue incubating at 42°C for 30 min.
Expression Using the T7 RNA Polymerase/ Promoter System
The E. coli RNA polymerase can be inhibited by adding rifampicin to a final concentration of 200 ìg/ml; when used, it should be added after T7 RNA polymerase has been induced at 42°C for 30 min. Although rifampicin reduces the background of host proteins being expressed, in general it does not significantly increase the final accumulation of gene products, and in some cases it decreases the final yield. Thus, as a general rule, rifampicin is only added to cells when the plasmid-encoded proteins are being uniquely labeled with [35S]methionine (see first alternate protocol).
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8. Reduce temperature to 37°C and grow the cells an additional 90 min with shaking. 9. Harvest the cells by centrifuging and discarding the supernatant. For 1-ml cultures, microcentrifuge 20 sec at 10,000 rpm (14,000 × g), room temperature. For 2-ml to 100-ml cultures, centrifuge 5 min in a Sorvall SS-34 rotor at 5000 rpm (3000 × g), 4°C. For >100-ml cultures, centrifuge 10 min in a Sorvall GS-3 rotor at 5000 rpm (4000 × g), 4°C. 10. To analyze the induced proteins by SDS-PAGE, resuspend the equivalent of 1.0 ml of cells in 0.1 ml cracking buffer. Heat at 100°C for 5 min immediately prior to loading a 20-µl aliquot of each sample onto an SDS-polyacrylamide gel (UNIT 10.2). To analyze the cells for an induced enzymatic activity, prepare an appropriate cell extract from ∼10 ml of cells. One example of the preparation of an extract for the purification of T7 RNA polymerase is described in Tabor and Richardson (1985).
SELECTIVE LABELING OF PLASMID-ENCODED PROTEINS Plasmid-encoded proteins under the control of a pT7 (see basic protocol) can be exclusively labeled by inducing the T7 RNA polymerase in cells growing in minimal medium, inhibiting the host E. coli RNA polymerase with rifampicin, and labeling the newly synthesized proteins with [35S]methionine. This procedure provides an attractive alternative to maxicells or minicells for labeling of plasmid-encoded proteins (Dougan and Sherratt, 1977; Sancar et al., 1981).
ALTERNATE PROTOCOL
Additional Materials M9 medium (UNIT 1.1) without and with 5% (v/v) of 18 amino acid mixture 20 mg/ml rifampicin in methanol (e.g., Sigma #R-3501; store in dark at 4°C for 2 weeks; Table 1.4.1) 10 mCi/ml [35S]methionine (>800 Ci/mmol) diluted 1:10 in M9 medium Fluorographic enhancing agent (e.g., Enlightning from Du Pont NEN or Amplify from Amersham) 1. Repeat steps 3 to 6 of the basic protocol (using the T7-promoter expression plasmid obtained from steps 1 and 2 of the basic protocol). An alternative to the use of LB/ampicillin/kanamicin medium for growing cells is M9 medium containing 25 ìg/ml ampicillin and 25 ìg/ml kanamycin, and any required nutrients. The addition of one part in twenty of the 18 amino acid mixture (0.1% stock, 0.005% final concentration) stimulates the growth of cells in M9 medium without interfering with the subsequent labeling of the proteins with [35S]methionine. Note that to grow in this medium, the E. coli strain must be Cys+ and Met+.
2. When OD590 ≅ 0.4, remove 1 ml of cells, microcentrifuge 10 sec, and discard supernatant. 3. Wash cell pellet with 1 ml M9 medium, microcentrifuge 10 sec at room temperature, and discard supernatant. Washing the cells after growth in LB medium is very important in order to remove the unlabeled methionine present in LB medium that otherwise dilutes the [35S]methionine during labeling.
4. Resuspend cell pellet in 1 ml M9 medium containing 18 amino acid mixture. Grow cells 60 min at 30°C with shaking. A time of 30 to 180 min is adequate for adapting cells to M9 medium. Although the OD590 may not increase significantly during this step, induction of T7 RNA polymerase and
Protein Expression
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efficient labeling of the plasmid-encoded proteins will occur even in the absence of apparent cell growth.
5. Induce the gene for T7 RNA polymerase by placing the cells in a 42°C water bath for 20 min. 6. Add 20 mg/ml rifampicin to 200 µg/ml final. Keep cells at 42°C for an additional 10 min after adding rifampicin. It is important to incubate the cells at 42°C for an additional 10 min after adding rifampicin, since rifampicin is more effective at inhibiting expression of host proteins at 42°C, possibly because the cells are more permeable to it at this temperature. The temperature of the cells is subsequently reduced for the labeling since in general the labeling is less efficient at 42°C than at 30° or 37°C.
7. Shift cells to a 30°C water bath for an additional 20 min. Remove 0.5 ml of cells for labeling with [35S]methionine. The other 0.5 ml can be used to label the cells at a later time point (e.g., after an additional 30 min) in order to follow the duration of protein synthesis.
8. Label newly synthesized proteins by adding 10 µl (10 µCi) diluted [35S]methionine to 0.5 ml of cells and incubating for 5 min at 30°C. 9. Microcentrifuge cells 10 sec and discard supernatant. (CAUTION: the supernatant is radioactive; discard properly.) Resuspend cell pellet in 100 µl cracking buffer. 10. Heat samples to 100°C for 5 min. Load a 20-µl aliquot onto an SDS-polyacrylamide gel and electrophorese (UNIT 10.2). 11. Treat the gel with a fluorographic-enhancing agent by soaking it in the fluor for 30 min. Dry the gel under vacuum 2 hr at 65°C and autoradiograph (APPENDIX 3). A 1-hr exposure should be adequate to visualize most proteins induced with this system. To determine whether the plasmid-encoded proteins are susceptible to proteases in the E. coli cell, prepare and induce the cells as described above; however, reduce the duration of the labeling step to 1 min (step 8), and follow this with a chase of nonradioactive methionine at 0.5% final concentration. Remove an aliquot for analysis both immediately prior to the chase, and after a chase reaction of 5, 15, and 60 min. After removing each aliquot, immediately pellet the cells by centrifugation, resuspend in cracking buffer, and heat the aliquot to 100°C for 5 min to inactivate the proteases. Analyze as in step 10. ALTERNATE PROTOCOL
Expression Using the T7 RNA Polymerase/ Promoter System
EXPRESSION BY INFECTION WITH M13 PHAGE mGP1-2 Whenever the gene for T7 RNA polymerase is present in E. coli cells, low levels of T7 RNA polymerase are constitutively produced. This can be a problem when the gene products under the control of pT7 are toxic. One strategy to avoid this is to keep the gene for T7 RNA polymerase out of the cell until the time of induction. In the protocol presented here, T7 RNA polymerase is introduced into the cell by infection with the M13 phage mGP1-2. This phage contains the gene for T7 RNA polymerase under the control of the lac promoter (Fig. 16.2.3). Host cells for this phage must carry the F factor so that they are susceptible to M13 infection (e.g., JM101 or K38). The cells are transformed with the single plasmid that contains the gene to be expressed under the control of pT7. The cells are grown at 37°C, and induction occurs by infection with a high multiplicity of mGP1-2 in the presence of IPTG. A λ vector, CE6, that contains the gene for T7 RNA polymerase has also been used to express toxic genes (Studier and Moffatt, 1986; Studier et al., 1990).
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Additional Materials M13 phage mGP1-2 (available from author) PEG solution (UNIT 1.7) 100 mM IPTG (Table 1.4.2) Additional reagents and equipment for preparing M13 phage (UNIT 1.15) and titering phage (UNIT 1.11) 1. Prepare a stock of M13 phage mGP1-2 and concentrate the phage by precipitation with PEG solution. (DO NOT proceed to add TE buffer or phenol.) Resuspend phage in M9 medium and titer. If the cell proteins are to be labeled, it is important that the phage used to infect the cells are free of unlabeled methionine. In this case, precipitate the phage with PEG twice, each time resuspending the pellet in M9 medium. For long-term storage of the M13 phage mGP1-2, it is best to purify the phage through a CsCl gradient (Nakai and Richardson, 1986).
2. Transform E. coli cells susceptible to M13 infection (e.g., JM101 or K38) with the T7-promoter expression plasmid obtained from steps 1 and 2 of the basic protocol. Plate the transformants on LB/ampicillin plates and grow overnight at 37°C. 3. Pick a single colony and grow in LB/ampicillin medium overnight at 37°C. 4. Dilute the overnight culture of cells 1:100 in LB/ampicillin medium and grow several hours at 37°C with gentle shaking to OD590 ≅ 0.5. It is very important that only gentle shaking is used when growing cells for M13 infection. Vigorous agitation results in shearing of the pili on the surface of the E. coli cells, resulting in inefficient infection.
5. Infect cells with M13 phage mGP1-2 (from step 1) at a ratio of ∼10 phage for each E. coli cell. Add 100 mM IPTG to 1 mM final (a 1:100 dilution) to induce production
T7 (5840) Sall Pst1 Hindlll T7 RNA polymerase
T7 (3133) mGP1-2 9960 bp
Smal Sac l EcoRI
plac
M13 ori
Figure 16.2.3 mGP1-2. M13 phage mGP1-2 contains the gene for T7 RNA polymerase under the control of the E. coli plac promoter. It is especially useful for the production of gene products that are toxic to the E. coli cell. When E. coli cells are infected with this phage, and IPTG is added to induce the plac promoter, T7 RNA polymerase is produced. As a result, any genes within the cell under the control of pT7 will be induced. mGP1-2 is described in Tabor and Richardson (1987).
Protein Expression
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of T7 RNA polymerase. Incubate the cells 2 hr at 37°C. At OD590 ≅ 0.5, the density of E. coli cells will be ∼2 × 108 cells/ml. Thus, it is necessary to add M13 mGP1-2 phage at a final concentration of 2 × 109 phage/ml to obtain a multiplicity of infection of 10. Small cultures (∼50 ml) can be incubated in a water bath without shaking. Larger cultures should be incubated at 37°C with gentle shaking.
6. Harvest cells and analyze induced proteins as in steps 9 and 10 of the basic protocol. REAGENTS AND SOLUTIONS 18 amino acid mixture Prepare a solution containing 0.1% (v/v) of each amino acid except cysteine (minus cysteine) and methionine (minus methionine). Filter sterilize through a 0.2-µm filter. Store at −20°C for several years. Cracking buffer 60 mM Tris⋅Cl, pH 6.8 1% 2-mercaptoethanol 1% sodium dodecyl sulfate (SDS) 10% glycerol 0.01% bromphenol blue COMMENTARY Background Information
Critical Parameters
Bacteriophage T7 and T7-related phage (e.g., SP6, T3) encode their own RNA polymerase (see UNIT 3.8). Compared to other known RNA polymerases, this RNA polymerase is both relatively simple and highly efficient. T7 RNA polymerase is a single polypeptide of 96,000 kDa. It initiates transcription specifically at a 23-nucleotide promoter sequence, a sequence not present on the E. coli genome. Transcription is very processive, producing transcripts that are many thousands of nucleotides in length. Transcription is relatively rapid—five times the rate of E. coli RNA polymerase. All of these properties make T7 RNA polymerase and its promoter an attractive system for controlling the expression of foreign genes in E. coli and in other organisms. Expression systems in E. coli based on the controlled induction of T7 RNA polymerase have been developed by Tabor and Richardson (1985) and Studier and his colleagues (Studier and Moffatt, 1986; Rosenberg et al., 1987; Studier et al., 1990). The vectors described here are those developed by Tabor and Richardson. T7 RNA polymerase/promoter expression systems have also been successfully applied in yeast (Chen et al., 1987) and mammalian cells (Dunn et al., 1988; Fuerst et al., 1986).
Choice of vector Questions that determine what vector to use to express a gene using T7 RNA polymerase include: Is there a ribosome-binding site upstream of the gene? What are the restriction sites available on each end of the gene? Is the gene product toxic to the E. coli cell? Examples of three standard vectors (pT7-5, pT7-6, and pT7-7) are shown in Figure 16.2.1. These vectors are derivatives of pBR322. The β-lactamase gene encoding ampr is in the opposite orientation of pT7; consequently the only plasmid-encoded genes expressed by T7 RNA polymerase are those cloned into the polylinker region. pT7-5 and pT7-6 contain the polylinker region located immediately downstream of pT7 in opposite orientations. There is no ribosomebinding sequence in these two plasmids; they should thus be used either for the production of transcripts without expectation of good translation of the protein, or for the expression of genes that already have strong ribosome-binding sequences. pT7-7 differs from pT7-5 and pT7-6 in that it contains a strong ribosome-binding sequence between pT7 and the polylinker region; it is recommended for the expression of genes that lack a strong ribosome binding sequence or for the production of fusion proteins.
Expression Using the T7 RNA Polymerase/ Promoter System
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An extensive series of additional vectors containing pT7, the pET series, have been described by Studier et al. (1990). These vectors are particularly useful for applications that require a greater selection of restriction endonuclease sites to insert the gene into, or that involve the expression of a gene that is toxic to the cell (see below). Some of these vectors contain other transcriptional regulatory elements (i.e., terminators, operators, RNase III cleavage sites) that could be of use for specific applications. A large number of commercially available vectors contain a T7 RNA promoter (e.g., pIBI vectors, available from IBI; pSP6/T7-19, available from GIBCO/BRL; pBluescript II vectors, available from Stratagene; and pTZ18R and pTZ19R, available from U.S. Biochemical). These are intended to be used for producing specific transcripts in vitro using T7 RNA polymerase. In principle, they should be useful for the expression of genes using T7 RNA polymerase in vivo as well. In practice, however, the use of some of these vectors can result in some unexpected problems. (1) Most commercial vectors have extremely high copy numbers within the cell; this can accentuate the problems encountered with toxic genes. (2) In most vectors, the β-lactamase gene is oriented in the same direction as pT7, complicating the analysis of radiolabeled proteins. (3) Some commercial vectors have pT7 oriented in a potentially deleterious direction. Derivatives of pBR322 that contain pT7 oriented clockwise with respect to the standard map are inviable in some E. coli strains that contain the gene for T7 RNA polymerase. This is due to the fact that high levels of transcription through the origin region of these plasmids in this orientation interferes with the replication of the plasmids. (4) Most commercial vectors have a lac operator sequence within them. This can titrate out the lac repressor (UNIT 1.4) and cause problems when the plac is used to control the T7 RNA polymerase gene. Toxic genes In some cases the gene to be expressed is toxic to the cells, even when it is not induced. This is due to a low level of constitutive expression present even under uninduced conditions. Although most genes are not toxic when expressed using the two-plasmid pT7 system, it is important to recognize the symptoms of toxicity to avoid selecting for mutations and to allow alternate systems for induction to be tried. The degree of toxicity varies greatly with each gene.
The symptoms encountered with toxic genes are discussed below, in order of increasing toxicity. Some genes are mildly toxic to the cells when expressed using the two-plasmid pT7 system. In such cases, the cells can be stably transformed with the two plasmids and the gene product is produced at a high level. However, after the cells are several days old, they no longer induce the expected gene product even though they remain resistant to ampicillin and kanamycin. To avoid this problem, it is recommended that the E. coli K38/pGP1-2 be stored in the absence of the plasmid containing pT7 as a glycerol stock at −80°C (UNIT 1.3). The plasmid containing pT7 and the gene to be expressed should be stored as DNA at −20°C or −70°C (UNIT 1.6). To prepare the strain for induction, streak K38/pGP1-2 on an LB/kanamycin plate at 30°C, grow up a single colony, transform with the plasmid containing pT7 and the gene to be expressed, and plate the transformants on LB/ampicillin/kanamycin plates at 30°C. A single colony should then be grown at 30°C and induced as described above. This procedure is not necessary for genes that are not toxic. Strains that do not induce toxic genes can be stored in glycerol at −80°C for many months (UNIT 1.3). A more toxic class of genes consists of those that can be successfully cloned into a plasmid under the control of pT7, but that render the resulting plasmid unable to stably transform a cell that contains the gene for T7 RNA polymerase. Genes that are toxic to the cells only in the presence of pGP1-2 (which expresses the T7 RNA polymerase) are relatively common, occurring on the average ∼5% of the time (S. Tabor, unpublished observation). Note that such plasmids will give transformants in E. coli cells containing pGP1-2, but that the frequency of transformation will be greatly reduced (>50fold) compared to the frequency of transformation by the parent vector alone. The cells that do grow in the presence of ampicillin and kanamycin will invariably contain deletions or other mutations in one of the two plasmids, and the desired gene product will not be produced. When genes are toxic at this level, it is necessary to use an alternative strategy that reduces the expression of the gene under uninduced conditions. One strategy is to remove the gene for T7 RNA polymerase from the cell until induction is desired, and then introduce it by a phage infection. Such an alternate protocol is described using an M13 phage harboring the gene for T7 polymerase, mGP1-2. A lambda
Protein Expression
16.2.9 Current Protocols in Molecular Biology
Supplement 11
Expression Using the T7 RNA Polymerase/ Promoter System
vector, CE6, that contains the gene for T7 RNA polymerase has also been used for this purpose (Studier and Moffatt, 1986; Studier et al., 1990). Another strategy is to retain the gene for T7 RNA polymerase in the cell but reduce the level of transcription by T7 RNA polymerase under uninduced conditions. For example, a system has been developed that expresses an inhibitor of T7 RNA polymerase—the T7 lysozyme—to reduce the activity of T7 RNA polymerase until it is induced (Studier et al., 1990). Another recent modification is the placement of pT7 under the control of the lac repressor, reducing the activity of T7 RNA polymerase until IPTG has been added (Studier et al., 1990). Finally, some genes are difficult to clone in multicopy plasmids even in the absence of a known E. coli promoter. The difficulty in cloning these genes arises from the fact that their products are extremely toxic and that the residual low level of transcription by E. coli RNA polymerase in most plasmids is sufficient to direct the synthesis of small amounts of these proteins. One strategy that can be used to clone such toxic genes is to insert the gene near a strong E. coli promoter that is oriented so that transcription by the E. coli RNA polymerase results in the accumulation of RNA that is antisense to the toxic gene, reducing the level of its gene product. It is important to remember that the amount of a gene product synthesized is a function not only of the level of transcription but also of the efficiency at which translation is initiated. This is determined primarily by the ribosomebinding sequence located upstream of the start codon. Thus, some toxic genes with relatively weak ribosome-binding sequences can be cloned into multicopy plasmids, but not into a multicopy plasmid that also introduces a strong ribosome-binding sequence (S. Tabor, unpublished observations). In summary, the first step in using the T7 RNA polymerase/promoter system is to clone the gene into an appropriate vector containing a pT7 and be certain it has an efficient ribosomebinding sequence. Once this is accomplished, the next step is determining whether the plasmid can stably transform an E. coli cell containing pGP1-2 at an efficiency comparable to that of the parent vector alone. If this is successful, the system is ready to be induced. If unsuccessful, it is necessary to induce the gene either by infection with M13 phage mGP1-2
(see second alternate protocol), or to use one of the more specialized vectors that further reduce the expression of T7 RNA polymerase in the cell under uninduced conditions (Studier et al., 1990).
Troubleshooting For gene expression, one of the major advantages of the T7 RNA polymerase/promoter system over an E. coli RNA polymerase system is the ability to exclusively label the gene products under the control of pT7. If the level of induction of the gene is estimated by inspection of a standard SDS-polyacrylamide gel, and it is difficult to see the expected induced product, then it is recommended that the induced proteins be labeled using [35S]methionine as described in the first alternate protocol. This is a much more sensitive and specific assay for the specific protein production. Be sure that there is at least one methionine codon in the gene other than the one at the start of the protein (which is often removed in E. coli; Kirel et al., 1989); if not, then it is necessary to label with a cysteine or some other amino acid. If it is not possible to detect the expected labeled product, there may be a problem with one of the two plasmids in the cell. One possibility is that the expressed protein is toxic to the cell, and as a result, a mutation has been selected for such that the toxic product is not synthesized. For more information on determining whether a gene is toxic, see the discussion on toxic genes in critical parameters. To determine if the cells and T7 RNA polymerase gene (e.g., pGP1-2) are inducing T7 RNA polymerase, attempt to induce a control protein that has been shown to work well in this system (e.g., the β-lactamase gene in pT7-1; Tabor and Richardson, 1985). If the expressed protein does not accumulate significantly after induction, determine its stability in E. coli cells by pulse labeling with [35S]methionine and chasing for various time periods with unlabeled methionine. If it is rapidly degraded, try to induce the gene in a protease-deficient strain. It should be noted that there are no known mutations that inactivate several very active E. coli proteases, and thus there is a strong probability that the mutant strains available (e.g., lon−) will have no effect on the stability of the gene product. In addition, such mutant strains generally grow poorly, and as a consequence the gene products are poorly produced upon induction of T7 RNA polymerase. The most common reason for poor induc-
16.2.10 Supplement 11
Current Protocols in Molecular Biology
tion of a gene is that the translation does not initiate efficiently. Therefore, it is very important that there be an efficient ribosome-binding sequence the proper distance upstream of the gene. If a gene product does not induce well, and the problem is not the stability of the product, try a different ribosome-binding sequence—one that is known to work efficiently. The sequence and spacing between the ribosome-binding sequence and the start codon is critical. Because of this, it is recommended that the gene be inserted into a vector such as pT7-7, without altering any of the sequences between the ribosome-binding sequence and the start codon.
Anticipated Results Under optimal conditions, the gene product expressed by the T7 RNA polymerase/ promoter system can accumulate to >25% of the total cellular protein. However, in most instances the amount of gene product that accumulates is significantly less than this. There are numerous reasons for poor yields of gene product, as discussed in troubleshooting (see above).
Time Considerations
It should take ∼1 week to insert the gene of interest into the pT7 vector, prepare minipreps of the DNA, and characterize the recombinants for the correct size and orientation of the insert. It should then take 3 days to transform the recombinant plasmid into the E. coli strain containing pGP1-2, induce the cells, and test the extracts for the production of the expected gene product.
Literature Cited Chen, W., Tabor, S., and Struhl, K. 1987. Distinguishing between mechanisms of eukaryotic transcriptional activation with bacteriophage T7 RNA polymerase. Cell 50:1047-1055. Dougan, G. and Sherratt, D. 1977. The transposon Tn1 as a probe for studying ColE1 structure and function. Mol. Gen. Genet. 151:151-160. Dunn, J.J., Krippl, B., Bernstein, K.E., Westphal, H., and Studier, F.W. 1988. Targeting bacteriophage T7 RNA polymerase to the mammalian cell nucleus. Gene 68:259-266. Fuerst, T.R., Niles, E.G., Studier, F.W., and Moss, B. 1986. Eukaryotic transient-expression system based on recombinant vaccinia virus that synthe-
sizes bacteriophage T7 RNA polymerase. Proc. Natl. Acad. Sci. U.S.A. 83:8122-8126. Kirel, P.-H, Schmitter, J.-M., Dessen, P., Fayat, G., and Blanquet, S. 1989. Extent of N-terminal methionine excision from Escherichia coli proteins is governed by the side-chain length of the penultimate amino acid. Proc. Natl. Acad. Sci. U.S.A. 86:8247-8251. Nakai, H. and Richardson, C.C. 1986. Interactions of the DNA polymerase and gene 4 protein of bacteriophage T7. J. Biol. Chem. 261:1520815216. Rosenberg, A.H., Lade, B.N., Chui, D., Lin, S., Dunn, J.J., and Studier, F.W. 1987. Vectors for selective expression of cloned DNAs by T7 RNA polymerase. Gene 56:125-135. Russel, M. and Model, P. 1984. Replacement of the flp gene of Escherichia coli by an inactive gene cloned on a plasmid. J. Bacteriol. 159:10341039. Sancar, A., Wharton, R.P., Seltzer, S., Kacinsky, B.M., Clark, N.D., and Rupp, W.D. 1981. Identification of the uvrA gene product. J. Mol. Biol. 184:45-62. Studier, F.W. and Moffatt, B.A. 1986. Use of bacteriophage T7 RNA polymerase to direct selective high-level expression of cloned genes. J. Mol. Biol. 189:113-130. Studier, F.W., Rosenberg, A.H., Dunn, J.J., and Dubendorff, J.W. 1990. Use of T7 RNA polymerase to direct the expression of cloned genes. Methods Enzymol. 185. In press. Tabor, S. and Richardson, C.C. 1985. A bacteriophage T7 RNA polymerase/promoter system for controlled exclusive expression of specific genes. Proc. Natl. Acad. Sci. U.S.A. 82:10741078. Tabor, S. and Richardson, C.C. 1987. DNA sequence analysis with a modified bacteriophage T7 DNA polymerase. Proc. Natl. Acad. Sci. U.S.A. 84:4767-4771.
Key References Studier et al., 1990. See above. Gives extensive list of vectors and protocols for expression using T7 RNA polymerase. Tabor and Richardson, 1985. See above. Describes the use of the two-plasmid system for expression of genes using T7 RNA polymerase.
Contributed by Stanley Tabor Harvard Medical School Boston, Massachusetts
Protein Expression
16.2.11 Current Protocols in Molecular Biology
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UNIT 16.3
Expression Using Vectors with Phage ë Regulatory Sequences Many expression systems have been developed that utilize pBR322-based plasmids into which transcriptional and translational regulatory signals have been inserted. In the system described here, however, plasmids (pSKF) utilize regulatory signals—such as the powerful promoter pL—from the bacteriophage λ. Transcription from pL can be fully repressed and plasmids containing it are thus stabilized by the λ repressor, cI. The repressor is supplied by an E. coli host which contains an integrated copy of a portion of the λ genome. This so-called defective lysogen supplies the λ regulatory proteins cI and N but does not provide the lytic components that would normally lead to cell lysis. Thus, cells carrying these plasmids can be grown initially to high density without expression of the cloned gene and subsequently induced to synthesize the product upon inactivation of the repressor. This system also ensures that pL-directed transcription efficiently traverses any gene insert, which is accomplished by providing the phage λ antitermination function, N, to the cell and by including on the pL transcription unit a site necessary for N utilization (Nut site). The N protein interacts with and modifies the RNA polymerase at the Nut site so as to block transcription termination at distal sites in the transcription unit. In order to express the coding sequence, efficient ribosome-recognition and translationinitiation sites have been engineered into the pL transcription unit. Expression occurs after temperature or chemical induction inactivates the repressor (see first and second basic protocols). Restriction endonuclease sites for insertion of the desired gene have been introduced both upstream and downstream from an ATG initiation codon. Thus, the system allows either direct expression or indirect expression (via protein fusion) of any coding sequence, thereby potentially allowing expression of any gene insert. Direct expression generates “authentic” gene products (first support protocol), while expression of heterologous genes fused to highly expressed gene partners generates chimeric proteins that differ from the native form. In the latter case, the fusion partner can be removed to obtain an unfused version of the gene product (second support protocol).
BASIC PROTOCOL
TEMPERATURE INDUCTION OF GENE EXPRESSION Expression from pL-containing vectors can be induced by raising the temperature. The E. coli lysogens used with these vectors are typically defective for phage replication and carry a temperature-sensitive mutation in the phage λ cI gene (cI857). After transformation and growth, induction is accomplished by raising the temperature of the culture from 32° to 42°C. Materials Expression vector (e.g., pSKF series; see support protocols) E. coli AR58 or equivalent (Table 1.4.5) LB plates containing the appropriate antibiotic (UNIT 1.1) LB medium containing appropriate antibiotic (room temperature and prewarmed to 65°C; UNIT 1.1) SDS/sample buffer (UNIT 10.2) Gyrotory air or water shaker, 32° and 42°C Additional reagents and equipment for transformation (UNIT 1.8)
Expression Using Vectors with Phage ëRegulatory Sequences
16.3.1 Supplement 11
1. Transform the expression vector into an E. coli λ lysogen (such as AR58) carrying a temperature-sensitive mutation in its repressor gene (λ cI857). Plate on LB/antibiotic plates and incubate transformants at 32°C. Contributed by Allan R. Shatzman, Mitchell S. Gross, and Martin Rosenberg Current Protocols in Molecular Biology (1994) 16.3.1-16.3.11 Copyright © 2000 by John Wiley & Sons, Inc.
Heat-shock at 37° or 42°C for ≤90 sec during transformation is not a problem.
2. Grow the transformed cells overnight at 32°C in LB/antibiotic medium. 3. Dilute the overnight culture ≥1:20 into fresh LB/antibiotic medium. Grow the culture at 32°C in a gyrotory shaker at 250 to 300 rpm until OD650 = 0.6 to 0.8. 4. Add 1⁄3 vol of 65°C LB/antibiotic medium with swirling in order to elevate the culture temperature rapidly to 42°C. In our experience, a rapid increase in temperature favors production. Small shake-flask cultures (≤25 ml) are more easily induced by transfer to a 42°C gyrotory water bath without addition of prewarmed media. This generally raises the culture temperature to 42°C within 3 to 5 min.
5. Continue growing the culture 2 to 3 hr at 42°C. 6. Remove a 1-ml aliquot for analysis and harvest the remainder of cells by centrifuging 15 min in a low-speed rotor at 3000 × g, 4°C. Discard the supernatant. Freeze cell pellet at −70°C until ready to isolate the gene product.
7. Spin the 1-ml aliquot 1 min at top speed in a microcentrifuge, then resuspend the pellet in 50 µl SDS/sample buffer. Boil 5 to 10 min and analyze gene product by SDS–polyacrylamide gel electrophoresis. CHEMICAL INDUCTION OF GENE EXPRESSION Expression using the pSKF system can also be induced chemically in lysogens that carry a wild-type (ind+) repressor gene (cI857 cannot be used as it is ind−). This is accomplished by treating the bacterial host with an agent such as nalidixic acid. Nalidixic acid inhibits DNA gyrase and leads to DNA damage, which induces the SOS response. During the SOS response, wild-type repressor protein is cleaved. In this case, the wild-type repressor protein is cleaved by the RecA protease, which is induced by the SOS response. In contrast to induction by heat (product accumulates in 45 to 90 min) nalidixic acid–mediated induction of protein expression is comparatively slow (product accumulates in 5 to 6 hr).
BASIC PROTOCOL
Materials Expression vector (e.g., pSKF series; see support protocols) E. coli AR120 or equivalent (Table 1.4.5) LB plates containing appropriate antibiotic (UNIT 1.1) LB medium containing appropriate antibiotic (UNIT 1.1) 60 mg/ml nalidixic acid in 1 N NaOH (not necessary to filter sterilize; Table 1.4.1) Additional reagents and equipment for transformation (UNIT 1.8) 1. Transform the expression vector into a replication-defective, E. coli cI+ lysogen (e.g., AR120). Plate on LB/antibiotic plates and incubate the transformants at 37°C. 2. Grow the transformed cells overnight at 37°C in LB/antibiotic medium. 3. Dilute the overnight culture ≥1:20 into fresh LB/antibiotic medium. Grow the culture at 37°C in a gyrotory shaker at 250 to 300 rpm until OD650 = 0.4. 4. Add 1/1000 vol of 60 mg/ml nalidixic acid solution to give 60 µg/ml final concentration. 5. Continue growing the culture 5 to 6 hr at 37°C. 6. Harvest cells and analyze gene product (steps 6 and 7 of first basic protocol).
Protein Expression
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SUPPORT PROTOCOL
AUTHENTIC GENE CLONING USING pSKF VECTORS It is often most desirable to express a gene product in a form as similar to the native protein as possible. Such an “authentic” gene product will have the greatest chance of having a structure and activity identical to that of the native protein. Efficient translation of a coding sequence for an authentic gene product is typically accomplished by placing the inserted information immediately adjacent to a ribosome-binding site (a translational regulatory signal that interacts with the 16S rRNA of E. coli and contains an ATG initiation codon; Gold et al., 1981). Strategic Planning The translation-initiation signal utilized here is that of the phage λ cII gene. In order to make the translational information generally useful, the coding region of the gene has been removed from the vectors, leaving only their initiator fMet codon and upstream translational regulatory sequences. Additionally, these vectors have been engineered to provide restriction endonuclease sites on either side of the ATG, such that the initiation codon can be supplied by either the plasmid or the gene being inserted. Finally, restriction sites have also been engineered upstream of the translational regulatory region to permit insertion of other ribosome-binding sites. Those genes that contain restriction sites compatible with the sites on the vector may be inserted directly into the vector. As most genes do not contain appropriately positioned restriction sites, it is often necessary to adapt existing restriction cloning sites within the gene to fuse it to the translation-initiation signals provided by the vectors. For example, pSKF101 (Fig. 16.3.1) and pSKF102 both have a BamHI site adjacent to the initiation codon (ATGgatcc), while pSKF201 has an NcoI site (ccATGg) and pSKF301 (Fig. 16.3.2) has an NdeI site (catATG). The protocol presented below summarizes the steps to obtain an authentic gene clone using pSKF101 as an example. Sample Protocol Materials Appropriate restriction endonucleases and buffers (UNIT 3.1) pSKF101 vector (available from A. Shatzman; Fig. 16.3.1) Competent E. coli AS1 (Table 1.4.5; also known as MM294cI+) Additional reagents and equipment for restriction digestion, (UNIT 3.1), oligonucleotide synthesis and purification (UNITS 2.11 & 2.12), nondenaturing PAGE (UNIT 2.7), isolation, recovery, and quantitation of DNA (UNIT 2.6 & APPENDIX 3), subcloning DNA fragments (UNIT 3.16), transforming, plating, and growing E. coli (UNITS 1.8, 1.1, & 1.3), and DNA miniprep (UNIT 1.6) 1. Identify a unique restriction endonuclease site close to the 5′ end of the coding sequence of the gene to be expressed, as well as another unique site 3′ to this gene’s termination codon. 2. Synthesize two single-stranded DNA oligonucleotides, recreating the coding sequence immediately preceding the unique restriction endonuclease site near the 5′ end of the gene to be expressed. Purify and quantitate the DNA, then anneal in order to obtain double-stranded DNA.
Expression Using Vectors with Phage ëRegulatory Sequences
This synthetic DNA sequence is used to link the gene to be expressed to the initiating ATG of the pSKF expression vector. The double-stranded oligonucleotide should be designed to have ends that are complimentary to the restriction sites identified at the 5′ end of the gene to be expressed as well as the chosen restriction site in the expression vector.
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Current Protocols in Molecular Biology
Eco Rl 1 Clal 27
Aat ll 5796
Sspl 5678 start 5662 Scal 5354
Hin dlll 32 / border 32, 33 BstEll 552 start 673
Pstl 5129
rexB
Apr
end 4901
λ pSKF101 5866 bp ori 4044 Afl lIl 3981
pBR322
Pvu ll 3574
end 1107 Bgl ll 1216 p L(–35) 1324-1329 p L(–10) 1338-1343 message start 1353 Nut L1398 -1414
border 1670, 1672 NutR 1785-1801 BstXI 1819 tRI 1822-1842 c ll rbs 1863-1868 c ll start 1880 BamHl 1882 / border 1882, 1883 Sphl 2072 Nrul 2480 Sall 2157
Figure 16.3.1 pSKF101. pSKF101 is a vector used for authentic gene cloning which allows direct expression of the inserted gene. It is a derivative of pBR322 (UNIT 1.5) containing sequences inserted between HindIII and BamHI sites of pBR322. The inserted λ sequences contain the p L promoter and cII ribosome-binding site (rbs); these are the transcriptional and translational requlatory sequences necessary to express heterologous genes in E. coli. Within this region are several unique restriction sites that permit insertion of the gene. The regions derived from pB322 and λ are indicated. This plasmid can be maintained stably in a λ-lysogenized E. coli strain. The selectable marker is ampicillin, encoded by β-lactamase. An alternate name for pSKF101 is pASI (Rosenberg et al., 1983). Alternative names of related vectors are as follows: pSKF102 is pOTSV (Shatzman and Rosenberg, 1987); pSKF201 is pOTS-Nco (Shatzman and Rosenberg, 1987); and pSKF301 is pMG1.
3. Digest 25 to 50 µg plasmid DNA containing the gene to be expressed with the restriction endonucleases identified in step 1. To ensure complete digestion, determine that the restriction endonuclease buffer is appropriate for each enzyme to be used. If the endonucleases require different buffers, then each restriction digestion must be done separately.
4. Electrophorese the doubly digested plasmid DNA on a polyacrylamide gel. If the DNA fragment to be isolated is between 150 and 1100 bp, a 6% gel can be used. Either a borate- or acetate-buffer system can be used. If digestion was done in a large volume, ethanol precipitate the DNA (UNIT 2.1) and resuspend in 40 to 100 ìl TE buffer. Mix with loading dye and load.
5. Locate the fragment of interest by staining with an agent such as ethidium bromide and cut the DNA fragment out of the gel. 6. Recover the DNA by electroelution and quantitate the amount of DNA. Confirm that the correct fragment has been isolated by running a small aliquot on an agarose gel. Be sure to run appropriate size markers in an adjacent lane.
7. Digest 10 µg pSKF101 with BamHI and a restriction endonuclease that generates ends compatible with the 3′ end of the coding sequence (to accommodate the 3′ end of the gene to be expressed). Confirm that complete digestion of the vector has occurred by analysis of digested DNA
Protein Expression
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Sspl Aat ll Scal 5000
Ps tl
Bgl ll pL
5192
BstXI
Ap r 1000 4000
NS1-81
pSKF301 5192 bp Eagl/Xmalll
ori 2000
Nde l BamHI HindIII
Ncol Ndel Hpal EcoRV Stul 3 reading frame insertion sites Banll/Sac l Xhol Xbal Clal 3 reading frame termination sites Hin dlll
3000 Accl Tth111l Pvull
Nar l
Ball
Figure 16.3.2 pSKF301. pSKF301 is a vector that can be used for both indirect and direct expression. it is similar to pSKF101 in that it contains the same transcriptional and translational regulatory sequences as well as selectable markers; it differs in that it contains a shorter segment of λ DNA than pSKF101. pSKF301 also contains the coding sequence of the first 81 amino acids of the influenza protein, NS1, shown as NS1-81. This region is adjacent to the cll ribosome-binding site (fbs) and contains restriction sites at the 3′ end of NS1-81 that allow construction of translational fusions in any of the three reading frames. Removal of NS1-81 permits direct expression of the cloned gene. (This vector is also known as pmG1.)
on an agarose gel. Compare undigested pSKF101 with digested to make sure that pSKF101 has been completely linearized.
8. Prepare a ligation reaction (using the conditions described in UNIT 3.16) by combining the following ingredients: 1 ng digested pSKF101 vector DNA 10 ng of the gene fragment to be expressed (from step 6) 20 ng synthetic oligonucleotide (from step 2) T4 DNA ligase. Ligate 10 to 12 hr at 4°C. There is no need to dephosphorylate pSKF101 as long as there is at least a 5-fold molar excess of vector DNA to isolated DNA fragment and synthetic DNA.
9. Remove one-third of the ligation reaction and transform 50 to 100 µl competent E. coli AS1. Plate on LB/ampicillin plates and incubate overnight at 37°C. 10. Pick 12 to 24 colonies and transfer with a sterile toothpick to 3 ml LB/ampicillin medium. Grow cells 5 to 18 hr and isolate DNA by a miniprep method. Cells may be harvested once the broth appears turbid. For best results, allow 8 to 12 hr of growth.
Expression Using Vectors with Phage ëRegulatory Sequences
11. Perform appropriate restriction endonuclease digests to determine which clones contain the desired construction of the gene to be expressed. 12. Transform an E. coli strain with the DNA and express the gene as in the basic protocols.
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Current Protocols in Molecular Biology
CONSTRUCTION AND DISASSEMBLY OF FUSED GENES IN pSKF301 By fusing the gene to be expressed to a coding region of another gene (the fusion partner), a chimeric gene can be constructed in an appropriate vector. Numerous vectors are available for this purpose but most share the common feature of a fusion partner that is a highly expressed gene. When expression of the chimeric gene is induced, the resulting proteins carry additional peptide information at the N terminus. Although the fusion product may have physical and/or functional properties that differ from the “authentic” protein, advantages of the approach include highly efficient expression (up to 30% of total cell protein) without complicated alterations on the gene, and the presence of a “handle” on the expressed protein which can help to identify and purify it. Such proteins are often used to develop antisera to specific proteins that have diagnostic potential, and have been used successfully to identify and define a variety of gene products (Casadaban et al., 1983; Rose and Botstein, 1983; Guarente, 1983).
SUPPORT PROTOCOL
Strategic Planning Plasmid pSKF301 has been constructed to permit initial expression of a gene as a fusion product, followed by removal of the DNA encoding the fusion portion by restriction digestion. Finally, the unfused version of the gene is expressed as an authentic protein. pSKF301 contains an NdeI restriction site adjacent to the ATG following the cII ribosome-binding site (Fig. 16.3.3). This ATG also serves as the translational start (Gold et al., 1981) of the NS1 gene derived from the influenza nonstructural gene. This gene has been truncated to express only its first 81 amino acids. Just beyond the coding sequence for the 81st amino acid is a second NdeI site followed by three unique blunt-ended restriction sites, HpaI, EcoRV, and StuI, which allow for the insertion of genes into any of three reading frames. Immediately following the StuI site are sequences coding for translational stops in any of the three reading frames.
A Ndel/BamHI
cllrbs
NS1-81 amino acids
Ndel Hpal
Eco RV
Stul
ccatg gat cat atg tta aca gat atc aag gcc tga ctg act gag
stop stop stop
B ccatg gat cat atg tt H pal-digested pSKF301 supplies
2 3
of a codon
ccatg gat cat atg tta aca gat Eco RV-digested pSKF301 supplies a complete codon
ccatg gat cat atg tta aca gat atc aag g 1 Stu l-digested pSKF301 supplies 3 of a codon
Figure 16.3.3 Sequence and restriction endonuclease sites (in the region used for cloning) of pSKF301 (A). Restriction endonuclease digestion shows the strategy utilized to obtain pSKF301 as a vehicle for expression in all three reading frames (B). Protein Expression
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The expression of a gene of interest as a fusion protein may be achieved by utilizing any of the following restriction sites in pSKF301: NcoI, HpaI, EcoRV, or StuI. Choice of restriction site depends upon the reading frame necessary for the translation of a specific protein sequence. First, a unique restriction site close to the 5′ end of the gene (or portion of the gene) to be expressed must be identified. Second, the appropriate restriction endonuclease is selected for digesting pSKF301 such that the gene will be expressed. If the chosen restriction site is a blunt-end cutter, no further manipulation of that end is required. In the event the restriction site identified leaves either a 5′ or 3′ protruding end, further manipulation is required. “Filling in” using the Klenow fragment of E. coli DNA polymerase for 5′ protrusions, or T4 DNA polymerase, S1, or mung bean nuclease for 3′ protrusions, are methods of choice (see UNIT 3.16). Sample Protocol Materials Appropriate restriction endonucleases and buffers (UNIT 3.1) Klenow fragment of E. coli DNA polymerase I (UNIT 3.5) pSKF301 vector (available from A. Shatzman; Figs. 16.3.1 & 16.3.2) T4 DNA ligase (UNIT 3.14) Competent E. coli AS1 (Table 1.4.5; also known as MM294cI+) Additional reagents and equipment for large-scale plasmid prep (UNIT 1.7), agarose gel electrophoresis (UNIT 2.5), extraction and precipitation of DNA (UNIT 2.1), transformation of competent cells (UNIT 1.8), and restriction digestion and mapping (UNITS 3.1-3.3) Construct a gene fusion in pSKF301 1. Assume the restriction site identified in the gene is a BamHI site. Digest with BamHI to obtain: GATCC
XXX
XXX
XXX
XXX
G
YYY
YYY
YYY
YYY
2. Treat with Klenow fragment to fill in the unpaired bases to obtain: GATCC
XXX
XXX
XXX
XXX
CTAGG
YYY
YYY
YYY
YYY
As noted above, Klenow fragment is used to fill in for 5′ protrusions. For 3′ protrusions, use T4 DNA polymerase (UNIT 3.5) or S1 or mung bean nuclease (UNIT 3.12).
3. Determine the proper reading frame of the gene. In this example assume XXX XXX XXX XXX is the proper reading frame; therefore, the coding sequence of the filled-in fragment should read: GA
TCC
XXX
XXX
XXX
4. Determine which restriction endonuclease should be used to digest pSKF301 to allow expression of the fusion protein. For this example, StuI is required to yield: ccatg gat cat atg tta aca gat atc aag gGA TCC XXX XXX XXX XXX pSKF301 Expression Using Vectors with Phage ëRegulatory Sequences
fusion gene
5. Prepare the vector and the fragment of the gene to be expressed as in the first support protocol, steps 3 to 12 (except no synthetic DNA is required).
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Generate an authentic version of the gene Once a gene has been expressed as a fusion protein, it may be desirable to obtain an unfused version of the gene product. If this is useful, follow steps 6 to 12. To convert a fusion protein to an unfused protein when using pSKF301, be certain that the gene of interest does not contain an NdeI site. The following theoretical fusion construct will be used as an example in these steps: NdeI
NdeI
CATATGGATCC- - -NS1-81- - -CCATGGATCATATGTT- - -fusion gene- - -tga
6. Set up a large-scale plasmid preparation of the fusion construct to yield ∼100 µg plasmid DNA. 7. Digest 10 µg of the construct with NdeI. Verify that all of the vector DNA has been completely digested by taking a small aliquot of the digested material and running it on an agarose gel next to lanes containing uncut plasmid and appropriate size markers. A 280-bp fragment should be observed; this contains the NS1-81 gene sequence being liberated from the construct. Confirmation of complete digestion is extremely important.
8. Purify the digested construct by phenol/chloroform/isoamyl alcohol extraction followed by ethanol precipitation. 9. Add T4 DNA ligase to 1 µg of the NdeI-digested construct and incubate overnight at 4°C. 10. Transform ligated DNA into competent E. coli AS1 cells (or any other suitable cI+ lysogen). 11. Determine that the construct no longer contains the NS1-81 gene sequence by restriction analysis. Consult the restriction map of pSKF301 and the gene to be expressed to determine which endonucleases are diagnostic for identifying the construct devoid of the NS1-81 gene. If the NdeI digestion was complete upon ligation, reclosure is highly efficient. Expect 95% to 100% of the resulting transformants to contain the unfused construct.
12. Transform the DNA and express the gene by temperature or chemical induction as in the basic protocols. COMMENTARY Background Information Expression of a heterologous gene or gene fragment in E. coli requires that the coding sequence be placed under the transcriptional and translational control of regulatory elements recognized by the bacterial cell. The pSKF vectors were designed specifically to direct gene expression by providing regulatory signals from bacteriophage λ. Phage regulatory signals were chosen because of their high efficiency and ability to be tightly regulated. This system uses a promoter that can be tightly controlled, eliminating problems with “leaky”
basal expression sometimes found in other expression systems (see below). This system uses an antitermination mechanism to help assure efficient transcription across any gene insert. The different vectors used with this system offer several choices of antibiotic selection markers, contain elements that optimize plasmid stability, and carry a variety of restriction sites that permit relatively easy insertion of the gene of interest adjacent to the efficient translation regulatory information. The pSKF system offers some advantages that differentiate it from many other expression
Protein Expression
16.3.8 Current Protocols in Molecular Biology
Supplement 11
Expression Using Vectors with Phage ëRegulatory Sequences
systems. Perhaps most important is the “tightness” of regulation of the pL promoter. Several other strong regulatable promoters—ptac (de Boer et al., 1982), ptrp (Edman et al., 1981), and pT7 (promoter of T7 gene 10; Studier and Moffatt, 1986)—are also used routinely for optimizing heterologous gene expression in E. coli. These promoters, along with pL, are all of comparable strength and are sufficient to achieve very high levels of mRNA production (UNIT 16.1). In fact, these promoters are so powerful that further enhancement of promoter strength would not be expected to result in an increase of protein production; indeed, these promoters are so strong that it is very difficult to keep them fully turned off even in the “repressed” state. Because of this basal transcription under repressed conditions, use of the ptac, ptrp, or pT7 (coupled with the plac-T7 polymerase) systems often leads to some expression of the cloned gene even under nonpermissive conditions. This may lead to plasmid loss or rearrangement, or possibly cell death, if small amounts of the gene product are lethal to E. coli. In contrast, one does not typically see expression of the cloned gene in the pL system until cultures have been induced. A second advantage of the pL system over other promoter systems is the flexibility gained from having completely different induction systems (thermal and chemical). In contrast, the ptac and pT7 systems mentioned above permit induction only by a chemical route. Different routes of induction lead to completely different cellular states (e.g., different physiology, morphology, and growth patterns) and these variations can lead to significant differences in gene product accumulation and stability (unpub. observ.). It should be pointed out that ptrp also permits dual modes of induction (by β-indolyl acetic acid or Trp starvation) and that a different version of the pT7 system has been developed (UNIT 16.2; Tabor and Richardson, 1985) in which the T7 RNA polymerase is thermally regulated via the pL-cI857 system (however, this system is not chemically inducible as well). The third major advantage of the pSKF system is the availability of a single vector that permits expression of either an authentic or a fusion gene product; furthermore, this vector allows the fusion gene to be converted to an authentic gene by a simple restriction digest followed by self ligation. Thus, a gene may be rapidly expressed at high levels as a protein fusion to give an initial reagent for use in activity studies and antisera preparation. Time
may then be taken to optimize the expression of the authentic (nonfusion) gene product, which will be better suited for functional and structural studies. Most other expression systems do not provide this flexibility.
Critical Parameters Gene expression is not solely a function of message levels. The efficiency of the ribosomebinding region—including the sequences both upstream and downstream of the ATG initiation codon—also play a role in determining the extent to which a protein is made. Alterations in these sequences may affect the secondary structure of a message and the conformational presentation of the initiation signals which, as a result, can alter translational efficiency (Gold et al., 1981). From our experience, the host strain plays a major role in determining the ultimate levels of gene expression. The reasons for the rather dramatic differences seen in product yield from different host strains are poorly understood. Product stability is, however, one determining factor that has been somewhat characterized. Host strains have been developed that are defective in certain proteases (UNIT 16.6). These specialized host strains can have a significant impact on the expression of certain gene products. However, proteases are not the only factor involved in strain-to-strain variations observed in protein expression. Other uncharacterized factors can have equally dramatic effects. It is therefore recommended that expression be tried in a number of different E. coli strains. Following the induction of cultures carrying the desired expression vector, cells may be analyzed in a variety of ways to detect the presence of the cloned gene product. Most typically, the presence of the novel gene product is determined directly by observing in SDS– polyacrylamide gels a new, inducible protein band not present in lanes from control cultures. The expression of any gene insert can also be identified and/or confirmed in several ways related to the activity or function of the protein including: (1) direct detection of a novel function or activity imparted to the living bacterial host; (2) genetic complementation of the appropriate mutant host; (3) assay of whole-cell extracts for the activity of the cloned gene product; and (4) assay after partial or complete purification of the cloned gene product. Immunochemical methods such as immunoprecipitation (UNIT 10.16) or western blotting (UNIT 10.8) are some of the most sensitive meth-
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ods available to detect expression of a gene product. These methods, of course, require that an antiserum be available which is specific for the protein to be expressed. These methods, however, are primarily quantitative and do not necessarily indicate anything about the level of expression, homogeneity, or activity of the gene product. If a good antiserum to the protein of interest is not available, purification of sufficient amounts of a gene product allows generation of high-titer, antigen-specific mono- or polyclonal antisera (UNITS 11.3-11.13). One approach to generate an antiserum is to produce the desired heterologous gene product in bacteria as a native protein, as a fusion, or as a protein fragment. The protein may then be purified and used to produce high-titer mono- or polyclonal antisera. Such antisera have been used to (1) map natural expression of the gene product with respect to cell type, subcellular distribution, and temporal regulation; (2) determine relative levels of expression in various cell types; (3) study protein processing and stability; (4) map immuno-dominant domains; (5) purify by immunoaffinity both the native and modified forms of the protein; and (6) provide in vivo diagnostic reagents for examining tissue distribution and expression of the gene product by immunofluorescent methods.
Troubleshooting There is never a guarantee that a gene will be expressed at high levels, but poor expression upon initial trials does not signify defeat. As mentioned earlier, transcription is rarely limiting and is, therefore, not the first parameter to be addressed in attempting to improve expression. Instead, the easiest parameter to change is the host strain being used for production. Typically, five or six different strains (which might or might not be closely related to each other) may have to be tested in pilot experiments to see which gives optimal production. The next parameter to examine in the event of poor expression is translation. Expression may be increased by altering ribosome-binding sites to improve complimentarity to the 16S rRNA, or by increasing the A-T richness of the 5′-end of the gene’s coding region. After steps have been taken to optimize translation, it is often helpful to alter the promoter and repressor system in order to change the induction system and the physiology of the cells during the production phase. For example, inducing the cI857-containing pL system via a temperature shift generates a cellular heat-
shock response and protein synthesis at 42°C. Induction of this system with nalidixic acid leads to a cellular SOS response (see glossary, Chapter 1 introduction) and protein production at 37°C. Induction of the trp system by tryptophan starvation turns on the host stringent response (a generalized response of E. coli to amino acid starvation). Thus, in each case, a different host response leads to induction of a different set of host proteins as well as to greatly different physiological effects (such as changes in respiration, filamentation, and growth rate). Finally, it may be possible to improve expression by optimizing the temperature at which the protein is made, as this parameter has often been shown to affect the proteins’ solubility, stability, and activity.
Anticipated Results Expression of most gene products as fusions with the first 81 amino acids of the NS1 protein (using pSKF301) can be achieved at levels between 5% and 30% of total cellular protein. Expression levels of nonfusion proteins (authentic) are less predictable and may vary from 50 kDa), or when the protein contains regions that are strongly hydrophobic or highly charged (D.B.S. and L.M.C., unpub. observ.). Insoluble fusion proteins can sometimes be coaxed into solution (see Critical Parameters and Troubleshooting) or can otherwise be purified after solubilization in denaturing reagents. If necessary, the GST moiety can be removed from fusion proteins by cleavage with site-specific proteases (UNIT 16.4B). Note however, that often the GST carrier does not compromise the antigenicity or functional activity of the foreign polypeptide. Modified versions of the original pGEX vectors have been produced that simplify cloning, cleavage or detection of fusion proteins (for review, see Smith, 1993).
Critical Parameters and Troubleshooting
Expression and Purification of Glutathione-STransferase Protein Fusions
Contamination of fusion proteins by host cell proteins is usually a sign that sonication has been too severe, perhaps because denaturation exposes regions on proteins that are more likely to cause aggregation. Some contaminating species may represent degraded fragments of the fusion protein that bind to glutathione. Such fragments are not easy to eliminate, except by increasing the stability of the fusion protein. The addition of protease inhibitors [e.g., 1% (w/v) aprotinin and 1 mM phenylmethylsulfonyl fluoride (PMSF)] may help in this regard, and inclusion of 50 to 100 mM EDTA in the lysis buffer has also been beneficial. The E. coli host strain can have a major and unpredictable effect on stability, and it is worth testing several different strains, including the protease-deficient lon− strains (D.B.S. and L.M.C., unpub. observ.; UNIT 1.4). Alterna-
tively, degradation of fusion proteins can be minimized by adding isopropyl-1-thio-β-Dgalactoside (IPTG) later in the course of the culture and keeping the induction period to a minimum. The overall yield of fusion protein can sometimes be improved by increasing the quantity of glutathione-agarose beads, minimizing the volume of liquid during absorption, and extending the period of absorption to 1 hr. Insolubility of fusion proteins can be addressed by several means. In some cases, growth of cells at 30°C is sufficient to alter solubility (D.B.S., unpub. observ.), but in other cases it may be necessary to investigate the effect of mild detergent treatment after cell lysis. Examples of conditions under which binding of GST to glutathione-agarose is unaffected are 1% Triton X-100, 1% Tween-20, 1% CTAB, 10 mM DTT, and 0.03% SDS. Alternatively, fusion proteins can be solubilized in 1% to 2% Sarkosyl prior to sonication; they can be solubilized in 2% to 4% Triton X-100 prior to binding to glutathione agarose (Frangioni and Neel, 1993). Even if a fusion protein is largely insoluble, it may be possible to purify the small proportion that is still soluble, with a yield of perhaps 50 µg/liter. Otherwise, a fusion protein that is abundant, but stubbornly insoluble, can be purified by gel filtration after solubilization under denaturing conditions or by electroelution from a gel after SDS-PAGE. A different approach is to express the polypeptide as smaller fragments, particularly if it is possible to express parts of the protein that do not contain severely hydrophobic or highly charged regions. If the fusion protein is not of the expected size, it may be worth sequencing across the cloning site to ensure that the reading frame of the vector and the insert are matched. There is a hairpin in GST sequences near the cloning sites, so a sequencing primer complementary to either the extreme 3′ end of the GST gene or to sequences within the DNA insert should be used.
Anticipated Results Yields of fusion protein vary from more typical yields of 1 to 3 mg/liter up to 10 mg/liter, and can be as low as 50 µg/liter if most of the fusion protein is insoluble. The single affinity chromatography step can generate fusion protein preparations that are >90% pure.
Time Considerations A full day is required to screen colonies of transformants for expression of fusion proteins,
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although continuous attention is not required. Large-scale purification and cleavage of fusion proteins require another day of intermittent work. Cells can be stored as pellets at −70°C before lysis, although with fusion proteins that are unstable, this may be undesirable. Purification is best completed in one session.
Literature Cited Frangioni, J.V. and Neel, B.G. 1993. Solubilization and purification or enzymatically active glutathione-S-transferase (pGEX) fusion proteins. Anal. Biochem. 210:179-187. Smith, D.B. 1993. Purification of glutathione-Stransferase fusion proteins. Methods Mol. Cell Biol. 4:220-229. Smith, D.B., Davern, K.M., Board, P.G., Tiu, W.U., Garcia, E.G., and Mitchell, G.F. 1986. Mr 26,000 antigen of Schistosoma japonicum recognized by resistant WEHI 129/J mice is a parasite glutathione S-transferase. Proc. Natl. Acad. Sci. U.S.A. 83:8703-8707.
Key Reference Smith et al., 1986. See above. Original description of the pGEX system. Smith, 1993. See above. Summary of modified pGEX vectors and alternative purification methods. Smith, D.B. and Johnson, K.S. 1988. Single-step purification of polypeptides expressed in Escherichia coli as fusions with glutathione Stransferase. Gene 67:31-40. First description of GST fusion system.
Contributed by Donald B. Smith University of Edinburgh Edinburgh, Scotland Lynn M. Corcoran Walter & Eliza Hall Institute Victoria, Australia
Protein Expression
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Expression and Purification of Thioredoxin Fusion Proteins
UNIT 16.8
This unit describes a gene fusion expression system that uses thioredoxin, the product of the Escherichia coli trxA gene, as the fusion partner. The system is particularly useful for high-level production of soluble fusion proteins in the E. coli cytoplasm; in many cases heterologous proteins produced as thioredoxin fusion proteins are correctly folded and display full biological activity. Although the thioredoxin gene fusion system is routinely used for protein production, high-level production of peptides—i.e., for use as antigens— is also possible because the prominent thioredoxin active-site loop is a very permissive site for the introduction of short amino acid sequences (10 to 30 residues in length). The inherent thermal stability of thioredoxin and its susceptibility to quantitative release from the E. coli cytoplasm by osmotic shock can also be exploited as useful tools for thioredoxin fusion protein purification. In addition, a more generic method for purification of any soluble thioredoxin fusion employs a modified form of thioredoxin (called “His-patch Trx”), which has been designed to bind to metal chelate resins. Protein fusions to His-patch Trx can usually be purified in a single step from cell lysates (see Strategic Planning). The basic protocol outlines the construction of a fusion of trxA to any desired gene and expression of the fusion protein in an appropriate host strain at 37°C. Additional protocols describe E. coli cell lysis using a French pressure cell and fractionation (first support protocol), osmotic release of thioredoxin fusion proteins from the E. coli cytoplasm (second support protocol), and heat treatment to purify some thioredoxin fusion proteins (third support protocol). STRATEGIC PLANNING The thioredoxin gene fusion expression vectors pTRXFUS and hpTRXFUS, both of which carry the E. coli trxA gene (Fig. 16.8.1), are used for high-level production of C-terminal fusions to thioredoxin. The vector hpTRXFUS differs from pTRXFUS in that it contains a modified E. coli trxA gene which produces a mutant protein (“His-patch” thioredoxin) that can specifically bind to metal chelate matrices charged with nickel or cobalt, otherwise known as native metal-chelate affinity chromatography (MCAC; UNIT 10.11B). The trxA translation termination codon has been replaced in both vectors by DNA encoding a ten-residue peptide linker sequence that includes an enterokinase (enteropeptidase; LaVallie et al., 1993a) cleavage site. This highly specific site can be cleaved with enterokinase following purification of the fusion protein to release the protein of interest from its thioredoxin fusion partner (cleavage of the fusion protein is covered in UNIT 16.4B). Immediately downstream of the DNA encoding the enterokinase site in pTRXFUS and hpTRXFUS lies a DNA polylinker sequence containing a number of unique restriction endonuclease sites that can be used for forming in-frame translational fusions of any desired gene to trxA. Downstream of the DNA polylinker lies the E. coli aspA transcription terminator. Replication of these vectors is controlled by a modified colE1 replication origin similar to that found in pUC vectors (Norrander et al., 1983). Plasmid selection and maintenance is ensured by the presence of the β-lactamase gene on the vector. The vector pALtrxA-781 (Fig. 16.8.1) is very similar to pTRXFUS. However in this plasmid the trxA gene is followed by a translation termination codon, and the sequences encoding the enterokinase-site peptide linker are absent. A unique RsrII site, present in both pALtrxA-781 and pTRXFUS, allows for the easy insertion of short peptide-encoding DNA sequences into trxA within the region that encodes the active-site loop. Contributed by John McCoy and Edward LaVallie Current Protocols in Molecular Biology (1994) 16.8.1-16.8.14 Copyright © 2000 by John Wiley & Sons, Inc.
Protein Expression
16.8.1 Supplement 28
ori
BLA
pALtrxA–781 pTRXFUS p L hpTRXFUS
trxA aspA TGGTGCGGTCCGTGCAAA W C G33 P34 C K pALtrxA–781:
Rsr II
Sfi I
Xba I
Sal I
Pst I
AACCTGGCCTAGCTGGCCATCTAGAGTCGACCTGCAG N L A *
aspA terminator
thioredoxin
KpnI BamHI XbaI
pTRXFUS:
Sal I
Pst I
AACCTGGCCGGTTCTGGTTCTGGTGATGACGATGACAAGGTACCCGGGGATCCTCTAGAGTCGACCTGCAG N L A G S G S G D D D D K fusion point thioredoxin
aspA terminator
linker peptide enterokinase site
Figure 16.8.1 Thioredoxin gene fusion expression vectors pTRXFUS, hpTRXFUS, and pALtrxA-781. pALtrxA-781 contains a polylinker sequence at the 3′ end of the trxA gene. pTRXFUS and hpTRXFUS contain a linker region encoding a peptide that includes the enterokinase cleavage site between the trxA gene and the polylinker. The sequence surrounding the active site loop of thioredoxin has a single RsrII site that can be used to insert peptide coding sequence. The asterisk indicates a translational stop codon. Abbreviations: trxA, E. coli thioredoxin gene; BLA, β-lactamase gene; ori, colE1 replication origin; pL, bacteriophage λ major leftward promoter; aspA terminator, E. coli aspartate amino-transferase transcription terminator.
Expression and Purification of Thioredoxin Fusion Proteins
pTRXFUS, hpTRXFUS, and pALtrxA-781 carry the strong bacteriophage λ promoter pL (Shimatake and Rosenberg, 1981) positioned upstream of the trxA gene. Transcription initiation at the pL promoter is controlled by the intracellular concentration of λ repressor protein (cI). UNIT 16.3 describes λ strains that carry either a temperature-sensitive form of cI (cI857) or a wild-type cI repressor protein. cI857-containing strains can be used for heat inductions of pL at 42°C; alternatively, in the strains carrying the wild-type repressor, pL can be induced by a prior induction of the E. coli SOS stress response. However, it is often desirable to express heterologous genes in E. coli at temperatures considerably lower than 42°C, or under conditions where cells are not undergoing a physiological stress. Strains GI698, GI724 and GI723 were designed to allow the growth and induction of pL expression vectors, including pTRXFUS, hpTRXFUS, and pALtrxA-781, under mild conditions over a wide range of temperatures (see Table 16.8.1; Mieschendahl et al., 1986). Each of these strains carries a wild-type allele of cI stably integrated into the E. coli chromosome at the nonessential ampC locus. A synthetic trp promoter integrated into ampC upstream of the cI gene in each strain directs the synthesis of cI repressor only when intracellular tryptophan levels are low. When tryptophan levels are high, synthesis of cI is switched off; therefore, the presence of tryptophan in the growth medium of GI698, GI723, or GI724 will block expression of λ repressor and thus will turn on pL. Because
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Table 16.8.1 E. coli Strains for Production of Thioredoxin Fusion Proteins at Varying Temperatures
Strain
Desired production temperature (°C)
Pre-induction growth temperature (°C)
Induction period (hr)
GI698 GI698 GI698
15 20 25
25 25 25
20 18 10
GI724 GI724 GI723
30 37 37
30 30 37
6 4 5
the three strains carry ribosome-binding sequences of different strengths at the 5′-end of their respective cI genes, they maintain intracellular concentrations of λ repressor that increase in the order GI698 < GI724 < GI723. The choice of which strain to use for a particular application is dependent on the desired culture conditions as described below. Although some thioredoxin fusion proteins produced at 37°C are insoluble, expression at lower temperatures can often result in the fusion protein being produced in a soluble form. Each of the three pL host strains GI698, GI723, and GI724 is suitable for the production of thioredoxin fusion proteins over a particular temperature range. Table 16.8.1 indicates the correct strain for expression of thioredoxin fusion proteins at any temperature between 15°C and 37°C. The induction protocol at any of these temperatures is the same as that described in the basic protocol for induction of GI724 at 37°C, except the preinduction growth temperature and the length of the induction period vary according to the strain used and the temperature chosen. Cultures should be grown at the indicated preinduction growth temperature until they reach a density of 0.4 to 0.6 OD550/ml. They should then be moved to the desired induction temperature and induced by the addition of 100 µg/ml tryptophan. Low-temperature inductions are best performed in strain GI698. However, this strain makes only enough cI repressor protein to maintain the vectors in an uninduced state at temperatures below 25°C. GI698 should therefore never be grown above 25°C when it carries a pL plasmid. A nonrefrigerated water bath can be maintained below room temperature by placing it in a 4°C room and setting the thermostat to the desired temperature. It is often a good idea to collect timepoints during the course of a long induction period and to fractionate cells from these timepoints using the procedure in the first support protocol (steps 9 to 13). Although a particular fusion protein may be soluble during the early part of an induction, during the later phases of induction, it may become unstable or its concentration inside the cell may exceed a critical threshold above which it will precipitate and appear in the insoluble fraction.
Protein Expression
16.8.3 Current Protocols in Molecular Biology
Supplement 28
BASIC PROTOCOL
CONSTRUCTION AND EXPRESSION OF A THIOREDOXIN FUSION PROTEIN This protocol describes construction and subsequent expression of a gene fusion between trxA (encoding thioredoxin) and a gene encoding a particular protein or peptide. After a clone carrying the correct fusion sequence is constructed, analyzed, and isolated, cultures are grown and expression is induced. The protocol is described in terms of the E. coli host strain GI724 with expression at 30°C; it may also be applied to strains GI698 and GI723 (also available from Genetics Institute) for expression at other temperatures by using the parameters specified in Table 16.8.1 (see Strategic Planning). Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
DNA fragment encoding desired sequence Thioredoxin expression vectors (Fig. 16.8.1): pTRXFUS or pALtrxA-781 (Genetics Institute or Invitrogen) or hpTRXFUS (Genetics Institute) E. coli strain GI724 (Genetics Institute or Invitrogen), grown in LB medium and made competent (UNIT 1.8) LB medium (UNIT 1.1) IMC plates containing 100 µg/ml ampicillin (see recipe) CAA/glycerol/ampicillin 100 medium (see recipe) IMC medium containing 100 µg/ml ampicillin (see recipe) 10 mg/ml tryptophan (see recipe) SDS-PAGE sample buffer (see recipe) 30°C convection incubator 18 × 50–mm culture tubes Roller drum (New Brunswick Scientific) 250-ml culture flask 70°C water bath Microcentrifuge, 4°C Additional reagents and equipment for subcloning of DNA fragments (UNIT 3.16), transforming competent E. coli cells (UNIT 1.8), preparing miniprep DNA (UNIT 1.6), restriction mapping (UNIT 3.2), direct sequencing of plasmid DNA (UNITS 7.3 & 7.4), SDS-PAGE (UNIT 10.2), and Coomassie brilliant blue staining (UNIT 10.6) Construct the trxA gene fusion 1. Use DNA fragment encoding the desired sequence to construct either an in-frame fusion to the 3′-end of the trxA gene in pTRXFUS or hpTRXFUS, or a short peptide insertion into the unique RsrII site of pALtrxA-781. A precise fusion of the desired gene to the enterokinase linker sequence in pTRXFUS or hpTRXFUS can be made by using the unique KpnI site trimmed to a blunt end with the Klenow fragment of E. coli DNA polymerase. The desired gene can usually be adapted to this blunt-end construct by using a synthetic oligonucleotide duplex ligated between it and any convenient downstream restriction site close to the 5′ end of the gene. When designing the fusion junction, note that enterokinase is able to cleave —DDDDK↓X—, where X is any amino acid residue except proline. Synthetic oligonucleotides encoding short peptides for insertion into the thioredoxin active-site loop at the RsrII site will insert only in the desired orientation, because the RsrII sticky end consists of three bases. Expression and Purification of Thioredoxin Fusion Proteins
2. Transform the ligation mixture containing the new thioredoxin fusion plasmid into competent GI724 cells. Plate transformed cells onto IMC plates containing 100 µg/ml
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ampicillin to select transformants. Incubate plates in a 30°C convection incubator until colonies appear. Strains GI698, GI723, and GI724 are all healthy prototrophs that can grow under a wide variety of growth conditions, including rich and minimal media and a broad range of growth temperatures (see Table 16.8.1). These strains can be prepared for transformation with pL-containing vectors by growing them in LB medium at 37°C. LB medium may also be used for these strains during the short period of outgrowth immediately following transformation. This growth period of 30 min to 1 hr is often used to express drug resistance phenotypes before plating out plasmid transformations onto solid medium. Subsequently, however, these strains should be grown only on minimal or tryptophan-free rich media, such as IMC medium containing 100 ìg/ml ampicillin (for expression of the fusion protein) or CAA/glycerol/ampicillin 100 medium (for plasmid DNA preparations). Except during transformation, LB medium should never be used with these three strains when they carry pL plasmids because LB contains tryptophan. The pL promoter is extremely strong and should be maintained in an uninduced state until needed so that expression of the protein will not lead to selection of mutant or variant cells with lower expression due to undesirable genetic selections or rearrangements in the expression strain.
3. Grow candidate colonies in 5 ml CAA/glycerol/ampicillin 100 medium overnight at 30°C. Prepare minipreps of plasmid DNA and check for correct gene insertion into pTRXFUS by restriction mapping. 4. Sequence plasmid DNA of candidate clones to verify the junction region between thioredoxin and the gene or sequence of interest. Induce expression 5. Streak out frozen stock culture of GI724 containing thioredoxin expression plasmid to single colonies on IMC plates containing 100 µg/ml ampicillin. Grow 20 hr at 30°C. Occasionally there is induction of pL plasmids grown in GI698 and GI724 at 37°C, even in medium containing no tryptophan. Such induction appears to be a temperature-dependent phenomenon. If growth at 37°C prior to pL induction is essential, then GI723 should be used as the host strain because GI723 produces higher levels of cI repressor than both GI698 and to GI724. Otherwise, plasmid-containing GI698 should be grown at 25°C and plasmid-containing GI724 should be grown at 30°C prior to induction (see Table 16.8.1).
6. Pick a single fresh, well-isolated, colony from the plate and use it to inoculate 5 ml IMC medium containing 100 mg/ml ampicillin in an 18 × 150–mm culture tube. Incubate overnight at 30°C on a roller drum. 7. Add 0.5 ml overnight culture to 50 ml fresh IMC medium containing 100 µg/ml ampicillin in a 250-ml culture flask (1:100 dilution). Grow at 30°C with vigorous aeration until absorbance at 550 nm reaches 0.4 to 0.6 OD/ml (∼3.5 hr). 8. Remove a 1-ml aliquot of the culture (uninduced cells). Measure the absorbance at 550 nm and harvest the cells by microcentrifuging 1 min at maximum speed, room temperature. Carefully remove all the spent medium with a pipet and store the cell pellet at −80°C. 9. Induce pL by adding 0.5 ml of 10 mg/ml tryptophan (100 µg/ml final) to remaining cells immediately. 10. Incubate 4 hr at 37°C. At hourly intervals during this incubation, remove 1-ml aliquots of the culture and harvest cells as in step 8. Protein Expression
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11. Harvest the remaining cells from the culture 4 hr post-induction by centrifuging 10 min at 3000 rpm (e.g., in a Beckman J6 rotor), 4°C. Store the cell pellet at −80°C. Procedures for further analysis of these cells are outlined in the support protocols.
Verify induction 12. Resuspend the pellets from the induction intervals (steps 8 and 10) in 200 µl of SDS-PAGE sample buffer/OD550 cells. Heat 5 min at 70°C to completely lyse the cells and denature the proteins. Run the equivalent of 0.15 OD550 cells per lane (30 µl) on an SDS-polyacrylamide gel. 13. Stain the gel 1 hr with Coomassie brilliant blue. Destain the gel and check for expression. Most thioredoxin fusion proteins are produced at levels that vary from 5% to 20% of the total cell protein. The desired fusion protein should exhibit the following characteristics: it should run on the gel at the mobility expected for its molecular weight; it should be absent prior to induction; and it should gradually accumulate during induction, with maximum accumulation usually occurring 3 hr post-induction at 37°C. SUPPORT PROTOCOL 1
E. COLI LYSIS USING A FRENCH PRESSURE CELL A small 3.5-ml French pressure cell can be used as a convenient way to lyse E. coli cells. The whole-cell lysate can be fractionated into soluble and insoluble fractions by microcentrifugation. Other lysis procedures may be used—for example, sonication (UNITS 4.4 & 16.6) or treatment with lysozyme-EDTA (UNIT 4.4). Additional Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Cell pellet from 4-hr post-induction culture (basic protocol) 20 mM Tris⋅Cl, pH 8.0 (APPENDIX 2), 4°C Lysis buffer: 20 mM Tris⋅Cl (pH 8.0) with protease inhibitors (optional)—0.5 mM phenylmethylsulfonyl fluoride (PMSF), 1 mM p-aminobenzamidine (PABA), and 5 mM EDTA French press and 3.5-ml mini-cell (Fig. 16.8.2; SLM Instruments), 4°C Lyse the cells 1. Resuspend cell pellet from 4-hr post-induction culture in 20 mM Tri⋅Cl, pH 8.0, to a concentration of 5 OD550/ml. Protease inhibitors can be included in the resuspension if desired. Cells can also be resuspended at densities of 100 OD550/ml or greater; however, at high densities cell lysis may be less efficient.
2. Place 1.5 ml resuspended cell pellet in the French pressure cell. Hold the cell upside down with the base removed, the piston fully extended downwards, and the outlet valve handle that holds the nylon ball seal in the open position (loose). Before filling the pressure cell, check that the nylon ball, which seals the outlet port and sits on the end of the outlet valve handle, is not deformed. If it is, replace it with a new one. Both the condition of the nylon ball and its seat in the pressure cell body are critical for the success of the procedure.
Expression and Purification of Thioredoxin Fusion Proteins
3. Bring the liquid in the pressure cell to the level of the outlet port by raising the piston slowly to expel excess air from the cell. With the outlet valve open and at the same time maintaining the piston in position, install the pressure cell base. Gently close the outlet valve.
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Figure 16.8.2 French pressure cell, equipped with 3.5-ml mini-cell. piston
cell body
outlet valve
outlet port
base
CAUTION: Do not over-tighten the valve as this will deform the nylon ball and may irreparably damage its seat on the pressure cell body.
4. Turn the sealed cell right side up and place it in the hydraulic press. 5. Turn the pressure regulator on the press fully counter-clockwise to reset it to zero pressure. Set the ratio selector to medium. Turn on the press. CAUTION: The larger (50-ml) pressure cell is usually used with the selector set on high. The small (3.5-ml) cell is only used on medium ratio.
6. Slowly turn the pressure regulator clockwise until the press just begins to move. Allow the press to compress the piston. It will stop moving after a few seconds. 7. Position a collection tube under the pressure cell outlet. Slowly increase the pressure in the cell by turning the pressure regulator clockwise. Monitor the reading on the gauge and increase the pressure to 1000 on the dial, corresponding to an internal cell pressure of 20,000 lb/in2. 8. While continuously monitoring the gauge, very slowly open the outlet valve until lysate begins to trickle from the outlet. The lysate should flow slowly and smoothly, and the cell pressure should not drop more than 100 divisions on the dial. At 20,000 lb/in2 and 5 OD550/ml, cell lysis will be complete after one passage through the press. Lower pressures and/or higher cell densities may require a second passage.
Fractionate the lysate 9. Remove a 100-µl aliquot of the lysate and freeze at −80°C (whole-cell lysate). 10. Fractionate the remainder of the lysate by microcentrifuging 10 min at maximum speed, 4°C. 11. Remove a 100-µl aliquot of the supernatant and freeze at −80°C (soluble fraction). Discard the remainder of the supernatant. Because this is a pilot experiment, it would not produce enough material to warrant saving any remaining supernatant.
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12. Resuspend the pellet in an equivalent volume of lysis buffer. Remove a 100-µl aliquot and freeze at −80°C (insoluble fraction). 13. Lyophilize the 100-µl aliquots to dryness in a Speedvac evaporator. Solubilize in 100 µl SDS-PAGE sample buffer. Analyze 30-µl samples by SDS-PAGE. This crude fractionation provides a fairly reliable indication of whether a protein has folded correctly. Usually proteins in the soluble fraction have adopted a correct conformation and proteins in the insoluble fraction have not. However, occasionally proteins found in the soluble fraction are not truly soluble; instead they form aggregates that do not pellet in the microcentrifuge. Conversely, sometimes a protein found in the insoluble fraction may be there because it has an affinity for cell wall components and cell membranes, and it may not be intrinsically insoluble. Occasionally proteins can be recovered from these insoluble fractions by extracting with agents such as mild detergents. SUPPORT PROTOCOL 2
OSMOTIC RELEASE OF THIOREDOXIN FUSION PROTEINS Thioredoxin and some thioredoxin fusion proteins can be released with good yield from the E. coli cytoplasm by a simple osmotic shock procedure. Additional Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Cell pellet from 4-hr post-induction cultures (basic protocol) 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA/20% (w/v) sucrose, ice-cold 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA, ice-cold 1. Resuspend cell pellet from 4-hr post-induction cultures at a concentration of 5 OD550/ml in ice-cold 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA/20% sucrose. Incubate 10 min on ice. 2. Microcentrifuge 30 sec at maximum speed, 4°C, to pellet the cells. 3. Discard the supernatant and gently resuspend the cells in an equivalent volume of ice-cold 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA. Incubate 10 min on ice and mix occasionally by inverting the tube. Osmotic release from the cytoplasm occurs at this stage.
4. Microcentrifuge 30 sec at maximum speed, 4°C. Save the supernatant (osmotic shockate). Resuspend the cell pellet in an equivalent volume 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA (retentate). 5. Lyophilize 100-µl aliquots of osmotic shockate and retentate to dryness in a Speedvac evaporator. 6. Solubilize each in 100 µl SDS-PAGE sample buffer. Analyze 30-µl aliquots by SDS-PAGE. The osmotic shock procedure provides a substantial purification step for some thioredoxin fusion proteins. This procedure will remove most of the contaminating cytoplasmic proteins as well as almost all of the nucleic acids. However the shockate will contain as contaminants about half of the cellular elongation factor-Tu (EFTu) and most of the E. coli periplasmic proteins.
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PURIFICATION OF THIOREDOXIN FUSION PROTEINS BY HEAT TREATMENT
SUPPORT PROTOCOL 3
Wild-type thioredoxin is resistant to prolonged incubations at 80°C. A subset of thioredoxin fusion proteins also exhibit corresponding thermal stability, and heat treatment at 80°C can sometimes be used as an initial purification step. Under these conditions the majority of contaminating E. coli proteins are denatured and precipitated. Additional Materials For recipes, see Reagents and Solutions in this unit (or cross-referenced unit); for common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Cell pellet from 4-hr post-induction cultures (basic protocol) 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA 80°C water bath 10-ml glass-walled tube 1. Resuspend cell pellet from 4-hr post-induction cultures at a concentration of 100 OD550/ml in 20 mM Tris⋅Cl (pH 8.0)/2.5 mM EDTA. It is important to start off with a high protein concentration in the lysate to ensure efficient precipitation of denatured proteins.
2. Lyse the cells at 20,000 lb/in2 in a French pressure cell as described in steps 2 to 8 of the first support protocol. Collect whole-cell lysate in a 10-ml glass-walled tube. 3. Incubate whole-cell lysate 10 min at 80°C. Remove 100-µl aliquots after 30 sec, 1 min, 2 min and 5 min and plunge immediately into ice. At 10 min plunge the remaining heated lysate into ice. A glass-walled tube (not plastic) provides good thermal conductivity to provide a rapid rise in temperature to 80°C and then a rapid drop in temperature to 4°C. A suitable volume to use in a 10-ml glass tube is 1.5 ml lysate. For large-scale work, a glass-walled vessel should be used and the lysate should be mixed well during both heat treatment and cooling.
4. Microcentrifuge the aliquots 10 min at maximum speed, 4°C to pellet heat-denatured, precipitated proteins. 5. Remove 2-µl aliquots of the supernatants and add 28 µl SDS-PAGE sample buffer. Analyze the samples by SDS-PAGE to determine the heat stability of the fusion protein and the minimum time of heat treatment required to obtain a good purification.
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REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Casamino Acids (CAA), 2% (w/v) 20 g Casamino Acids (Difco certified) H2O to 1 liter Autoclave or filter sterilize through a 0.45-µm filter Store ≤2 months at room temperature Do not use technical-grade Casamino Acids because it has a higher NaCl content.
CAA/glycerol/ampicillin 100 medium 800 ml 2% (w/v) Casamino Acids (see recipe; 1.6% final) 100 ml 10× M9 salts (see recipe; 1× final) 100 ml 10% (v/v) glycerol (sterile; 1% final) 1 ml 1 M MgSO4(sterile; 1 mM final) 0.1 ml 1 M CaCl2 (sterile; 0.1 mM final) 1 ml 2% (w/v) vitamin B1 (sterile; 0.002% final) 10 ml 10 mg/ml ampicillin (sterile; 100 µg/ml final) Prepare fresh IMC medium 200 ml 2% (w/v) Casamino Acids (see recipe; 0.4% final) 100 ml 10× M9 salts (see recipe; 1× final) 40 ml 20% (w/v) glucose (sterile; 0.5% final) 1 ml 1 M MgSO4 (sterile; 1 mM final) 0.1 ml 1 M CaCl2 (sterile; 0.1 mM final) 1 ml 2% (w/v) vitamin B1 (sterile; 0.002% final) 658 ml glass-distilled H2O (sterile) 10 ml 10 mg/ml ampicillin (sterile; optional; 100 µg/ml final) Use fresh IMC plates 15 g agar [Difco; 1.5% (w/v)] 4 g casamino acids [Difco-certified; 0.4% (w/v)] 858 ml glass-distilled H2O(sterile) Autoclave 30 min Cool in a 50°C water bath 100 ml 10× M9 salts (see recipe; 1× final) 40 ml 20% (w/v) glucose (sterile; 0.5% final) 1 ml 1 M MgSO4 (sterile; 1 mM final) 0.1 ml 1 M CaCl2 (sterile; 0.1 mM final) 1 ml 2% (w/v) vitamin B1 (sterile; 0.002% final) 10 ml 10 mg/ml ampicillin (sterile; optional; 100 µg/ml final) Mix well and pour into Petri plates Store ≤1 month at 4°C
Expression and Purification of Thioredoxin Fusion Proteins
M9 salts, 10× 60 g Na2HPO4 (0.42 M) 30 g KH2PO4 (0.24 M) 5 g NaCl (0.09 M) 10 g NH4Cl (0.19 M) H2O to 1 liter continued
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Adjust pH to 7.4 with NaOH Autoclave or filter sterilize through a 0.45-µm filter Store ≤6 months at room temperature SDS-PAGE sample buffer 15% (v/v) glycerol 0.125 M Tris⋅Cl, pH 6.8 (APPENDIX 2) 5 mM Na2EDTA 2% (w/v) SDS 0.1% (w/v) bromphenol blue 1% (v/v) 2-mercaptoethanol (2-ME; add immediately before use) Store indefinitely at room temperature Tryptophan, 10 mg/ml Heat 500 ml glass-distilled H2O to 80°C. Stir in 5 g L-tryptophan until dissolved. Filter sterilize the solution through a 0.45 µm filter and store ≤6 months in the dark at 4°C. COMMENTARY Background Information Two significant problems plague researchers who hope to express heterologous proteins in Escherichia coli: inefficient initiation of translation of many eukaryotic mRNA sequences on bacterial ribosomes (Stormo et al., 1982), and proteins that often form insoluble aggregates, called inclusion bodies, that are composed of misfolded or denatured proteins (Mitraki and King, 1989). Although successful protocols for refolding eukaryotic proteins from inclusion bodies can be developed, the process is always uncertain and usually time-consuming; in most instances it is preferable to prevent inclusion-body formation in the first place. The use of trxA fusions provides a solution to both problems. Inefficient initiation of translation of eukaryotic messages in E. coli can often be improved by modifying sequences at the 5′ end of the gene. A more reliable technique that avoids the problem entirely is to use a gene fusion strategy in which the gene of interest is linked in-frame to the 3′ end of a highly translated partner gene. In this case protein synthesis always initiates on the same efficiently translated fusion partner mRNA, thus high-level expression is assured. Some earlier gene fusion expression systems, for example the trpE and lacZ systems described in UNIT 16.5, offer very reliable ways of producing large quantities of any desired eukaryotic protein. However, these gene fusion systems still suffer from the pervasive inclusion-body problem. They are thus mainly useful for the production of antigens, rather than correctly folded, biologically active
proteins. More recently the maltose binding protein (MBP) and glutathione-S-transferase (GST) gene fusion expression systems (see UNITS 16.6 & 16.7) have proven more successful in producing soluble fusion proteins; these systems retain the translation advantage of the earlier fusion systems. Apart from the obvious advantages in making a correctly folded product, the synthesis of soluble fusion proteins also allows for the development of generic purification schemes based on some unique property of the fusion partner. Why would any particular eukaryotic protein produced in the E. coli cytoplasm be more soluble when it is linked to a fusion partner than it would be by itself? It is likely that physical properties of the fusion partner protein are important, with efficient self-folding and high solubility being useful in this role. It is possible that some good fusion partners (proteins that fold efficiently and are highly soluble), by virtue of their desirable physical qualities, are able to keep folding intermediates of linked heterologous proteins in solution long enough for them to adopt their correct final conformations. In this respect the fusion partner may serve as a covalently joined chaperon protein, in many ways fulfilling the role of authentic chaperon proteins (McCoy, 1992), analogous to the covalent chaperon role proposed for the N-terminal pro regions of a number of protein precursors (Silen et al., 1989; Shinde et al., 1993). Many of the known properties of E. coli thioredoxin (Holmgren, 1985) suggested that it would make a particularly effective fusion partner in an expression system. First, thiore-
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Expression and Purification of Thioredoxin Fusion Proteins
doxin, when overproduced from plasmid vectors, can accumulate to 40% of the total cellular protein, yet even at these expression levels all of the protein remains soluble. Second, the molecule is small (11,675 Mr) and would contribute a relatively modest amount to the total mass of any fusion protein, in contrast to other systems such as the lacZ system. Third, the tertiary structure of thioredoxin (Katti et al., 1990) reveals that both the N- and C-termini of the molecule are accessible on the surface and in good position to link to other proteins. The structure also shows that the molecule has a very tight fold, with >90% of its primary sequence involved in strong elements of secondary structure. This provides an explanation for thioredoxin’s observed high thermal stability (Tm 85°C), and suggests that the molecule might possess the robust folding characteristics that could make it a good fusion partner protein. In support of this view, complete thioredoxin domains are found in a number of naturally occurring multidomain proteins, including E. coli DsbA (Bardwell et al., 1991), the mammalian endoplasmic reticulum proteins ERp72 (Mazzarella et al., 1990), and protein disulfide isomerase (PDI; Edman et al., 1985). These proteins can all be considered as natural precedents for thioredoxin fusion proteins. The synthesis of small peptides in E. coli is often difficult, with the products frequently being extensively degraded or insoluble. The thioredoxin tertiary structure revealed that the characteristic active site, —CGPC—, protrudes from the body of the protein as a surface loop, with few interactions with the rest of the molecule. The loop does not seem to contribute to the overall stability of thioredoxin, so the production of peptides as insertions at this site was an attractive possibility. In this location they would be protected from host-cell amino- and carboxypeptidases, and thioredoxin’s high solubility should help keep them in solution. In addition, the conformation of peptides inserted at this position would be constrained, which could be an advantage for applications in which it is desirable for the peptide to adopt a particular form. Thioredoxin has indeed proven to be an excellent partner for the production of soluble fusion proteins in the E. coli cytoplasm (LaVallie et al., 1993b). Figure 16.8.3 demonstrates the production of soluble fusion proteins between thioredoxin and eleven human and murine cytokines and growth factors using the trxA vectors. All of these mammalian proteins had been previously produced in E. coli only as
insoluble inclusion bodies. As thioredoxin fusions, the growth factors are not only made in a soluble form, but in most cases they are also biologically active in in vitro assays. Experience gained while working with these and a number of other trxA fusion proteins shows that two further characteristics of thioredoxin can be exploited as purification tools. The first is the inherent thermal stability of the molecule, a property that is retained by some thioredoxin fusion proteins. This enables heat treatment to be used as an effective purification step. The second additional property relates to thioredoxin’s cellular location. Although E. coli thioredoxin is a cytoplasmic protein, it has been shown to occupy a special position within the cell—it is primarily located on the cytoplasmic face of the adhesion zones that exist between the inner and outer membranes of the E. coli cell envelope (Lunn and Pigiet, 1982). From this location thioredoxin is quantitatively released to the exterior of the cell by simple osmotic shock or freeze/thaw treatments, a remarkable property that is retained by some thioredoxin fusion proteins, thus providing a simple purification step. A more generic method for purification of any soluble thioredoxin fusion employs a modified form of thioredoxin (called “His-patch Trx”), which has been designed to bind to metal chelate resins (E.A. DiBlasio, J.M. McCoy, and E.R. LaVallie, manuscript in preparation).
Critical Parameters Lack of protein solubility leading to inclusion-body formation in E. coli is a complex phenomenon with many contributing factors: simple insolubility as a result of high-level expression, insolubility of protein-folding intermediates, lack of appropriate bacterial chaperon proteins, and lack of glycosylation mechanisms in the bacterial cytoplasm. Fusion of heterologous proteins to thioredoxin or to other fusion partners can help address most of these solubility issues. However, another important factor contributing to inclusion body formation is the inability to form essential disulfide bonds in the reducing environment of the bacterial cytoplasm, which leads to incorrect folding. Thermal lability of even correctly folded heterologous proteins in the absence of these stabilizing disulfide cross-links is a significant problem, so the expression of fusion genes should be attempted over a wide range of temperatures, even as low as 15°C (the limit for E. coli growth is ∼8°C). Thermal denaturation is
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1
2
3
4
5
6
7
8
9
10
11
12
MW(kDa) - 97.4 - 66.2
- 45.0
- 31.0
- 21.5
- 14.4
Figure 16.8.3 Expression of thioredoxin gene fusions. The gel shows proteins found in the soluble fractions derived from E. coli cells expressing eleven different thioredoxin gene fusions. Lane 1, host E. coli strain GI724 (negative control, 37°C); lane 2, murine interleukin-2 (IL-2; 15°C); lane 3, human IL-3 (15°C); lane 4, murine IL-4 (15°C); lane 5, murine IL-5 (15°C); lane 6, human IL-6 (25°C); lane 7, human MIP-1a (37°C); lane 8, human IL-11 (37°C); lane 9, human macrophage colony-stimulating factor (M-CSF; 37°C); lane 10, murine leukemia inhibitory factor (LIF; 25°C); lane 11, murine steel factor (SF; 37°C); and lane 12, human bone morphogenetic protein-2 (BMP-2; 25°C). Temperatures in parentheses are the production temperature chosen for expressing each fusion. This is a 10% SDS-polyacrylamide gel, stained with Coomassie brilliant blue.
a time-dependent process, so it is also prudent to monitor the solubility of the expressed fusion protein over the time course of induction. A great many proteins contain distinct structural domains. For example, hormone receptor proteins usually have an extracellular ligandbinding domain, a transmembrane region, and an intracellular effector domain. Sometimes expressing these domains individually as fusion proteins can yield better results than expressing the entire protein. The exact positions chosen for boundaries of the domains to be expressed in the fusion protein are important and can be determined from a knowledge of the tertiary structure of the protein of interest, by homology comparisons with similar proteins, by limited proteolysis or other domain-mapping experiments, or empirically by generating multiple fusions that test different boundary positions. It is important to be consistent in treating samples for loading on gels. For example, using different heating conditions from one experi-
ment to the next can result in a mobility shift for the protein of interest.
Anticipated Results Thioredoxin fusion protein yields are usually in the range of 5% to 20% of total cell protein. At these expression levels a 1-liter induction culture in a shaker flask will yield ∼3 g (wet weight) of cells, 300 mg total protein, and 15 to 60 mg of thioredoxin fusion protein. The final recovered yield will depend on factors such as solubility of the fusion protein and the efficiency of downstream purification procedures.
Time Considerations From a single colony on a plate, the basic induction protocol requires an overnight growth to prepare a liquid inoculum and a 3.5-hr preinduction growth at 30°C the next day, followed by a 4-hr 37°C induction period. These times are significantly longer if lower induction temperatures are required (see Table
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16.8.1). Lysis of a sample in the French pressure cell should require ≤ 5 min, and both the heattreatment and osmotic-shock procedures require 99.9%, that are derived from these cotransfections contain the repaired virus with the target gene, thus minimizing the need to screen and plaque-purify recombinants. Several companies (Pharmingen, Invitrogen, Clontech, and Novagen; see APPENDIX 4) market linearized ORF 1629–deleted AcMNPV DNA. To further facilitate the identification of recombinants, several of these commercially available baculovirus DNAs contain the bacterial lacZ gene, which codes for β-galactosidase in lieu of the AcMNPV polyhedrin gene, thereby allowing lacZ-negative recombinants to be distinguished visually from any residual nonrecombinant viruses via a plaque assay. Nonrecombinant viruses form blue plaques on Xgal plates because they contain a functional lacZ gene, whereas recombinants form colorless, opaque plaques. Recombinant viruses can also be identified by DNA hybridization and polymerase chain reaction (PCR) amplification. Another rapid and efficient method for generating recombinant baculoviruses uses site-specific transposition to insert foreign genes by homologous recombination into a bacmid propagated in E. coli rather than in insect cells. In this case, recombinant viral DNA is isolated from individual bacterial colonies and is free of any wild-type viral DNA. Upon transfection of insect cells, recombinant virus is generated free of parental nonrecombinant virus, thereby eliminating the need for multiple rounds of plaque purification. This is the basis of the Bac-to-Bac
baculovirusexpressionsystem, which is available commercially from Invitrogen.
POSTTRANSLATIONAL MODIFICATION OF PROTEINS IN INSECT CELLS Because baculoviruses infect invertebrate cells, it is possible that the processing of proteins produced by them is different from the processing of proteins produced by vertebrate cells. Although this seems to be the case for some posttranslational modifications, it is not the case for others. For example, two of the three posttranslational modifications of the tyrosine protein kinase, pp60c-src, that occur in higher eukaryotic cells (myristylation and phosphorylation of serine 17) also take place in insect cells. However, another modification of pp60c-src observed in vertebrate cells, phosphorylation of tyrosine 527, is almost undetectable in insect cells (Piwnica-Worms et al., 1990). In addition to myristylation, palmitylation has been shown to take place in insect cells. However, it has not been determined whether all or merely a subfraction of the total recombinant protein contains these modifications. Cleavage of signal sequences, removal of hormonal prosequences, and polyprotein cleavages have also been reported, although cleavage varies in its efficiency. Internal proteolytic cleavages at arginine- or lysine-rich sequences have been reported to be highly inefficient, and alpha-amidation, although it does not occur in cell culture, has been reported in larvae and pupae (Hellers et al., 1991). In most of these cases a cell- or species-specific protease may be necessary for cleavage. Protein targeting seems conserved between insect and vertebrate cells. Thus, proteins can be secreted and localized faithfully to either the nucleus, cytoplasm, or plasma membrane. Although much remains to be learned about the nature of protein glycosylation in insect cells, proteins that are glycosylated in vertebrate cells will also generally be glycosylated in insect cells. However, with few exceptions the N-linked oligosaccharides in insect cell–derived glycoproteins are only high-mannose type and are not processed to complex-type oligosaccharides containing fucose, galactose, and sialic acid. O-linked glycosylations have been even less well characterized in Sf 9 cells, but have been shown to occur. For further information on posttranslational modifications of proteins and protein processing in insect cells, see Davidson et al. (1990), O’Reilly et al. (1992), Jarvis and Sum-
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mers (1992), Grabenhorst et al. (1993), James et al. (1995), Jarvis and Finn (1995), Davis and Wood (1995), and Ogonah et al. (1996).
STEPS FOR OVERPRODUCING PROTEINS USING THE BACULOVIRUS SYSTEM The use of the baculovirus expression system is presented in detail in UNITS 16.10 & 16.11. The following steps comprise a brief overview (also see Fig. 16.9.2). 1. Clone the gene of interest into the appropriate baculovirus expression vector and cotransfect with linearized baculoviral DNA (available from various vendors) or use transposition to create recombinant bacmid DNA in E. coli (BAC-TO-BAC system; Invitrogen). Alternatively, purify circular wild-type baculovirus DNA (UNIT 16.10, Alternate Protocol 1). 2. Cotransfect baculovirus DNA with the recombinant baculovirus plasmid or transfect purified bacmid DNA into Sf 9 insect cells (UNIT 16.10, Basic Protocol 2). 3. Collect the medium, which contains the baculoviral particles (UNIT 16.10, Basic Protocol 2), and plaque the virus on Sf 9 cells to separate recombinant from nonrecombinant virus (UNIT 16.10, Basic Protocol 4). This and subsequent rounds of plaque purification are optional when linearized virus that contains a lethal deletion is used (e.g., BaculoGold from Pharmingen), as in that case >99.9% of all amplified virus particles will be recombinant because of selection pressure. Similarly, when bacmid DNA is used, no purification is required. 4. Amplify the virus stock by infecting fresh insect cells (UNIT 16.10, Basic Protocol 3) and determine titer of the amplified virus stock (UNIT 16.10, Basic Protocol 4). 5. Express the protein of interest by infecting a new batch of insect cells with the hightiter baculovirus stock (UNIT 16.11, Basic Protocol 1). Determine the expression level of the recombinant protein of interest and analyze its biological activity (UNIT 16.11, Support Protocols 1 and 2).
CHOOSING A BACULOVIRUS TRANSFER VECTOR
Overview of the Baculovirus Expression System
The majority of available baculovirus vectors are pUC-based and confer ampicillin resistance. Most contain the polyhedrin gene promoter and insertion site(s) for cloning a foreign gene of interest, flanked by viral sequences that lie 5′ to the promoter and 3′ to the foreign gene insert. These flanking sequences facilitate ho-
mologous recombination between the vector and baculovirus DNA. Other baculovirus vectors contain the p10 promoter, another strong, very late promoter, or the basic protein promotor expressed late in the infection process. Some vectors are designed to express more than one heterologous gene or to express genes as fusions to N-terminal signal sequences or leader peptides, which facilitate secretion and purification of the recombinant protein. For more information on these vectors, see Table 16.9.1, O’Reilly et al. (1992), and relevant catalogs from Pharmingen, Clontech, Invitrogen, Novagen, and Stratagene. A major consideration when choosing the appropriate baculovirus expression vector is whether to express the recombinant protein as a fusion or nonfusion protein in insect cells. Fusion proteins containing a specific tag have the advantage of easy purification and detection. For nonfusion proteins, there are several vectors available—notably pVL1392 and pVL1393 (Pharmingen and Invitrogen)—each of which contains a polylinker in the opposite orientation from the other. These vectors are derived from pAcYM1 (Matsuura et al., 1987) and pAcCL29 (Livingston and Jones, 1989) and differ only in the order of the cloning sites. Other nonfusion vectors include pBacPAK8 and pBacPAK9 (Clontech), pBac-1 (Novagen), pFAST Bac 1 (Invitrogen), and pAcSG2 (Pharmingen). To express the protein as a polyhistidine fusion protein, there are several available vectors: pAcHLT-A, -B, and -C (Pharmingen), pBlueBacHis-A, -B, and -C and pFAST BacHT -A, -B, and -C (Invitrogen), and pBac2cp (Novagen). These vectors allow easy purification of the recombinant protein using a nickel-chelating resin (UNIT 10.11B). To express the protein as a glutathione-S-transferase (GST) fusion protein, the pAcGHLT-A, -B, and -C vectors are commercially available (Pharmingen). These produce an N-terminal GST fusion protein. As with all fusion protein vectors, the gene of interest has to be cloned in the proper reading frame with respect to the tag. Several other affinity tags, such as FLAG, HA, and myc, have also been used successfully for purification using commercially available affinity resins. These tags can be fused to coding sequences using PCR. Secretion of recombinant proteins into insect cell medium simplifies purification and characterization of expressed recombinant proteins. The ease of purification is further enhanced by the use of serum-free medium. There are several vectors that contain signal se-
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prepare insect cell (e.g., Sf 9) culture (UNIT 16.10, Basic Protocol 1)
prepare wild-type baculoviral DNA (UNIT 16.10, Alternate Protocol 1)
subclone gene of interest into baculoviral vector (UNIT 16.10)
cotransfect insect cell with wild-type viral DNA and recombinant vector (UNIT 16.10, Alternate Protocol 1)
buy linearized ORF 1629–negative baculoviral DNA
cotransfect insect cell with linearized viral DNA and recombinant vector (UNIT 16.10, Basic Protocol 2)
visually screen for recombinants by plaque assay (UNIT 16.10, Basic Protocol 4)
obtain recombinant virus by several rounds of plaque purification (UNIT 16.10, Basic Protocol 4)
purify recombinant virus by one round of plaque purification (optional; UNIT 16.10, Basic Protocol 4)
amplify recombinant virus by infecting new insect cells (UNIT 16.10, Basic Protocol 3)
analyze recombinant protein expression level (UNIT 16.11, Basic Protocol 1 and Support Protocols 1 and 2)
produce and purify recombinant protein on a large scale (UNIT 16.11, Basic Protocol 2)
Figure 16.9.2 Flow chart for the expression of proteins in insect cells using the baculovirus system. The flow chart describes the production of recombinants using circular wild-type baculovirus DNA and the more recently developed linearized ORF 1629–negative variants. The use of linearized baculovirus DNA, now widely accepted, simplifies the whole protocol and produces viral-infection stocks with little or no background of nonrecombinants, which may be used directly without plaque purification.
Protein Expression
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Table 16.9.1
Baculoviral Expression Vectorsa
Promoter type
Fusion protein
Polyhedrin locus–based Single-baculovirus promoter vectors pVL1392/1393 (pair) P pAcSG2 P pAcMP2/3 (pair) BP
VL VL L
No —c No
pAcUW21 pBacPak 8/9 (pair) pBAC-1 pBacgus-1 pBlue Bac III pAcGHLT-A, B, C (set)
P P P P P P
VL VL VL VL VL VL
No No —c —c No Yes
pAcHLT-A, B, C (set) pBac-2cp pBACgus-2cp
P P P
VL VL VL
Yes Yes Yes
pBlue Bac His, A, B, C (set) P pAc 360 P
VL VL
Yes Yes
pAcG1 pAcG2T pAcG3X BioColors BV Control (set) BioColors His (set)
P P P P P
VL VL VL VL VL
Yes Yes Yes Yes Yes
Secretory-signal vectors pAcGP67, A, B, C (set) pAcSecG2T
P P
VL VL
Yes Yes
pPbac
P
VL
Yes
pMbac
P
VL
Yes
pBAc surf-1
P
VL
Yes
Ligation-independent cloning vectors pAcSG2-LIC P pAcGST-LIC-2T P
VL VL
No No
pAcGST1-LIC
VL
No
Vector
Promoter
P
Features
Sourceb
Standard transfer vectors Recommended for large inserts Facilitates post-translational modifications Polyhedrin gene, F1 origin Standard transfer vectors F1 origin gus reporter gene lacZ gene GST and 6×His tags, thrombin cleavage site 6×His tag, thrombin cleavage site 6×His and S tags, F1 origin 6×His and S tags, F1 origin, gus reporter gene lacZ gene, 6×His tag Translational fusion with polyhedrin gene GST tag GST tag, thrombin cleavage site GST tag, factor Xa cleavage site Fusion with GFP or its variants Fusion with GFP or its variants, 6×His tag, thrombin cleavage site
PG, IN PG PG
gp67 signal sequence for secretion gp67 signal sequence for secretion, GST tag Placental AKP signal sequence for secretion Melittin signal sequence for secretion gp67 signal sequence for secretion, F1 origin LIC site for fast PCR cloning GST tag, thrombin cleavage, and LIC sites GST tag, LIC site
PG CT NG NG IN PG PG NG NG IN IN PG PG PG PG PG
PG PG SG SG NG
PG PG PG continued
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Table 16.9.1
Baculoviral Expression Vectorsa, continued
Vector
Promoter
Promoter type
Fusion protein
Features
Sourceb
gus reporter gene, 6×His and S tags, thrombin cleavage site, F1 origin, LIC site 6×His and S tags, thrombin cleavage site, F1 origin, LIC site
NG
Ligation-independent cloning vectors, continued pBACgus-2cp LIC
P
VL
No
pBAC-2cp LIC
P
VL
No
Multiple baculovirus promoter vectors pAcUW51 P, p10
VL
No
p2Bac pAcAB3 pAcAB4 pAcUW31
P, p10 P, p10 P, p10 P, p10
VL VL VL VL
No No No No
pBAC4x-1
P, p10
VL
—c
pBACgus 4x-1
P, p10
VL
—c
p10 locus-based Single-baculovirus promoter plasmids pAcUW1 p10
VL
No
Standard p10 locus vector
PG
Multiple promoter vector pAcUW32/43 (pair)
VL
No
Expression of 2 foreign genes, F1 origin
PG
Bacmid expression vectors FastBac1 P FastBacHT A,B,C (set) P
VL VL
No Yes
FastBacDUAL
VL
Yes
Bacmid expression system IN Bacmid expression system, 6×His IN tag Bacmid expression system IN
P, p10
P
Expression of 2 foreign genes, F1 origin Expression of 2 foreign genes Expression of 3 foreign genes Expression of 4 foreign genes Expression of 2 foreign genes, M13 origin Expression of 4 foreign genes, F1 origin Expression of 4 foreign genes, gus reporter gene, F1 origin
NG
PG IN PG PG CT NG NG
aAbbreviations: 6×His, six-histidine (tag); BP, basic protein (promoter); CT, Clontech; GFP, green fluorescent protein; GST, glutathione-S-
transferase (tag); gus, β-glucuronidase; IN, Invitrogen; L, late (promoter); LIC, ligation-independent cloning; NG, Novagen; P, polyhedrin; PG, Pharmingen; SG, Stratagene; S tag, peptide tag from Novagen; VL, very late (promoter). bSee APPENDIX 4 for source addresses and telephone numbers. cFusion protein optional.
quences under strong polyhedrin-promoter control that direct the nascent polypeptide chain toward the secretory pathway of the cell. The sequences to be expressed are inserted downstream with respect to the signal sequences to generate a fusion gene that is transcribed under strong polyhedrin-promoter control. The secretion of biologically active protein from insect cells is the final step in a complex pathway of posttranslational modifications performed in the endoplasmatic reticulum (ER) and the Golgi complex (GC). Proteins destined for secretion are first cotranslationally translo-
cated into the lumen of the ER, where initial steps of carbohydrate processing occur. Later, the protein is transported to the GC, where further modifications take place. During translocation, the amino-terminal leader peptide sequence is, in most cases, proteolytically removed. The major determinants for the final form of posttranslational modification of a protein are its primary structure and the conformation presented to successive processive steps. Each secreted protein will present its own characteristics and potential problems for efficient secretion in a biologically active form. In gen-
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eral, the protein produced by baculovirus-infected insect cells carries modifications that are very similar to those of the native protein. With no need for cell lysis, purification of the secreted recombinant proteins is extremely easy. For an example of a recombinant protein expressed and purified in this way, see Murphy et al. (1993). Several vectors have been developed that utilize the gp67 secretory sequence of the baculovirus envelope protein: pAcGP67-A, -B, or -C (Pharmingen) and pBac surf-1 (Novagen). Two additional vectors, pMbac and pPbac (Stratagene), have insertions, respectively, of the melittin and human placental alkaline phosphatase secretory signal sequences. Additionally, these two vectors contain a p10promoter-driven lacZ gene, allowing color selection of recombinants on Xgal plates. To improve screening of recombinant baculoviruses when using wild-type AcMNPV DNA, pBlueBacIII (Invitrogen), derived from pJVNheI (Vialard et al., 1990), was developed. pBlueBacIII has a multiple cloning site and contains two promoters—the polyhedrin promoter and the early-to-late (ETL) promoter— downstream with respect to which the lacZ gene has been inserted. As with the other baculovirus vectors, the gene of interest is cloned downstream of the polyhedrin promoter, which then controls synthesis of the recombinant protein. The recombinant virus is plaqued using agarose overlays containing 150 µg/ml Xgal; the plaques are visualized as described in Basic Protocol 4 of UNIT 16.10. Recombinant viruses generate plaques that are blue and lack occlusion bodies.
Cotransfection of the linearized baculoviral DNA with a complementing plasmid construct rescues the lethal deletion of the essential gene—ORF 1629—that lies downstream of the AcMNPV polyhedrin gene of the baculovirus DNA (see Fig. 16.9.3). Therefore, the baculovirus transfer vector must be polyhedrin-locus based to rescue this deficiency. This means that the flanking sequences of its promoter region must be derived from the polyhedrin locus of the AcMNPV wild-type virus, otherwise it will not recombine with the polyhedrin locus of linearized ORF 1629–deleted baculoviral DNA. Protocols for both methods are covered in UNIT 16.10, which also discusses a new baculovirus variant allowing the gene of interest to be cloned directly into the baculovirus genome, thus obviating the need for transfer vectors. Another method for avoiding the time-consuming purification by plaque assay is the Bacto-Bac system (Invitrogen). The gene of interest is cloned into a donor plasmid, pFastBac1, and transformed into competent E. coli cells containing a helper plasmid and a baculovirus shuttle vector (bacmid). pFastBac1 contains Tn7 sites, and is transposed into the bacmid using functions supplied by the helper plasmid in trans. The recombinant bacmid is isolated from the competent bacteria by miniprep and transfected into insect cells using a cationic lipid reagent (Cellfectin). Screening for recombinants is done in E. coli and can therefore be done much more quickly than via multiple rounds of plaque purification, as is done in generation of recombinant baculovirus using wild-type virus.
CHOOSING A BACULOVIRUS DNA
Overview of the Baculovirus Expression System
There are several methods for generating recombinant baculovirus. Initially it was required that researchers cotransfect the recombinant transfer vector with wild-type baculovirus DNA, generate a supernatant containing recombinant baculovirus, and screen out the nonrecombinant wild-type virus background through several rounds of plaque purification. This was a time-consuming process requiring technical expertise developed over a long period of time. The development of modified linearized baculovirus DNA allowed the generation of an initial viral stock containing little or no nonrecombinant virus, thus abolishing the need for plaque purification. The principle of this technique lies in the construction of a modified type of baculovirus DNA which, after linearization, contains a lethal deletion and no longer codes for any viable virus.
REAGENTS, SOLUTIONS, AND EQUIPMENT FOR THE BACULOVIRUS SYSTEM Reagents and solutions commonly used for expression of proteins using baculovirus vectors are summarized below. See APPENDIX 4 for supplier contact information. 1. Suitable insect cell lines. Sf 9 cells are derived from the ovaries of the fall armyworm (Spodoptera frugiperda) and are available from American Type Culture Collection, Pharmingen, or Invitrogen. A similar cell line, Sf21, is available from the same vendors. As an alternative to S. frugiperda cell lines, the Trichoplusia ni High Five line, derived from T. ni egg-cell homogenates, is available from Invitrogen. Several proteins have been reported to have significantly higher expression using this T. ni cell line. Additionally, High Five cells have a
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AcMNPV wild-type DNA
polyhedrin gene
p10 gene
Bsu 36I Bsu 36I
Bsu 36I uncut BaculoGold DNA
ORF 603
lacZ
ORF 1629
polyhedrin promoter
∆ORF 603
∆lacZ
∆ORF 1629
cleaved BaculoGold DNA (ORF 1629 –deleted)
Figure 16.9.3 Generation and purification of recombinant baculovirus.
rapid doubling time as adherent cultures and adapt quickly to serum-free media. 2. Fully prepared TNM-FH insect medium. This will contain trace metals, lactalbumin hydrolysate, yeastolate, 10% fetal bovine serum (FBS), and gentamicin. The medium can be purchased from Pharmingen and several other vendors. It can also be prepared from Grace’s insect-cell culture medium (available at 1× and 2× concentration in powdered or liquid form from Invitrogen). For instructions on preparing media from individual components, see O’Reilly et al. (1992). FBS is available from many vendors. Obtain and test different lots of serum from a number of suppliers. The lot that promotes the best growth rate and cell viability should be purchased in bulk. See APPENDIX 3F for additional discussion of FBS. Alternatively, a serum-free insect cell culture medium can be purchased from several vendors (BaculoGold medium from Pharmingen, Sf-900 II from Invitrogen, HYQ-CCM3 and SFX-Insect from Hyclone, or ExCell 401 from JRH Biosciences). These synthetic, lowprotein media are recommended for secreted proteins and facilitate subsequent purification. 3. Incubator at 27°C ± 1°C. CO2 is not required and humidification is optional. The Biological Oxygen Demand (B.O.D.) low-temperature incubator (VWR Scientific) or the larger Isotemp (Fisher) are good examples. 4. Magnetic spinner flasks. These are available in a variety of sizes from Techne or Bellco.
5. Stir plate for multiple spinners. This is available from Techne or Bellco. 6. SeaKem ME agarose. (FMC Bioproducts). 7. 60-mm, 100-mm, and 150-mm tissue culture plates. (Falcon or Corning). 8. Antibiotics (optional). Gentamicin (available from numerous vendors) and amphotericin B (Fungizone from Flow Laboratories) are used. 9. Microscope. Either an inverted light microscope or a dissecting microscope is required. 10. Appropriate cloning vectors. These are available from many vendors (see Table 16.9.1). Several additional vectors, a manual of methods, and wild-type baculovirus DNA are also available upon request from Dr. Max D. Summers, Department of Entomology, Texas Agricultural Experiment Station, Texas A & M University, College Station, Texas 77843 (Phone: 409-845-9730). It is necessary to sign a licensing agreement before the material will be sent. Commercial kits are available from Pharmingen, Invitrogen, Clontech, Novagen and Stratagene. 11. Linearized ready-to-use baculovirus DNA. This can be purchased from many of the same vendors. 12. Shaking incubators (90 to 150 rpm) can also be used to culture Sf9 cells. In this case, disposable polycarbonate Erlenmeyer flasks (Corning) or Fernbach flasks with screw caps
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(Bellco) should be used instead of magnetic spinner flasks.
LITERATURE CITED Carbonell, L.F., Klowden, M.J., and Miller, L.K. 1985. Baculovirus-mediated expression of bacterial genes in dipteran and mammalian cells. J. Virol. 56:153-160. Davidson, D.J.C., Fraser, M.J., and Castellino, F.J. 1990. Oligosaccharide processing in the expression of human plasminogen cDNA by lepidopteran insect (Spodoptera frugiperda) cells. Biochemistry 29:5584-5590. Davis, T.R. and Wood, H.A. 1995. Intrinsic glycosylation potentials of insect-cell cultures and insect larvae. In Vitro Cell. Dev. Biol. Animal 31:659-663. Doerfler, W. and Bohm, P. 1986. The Molecular Biology of Baculoviruses. Springer-Verlag, New York. Grabenhorst, E., Hofer, B., Nimitz, M., Jager, V., and Contradt, H.S. 1993. Biosynthesis and secretion of human interleukin-2 glycoprotein variants from baculovirus-infected insect Sf21 cells: Characterization of polypeptides and post-translational modifications. Eur. J. Biochem. 215:189-197. Hellers, M., Gunne, H., and Steiner, H. 1991. Expression and post-translational processing of preprocecropin-A using a baculovirus vector. Eur. J. Biochem. 199:435-439. James, D.C., Freedman, R.B., Hoare, M., Ogonah, O.W., Rooney, B.C., and Larionov, O.A. 1995. N-glycosylation of recombinant human interferon-γ produced in different animal expression systems. Bio/Technology 13:592-596. Jarvis, D.L. and Finn, E.E. 1995. Biochemical analysis of the N-glycosylation pathway in baculovirus-infected lepidopteran insect cells. Virology 212:500511. Jarvis, D.L. and Summers, M.D. 1992. Baculovirus expression vectors. In Recombinant DNA Vaccines: Rationale and Strategies (R.E. Isaacson, ed.) pp. 265-291. Marcel Dekker, New York. Kitts, P.A. and Possee, R.D. 1993. A method for producing recombinant baculovirus expression vectors at high frequency. BioTechniques 14:810-817. Livingston, C. and Jones, I. 1989. Baculovirus expression vector with single-strand capability. Nucl. Acids Res. 17:2366. Luckow, V.A. 1991. Cloning and expression of heterologous genes in insect cells with baculovirus vectors. In Recombinant DNA Technology and Applications (A. Prokop, R.K. Bajpai, and C. Ho, eds.) pp. 97-152. McGraw-Hill, New York. Luckow, V.A. and Summers, M.D. 1988. Trends in the development of baculoviral expression vectors. Bio/Technology 6:47-55.
Overview of the Baculovirus Expression System
Matsuura, Y., Possee, R.D., and Bishop, D.H.L. 1987. Baculovirus expression vectors: The requirements for high level expression of proteins, including glycoproteins. J. Gen. Virol. 68:12331250.
Miller, L.K. 1988. Baculoviruses as gene expression vectors. Annu. Rev. Microbiol. 42:177-199. Miller, D.W., Safer, P., and Miller, L.K. 1986. An insect baculovirus host vector for high-level expression of foreign genes. In Genetic Engineering, Vol. 8 (J.K. Setlow and A. Hollaender, eds.) pp. 277-298. Plenum, New York. Murphy, C.I., McIntire, J.R., Davis, D.R., Hodgdon, H., Seals, J.R., and Young, E. 1993. Enhanced expression, secretion, and large-scale purification of recombinant HIV-1 gp120 in insect cells using the baculovirus egt and p67 signal peptides. Protein Expr. Purif. 4:349-357. Ogonah, O.W., Freedman, R.B., Jenkins, N., Patel, K., and Rooney, B.C. 1996. Isolation and characterization of an insect cell line able to perform complex N-linked glycosylation on recombinant proteins. Bio/Technology 14:197-202. O’Reilly, D.R., Miller, L.K., and Luckow, V.A. 1992. Baculovirus Expression Vectors. W.H. Freeman, New York. Piwnica-Worms, H., Williams, N.G., Cheng, S.H., and Roberts, T.M. 1990. Regulation of pp60 c-src and its association with polyoma virus middle T antigen in insect cells. J. Virol. 64:61-68. Summers, M.D. and Smith, G.E. 1987. A manual of methods for baculovirus vectors and insect cell culture procedures. Texas Agricultural Experiment Station Bulletin No.1555. College Station, Texas. Vialard, J., Lalumiere, M., Vernet, T., Briedis, D., Alkhatib, G., Henning, D., Levin, D., and Richardson, C. 1990. Synthesis of the membrane fusion and hemagglutinin proteins of measle virus, using a novel baculovirus vector containing the β-galactosidase gene. J. Virol. 64:37-50.
KEY REFERENCE O’Reilly et al., 1992. See above. A guide assembled to aid researchers using the baculoviral expression system, containing detailed protocols for using this system effectively.
Contributed by Cheryl Isaac Murphy Aquila Biopharmaceuticals Worcester, Massachusetts Helen Piwnica-Worms Washington University School of Medicine St. Louis, Missouri Stefan Grünwald Pharmingen San Diego, California William G. Romanow Corning Costar Portsmouth, New Hampshire Nicole Francis and Hua-Ying Fan Massachusetts General Hospital Boston, Massachusetts
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Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
UNIT 16.10
This unit describes the maintenance and care of insect cell cultures as well as the generation, purification, and storage of recombinant baculoviruses. Procedures are included for maintenance and subculturing of insect cells (see Basic Protocol 1) and cotransfection of insect cells with linearized baculovirus DNA and recombinant transfer plasmid containing the gene of interest (see Basic Protocol 2). Bacmid DNA may be prepared for this procedure (see Alternate Protocol 1). In the event that the linearized virus is not available, wild-type baculovirus (AcMNPV) DNA may be used to produce recombinant baculoviruses (see Alternate Protocol 2). A procedure is also included for the generation of recombinant baculoviruses using a novel method, direct cloning (see Alternate Protocol 3), which eliminates the need to first clone the gene of interest into a baculoviral transfer vector. Preparation of baculovirus infection stocks from both monolayer and suspension cultures is also described (see Basic Protocol 3). Finally, a protocol is given for a plaque assay to be used for determining the titer of baculoviral stocks as well as for selection of recombinants and plaque purification (see Basic Protocol 4). NOTE: All reagents and equipment coming into contact with live cells must be sterile and proper sterile technique should be used accordingly. MAINTENANCE AND CULTURE OF INSECT CELLS This protocol describes how to maintain and subculture Spodoptera frugiperda (Sf 9) cells in both monolayer and suspension (spinner) cultures, in either serum-containing or serum-free medium. A culture of insect cells is begun using frozen Sf 9 cells. Cultures are maintained by subculturing and their viability is checked periodically. Aliquots of these cultures can be frozen in a liquid nitrogen freezer for long-term storage.
BASIC PROTOCOL 1
Materials TNM-FH insect medium (see recipe) containing 10% fetal bovine serum (FBS), with and without 20% (v/v) DMSO Spodoptera frugiperda (Sf 9) cells (ATCC #CRL 1711) derived from fall armyworm ovaries (also see UNIT 16.9) 70% ethanol 0.4% trypan blue stain (Life Technologies) Serum-free insect cell culture medium (BaculoGold Protein-Free Insect Medium from Pharmingen; Sf-900 II from Life Technologies; HyQ-CCM3 and HyQSFX-Insect from Hyclone; or ExCell 401 from JRH Biosciences) 60-mm tissue culture plates or 25-cm2 flasks 27°C incubator (humidification optional) Spinner culture flasks (for suspension cultures) and stir plate for multiple spinner flasks (all available from Techne or Bellco) or disposable (plastic) shaker flasks (Corning) and shaking incubator set to 27°C, 90 to 150 rpm Beckman GPR centrifuge with GH-3.7 horizontal rotor (or equivalent) Screw-top cryostat freezing vials Liquid nitrogen freezer Additional reagents and equipment for counting cells with a hemacytometer (APPENDIX 3F) Protein Expression Contributed by Cheryl Isaac Murphy, Helen Piwnica-Worms, Stefan Grünwald, William G. Romanow, Nicole Francis, and Hua-Ying Fan
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Begin culture of Sf 9 cells 1. Place 3 ml TNM-FH medium containing 10% FBS in a 60-mm tissue culture plate or 25-cm2 flask. 2. Thaw a frozen ampule of Sf 9 cells rapidly in a 37°C water bath by moving it back and forth by hand. When ampule contents are almost completely thawed, immerse ampule in 70% ethanol to sterilize the outside. 3. Break the neck of the ampule and transfer contents to 60-mm tissue culture plate or 25-cm2 flask from step 1. Rock plate gently by hand to distribute the cells evenly and incubate 2 to 3 hr at 27°C until cells have attached. 4. Remove old medium and replace with 3 ml fresh TNM-FH/10% FBS. Continue incubation, feeding culture every 3 days (by removing old medium and replacing with fresh) until cells reach confluency (form a packed monolayer). It is important to hold the plates at an angle and remove and add medium at one corner so as not to dislodge the cells from the monolayer.
Maintain and subculture monolayer cultures 5. Remove the old medium from a confluent plate or flask of Sf 9 cells and resuspend cells by gently spritzing them with medium from a pipet. 6. Count the cells using a hemacytometer designed for tissue culture cells (APPENDIX 3F). Each cell in a small square of the hemacytometer is equivalent to 104 cells/ml.
7. Seed 1–2 × 106 cells from step 5 in new 60-mm plates or 25-cm2 flasks and rock to evenly distribute the cells (or use a larger plate or flask with more medium if preparing a larger culture of cells). Add fresh TNM-FH/10% FBS to bring the volume to 3 ml. 8. Incubate at 27°C, feeding the culture every 3 days with TNM-FH/10% FBS, until the cells reach confluency. Maintain and subculture suspension cultures 9. Remove medium and resuspend cells from confluent monolayer culture as described in step 5. Count the cells using a hemacytometer (APPENDIX 3F). 10a. Seed cells in a spinner culture flask at ∼4–5 × 105 cells/ml. Incubate at 27°C with constant stirring on a stir plate set at 60 to 80 rpm. Leave the side-arm caps slightly loosened to ensure adequate aeration. 10b. Alternatively, seed cells in a shaker flask and incubate at 27°C in a shaking incubator. When initially adapting cells, shake at 80 to 90 rpm. Increase by 10 rpm each time cells are split until cultures are shaking at 150 rpm. The volume of cell suspension in the shaker flasks should not exceed 40% of the total capacity.
11. Count cells every 2 to 3 days using a hemacytometer (APPENDIX 3F). Subculture when cells reach a concentration of 2–2.5 × 106 cells/ml by transferring the appropriate number of cells to a new flask containing fresh TNM-FH/10% FBS to achieve a final density of 4–5 × 105 cells/ml. Alternatively, pour out the appropriate volume of cell suspension and replace it with fresh medium. Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
12. Determine cell viability by adding 0.1 ml of 0.4% trypan blue to 1 ml log-phase cells and examining the cells under a microscope at low power. Count the number of cells that take up trypan blue (dead cells) and count the total number of cells, then calculate the percentages of dead cells and viable cells.
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A healthy culture of cells should be >97% viable. To maintain a sufficient transfer of oxygen to the cells in suspension, a minimum ratio of surface area to volume of culture must be maintained (Maiorella et al., 1988). If this value decreases, the cells will not grow exponentially and will stop growing at a lower cell density. This ratio is adequate for a 100-ml culture grown in a 100-ml spinner flask, but decreases when larger spinner flasks are used at maximum volume. Thus, to maintain sufficient oxygen transfer, smaller volumes should be used in the larger flasks unless an outside source of air is introduced into the flask (UNIT 16.11).
Adapt cells to serum-free medium 13. Subculture monolayer cells (from step 5) or suspension cells (from step 11) into medium composed of one part complete TNM-FH/10% FBS and one part serum-free medium (BaculoGold, Sf-900 II, or ExCell 401). Allow cells to grow to confluency (monolayer cultures) or to a density of 2–3 × 106 cells/ml (suspension cultures). Other commercially available serum- or protein-free media besides BaculoGold, Sf-900 II, and ExCell 401 may be used. The final choice of serum-free medium should be based on a comparison of cell growth curves and production of recombinant protein in different media.
14. Repeat the subculture and growth procedure as in step 13 using a medium composed of one part FBS-containing complete medium and three parts serum-free medium. 15. Repeat the subculture and growth procedure as in step 13 using a medium composed of one part FBS-containing complete medium and between 7 and 9 parts serum-free medium. 16. Subculture the cells into serum-free medium. Cells may adapt slowly to serum-free medium and may require several passages before growth rates and viability return to normal.
Freeze cells 17. Count cells to be frozen from an exponentially growing culture using a hemacytometer (APPENDIX 3F). 18. Centrifuge cells 10 min at 1000 × g (2000 rpm in a GH-3.7 rotor), room temperature, and discard supernatant. 19. Resuspend cell pellet at 1–2 × 107 cells/ml in TNM-FH/10% FBS. Add an equal volume of TNM-FH/10% FBS containing 20% DMSO and place cells on ice. Dispense 1-ml aliquots of this cell suspension into screw-top cryostat freezing vials and incubate 1 hr at −20°C, then overnight at −70°C. Alternatively, cells can be frozen using serum-free medium (with and without DMSO) if the cells have been adapted to serum-free medium as described in steps 13 to 16.
20. Transfer frozen cells to a liquid nitrogen freezer for long-term storage. COTRANSFECTION OF INSECT CELLS USING LINEARIZED BACULOVIRAL DNA
BASIC PROTOCOL 2
There are several protocols available for preparing baculoviral supernatant, including transfection with bacmid DNA, transfection with linearized baculoviral DNA, and cotransfection with wild-type baculoviral DNA. Reagents and protocols for these methods are available commercially, and are summarized below. In general, the authors have found that preparation of bacmid DNA is the simplest means of generating baculovirus (Bac-to-Bac system, Invitrogen; see Alternate Protocol 1); however, cotransfection with linearized baculoviral DNA is still also used widely.
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One of the most commonly used methods of introducing baculovirus and transfer-plasmid DNA into susceptible insect cells is to coprecipitate the DNA with calcium phosphate and present the mixture to insect cells. For cotransfection, prepare ≥10 µg of purified plasmid DNA. Care must be taken that the plasmid be as clean as possible. With impure plasmids, cells may lyse shortly after transfection, resulting in a lower viral titer. At ∼24 hr post-transfection, Sf 9 cell viability should be greater than 97%. It should be noted that this protocol is optimized for use with Pharmingen linear DNA. Clontech and Invitrogen (see APPENDIX 4) have their own protocols for use with the linear DNA that they offer. Materials Spodoptera frugiperda (Sf 9) cells growing in tissue culture at 50% to 70% confluence or growing in suspension culture at 1–1.5 × 106 cells/ml (see Basic Protocol 1) TNM-FH insect medium (see recipe) containing 10% fetal bovine serum (FBS) Linearized ORF 1629–deleted AcMNPV DNA (e.g., BaculoGold from Pharmingen; see Fig. 16.9.3) Recombinant baculovirus transfer vector containing gene of interest (UNIT 16.9) Transfection buffer B (see recipe) Control transfer vector: pVL1392-XylE (Pharmingen) Transfection buffer A (see recipe) 500 mM catechol/50 mM sodium bisulfate 60-mm tissue culture plates 27°C incubator (humidification optional) Inverted microscope 15-ml conical centrifuge tubes Beckman GPR centrifuge with GH-3.7 horizontal rotor (or equivalent), 4°C Additional reagents and equipment for amplification of viral supernatants to produce baculoviral stocks (see Basic Protocol 3) and plaque assay of baculovirus (see Basic Protocol 4) Prepare cells and DNA 1. In each of three 60-mm tissue culture plates seed 2 × 106 Sf 9 cells in TNM-FH medium containing 10% FBS. Incubate in a 27°C incubator until cells attach. Cell attachment should be done on a flat and even surface, allowing the cells to attach firmly, which usually takes ∼5 min. If cells do not attach after that time, either they are not healthy or the wrong plates (e.g., petri dishes that have not been tissue culture treated) have been used. The quality of the insect cells is very important and only rapidly dividing cells should be used.
2. While cells are attaching, combine in a microcentrifuge tube 0.5 µg linearized ORF 1629–deleted AcMNPV DNA and 2 to 5 µg recombinant baculovirus transfer vector containing the gene of interest. Mix well by gently vortexing or flicking the tube. Let mixture sit 5 min, then add 1 ml transfection buffer B.
Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
3. Prepare positive control by combining, in a microcentrifuge tube, 0.5 µg linearized ORF 1629–deleted AcMNPV DNA and 2 µg pVL1392-XylE control transfer vector DNA. Mix well by gently vortexing or flicking the tube. Let mixture sit 5 min, then add 1 ml transfection buffer B.
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4. Label the first plate (from step 1) as the cotransfection plate. Aspirate old medium and replace with 1 ml transfection buffer A, making sure that the entire surface of the plate is covered to prevent the cells from drying out. 5. Label the second plate (from step 1) as the positive control. Aspirate old medium and replace with 1 ml of transfection buffer A, as in step 4. 6. Label the third plate (from step 1) as negative control. Aspirate old medium and replace with 3 ml fresh TNM-FH/10% FBS, without adding any DNA. Transfect cells 7. Add 1 ml of the solution prepared in step 2 (containing the vector with the gene of interest) drop-by-drop to the cotransfection plate. After every three to five drops, gently rock the plate back and forth to mix the drops with the medium. During this procedure, a fine calcium phosphate/DNA precipitate should form. The quality of the precipitate that is optimal for transfection can be assessed visually. It should be of a fine white milky appearance.
8. Add 1 ml of the solution prepared in step 3 (containing the positive control vector) drop-by-drop to the positive control plate, repeating the procedure in step 7. 9. Incubate all three plates 4 hr in a 27°C incubator. The time of exposure to the calcium precipitate is critical for optimal transfection results. If the incubation time is too long, cell viability will be dramatically reduced. For different cell lines, the optimal incubation time varies. For Sf 9 cells, the optimal time is 4 hr.
10. After 4 hr, remove the medium from the cotransfection plate and the positive control plate (but not the negative control plate). Add 3 ml fresh TNM-FH/10% FBS to each plate, rock the plate back and forth several times, then remove all the medium again. Add 3 ml of fresh TNM-FH/10% FBS to each plate and incubate all three plates 4 to 5 days at 27°C. It is not necessary to change the medium of the negative control plate.
Check for successful transfection 11. After 4 days, check the three plates for signs of infection using an inverted microscope. Compare the negative and positive controls to the cotransfection plate. Infected cells are much larger than uninfected cells and have enlarged nuclei. Because they stop dividing early in infection, their cell density will be much lower as compared to the uninfected population. Furthermore, infected cells do not attach well to the plate and a high percentage of them will float in the medium. Many of these infection signs may not be visible at this time because the virus titer during the cotransfection is usually low. A further amplification step (see below) may be needed to visualize these changes.
12. After 5 days, collect the supernatants of the cotransfection and positive control plates. Determine viral titer by plaque screening (see Basic Protocol 4). Alternatively, cotransfection efficiency may by assessed by endpoint dilution assay (see Basic Protocol 4, step 7 annotation).
13. Check the expression of the protein of interest by lysing the transfected cells (for recombinant proteins that are not secreted) or using an aliquot of the supernatant (for recombinant proteins that are secreted), and performing an appropriate assay. Unless a sensitive assay is available for the protein of interest, expression of the recombinant protein may not be detectable at this stage. Protein Expression
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14. Assay for cells expressing the XylE protein in the positive control plate by adding 100 µl of 500 mM catechol/50 mM sodium bisulfate. Infected cells expressing XylE protein will turn bright yellow in ∼5 min.
15. Transfer the transfection supernatants from each plate to sterile conical 15-ml centrifuge tubes and centrifuge 10 min at 1000 × g (2000 rpm in GH-3.7 rotor), 4°C. Transfer viral supernatant to new sterile tubes and store at 4°C in the dark. 16. Amplify viral supernatant to produce a high-titer virus stock for production of the recombinant protein by infection of insect cells (see Basic Protocol 3). Alternatively, a single recombinant virus, obtained by plaque purification (see Basic Protocol 4), may be used for virus amplification. ALTERNATE PROTOCOL 1
GENERATION OF RECOMBINANT BACULOVIRUS BY PREPARATION AND TRANSFECTION OF BACMID DNA USING THE BAC-TO-BAC SYSTEM The Bac-to-Bac manual from Invitrogen provides detailed protocols for preparation and transfection of bacmid DNA. The following is a brief overview of the protocol starting from the cDNA being cloned into a pFastBac vector. All protocols and reagents are available from Invitrogen (http://www.invitrogen.com). Materials cDNA of interest (e.g., UNIT 5.5) Bac-to-Bac Baculovirus Expression System (Invitrogen) including: pFastBac plasmid vector and control plasmid DH10Bac competent E. coli cells Bac-to-Bac manual (available online at http://invitrogen.com/content/sfs/ manuals/bactobac_man.pdf) Cellfectin lipid transfection reagent Selection plates for transposition (see recipe) Bacmid selection medium (see recipe) Sf9 cells in serum-free medium Serum-free medium 27°C incubator 6-well tissue culture plates Additional reagents and equipment for cloning DNA into plasmids (UNIT 3.16), introduction of plasmid DNA into E. coli cells (UNIT 1.8), growing E. coli on solid (UNIT 1.3) and in liquid (UNIT 1.2) medium, alkaline lysis miniprep (UNIT 1.6), and preparation of insect cells for transfection (see Basic Protocol 2, steps 1 to 6, in this unit), and amplification of recombinant baculovirus to produce stocks (see Basic Protocol 3 in this unit) 1. Transpose cDNA into bacmid to create recombinant bacmid as follows (also see Bac-to-Bac manual). a. Clone the cDNA of interest into the pFastBac plasmid vector (also see UNIT 3.16). b. Transform plasmid DNA into DH10Bac competent cells (also see UNIT 1.8).
Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
The DH10Bac competent cells carry the bacmid DNA and helper plasmid (encoding proteins required for transposition).
c. Incubate 4 hr at 37°C to allow transposition of the gene of interest into the bacmid.
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2. Grow colonies (also see UNIT 1.2) for 24 hr on selection plates for transposition. Identify recombinant bacmid-containing clones by blue-white selection. Transposition disrupts the bacmid lacZα gene, enabling selection via Bluo-Gal, which is included in the selection plates. Also see UNIT 1.4.
3. Pick colonies and culture 24 hr in bacmid selection medium. 4. Prepare bacmid DNA by alkaline lysis miniprep (UNIT 1.6). 5. Prepare Sf9 cells (see Basic Protocol 2, steps 1 to 6, in this unit). Use Cellfectin lipid transfection reagent to transfect cells with the bacmid DNA (see Bac-to-Bac manual), then incubate for 3 days at 27°C. 6. Collect transfection supernatant (containing recombinant baculovirus) and amplify to produce baculovirus stock (see Basic Protocol 3). GENERATION OF RECOMBINANT BACULOVIRUS USING WILD-TYPE BACULOVIRAL DNA
ALTERNATE PROTOCOL 2
As an alternative to linearized ORF 1629–deleted baculoviral DNA, circular wild-type AcMNPV DNA can be used for contransfection of insect cells with baculoviral transfer plasmids (also see Basic Protocol 2). However, recombination efficiency is dramatically lower (usually around 0.1% to 0.2%) as compared to that obtained with linearized DNA. The technique described here thus requires the identification and purification of recombinants by multiple rounds of plaque assay. The following protocol describes how to isolate and purify AcMNPV wild-type baculoviral DNA, which can then be used to cotransfect susceptible insect cells (e.g., Sf9) with an appropriate plasmid vector to generate recombinant baculoviruses. The cotransfection of wild-type baculovirus DNA and recombinant transfer plasmid should be performed as described in Basic Protocol 2 (using wild-type baculoviral DNA instead of linearized ORF 1629–negative AcMNPV DNA). To facilitate screening between wild-type and recombinant viral plaques, pBlueBacIII (Invitrogen) or pVL1393-XylE (Pharmingen) can be used as a positive control, making it possible to visualize differences between nonrecombinant wild-type viral plaques and recombinant viral plaques. Wild-type baculovirus available upon request from Dr. Max D. Summers, Department of Entomology, Texas Agricultural Experiment Station, Texas A & M University, College Station, Texas 77843 (Phone: 409-845-9730). Additional Materials (also see Basic Protocol 2) Wild-type baculovirus (Dr. Max D. Summers, see above, or Pharmingen) Sucrose cushion solution (see recipe) 0.1× and 1× TE buffer, pH 7.4 (APPENDIX 2) 25% and 56% (w/v) sucrose (ultrapure) in 0.1× TE buffer (filter sterilize and store up to 1 month at 4°C) Extraction buffer (see recipe) 10 mg/ml proteinase K (prepare fresh) 10% N-lauroylsarcosine (sodium salt; filter sterilize and store up to 1 year at 4°C) 25:24:1 phenol/chloroform/isoamyl alcohol (UNIT 2.1) 100% (ice-cold) and 70% (room temperature) ethanol 3 M sodium acetate, pH 5.2 (APPENDIX 2) Protein Expression
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150-mm tissue culture dishes 50-ml conical centrifuge tubes Beckman GPR centrifuge with GH-3.7 horizontal rotor (or equivalent), 4°C Beckman ultracentrifuge with SW-27 or SW-28 rotor and SW-41 rotor (or other ultracentrifuge with equivalent rotors), 4°C, and appropriate tubes Sucrose gradient maker 15-ml polypropylene centrifuge tubes 50°C water bath 5- or 10-ml wide-mouth pipets Additional reagents and equipment for phenol/chloroform extraction and ethanol precipitation of DNA (UNIT 2.1) and quantitating DNA by absorbance spectrometry (APPENDIX 3D) Prepare viral supernatant and collect virus 1. Seed at least ten 150-mm plates with 2.0 × 107 Sf 9 cells/plate in 30 ml TNM-FH medium containing 10% FBS. Incubate 1 hr at 27°C to allow the cells to attach firmly, then infect them with AcMNPV wild-type virus at a multiplicity of infection (MOI) of 0.1. Incubate 3 to 5 days at 27°C, examining plates periodically with an inverted microscope for the presence of occlusion bodies. Dr. Max D. Summers provides a manual with his materials that includes details for these procedures. Pharmingen provides similar documentation with its wild-type baculovirus. Also see O’Reilly et al. (1992). MOI is equal to plaque-forming units (pfu; see Basic Protocol 4) divided by the number of cells (pfu/cell). If the supplier of the virus does not provide adequate information regarding pfu, this can be determined by plaque assay (see Basic Protocol 4). Occlusion bodies are highly refractile, giving them a yellowish-green crystalline appearance that is readily detected under a light microscope.
2. When occlusion bodies are observed in most cells, pool the viral supernatant (∼30 ml per plate) in six 50-ml conical tubes. Centrifuge 10 min at 1000 × g (2000 rpm in GH-3.7), 4°C, then pour viral supernatant into six new tubes. Repeat centrifugation to completely remove any remaining cells. Sterile technique is not required in this step nor for any of the remaining steps of this protocol.
3. Place 6 to 35 ml of viral supernatant in each of an appropriate number of ultracentrifuge tubes for the SW-27 or SW-28 rotor and balance the tubes. Underlay with 3 ml sucrose cushion solution. Precool the ultracentrifuge and rotor to 4°C. The amount of sucrose cushion may have to be increased for larger volumes of viral supernatant.
4. Centrifuge 60 min at 100,000 × g (24,000 rpm in an SW-27 or -28 rotor), 4°C, to pellet the virus. Pour off supernatants and invert tubes on a Kimwipe to drain as much liquid as possible.
Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
Separate virus from cellular contaminants (if necessary) 5. Examine the viral pellets carefully. If the viral pellet is pure (i.e., has an light bluish appearance), proceed to DNA isolation (step 11). If the pellet appears yellowish, separate the virus from cellular contaminants by steps 6a to 10a or 6b to 10b.
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To purify viral pellet by sucrose-gradient fractionation 6a. Add 2 ml of 0.1 × TE buffer to one of the viral pellets and repeatedly pipet up and down with a Pasteur pipet to resuspend. Transfer the buffer with the resuspended virus to a second tube containing a pellet and repeat the resuspension, then repeat in turn for each pellet until all pellets are pooled in the same 2 ml. If pellet is difficult to resuspend, incubate preparation overnight at 4°C.
7a. Place 25% and 56% sucrose solutions in 0.1× TE buffer in the reservoirs of a gradient maker and prepare two linear 25% to 56% sucrose gradients in SW-41 ultracentrifuge tubes. If a gradient maker is unavailable, simply layer the 25% sucrose carefully atop the 56% sucrose to form a step gradient. Some investigators report that a step gradient gives a sharper band.
8a. Carefully layer 1 ml of the pooled viral suspension on top of each sucrose gradient. Centrifuge 90 min at 100,000 × g (28,000 rpm in an SW-41 rotor), 4°C. After centrifugation, virus should be visible as a broad bluish-white band inside the gradient.
9a. Using a Pasteur pipet, transfer the viral bands to a new SW-41 ultracentrifuge tube. Add enough 0.1× TE buffer to fill the tube (∼35 ml), then centrifuge 30 min at 100,000 × g (28,000 rpm in an SW-41 rotor), 4°C, to pellet the virus. Decant supernatant and invert tube on a Kimwipe to drain as much liquid as possible. 10a. Resuspend the virus pellet in 9 ml extraction buffer and transfer 4.5-ml aliquots to two 15-ml polypropylene centrifuge tubes. Proceed to step 11. To purify viral pellet by microcentrifugation 6b. Add 3 ml extraction buffer to one of the viral pellets and repeatedly pipet up and down with a Pasteur pipet to resuspend. Transfer the buffer with the resuspended virus to a second tube containing a pellet and repeat the suspension, then repeat in turn for each pellet until all pellets are pooled in the same 3 ml. If pellet is difficult to resuspend, incubate preparation overnight at 4°C.
7b. Transfer 1.5 ml viral suspension into each of two 1.5-ml microcentrifuge tubes. Microcentrifuge 5 min at maximum speed and pool supernatants in one 15-ml polypropylene centrifuge tube. 8b. Resuspend each pellet in 1 ml extraction buffer. 9b. Microcentrifuge the pellets 5 min at maximum speed and combine the two supernatants with the pooled supernatants in the 15-ml tube. 10b. Bring volume in the 15-ml tube to 9 ml with extraction buffer and transfer 4.5-ml aliquots to two new 15-ml polypropylene centrifuge tubes. Proceed to step 11. Isolate DNA from purified virions 11. Add 200 µl of 10 mg/ml proteinase K to each tube and incubate 1 to 2 hr at 50°C. 12. Add 0.5 ml of 10% N-lauroylsarcosine to each tube and incubate 2 hr or overnight at 50°C. 13. Extract DNA twice with an equal volume of 25:24:1 phenol/chloroform/isoamyl alcohol (see UNIT 2.1 for additional details on extraction of DNA). Extreme care should be taken to be as gentle as possible to avoid shearing the DNA at this point. Use a wide-bore Pasteur pipet and mix DNA solutions by inverting the tubes rather than by vortexing.
Protein Expression
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14. Transfer the aqueous phase containing the DNA to another 15-ml tube using a wide-mouth 5- to 10-ml pipet. Add 10 ml of ice-cold 100% ethanol to each tube and mix gently by inverting the tubes several times. Incubate 10 min at −80°C (see UNIT 2.1 for additional details on ethanol precipitation of DNA). 15. Centrifuge 20 min at 1500 × g (2500 rpm in GH-3.7 rotor), 4°C, and discard supernatant. Rinse the DNA pellet with 70% ethanol and air dry pellet for 30 to 60 min. Resuspend pellet in 800 µl of 1× TE buffer. 16. Transfer 400 µl of the resuspended DNA to each of two microcentrifuge tubes and reprecipitate the DNA by adding 40 µl of 3 M sodium acetate and 2 vol of ice-cold 100% ethanol to each tube. Incubate 10 min at −80°C. 17. Microcentrifuge 10 min and discard supernatant. Rinse DNA pellet with 70% ethanol and air dry pellet. Resuspend DNA in 0.3 to 1.0 ml of 1× TE buffer. 18. Quantitate DNA by measuring A260 (APPENDIX 3D) and calculate yield. Store the circular wild-type baculoviral DNA at 4°C (stable for several months). This method should yield 50 to 100 ìg viral DNA per ten 150-mm dishes. If difficulty is encountered resuspending the DNA, heat mixture for ∼15 min at 65°C.
19. Cotransfect Sf 9 cells with the circular wild-type baculoviral DNA and a baculoviral transfer plasmid containing a gene of interest as in Basic Protocol 2, substituting the wild-type DNA for the linearized ORF 1629–deleted DNA in step 2 of that protocol. Recombination efficiency is dramatically lower with wild-type DNA as compared to that obtained with linearized DNA. The technique described here thus requires the identification and purification of recombinants by multiple rounds of plaque assay (see Basic Protocol 4). ALTERNATE PROTOCOL 3
Maintenance of Insect Cell Cultures and Generation of Recombinant Baculoviruses
GENERATION OF RECOMBINANT BACULOVIRUSES BY DIRECT CLONING This protocol describes the generation of recombinant virus by direct cloning methods (see Fig. 16.10.1), which may be applicable to the production of high-diversity expression libraries in baculovirus. Two modified AcMNPV baculoviruses—vEHuni and vECuni— have been constructed that have two Bsu36I sites downstream, respectively, of the hsp70 promoter and the synthetic promoter PcapminXIV (Lu and Miller, 1996). Cleavage of the Bsu36I sites produces overhanging TTA ends, which are filled in by incubation with the Klenow fragment of DNA polymerase I in presence of dTTP, thus leaving a TT overhang. Ready-to-use vEHuni and vECuni baculoviral DNA can be purchased from Pharmingen. The gene to be cloned must be flanked by EcoRI sites and the EcoRI ends produced by cleavage at these sites have to be partially filled in with Klenow fragment in the presence of dATP to leave AA overhangs. The gene can then be cloned directly into the vEHuni or vECuni genome. Additional Materials (also see Basic Protocol 2) Purified vector containing gene of interest, flanked by EcoRI sites Klenow fragment of DNA polymerase I (UNIT 3.5) Reaction buffer for Klenow fragment (see recipe) 25:24:1 phenol/chloroform/isoamyl alcohol (UNIT 2.1) 1× TE buffer (APPENDIX 2) vEHuni or vECuni (Pharmingen), linearized, partially filled in by Klenow fragment treatment, and containing TTA ends T4 DNA ligase (UNIT 3.14) 15°C water bath Additional reagents and equipment for agarose gel electrophoresis (UNIT 2.5A) and phenol/chloroform extraction and ethanol precipitation of DNA (UNIT 2.1)
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Bsu36I promoter
Bsu36I
CCTAAGG CCTTAGG MCS GGATTCC GGAATCC
vector GAATTC CTTAAG
AcMNPV genome
gene
GAATTC CTTAAG
+ Bsu36I EcoRI promoter
TTAGG CC
CC GGATT
+ EcoRI
AcMNPV genome
+ dTTP + Klenow fragment promoter
AATTC G
AcMNPV genome
gene
G CTTAA
+ dATP + Klenow fragment
TTAGG TCC
CCT GGATT
EcoRI
AATTC AAG
gene
GAA CTTAA
ligate promoter
CCTAAT TC GGATTAAG
gene
GAATTAGG CTTAATCC
AcMNPV genome
Figure 16.10.1 Direct cloning of a gene of interest into baculovirus DNA. At the left is a diagram representing an AcMNPV recombinant containing two different Bsu36I sites. Digestion of this viral DNA with Bsu36I followed by a partial fill-in reaction with dTTP and Klenow fragment of DNA polymerase I generates a linear viral DNA with TT overhanging ends. At the right, a foreign gene with flanking EcoRI sites is digested with EcoRI to generate overhanging ends, which are then partially filled in using dATP. The resulting AA overhanging ends are then compatible with the TT overhanging ends of the viral DNA. The viral DNA and foreign gene DNAs are then combined, ligated, and transfected into insect cells.
1. Digest vector containing gene of interest with EcoRI in a total volume of 50 µl for 16 hr at 37°C and isolate gene of interest that contains flanking EcoRI sites by agarose gel electrophoresis (UNIT 2.5A). If gene of interest does not contain flanking EcoRI sites, a polymerase chain reaction with specific primers containing EcoRI sites can be used to insert flanking EcoRI sites. Alternatively, EcoRI linkers can be ligated to the purified gene.
2. In a total volume of 50 µl, incubate up to 5 µg of EcoRI-digested and purified gene of interest with 5 U Klenow fragment in reaction buffer for Klenow fragment for 20 min at 37°C. 3. Extract reaction mix with 1 vol of 25:24:1 phenol/chloroform/isoamyl alcohol, then ethanol precipitate DNA and resuspend in 10 µl of 1× TE buffer (UNIT 2.1). 4. In a total volume of 50 µl, mix 0.5 µg vEHuni or vECuni DNA with 0.1 to 1 µg of the treated gene fragment from step 3 (to obtain an ∼1:60 molar ratio of baculoviral DNA to gene fragment), and add 2 U T4 DNA ligase. Incubate overnight at 15°C. The ligated AcNPV DNA is now ready to be transfected into susceptible insect cells. Protein Expression
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5. Transfect the entire ligation mixture from step 4 into 2 × 106 Sf 9 cells as described in Basic Protocol 2. The ligation mixture from this protocol replaces the reaction mixture composed of ORF 1629–deleted AcMNPV DNA and recombinant baculovirus transfer vector used in step 2 of Basic Protocol 2. As in Basic Protocol 2, use pVL1392-XylE as a control. To purify recombinant plaques obtained by direct cloning, see Basic Protocol 4. BASIC PROTOCOL 3
PREPARATION OF BACULOVIRUS STOCKS This protocol describes how to prepare a large-scale stock of wild-type or recombinant AcMNPV virus from either monolayer or suspension culture. Growing insect cells (Sf 9) are infected with virus at a low MOI (100 ml), an MOI of 1 or 2 is sufficient. The virus is stable for 6 months at 4°C but must be shielded from light to maintain the titer (Jarvis and Garcia, 1994). Placement of 1 ml of each stock in liquid
nitrogen is recommended for long-term storage. It is highly recommended that linear wildtype DNA be used for cotransfection or that bacmid DNA be prepared in E. coli and used for transfection. This greatly simplifies the screening procedure and saves time in obtaining a pure recombinant viral stock. If circular wild-type DNA is used, it is important to purify the recombinant viral DNA through several rounds of plaque assay. Degradation by mechanical shearing and by nuclease contamination are the major problems encountered when trying to purify viral DNA from intact virions. Use of wide-mouth 5- or 10-ml pipets or Pasteur pipets with the necks broken off to handle the viral DNA helps to avoid shearing. In addition, tubes containing viral DNA should be mixed by inversion rather than by vortexing when performing phenol extractions and ethanol precipitations, and the purified DNA should be stored at 4°C rather than frozen and thawed. It is very useful to practice visualizing recombinant plaques using the β-Gal recombinant virus as described in Basic Protocol 4. As mentioned, screening for recombinants is the most time-consuming and variable part of the entire expression system. The cell density at the time of plaquing seems to be the most critical parameter for achieving good plaques in the shortest period of time. Thus, it is helpful to seed duplicate plates at a couple of different cell densities. A perfect plaque assay will yield easily visualized plaques within 6 to 8 days.
Anticipated Results Maintenance and culture of insect cells When grown at 27°C, healthy insect cells should double every 18 to 24 hr. Healthy, logarithmically growing cells will be maintained in suspension cultures most readily when they are cultured at densities between 5 × 105 and 2 × 106 cells/ml. Cell viability should be ∼97%. Cotransfection After 5 days, 20% to 100% of the cells should be showing signs of infection. Infected cells are of increased size with enlarged nuclei. They stop dividing and do not attach well to the plate. Many of the signs of infection may not be obvious if the transfection efficiency results in a low viral titer. In this case, it is necessary to amplify the virus in order to verify the success of the cotransfection. When generating recombinant baculovirus using wild-type bacu-
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loviral DNA (see Alternate Protocol 2), the recombination frequency will be 0.1% to 0.2%. This method requires selection based upon identification of viral occlusion bodies. The direct cloning method (see Alternate Protocol 3) results in ∼99% recombinant virus with ∼50% containing the foreign gene in the correct orientation. Plaques can generally be identified within 5 to 10 days post-infection and counted to determine viral titers. When individual plaques are picked to isolate recombinant virus, the resulting viral titer is generally low (5 × 105 pfu/ml) and will need to be amplified several times to produce a high-titer stock.
Time Considerations Purification of wild-type viral DNA takes ∼7 days to complete. Subcloning and largescale production of recombinant DNA should take 1 to 2 weeks. The transfection procedure takes 1 day, and the transfection supernatant is harvested after 4 to 5 days. Plaquing takes l day, and it generally takes 1 week before the plaques are sufficiently formed to begin screening. Screening takes an additional day. It takes 4 to 5 days from the time of infecting Sf 9 cells with a plaque to the time of harvesting the culture supernatant for an expanded virus stock. Purifying recombinants takes from 1000 Ci/mmol; Du Pont NEN) Additional reagents and equipment for one-dimensional SDS-PAGE (UNIT 10.2) and autoradiography (APPENDIX 3A) 1. Seed 2.5 × 106 cells into 60-mm tissue culture plates containing 3 ml TNM-FH medium with 10% FBS. Prepare one plate to be infected with each putative recombinant virus and one control plate to be infected with wild-type baculovirus. 2. Incubate 1 hr at 27°C to allow cells to attach. Aspirate medium, then add 1 ml recombinant virus or 1 ml medium containing wild-type virus at an MOI of 5 to 10. Incubate 1 hr at room temperature. 3. Remove the medium from each plate by aspiration. Add 3 ml TNM-FH/10% FBS medium to the cells and incubate 24 to 48 hr at 27°C. 4. Carefully remove medium from each plate, then rinse cells once with methionine-free or methionine-free/cysteine-free medium. Add 1 ml methione-free or cysteine-free medium to each plate. Incubate cells 30 min at 27°C, then add 0.25 to 0.5 µCi EXPE35S35S per plate and incubate 3 to 4 hr at 27°C. It is important to hold the plates at an angle and remove and add medium at one corner so as not to dislodge the cells from the monolayer.
5. Transfer cells and culture supernatant from each plate to a separate 15-ml polypropylene centrifuge tube and centrifuge 10 min at 1000 × g (2000 rpm in a GH-3.7 rotor), 4°C. 6. Process and analyze the cells (if the recombinant protein is not secreted) or the supernatant (if the recombinant protein is secreted) according to the appropriate procedures in Basic Protocol 1 (see Basic Protocol 1, steps 5a or 5b, 6a or 6b, and 7). If an antibody is available, it is recommended that the labeled lysates be immunoprecipitated (UNIT 10.16) prior to boiling in SDS sample buffer and resolution by SDS-PAGE. Immunoprecipitation will help detect the recombinant protein if it comigrates with a labeled host or late-viral-specific protein. Purification of Proteins Using Baculovirus
Polyhedra (UNIT 16.9) are solubilized only under very alkaline conditions (0.1 M final NaOH concentration). Without prior disruption under alkaline conditions, 97% viability can be achieved. Suspension cultures are also preferred when growing large quantities of cells (e.g., to prepare large viral stocks or large
quantities of recombinant protein), because the entire process can be carried out in one flask. For small-scale procedures, by contrast, monolayer cultures are preferred because they seem less susceptible to contamination and can more easily be assessed to monitor the progress of viral infections. Monolayer cultures are absolutely required for transfections as well as for plaquing viral supernatants, and they are also preferred for preparing viral stocks or producing recombinant protein.
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Most cell lines are readily adaptable to serum-free medium when in suspension. The use of serum-free medium has several advantages. First, cells can be grown to higher cell densities in serum-free medium. Second, serum-free medium is more consistent and less expensive than serum-supplemented medium. In addition, serum-free medium has a low protein content, which aids in the purification of secreted recombinant proteins. It should be determined empirically which cell line and medium formulation is most productive for the particular recombinant protein of interest. Spodoptera frugiperda (Sf 9, Sf 21) cells grown in serumfree medium are more sensitive to centrifugation, stick more tightly to plasticware, undergo a growth lag when seeded at too low a density, and cannot be passaged more than ∼50 times. In regard to purification, secreted recombinant proteins are much easier to purify than nonsecreted proteins, because the ratio between recombinant protein and host proteins in the medium is much higher than in lysates, especially when protein-free medium has been used. The general strategy for purifying protein from the medium depends on the nature of the recombinant protein. If an antibody against the desired protein is available in large quantities, it can be used for affinity purification. Otherwise, conventional ion-exchange chromatography may perform equally well.
Critical Parameters The efficiency of heterologous gene expression in the baculovirus system can differ 1000fold as a result of the intrinsic nature of the gene and the encoded protein. Modifying the heterologous gene will generally influence gene expression by only 2- to 5-fold. Researchers should not feel compelled to modify their gene excessively. For some general rules regarding the improvement of gene expression, see O’Reilly et al. (1992). When expressing protein, a time-course experiment should be conducted to determine peak protein production. To optimize for protein production, it is recommended that log-phase Sf9 cells be infected at high multiplicity of infections (MOIs)—e.g., 5 to 10 for monolayer culture and 1 to 2 for suspension culture. Cells to be infected should be >97% viable. Use of highquality media and FBS also seems to contribute to high protein yields. The most common method for radiolabeling protein involves incorporating [35S]methionine or [35S]cysteine. To improve results, the intra-
cellular pool of these essential amino acids should be depleted prior to radiolabeling by incubating the cells in methionine/cysteinefree medium. If an antibody is available, the labeled lysate should be immunoprecipitated to help detect the recombinant protein if it comigrates with a labeled host protein. To maximize large-scale production of recombinant proteins, Sf 9 cells should be grown in suspension in serum-free medium. Flasks should remain 1, it is expected that most cells in the culture will become infected at the same time and thus produce protein within the same time frame. A time course can be run to test for peak protein production. Cells infected during logphase growth with a high surface-to-volume ratio to allow for proper gas exchange will produce protein optimally. Approximately 1 to 2 mg of 6×His and GST fusion protein are routinely obtained per liter of insect-cell culture. Affinity purifications to
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>90% homogeneity are easily achieved using single-step affinity purification with Ni-NTA, glutathione agarose, or other epitope tags.
Time Considerations It takes 1.5 to 2 weeks to obtain a spinner culture from a frozen vial of Sf9 cells. A viral stock can be obtained in ∼4 days and plaqued in 1 day. It takes ∼1 week before the plaques are formed and ready to be counted. It takes an additional 2 days to infect Sf9 cells to obtain cells or medium for protein analysis. Analyzing for the recombinant protein after harvesting should take l to 3 days. Finally, it will take ∼4 days to complete the time course and another 2 to 3 days to harvest and analyze the infected cell pellets or culture supernatants.
Literature Cited Coligan, J.E., Dunn, B.M., Ploegh, H.L., Speicher, D.W., and Wingfield, P.T. (eds.) 1997. Current Protocols in Protein Science. John Wiley & Sons, New York. Guan, K. and Dixon, J.E. 1991. Eukaryotic proteins expressed in Escherichia coli: An improved thrombin cleavage and purification procedure of fusion proteins with glutathione S-transferase. Anal. Biochem. 192:262-267. Haun, R.S. and Moss, J. 1993. Ligation-independent cloning of glutathione S-transferase fusion genes for expression in Escherichia coli. Gene 112:3743. O’Reilly, D.R., Miller, L.K., and Luckow, V.A. 1992. Baculovirus Expression Vectors. W.H. Freeman, New York. Phelan, M.L., Sif, S., Narlikar, G.J., and Kingston, R.E. 1999. Reconstitution of a core chromatin remodeling complex from SWI/SNF subunits. Mol. Cell 3:247-53.
Summers, M.D. and Smith, G.E. 1987. A manual of methods for baculovirus vectors and insect cell culture procedures. Texas Agricultural Experiment Station Bulletin No. 1555. College Station, Texas. Wu, H., White, G.C., Workman, E.F., Jenzano, J.W., and Lundblad, R.L. 1992. Affinity chromatography of platelets on immobilized thrombin: retention of catalytic activity by platelet-bound thrombin. Thrombosis Res. 67:419-427. Zhang, Y., Ng, H.-H., Erdjument-Bromage, H., Tempst, P., Bird, A., and Reinberg, D. 1999. Analysis of the NuRD subunits reveals a histone deacetylase core complex and a connection with DNA methylation. Genes Dev. 13:1924-1935.
Key Reference O’Reilly et al., 1992. See above. A laboratory manual that will aid researchers in the expression and purification of recombinant proteins using the baculovirus system.
Contributed by Cheryl Isaac Murphy Aquila Biopharmaceuticals Worcester, Massachusetts Helen Piwnica-Worms Washington University School of Medicine St. Louis, Missouri Stefan Grünwald Pharmingen San Diego, California William G. Romanow Corning Costar Portsmouth, New Hampshire Nicole Francis and Hua-Ying Fan Massachusetts General Hospital Boston, Massachusetts
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EXPRESSION OF PROTEINS IN MAMMALIAN CELLS
SECTION III
Transient Expression of Proteins Using COS Cells
UNIT 16.12
This unit describes the use of COS cells to efficiently produce a desired protein in a short period of time. These cells express high levels of the SV40 large tumor (T) antigen, which is necessary to initiate viral DNA replication at the SV40 origin. Three factors contribute to make COS cell expression systems appropriate for the high-level, short-term expression of proteins: (1) the high copy number achieved by SV40 origin–containing plasmids in COS cells 48 hr post-transfection, (2) the availability of good COS cell expression/shuttle vectors, and (3) the availability of simple methods for the efficient transfection of COS cells. Each COS cell transfected with DNA encoding a cell-surface antigen (in the appropriate vector) or cytoplasmic protein will express several thousand to several hundred thousand copies of the protein 72 hr post-transfection. If the transfected DNA encodes a secreted protein, up to 10 µg of protein can be recovered from the supernatant of the transfected COS cells 1 week post-transfection. COS cell transient expression systems have also been used to screen cDNA libraries, to isolate cDNAs encoding cell-surface proteins, secreted proteins, and DNA binding proteins, and to test protein expression vectors rapidly prior to the preparation of stable cell lines (UNIT 9.5).
BASIC PROTOCOL
This transfection protocol is a modification of that presented in UNIT 9.2 and gives conditions for optimal transfection of COS cells. The main difference between this procedure and that in UNIT 9.2 is the composition of the DEAE-dextran/chloroquine solution, which is prepared here in PBS, not TBS, and contains chloroquine to prevent the acidification of endosomes presumed to carry the DEAE-dextran/DNA into the cell. (This acidification results in acid hydrolysis of the DNA, giving rise to mutations and destruction of the DNA.) With this protocol, 40% to 70% of the cells can be routinely transfected. Materials Appropriate vector (see Background Information) COS-7 cells to be transfected (see Critical Parameters), adapted to growth in 2% serum (see annotation to step 2) Dulbecco’s minimum essential medium (DMEM) with 2% calf serum (DMEM-2 CS) DMEM with 2% NuSerum (Collaborative Research) (DMEM-2 NS), 37°C Phosphate-buffered saline (PBS; APPENDIX 2) DEAE-dextran/chloroquine solution: PBS containing 10 mg/ml DEAE-dextran (Sigma) + 2.5 mM chloroquine (Sigma) 10% dimethyl sulfoxide (DMSO; Sigma) in PBS (see APPENDIX 2 for PBS) 0.5 mM EDTA in PBS 100-mm tissue culture dishes Humidified 37°C, 6% CO2 incubator Phase-contrast microscope Sorvall RT-6000B rotor (or equivalent) Protein Expression Contributed by Alejandro Aruffo Current Protocols in Molecular Biology (1998) 16.12.1-16.12.7 Copyright © 2002 by John Wiley & Sons, Inc.
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Additional reagents and equipment for subcloning of DNA (UNIT 3.16), preparation of miniprep DNA (UNIT 1.6), purification of DNA by CsCl/ethidium bromide equilibrium centrifugation (UNIT 1.7), and flow cytometric analysis (Otten et al., 1995) NOTE: All cell culture incubations should be carried out in a humidified 37°C, 6% CO2 incubator unless otherwise stated. 1. Subclone the gene of interest into the appropriate vector to obtain the desired recombinant DNA (UNIT 3.16). Purify the recombinant DNA by a miniprep procedure (5-ml culture UNIT 1.6), or by CsCl/ethidium bromide centrifugation (UNIT 1.7). 2. Seed COS-7 cells in DMEM-2 CS at ∼20% confluence per 100-mm dish the day prior to transfection (so they will be ∼50% confluent the next day). Grow cells overnight to ∼50% confluence. The COS-7 cells should be adapted to 2% serum (from the usual 10% serum) by passaging them through decreasing concentrations of serum, in 2% increments, over ∼1 month. The procedure can be carried out in 10% serum, but at much greater expense. A confluent dish of COS-7 cells (∼106 cells) is usually split 1:5 on the day prior to transfection to give 2 × 105 cells/100-mm dish in 10 ml of DMEM-2 CS. In some cases cells may benefit from an extra day of growth prior to transfection (see step 6 annotation).
3. Just before use (for each 100-mm dish of COS cells to be transfected), thoroughly mix 5 ml of 37°C DMEM-2 NS with 0.2 ml of DEAE-dextran/chloroquine solution. Add 5 to 10 µg recombinant DNA and mix. It is important that the DEAE-dextran be well mixed with the medium before the DNA is added—otherwise, the DNA, a negatively charged polymer, will form large precipitates with DEAE-dextran, a positively charged polymer. These large precipitates cannot be taken up by the cell, resulting in a reduced transfection efficiency. When larger dishes are used, the amount of medium/DEAE/DNA should be sufficient to easily cover the cells and should include 400 ìg/ml DEAE-dextran, 100 ìM chloroquine, and 1 to 2 ìg/ml DNA in DMEM-2 NS. Either CsCl-purified (UNIT 1.7) or miniprep (UNIT 1.6) plasmid DNA can be used for the transfections. If miniprep DNA is used, use one-fifth of the miniprep per transfection.
4. Aspirate medium from COS cells and add the DMEM-2 NS/DEAE-dextran/DNA prepared in step 3. Incubate cells 3 to 4 hr. Observe the cells using a phase-contrast microscope. The DEAE-dextran will cause cells to retract and become vacuolated. Efficiency of transfection increases with longer incubation periods; on the other hand, so does cell death. The 3- to 4-hr incubation suggested here is a good starting point. However, several time points should be tried to optimize transfection of the particular population of cells used.
5. Aspirate DMEM/DEAE-dextran/DNA and add 5 ml of 10% DMSO (prepared in PBS). Incubate cells 2 min at room temperature. The DMSO shock results in increased transfection efficiencies. Without this step, transfection efficiencies might be lower by a factor of two or more.
6. Aspirate DMSO and add 10 ml DMEM-2 CS. Grow cells overnight (12 to 20 hr). If a significant number of the cells have lifted from the plate after overnight growth, the procedure will need to be repeated, but cells should be allowed to grow an extra day at step 2. Transient Epression of Proteins Using COS Cells
7. Passage (split and replate) each 100-mm dish of transfected COS cells into two new 100-mm dishes. After transfection, the COS cells will look unhealthy. Passaging them the day after transfection facilitates recovery, resulting in better levels of protein expression. In addition,
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DEAE-dextran treatment makes the cells sticky, and passaging the cells the morning after transfection restores their adhesion characteristics so that they may be once again lifted by a gentle treatment with PBS and EDTA (see step 8b).
8a. When expressing secreted proteins: 96 hr (4 days) after completing step 7, add 5 ml DMEM-2 CS and incubate 4 days. Harvest the medium, remove dead cells and debris by centrifuging 10 min in a Sorvall RT-6000B rotor at ∼2000 rpm (∼1000 × g), room temperature, and save the supernatant (see Anticipated Results). Detect secreted proteins by bioassay (UNIT 9.5) or by metabolic labeling and immunoprecipitation (UNITS 10.18 & 10.16), immunoaffinity chromatography (UNIT 10.11), radioimmunoassay (UNIT 11.17), or western blotting (UNIT 10.8). Do not aspirate the old medium prior to addition of 5 ml DMEM-2 CS because this medium contains the secreted protein. Addition of extra medium 96 hr post-transfection results in better yield of expressed protein; however, it also increases the level of total protein (since the medium contains 2% serum), which could complicate protein purification. To eliminate this problem, COS cells can be placed in serum-free medium 10 to 12 hr after they have been replated, although (in our hands) this results in a 10-fold lower yield of expressed protein than in the presence of serum. Thus, unless it is absolutely necessary to remove additional contaminating protein, serum should be present in the medium even at reduced levels (1%).
8b. When expressing cell-surface or intracellular proteins: Aspirate medium from cells 72 hr (3 days) after transfection in step 6. Add 5 ml PBS, swirl, and aspirate PBS. Add 5 ml of 0.5 mM EDTA in PBS and incubate 15 min. Lift cells from the dish by gently dislodging them with a Pasteur pipet. Stain cell-surface proteins with the appropriate fluorescent antibody and detect by microscopy or flow cytometry (Otten et al., 1995). Transfected COS cells will tend to clump when lifted from the dish. Pipetting the cells up and down will tend to disrupt these clumps. More effective dispersion of the clumps can be obtained by forcing the cells through a 100-ìM nylon mesh.
COMMENTARY Background Information COS cells COS cells are African green monkey kidney cells (CV-1) that have been transformed with an origin-defective SV40 virus, which has integrated into COS cell chromosomal DNA. Therefore, COS cells produce wild-type SV40 large T antigen but no viral particles. Since SV40 large T antigen is the only viral protein required in trans for viral replication (i.e., its coding sequence need not be located on the DNA molecule on which it acts), SV40 origin– containing plasmids replicate in these cells to a high copy number (10,000 to 100,000 copies/cell) 48 hr post-transfection. If the plasmid carries a cDNA or genomic insert encoding a desired protein (under the control of the appropriate promoter), COS cells will express the protein at relatively high levels over a short period of time. Transfected COS cells produce protein in a burst that starts ∼24 hr post-trans-
fection and can last for up to a week. However, due to the excessive burden placed on the transfected cell by the replicating plasmid and the high levels of protein production, the transfected cells typically either die or lose the plasmid a week after transfection. COS cells were developed by Yakov Gluzman (1981) as a host for the propagation of SV40 virus early-region mutants. The first SV40 origin–containing plasmids to be used in conjunction with COS cells were made by Lusky and Botchan (1981). Short-term expression systems using both COS cells and SV40 origin–containing plasmids were initially used to identify DNA sequences required for transcription of the human α1-globin gene (Mellon et al., 1981). COS cells were first used to produce cellsurface and secreted proteins by Rose and Bergmann (1982), who looked at the expression of wild-type and mutant vesicular stomatitis virus glycoprotein in transfected cells. This
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Transient Epression of Proteins Using COS Cells
technique was subsequently used to study the expression of insulin (Laub and Rutter, 1983), somatostatin (Warren and Shields, 1984), and acetylcholine receptors (Mishina et al., 1984). These experiments demonstrated that COS cells could be used to express biologically active cell-surface and secreted proteins. Furthermore, these proteins were correctly processed although they are normally not produced by COS cells. COS cell expression was initially used to screen a cDNA library to isolate a cDNA encoding human granulocyte/macrophage colony-stimulating factor (Wong et al., 1985). This was subsequently extended to isolation of cDNAs encoding cell-surface proteins (Aruffo and Seed, 1987a; Seed and Aruffo, 1987) and DNA-binding proteins (Tsai et al., 1989). The expressed protein produced in COS cells is in most cases biologically active. However, although COS cells are able to carry out some post-translational modifications, they may not modify the expressed protein in exactly the same way as the cell that would normally produce it. For example, COS cells do not express the α-(1,3)fucosyltransferase, which is capable of transferring fucose to either sialyl or asialyl precursors (Goelz et al., 1990). In addition, insufficient post-translational modification occurs in the case of lymphocyte cell-surface proteins, which tend to be underglycosylated in COS cells (Aruffo and Seed, 1987b). This might be due to an overburdening of the COS cell glycosylation machinery by the high levels of protein expression and/or by the lack of enzymes required to carry out the full posttranslational modifications. An alternative to COS cells is provided by WOP cells (Dailey and Basilico, 1985), which are mouse 3T3 cells transformed by an origindefective polyoma virus. Like COS cells, they produce no viral particles. However, they produce polyoma large T antigen and are therefore capable of replicating a plasmid containing a polyoma origin of replication to a copy number that is typically ten times lower than that obtained in COS cells. WOP cells are also more delicate than COS cells, making them harder to transfect. For these two reasons, COS cells should be used whenever possible. However, WOP cells should be used in those cases where a monoclonal antibody that is used to identify and/or purify the protein being expressed crossreacts with COS cell proteins. This situation has arisen when transient expression in COS cells was carried out in order to clone a human
protein with a mouse anti-human monoclonal antibody that also recognizes the equivalent monkey protein (Seed and Aruffo, 1987). In this situation, the mouse cell line presented a useful alternative to COS cells by avoiding monoclonal antibody cross-reactivity. Vectors The main requirements of any COS cell expression/shuttle vector are: (1) an SV40derived origin of replication, (2) appropriate eukaryotic transcription regulatory elements (i.e., enhancer, promoter, and polyadenylation signal sequences), (3) a prokaryotic origin of replication, and (4) a prokaryotic genetic marker for selection in Escherichia coli. A particularly useful example of such a vector is CDM8 (Fig. 16.12.1; Seed, 1987). The eukaryotic transcription element of CDM8 is composed of the cytomegalovirus (CMV) enhancer-promoter, with an SV40 virus–derived intron and polyadenylation signal; the CMV promoter is a cis element (i.e., one that must be located adjacent to the DNA it acts on) that directs transcription of the DNA subcloned downstream from it. The prokaryotic genetic marker in CDM8 is provided by the supF (amber suppressor) gene. CDM8 is propagated in host bacterial cells containing helper plasmid P3, which contains amber mutations in the genes responsible for tetracycline and ampicillin resistance. (P3 has been introduced into many E. coli strains.) When CDM8 is transformed into an E. coli strain containing P3, the amber mutations are suppressed, rendering the host resistant to tetracycline and ampicillin. In addition to these elements, the CDM8 expression/shuttle vector contains an M13 origin of replication so that it can be used for the production of single-stranded DNA, a T7 RNA polymerase promoter for preparation of mRNA in vitro, and a polyoma virus–derived origin of replication which permits plasmid replication in WOP cells. Other vectors commonly used for COS cell transient expression include pXM (Yang et al., 1986) and pDC201 (Sims et al., 1988). These two plasmids contain the adenovirus-2 major late promoter and tripartite mRNA leader. This element acts in conjunction with the adenovirus VA RNA (also produced by the vectors) to increase the translatability of the mRNA encoding the desired protein. It is thought that adenovirus VA RNAs increase translation efficiency of mRNAs containing the major late promoter tripartite leader by facilitating the
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enhancer- pcmv/ T7
supF
Hin dIII XbaI Xho I Bst XI
M13 ori vx ori
stuffer CDM8 ~ 4800 bp
SV40 ori
splice and poly(A)+
Bst XI XbaI Pst I Not I
Py ori
Figure 16.12.1 CDM8. CDM8 (Seed, 1987) is a COS cell expression/shuttle vector that contains an SV40-derived origin of replication (SV40 ori), eukaryotic transcription regulatory elements [splice and poly(A)+], a prokaryotic origin of replication (πvx; derived from the pBR322 ori), and a prokaryotic genetic marker (supF). In addition, CDM8 contains an M13 origin of replication (M13 ori), a T7 RNA polymerase promoter (pCMV/T7), and a polyoma-derived origin of replication (Py ori). Any restriction endonuclease sites shown can be used for cloning, but the inserted fragment must have its 5′ end nearest the enhancer pCMV/T7. The stuffer sequence is used to detect a size difference after restriction digestion.
interaction between mRNA and a 43S ribosomal protein translation preinitiation complex (Kaufman, 1985).
Critical Parameters Efficiency of transfection depends critically on the length of time that the cells (COS or other cells) are incubated in the presence of DEAEdextran/DNA. Longer periods of time result in higher transfection efficiencies. However, the DEAE-dextran/chloroquine solution is quite toxic to cells and in general, cells should not be in its presence for >4 hr. In the past, DEAEdextran transfections were carried out in the absence of serum, because a precipitate of unknown composition that seemed to be very toxic formed in DEAE-dextran/calf serum mixtures. Medium containing NuSerum, on the other hand, does not form this precipitate and tends to enhance the ability of the cells to tolerate DEAE-dextran; thus NuSerum should always be included in transfection medium. Efficiency of transfection can also be affected by the quality of the DNA and the age of the DEAE-dextran/chloroquine solution. It is preferable to use CsCl-purified or other highly purified DNA whenever possible (UNIT
1.7). However, miniprep DNA (UNIT 1.6) or DNA
purified using a pZ523 column (5′→3′) or by other methods can also be used. The DEAEdextran chloroquine solution can be kept at 4°C for several months but it is wise to prepare it fresh about every 3 months. COS cells can be obtained from the American Type Culture Collection; several sublines exist, including COS-1 and COS-7. The COS-7 subline is recommended because it produces a higher plasmid copy number. These cells grow as a monolayer in DMEM-2 CS in a humidified 37°C, 5% CO2 incubator; however, the ATCC grows its COS cells in DMEM-10 fetal bovine serum (FBS). Since serum is expensive, changing the growth medium is worthwhile. The change from FBS to CS should be done slowly over 1 to 2 weeks; the concentration of CS should then be reduced to 2% in 2% increments over ∼1 month. Because the growth characteristics, transfectability, and protein expression properties of COS cells change with time and with repeated subculturing, and because these changes tend not to favor the production of high levels of proteins, it is prudent to freeze aliquots of the original COS cell stock in DMEM-10 CS/10%
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DMSO for later use in a −70°C freezer for 24 hr and then transfer them to a −150°C (liquid nitrogen) freezer. COS cells grow rapidly requiring passage every 4 to 5 days; typically, a confluent plate of cells is split 1 to 10. To obtain good levels of transient protein production from transfected COS cells, it is very important to replate the transfected cells onto new dishes with fresh medium the morning after tranfection. In addition to enhancing protein production, replating the transfected cells allows lifting of the cells from the dish using only PBS/EDTA (without trypsin). This is very important when transient expression by COS cells is used to produce cell-surface proteins.
Anticipated Results The Basic Protocol should yield transfection efficiencies of 40% to 70%. When COS cells are being used to produce cell-surface or intracellular proteins, it can be expected that each transfected cell will express several thousand copies of this protein (10,000 to 100,000 copies/cell) 72 hr post-transfection. If COS cells are used to produce secreted proteins, up to 1 µg/ml of protein can be recovered from the supernatant of a 100-mm dish of transfected cells 1 week post-transfection. However, the amount of protein produced by COS cells can vary dramatically depending on the protein being produced. This was the case when COS cells were used to produce soluble immunoglobulin fusion forms of cell-surface proteins (Aruffo et al., 1990). In this case, one of the fusion proteins, CD8 immunoglobulin, was secreted from COS cells at high levels (1 µg/ml) while the other, CD44 immunoglobulin, was secreted very poorly if at all. It was found that the CD44 fusion protein was sequestered inside the cell. To obtain efficient secretion of the CD44 fusion protein, it was necessary to change the amino-terminal signal sequence of the CD44 fusion protein. Interestingly, the native cell-surface forms of both CD8 and CD44 are expressed equally efficiently on the surface of transfected COS cells. For some of these immunoglobulin fusion proteins, 0.5 ml of medium contained plenty of protein (∼500 ng) after concentration using a protein A affinity matrix. In some cases, it is possible to use such COS cell supernatants directly without further purification (Aruffo et al., 1990). Transient Epression of Proteins Using COS Cells
Time Considerations It is important not to transfect the cells too soon after replating; >8 to 12 hr should pass
between the time the cells are seeded on the plate and the time of transfection. Once the transfection has started, the DEAE-dextran/DNA mixture should be left on the cells for a minimum of 2 hr and a maximum of 4 hr; because the mixture increases transfection efficiency it should remain in contact with the cells as long as they appear viable. After transfection, the cells look quite unhealthy, and 12 to 24 hr post-transfection they should be replated in new dishes with fresh medium. The peak of plasmid replication in transfected COS cells occurs 48 to 72 hr post-transfection. Protein production starts 24 hr posttransfection but peaks 72 to 96 hr post-transfection. Thus, when expressing cell-surface or cytoplasmic proteins, the cells should be harvested 72 to 96 hr post-transfection. However, transfected cells continue to produce protein for up to a week post-transfection and when expressing secreted proteins, the supernatants should be harvested a week post-transfection.
Literature Cited Aruffo, A. and Seed, B. 1987a. Molecular cloning of a CD28 cDNA by a high-efficiency COS cell expression system. Proc. Natl. Acad. Sci. U.S.A. 84:8573-8577. Aruffo, A. and Seed, B. 1987b. Molecular cloning of two CD7 (T cell leukemia antigen) cDNAs by a COS cell expression system. EMBO J. 6:33133316. Aruffo, A., Stamenkovic, I., Melnick, M., Underhill, C.B., and Seed, B. 1990. CD44 is the principal cell-surface receptor for hyaluronate. Cell 61:1303-1313. Dailey, L. and Basilico, C.J. 1985. Sequences in the polyoma virus DNA regulatory region involved in viral DNA replication and early gene expression. J. Virol. 54:739-749. Gluzman, Y. 1981. SV-40 transformed simian cells support the replication of early SV40 mutants. Cell 23:175-182. Goelz, S.E., Hession, C., Goff, D., Griffiths, B., Tizard, R., Newman, B., Chi-Rosso, G., and Lobb, R. 1990. ELFT: A gene that directs the expression of an ELAM-1 ligand. Cell 63:13491356. Kaufman, R.J. 1985. Identification of the components necessary for adenovirus translational control and their utilization in cDNA expression vectors. Proc. Natl. Acad. Sci. U.S.A. 82:689693. Laub, O. and Rutter, W.J. 1983. Expression of the human insulin gene and cDNA in a heterologous mammalian system. J. Biol. Chem. 258:60436050. Lusky, M. and Botchan, M. 1981. Inhibition of SV40 replication in simian cells by specific pBR322 DNA sequences. Nature 293:79-81.
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Mellon, P., Parker, V., Gluzman, Y., and Maniatis, T. 1981. Identification of DNA sequences required for transcription of the human α1-globin gene in a new SV40 host-vector system. Cell 27:279288.
Tsai, F., Martin, D.I.K., Zon, L.I., D’Andrea, D., Wong, G.G., and Orkin, S.H. 1989. Cloning of cDNA for the major DNA-binding protein of the erythroid lineage through expression in mammalian cells. Nature 339:446-451.
Mishina, M., Kurosaki, T., Tobimatsu, T., Morimoto, Y., Noda, M., Yamamoto, T., Terao, M., Lindstrom, J., Takahashi, T., Kuno, M., and Numa, S. 1984. Expression of functional acetylcholine receptor from cloned cDNAs. Nature 307:604-608.
Warren, T.G. and Shields, D. 1984. Expression of preprosomatostatin in heterologous cells: Biosynthesis, posttranslational processing, and secretion of mature somatostatin. Cell 39:547-555.
Otten G., Yokoyama, W.M. and Holmes, K.H. 1995. Flow cytometry analysis using the Becton Dickinson FACScan. In Current Protocols in Immunology (J.E. Coligan, A.M. Kruisbeek, D.H. Margulies, E.M. Shevach, and W. Strober, eds.) pp. 5.4.1-5.4.19. John Wiley & Sons, New York. Rose, J.K. and Bergmann, J.E. 1982. Expression from cloned cDNA of cell-surface, secreted forms of the glycoprotein of vesicular stomatitis virus in eucaryotic cells. Cell 30:753-762. Seed, B. 1987. An LFA-3 cDNA encodes a phospholipid-linked membrane protein homologous to its receptor CD2. Nature 329:840-842. Seed, B. and Aruffo, A. 1987. Molecular cloning of the CD2 antigen, the T cell erythrocyte receptor, by a rapid immunoselection procedure. Proc. Natl. Acad. Sci. U.S.A. 84:3365-3369. Sims, J.E., March, C.J., Cosman, D., Widmer, M.B., MacDonald, H.R., McMahan, C.J., Grubin, C.E., Wignall, J.M., Jackson, J.L., Call, S.M., Friend, D., Alpert, A.R., Gillis, S., Urdal, D.L., and Dower, S.K. 1988. cDNA expression cloning of the IL-1 receptor, a member of the immunoglobulin superfamily. Science 241:585-589.
Wong, G.G., Witek, J.S., Temple, P.A., Wilkens, K.M., Leary, A.C., Luxenber, D.P., Jones, S.S., Brown, E.L., Kay, R.M., Orr, E.C., Shoemaker, C., Golde, D.W., Kaufman, R.J., Hewick, R.M., Wang, E.A., and Clark, S.C. 1985. Human GMCSF: Molecular cloning of the complementary DNA and purification of the natural and recombinant proteins. Science 228:810-815. Yang, Y., Ciarletta, A.B., Temple, P.A., Chung, M.P., Kovacic, S., Witek-Giannotti, J.S., Leary, A.C., Kriz, R., Donahue, R.E., Wong, G.G., and Clark, S.C. 1986. Human IL-3 (multi-CSF): Identification by expression cloning of a novel hematopoietic growth factor related to murine IL3. Cell 47:3-10.
Key Reference Warren and Shields, 1984. See above. This article shows that COS cells can be used as an efficient, short-term, mammalian epression system for the production of proteins.
Contributed by Alejandro Aruffo Bristol-Myers Squibb Princeton, New Jersey
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Expression and Purification of EpitopeTagged Multisubunit Protein Complexes from Mammalian Cells
UNIT 16.13
Biochemical characterization and functional studies of mammalian proteins are often hampered by the availability of the purified protein, in particular, when the functional entity is present as a multisubunit protein complex in the cell. To overcome the difficulties in the purification of multisubunit protein complexes from mammalian cells, one may create stable cell lines containing epitope-tagged protein. In general, a stable cell line that expresses an epitope-tagged subunit of a protein complex is first established either by retrovirus-mediated gene transfer (for constitutive expression) or by a tetracycline-regulated system (for inducible expression). Immunoaffinity purification using epitope-specific monoclonal antibody-conjugated beads is then performed to pull down the epitope-tagged multisubunit protein complex, which is finally recovered by peptide elution under neutral pH or physiological conditions and is readily available for functional assays. Several commonly used epitopes are listed in Table 10.15.1 (UNIT 10.15). For simplicity, only protocols using the FLAG epitope (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys, or DYKDDDDK) are described here. The authors normally use human HeLa cells, which can grow in monolayer as well as in suspension, as parental cells for establishing stable cell lines, although other cells, such as human 293, can also be used for the experiments. By and large, HeLa-derived cell lines expressing an epitope-tagged protein are initially isolated in monolayer culture and then adapted to suspension culture for the purpose of collecting large quantities of cells for protein purification. The first protocol in this unit (see Basic Protocol 1) describes the procedures involved in the establishment of a stable cell line constitutively expressing the FLAG-tagged protein by retrovirus-mediated gene transfer and immunoaffinity purification of the epitope-tagged multisubunit protein complex. The next protocol (see Basic Protocol 2) outlines the steps involved in the establishment of an inducible cell line conditionally expressing the FLAG-tagged protein by a tetracycline-regulated system, and the one-step immunoaffinity purification of the multisubunit protein complex. Since an epitope-tagged protein may exist as different complexes in the cell, it is also important to isolate various forms of multisubunit protein complexes. This can sometimes be achieved simply by careful selection of the starting material (e.g., using nuclear extract, cytoplasmic S100 fraction, or the chromosomal fraction) for immunoaffinity purification or by manipulating the wash conditions with different concentrations of salt, detergent, and chaotropic agents during immunoaffinity purification. The Alternate Protocol provides an excellent example for the purification of different forms of human RNA polymerase II complexes, achieved simply by choosing the appropriate starting material and by varying wash conditions. Frequently, multiple forms of the epitope-tagged protein complexes coexist in the same starting material. In that case, a simple chromatographic column, such as the P11 ion-exchange resin, can be used to separate multiple forms of protein complexes into different fractions prior to immunoaffinity purification. The isolation of various human TATA-binding protein (TBP)–containing complexes, described in the Support Protocol, is a good example of combining the P11 column and immunoaffinity purification. These protocols, collectively, illustrate a powerful methodology in applying epitope tagging and stable cell line approaches for the purification of multisubunit protein complexes from mammalian cells. Protein Expression Contributed by Shwu-Yuan Wu and Cheng-Ming Chiang Current Protocols in Molecular Biology (2001) 16.13.1-16.13.17 Copyright © 2002 by John Wiley & Sons, Inc.
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BASIC PROTOCOL 1
PURIFICATION OF MULTISUBUNIT PROTEIN COMPLEXES FROM CLONAL CELL LINES CONSTITUTIVELY EXPRESSING A FLAG-TAGGED PROTEIN To purify a multisubunit protein complex, the authors usually begin with a constitutive expression system using retrovirus-mediated gene transfer for the establishment of clonal cell lines expressing the FLAG-tagged subunit of a protein complex. Retrovirus-mediated gene transfer (UNITS 9.9-9.14) provides a gene delivery system with few copies of the viral integrant to ensure the expressed FLAG-tagged protein can be efficiently assembled into a large protein complex, in a stoichiometric ratio, with endogenous cellular proteins. A retroviral vector containing a drug-selection marker and the FLAG sequence introduced either at the N-terminus or at the C-terminus of the protein-coding region is first created and delivered into a packaging cell line that provides all the viral proteins essential for virion assembly. Viral particles are then collected from the cell culture medium and used to infect HeLa or other chosen cells. Cellular clones potentially expressing the tagged protein are initially selected by drug resistance and further identified by immunoblotting from individually expanded colonies. Clonal cell lines expressing the FLAG-tagged protein are then adapted to suspension culture and used for immunoaffinity purification of the epitope-tagged protein complexes. NOTE:All culture incubations should be performed in a humidified 37°C, 5% CO2 incubator unless otherwise specified. Materials FLAG-tagged protein-coding sequence Retroviral vector (e.g., pBabe neo, Morgenstern and Land, 1990; also see UNITS 9.9-9.14) 2× HEPES-buffered saline (HBS), pH 7.12 (UNIT 16.14) 0.25 M CaCl2 Retrovirus-packaging cells (e.g., ψCRIP, Danos and Mulligan, 1988) Dulbecco’s modified Eagle’s medium (DMEM) with and without 10% calf serum 15% (v/v) glycerol/DMEM DMEM with 10% fetal bovine serum (FBS; APPENDIX 3F) 2.5 mg/ml polybrene stock solution 50% confluent HeLa cells Drug-containing selection medium (e.g., 1 mg/ml of G418 (total weight), 0.2 mg/ml of hygromycin, or 0.5 µg/ml of puromycin; see UNIT 9.5) Trypsin-EDTA (APPENDIX 3F) 1× PBS (APPENDIX 2) 1× SDS-PAGE protein sample buffer (UNIT 10.2A) Anti-FLAG M2 monoclonal antibody (and/or anti-protein antibodies; Sigma) Joklik’s medium (Sigma) with 10% calf serum and 5% calf serum Anti-FLAG M2 monoclonal antibody-conjugated beads (Sigma) BC100 (see recipe) BC300 (see recipe)/0.1% Nonidet P-40 FLAG peptide (100 mg/ml in water) Liquid nitrogen
Purification of Epitope-Tagged Multisubunit Protein Complexes
5-ml round-bottom Falcon tube 60-mm and 100-mm tissue culture plates 15-ml sterile tube 0.45-µm cellulose acetate syringe filter 24-well tissue culture plate 1.5-ml and 0.5-ml microcentrifuge tubes
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250-ml, 500-ml, 1-liter, 3-liter, and 12-liter spinner flasks End-over-end tube rotator Rotor (e.g., Sorvall H-6000A or equivalent) Microcentrifuge spin column Additional reagents and equipment for use of retroviral vectors (UNITS 9.9-9.14), preparation of nuclear extracts (UNIT 12.1), and immunoblotting (UNIT 10.8) Establish stable cell lines by retrovirus-mediated gene transfer 1. Clone the FLAG-tagged protein-coding sequence into a retroviral vector containing a drug-selection marker (e.g., neomycin). UNITS 9.9-9.14 describe
the use of retroviral vectors.
2. Transfer 20 µg plasmid DNA into a 5-ml round-bottom Falcon tube and mix with 0.5 ml of 2× HBS, pH 7.12. The following transfection procedure works well for ψCRIP and many other packaging cell lines.
3. Add 0.5 ml of 0.25 M CaCl2 dropwise while vortexing. Leave for 20 to 30 min at room temperature. 4. Vortex the calcium phosphate–DNA coprecipitates vigorously and then add the solution to 50% confluent ψCRIP cells, which are maintained in DMEM plus 10% calf serum in a 100-mm tissue culture plate. Leave cells for 3 to 4 hr in a 37°C, 5% CO2 incubator. 5. Remove medium and add 3 ml of 15% (v/v) glycerol/DMEM for 3 min at room temperature. 6. Add 8 ml DMEM to quickly dilute glycerol, mix well, and remove solution. 7. Repeat washes three additional times with 3 ml DMEM for each wash. 8. Add 10 ml DMEM with 10% calf serum to transfected cells and leave plates for 18 hr in a 37°C, 5% CO2 incubator. 9. Collect viral particles in a 15-ml sterile tube after filtering cell culture supernatant (18-hr post glycerol shock) through a 0.45-µm cellulose acetate syringe filter to remove floating cells and large cellular debris. 10. Mix 1 ml of filtered viral stock with 1 ml of DMEM plus 10% FBS and polybrene (final concentration 8 µg/ml), and incubate mixture with 50% confluent HeLa cells, which are grown in DMEM with 10% FBS in 100-mm cell culture plates and have been prewashed with medium two times, for 2.5 hr at 37°C with intermittent mixing. 11. Add an additional 8 ml of DMEM with 10% FBS, without removing the original infection medium, and leave cells for 1 to 1.5 days in a 37°C, 5% CO2 incubator. 12. Split cells 1:5 in drug-containing medium and change the selection medium every 3 to 4 days. The drug concentrations normally used for HeLa cells, depending on individual drug-selection markers, are: 1 mg/ml of G418 (total weight), 0.2 mg/ml of hygromycin, or 0.5 ìg/ml of puromycin. See UNIT 9.5 for additional details. Drug-resistant colonies are usually visible after 2 to 3 weeks. Protein Expression
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Identify clonal cell lines expressing FLAG-tagged protein 13. Prepare a 24-well plate and add 2 drops (∼100 µl) of trypsin-EDTA to each well at alternate positions to avoid cross contamination between neighboring wells. 14. Circle the drug-resistant colonies (∼2- to 3-mm diameter), to be picked up, on the bottom of the plate with a black (or blue) marker pen, remove medium, wash cells once with 1× PBS, and then remove solution. 15. Take ∼50 µl of trypsin-EDTA from the first well using a 200-µl pipet tip and transfer a circled colony back to the first well after pipetting up and down several times. 16. Repeat the transfer process for a total of 12 colonies. 17. Add 1 to 2 ml of selection medium to each well and put the 24-well cell culture plate back to a 37°C, 5% CO2 incubator for cells to attach and grow. Depending on the number of viable cells transferred, it may take 3 to 14 days for cells to become confluent in the wells.
18. Transfer cells to a 60-mm cell culture plate when they become confluent in the 24-well plate and continue the expansion for individual cellular clones. 19. When cells are nearly confluent on the 60-mm cell culture plate, split cells again to one 60-mm cell culture plate and two 100-mm cell culture plates. The cells grown on the 100-mm plates will be frozen down as individual cloned cell lines.
20. Prepare whole-cell lysates from 80% to 90% confluent cells still in log phase grown on the 60-mm cell culture plate by adding 300 µl of 1× SDS-PAGE protein sample buffer to plate, after washing cells two times with 1× PBS. Pipet lysate up and down many times until the sample is no longer viscous, and then transfer the lysate to a 1.5-ml microcentrifuge tube. 21. Perform immunoblotting (UNIT 10.8) on individually collected whole-cell lysates with anti-FLAG M2 monoclonal antibody and/or anti-protein antibodies to identify the cellular clones that express FLAG-tagged protein. Purify FLAG-tagged protein complexes from established cell lines 22. Select a positive clone expressing FLAG-tagged protein for further expansion by adapting it to suspension culture. Combine cells from twelve 100-mm cell culture plates into a 250-ml spinner flask and adjust the cell density to 0.5 × 106 cells/ml using Joklik’s medium with 10% FBS. Maintain cells at the same density using Joklik’s medium with 10% FBS for ≥3 days. If cells are healthy and nearly grow exponentially, start to feed cells using Joklik’s medium with 10% calf serum. Continue feeding cells using Joklik’s medium with 10% calf serum for at least three days. If cells still look healthy and grow well, begin to feed cells using Joklik’s medium with 7.5% (or 5%) calf serum. During the expansion and serum-switching/reducing process, if cells look unhealthy or cell density drops, centrifuge cells for 5 min at 300 × g and resuspend cells in fresh Joklik’s medium with the same concentration of calf serum, this helps to remove cell debris. Readjust the cell density to 0.5 × 106 cells/ml. Once cells are healthy and continue to grow, begin to lower the concentration of serum as previously described. Repeat the process until cells are maintained in Joklik’s medium with 5% calf serum. Purification of Epitope-Tagged Multisubunit Protein Complexes
23. Grow and expand the cells growing in Joklik’s medium with 5% calf serum continually to 20 liters or any other desired volume.
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24. Prepare nuclear extract, cytoplasmic S100, and nuclear pellet (or chromosomal fraction) according to UNIT 12.1 (Dignam et al., 1983). 25. Perform immunoblotting (UNIT FLAG-tagged protein.
10.8)
to identify the cellular fractions containing
26. Add 0.4 ml of anti-FLAG M2 monoclonal antibody-conjugated beads, which have been prewashed several times with BC100, to a 15-ml tube containing 10 to 14 ml of nuclear extract (or S100, or solubilized nuclear pellet). Incubate the sample for 6 to 12 hr at 4°C on an end-over-end tube rotator. 27. Centrifuge for 2 min at 300 × g (Sorvall H-6000A rotor 1000 rpm), 4°C, to separate supernatant from beads. 28. Remove the supernatant and wash protein-bound beads five times with BC300/0.1% Nonidet P-40. Inclusion of Nonidet P-40 prevents protein/bead aggregation and also helps the immunoaffinity beads rotate better during the elution step.
29. Transfer the M2-conjugated beads with bound proteins to a microcentrifuge spin column. 30. Microcentrifuge for 10 sec at maximum speed (10,000 rpm) to remove the residual liquid. 31. Add 0.5 ml of BC300/0.1% Nonidet P-40 plus 0.2 mg/ml of FLAG peptide to the dried beads. Mix well and incubate for 20 to 60 min at 4°C with continuous rotation. 32. Collect the eluate (i.e., the first elution) by centrifuging for 10 sec at 4°C. 33. Repeat the elutions (steps 31 and 32) for a total of four times to recover the majority of FLAG-tagged protein complexes. 34. Dispense samples into small aliquots (e.g., 20 µl) in 0.5-ml microcentrifuge tubes and freeze in liquid nitrogen. Store aliquots of the purified complexes at −80°C until use. PURIFICATION OF MULTISUBUNIT PROTEIN COMPLEXES FROM CLONAL CELL LINES CONDITIONALLY EXPRESSING THE FLAG-TAGGED PROTEIN
BASIC PROTOCOL 2
Sometimes, the introduced FLAG-tagged protein-coding sequence is not expressed in the established clonal cell lines following retrovirus-mediated gene transfer and drug selection. This is likely due to cytotoxicity resulting from unregulated or constitutive expression of the ectopic gene product. To overcome this problem, an inducible mammalian expression system can be used to turn “on” or “off” expression of the FLAG-tagged protein based on the use of tetracycline. In the process of establishing FLAG-tagged protein-expressing cell lines, tetracycline is continuously added to silence ectopic gene expression. The inducibility of individual cellular clones is then screened by immunoblotting using whole-cell lysates collected in the presence and absence of tetracycline. Once identified, the inducible cell line is adapted to suspension culture and used for immunoaffinity purification of FLAG-tagged multisubunit protein complexes after removing tetracycline.
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Materials Protein-coding sequence Tetracycline-regulated expression plasmid (e.g., pTetCMV-F°(S), Wu and Chiang, 1996; UNIT 16.14) Sheared calf thymus DNA SacI-linearized plasmid containing a drug-selection marker (e.g., pREP4, Invitrogen) Cell line expressing a tetracycline-controlled transactivator (e.g., HtTA-1, Gossen and Bujard, 1992) DMEM with 10% FBS (APPENDIX 3F) G418 sulfate Trypsin-EDTA (APPENDIX 3F) 1 M BES buffer, pH 7.2 (see recipe) Hygromycin B Tetracycline 1× SDS-PAGE protein sample buffer (UNIT 10.2A) 1× PBS (APPENDIX 2) Anti-FLAG M2 monoclonal antibody and/or anti-protein antibodies Joklik’s medium (Sigma) with 10% calf serum Joklik’s medium with 5% calf serum HeLa cells Anti-FLAG M2-conjugated beads BC100 and BC300 (see recipe) 10% Nonidet P-40 FLAG peptide Liquid nitrogen 0.4-cm cuvettes for electroporation 24-well, 60-mm, and 100-mm tissue culture plates 15-ml and 50-ml Falcon tubes Rotor (e.g., Sorvall H-6000A or equivalent) Electroporator (e.g., Bio-Rad Gene Pulser II) Disposable glass pipet 1.5-ml and 0.5-ml microcentrifuge tubes 250-ml, 500-ml, 1-liter, 3-liter, and 12-liter spinner flasks 250-ml sterile conical centrifuge tubes Microcentrifuge spin column Additional reagents and equipment for tissue culture (APPENDIX 3F) and immunoblotting (UNIT 10.8) Establish tetracycline-regulated cell lines by electroporation 1. Clone the protein-coding sequence into a tetracycline-regulated expression plasmid containing the FLAG epitope sequence, such as pTetCMV-F°(S). 2. Mix 10 µg of linearized tetracycline-regulated FLAG-tagged protein-expressing plasmid (e.g., PvuI used to linearize most pTetCMV-F°(S)-derived constructs), 50 µg of sheared calf thymus DNA, and 0.5 µg of SacI-linearized pREP4, which contains a hygromycin-selection marker, into an electroporation cuvette.
Purification of Epitope-Tagged Multisubunit Protein Complexes
3. Combine HtTA-1 cells (or other cells expressing a tetracycline-controlled transactivator), which are grown in DMEM with 10% FBS containing 0.6 mg/ml of G418, from several 100-mm cell culture plates into a 50-ml Falcon tube after trypsin-EDTA treatment, centrifuge for 5 min at 300 × g (Sorvall H-6000A rotor 1000 rpm), and
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resuspend cell pellets in DMEM with 10% FBS and 5 mM BES buffer, pH 7.2, at a density of 2 × 107 cells/ml. 4. Add 250 µl of resuspended cells, using a 200-µl pipet tip to transfer 125 µl of cells two times, to the electroporation cuvette containing plasmids and carrier DNA (see step 2). Mix well by pipetting up and down several times. 5. Electroporate DNAs into cells using the Bio-Rad Gene Pulser II with setting at 960 µF and 200 V. Leave cuvette for 10 min at room temperature and then transfer cells, using a disposable glass pipet, to a 15-ml centrifuge tube containing 10 ml DMEM with 10% FBS. 6. Centrifuge for 5 min at 300 × g. Aspirate and discard the supernatant, which contains untransfected DNAs and cell debris generated by electroporation. 7. Resuspend cells in 5 ml DMEM with 10% FBS and distribute cells onto five 100-mm cell culture plates, each containing 9 ml DMEM with 10% FBS, and leave for 2 days in a 37°C, 5% CO2 incubator. 8. Initiate drug selection by changing medium to DMEM with 10% FBS containing 400 µg/ml of G418, 200 µg/ml of hygromycin B, and 2 µg/ml of tetracycline. 9. Change medium every 3 or 4 days and continue drug selection for ∼3 weeks until drug-resistant colonies are clearly visible. Identify clonal cell lines conditionally expressing FLAG-tagged protein 10. Pick up 12 to 24 drug-resistant colonies and expand them into cell lines following the procedures described in Basic Protocol 1, steps 13 to 18. 11. When cells are nearly confluent on the 60-mm cell culture plate during expansion, split cells to one 100-mm cell culture plate and two 60-mm cell culture plates. The cells grown on the 100-mm cell culture plate will be frozen down as individual cloned cell lines. The cells grown on the two 60-mm cell culture plates are plated in selection medium, one with and the other without tetracycline, for 48 to 72 hr.
12. Prepare whole-cell lysates from cells grown on the two 60-mm cell culture plates by adding 300 µl of 1× SDS-PAGE protein sample buffer to each plate, after washing the cells two times with 1× PBS. Pipet the lysate up and down many times until the sample is no longer viscous and then transfer lysate to a 1.5-ml microcentrifuge tube. 13. Perform immunoblotting (UNIT 10.8) on an aliquot of the individually collected whole cell lysates with anti-FLAG M2 monoclonal antibody and/or anti-protein antibodies to identify the cellular clones that express FLAG-tagged protein only in the absence of tetracycline. Purify FLAG-tagged protein complexes from established cell lines 14. Select a positive clone expressing FLAG-tagged protein for further expansion by adapting it to suspension culture. Combine cells from twelve 100-mm cell culture plates into a 250-ml spinner flask (at a density of 0.5 × 106 cells/ml) and gradually replace medium from DMEM with 10% FBS to Joklik’s medium with 10% calf serum, and eventually to Joklik’s medium with 5% calf serum, but include 1 µg/ml of tetracycline and 0.6 mg/ml of G418 to the medium throughout the expansion process until culture volume reaches 250 ml. 15. Continue to expand cells in tetracycline-containing medium, but omit G418 after 250 ml culture.
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16. After expansion of cell culture to 12 liters, remove tetracycline by centrifuging cells for 5 min at 300 × g (Sorvall H-6000A rotor 1000 rpm) using 250-ml sterile conical tubes. Repeat the process until all of the cells are consolidated into two 250-ml conical tubes. 17. Wash cell pellets with 250 ml of 1× PBS in each tube. Centrifuge for 5 min at 300 × g (Sorvall H-6000A rotor 1000 rpm). Repeat washes for a total of 4 to 6 times. 18. Resuspend cell pellets in ∼120 ml of Joklik’s medium with 5% calf serum without tetracycline, and dispense cells into six 12-liter flasks, each containing 1.5 liters of Joklik’s medium with 5% calf serum. 19. Four days after protein induction with medium doubling every day, prepare nuclear extract, cytoplasmic S100, and nuclear pellet (or chromosomal fraction) according to UNIT 12.1. Protein induction in suspension culture usually takes longer than induction in monolayer culture. From the time course study, protein induction after 4 days maximizes the quantity of the induced FLAG-tagged protein, in part due to a reduced doubling time in a larger volume of suspension culture using less expensive and lower concentrations of calf serum. Induction also appears to work better under a slightly dilute condition with medium doubling every day.
20. Perform immunoblotting (UNIT FLAG-tagged protein.
10.8)
to identify the cellular fractions containing
21. For immunoaffinity purification of FLAG-tagged protein complexes, follow the same procedures as described in Basic Protocol 1, steps 26 to 34. ALTERNATE PROTOCOL
PURIFICATION OF MULTIPLE FORMS OF EPITOPE-TAGGED PROTEIN COMPLEXES BY VARYING THE STARTING MATERIAL AND WASH CONDITIONS Using human RNA polymerase II (pol II) as an example, this protocol describes the purification of epitope-tagged protein complexes. RNA pol II is a multisubunit protein complex containing 12 polypeptides (RPB1-12). The largest subunit (RPB1) of pol II has a unique C-terminal domain (CTD) containing heptapeptide sequence, Tyr-Ser-Pro-ThrSer-Pro-Ser (YSPTSPS), which occurs 26 times in yeast and 52 times in humans. This CTD can be phosphorylated by many protein kinases, resulting in three forms of pol II commonly found in eukaryotes. The IIO form of pol II contains a highly or hyper-phosphorylated CTD, whereas the IIA form of pol II has a non- or hypo-phosphorylated CTD. The IIB form of pol II does not have the CTD, likely generated due to protease cleavage. In addition, pol II can form an extremely large protein complex, so-called pol II holoenzyme, by association with a subset of general transcription factors and other cellular proteins involved in chromatin remodeling, mRNA processing, and DNA repair. This Alternate Protocol exemplifies an application of the epitope-tagging and stable cell line approaches for the simultaneous purification of the IIO, IIA, and IIB forms of pol II as well as pol II holoenzyme, which contains only the nonphosphorylated form of RPB1. This is achieved simply by careful selection of the starting material and by modifying the composition of protein buffer used for immunoaffinity purification (see Fig. 16.13.1). Please note that the conditions outlined herein will vary with other tagged protein systems.
Purification of Epitope-Tagged Multisubunit Protein Complexes
Additional Materials (also see Basic Protocols 1 and 2) 10 M urea BC850 (see recipe) Buffer B (see recipe)
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Buffer D (see recipe) FLAG peptide elution buffer (see recipe) Ammonium sulfate Sonicator Rotor (e.g., Bechman 45-Ti or equivalent) Purify FLAG-tagged human pol II holoenzyme 1a. Establish a tetracycline-regulated HeLa-derived cell line conditionally expressing a FLAG-tagged subunit of human pol II, and prepare nuclear extract, cytoplasmic S100, and nuclear pellet according to the procedures described in Basic Protocol 2, steps 1 to 20. 2a. For immunoaffinity purification of FLAG-tagged human pol II holoenzyme from nuclear extract or S100, follow Basic Protocol 1, steps 26 to 34, except use BC100/0.1% Nonidet P-40 as wash and elution buffer. Purify IIA form of FLAG-tagged human pol II 1b. For immunoaffinity purification of FLAG-tagged human pol II from nuclear extract or S100, follow Basic Protocol 1, steps 26 to 34, except wash the protein-bound beads sequentially five times each with BC850/0.1% Nonidet P-40 containing 1.0 M urea,
cell line expressing FLAG-tagged human RPB9 (hRPB9-3)
cytoplasmic S100
nuclear extract
nuclear pellet
p low salt
GCN5
pol II
TFIIB
TFIIF TFIIE TFIIH
pol II
p p
p
high salt pol II
SWI/SNF SRBs
p
IIO
pol II
IIA
pol II
IIB
pol II holoenzyme
Figure 16.13.1 Purification of human RNA polymerase II complexes. The hRPB9-3 cells (Wu and Chiang, 1998), derived from human HeLa cells that conditionally express the FLAG-tagged RPB9 subunit of human RNA polymerase II, are first separated into cytoplasmic S100, nuclear extract, and nuclear pellet. Immunoaffinity purification is then performed with S100 or nuclear extract under either low salt (100 mM KCl-containing buffer) or high salt (850 mM KCl and 1.0 M urea-containing buffer) wash conditions, which results in the purification of human RNA polymerase II (pol II) holoenzyme and pol II, respectively. The pol II holoenzyme complex purified from S100 or nuclear extract contains pol II, a subset of general transcription factors (TFIIB, TFIIE, TFIIF, and TFIIH), SRBs, histone acetyltransferase GCN5, and chromatin remodeling factor SWI/SNF (Wu and Chiang, 1998; Wu et al., 1999). Additional forms of pol II complexes with different phosphorylation status on the largest subunit (RPB1) of pol II can also be purified from the nuclear pellet under the high salt wash condition. The IIO form of pol II contains the highly phosphorylated carboxy-terminal domain (CTD), indicated by a tail, in RPB1, whereas the IIA and IIB forms of pol II have non-phosphorylated or truncated CTD in RPB1, respectively.
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and BC100, then finally elute the bound proteins with BC100 containing 0.2 mg/ml of FLAG peptide (FLAG peptide elution buffer). Inclusion of high salt and urea during the washes disrupts the interactions between pol II and other weakly associated polypeptides found in the pol II holoenzyme, thereby allowing the recovery of only the IIA form of the core pol II, which forms a tight complex resistant to high salt and urea washes.
Purify FLAG-tagged human pol II containing a mixture of IIA, IIB, and IIO forms 1c. Slowly agitate 35 ml of nuclear pellet isolated from hRPB9-3, which is a HeLa-derived cell line conditionally expressing the FLAG-tagged RPB9 subunit of human pol II, with 70 ml of buffer B at 4°C (or on ice) and then with 11 ml of 3 M (NH4)2SO4 to adjust the ammonium sulfate concentration to 0.3 M. Stir for an additional 30 min. The use of buffer B together with ammonium sulfate provides both a magnesium ion and high salt concentration to help dissociate pol II complexes from the chromosomal fraction and at the same time preserve pol II activity.
2c. Sonicate five times, for 1 min each, on an ice water slurry with 20-sec intervals between bursts. 3c. Centrifuge the mixture for 90 min at 185,000 × g (Beckman 45-Ti rotor 40,000 rpm), 4°C, and then pour the supernatant into a 500-ml beaker. 4c. Add 2 volumes of buffer B dropwise with a syringe to gradually adjust the ammonium sulfate concentration to 0.1 M. 5c. Remove precipitated material by centrifuging for 60 min at 185,000 × g (Beckman 45-Ti rotor 40,000 rpm) and pour supernatant to another 500-ml beaker. 6c. Precipitate pol II by slowly adding solid ammonium sulfate to 65% saturation (i.e., 0.42 g/ml of suspension) and stir for an additional 30 min. 7c. Centrifuge for 60 min at 142,000 × g (Beckman 45-Ti rotor 35,000 rpm) and resuspend the pellet in 40 ml of buffer D. 8c. Dialyze the sample against 4 liters of BC100 for 6 hr with at least one change of buffer. The final recovered sample is ∼46 ml.
9c. For immunoaffinity purification of FLAG-tagged human pol II complexes containing a mixture of IIA, IIB, and IIO forms, follow Basic Protocol 1, steps 26 to 34, except sequentially wash the protein-bound beads with five times each of BC850/0.1% Nonidet P-40 with 1.0 M urea, and BC100, before final elutions with BC100 containing 0.2 mg/ml of FLAG peptide (FLAG peptide elution buffer). The inclusion of high salt and urea during the initial washes disrupts the interactions between pol II and other weakly associated polypeptides found in the pol II holoenzyme, thereby allowing the recovery of core pol II complexes with different phosphorylation states. Equilibration of the immobilized complexes to BC100 is necessary before final elutions with FLAG peptide BC100 elution buffer.
Purification of Epitope-Tagged Multisubunit Protein Complexes
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PURIFICATION OF MULTIPLE FORMS OF EPITOPE-TAGGED PROTEIN COMPLEXES FOLLOWING A P11 ION-EXCHANGE CHROMATOGRAPHIC COLUMN
SUPPORT PROTOCOL
Human TATA-binding protein (TBP) can associate with different classes of cellular proteins to form distinct protein complexes, such as SL1, TFIID, and TFIIIB, which are required for transcription by RNA polymerase I, II, and III, respectively. These distinct protein complexes are all found in nuclear extract. A simple chromatographic step, such as a P11 ion-exchange column, is then used to separate various TBP-containing complexes into different fractions prior to immunoaffinity purification (see Fig. 16.13.2). This Support Protocol illustrates an application of conventional chromatography together with immunoaffinity purification for the isolation of multiple protein complexes containing a common subunit. Additional Materials (also see Basic Protocol 1) P11 ion-exchange resin Various BC buffers (BC100, BC300, BC500, BC850, and BC1200; see recipe) Chromatographic column (∼100 ml capacity) Flow adapter Chart recorder Fraction collector Dialysis tubing (MWCO 12,000; Sigma) Additional reagents and equipment for dialysis (APPENDIX 3C) Purify TFIID and TFIIIB from a cell line expressing FLAG-tagged human TBP 1. Establish a HeLa-derived cell line expressing FLAG-tagged human TBP by retrovirus-mediated gene transfer (UNITS 9.9-9.14) and prepare nuclear extract, S100, and nuclear pellet from the established cell line using the procedures described in Basic Protocol 1, steps 1 to 24. 2. Pack a P11 ion-exchange column containing ∼60 ml of resin in a chromatographic column. 3. Connect the column to a flow adapter, a chart recorder, and a fraction collector, and equilibrate the whole system with BC100 overnight. Set the flow rate at ∼1 column volume (CV) per hr (i.e., 1 ml/min).
cell line expressing FLAG-tagged human TBP (3-10) nuclear extract
P11 0.1
0.3
0.5
TBP complex 1
M2-agarose
TFIIIB
0.85 M KCl TBP complex 2
M2-agarose
TFIID
Figure 16.13.2 Purification of different FLAG-tagged TBP complexes. Nuclear extract prepared from 3-10 (Chiang et al., 1993), which is a HeLa-derived cell line constitutively expressing FLAG-tagged TBP, is loaded onto a P11 ion-exchange column. Proteins bound to the P11 resin are sequentially eluted off the column by step elutions with 0.1, 0.3, 0.5, and 0.85 M KCl-containing buffer. The FLAG-tagged TBP complexes are mainly detected at the 0.3 and 0.85 M KCl fractions, which are then used for immunoaffinity purification by anti-FLAG M2 monoclonal antibody-conjugated beads to isolate TFIIIB and TFIID complexes, respectively.
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4. Thaw ∼100 ml of nuclear extract in an ice bucket overnight at 4°C. 5. Centrifuge nuclear extract for 20 min at 105,000 × g (Beckman 45-Ti rotor 30,000 rpm), 4°C. 6. Pour supernatant into 50-ml tubes and load the sample onto an equilibrated P11 column at a flow rate of 1 CV/hr. Collect the eluate, ∼12 ml/tube, with a fraction collector. Remember to save a small aliquot (∼100 ìl) for immunoblotting analysis.
7. Wash the column with ∼2 CV of BC100 or until the absorption profile, monitored at 280 nm, almost reaches the baseline. 8. Step elute the bound proteins by sequentially switching to BC300, BC500, BC850, and BC1200, when the protein curve of each elution buffer nearly reaches the baseline. 9. Combine the fractions representing each of the BC100, BC300, BC500, and BC850 protein peaks. The BC1200 fractions, containing a minor amount of SL1, are not collected for further purification.
10. Dialyze BC500 and BC850 samples against 4 liters of BC100 for 5 hr at 4°C. 11. Centrifuge samples for 15 min at 46,000 × g (Beckman 45-Ti rotor 20,000 rpm), 4°C. 12. Aliquot the supernatant of BC500 and BC850, as well as the BC100 and BC300 fractions, individually, into several 15-ml tubes (for immunoaffinity purification) and 1.5-ml microcentrifuge tubes (for immunoblotting and protein analysis). 13. Freeze the aliquots in liquid nitrogen and store samples at −80°C until use. 14. Perform immunoblotting (UNIT 10.8) to locate the P11 fractions containing FLAGtagged TBP. 15. Purify TFIIIB from the P11 0.3 M KCl fraction and TFIID from the P11 0.85 M KCl fraction according to the procedures described in Basic Protocol 1, steps 26 to 34. REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
BES buffer, 1 M (pH 7.2) 21.32 g of BES (N, N-bis[2-hydroxyethyl]-2-aminoethanesulfonic acid) (Sigma) 60 ml H2O Adjust pH to 7.12 with 1 M NaOH and bring up total volume to 100 ml Autoclave and store up to 6 months at room temperature
Purification of Epitope-Tagged Multisubunit Protein Complexes
BC100 20 mM Tris⋅Cl, pH 7.9 at 4°C (APPENDIX 2) 20% glycerol 0.2 mM EDTA, pH 8.0 (APPENDIX 2) 100 mM KCl 1 mM DTT (added prior to use) 0.5 mM PMSF (added prior to use) Store up to 6 months at 4°C
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BC300, BC500, BC850, and BC1200 Prepare as for BC100 (see recipe) except the KCl concentration is adjusted to 300, 500, 850, and 1200 mM, respectively. Buffer B 50 mM Tris⋅Cl, pH 7.9 at 4°C (APPENDIX 2) 25% glycerol 5 mM MgCl2 5 mM EDTA, pH 8.0 (APPENDIX 2) 5 mM EGTA, pH 8.0 5 mM DTT (added prior to use) 0.5 mM PMSF (added prior to use) Store up to 6 months at 4°C Buffer D 50 mM Tris⋅Cl, pH 7.9 at 4°C (APPENDIX 2) 25% glycerol 5 mM EDTA, pH 8.0 (APPENDIX 2) 5 mM EGTA, pH 8.0 2 mM DTT (added prior to use) 0.5 mM PMSF (added prior to use) Store up to 6 months at 4°C FLAG peptide elution buffer Diluting FLAG peptide from 100 mg/ml of FLAG peptide (in water) in BC100 or BC300 (see recipe) prior to use. COMMENTARY Background Information The FLAG epitope consists of 8 amino acids that are mostly charged residues (Hopp et al., 1988). The popularity of this epitope lies in the following facts. 1. It is synthetic, so that very few, if any, cross-reacting species are detected in bacterial, yeast, insect, or mammalian cells. 2. It is very hydrophilic and tends to help proteins stay soluble in aqueous solution. 3. It is small and highly antigenic. The introduction of the FLAG sequence rarely changes protein conformation and has a tendency to stimulate the immune response when injected into animals for antibody production (Chiang and Roeder, 1995). 4. A heart muscle kinase site linked to the FLAG sequence offers a unique way to label proteins with 32P after purification (Blanar and Rutter, 1992). The labeled proteins can be used for interaction studies, expression library screening and protein tracking (Chiang and Roeder, 1995). 5. A peptide elution method has been developed to elute the FLAG-tagged protein (Chiang and Roeder, 1993) or protein complexes (Chiang et al., 1993) off the anti-FLAG M2
monoclonal antibody-conjugated beads under neutral pH or physiological conditions, thereby allowing the recovery of fully active proteins ready for functional assays. 6. The fold purification achieved by M2agarose beads, which are commercially available, is exceedingly high, making it possible to purify FLAG-tagged multisubunit protein complexes to near homogeneity via one-step immunoaffinity purification (Chiang et al., 1993; Fondell et al., 1996; Kershnar et al., 1998; Ogryzko et al., 1998; Sif et al., 1998; Wu et al., 1998; Gu et al., 1999; Ikura et al., 2000). Although purification of recombinant FLAG-tagged protein can be easily achieved by using bacterial and baculovirus expression systems (Chiang and Roeder, 1993; Wu et al., 1999), the reconstitution of a fully functional protein complex from individually purified subunits or from insect cells coinfected with several recombinant baculoviruses expressing different subunits remains challenging (Bell et al., 1995; Chen and Tjian, 1996; Guermah et al., 1998; Phelan et al., 1999). Therefore, we and others have applied retrovirus-mediated gene transfer to deliver an epitope-tagged subunit of a large protein complex into human
Protein Expression
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HeLa cells for the natural assembly of multisubunit protein complexes in a mammalian cell. Isolation of the in vivo-assembled large protein complexes is then facilitated by immunoaffinity purification using epitope-specific monoclonal antibody-conjugated beads and peptide elution (Zhou et al., 1992; Chiang et al., 1993). This methodology, combining epitope-tagging and stable cell line approaches, has made it possible to purify many multisubunit protein complexes, as long as a clonal cell line expressing the tagged subunit of a protein complex is available. Sometimes, cytotoxicity caused by constitutive expression of the tagged protein prevents the establishment of a stable cell line by retrovirus-mediated gene transfer. The application of an inducible mammalian expression system, in combination with FLAG epitopetagging and peptide elution methods, has further expanded our ability to establish stable cell lines expressing essentially any epitope-tagged protein (Wu and Chiang, 1996). The purification of human pol II and TFIIH from tetracycline-regulated clonal cell lines conditionally expressing FLAG-tagged RPB9 and the FLAG-tagged p62 subunit of human TFIIH, respectively, was the first documentation detailing the procedures for biochemical purification of multisubunit protein complexes from an inducible mammalian expression system (Kershnar et al., 1998). The protocols described here not only greatly simplify the purification steps, but also dramatically reduce the cost of preparation.
Critical Parameters and Troubleshooting
Purification of Epitope-Tagged Multisubunit Protein Complexes
Location of the epitope tag. In general, the FLAG epitope sequence is introduced at either the N-terminus or the C-terminus of the protein-coding region. A critical parameter is to keep the tag away from essential regions of the protein. In many cases, however, it is hard to predict where the best location is in a protein and which subunit may be exposed on the surface of a protein complex. The suggestion is to tag different subunits in parallel to enhance the successful rate. Isolation of drug-resistant colonies. The cellular colonies are ready to be picked up when their diameter is between 2-3 mm. However, some slow-growing colonies should not be ignored, as expression of potentially toxic proteins may reduce the growth rate of the cells. The suggestion is to pick up colonies of various
sizes, especially if there are clear differences in the diameter of the colonies. Identification of positive clones. It is very important to use anti-FLAG M2, not M1, monoclonal antibody for immunoblotting, as anti-FLAG M2 monoclonal antibody can recognize the FLAG sequence at any location on the protein, whereas the M1 monoclonal antibody only recognizes the FLAG sequence at the very N-terminal region of the protein. In addition, a negative result with the M2 monoclonal antibody does not necessarily indicate a failure in establishing clonal cell lines. The FLAG epitope may sometimes be masked by the fusion partner, thereby preventing its detection by anti-epitope antibodies. In this case, it is essential to further confirm the results by anti-protein antibodies. Induction in suspension culture. It often happens that the tagged protein is not successfully induced after continued expansion in suspension culture. This is probably caused by residual tetracycline, which prevents tetracycline-dependent transactivator (tTA) from binding to the regulated gene, or by a reduced expression of tTA in the clonal cell line. To overcome these problems and to ensure a high level of protein induction, we usually follow these four guidelines. First, preselect cells with antibiotic (G418) in a small culture (∼100 ml) in the presence of tetracycline for at least three days before further expansion in the absence of antibiotic (G418). Presumably, this will boost the expression of tTA in the cell, thereby enhancing the level of transactivation from the tetracycline-regulated promoter. Second, wash cells at least 4 to 6 times with 1× PBS to remove tetracycline prior to induction. Third, use 250ml conical centrifuge tubes instead of 1-liter flat bottom bottles to spin down cells when washing with 1× PBS. Conical centrifuge tubes allow for better cell adhesion; thus medium and PBS washes can be completely removed after each successive centrifugation without losing cell volume. The 1-liter flat bottom centrifuge bottles, although convenient to use, do not permit complete removal of the original medium without disturbing the cell pellet; a minor residual tetracycline contaminant is sufficient to prohibit the induction. Fourth, use tetracyclinefree serum as a growth supplement when protein induction is desired. The authors have discovered that some batches of calf serum contain endogenous tetracycline, likely resulting from the use of tetracycline in cattle feed to protect animals from bacterial infection.
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Removal of FLAG peptide. The presence of the free FLAG peptide in the purified protein complexes, following elution from the M2conjugated beads, is unlikely to cause problems in functional assays. In fact, the free peptide can enhance the stability of the purified protein complexes, which are often present in dilute concentrations after the final step of purification. However, if the presence of FLAG peptide presents a problem, e.g., by masking the surfaces of protein complexes or by competing for resin binding to an ion-exchange chromatographic column, a dialysis step can be included to remove the free peptide. Thus, depending on the purpose, FLAG peptide may or may not be removed after immunoaffinity purification. Specificity of purified protein complexes. Normally, a control purification with extract prepared from the parental cells is conducted in parallel to identify polypeptides binding non-specifically to the M2-conjugated beads. This is critical, especially for the characterization of the protein composition in a purified protein complex. It is also a good idea to perform a small-scale purification, e.g., by using 200 µl of protein sample and 10 µl of M2-conjugated beads in each tube (Chiang et al., 1993), in order to compare different washing conditions. Once the condition is optimized, a large-scale purification is then conducted. In this way, the specificity of the protein complexes is enhanced, while the nonspecific background is minimized. Consideration of elution buffer. The elution conditions also play an important role in immunoaffinity purification. Ideally, proteins are stored in low salt-containing buffers which allow for better manipulation of the salt conditions in later functional assays. But when comparing the elution conditions, we find that some proteins, such as the FLAG-tagged human papillomavirus E2 protein, elute better in 300 mM KCl–containing buffer (Hou et al., 2000), while the majority of other proteins elute well in 100 mM KCl–containing buffer. Inclusion of a minor amount (0.1% to 0.01%) of detergents, such as Nonidet P-40 or Triton X-100, in the elution buffer prevents protein/bead aggregation, allowing the immunoaffinity beads to rotate better during the elution step. A caveat in using detergent, however, is that the activity of some proteins (or complexes) may be sensitive to different concentrations of detergent. Luckily, protein complexes purified with these procedures, including human TFIID, TFIIH,
RNA polymerase II, and RNA polymerase II holoenzyme, are fully functional in both basal and activated transcription (Chiang et al., 1993; Chiang and Roeder, 1995; Kershnar et al., 1998; Wu and Chiang, 1998; Wu et al., 1998, 1999). Combination of immunoaffinity purification and conventional column chromatography. Additional steps of conventional column chromatography, such as ion-exchange and gel filtration, can be included prior to or following immunoaffinity purification to further enhance the purity of the target protein complex.
Anticipated Results The percentage of drug-resistant colonies exp ressing FLAG-tagg ed pr otein via retrovirus-mediated gene transfer is theoretically 100%. However, integration of the coding cassette into a transcriptionally silenced region of the cellular chromosomes may prevent the expression of FLAG-tagged protein, thereby reducing the number of positive clones. Even so, there are usually several clonal cell lines expressing FLAG-tagged protein from a dozen cellular isolates. In contrast, only 5 to 10% of clonal cell lines expressing FLAG-tagged protein were identified when the tetracycline-regulated expression system was used to establish stable cell lines. This is in part due to random integration of the tetracycline-regulated expression plasmid into cellular chromosomes. Thus, more drug-resistant colonies (>12) should be expanded when using the inducible expression system. The level of induced FLAG-tagged protein, which can vary over three orders of magnitude, is modulated by the amount of tetracycline included in the growth medium (Wu and Chiang, 1996). A maximal induction is usually achieved by complete removal of tetracycline. Normally the concentration of tetracycline is kept at either zero or 1 µg/ml to turn “on” or “off” the expression of the tagged protein throughout the entire experiment. The purified protein complexes are finally analyzed, in comparison with a control sample purified in parallel, by silver staining and immunoblotting. A purification table listing variables, including protein amount of the starting material, units, specific activity, fold purification, and yield, is typically generated. This is helpful for estimating the efficiency of immunoaffinity purification and other chromatographic steps (Kershnar et al., 1998). Protein Expression
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Time Considerations It usually takes three to four weeks to see drug-resistant colonies and an additional three weeks to identify clonal cell lines expressing FLAG-tagged protein by either retrovirus-mediated gene transfer or the tetracycline-regulated expression system. Expansion of a clonal cell line in suspension culture ready for extract preparation may take another 3 to 4 weeks. Once cells are ready for harvesting, it takes only one day to prepare nuclear extract, S100, and nuclear pellet, and another two days for immunoaffinity purification. If a chromatography step in involved, only one additional day is needed. Taken together, it may take 2 to 3 months to purify a multisubunit protein complex when beginning with an expression plasmid containing the FLAG-tagged protein-coding sequence.
Literature Cited Bell, S.P., Mitchell, J., Leber, J., Kobayashi, R., and Stillman, B. 1995. The multidomain structure of Orc1p reveals similarity to regulators of DNA replication and transcriptional silencing. Cell 83:563–568. Blanar, M.A. and Rutter, W.J. 1992. Interaction cloning: identification of a helix-loop-helix zipper protein that interacts with c-Fos. Science 256:1014-1018. Chen, J.L. and Tjian, R. 1996. Reconstitution of TATA-binding protein-associated factor/TATAbinding protein complexes for in vitro transcription. Methods Enzymol. 273:208-217. Chiang, C.-M. and Roeder, R.G. 1993. Expression and purification of general transcription factors by FLAG epitope-tagging and peptide elution. Peptide Res. 6:62-64. Chiang, C.-M. and Roeder, R.G. 1995. Cloning of an intrinsic human TFIID subunit that interacts with multiple transcriptional activators. Science 267:531-536. Chiang, C.-M., Ge, H., Wang, Z., Hoffmann, A., and Roeder, R.G. 1993. Unique TATA-binding protein-containing complexes and cofactors involved in transcription by RNA polymerases II and III. EMBO J. 12:2749-2762. Danos, O. and Mulligan, R.C. 1988. Safe and efficient generation of recombinant retroviruses with amphotropic and ecotropic host ranges. Proc. Natl. Acad. Sci. U.S.A. 85:6460-6464. Dignam J.D., Lebovitz, R.M., and Roeder, R.G. 1983. Accurate transcription initiation by RNA polymerase II in a soluble extract from isolated mammalian nuclei. Nucleic Acids Res. 11:14751489. Purification of Epitope-Tagged Multisubunit Protein Complexes
Fondell, J.D., Ge, H., and Roeder, R.G. 1996. Ligand induction of a transcriptionally active thyroid hormone receptor coactivator complex. Proc. Natl. Acad. Sci. U.S.A. 93:8329-8333.
Gossen, M. and Bujard, H. 1992. Tight control of gene expression in mammalian cells by tetracycline-responsive promoters. Proc. Natl. Acad. Sci. U.S.A. 89:5547-5551. Gu, W., Malik, S., Ito, M., Yuan, C.X., Fondell, J.D., Zhang, X., Martinez, E., Qin, J., and Roeder, R.G. 1999. A novel human SRB/MED-containing cofactor complex, SMCC, involved in transcription regulation. Mol. Cell 3: 97-108. Guermah, M., Malik, S., and Roeder, R.G. 1998. Involvement of TFIID and USA components in transcriptional activation of the human immunodeficiency virus promoter by NF-κB and Sp1. Mol. Cell. Biol. 18:3234-3244. Hopp, T.P., Prickett, K.S., Price, V.L., Libby, R.T., March, C.J., Cerretti, D.P., Urdal, D.L., and Conlon, P.J. 1988. A short polypeptide marker sequence useful for recombinant protein identification and purification. Bio-Technology 6:12041210. Hou, S.Y., Wu, S.-Y., Zhou, T., Thomas, M.C., and Chiang, C.-M. 2000. Alleviation of human papillomavirus E2-mediated transcriptional repression via formation of a TATA binding protein (or TFIID)-TFIIB-RNA polymerase II-TFIIF preinitiation complex. Mol. Cell. Biol. 20:113-125. Ikura, T., Ogryzko, V.V., Grigoriev, M., Groisman, R., Wang, J., Horikoshi, M., Scully, R., Qin, J., and Nakatani, Y. 2000. Involvement of the TIP60 histone acetylase complex in DNA repair and apoptosis. Cell 102:463-473. Kershnar, E., Wu, S.-Y., and Chiang, C.-M. 1998. Immunoaffinity purification and functional characterization of human transcription factor IIH and RNA polymerase II from clonal cell lines that conditionally express epitope-tagged subunits of the multiprotein complexes. J. Biol. Chem. 273:34444-34453. Morgenstern, J.P. and Land, H. 1990. Advanced mammalian gene transfer: High titre retroviral vectors with multiple drug selection markers and a complementary helper-free packaging cell line. Nucleic Acids Res. 18:3587-3596. Ogryzko, V.V., Kotani, T., Zhang, X., Schlitz, R.L., Howard, T., Yang, X.J., Howard, B.H., Qin, J., and Nakatani, Y. 1998. Histone-like TAFs within the PCAF histone acetylase complex. Cell 94:35-44. Phelan, M.L., Sif, S., Narlikar, G.J., and Kingston, R.E. 1999. Reconstitution of a core chromatin remodeling complex from SWI/SNF subunits. Mol. Cell 3:247-253. Sif, S., Stukenberg, P.T., Kirschner, M.W., and Kingston, R.E. 1998. Mitotic inactivation of a human SWI/SNF chromatin remodeling complex. Genes Dev. 12:2842-2851. Wu, S.-Y. and Chiang, C.-M. 1996. Establishment of stable cell lines expressing potentially toxic proteins by tetracycline-regulated and epitopetagging methods. BioTechniques 21:718-725. Wu, S.-Y. and Chiang, C.-M. 1998. Properties of PC4 and an RNA polymerase II complex in
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directing activated and basal transcription in vitro. J. Biol. Chem. 273:12492-12498. Wu, S.-Y., Kershnar, E., and Chiang, C.-M. 1998. TAFII-independent activation mediated by human TBP in the presence of the positive cofactor PC4. EMBO J. 17:4478-4490.
Chiang and Roeder, 1993. See above. This paper documents the development of the FLAG peptide elution method for purification of recombinant FLAG-tagged protein from bacteria. Kershnar, et al., 1998. See above.
Wu, S.-Y., Thomas, M.C., Hou, S.Y., Likhite, V., and Chiang, C.-M. 1999. Isolation of mouse TFIID and functional characterization of TBP and TFIID in mediating estrogen receptor and chromatin transcription. J. Biol. Chem. 274:2348023490.
This paper is the first documentation detailing the procedures for biochemical purification of multisubunit protein complexes from an inducible mammalian expression system.
Zhou, Q., Lieberman, P.M., Boyer, T.G., and Berk, A.J. 1992. Holo-TFIID supports transcriptional stimulation by diverse activators and from a TATA-less promoter. Genes Dev. 6:1964-1974.
This paper describes the construction of tetracycline-regulated FLAG-tagged expression plasmids, and the isolation and characterization of the inducible cell lines.
Wu and Chiang, 1996. See above.
Key References Chiang et al., 1993. See above. This paper describes the purification of human TFIID and TFIIIB complexes from a stable cell line constitutively expressing FLAG-tagged TBP.
Contributed by Shwu-Yuan Wu and Cheng-Ming Chiang Case Western Reserve University School of Medicine Cleveland, Ohio
Protein Expression
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Inducible Gene Expression Using an Autoregulatory, Tetracycline-Controlled System
UNIT 16.14
Tetracycline-regulated gene expression systems have been developed to overcome some of the obstacles encountered using other strategies for inducible gene expression in mammalian cells. These difficulties include pleiotropic, nonspecific effects or toxicity of inducing agents or treatments, and high uninduced background levels of expression. This unit describes protocols for using a modified tetracycline-regulated system in which a transcriptional transactivator drives expression of itself and a target gene in cultured cells and, to some extent, in transgenic mice. This transactivator (tTA) is a fusion protein consisting of the tetracycline-repressor of E. coli and the transcriptional activation domain of the VP16 protein of herpes simplex virus. In the absence of tetracycline, tTA binds to and activates genes preceded by a heptamerized version of the tetracycline-resistance operator of Tn10 plus a minimal CMV promoter (here collectively referred to as Tet P). Binding of tTA to Tet P and subsequent gene activation are blocked in the presence of tetracycline. The plasmid pTet-Splice (Fig. 16.14.1A) contains Tet P upstream, and SV40 splice and polyadenylation signals downstream, of a multiple cloning site into which sequences encoding the open reading frame (ORF) of a target gene of choice is easily inserted. Autoregulatory tTA expression is driven from the plasmid pTet-tTAk (Fig. 16.14.1B), in which the tTA ORF (including an optimal sequence for initiation of translation according to Kozak) has been inserted into pTet-Splice. The protocols in this unit describe the transfection of adherent cells and the testing of resultant clones for inducible transactivator or target gene protein expression. Stably transfected fibroblast cell lines expressing transactivator and target gene(s) can be derived by first cotransfecting pTet-tTAk and a plasmid encoding a selectable marker and obtaining stable lines with inducible transactivator expression (see Basic Protocol). These lines are subsequently stably cotransfected with plasmids encoding the target gene(s) and a second selectable marker. The procedure may also be used to cotransfect pTet-tTAk with the target gene–encoding plasmid(s) and a single selectable marker plasmid. The choice of method depends upon the feasibility of screening for the protein products of the target genes. While the consecutive method is more systematic, cotransfection may be faster given a relatively straightforward screening method for expression of the target gene (see Critical Parameters). A Support Protocol also describes methods to test stably transfected cell lines for inducible gene expression, for transient transfection and induction of tet-regulated plasmids, and for detection of the tTAk gene in cells (or transgenic mice). CALCIUM PHOSPHATE-MEDIATED STABLE TRANSFECTION OF NIH3T3 CELLS WITH pTet-tTAk AND TETRACYLINE-REGULATED TARGET PLASMIDS This protocol describes the stable transfection of adherent cells with pTet-tTAk for the derivation of cell lines expressing inducible tTA. In the first round of transfection stable cell lines expressing inducible tTA alone are produced. The single transfection procedure may also be used for stable cotransfection of pTet-tTAk and plasmids expressing the target gene(s). In the second round of transfection tTA expressing lines are transfected with plasmids expressing the target gene(s).
BASIC PROTOCOL
Protein Expression Contributed by Penny Shockett and David Schatz Current Protocols in Molecular Biology (1997) 16.14.1-16.14.9 Copyright © 2002 by John Wiley & Sons, Inc.
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A PvuI
ScaI
SspI SspI XmnI
BssH II PvuI PvuII
ampr
SstI Bst XI NotI Xbal PstI
poly(A) pTet–Splice (5178 bp)
BamHI PvuII
TetP
BssH II
+
splice
mcs XhoI PstI
EcoRI
Xbal SaII
BamHI
Clal EcoRV HindIII
Xbal SpeI BamHI
EcoRI
B ampr
poly(A) pTet–tTAk (6206 bp)
splice TetP +
XbaI
tTAk SalI
Figure 16.14.1 (A) The plasmid pTet-Splice (Shockett et al., 1995) is designed to drive tetracycline-regulated expression of a target gene inserted into the multiple cloning site (mcs). The tetracycline-regulated promoter (TetP) consists of a heptamerized tetracycline operator (doubleended arrow) upstream of a minimal human CMV promoter that includes bases −53 (triangle) to +75. The transcriptional start site (+) and TATAA box (small rectangle) are also indicated. This TetP fragment is an Xhol-SaII fragment derived from pUHC13-3 (Gossen and Bujard, 1992). SV40-derived sequences downstream of the MCS drive mRNA splicing and polyadenylation. The backbone of the plasmid is from Bluescript II KS+ (Stratagene) and carries the ampicillin resistance gene (ampr). (B) pTet-tTAk (Shockett et al., 1995) consists of the tTAk open reading frame inserted into the HindIII-EcoRV sites of pTet-Splice.
Inducible Gene Expression
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Materials NIH3T3 cells Complete DMEM-10 medium (see recipe) Complete DMEM/tet: complete DMEM-10 medium (see recipe) containing 0.5 µg/ml tetracycline hydrochloride (Sigma; dilute 10 mg/ml stock in 70% ethanol and store protected from light at −20°C) Selection medium (see recipe) containing 125 µM, 250 µM, or 500 µM L-histidinol Plasmids for first-round or cotransfection procedure: pTet-tTAk (Life Technologies) and plasmids containing target gene ORF(s) cloned into pTet-Splice (Life Technologies), pSV2-His, or another selectable marker plasmid; purified by CsCl banding (UNIT 1.7) or anion-exchange chromatography (UNIT 2.1B) Plasmids for second round transfection procedure: plasmids containing target gene ORF(s) cloned into pTet-Splice, pPGKPuro, or another selectable marker plasmid; purified by CsCl banding (UNIT 1.7) or anion-exchange chromatography (UNIT 2.1B) 2 M CaCl2 HEPES-buffered saline (HeBS; see recipe) 10 mg/ml chloroquine (19 mM; optional; Sigma); dilute in water and store at −20°C 85% HeBS/15% glycerol, prewarmed to 37°C 3 mg/ml puromycin (Sigma) diluted in PBS (APPENDIX 2) Phosphate-buffered saline (PBS; APPENDIX 2) 1× trypsin/EDTA (Life Technologies) 10-cm and 6-cm tissue culture plates 4-ml polystyrene tubes (Falcon) 24-well and 6-well tissue culture plates NOTE: All tissue culture incubations are performed in a humidified 37°C, 5% CO2 incubator. Grow the cells 1. First round only: Grow cells in complete DMEM-10 medium. The day before transfection split cells into 10-cm tissue culture plates in complete DMEM/tet to achieve one-third confluence on the day of the transfection. From this point on cells are kept in the presence of 0.5 ìg/ml tet. One plate per transfection is needed at this stage. A typical experiment might include one plate for tTA only, one for tTA plus target gene, and one to serve as the untransfected control plate.
Second round only: Grow stable cell lines that inducibly express autoregulatory tTA in selection medium/500 µM L-histidinol. The day before transfection split into 10-cm plates in this same medium to achieve one-third confluence on the day of transfection. Transfect the cells 2. Linearize plasmids prior to transfection and adjust concentration to ≥0.5 mg/ml. See Damke et al. (1995) for discussion of other selectable markers. All plasmids should be purified by CsCl banding (UNIT 1.7) or on a Qiagen column (UNIT 2.1B).
3. First round only: Mix 10 to 20 µg of pTet-tTAk (in the presence or absence of an equimolar amount of target gene plasmids) plus 1 to 2 µg pSV2-His (a molar ratio
Protein Expression
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of ∼10:1 of each tet plasmid to selectable marker plasmid) with 500 µl HeBS in a clear 4-ml polystyrene tube. A control mock transfection should be performed with no DNA added to the transfection. All of these cells should die in the selection medium/125 ìM L-histidinol introduced in step 14.
Second round only: Mix 10 to 20 µg each of target gene plasmid(s) plus 1 to 2 µg pPGKPuro (a molar ratio of ∼10:1 of each tet plasmid to selectable marker plasmid) with 500 µl HeBS in a clear 4-ml polystyrene tube. A control mock transfection should be performed with no DNA added to the transfection. All of these cells should die in the presence of the puromycin introduced in step 14. The optimal killing concentration for puromycin (lowest dose between 0.1 ìg/ml to 10 ìg/ml that kills all untransfected cells within a few days) should be determined empirically prior to the transfection and varies with the cell type.
4. Add 32.5 µl of 2 M CaCl2 to plasmid DNA and mix immediately by gentle vortexing. With occasional gentle mixing, allow precipitate to form for 15 to 30 min at room temperature or until solution is visibly cloudy when compared to a tube containing water. 5. Aspirate all of the medium from cells, doing one plate at a time. Mix precipitate a few times by pipetting with a Pasteur pipet, and apply dropwise and evenly over cells. 6. Incubate 30 min, gently rocking the plate after 15 min to ensure even coverage over entire plate. 7. First round only: Add 10 ml complete DMEM/tet, with or without 25 µM chloroquine (final), to each plate. Although the use of chloroquine may further reduce cell integrity during the glycerol shock (step 9), it can improve transfection efficiency.
Second round only: Add 10 ml selection medium/500 µM L-histidinol, with or without 25 µM chloroquine (final), to each plate. 8. Incubate 4 to 5 hr. The optimal length of incubation may vary for different cell types.
9. Gently aspirate medium from cells with minimal disruption of the precipitate that has settled onto the cells. Shock cells by adding dropwise 2.5 ml of prewarmed 85% HeBS/15% glycerol. It is normal for the cells to look somewhat ragged before and especially after glycerol shock. Two to four plates may be shocked at one time, depending on the speed of the researcher.
10. Aspirate HeBS/glycerol after exactly 2.5 min. Work quickly, as glycerol can be very toxic to the cells. The length of time cells are exposed to glycerol solution can be varied and increased up to 4 to 5 min to optimize transfection efficiency for different cell types. Cells should be shocked the maximal length of time which results in the least cell death.
11. First round only: Immediately, gently, and quickly wash cells twice by adding 10 ml complete DMEM/tet and immediately aspirating. Because cells tend to come loose from the plate after glycerol addition, add all medium to a single spot on the plate.
Inducible Gene Expression
Second round only: Immediately, gently, and quickly wash cells twice by adding 10 ml selection medium/500 µM L-histidinol and immediately aspirating. Again, add medium to a single spot on the plate to avoid loosening the cells.
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12. First round only: Add 10 ml complete DMEM/tet. Incubate cells overnight. Second round only: Add 10 ml selection medium/500 µM L-histidinol. Incubate cells overnight. 13. First round only: The morning after the transfection, aspirate the medium and replace with 10 ml complete DMEM/tet. Continue incubation. Second round only: The morning after the transfection, aspirate the medium and replace with 10 ml selection medium/500 µM L-histidinol. Continue incubation. Select and clone transfected cells 14. First round only: At 48 hr posttransfection, split cells into selection medium/125 µM 4 6 L-histidinol at several dilutions ranging from 3 × 10 to 1 × 10 cells per 10-cm plate. Make more than one plate in the mid-range that corresponds to an approximate split from one confluent plate of 1:16 to 1:32. Second round only: At 48 hr posttransfection, split cells as above, using selection medium/500 µM L-histidinol containing 3 µg/ml puromycin (final). The optimal killing concentration for puromycin (lowest dose between 0.1 ìg/ml to 10 ìg/ml that kills all untransfected cells within a few days) should be determined empirically prior to the transfection and varies with the cell type. The concentration of 3 ìg/ml puromycin is sufficient for selection of transfected NIH3T3 cells.
15. First round only: Refeed cells 4 days later with selection medium/125 µM L-histidinol. When colonies have formed, increase the concentration of L-histidinol in the selection medium to 250 µM. L-histidinol
is normally toxic to cells. The concentration of L-histidinol in the selection medium is therefore kept low initially and is raised as the number of cells expressing pSV2-His at high levels reaches a critical mass.
Second round only: Refeed cells 4 days later with selection medium/500 µM L-histidinol/puromycin. 16. When colonies are well established (at about day 12 to 14 of selection), circle their borders with a marker. Aspirate medium from plate and place a plastic cloning ring (autoclaved upright in vacuum grease) on the plate to surround an individual clone. Wash clones quickly with ∼100 µl PBS and add 2 drops of trypsin (∼100 µl) for 30 sec to 1 min. Pick cells from plates on which individual colonies are moderately spaced and can easily be distinguished.
17. First round only: Loosen cells by pipetting up and down with a Pasteur pipet and transfer colonies to wells of a 24-well plate into 1 ml selection medium/250 µM L-histidinol. Second round only: Loosen cells as for first round, transferring them into 1 ml selection medium/500 µM L-histidinol/puromycin. 18. First round only: When cells are heavy in wells, split into 6-cm dishes in selection medium/500 µM L-histidinol. Second-round only: When cells are heavy in wells, split into 6-cm dishes in selection medium/500 µM L-histidinol/puromycin. All trypsinization is performed by standard methods (APPENDIX 3F), involving a quick PBS wash, a 1 to 3 min trypsin/EDTA incubation (2 ml per confluent 10-cm plate), and using 3rd selection medium/500 ìM L-histidinol (± puromycin) and containing 10% calf serum to dilute and stop the trypsin.
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19. First round only: Expand cells for testing in selection medium/500 µM L-histidinol. Freeze aliquots of cells for storage in liquid nitrogen and grow in selection medium/500 µM L-histidinol from this point on. Test for tTA or target gene expression (if applicable; see Support Protocol for methods that may be used). Or, if applicable, repeat transfection procedure with target gene plasmid(s), following steps 1 to 18 and using the options listed for second-round transfection. Second round only: Test for target gene expression by northern or immunoblotting after induction (see Support Protocol). Freeze aliquots for storage in liquid nitrogen and grow in selection medium/500 µM L-histidinol/puromycin from this point on. SUPPORT PROTOCOL
ANALYSIS OF TARGET GENE PROTEIN EXPRESSION This protocol outlines methods for the analysis of target gene expression and inducibility. Instructions for inducing stable cell lines, for examining transient target gene expression with and without induction, and for PCR amplification of the tTA gene are included, with references to detection procedures such as Southern, northern, and immunoblotting techniques. Induction of Stable Cell Lines Stable cell lines can be tested for tTA or target gene expression by comparing induced to uninduced cells for tTA mRNA or target gene mRNA (see Detection of tTA Transgene in Cellular or Tail DNA by Southern Blotting), or protein expression or protein activity. Multiple lines may be screened at a time. The night before induction, the cells are plated in selection medium/500 µM L-histidinol (see recipe) containing 3 µg/ml puromycin at an appropriate density such that cells will be subconfluent to confluent at the time of harvest. Cells are washed three times with PBS (APPENDIX 2), with gentle swirling. Immediately, the medium is replaced with selection medium without 0.5 µg/ml tetracycline hydrochloride (tet). (For tet+ controls, simply aspirate medium and replace with fresh selection medium containing tet.) Cells are incubated 6 to 48 hr in a humidified 37°C, 5% CO2 incubator, then trypsinized (APPENDIX 6 3F) and harvested at 4°C, and an aliquot of 0.15–0.4 × 10 cells is analyzed by immunoblotting (see UNIT 10.8). Alternatively, cells may be grown in selection medium in the presence of tet, transferred to tubes [with a quick wash with cold PBS followed by trypsinization (APPENDIX 3F) and stopping of the trypsin by addition of selection medium containing tet], washed three times with PBS (or just pelleted, for tet+ controls), and replated into selection medium with and without tet at an appropriate density such that the cells will be subconfluent to confluent at the time of harvest. Induction of Gene Expression in Transiently Transfected Cells Transient transfection of tet-regulated plasmids is useful in several situations, including the initial testing of the autoregulatory system in a given cell line, screening stable tTA expressors for inducible expression, and biological applications where transient expression is specifically desired.
Inducible Gene Expression
The night before the transfection, cells are split into medium containing 0.5 µg/ml tetracycline hydrochloride; the following day they are then transfected by methods appropriate for the cells being used (UNITS 9.1-9.4). Cells are induced by washing them three times in medium without tet. For CaPO4 transfection, washes are incorporated into those normally performed after glycerol shock (see Basic Protocol, step 11). Uninduced cell
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controls are washed with medium containing tet. Medium with and without tet is added to the appropriate plates, then the cells are incubated for 12 to 48 hr in a humidified 37°C, 5% CO2 incubator. The cells are harvested at 4°C and, if trypsinized (APPENDIX 3F), cold medium containing 10% FBS (with and without tet, as appropriate) is used to stop the action of the trypsin. Cells are pelleted for freezing or lysis, and tTA or target gene (experimental or reporter) expression can be analyzed by northern blotting (UNIT 4.9), immunoblotting (UNIT 10.8), or by an appropriate activity assay (see Commentary). Detection of tTA Transgene in Cellular or Tail DNA by PCR PCR is routinely used to detect the Tet-tTAk transgene in candidate transgenic mouse tail DNA. The forward primer derives from the minimal human CMV promoter, CMV-F1: 5′-TGACCTCCATAGAAGACACC-3′ The reverse primer, TTA-REV1, is specific for the tTA ORF: 5′-ATCTCAATGGCTAAGGCGTC-3′ Hot-start PCR (UNIT 15.1) is performed on 150 ng of each tail DNA to be analyzed in a reaction mix containing 1.5 mM MgCl2, 0.5 µM each primer, and 0.2 mM each dNTP. PCR cycling conditions are as follows: 1 cycle: 30 cycles:
1 cycle
3 min 45 sec 45 sec 90 sec 10 min
94°C 80°C 94°C 58°C 72°C 72°C 8°C
(pause) add Taq polymerase (denaturation) (annealing) (extension) (extension) (end).
Products are analyzed on a 1% to 1.3% agarose gel; the main product of interest is visible as a 290-bp band after ethidium bromide staining. Detection of tTA Transgene in Cellular or Tail DNA by Southern Blotting The tTA transgene may also be detected by Southern blot analysis (UNIT 2.9). Tail DNA is digested with EcoRI and blots are probed with a 761-bp XbaI-SalI tTA insert from pTet-tTA. This fragment detects a 1094-bp tTA fragment of the transgene. This probe may also be used to detect tTA mRNA by northern blotting (UNIT 4.9). REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Complete DMEM-10 Dulbecco’s minimal essential medium containing: 10% donor bovine calf serum (JRH Biosciences) 100 U/ml penicillin/100 µg/ml streptomycin (Life Technologies) 2 mM glutamine (Life Technologies) All DMEM complete medium used in this unit (with or without selection reagents or 0.5 ìg/ml tetracycline hydrochloride) may be stored protected from light ∼1 month at 4°C). Fetal bovine serum (FBS) may also be used in place of donor bovine calf serum, but the latter is less expensive. Protein Expression
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HEPES-buffered saline (HBS) 6 mM dextrose 137 mM NaCl 5 mM KCl 0.7 mM Na2HPO4⋅7H2O 21 mM HEPES (free acid) Adjust final pH to 7.05 with NaOH Filter sterilize and store in aliquots at −20°C Selection medium Complete histidine-free DMEM (Irvine Scientific, purchased without glutamine), containing: 10% donor bovine calf serum (JRH Biosciences) 100 U/ml penicillin/100 µg/ml streptomycin (Life Technologies) 2 mM glutamine (Life Technologies) 0.5 µg/ml tetracycline⋅HCl (Sigma; dilute 10 mg/ml stock in 70% ethanol and store protected from light at −20°C) 125 µM, 250 µM, or 500 µM L-histidinol (Sigma, dilute in water as a 125 mM stock and store at −20°C) COMMENTARY Background Information
Inducible Gene Expression
Inducible, tetracycline-regulated gene expression systems were initially developed to allow the controlled expression in eukaryotic cells of foreign genes not tolerated constitutively in cultured cells or during the development of transgenic animals. The general features of tetracycline-regulated gene expression strategies and their improvements over previous inducible expression systems have been addressed in current review articles (Gossen et al., 1993; Barinaga, 1994; Damke et al., 1995; Shockett and Schatz, 1996). The autoregulatory tTA system used in this protocol derives directly from a constitutive tTA system described by Gossen and Bujard (1992). Although tight regulatory control and high inducibility was achieved with the original system in HeLa cells, the inability to detect clones expressing moderate to high levels of tTA by immunoblotting suggested that the tTA was toxic when expressed constitutively. The autoregulatory tTA system was designed to overcome possible toxic effects of constitutive tTA expression by making tTA expression itself tetracycline regulated. Autoregulated tTA expression theoretically allows for the selection of clones expressing higher levels of tTA via an autoregulatory feed-forward mechanism that is activated only in the absence of tetracycline. In the presence of tetracycline, low-level tTA and target gene expression are driven from the minimal human CMV promoter. However, any tTA produced is
unable to bind to tet operators upstream of the tTA or target gene. Conversely, when tetracycline is removed from the system, the small amounts of tTA protein expressed from the minimal promoter can bind the tet operators upstream of the tTA gene, driving higher levels of tTA (for controlled periods of time) and, subsequently, target gene expression. The theoretical benefits of the autoregulatory tTA system have been confirmed by experiments in stably transfected NIH3T3 cell lines (Shockett et al., 1995). In these experiments, expression of the recombination activating genes RAG-1 and RAG-2, and subsequent DNA recombination activated by these proteins, was higher and more frequently detected among stable transfectants expressing autoregulatory tTA than in constitutive tTA expressors. In transgenic mice expressing a luciferase reporter target transgene, the levels of expression appear to be 1 to 2 orders of magnitude greater with the autoregulatory system, although the uninduced levels also appear to be higher. Since the description of the early tTA systems, several laboratories have created modified vectors, including streamlined versions containing both tTA and the target gene, viral vectors, and vectors in which expression of two different target genes may be differentially or co-regulated. Some of these systems and their applications have recently been reviewed (Shockett and Schatz, 1996).
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Critical Parameters and Troubleshooting Cell lines stably expressing both autoregulatory tTA and target genes have been derived at fairly high efficiencies by simultaneous transfection of all plasmids. This method may be faster, but it may require the screening of more clones than if stable lines with low basal and high induced levels of tTA are first derived and subsequently transfected with plasmids encoding the target genes. For the derivation of these clones, any selectable marker combination should theoretically work for consecutive cotransfection. Additionally, although the Basic Protocol describes calcium phosphate–mediated transfection of adherent fibroblast cell lines, the procedure can be adapted for other cell types using their optimal methods of transfection and selection. The protocol can also be scaled down to require fewer cells by using smaller dishes or wells and reducing all components proportionately. Using the autoregulatory tTA system, tTA mRNA induction appears to be a good indicator of induced tTA expression (see Support Protocol). Alternatively, the vector pUHC13-3 (Life Technologies) encoding luciferase under tet control may be transiently transfected into putative stable tTA expressors as previously described (see Support Protocol and Damke et al., 1995). Cells are then cultured for 12 to 48 hr in the presence and absence of tetracycline. Luciferase activity is easily measured in cell lysates using a kit (Luciferase Assay System and Dual-Luciferase Reporter Assay System; Promega) in which luciferase activity in cell lysates is normalized either to total protein determined using a Bradford protein assay (UNIT 10.1), or to a transfection control, respectively. Although basal expression of target plasmids tends to be higher when transiently transfected and luciferase detection is extremely sensitive, this method can be useful for the initial testing of the system in a given cell type (Damke et al., 1995). It is imperative after stable transfection with pTet-tTAk that cells be maintained in medium containing 0.5 µg/ml tetracycline to prevent any toxic effects of tTA expression and subsequent selection against clones expressing high levels of tTA.
Anticipated Results In the authors’ experience with stably transfected NIH3T3 cells, expression of induced
tTA and target gene has been observed by 6 hr and peaks at ∼12 hr after induction. In cells that stably express tTA, transient target gene expression has been observed by 12 hr. In cells transiently expressing tTA and a tet-sensitive luciferase reporter (pUHC13-3), luciferase activity induced by 2 orders of magnitude has been observed by 20 hr.
Time Considerations Starting with the plasmid vectors and following the transfection protocols above, stable clones expressing tTA (or tTA + target gene(s) if cotransfecting) are obtained in ∼12 to 14 days. Approximately 2 additional weeks are required for expansion and testing of candidate clones. Subsequent transfection of a stable inducible tTA clone with vectors expressing target genes will require the same amount of time. Transient transfection and inducible gene expression may be achieved within 48 hr.
Literature Cited Barinaga, M. 1994. Researchers devise a master gene control switch. Science 265:26-28. Damke, H., Gossen, M., Freundlieb, S., Bujard, H., and Schmid, S.L. 1995. Tightly regulated and inducible expression of dominant interfering dynamin mutant in stably transformed HeLa cells. Methods Enzymol. 257:209-220. Gossen, M. and Bujard, H. 1992. Tight control of gene expression in mammalian cells by tetracycline-responsive promoters. Proc. Natl. Acad. Sci. U.S.A. 89:5547-5551. Gossen, M., Bonin, A.L., and Bujard, H. 1993. Control of gene activity in higher eukaryotic cells by prokaryotic regulatory elements. Trends Biochem. Sci. 18:471-475. Shockett, P.E. and Schatz, D.G. 1996. Commentary: Diverse strategies for tetracycline-regulated inducible gene expression. Proc. Natl. Acad. Sci. U.S.A. 93:5173-5176. Shockett, P., Difilippantonio, M., Hellman, N., and Schatz, D. 1995. A modified tetracycline-regulated system provides autoregulatory, inducible gene expression in cultured cells and transgenic mice. Proc. Natl. Acad. Sci. U.S.A. 92:65226526.
Contributed by Penny Shockett Yale University School of Medicine New Haven, Connecticut David Schatz Howard Hughes Medical Institute and Yale University School of Medicine New Haven, Connecticut Protein Expression
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Overview of the Vaccinia Virus Expression System Vaccinia virus was introduced in 1982 as a vector for transient expression of genes in mammalian cells (Mackett et al., 1982; Panicali and Paoletti, 1982). This expression system differs from others in that transcription occurs in the cytoplasm of the cell rather than in the nucleus. As a vector, vaccinia virus has a number of useful characteristics, including a capacity that permits cloning large fragments of foreign DNA (>20 kbp) with retention of infectivity, a wide host range, a relatively high level of protein synthesis, and “appropriate” transport, secretion, processing, and posttranslational modifications as dictated by the primary structure of the expressed protein and the cell type used. For example, N- and O-glycosylation, phosphorylation, myristylation, and cleavage, as well as assembly of expressed proteins, occur in an apparently faithful manner. Laboratory applications of vaccinia virus vectors include production of biologically active proteins in tissue culture, analysis of mutant forms of proteins, and determination of transport and processing signals. In addition, recombinant vaccinia viruses have been important for immunological studies (Bennink and Yewdell, 1990). Infected cells can serve as targets to analyze the antigenic specificity of cytotoxic T cells. Recombinant viruses can be used to infect animals in order to determine cell-mediated and humoral responses to specific proteins, and are being evaluated as candidate vaccines for human and veterinary uses. Several variations of the vaccinia vector system have been developed and either standard or more attenuated, host-restricted strains may be used. After obtaining the virus stock (UNIT 16.16), the gene of interest is placed under control of a vaccinia virus promoter and integrated into the genome of vaccinia so as to retain infectivity (UNIT 16.17). Alternatively, expression can be achieved by transfecting a plasmid containing the vaccinia promoter– controlled gene into a cell that has been infected with vaccinia virus. These recombinant viruses are then characterized using various methods (UNIT 16.18). In still another variation, the bacteriophage T7 RNA polymerase gene can be integrated into the genome
UNIT 16.15
of vaccinia so that a gene controlled by a T7 promoter, either in a transfected plasmid or a recombinant vaccinia virus, will be expressed.
VACCINIA REPLICATION CYCLE Vaccinia is the prototypic member of the Orthopoxvirus genus of the Poxviridae family. Poxviruses differ from other eukaryotic DNA viruses in that they replicate in the cytoplasm rather than in the nucleus. Vaccinia virus has a linear, double-stranded DNA genome of nearly 200,000 bp that encodes most of the proteins needed for replication and transcription in the cytoplasm. The replication cycle of poxviruses is represented in Figure 16.15.1. The virus particle or virion consists of a complex core structure surrounded by a lipoprotein envelope. Remarkably, all proteins necessary for transcription of the early class of genes are packaged with the genome in the core. These include the following virus-encoded proteins: a multisubunit, DNA-dependent RNA polymerase, an early transcription factor, capping and methylating enzymes, and a poly(A) polymerase. The transcription system is activated upon infection, and early mRNAs and proteins can be detected within the first hour. The early mRNAs closely resemble their eukaryotic counterparts—they are capped, methylated, polyadenylated, and of discrete size. Termination of transcription o c c u r s ∼5 0 b a s e s a f te r t h e se q u e n c e TTTTTNT (where N can be any nucleotide; the termination signal is actually recognized in the nascent RNA as UUUUUNU). There is no evidence for splicing or other kinds of processing involving RNA cleavage. DNA replication begins within a few hours postinfection and leads successively to the intermediate and late phases of gene expression. The promoters associated with intermediate and late genes differ in sequence from early promoters and have different transcription factor requirements. The intermediate factors are synthesized early in order to facilitate transcription of newly replicated viral DNA. Some of the late factors are products of intermediate genes and hence are made and used after viral DNA replication. Protein Expression
Contributed by Bernard Moss and Patricia L. Earl Current Protocols in Molecular Biology (1998) 16.15.1-16.15.5 Copyright © 2002 by John Wiley & Sons, Inc.
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envelope RNA polymerase transcription factor poly(A) polymerase capping enzyme methylating enzymes
e tor as tiva c er a ns early mRNA lym tra po e t A dia DN erme t in early enzymes
growth factor
nucleus
mature virion
morphogenesis (4-20 hr)
intermediate mRNA
late transactivators
n
uncoating
DNA replication (2-12 hr)
er resolutio
core
concatem
DNA
late mRNA late enzymes early transcription factor structural proteins cleavage glycosylation acylation phosphorylation
cell
Figure 16.15.1 Replication cycle of vaccinia virus. After entry of vaccinia virus into the cells, early genes are expressed, leading to secretion of several proteins (including a growth factor), uncoating of the virus core, and the synthesis of DNA polymerase (and other replication proteins), RNA polymerase subunits, and transcriptional transactivators of the intermediate class of genes. After DNA replication, intermediate mRNAs are made, some of which encode late transcriptional transactivators. This in turn leads to expression of late genes, which encode structural proteins, enzymes, and early transcription factors that are packaged in the assembling virus particles. Some of the mature virions are wrapped in Golgi-derived membranes and are released from the cell. The bold arrows indicate products that exit the cell. Reprinted with permission from Raven Press (Moss, 1996a).
Overview of the Vaccinia Virus Expression System
The mRNAs transcribed from intermediate and late genes differ from typical early mRNAs as follows. First, the termination signal UUUUUNU is not recognized; consequently the mRNAs are long and heterogeneous in length, making procedures like northern blotting virtually useless. Second, the 5′ ends of intermediate and late mRNAs contain a capped poly(A) leader of ~35 nucleotides that is probably the result of an RNA polymerase slippage mechanism within the conserved AAA at the initiation site. Many early proteins are not synthesized beyond ∼6 hr postinfection (unless an inhibitor of DNA replication such as cytosine arabinoside is added) because of the cessation of early gene transcription and the relatively short half-life of all mRNAs at late times. Some genes, however, have tandem early and late promoters so that they are expressed throughout the growth cycle. Intermediate
genes are expressed for a relatively short period after DNA replication. Either because of intrinsic promoter strength, DNA copy number, or prolonged expression (>20 hr), much more protein is made from the strongest late promoters than from early or intermediate promoters.
EFFECTS OF VACCINIA INFECTION Standard vaccinia virus strains can productively infect most mammalian and avian cell lines, with a few exceptions such as Chinese hamster ovary (CHO) cells, but may not complete the replication cycle in primary lymphocytes or macrophages. The host-restricted modified vaccinia virus Ankara (MVA) strain has been shown to replicate efficiently only in chick embryo fibroblasts and BHK-21 cells, but viral and recombinant protein expression occurs in all cell lines infectable by standard strains. Vaccinia infection generally results in
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rapid inhibition of host nucleic acid and protein synthesis. Inhibition of host protein synthesis is dramatic and probably results from several factors whose identities are uncertain; the relative contribution of each factor may depend on the virus multiplicity, cell type, and time of analysis. At the time of maximal late gene expression, host protein synthesis has been largely suppressed, facilitating the identification of viral or recombinant proteins by pulselabeling with radioactive amino acids (UNIT 10.18). In fibroblasts, the initial cytopathic effect is cell rounding, and is obvious by several hours postinfection. Nevertheless, the majority of cells remain intact for ≥48 hr. Some virus strains may cause less cytopathic effects than others, but mutants that efficiently express recombinant proteins but do not inhibit host gene expression have not been identified. Approximately 100 to 200 plaque-forming units (pfu), equivalent to ∼2500 to 5000 particles, are made per cell within a 20- to 40-hr period. With the commonly used vaccinia virus Western Reserve (WR) strain, >95% of the infectious virus remains cell-associated. With some other vaccinia virus strains, notably IHD-J, larger amounts of extracellular virus are produced.
VACCINIA VECTOR EXPRESSION SYSTEM Genes or cDNAs containing open reading frames derived from prokaryotic, eukaryotic, or viral sources have been expressed using vaccinia virus vectors. The gene of interest is usually placed next to a vaccinia promoter and this expression cassette is then inserted into the virus genome by homologous recombination or direct ligation (UNIT 16.17). Use of poxvirus promoters is essential because cellular and other viral promoters are not recognized by the vaccinia transcriptional apparatus. Strong late promoters are preferable when high levels of expression are desired. An early promoter, however, may be of use if it is desirable to express proteins prior to the occurrence of major cytopathic effects, or when the purpose is to make cells that express antigens in association with major histocompatibility class I molecules so they form cytotoxic T cell targets or prime animals for a cytotoxic T cell response. (The ability of vaccinia viral vectors to direct this type of antigen presentation seems to diminish late in infection.) The most versatile and widely used promoters contain early and late promoter elements. Transcripts originating early will terminate after the sequence
TTTTTNT; thus, any cryptic TTTTTNT termination motifs within the coding sequence of the gene should be altered by mutagenesis if an early poxvirus promoter is used (Earl et al., 1990). To mimic vaccinia virus mRNAs, untranslated leader and 3′-terminal sequences are usually kept short. A number of plasmids have been designed with restriction endonuclease sites for insertion of foreign genes downstream of vaccinia promoters (UNIT 16.17). The expression cassette is flanked by vaccinia DNA to permit homologous recombination when the plasmid is transfected into cells that have previously been infected with a vaccinia virus. The flanking vaccinia virus DNA is chosen so that recombination will not interrupt an essential viral gene. Without selection, the ratio of recombinant to parental vaccinia virus is usually ∼1:1000. Although this frequency is high enough to permit the use of plaque hybridization (UNITS 6.3 & 6.4) or immunoscreening (UNIT 6.7) to pick recombinant viruses, a variety of methods have been employed to facilitate identification of recombinant viruses. Some widely used selection or screening techniques are described in UNIT 16.17. Commonly, the expression cassette is flanked by segments of the vaccinia thymidine kinase (TK) gene so that recombination results in inactivation of TK. Virus with a TK− phenotype can then be distinguished from those with a TK+ phenotype by infecting a TK− cell line in the presence of 5-bromodeoxyuridine (BrdU), which must be phosphorylated by TK to be lethally incorporated into the virus genome. Alternatively, recombinant viruses can be selected by the co-expression of a bacterial antibiotic resistance gene such as guanine phosphoribosyltransferase (gpt). Co-expression of the Escherichia coli lacZ gene or other color markers allows rapid screening of recombinant virus plaques. Complementation of host-range and plaque-forming defects are also useful for isolation of recombinant viruses (UNIT 16.17).
STEPS FOR EXPRESSION OF GENES USING VACCINIA VECTORS The expression of genes using the vaccinia expression system is presented in detail in UNITS 16.16-16.18 and is outlined in the flowchart in Figure 16.15.2. A brief overview is presented below. 1. Prepare a stock of standard, host-rangerestricted, or bacteriophage T7 RNA polym-
Protein Expression
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culture cell lines (UNIT 16.16, Basic Protocols 1 & 2)
infect cell line and prepare wild-type vaccinia stock (UNIT 16.16, Basic Protocol 3)
purify vaccinia virus (UNIT 16.17, Support Protocol 1)
titer vaccinia virus stock using plaque assay (UNIT 16.16, Support Protocol 1)
insert gene into transfer vector (UNIT 16.17, Basic Protocol 1)
isolate vaccinia virus DNA (UNIT 16.17, Support Protocol 2)
infect cells with vaccinia virus and transfect with vector (UNIT 16.17, Basic Protocol 1)
amplify a plaque (UNIT 16.17, B asic Protocol 3)
select and screen recombinant virus plaques (UNIT 16.17, Basic Protocol 2)
titer the amplified plaque (UNIT 16.16, Support Protocol 1)
prepare recombinant virus stock (UNIT 16.16, Basic Protocol 3)
express gene and analyze protein by • polyacrylamide gel electrophoresis • immunoblotting • immunoprecipitation
analyze recombinant virus by • polymerase chain reaction (UNIT 16.18, Basic Protocol 1) • Southern blot hybridization (UNIT 16.18, Basic Protocol 2) • DNA dot-blot hybridization (UNIT 16.18, Basic Protocol 3) • immunoblotting (UNIT 16.18, Alternate Protocol) • immunostaining (UNIT 16.16 )
Figure 16.15.2 Flowchart showing protocols for gene expression using the recombinant vaccinia virus system.
Overview of the Vaccinia Virus Expression System
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erase–expressing vaccinia virus. At the same time, subclone the gene of interest into a plasmid transfer vector (UNITS 16.16 & 16.17). 2. Infect cells with vaccinia virus and transfect with the recombinant plasmid (UNIT 16.17). 3. Lyse the cells and plaque the virus under suitable selection or screening conditions (UNIT 16.17). 4. Pick plaques and confirm the presence and/or expression of the foreign gene (UNIT 16.18). 5. Amplify the plaque and prepare recombinant virus stock (UNIT 16.17). 6. Infect cells and analyze proteins synthesized (UNIT 16.18).
SAFETY PRECAUTIONS FOR USING VACCINIA Vaccinia virus is not to be confused either with variola virus, another member of the Orthopoxvirus genus that caused smallpox prior to its eradication, or with varicella virus, a herpes virus that causes chicken pox. Until 1972, vaccinia virus was routinely used in the United States as a live vaccine to prevent smallpox, and a residual scar, commonly on the upper arm, is evidence of that vaccination. To prevent laboratory infections, the Centers for Disease Control (CDC) and the National Institutes of Health (NIH) recommend that individuals who come into contact with vaccinia virus receive vaccinations at 10-year intervals (Richmond and McKinney, 1993). The CDC has supplied vaccine for such purposes when requested by qualified health workers. Eczema or an immunodeficiency disorder in the laboratory worker or a close contact, however, may be a contraindication to vaccination, which should only be given under medical supervision. The benefits of routine vaccination for healthy investigators have also been questioned (Baxby, 1989; Wenzel and Nettelman, 1989). Vaccinia virus is very stable and parenteral inoculation, ingestion, and droplet or aerosol exposure of mucous membranes are the primary hazards to laboratory or animal care personnel. Standard biosafety level 2 (BL-2) practices and class I or II biological safety cabinets should be employed (Richmond and McKinney, 1993). The NIH intramural program has lowered the safety requirements for highly attenuated vaccinia virus strains such as modified vaccinia virus Ankara (MVA; UNIT 16.6) and NYVAC (Tartaglia et al., 1992). However, local institutional biosafety offices should be contacted to determine current policy regarding vaccination and physical containment.
Additional precautions may be necessary for expression of certain genes such as toxins or large segments of other viral genomes, and guidelines for recombinant DNA work should be consulted. Approval of local biosafety committees may be necessary.
LITERATURE CITED Baxby, D. 1989. Smallpox vaccination for investigators. Lancet 2:919. Bennink, J.R. and Yewdell, J.W. 1990. Recombinant vaccinia viruses as vectors for studying T lymphocyte specificity and function. Curr. Topics Microbiol. Immunol. 163:153-184. Earl, P.L., Hügin, A.W., and Moss, B. 1990. Removal of cryptic poxvirus transcription termination signals from the human immunodeficiency virus type 1 envelope gene enhances expression and immunogenicity of a recombinant vaccinia virus. J. Virol. 64:2448-2451. Mackett, M., Smith, G.L., and Moss, B. 1982. Vaccinia virus: A selectable eukaryotic cloning and expression vector. Proc. Natl. Acad. Sci. U.S.A. 79:7415-7419. Moss, B. 1996a. Poxviridae: The viruses and their replication. In Virology (B.N. Fields, D.M. Knipe, and P. M. Howley, eds.) pp. 2637-2671. Raven Press, New York. Panicali, D. and Paoletti, E. 1982. Construction of poxviruses as cloning vectors: Insertion of the thymidine kinase gene from herpes simplex virus into the DNA of infectious vaccinia virus. Proc. Natl. Acad. Sci. U.S.A. 79:4927-4931. Richmond, J.Y. and McKinney, R.W. 1993. Biosafety in Microbiological and Biomedical Laboratories, 3rd ed. U.S. Government Printing Office, Washington, D.C. Tartaglia, J., Perkus, M.E., Taylor, J., Norton, E.K., Audonnet, J.C., Cox, W.I., Davis, S.W., Vanderhoeven, J., Meignier, B., Riviere, M., Languet, B., and Paoletti, E. 1992. NYVAC–A highly attenuated strain of vaccinia virus. Virology 188:217-232. Wenzel, R.P. and Nettelman, M.D. 1989. Smallpox vaccination for investigators using vaccinia recombinants. Lancet 2:630-631.
KEY REFERENCES Johnson, G.P., Goebel, S.J., and Paoletti, E. 1993. An update on the vaccinia virus genome. Virology 196: 381-401. Moss, B. 1996b. Genetically engineered poxviruses for recombinant gene expression, vaccination, and safety. Proc. Natl. Acad. Sci. U.S.A. 93:11341-11348.
Contributed by Bernard Moss and Patricia L. Earl National Institute of Allergy and Infectious Diseases Bethesda, Maryland
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Preparation of Cell Cultures and Vaccinia Virus Stocks
UNIT 16.16
This unit describes the maintenance of cell lines used with vaccinia virus, both in monolayer cultures (see Basic Protocol 1) and in suspension (see Basic Protocol 2). The suspended cell culture is then used in the preparation of vaccinia virus stocks (see Basic Protocol 3). The preparation of chick embryo fibroblasts (CEF) is also presented (see Basic Protocol 4), for use in the production of the highly attenuated and host range–restricted modified vaccinia virus Ankara (MVA) strain of vaccinia virus (see Basic Protocol 5). Additionally, support protocols are presented for the titration of standard and MVA vaccinia virus stocks (see Support Protocols 1 and 2, respectively). Because standard vaccinia virus strains have a broad host range, there is considerable latitude in the selection of cell lines; those described below (see Basic Protocols 1 and 2) have been found to give good results. BS-C-1 cells give the best results for a plaque assay, whereas HeLa cells are preferred for preparation of virus stocks. CV-1 cells can be used for both procedures, but they are generally used for transfection (UNIT 16.17). Human thymidine kinase–negative (TK−) 143B cells are used when TK selection is employed (UNIT 16.17), but they can be used for transfection as well as for a plaque assay. Either CEF or BHK-21 cells are used to propagate MVA and prepare recombinant MVA. Table 16.16.1 presents a summary of the uses for specific cell lines. NOTE: Carry out all procedures in this unit using sterile technique, preferably in a biosafety cabinet.
Table 16.16.1
Cell Lines Used in Specific Vaccinia Protocols
Cell line
Usea
Procedure
HeLa S3
Virus stock preparation Virus purification Plaque amplification
UNIT 16.16 Basic
Plaque assay Transfection (optional) XGPRT selection Plaque amplification (optional)
UNIT 16.16
Transfection Virus stock preparation (optional) Plaque assay (optional)
UNIT 16.17 Basic
TK selection Plaque assay (optional) Transfection (optional)
UNIT 16.17 Basic
MVA procedures
UNIT 16.16 Basic Protocol 5, Support Protocol 2; UNIT 16.17 Basic Protocol
4
UNIT 16.16 Basic Protocol 5, Support Protocol 2; UNIT 16.17 Basic Protocol
4
BS-C-1
CV-1
HuTK− 143B
CEF BHK-21
MVA procedures
Protocol 3 Support Protocol 1 UNIT 16.17 Basic Protocol 3 UNIT 16.17
Support Protocol 1 Protocol 1 UNIT 16.17 Basic Protocol 2 UNIT 16.17 Basic Protocol 3 UNIT 16.17 Basic
Protocol 1 Protocol 3 UNIT 16.16 Support Protocol 1 UNIT 16.16 Basic
Protocol 2 Support Protocol 1 UNIT 16.17 Basic Protocol 1 UNIT 16.16
aThe preferred use(s) for each cell line is listed first; if optional is indicated, the cell line can be used for the indicated
procedure, but the results may not be as good as those from the preferred cell line.
Protein Expression Contributed by Patricia L. Earl, Norman Cooper, Linda S. Wyatt, Bernard Moss, and Miles W. Carroll
16.16.1
Current Protocols in Molecular Biology (1998) 16.16.1-16.16.13 Copyright © 1998 by John Wiley & Sons, Inc.
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BASIC PROTOCOL 1
CULTURE OF MONOLAYER CELLS Frozen cells are thawed and grown in appropriate complete medium containing twice the maintenance amount of serum (see below). When the cells are confluent, they are treated with trypsin/EDTA, diluted, and maintained in appropriate complete medium containing 10% FBS (see Table 16.16.2). Table 16.16.2
Media Used for Growth and Maintenance of Cell Linesa
Cell line
Maintenance medium
Start-up mediuma
BHK-21 BS-C-1 CEF CV-1 HeLa S3 HuTK− 143B
Complete MEM-10 Complete MEM-10 Complete MEM-10 Complete DMEM-10 Complete spinner medium-5 Complete MEM-10/BrdU
Complete MEM-20 Complete MEM-20 Complete MEM-10 Complete DMEM-20 Complete MEM-10 Complete MEM-20/BrdU
aSee Reagents and Solutions for recipes.
Materials Frozen ampule of cells (Table 16.16.2): BS-C-1 (ATCC no. CCL26), CV-1 (ATCC no. CCL70), HuTK− 143B (ATCC no. CRL8303), or BHK-21 (ATCC no. CCL10) cells 70% ethanol Start-up medium (Table 16.16.2): complete MEM-20, complete DMEM-20, or complete MEM-20/BrdU (see recipes), 37°C Maintenance medium (Table 16.16.2): complete MEM-10, complete DMEM-10, or complete MEM-10/BrdU (see recipes), 37°C PBS (optional; APPENDIX 2) Trypsin/EDTA: 0.25% (w/v) trypsin/0.02% (w/v) EDTA, 37°C 25-cm2 and 150-cm2 tissue culture flasks Humidified, 37°C, 5% CO2 incubator Begin the culture 1. Thaw a frozen ampule of cells in a 37°C water bath. 2. Sterilize the ampule tip with 70% ethanol, break the neck, and transfer the cells with a pipet into a 25-cm2 tissue culture flask containing 5 ml start-up medium. Rotate the flask to evenly distribute the cells and place overnight in a humidified, 5% CO2 incubator at 37°C. 3. Aspirate the start-up medium and replace with appropriate maintenance medium. Return cells to the CO2 incubator at 37°C and check daily for confluency. Cells should be passaged when they become confluent. Generally, if cells are split 1:20, they reach confluence in 1 week and need not be counted.
Maintain the culture 4. When the cells are a confluent monolayer, aspirate medium.
Preparation of Cell Cultures and Vaccinia Virus Stocks
5. Wash cells once with PBS or trypsin/EDTA to remove remaining serum from the cells by covering cells with the solution and pipetting it off.
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6. Overlay cells with 37°C trypsin/EDTA using a volume that is just enough to cover the monolayer (e.g., 0.3 ml for a 25-cm2 flask). Allow to sit 30 to 40 sec (cells should become detached) and shake the flask to completely detach cells. 7. Add 1.4 ml appropriate maintenance medium. Pipet the cell suspension up and down several times to disrupt clumps (these cells are ready for passage). 8. Remove 0.5 ml cell suspension and add it to a new 150-cm2 tissue culture flask containing 30 ml maintenance medium. Rotate the flask to evenly distribute the cells and place in a CO2 incubator at 37°C until the cells are confluent (∼1 week). Maintain the cells by splitting ∼1:20 in maintenance medium at approximately weekly intervals. Cells can be maintained in smaller flasks if desired. If so, volumes should be adjusted proportionately.
CULTURE OF CELLS IN SUSPENSION HeLa S3 cells are maintained in complete spinner medium-5.
BASIC PROTOCOL 2
Materials Frozen ampule of HeLa S3 cells (ATCC no. CCL2.2) 70% ethanol Complete MEM-10 (see recipe), 37°C Trypsin/EDTA: 0.25% (w/v) trypsin/0.02% (w/v) EDTA, 37°C Complete spinner medium-5 (see recipe), 37°C 25-cm2 tissue culture flask Humidified, 37°C, 5% CO2 incubator Sorvall H-6000A rotor (or equivalent) 100- or 200-ml vented spinner bottles and caps with filters (Bellco) Additional reagents and equipment for counting cells with a hemacytometer (APPENDIX 3F) Begin the culture 1. Thaw a frozen ampule of HeLa S3 cells in a 37°C water bath. 2. Sterilize the ampule tip with 70% ethanol, break the neck, and transfer the cells with a pipet to a 25-cm2 tissue culture flask containing 5 ml complete MEM-10. Rotate the flask to evenly distribute the cells and place overnight in a humidified, 5% CO2 incubator at 37°C. 3. Aspirate medium. Overlay cells with 0.5 ml of 37°C trypsin/EDTA and let sit 30 to 40 sec. Since these cells do not attach firmly to the flask, they should not be washed prior to trypsinization.
4. Add 10 ml complete spinner medium-5 and transfer cells to a 50-ml centrifuge tube. Centrifuge 5 min in a Sorvall H-6000A rotor at 2500 rpm (1800 × g), room temperature, and discard supernatant. 5. Suspend cell pellet in 5 ml complete spinner medium-5 by pipetting up and down to disrupt clumps. 6. Add 50 ml complete spinner medium-5 to a 100- or 200-ml vented spinner bottle and transfer the cell suspension to this bottle.
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7. Remove 1 ml cell suspension and count the cells using a hemacytometer (APPENDIX 3F). Add complete spinner medium-5 to adjust the cell density to 3–4 × 105 cells/ml. Place cells in a 37°C incubator without CO2 and stir continuously. The initial high density is used because some cells are not viable.
8. Grow cells for two successive days, counting cells daily and adding complete spinner medium-5 as necessary to maintain a concentration of 3–4 × 105 cells/ml. Maintain the culture 9. Remove 1 ml cell suspension and monitor the cells using a hemacytometer. 10. When the density is 4–5 × 105 cells/ml, dilute the cells to 1.5 or 2.5 × 105 cells/ml with complete spinner medium-5 for alternate day or daily feeding, respectively. 11. Place a 100- or 200-ml vented spinner bottle containing 50 ml or 100 ml cells, respectively, in 37°C incubator without CO2 and stir continuously. Passage every 1 to 2 days. HeLa S3 cells are grown and maintained in complete spinner medium-5 in vented spinner bottles at 37°C without CO2. Cells are diluted with fresh medium at 1- to 2-day intervals to keep the cell density between 1.5 × 105 and 5 × 105 cells/ml. BASIC PROTOCOL 3
PREPARATION OF A VACCINIA VIRUS STOCK To prepare a vaccinia virus stock, HeLa S3 cells from a spinner culture (see Basic Protocol 2) are plated the day before infection and allowed to attach. They are then infected with trypsinized virus. After several days, the infected cells are harvested and lysed during repeated freeze-thaw cycles. The virus stock is then aliquoted and stored at −70°C. To titer this stock, see Support Protocol 1. The protocol can be modified for monolayer cultures. Materials HeLa S3 cells from suspension culture (see Basic Protocol 2) Complete MEM-10 and -2.5 (see recipe), 37°C Vaccinia virus (ATCC no.VR1354 or equivalent) 0.25 mg/ml trypsin (2× crystallized and salt-free; Worthington; filter sterilize and store at −20°C) Sorvall H-6000A rotor (or equivalent) 150-cm2 tissue culture flask Humidified, 37°C, 5% CO2 incubator Additional reagents and equipment for counting cells with a hemacytometer (APPENDIX 3F) Prepare cells 1. Count HeLa S3 cells from a suspension culture using a hemacytometer (APPENDIX 3F). 2. Centrifuge 5 × 107 cells 5 min in a Sorvall H-6000A rotor at 2500 rpm (1800 × g), room temperature, and discard supernatant. 3. Resuspend cells in 25 ml of 37°C complete MEM-10, dispense in a 150-cm2 tissue culture flask, and place overnight in a humidified, 5% CO2 incubator at 37°C.
Preparation of Cell Cultures and Vaccinia Virus Stocks
Increase the number of HeLa cells proportionately if more than one 150-cm2 flask is to be infected. As an alternative to HeLa suspension cells, prepare 150-cm2 flasks of monolayer cells as described (see Basic Protocol 1) and continue with step 4 below.
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Trypsinize virus 4. Just prior to use, mix an equal volume of vaccinia virus stock and 0.25 mg/ml trypsin, and vortex vigorously. Incubate 30 min in a 37°C water bath, vortexing at 5- to 10-min intervals throughout the incubation. Virus stocks are usually at a titer of ∼2 × 109 pfu/ml, but may be significantly lower depending on the source. Vortexing usually breaks up any clumps of cells. However, if there are still visible clumps, chill to 0°C and sonicate 30 sec on ice (UNIT 16.17). Sonication can be repeated several times but the sample should be allowed to cool on ice between sonications.
Infect cells 5. Dilute trypsinized virus in complete MEM-2.5 at 2.5–7.5 × 107 pfu/ml. Decant or aspirate medium from the 150-cm2 flask of cells and add 2 ml diluted, trypsinized virus. Place 2 hr in a CO2 incubator at 37°C, rocking flask by hand at 30-min intervals. The optimal multiplicity of infection (MOI) is 1 to 3 pfu/cell. Multiplicities of 0.1 pfu/cell may be necessary if the titer of the initial virus stock is low. The trypsinized virus must be diluted ≥10-fold to avoid detaching the cells.
6. Overlay cells with 25 ml complete MEM-2.5 and place 3 days in a CO2 incubator at 37°C. 7. Detach the infected cells from the flask by shaking and pour or pipet into a sterile plastic screw-cap tube. Centrifuge 5 min at 1800 × g, 5° to 10°C, and discard supernatant. 8. Resuspend cells in 2 ml complete MEM-2.5 (per initial 150-cm2 flask) by gently pipetting or vortexing. Harvest virus stock 9. Lyse the cell suspension by freeze-thaw cycling as follows: freeze in dry ice/ethanol, thaw in a 37°C water bath, and vortex. Carry out the freeze-thaw cycling a total of three times. 10. Keep the virus stock on ice and divide it into 0.5- to 2-ml aliquots. Store the aliquots indefinitely at −70°C. TITRATION OF VACCINIA VIRUS STOCKS BY PLAQUE ASSAY Serial dilutions of the trypsinized virus stock (see Basic Protocol 3) are used to infect the appropriate cell line. After several days growth, the medium is removed and the cells are stained with crystal violet. Plaques appear as 1- to 2-mm-diameter areas of diminished staining due to the retraction, rounding, and detachment of infected cells.
SUPPORT PROTOCOL 1
Additional Materials (also see Basic Protocols 1 and 3) BS-C-1 cells from confluent monolayer culture (see Basic Protocol 1) Virus stock (see Basic Protocol 3) 0.1% (w/v) crystal violet (Sigma) in 20% ethanol (store indefinitely at room temperature) 6-well, 35-mm2 tissue culture dishes Additional reagents and equipment for counting cells with a hemacytometer (APPENDIX 3F) Protein Expression
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Prepare cell and virus stocks 1. Trypsinize confluent monolayer of BS-C-1 cells as described (see Basic Protocol 1, steps 4 to 7). 2. Count the cells using a hemacytometer (APPENDIX 3F). 3. Seed wells of a 6-well, 35-mm2 tissue culture dish with 5 × 105 cells per well BS-C-1 cells in 2 ml complete MEM-10. Place overnight in a humidified, 5% CO2 incubator at 37°C to reach confluency. 4. Trypsinize virus stock (see Basic Protocol 3, step 4). 5. Make nine 10-fold serial dilutions (UNIT 1.11) of the trypsinized virus in complete MEM-2.5, using a fresh pipet for each dilution. Perform assay 6. Remove medium from BS-C-1 cells and infect cells in duplicate wells with 0.5 ml of the 10−7, 10−8, and 10−9 trypsinized virus dilutions. Place 1 to 2 hr in a CO2 incubator at 37°C, rocking dish at 15- to 30-min intervals to spread virus uniformly and keep cells moist. 7. Overlay cells in each well with 2 ml complete MEM-2.5 and place 2 days in a CO2 incubator at 37°C. 8. Remove medium and add 0.5 ml of 0.1% crystal violet to each well. Incubate 5 min at room temperature. 9. Aspirate crystal violet and allow wells to dry. 10. Determine the titer by counting plaques within the wells and multiplying by the dilution factor. Most accurate results are obtained from wells with 20 to 80 plaques. In determining the titer, take into account the 1:1 dilution of the virus stock with trypsin. BASIC PROTOCOL 4
PREPARATION OF CHICKEN EMBRYO FIBROBLASTS The attenuated, replication-deficient modified vaccinia virus Ankara (MVA) can be used as an alternative to standard strains of vaccinia virus, for use as a vector to express foreign genes. MVA was derived from the vaccinia virus strain Ankara by multiple (>570) passages in chick embryo fibroblasts (CEF). Although MVA expresses recombinant proteins efficiently, the assembly of infectious particles is interrupted in human and most other mammalian cells lines, providing an added degree of safety to laboratory personnel. The NIH Intramural Biosafety Committee has determined that individuals do not need to be vaccinated to work with MVA, and can do so under biosafety level 1 (BL-1) conditions if no other vaccinia virus strains are being manipulated at the same location by other investigators. Recombinant MVA expressing bacteriophage T7 RNA polymerase, which can be used for high level transient expression, has also been constructed (UNIT 16.19) CEF or Syrian hamster kidney cells (BHK-21, see Basic Protocol 1) are permissive for growth of MVA. MVA is titered by immunostaining (see Support Protocol 2), because it does not form distinct plaques for accurate quantitation.
Preparation of Cell Cultures and Vaccinia Virus Stocks
In this protocol, nine-day-old chicken embryos are trypsinized and plated in appropriate medium. After the cells form a confluent monolayer, they are transferred directly to a 31°C incubator where they can be held for 2 to 3 weeks before making secondary CEF for virus infection. With each passage, CEF require longer to become confluent; therefore use of the second passage is recommended.
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Materials Nine-day-old embryonated eggs (Specific Pathogen Free Eggs, SPAFAS) 70% ethanol MEM with no additives, 37°C Trypsin/EDTA: 0.25% (w/v) trypsin/0.02% (w/v) EDTA, 37°C Complete MEM-10 (Table 16.16.2; see recipe), 37°C Sterile dissecting scissors and forceps 100-cm2 sterile petri dishes 10-ml syringes Sterile trypsinization flask with magnetic stir bar Humidified, 37° and 31°C, 5% CO2 incubators 500-ml beakers with two layers of gauze taped over tops Sorvall RC-3B centrifuge and 250-ml centrifuge bottles (or equivalent) 150-cm2 tissue culture flasks Prepare tissue 1. Position ten 9-day-old embryonated eggs with air space (blunt end) up and spray with 70% ethanol. 2. Crack the top of an egg with sterile dissecting scissors, and cut off shell to just above the membrane while keeping the latter intact. Remove membrane with sterile forceps. Repeat with remaining eggs. NOTE: Healthy eggs have well-formed blood vessels.
3. Remove the embryo from each egg and combine in a 100-cm2 sterile petri dish. 4. Remove head and feet from each embryo and place the rest of the body in a 100-cm2 sterile petri dish containing 10 ml MEM with no additives. Mince embryos by squeezing through 10-cc syringes (∼5 embryos/syringe) into a sterile trypsinization flask. Dissociate cells 5. Add 100 ml of 37°C trypsin/EDTA and incubate 5 min in a humidified, 5% CO2 incubator at 37°C, with stirring. 6. Decant fluid from the trypsinization flask into a 500-ml beaker covered with gauze. Transfer filtrate to a 250-ml centrifuge bottle. 7. Add 100 ml fresh trypsin/EDTA to the remaining tissue in the trypsinization flask, and incubate 5 min at 37°C. 8. Pour digest into a second 500-ml beaker covered with gauze, and add filtrate to the first filtrate in the 250-ml centrifuge bottle. 9. Centrifuge 10 min at 1200 × g, 4°C, in a Sorvall RC-3B centrifuge. 10. Aspirate and discard supernatant from pellet, add 10 ml complete MEM-10, resuspend by pipetting ∼10 to 15 times, and adjust volume to 100 ml. 11. Transfer to a 250-ml centrifuge bottle and centrifuge 10 min at 1200 × g, 4°C. 12. Resuspend pellet in 5 ml complete MEM-10 and adjust volume to 30 ml. Prepare confluent culture 13. Add 1 ml cell suspension to each of thirty 150-cm2 tissue culture flasks containing 30 ml complete MEM-10.
Protein Expression
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14. Incubate at 37°C for several days until confluent, and move flasks to a humidified, 31°C, 5% CO2 incubator for storage. The CEF cell cultures can be held for 2 to 3 weeks at 31°C without further attention. Primary CEF can also be used directly for virus growth, or the trypsinized stock of CEF can be frozen in liquid nitrogen for future use. BASIC PROTOCOL 5
PREPARATION OF AN MVA STOCK To prepare an MVA stock, CEF (see Basic Protocol 4) or BHK-21 (see Basic Protocol 1) cells are plated several days before infection and allowed to approach confluency. They are then infected with MVA. After several days, the infected cells are harvested, lysed by repeated freezing and thawing, sonicated, dispensed in small aliquots, and stored at −70°C. NOTE: BHK-21 cells should be acquired from ATCC, as cells from alternative sources may support lower levels of MVA replication. Materials 150-cm2 tissue culture flasks of nearly confluent CEF (see Basic Protocol 4) or BHK-21 cells (see Basic Protocol 1) Complete MEM-10 and -2.5 (see recipe), 37°C Modified vaccinia virus Ankara (MVA; A. Mayr, Institut für Med. Mikrobiologie, Munich, Germany, or B. Moss, email
[email protected]) 150-cm2 tissue culture flasks Humidified, 37°C, 5% CO2 incubator Sorvall RC-3B centrifuge and sterile 250-ml centrifuge bottles (or equivalent) Additional reagents and equipment for trypsinizing cells (see Basic Protocol 1) 1. Trypsinize seven 150-cm2 tissue culture flasks of nearly confluent CEF or two of BHK-21 cells as described (see Basic Protocol 1, steps 4 to 7), using 1.5 ml trypsin/EDTA and 8.5 ml maintenance medium for the larger flask. Distribute to twenty 150-cm2 flasks. Incubate at 37°C in a humidified, 5% CO2 incubator until nearly confluent monolayers have formed (usually 2 days). CEF and BHK-21 cells are split 1:3 and 1:10, respectively. It is advisable to use monolayers of BHK-21 cells at 90% confluency, as completely confluent cultures degrade within 48 hr.
2. Remove medium and add 30 ml complete MEM-2.5 to each 150-cm2 flask. 3. Thaw MVA virus and sonicate 30 sec on ice to break up clumps. Trypsinizaton of virus is avoided with MVA because it may lower the titer.
4. Add 1 to 3 infectious units of virus/cell to the medium and place cells in a CO2 incubator for 3 days at 37°C. A monolayer of CEF or BHK-21 cells in a 150 cm2 flask contains ∼1–2 × 107 cells. If there is insufficient virus, then use fewer or smaller flasks of confluent cells.
5. Detach the cells with a cell scraper or, if possible, by shaking. Transfer cells and medium to a sterile 250-ml centrifuge bottle and centrifuge 10 min at 1200 × g, 4°C, in a Sorvall RC-3B centrifuge. Discard supernatant. 6. Resuspend cells in 1 ml complete MEM-2.5 (per 150-cm2 flask) by pipetting or vortexing. Preparation of Cell Cultures and Vaccinia Virus Stocks
7. Lyse cells by freeze-thaw cycling: freeze in dry ice/ethanol, thaw in a 37°C water bath, and vortex. Carry out the freeze-thaw cycling a total of three times.
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8. Sonicate stock on ice for 1 min, pause for 1 min, and repeat sonication. Dispense in 0.5-ml aliquots and store indefinitely at −70°C. TITRATION OF MVA STOCKS BY IMMUNOSTAINING The titer of MVA is routinely determined by immunostaining, because MVA does not form distinct plaques in CEF or BHK-21 cells. To determine the MVA titer, serial dilutions of virus stock are prepared and used to infect CEF or BHK-21 cells. At 24 hr after infection, the cells are fixed with acetone/methanol and immunostained using a polyclonal vaccinia virus antiserum and a secondary peroxidase-conjugated antibody. A focus of infected cells appears as a 0.3- to 0.4-mm reddish brown–stained area. If the cells are left for two days before immunostaining, secondary foci may form by virus spread giving inaccurate titers.
SUPPORT PROTOCOL 2
Materials CEF (see Basic Protocol 4) or BHK-21 cells (see Basic Protocol 1) in a 150-cm2 tissue culture flask Complete MEM-10 and -2.5 (see recipe), 37°C MVA stock (see Basic Protocol 5) 1:1 (v/v) acetone/methanol PBS (APPENDIX 2), with and without 3% FBS Rabbit anti-vaccinia antibody (e.g., Access Biomedical & Diagnostic Research Labs or see Linscott, 1998) Horseradish peroxidase–conjugated whole anti–rabbit Ig antibody (HRP-anti-rabbit; Amersham) Dianisidine, or premade peroxidase substrate kit (Sigma) PBS/H2O2 (add 10 µl 30% H2O2 to 10 ml PBS immediately before use) 6-well, 35-mm2 tissue culture dishes Humidified, 37°C, 5% CO2 incubator Additional reagents and equipment for trypsinizing cells (see Basic Protocol 1) Infect and fix cells 1. Trypsinize a 150-cm2 tissue culture flask of CEF or BHK-21 cells as described (see Basic Protocol 1, steps 4 to 6), using 1.5 ml trypsin/EDTA for the larger flask. Resuspend cells by repeated pipetting in 5 ml complete MEM-10, and add an additional 60 ml complete MEM-10. 2. Add 2 ml cell suspension to each well of a 6-well, 35-mm2 tissue culture dish. Incubate overnight in a humidified, 5% CO2 incubator at 37°C to reach near confluency. 3. Remove medium and replace with 2 ml complete MEM-2.5. 4. Thaw MVA stock and sonicate 30 sec on ice. 5. Make eight 10-fold serial dilutions of the virus in complete MEM-2.5, using a fresh pipet for each dilution. 6. Add 0.1 ml of each of the 10−6, 10−7, and 10−8 dilutions to cells in duplicate wells. Swirl gently to mix, and incubate 24 hr in a CO2 incubator at 37°C. 7. Remove fluid and fix cells 2 min with 1 ml of 1:1 acetone/methanol. Remove fixative and add 2 ml PBS to each well. At this point, the plates can be immunostained immediately or stored at 4°C for several weeks.
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Perform immunostaining 8. Dilute rabbit anti-vaccinia antibody in PBS containing 3% FBS and add 1 ml/well. Incubate 1 hr at room temperature, with gentle rocking, if desired. The optimal dilution for each antibody must be determined empirically, but a 1:500 or 1:1000 dilution is a good place to start.
9. Wash twice with 2 ml PBS. 10. Dilute HRP-anti-rabbit secondary antibody in PBS containing 3% FBS and add 1 ml/well. Incubate for 30 to 45 min at room temperature, with gentle rocking, if desired. The optimal dilution for each antibody must be determined, but a 1:500 or 1:1000 dilution is a good place to start. If another anti-vaccinia antibody is used in step 8, the appropriate HRP-conjugated anti-species secondary antibody should be substituted here.
11. Wash twice with 2 ml PBS. 12. Make a saturated solution of dianisidine in 0.5 ml ethanol, vortex, incubate 5 min at 37°C, and clarify by microcentrifugation at maximum speed for 1 min. Add 0.2 ml dianisidine solution to 10 ml PBS/H2O2. Alternatively, use premade substrate tablets according to manufacturer’s instructions. CAUTION: Dianisidine is carcinogenic and the powder should be manipulated with gloves in a fume hood. 30% H2O2 is caustic to skin and should be handled with gloves.
13. Add 0. 5 ml dianisidine substrate solution to each well. Rotate dish gently and let stand ∼10 min. Check microscopically for foci of infected cells. When development is complete, wash dishes with water and overlay with 1 ml water to preserve stain. Weaker antibody may take longer to develop.
14. Count the number of stained foci and multiply by the dilution factor to express titer as infectious units/ml. Most accurate results are obtained from wells with 20 to 100 plaques. In determining the titer as infectious units/ml, be sure to mulitply by 10 to take into account the 0.1 ml addition of virus to the wells.
REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Complete DMEM-10 or -20 Dulbecco’s minimum essential medium (DMEM) containing: 10% or 20% fetal bovine serum (FBS) 0.03% glutamine 100 U/ml penicillin 100 µg/ml streptomycin sulfate Store up to several months at 4°C Add supplements from stock solutions prepared in water at the following initial concentrations: 3% glutamine (100×), 20,000 U/ml penicillin (200×), and 20 mg/ml streptomycin (200×). Filter sterilize stock solutions. Store 100× glutamine 4 months at 4°C, and 200× penicillin/streptomycin 4 months at −20°C. Preparation of Cell Cultures and Vaccinia Virus Stocks
Fetal bovine serum is added at 10% to complete medium for maintenance of cells and at 20% to encourage initial growth upon thawing. See Chapter 9 introduction for a full discussion concerning media preparation and use of serum (e.g., heat-inactivation, screening).
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Complete MEM-2.5, -10, or -20 Minimum essential medium (MEM) containing: 2.5%, 10%, or 20% FBS 0.03% glutamine 100 U/ml penicillin 100 µg/ml streptomycin sulfate Store up to several months at 4°C Add supplements from stock solutions as described for complete DMEM (see recipe). Also see annotation to DMEM recipe concerning growth versus maintenance levels of serum.
Complete MEM-10/BrdU or -20/BrdU Prepare as for complete MEM-10 or -20 (see recipe) and add a 5 mg/ml solution of 5-bromodeoxyuridine (BrdU) to 25 µg/ml final. Store up to several months at 4°C. Prepare the 5 mg/ml BrdU stock solution (200×) in water and filter sterilize. Store in the dark at −20°C. After thawing 5 mg/ml BrdU, vortex to be sure it is in solution before adding to MEM.
Complete spinner medium-5 MEM spinner medium 5% horse serum Store up to several months at 4°C Supplements are in MEM spinner medium; no addition is necessary. Horse serum is used because it is cheaper than FBS and may give less cell clumping.
COMMENTARY Background Information An overview of the vaccinia life cycle and expression system is presented in UNIT 16.15. Because HeLa cells consistently give high yields of virus, they are routinely used for preparation of virus stocks. HeLa S3 cells, as obtained from the ATCC, grow well in monolayer culture but can also be put into suspension culture. After repeated passages in suspension, they do not adhere well to flasks and grow poorly in monolayer cultures. The authors prefer HeLa cells adapted to suspension culture because large numbers can be grown in a single bottle. However, suspension cultures require more maintenance than monolayer cultures and the latter may be more convenient. Suspension cells are allowed to form a monolayer before infection to increase the chances of cell-to-cell spread if not all cells are initially infected with the viral inoculum. Thus, good yields of virus may be obtained even if the inoculum is 90% of the total counts), plot the decrease in soluble 35SO4 on a log scale for typical first-order decay kinetics. Determine if single or multiple kinetic classes exist, as indicators of the number of types of sulfate esters in the sample. See Figure 17.23.1 for examples of single and multiple kinetic classes.
Preparation and Analysis of Glycoconjugates
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ALTERNATE PROTOCOL 2
BASE HYDROLYSIS FOR RELEASE OF SULFATE ESTERS FROM GLYCOSAMINOGLYCANS This method will release sulfate esters that are sensitive to base hydrolysis (see Background Information). Additional Materials (also see Basic Protocol 2) Sample: 35SO4-labeled, borohydride-reduced (UNIT 17.5) oligosaccharide or glycopeptide 2 N NaOH 2 N HCl (freshly diluted from 12 N reagent-grade acid) 1. Dispense 1000 to 2000 cpm 35SO4-labeled, borohydride-reduced oligosaccharide or glycopeptide in 50 µl water into a series of at least six microcentrifuge tubes. Add 50 µl of 2 N NaOH to each tube and mix. 2. Incubate the tubes at 80°C in a heating block with wells filled with oil for 0, 1, 2, 4, 6, and 10 hr. IMPORTANT NOTE: Use prereduced oligosaccharides; otherwise the base may degrade the oligosaccharide chain. The base hydrolysis occurs at about the same rate as the loss of primary-linked sulfates (T1⁄2 = ∼1 to 2 hr).
3. At each time point remove the tube, wipe it clean, and place it on ice for a few minutes. Microcentrifuge 2 sec at 10,000 × g to collect the dispersed droplets. Add 50 µl of 2 N HCl to adjust the pH to 3 to 9, checking 1 to 2 µl of solution with pH paper. 4. After the time course has been completed for all samples and all samples have been neutralized, perform steps 5 to 8 of the acid hydrolysis protocol (see Basic Protocol 2) to precipitate the solubilized 35SO4. Completeness of any hydrolysis will be indicated by a flattening of the decay curve.
REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Dowex-50 X8 resin Prewash Dowex-50 resin by stirring 50 to 100 g into 2 liters of water, allowing it to stand 1 hr, then discarding water. Repeat twice, then leave overnight at room temperature. Pour off the final water and resuspend the resin in 3 to 4 vol water. The resin may be stored at 4°C for months under these conditions. COMMENTARY Background Information Solvolysis The conditions of the solvolysis reaction can be adjusted to preferentially remove either Nor O-linked sulfate esters, because N-sulfates (e.g., GlcNSO4 in heparin; not to be confused with sulfated N-linked oligosaccharides) are more sensitive than O-sulfate esters (Inoue and Nagasawa, 1976; Nagasawa et al., 1977). Analysis of Sulfate Esters by Solvolysis or Hydrolysis
Acid hydrolysis It is not possible to get an accurate compostional analysis of “sulfated sugars” by doing
an acid hydrolysis because the sulfate esters are cleaved faster than most glycoside bonds. By selecting the right conditions, acid hydrolysis can give information about the number and locations of sulfate esters because they are cleaved at different rates depending upon their location on the individual sugar residues (Freeze and Wolgast, 1984). The kinetics of sulfate liberation (see Fig. 17.23.1) can help to determine the number of classes of sulfate esters present and what their likely locations may be. These properties are not strongly influenced by other substituents on the sulfated sugar or by the position of the sugar within an oligosac-
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charide chain. Since the hydrolysis can also cleave glycosidic linkages and partially destroy the sugar chain, only the release of sulfate can be measured. The acid hydrolysis procedure differentiates between sulfates linked to different hydroxyl groups on the sugar ring. Under the hydrolysis conditions in Basic Protocol 2, the T1⁄2 of sulfates linked to equatorial hydroxyl groups (e.g., glucose-3-sulfate, galactose-3-sulfate, N-acetylglucosamine-3-sulfate, and mannose-3-sulfate) is 6 to 25 min; that of those linked to axial hydroxyl groups (e.g., galactose-4-sulfate, Nacetylglucosamine-4-sulfate, and mannose-2sulfate) is 60 to 84 min, and that of those linked to primary hydroxyl groups (e.g., glucose-6-sulfate, galactose-6-sulfate, N-acetylglucosamine6-sulfate, and mannose-6-sulfate) is 90 to 120 min (Rees, 1963). All 6-OH positions are primary, and all of the hydroxyl groups in glucose are equatorial. Any hexose isomer of glucose (e.g., mannose or galactose) will change one of the equatorial positions to an axial position. Even though the time ranges for hydrolysis listed above have not been extensively documented for sulfate-labeled oligosaccharides, they are useful because they will clearly differentiate between multiple classes of sulfate es-
ters. Using the acid hydrolysis technique, it is not possible to determine which sugar is sulfated or where it is found in the chain. If other data are available on the structure of the oligsoaccharide, this information may provide likely possibilities for the location of sulfate groups, especially if certain residues are resistant to exoglycosidase digestion. Base hydrolysis Only certain types of sulfate esters are sensitive to base hydrolysis (Freeze and Wolgast, 1984; Percival, 1978). Sensitivity is determined by the location of the sulfate on the sugar residue and—in contrast to acid hydrolysis described above—by the other substituents linked to that sugar residue. Base treatment will release sulfate esters from intact oligosaccharides under two different conditions. The first is when sulfate is located in the 6 (or 3)-OH position and the 3 (or 6)-OH group of the same sugar residue is not substituted. The second is when the sulfate ester is adjacent to an unsubstituted trans OH group. In the first case, the sulfate is eliminated and an acid-labile 3,6-anhydrosugar is formed (see Fig. 17.23.2). In the second case, the sulfate is released with the formation of an epoxide
O HSO3
O
CH2
OCH2 O OH OH OH
O OH
OHOH
OCH2
O OH OH OH
CH2
O R
mild H+ OH
OCH2 O OH OH OH
O R
O OH OH
+ HOCH2 O OH OH OH O R
strong H+
CH2
OH
O O OH OH
HOCH2
+
O OH OH OH OH
Figure 17.23.2 Formation of 3,6-anhydromannose by base treatment of an oligosaccharide containing mannose-6-sulfate. The formation of the anhydro sugar makes the glycosidic linkage very sensitive to acid; mild hydrolysis causes cleavage. Stronger hydrolysis degrades the usual glycosidic linkages, but leaves the anhydrosugar. When reduced with sodium borohydride, the modified sugar can be quantified to show how much mannose-6-sulfate was originally present.
Preparation and Analysis of Glycoconjugates
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ring on the sugar. This is destroyed during subsequent acid hydrolysis of the oligosaccharide. The modified sugar can be identified if it is metabolically labeled—e.g., as in the discovery of mannose-6-sulfate (Freeze and Wolgast, 1984)—but this is not routinely done. On the other hand, the loss of sulfate is easily measured. When data on the acid and base hydrolysis of sulfate esters are combined with other structural information on the molecules, several potential structures can be eliminated or supported, but these procedures will not provide proof of the structure.
Critical Parameters Solvolysis is a harsh treatment that also removes sialic acids and probably fucose residues. If these sugars are normally present, their loss may be an unacceptable side effect of this prcedure. On the other hand, hexoses, hexuronic acids, and hexosamines in glycosidic linkages are mostly stable to solvolysis. The incubated control is very important because it will monitor for any nonspecific destruction of the sugar chain. For acid hydrolysis, the most critical points to observe are that the pH should be kept between 2 and 3 and that the salt concentration be kept very high during the precipitation of sufate with barium. Only then is the precipitation specific for sulfate. Under less stringent conditions, the oligosaccharides or even monosaccharide sulfates can be precipitated.
Time Considerations Solvolysis itself takes only a few hours to complete. The rate-limiting step is the time needed to remove the water after the sample has passed through the Dowex-50 column and the pyridine has been added. This will only take a few minutes using a shaker-evaporator, but will require an overnight lyophilization. Beginning with the hydrolysis step, the procedure can be completed within a day. If necessary, the procedure can be stopped after taking all of the time points and neutralizing them with base prior to precipitation. Where this is done, the reaction mixtures can be stored at 0°-4°C for 24 hr or more.
Literature Cited Inoue, Y. and Nagasawa, K. 1976. Selective N-desulfation of heparin with dimethyl sulfoxide containing water or methanol. Carbohydr. Res. 46:87-95. Nagasawa, K., Inoue, Y., and Kamata, T. 1977. Solvolytic desulfation of glycosaminoglycuronan sulfates with dimethyl sulfoxide containing water or methanol. Carbohydr. Res. 58:47-55. Freeze, H. and Wolgast, D. 1984. Structural analysis of N-linked oligosaccharides from glycoproteins secreted by Dictyostelium discoideum. J. Biol. Chem. 261:127-133. Rees, D.A. 1963. A note on the characterization of carbohydrate sulphates by acid hydrolysis. Biochem J. 88:343-346. Percival, E. 1978. Sulfated polysaccharides of the Rhodophycea. In Carbohydrate Sulfates. (R.G. Schweiger, ed.) pp. 213-224. American Chemical Society, Washington, D.C.
Anticipated Results
Expect ∼80% to 90% efficiency in desulfation, and 90% of the total protein serine/threonine phosphatase activity in many cell extracts (Shenolikar and Nairn, 1991) and demonstrate a broad in vitro substrate specificity. Therefore, PP1 and PP2A are the most commonly used reagents for in vitro dephosphorylation of phosphoserine- and phosphothreonine-containing proteins. PP1 and PP2A are inhibited by microcystin-LR as well as several other recently discovered inhibitors, including okadaic acid, calyculin A, and tautomycin, albeit with differing IC50 values. Inhibitors, such as okadaic acid (with IC50 for PP2A of 0.1 to 1.0 nM and for PP1 of 10 to 100 nM), calyculin A (with an IC50 for PP1 of 1 nM and for PP2A of 10 nM), and microcystin-LR (with an IC50 for both PP1 and PP2A of 0.1 nM) are used to exclude nonspecific effects of PP1 and PP2A. These inhibitors represent important experimental tools for establishing the physiological importance of reversible protein phosphorylation. PP2B, also called calcineurin, is a Ca2+/calmodulin-dependent phosphatase and demonstrates a narrow in vitro substrate specificity compared to PP1 or PP2A. The enzyme purified from mammalian tissues consists of a catalytic subunit that is tightly associated with a calcium-binding regulatory subunit. PP2B has no activity in the absence of 1 µM calmodulin and 1 mM CaCl2 and can be activated by either Ca2+/calmodulin or 1 mM MnCl2. Many highly purified PP2B preparations show no activity in the presence of Ca2+/calmodulin but are fully activated by Mn2+. The immunosuppressive drugs cyclosporin A and FK506 selectively inhibit PP2B in vivo and in vitro. PP2B in the presence of Ni2+ ions and PP2A in association with an endogenous regulator protein or viral antigens can dephosphorylate proteins and peptides containing phosphotyrosines. However, under the conditions described in Basic Protocol 2, PP1, PP2A, and PP2B are highly selective for dephosphorylation of phosphoserine and phosphothreonine residues. PP1 and PP2A represent the major phosphohistidine phosphatase activity measured in tissue extracts, but the regulatory importance of this modification has not yet been established.
Analysis of Protein Phosphorylation
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Critical Parameters Some commercial preparations of potato acid phosphatase contain contaminating protease activity. Because phosphorylation sites reside near the surface of the substrate protein, they are often accessible to proteases, so functional changes that result from limited proteolysis can be incorrectly attributed to dephosphorylation. To limit the effects of proteases, high enzyme concentrations or prolonged incubations with the general phosphatases should be avoided. Aside from including protease inhibitors in the reaction, two strategies should be considered to distinguish proteolysis from dephosphorylation. Formation of a complex between trichloroacetic acid–soluble [32P]phosphate and ammonium molybdate (Support Protocol) is particularly useful in distinguishing [32P]phosphate from 32P-labeled phosphopeptides. The use of phosphatase inhibitors in control reactions also identifies those effects that can be attributed to reactions in which there are contaminating proteolytic enzymes. Commercial preparations of PP2A and PP1 contain only the free catalytic subunits. In vivo these subunits associate with regulatory subunits, which modulate their substrate specificity. However, the catalytic subunits are the most readily purified to homogeneity and can be stored for prolonged periods without significant changes in their enzymatic properties. Both PP1 and PP2A catalytic subunits retain high activity in the absence of divalent cations; PP2A activity may be further stimulated by 1 mM Mn2+ in the enzyme reaction. Long-term storage of PP1 and PP2A results in an apparent loss of enzyme activity. However, these inactive enzymes can be fully reactivated by including 1 mM MnCl2 in the assay. The Mn2+-dependent PP1 enzyme shows slightly altered substrate specificity when compared with PP1 purified from tissues. Moreover, PP1 is potentially inhibited by several endogenous inhibitors. Thus, PP2A may be the preferred reagent for in vitro dephosphorylation of substrates in many cell extracts. Recent studies show that some tissues may contain endogenous PP2A inhibitors (Li et al., 1995). These inhibitory proteins could hinder the ability of PP2A to dephosphorylate proteins in some tissue preparations.
Troubleshooting Detection of Phosphorylation by Enzymatic Techniques
The absence of a functional change following incubation of the substrate with one or more phosphatases may indicate either that the protein was not phosphorylated or that the dephosphorylation of existing phosphates has no effect
on protein function. It is also possible that components in the reaction mixture inhibited the phosphatases. The latter possibility can be readily established if the substrate protein has been metabolically labeled with [32P]phosphate. SDS-PAGE followed by autoradiography or phosphoimage analysis can directly monitor the time-dependent loss of radiolabel from substrate protein. Coincidence between the removal of radiolabeled phosphate and change of function of the substrate protein can be used to distinguish between important regulatory modifications and “silent” phosphorylations. In analyses of unlabeled substrates, the reaction should include an internal standard or a known 32P-labeled protein or peptide. Dephosphorylation of the radiolabeled marker in the presence or absence of the sample should establish the extent to which activity of the added phosphatase has been compromised by components in the sample. If several different phosphatases fail to elicit a change in protein function under conditions that dephosphorylate the marker, the substrate protein may contain no protein-bound phosphate or phosphorylation may not be functionally important for the protein.
Anticipated Results Most phosphoproteins should be dephosphorylated by the general phosphatases, leading to complete removal of protein-bound phosphate and functional changes indicating that reversible phosphorylation controls the protein function. In a number of cases, metabolic labeling and phosphoamino acid analysis or the use of specific protein (phosphoserine/phosphothreonine or phosphotyrosine) phosphatases has successfully identified the amino acids modified. Such studies have then located the phosphorylated residues, especially in proteins whose primary structures have been determined by amino acid sequencing or cDNA cloning. A combination of biochemical studies and site-directed mutagenesis has then been used to establish the functional role of phosphorylation at individual sites. Increasing evidence points to the presence of both positive and negative regulatory phosphorylations in proteins. Thus, it is difficult to predict the functional outcome of complete in vitro dephosphorylation of many substrates.
Time Considerations Approximately 1 hr is required to complete the dephosphorylation reaction. SDS-PAGE
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and analysis of the gel require 1.5 hr. The time required for functional analysis depends on the particular assay used.
Literature Cited
Shenolikar, S. and Ingebritsen, T.S. 1984. Protein (serine and threonine) phosphate phosphatases. Methods Enzymol. 107:102-129. Shenolikar, S. and Nairn, A.C. 1991. Protein phosphatases: Recent progress. Adv. Second Messenger Phosphoprotein Res. 23:1-123.
Berger, H.A., Travis, S.M., and Welsh, M.J. 1993. Regulation of the cystic fibrosis transmembrane conductance regulator Cl− channel by specific protein kinases and protein phosphatases. J. Biol. Chem. 268:2037-2047.
Swarup, G., Cohen, S., and Garbers, D.L. 1981. Selective dephosphorylation of proteins containing phosphotyrosine by alkaline phosphatases. J. Biol. Chem. 256:1447-1452.
Charbonneau, H. and Tonks, N.K. 1992. 1002 protein phosphatases? Annu. Rev. Cell Biol. 8:463493.
Van Etten, R.L. and Waymack, P.P. 1991. Substrate specificity and pH dependence of homogeneous wheat germ acid phosphatase. Arch. Biochem. Biophys. 288:634-645.
Li, M., Guo, H., and Damuni, Z. 1995. Purification and characterization of two potent heat-stable inhibitors of protein phosphatase 2A from bovine kidney. Biochemistry 34:1988-1996. Shenolikar, S. 1994. Protein serine/threonine phosphatases: New avenues for cell regulation. Annu. Rev. Cell Biol. 10:55-86.
Contributed by Shirish Shenolikar Duke University Medical Center Durham, North Carolina
Analysis of Protein Phosphorylation
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Production of Antibodies That Recognize Specific Tyrosine-Phosphorylated Peptides
UNIT 18.6
It is possible to produce anti-phosphopeptide antibodies (i.e., antibodies recognizing phosphorylated peptides) that recognize a protein only in its phosphorylated state, and that do not cross-react with either the cognate unphosphorylated protein or other phosphoproteins. Because the state of phosphorylation is often indicative of a protein’s functional state or activity, such antibodies provide a convenient probe of the functional state of a protein. Thus, unlike conventional antibodies, anti-phosphopeptide antibodies provide information regarding not only the abundance of a protein but also its activity. Unlike general anti–phosphoamino acid (e.g., anti-phosphotyrosine) antibodies, which have broad reactivity, anti-phosphopeptide antibodies may have unique specificity toward the cognate proteins. Such reagents not only facilitate conventional in vitro analysis of phosphoproteins, but also allow heretofore impossible applications—e.g., differential isolation of species of a particular protein that have been phosphorylated at individual phosphorylation sites, as well as analysis of the functional state of a protein in situ by immunohistochemical techniques. Basic Protocol 1 describes the production of polyclonal anti-phosphopeptide antibodies and Basic Protocol 2 describes the production of monoclonal anti-phosphopeptide antibodies. Both of these procedures are based upon immunizing animals with oligopeptides that have been synthesized containing phosphorylated tyrosine; hence, a knowledge of the sequence and phosphorylation site of the protein of interest is a prerequisite for either protocol. Such an immunization will generate an immune response with at least four components: (1) anti–carrier protein reactivity, (2) general anti-phosphotyrosine reactivity, (3) phosphorylation-independent anti-peptide reactivity, and (4) phosphorylation-dependent anti-peptide reactivity. For the production of polyclonal antibodies (see Basic Protocol 1), a multiple-step affinity chromatographic purification with several negativeselection steps is carried out to produce a final antibody preparation having the desired reactivity. For production of monoclonal antibodies (see Basic Protocol 2), ELISAs are performed to screen candidate hybridoma supernatants against the cognate phosphopeptide, as well as against related phosphorylated and nonphosphorylated peptides, until one is found with the desired reactivity and specificity. Although monoclonal antibodies have a number of advantages, production of polyclonal antibodies is likely to be more predictable, as in the authors’ experience a monoclonal hybridoma clone of stringent specificity may occur with very low frequency (see Critical Parameters). Production of monoclonal antibodies is also generally more time-consuming and expensive. The relative merits of monoclonal and polyclonal antibodies are discussed in the Chapter 11 introduction and in general antibody guides (see Key References). Support Protocols are provided for the coupling of peptides (Support Protocol 2) and phosphotyrosine (Support Protocol 3) to the affinity matrix (Affi-Gel 10); BSA-agarose affinity matrix is commercially available. General methods for the coupling of peptides to carrier proteins for use in immunization are described in UNIT 11.16. This unit describes production of antibodies against tyrosine-phosphorylated peptides, with which the authors have the most expertise, but the principles discussed here also apply to peptides phosphorylated on serine and threonine (see Key References).
Analysis of Protein Phosphorylation Contributed by Michael P. DiGiovanna, Robert R. Roussel, and David F. Stern Current Protocols in Molecular Biology (1996) 18.6.1-18.6.19 Copyright © 2000 by John Wiley & Sons, Inc.
18.6.1 Supplement 50
BASIC PROTOCOL 1
PRODUCTION OF POLYCLONAL ANTI-PHOSPHOPEPTIDE ANTIBODIES Strategic Planning The first step of the planning phase is design and synthesis of the required peptides (see Support Protocol 1 and Background Information). The authors employ a peptide containing fifteen amino acids with the phosphorylation site Tyr in the center (residue 8). This peptide consists of fourteen amino acids from the actual sequence of the protein plus an N-terminal Lys to enhance coupling of the peptide to a carrier protein (BSA is used as a carrier in this protocol). The peptide, coupled to BSA, is then used to immunize rabbits. The cognate phosphopeptide is also immobilized on an affinity-chromatography column support (see Support Protocol 2); this affinity matrix will ultimately be used to purify the antibody from the rabbit serum. In addition, a series of negative-selection affinity-chromatography columns are used to adsorb antibodies from the serum that cross-react with epitopes other than the cognate phosphopeptide. These columns are composed of matrix coupled to various synthetic peptides—principally the cognate nonphosphorylated peptide (to remove phosphorylation-independent antibodies) and phosphotyrosine itself (to remove indiscriminant anti-phosphotyrosine reactivity; see Support Protocol 3 for preparation of phosphotyrosine affinity columns). Additional negative-selection steps may be used if needed, depending upon the particular target chosen, to remove antibodies that cross-react with phosphorylation sites of other proteins having known homology to the target site. This cross-reactivity occurs particularly when the target protein is a member of a family of homologous proteins—e.g., a subgroup of related receptor tyrosine protein kinases. If such cross-reactivity is anticipated, it is possible to synthesize not only the cognate phosphorylated and nonphosphorylated peptides, but also phosphopeptides based upon the related homologous sequences, to be used for negative selection. The authors also pass the serum over a column consisting of immobilized carrier protein (commercially available BSA-agarose) to remove the majority of the antibodies generated against the much larger carrier protein. Thus, a typical purification scheme might consist of the following: Negative-selection affinity-purification columns • BSA (carrier) • phosphotyrosine • cognate nonphosphopeptide • homologous phosphopeptides (if desired) Positive-selection affinity-purification column • cognate phosphopeptide.
Production of Antibodies That Recognize Phosphorylated Peptides
The second step in planning is to raise and test antisera in rabbits. To prepare the phosphopeptide-carrier conjugate for immunization of rabbits, the authors couple a 30-fold molar excess of phosphopeptide (14.4 mg dissolved in water and neutralized with 1 N NaOH) to BSA (20 mg dissolved in 0.4 M sodium phosphate buffer, pH 7.5), using glutaraldehyde as the cross-linking reagent as described in Doolittle (1986). The reaction is allowed to proceed 30 min at room temperature; the development of a yellow color in the coupling solution is an indication that the glutaraldehyde has reacted. General methods for coupling of peptides to carrier proteins for use in immunization are described in UNIT 11.16. Conjugation can be confirmed by performing an anti-phosphotyrosine immunoblot (UNIT 18.4) on a small test aliquot of the conjugate (although a smear should be expected on the blot, resulting from variation in the amount of peptide coupled per BSA molecule as well as from possible multimers of BSA). Guidelines for immunization of rabbits are provided in UNIT 11.12, and many institutions have core facilities for the immunization of
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rabbits and production of antisera. The authors immunize with 1 mg of immunogen (the peptide-BSA conjugate prepared above, dissolved in PBS) in 1 ml of a 50% emulsion with complete Freunds adjuvant. This is administered subcutaneously at multiple sites, followed by four boosts at 2-week intervals using same quantity of immunogen in incomplete Freunds adjuvant. An adequate immune response can be seen as early as week 6. The specificity of the final antibody is demonstrated most simply by immunoblot analysis (UNIT 10.8) using a panel of relevant phosphorylated and nonphosphorylated proteins. Electrophoresis is carried out with purified cognate protein in both its phosphorylated and unphosphorylated states, as well as with any proteins toward which there could conceivably be cross-reactivity. These are most easily isolated by immunoprecipitation using conventional antibodies. The blot is probed with the purified anti-phosphopeptide antibody to demonstrate appropriate reactivity. This immunoblot assay may also be used after each chromatographic step to monitor the success of the purification. An assay also must be devised for estimating the titer of the immune response in the sera of the immunized rabbits. Convenient screening assays for this purpose include tests for the ability of crude sera to immunoprecipitate the target phosphoprotein from a cell lysate known to contain it, as well as immunoblotting of purified phosphorylated target protein with the crude serum. The relationship among the above procedures is illustrated by the flow chart in Figure 18.6.1. Purification Method The purification of polyclonal antibodies consists of multiple affinity-chromatography steps for the negative and positive selection of antibodies of the appropriate reactivity (see Fig. 18.6.1). All of the following chromatographic steps may be carried out at room temperature, with the solutions applied to the columns by gravity. Materials BSA-agarose affinity matrix (Sigma) packed (as in Support Protocol 2) in a 10-ml bed volume column Phosphotyrosine affinity matrix column (10-ml bed volume; see Support Protocol 3) Crude serum from rabbit immunized with phosphopeptide-BSA conjugate (refer to Strategic Planning, above) PBS/azide: PBS (APPENDIX 2) containing 0.02% (w/v) sodium azide (store indefinitely at 4°C or room temperature) Cognate nonphosphopeptide affinity matrix column (3-ml bed volume; see Support Protocols 1 and 2 and Strategic Planning) 3 M NaSCN Homologous phosphopeptide affinity matrix columns (optional; 3-ml bed volume; see Support Protocols 1 and 2 and Strategic Planning) Positive-selection phosphopeptide affinity matrix column (3-ml bed volume; see Support Protocols 1 and 2 and Strategic Planning) 3.5 M and 4.5 M MgCl2 (optional; see Critical Parameters) Spectrophotometer (optional) Dialysis tubing (MWCO 12,000 to 14,000; 10 mm width, 6.4 mm diameter; e.g., Spectra/Por 4 from Spectrum) Additional reagents and equipment for dialysis (APPENDIX 3C), protein quantitation (UNIT 10.1), and analysis of antibodies by ELISA (UNIT 11.2) or immunoblotting (UNIT 10.8)
Analysis of Protein Phosphorylation
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design and synthesize peptides of interest (core facilty / Support Protocol 1 / Strategic Planning)
P ~ peptide
test serum for reactivity by immunoprecipitation or immunoblotting
regenerate matrix
immunize rabbits to produce crude antiserum (core facility / UNIT 11.12 )
peptides for matrix preparation
crude serum pass serum through BSA-agarose affinity matrix (negative selection)
flowthrough regenerate matrix
pass serum through phosphotyrosine affinity matrix (negative selection)
test serum for loss of antiphosphotyrosine flowthrough reactivity by immunoblotting (optional; pass serum through repeat cognate nonphosphoUNIT 10.8 ) pass peptide affinity matrix (negative selection) regenerate matrix
prepare phosphotyrosine affinity matrix (Support Protocol 3 )
flowthrough test serum for loss of anti-nonphosphopeptide or anti–homologous phosphopeptide reactivity by immunoblotting (optional; UNIT 10.8 )
pass serum through homologous phosphopeptide matrices; (negative selection; optional )
pass serum through affinity matrix containing phosphopeptide of interest (positive selection)
prepare various peptide affinity matrices (Support Protocol 2 )
elute with NaSCN or MgCl2 ; collect fractions; dialyze vs. PBS/azide
assay by A 280 (UNIT 10.1 )
Production of Antibodies That Recognize Phosphorylated Peptides
assay by ELISA (UNIT 11.2 )
assay by immunoblotting (UNIT 10.8 )
Figure 18.6.1 Flow chart for production of polyclonal anti-phosphopeptide antibodies.
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Deplete reactivity of serum toward phosphotyrosine and carrier protein 1. Connect the washed BSA-agarose and phosphotyrosine affinity matrix columns in series. 2. Pass ∼15 ml crude serum through the two columns, allowing it to flow through by gravity. Wash with PBS/azide until all of the yellow color of the serum has passed through the columns (or monitor the A280 of the column effluent spectrophotometrically until baseline absorbance is reached), then wash with an additional 5 to 10 ml PBS/azide, collecting all washings in the flowthrough fraction, which will contain the antibody of interest. Aliquots of the crude serum as well as this first flowthrough may be saved for subsequent analysis and comparison of individual purification fractions. The serum may be analyzed at this time for elimination of anti-phosphotyrosine reactivity by performing immunoblotting (UNIT 10.8) of samples containing a variety of phosphotyrosyl proteins; alternatively it is possible to save an aliquot for analysis and proceed to the next step (see Troubleshooting). Expect that with each passage through a column, the volume of the serum will increase, which will prolong the amount of time needed for subsequent columns. Note that if flowthrough is monitored by the passage of the yellow color of the serum, this will become more difficult as the serum becomes more dilute. After the serum passes through, regenerate the columns by washing with 10 bed volumes of 3 M NaSCN followed by 10 bed volumes of PBS/azide, and store at 4°C in PBS/azide for future use.
Deplete reactivity of serum toward cognate nonphosphopeptide 3. Pass the flowthrough serum through the nonphosphopeptide affinity matrix column by gravity as many times as necessary to deplete cross-reactivity, regenerating the column between passes by washing with 10 bed volumes of 3 M NaSCN followed by 10 bed volumes PBS/azide. A 15-ml starting serum sample may have to be passed over this column several times to quantitatively deplete cross-reactivity (see Critical Parameters). The serum may be analyzed after each or several passes, or an aliquot can be saved for analysis at a later time.
4. If cross-reactivity with homologous phosphoprotein(s) is anticipated, pass the flowthrough serum through the homologous phosphopeptide affinity matrix column(s) using the methodology described in step 3. Refer to Strategic Planning, above, for discussion of cross-reactivity with homologous phosphoproteins.
Purify antibodies by positive-selection affinity chromatography and dialysis 5. Hydrate and wash ∼25-cm strips of dialysis tubing in advance for collection of fractions from the positive-selection affinity purification. Secure one end of each length of tubing with a dialysis clamp and check for leaks (APPENDIX 3C). Also prepare 6 liters PBS/azide and cool to 4°C in advance to use as the dialysis solution. To minimize the time that the antibodies are exposed to the elution solution, fractions (∼3 ml each) from the positive-selection affinity column will be collected directly into preprepared dialysis tubing.
6. Pass the flowthrough serum from the previous chromatography step through the positive-selection phosphopeptide affinity column three times, without washing between passes. Three passes are performed to maximize the interaction of the antibody with the affinity matrix.
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7. Collect the flowthrough serum from the final pass, then wash the column with 5 to 20 ml PBS/azide (depending upon the precolumn volume and column bed volume) and combine the washings with the flowthrough serum. Wash with an additional 20 ml PBS/azide, and collect the washings separately as the “wash” fraction. 8. Elute with chaotropic agent of choice: either 20 ml of 3 M NaSCN, or 10 ml of 3.5 M MgCl2 followed by 10 ml of 4.5 M MgCl2. Upon starting the elution, immediately begin collecting ∼3-ml fractions directly into the dialysis bags prepared in step 5, securing the proximal end of the tubing with dialysis clamps and dropping the bag immediately into the PBS/azide dialysis solution. Collect at least six fractions. Alternatively, ∼3-ml fractions can be collected in tubes and the liquid immediately placed into dialysis tubing upon completion of each fraction. Most of the antibody will elute in the first three fractions from a column of 2- to 3-ml bed volume. Regenerate the column as in step 3. See Critical Parameters for a discussion of the relative merits of NaSCN versus MgCl2 as eluants.
9. Dialyze all fractions collected in step 8 exhaustively against PBS/azide at 4°C (APPENDIX 3C). Analyze and store purified antibody 10. Determine protein concentration of dialyzed fractions by measuring absorbance at 280 nm or by a colorimetric protein assay (UNIT 10.1) and calculate yield. The authors have generally recovered ≥1 mg of purified antibody from a 15-ml serum sample.
11. Store the antibody in aliquots at −70°C; store working aliquot at 4°C. 12. Assay the reactivity and cross-reactivity of the final samples, as well as any aliquots saved from previous steps, using ELISA (UNIT 11.2) or immunoblotting (UNIT 10.8). BASIC PROTOCOL 2
PRODUCTION OF MONOCLONAL ANTI-PHOSPHOPEPTIDE ANTIBODIES Strategic Planning
Production of Antibodies That Recognize Phosphorylated Peptides
As in the production of polyclonal anti-phosphopeptide antibodies (see Basic Protocol 1), the first step of the planning phase for anti-phosphopeptide monoclonal antibodies is the design and synthesis of the required peptides (see Support Protocol 1 and Background Information). Here too, the authors employ a peptide containing fifteen amino acids with the phosphorylation site Tyr in the center (residue 8), consisting of fourteen amino acids from the actual sequence of the protein plus an N-terminal Lys to enhance coupling to a carrier protein. A different carrier protein must be employed for immunization than will be used as substrate for the ELISA screen, to avoid detection of anti-carrier antibodies in the ELISA (see Purification Method, below). The authors use keyhole limpet hemocyanin (KLH) as the carrier for immunization and BSA as the carrier for the ELISA substrate. General methods for coupling of peptides to carrier proteins for use in immunization are described in UNIT 11.16. The cognate phosphopeptide can also be immobilized to an affinity-chromatographic column support (see Support Protocol 2) and used ultimately to affinity-purify the antibody produced by the desired hybridoma clone. A number of additional peptides must be produced for use in ELISA screening of candidate hybridoma supernatants, to screen both for the presence of the desired reactivity and the absence of undesired cross-reactivity. For ELISA screening, the peptides are coupled to a carrier protein different from the one used for immunization. Bovine serum albumin (BSA) is
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used for screening, because many of the clones are likely to react against the carrier used in the immunization of the mice (KLH) and because unconjugated peptides have not been found to function well as ELISA substrates (Doolittle, 1986). In addition, because antibodies may be generated against the cross-linking reagent, the method used for coupling the phosphopeptide to the BSA to produce the substrate for ELISAs should be different from that used to link peptides to the KLH carrier for immunization (Czernik et al., 1991; Doolittle, 1986). The second step of planning is to raise antibodies in mice. General guidelines for the immunization of mice and the production of monoclonal antibodies are described in UNITS 11.4-11.11, and many institutions have core facilities that will perform that task. The authors have immunized BALB/c mice by intraperitoneal injection with 1 mg/ml of immunogen (dissolved in PBS) in a 50% emulsion with complete Freunds adjuvant on day 1 and boosted intraperitoneally on days 15 and 37 with immunogen in incomplete Freunds adjuvant. Test bleeds from day 47 were analyzed by ELISA. On day 57 the mouse with the best titer was boosted intravenously, and its spleen was harvested on day 60. Freshly harvested spleen cells were prepared for cell fusion to generate hybridoma lines, which were subsequently screened by ELISA (UNIT 11.7). The purpose of the ELISA screening is to identify clones that react with the cognate phosphopeptide and to eliminate clones that cross-react with other epitopes in addition to the cognate phosphopeptide. Epitopes screened by ELISA generally consist of the cognate nonphosphorylated peptide (to eliminate clones with phosphorylation-independent reactivities) and at least one unrelated phosphopeptide (to eliminate clones with indiscriminant anti-phosphotyrosine reactivity). In addition, depending upon the particular target chosen, it may be necessary to screen for reactivity with phosphopeptides from proteins of known homology. This occurs particularly when the target protein is a member of a family of homologous proteins—e.g., the subgroups of related receptor tyrosine protein kinases. If such cross-reactivity is anticipated, it is essential to synthesize not only the cognate phosphorylated and nonphosphorylated peptides and at least one unrelated phosphopeptide, but also phosphopeptides based upon the related homologous sequences. Thus, a typical ELISA screening scheme might consist of the following steps. 1. Screen for clones that exhibit reactivity against the cognate phosphopeptide. These clones, which will likely be 20 min to elapse from the time the resin is removed from the bottle to the time it is mixed with the phosphotyrosine solution.
4. Remove most of the excess liquid from the gel by vacuum aspiration without letting it dry completely, transfer it to a screw-cap centrifuge tube, and add the phosphotyrosine solution from step 2. 5. Incubate overnight at 4°C with end-over-end rotation. Quench and wash resin 6. Add 2 µl pure ethanolamine per ml resin and incubate 2 hr at room temperature with end-over-end rotation. Ethanolamine is added directly to the coupling reaction to quench unreacted ester groups.
7. Wash resin twice, each time by centrifuging at low speed (see Support Protocol 2, step 3), aspirating the supernatant, resuspending the resin in 5 vol of 0.4% sodium bicarbonate, then centrifuging again at low speed. 8. Wash resin twice with 0.1 M ethanolamine⋅HCl, pH 8.0, as in step 7. Remove the 0.1 M ethanolamine from the second wash and replace with fresh, incubate tube overnight at 4°C with end-over-end rotation, then centrifuge at low speed and aspirate the supernatant. 9. Wash, store, and pack resin in columns (see Support Protocol 2, steps 8 to 12). REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Screening diluent 6.18 g boric acid (100 mM final) 9.54 g sodium borate (47 mM final) 4.38 g NaCl (75 mM final) 10 g bovine serum albumin (BSA; 1% w/v final) 1 g sodium azide (1% w/v final) H2O to 1 liter The pH of the solution will be 8.4 to 8.5.
COMMENTARY Background Information
Production of Antibodies That Recognize Phosphorylated Peptides
Conventional antibodies have proven to be invaluable tools in numerous techniques for the biochemical analysis of proteins. In the past antibodies with specificity toward the phosphorylated forms of proteins were produced serendipitously. Using techniques such as those described in this unit, such antibodies may now be produced by design. Whereas
phosphorylated holoproteins might in general be poor candidates for immunogens in the production of such antibodies as a result of their susceptibility to dephosphorylation, it has been suggested that short phosphopeptides are relatively resistant to phosphatases (Czernik et al., 1991), thereby providing a better chance of success. A dramatic advance in the analysis of pro-
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tein tyrosine phosphorylation and the regulation of signal transduction pathways by such phosphorylation occurred with the production of polyclonal and monoclonal anti-phosphotyrosine antibodies (Frackelton et al., 1983; Ross et al., 1981; reviewed in Stern, 1991). These antibodies proved capable of recognizing phosphorylated tyrosine residues in the context of virtually any flanking peptide sequence, and were shown to be exquisitely specific in the requirement for a phosphorylated tyrosine yet at the same time remarkably promiscuous in their acceptance of any peptide sequence. Techniques using polyclonal and monoclonal antiphosphotyrosine antibodies have supplanted the standard methodology of metabolically labeling proteins with 32Pi, which was cumbersome, time-consuming, hazardous, and of relatively lower sensitivity. Antibodies that recognize a specific tyrosylphosphorylated peptide as described in this unit represent a marriage of anti-phosphotyrosine technology with the stringent sequence-dependent specificity of conventional antibodies (Bangalore et al., 1992; DiGiovanna and Stern, 1995; Epstein et al., 1992; Roussel, 1990). Conceptually, they may be thought of either as anti-phosphotyrosine antibodies that have strict sequence specificity, or as conventional antibodies possessing the additional specificity of phosphorylation dependence. Anti-phosphotyrosine antibodies have been most useful in the analysis of tyrosine phosphorylation of proteins in a technique using a combination of immunoprecipitation and immunoblotting (known as the “IP-western” method). In this procedure, a protein typically is immunoprecipitated either with conventional anti-protein antibody or with anti-phosphotyrosine antibody, then immunoblotted with whichever of these two antibodies was not used for the immunoprecipitation. Because antiphosphopeptide antibodies recognize the cognate protein, but only when it is phosphorylated, they are capable of performing both functions in a single step—i.e., either an immunoprecipitation alone or an immunoblot alone is sufficient to supply the desired information. The most useful functions for such antibodies are likely to be those that in fact are not possible using traditional reagents. For example, using anti-phosphopeptide antibodies, it is possible to identify and isolate distinct phosphorylated species of a phosphoprotein containing multiple phosphorylation sites. Thus, phosphorylation at each site can be examined
independently, and a preparative separation of individual phosphospecies of a holoprotein can be achieved. Another unique application is analysis of the abundance and phosphorylation state of individual proteins in situ in preparations of cells or tissues. Because phosphorylation is a major mode of regulation of protein function, the phosphorylation state is often an indicator of the functional status of a protein. The mere identification of a protein in a cell or tissue specimen gives no indication of its functional status. The ability of anti-phosphopeptide antibodies to demonstrate the phosphorylation state, and by extrapolation the functional state, of a single protein with high specificity places these reagents in a unique class. Hence, identity and functional status may be probed simultaneously using one simple assay. In tissue sections, one may probe to determine whether a particular protein is present, and, if present, in what functional state it is found. The authors have employed this strategy in immunohistochemical staining of formalin-fixed, paraffin-embedded human breast tumors using antibody to the phosphorylated form of the receptor tyrosine kinase Neu (DiGiovanna and Stern, 1995), and were able to demonstrate that Neu is phosphorylated, and therefore functionally active, in only a subset of the tumors that overexpress this protein. Anti-phosphopeptide antibodies possess unique properties that render them capable of performing functions not possible with conventional reagents. From the technical considerations regarding their production, it would follow that antibodies with specificity for the nonphosphorylated state should also be achievable, and the production of such antibodies has also been reported (Czernik et al., 1991; Epstein, 1995; Kawakatsu et al., 1996; Nairn et al., 1982; Roussel, 1990; Tzartos et al., 1995).
Critical Parameters In the production of polyclonal anti-phosphopeptide antibodies, a major challenge is the depletion of cross-reactivity. The first consideration in achieving this is the anticipation of potential cross-reacting proteins in the planning phase of the procedure. Cross-reactivity is most likely to occur when the target protein is a member of a family of closely related homologous proteins—e.g., of the subfamilies of tyrosine protein kinases. The authors have produced a monoclonal anti-phosphopeptide antibody with specificity for the Tyr-1248 autophosphorylation site of Neu (DiGiovanna and Stern, 1995). A homologous site exists in
Analysis of Protein Phosphorylation
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Production of Antibodies That Recognize Phosphorylated Peptides
the epidermal growth factor receptor (EGFR). The antibody produced in the authors’ laboratory does not cross-react with the homologous EGFR site in spite of having identical residues in seven of the 14 amino acids of the peptide sequence (five of which are N-terminal to the phosphotyrosine, one C-terminal, and the phosphotyrosine itself). Thus, specificity is achievable even with at least up to 50% identity. Nevertheless, such a close relationship among the phosphorylation sites of different proteins highlights the importance of considering all known related proteins. The second consideration in depleting cross-reactivity is the quantity of cross-reactive antibodies in a given aliquot to be purified relative to the capacity of the peptide columns used for the negative selection. Because of the expense of the peptides, the authors have generally prepared small columns (with 2- to 3-ml bed volumes) and passed the sera over the columns multiple times, empirically determining the number of passes necessary to quantitatively deplete cross-reacting material. In these columns, 3 µmol of ligand is coupled per milliliter of Affi-Gel 10. The manufacturer states that the resin contains ∼15 µmol of active ester per milliliter of gel, and that the gel has a capacity for 35 mg of protein or 15 to 20 µmol of a low-molecular-weight ligand per milliliter of gel. The BSA and phosphotyrosine columns are relatively inexpensive to produce; thus, a single large column of each is practical and sufficient. Because the authors elute the antibodies from the positive-selection affinity column in strongly chaotropic solutions that are potentially deleterious to the stability of the antibody (3 M NaSCN or 3.5 M followed by 4.5 M MgCl2), care is taken to collect the fractions in preprepared dialysis tubing so that they may be immediately placed into the PBS dialysate, thus minimizing the time that the antibodies are exposed to the eluting solutions. Although MgCl2 is thought to be “gentler,” the authors have found that gravity-driven flow rates from phosphopeptide columns eluted with this salt quickly become extremely slow, possibly because of precipitation of the salt in the column or an interaction of the Mg2+ ion with the phosphate groups. Thus use of NaSCN is preferred, and this salt appears to permit recovery of comparable activity. In the production of monoclonal anti-phosphotyrosyl peptide antibody, which the authors have carried out once, a major technical hurdle was the low frequency with which clones of the
desired specificity were produced (DiGiovanna and Stern, 1995). In that work, in which antibody was produced to the phosphorylated form of the receptor tyrosine kinase Neu, >1200 hybridomas (obtained from a single fusion) were screened to obtain a single clone that satisfied all requirements. The authors found 68 candidate clones that recognized the cognate phosphopeptide, of which only 20 were unreactive toward the cognate nonphosphopeptide. Of those, seven cross-reacted with an unrelated phosphopeptide (i.e, exhibited indiscriminant phosphotyrosine activity), and of the remainder three cross-reacted with the homologous EGFR. The remaining ten were subcloned by limiting dilution, after which only five continued to produce antibody. Of these five, only one reliably detected the phosphorylated holoprotein in immunoblots and immunoprecipitations. Thus, the “hit rate” for an antibody that satisfied all requirements was 150 mM. For kinases, it is a good idea to use preliminary experiments to identify the effects of pH, salt concentration, concentrations of cations such as Mg2+ or Mn2+, and temperature to optimize the assay conditions for the kinase of interest. It is often advantageous to perform kinase assays at 30°C rather than 37°C because the lower temperature makes it easier to stay within the linear range of the kinase, thus providing more control of the assay. Another factor to be considered when optimizing an assay for a newly identified kinase is the concentration of ATP in the reaction mixture. In these protocols and in general, detection of kinase activity is based on the transfer of radiolabeled phosphate from ATP to the substrate, so the concentration of ATP can be varied only a little. Also, the specific activity of the [γ-32P]ATP must be known in order to actually measure phosphotransfer. Most kinases have a Km for ATP of 1 to 100 µM. If there is too much ATP in the reaction mixture, it will be difficult to measure phosphotransfer. An ATP concentration of 50 to 100 µM tends to work well. At that concentration, the enzyme should be working at ≥50% of maximum, depending on its apparent Km for ATP, and addition of sufficient [γ-32P]ATP to measure phosphotransfer is not prohibitively expensive. Usually the substrate concentration is high so that the enzyme is working at or close to Vmax. Controls Including the correct controls for a kinase assay is critical to the success of the assay, especially when the enzyme source is a cell or tissue extract. The appropriate controls should always include a no substrate control, a no enzyme source control, a heat-denatured enzyme control, and, for enzymes that require an activator or cofactor, controls that use irrelevant activator or cofactor and controls without activator or cofactor. CAUTION: These assays use [γ-32P]ATP which should be handled and disposed of according to safety regulations. See APPENDIX 1F and the institutional Radiation Safety Office for guidelines for proper handling and disposal of 32P. BASIC PROTOCOL 1
ASSAY FOR CYCLIC NUCLEOTIDE–DEPENDENT PROTEIN KINASES Cyclic adenosine monophosphate (cAMP)–and cyclic guanosine monophosphate (cGMP)–dependent protein kinases (PrKs) are quite similar in structure and require similar assay conditions. cAMP-PrK is a heterotetramer consisting of two regulatory (R) subunits and two catalytic (C) subunits, R2C2. In each case the regulatory subunits have binding sites for two molecules of cAMP. When the cAMP concentration is elevated in cells after activation of receptor-linked adenylyl cyclases, cAMP binds to the R subunits, causing the affinity of the C subunits for R subunits to drop by about four orders of magnitude. The formation of R2cAMP4 dimers and free C-subunits is favored, and the C-subunits are now active. The case of cGMP-PrK is slightly different in that there are no free catalytic subunits released. cGMP-PrK is composed of two identical subunits. Each subunit has a regulatory domain and a catalytic domain, which are homologous to the R and C subunits of cAMP-PrK. On binding four molecules of cGMP the enzyme is activated, presumably by a conformational change.
Assays of Protein Kinases Using Exogenous Substrates
The assay procedures for these two kinases are very similar and the same protein substrate can be used for each one as detailed below. More recently, both of these kinases have been assayed using peptide substrates.
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Materials 5× cyclic nucleotide–dependent kinase reaction buffer (see recipe) 10 mg/ml histone 2B in H2O [γ-32P]ATP solution (see recipe) 20× cyclic nucleotide solution: 20 µM cyclic AMP in H2O/20 µM cyclic GMP in H2O Enzyme sample containing cyclic nucleotide–dependent kinase activity (see Support Protocol 1), kept on ice until use 30° water bath Additional reagents and equipment for TCA precipitation (see Support Protocol 2), adsorption onto P81 phosphocellulose paper (see Support Protocol 3), or electrophoretic analysis (see Support Protocol 4) 1. For each assay reaction, add the following to a 1.5-ml microcentrifuge tube kept on ice: 4 µl 5× cyclic nucleotide–dependent kinase reaction buffer 1 µl of 10 mg/ml histone 2B 1 µl [γ-32P]ATP solution (to give 5 µCi/µl and 5 µM ATP final) 1 µl 20× cyclic nucleotide solution 0 to 13 µl H2O. Cap tube and warm the mixture in a 30°C water bath. Perform each assay in triplicate and include no substrate, no enzyme, and no cyclic nucleotide controls. The total reaction mix volume is 20 ìl per reaction. The amount of water required depends on how much enzyme sample is used in step 2. Sufficient reaction mix can be prepared in a single tube for all the reactions by multiplying the quantities for a single reaction by the total number of reactions + 1. Then the total amount of reaction mixture (minus enzyme) for one reaction can be added to each tube. This is often very convenient and reduces the potential for pipetting errors.
2. Start the reaction by adding 1 to 14 µl ice-cold enzyme sample containing cyclic nucleotide–dependent kinase activity. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mix to 25 ìl immunoprecipitate bound to beads.
10.16),
3. Incubate 10 min in a 30°C water bath. 4. Stop the reaction using the reagent appropriate for the analytic method to be used—20 µl ice-cold 10% TCA for TCA precipitation (see Support Protocol 2), or 10 µl or 20 µl ice-cold 2× SDS-PAGE sample buffer for electrophoretic analysis (see Support Protocol 4). Use 10 µl of the reaction mix for adsorption to P81 phosphocellulose paper (see Support Protocol 3). Proceed with analysis by one of those methods.
Analysis of Protein Phosphorylation
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BASIC PROTOCOL 2
ASSAY FOR PROTEIN KINASE C ISOFORMS There are presently at least ten members of the protein kinase C (PKC) family of kinases. Protein kinase C was originally identified as a kinase activity that was reversibly activated by calcium (Ca2+), phospholipid (usually phosphatidylserine), and the neutral lipid diacylglycerol (DAG; Takai et al., 1979). Subsequently, these kinases were shown to be the sites of action for the tumor-promoting phorbol esters, which bind to some PKC isoforms and activate the kinase activity by substituting for DAG. The initial isoforms of PKC that were purified were called α, β, and γ, and it is these forms that are the most clearly regulated by Ca2+, phospholipid, and DAG or phorbol esters. They are termed the “conventional” subfamily of PKCs (cPKCs). Another subfamily contains δ, ε, ν, and θ, the “novel” PKCs (nPKCs); these kinases are not regulated by Ca2+ but are activated by phorbol esters and phospholipids. A third subfamily, the “atypical” PKCs (aPKCs), consist of ζ, λ, and ι; these are not regulated by Ca2+ nor do they bind phorbol esters or DAG, but there is still a requirement for phospholipids. When the isoforms are analyzed, the reaction buffer must include the appropriate activating cofactors. Materials 5× PKC reaction buffer (see recipe) 10 mg/ml histone H1 in H2O [γ-32P]ATP solution (see recipe) Enzyme sample containing PKC activity (see Support Protocol 1) 30°C water bath Additional reagents and equipment for TCA precipitation (see Support Protocol 2), adsorption onto P81 phosphocellulose paper (see Support Protocol 3), or electrophoretic analysis (see Support Protocol 4) 1. For each assay reaction, add the following to a 1.5-ml microcentrifuge tube: 4 µl 5× PKC reaction buffer 1 µl 10 mg/ml histone H1 1 µl [γ-32P]ATP solution (to give 5 µCi/µl and 5 µM ATP final) 0 to 14 µl H2O. Cap tube and warm the mixture 10 min in a 30°C water bath. Perform each assay in triplicate and include no substrate and no enzyme controls. The total reaction mix volume is 20 ìl per reaction. The amount of water required depends on how much enzyme sample is used in step 2. Sufficient reaction mix can be prepared in a single tube for all the reactions by multiplying the quantities for a single reaction by the total number of reactions + 1. Then the total amount of reaction mixture (minus enzyme) for one reaction can be added to each tube. This is often very convenient and reduces the potential for pipetting errors.
2. Start reaction by adding 1 to 14 µl enzyme sample containing PKC activity to the warmed reaction mixture. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. Assays of Protein Kinases Using Exogenous Substrates
For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT 10.16), prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mix to 25 ìl immunoprecipitate bound to beads.
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3. Incubate 10 min in a 30°C water bath. 4. Stop the reaction using the reagent appropriate for the analytic method to be used—20 µl ice-cold 10% TCA for TCA precipitation (see Support Protocol 2), 10 µl, or 20 µl ice-cold 2× SDS-PAGE sample buffer for electrophoretic analysis (see Support Protocol 4). Use 10 µl of the reaction mix for adsorption to P81 phosphocellulose paper (see Support Protocol 3). Proceed with analysis by one of those methods. ASSAY FOR CASEIN KINASES USING β-CASEIN Casein is a very acidic milk protein available in reasonable purity and quantity. Casein kinases were named for their ability to phosphorylate casein rather than other substrates available at the time. Because of its acidic nature, the use of casein as a substrate presents a number of problems, one of which is the fact that it does not carry a net positive charge at low pH and thus does not bind to P81 phosphocellulose paper. In recent years this problem has been circumvented by using short acidic peptides engineered to bind to P81 paper as substrates for the casein kinases (see Alternate Protocol).
BASIC PROTOCOL 3
There are currently two known mammalian casein kinases, casein kinase I and casein kinase II, named for their order of elution from anion-exchange chromatography columns. Casein kinases show a marked preference for substrates containing acidic residues close to the serine or threonine to be phosphorylated. Casein kinase I is a 37-kDa monomer that phosphorylates serine and threonine residues (see Table 18.7.1) and utilizes ATP only as phosphoryl group donor. The basic phosphorylation consensus sequence is Glu/Ser(P)XXSer (where the underlined residue is the phosphate acceptor, and may be either Ser or Thr). Mammalian casein kinase II is a tetramer of α2β2 structure: the α-subunit is 24 to 28 kDa and the β-subunit is 37 to 44 kDa. Its preferred substrate has aspartate or glutamate residues C-terminal to the phosphorylation site serine or threonine and a consensus of SerGluGlu/Asp. Casein kinase II is unusual in that it can utilize both ATP and GTP as phosphoryl group donor with similar Km values, ∼10 µM for ATP and ∼30 µM for GTP. Mammalian casein kinase II is also inhibited by heparin, whereas casein kinase I is not. The IC50 for the inhibition of casein kinase II by heparin is in the 10 to 20 nM range and at these concentrations casein kinase I is not inhibited. Assays of either casein kinase I or II can be performed using casein as substrate. When isolated from milk, casein is a phosphoprotein. However, it is preferable to use dephosphorylated casein as substrate; this is commercially available in a relatively pure state as either the α or β isoform. Materials 5× casein kinase reaction buffer (see recipe) 10 mg/ml β-casein in H2O [γ-32P]ATP solution (see recipe) Enzyme sample containing casein kinase activity (see Support Protocol 1), kept on ice 30°C water bath Additional reagents and equipment for TCA precipitation (see Support Protocol 2) or electrophoretic analysis (see Support Protocol 4) Analysis of Protein Phosphorylation
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1. For each assay reaction, add the following to a 1.5-ml microcentrifuge tube kept on ice: 4 µl 5× casein kinase reaction buffer 1 µl 10 mg/ml β-casein 1 µl [γ-32P]ATP solution (to give 5 µCi/µl and 5 µM ATP final) 1 to 14 µl H2O. Cap tube and warm the mixture 10 min in a 30°C water bath. This reaction mix can also be used for α-casein. The total reaction mix volume is 20 ìl per reaction. The amount of water required depends on how much enzyme sample is used in step 2. Sufficient reaction mix can be prepared in a single tube for all the reactions by multiplying the quantities for a single reaction by the total number of reactions + 1. Then the total amount of reaction mixture (minus enzyme) for one reaction can be added to each tube. This is often very convenient and reduces the potential for pipetting errors.
2. Start the reaction by adding 1 to 14 µl enzyme sample containing casein kinase activity. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT 10.16), prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mix to 25 ìl immunoprecipitate bound to beads.
3. Incubate 10 min in a 30°C water bath. 4. Stop the reaction using the reagent appropriate for the analytic method to be used—20 µl ice-cold 10% TCA for TCA precipitation (see Support Protocol 2), or 10 µl or 20 µl ice-cold 2× SDS-PAGE sample buffer for electrophoretic analysis (see Support Protocol 4). Proceed with analysis by one of those methods. ALTERNATE PROTOCOL
ASSAY FOR CASEIN KINASES USING A PEPTIDE SUBSTRATE Casein kinase I can be assayed with the peptide AspAspAspGluGluSerIleThrArgArg. Casein kinase II has most recently been assayed using specific peptide substrates, e.g., ArgArgArgGluGluGluThrGluGluGlu (where the underlined residue is the phosphate acceptor). In both cases the arginine residues allow binding to P81 phosphocellulose paper. The first peptide is relatively specific for casein kinase I, but the kinetics of its phosphorylation are not ideal; the Km is in the millimolar range. Thus, it is preferable to use this assay for casein kinase I only if the kinase is highly purified. Additional Materials (also see Basic Protocol 3) 10 mM synthetic peptide substrate solution (see recipe) for casein kinase: e.g., AspAspAspGluGluSerIleThrArgArg (for casein kinase I) or ArgArgArgGluGluGluThrGluGluGlu (for casein kinase II)
Assays of Protein Kinases Using Exogenous Substrates
Additional reagents and equipment for TCA precipitation (see Support Protocol 2), adsorption onto P81 phosphocellulose paper (see Support Protocol 3), or electrophoretic analysis (see Support Protocol 4)
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1. For each assay, add the following to a microcentrifuge tube: 10 µl 5× casein kinase reaction buffer 5 µl 10 mM synthetic peptide substrate solution for casein kinase 5 µl [γ-32P]ATP solution (to give 5 µCi/µl and 5 µM ATP final) 0 to 30 µl H2O. Cap the tube and warm 10 min in a 30°C water bath. The total reaction mix volume is 50 ìl per reaction. The amount of water required depends on how much enzyme sample is based. Sufficient reaction mix can be prepared in a single tube for all the reactions by multiplying the quantities for a single reaction by the total number of reactions + 1. Then the total amount of reaction mix minus enzyme for one reaction can be added to each tube. This is often very convenient and reduces the potential for pipetting errors.
2. Start the reaction by adding 1 to 30 µl enzyme sample containing casein kinase activity. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mix to 25 ìl immunoprecipitate bound to beads.
10.16),
3. Incubate 10 min in a 30°C water bath. 4. Stop the reaction using the reagent appropriate for the analytic method to be used—20 µl ice-cold 10% TCA for TCA precipitation (see Support Protocol 2), or 10 µl or 20 µl ice-cold 2× SDS-PAGE sample buffer for electrophoretic analysis (see Support Protocol 4). Use 10 µl of the reaction mix for adsorption to P81 phosphocellulose paper (see Support Protocol 3). Proceed with analysis by one of those methods. ASSAY FOR Ca2+/CALMODULIN–DEPENDENT KINASES 2+
2+
As the name implies, Ca /calmodulin–dependent kinases are dependent on Ca and calmodulin for their activity. Ca2+/calmodulin–dependent kinase I (CaM kinase I) is a monomeric protein of ∼40 kDa. Ca2+/calmodulin–dependent kinase II (CaM kinase II) is composed of the products of four distinct genes for α, β, γ, and δ subunits. In its native form, CaM kinase II is an oligomer of Mr ∼500,000 to 600,000. As its name suggests, both Ca2+ and calmodulin are required for activity.
BASIC PROTOCOL 4
Little is known of the physiological function of CaM kinase I. Activity can be assayed using the protein synapsin 1 as substrate, but both CaM kinase 1 and CaM kinase II phosphorylate full-length synapsin 1. This problem can be overcome by using synthetic peptide substrates that include the specific phosphorylation site for the enzyme being assayed. CaM kinase I can be assayed using a peptide containing site 1 of synapsin I, TyrLeuArgArgArgLeuSerAspSerAsnPhe (where the underlined residue is the phosphate acceptor). CaM kinase II can be assayed using autocamtide, LysLysAlaLeuArgGlnGluThrValAspAlaLeu (Hanson et al., 1989), which is modeled on the autophosphorylation site of the kinase itself. Analysis of Protein Phosphorylation
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Materials 5× CaM kinase reaction buffer (see recipe) 10 mM synthetic peptide substrate solution (see recipe): e.g., TyrLeuArgArgArgLeuSerAspSerAsnPhe (for CaM kinase I) or LysLysAlaLeuArgGlnGluThrValAspAlaLeu (autocamtide for CaM kinase II) [γ-32P]ATP solution (see recipe) 1 mg/ml calmodulin in Milli-Q-purified water (store small aliquots at −70°C) Enzyme sample containing CaM kinase activity (see Support Protocol 1), kept on ice 30°C water bath Additional reagents and equipment for adsorption onto P81 phosphocellulose paper (see Support Protocol 3) 1. Set up and label duplicate 1.5-ml microcentrifuge tubes. For each reaction, prepare a mixture containing: 5 µl 5× CaM kinase reaction buffer 5 µl 10 mM synthetic peptide substrate solution 5 µl [γ-32P]ATP solution (to give 5 µCi/µl and 5 µM ATP final) 5 µl 1 mg/ml calmodulin or 5 µl H2O 0 to 30 µl H2O. Cap tube and warm the mixture 10 min in a 30°C water bath. The reaction mix is described for 50 ìl per reaction, but it can be carried out in volumes of 25 to 100 ìl by adjusting the amounts of the ingredients proportionally. Control reactions should include one reaction without calmodulin so that any background phosphotransfer reactions can be accounted for and only calcium2+/calmodulin–dependent phosphotransfer is measured.
2. Start each set of reactions sequentially every 30 sec by adding 1 to 25 µl enzyme sample containing CaM kinase activity to one of the tubes. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT 10.16), prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mixture to 25 ìl immunoprecipitate bound to beads.
3. Incubate for 10 min in a 30°C water bath. 4. Once the desired incubation time has elapsed, pipet 30 µl of the contents of the tube quickly onto the surface of prepared P81 paper squares (see Support Protocol 3). Only adsorption onto P81 phosphocellulose (see Support Protocol 3) is used to measure CaM kinase–mediated transfer because peptides are not efficiently precipitated by TCA, and they require specialized techniques for analysis by PAGE.
Assays of Protein Kinases Using Exogenous Substrates
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ASSAY FOR TYROSINE KINASES Acid-denatured rabbit muscle enolase was first used as an exogenous tyrosine kinase substrate in the early 1980s, and it is still a useful substrate in a number of experimental situations. Usually enolase is used as a substrate for cytosolic tyrosine kinases such as pp60c-src. Once the reaction is complete, phosphorylation of enolase is detected by separating proteins in the reaction mixture by SDS-PAGE (UNIT 10.2).
BASIC PROTOCOL 5
The following method is optimized for pp60c-src, but may be applied to other tyrosine kinases. It is also useful as a framework for the assay of any tyrosine kinase, but optimization of the immunoprecipitation and assay buffers is required for the tyrosine kinase of interest. Materials Rabbit muscle enolase (enolase EC 4.2.1.11; ammonium sulfate precipitate or suspension purified from rabbit muscle) 1 mM DTT/50 mM HEPES, pH 7.0 Glycerol 100 mM acetic acid 5× tyrosine kinase reaction buffer (see recipe) [γ-32P]ATP solution (see recipe) Enzyme sample containing tyrosine kinase activity (see Support Protocol 1), kept on ice 2× SDS-PAGE sample buffer (UNIT 10.2), ice-cold 30°C and boiling water bath Additional reagents and equipment for immunoprecipitation (UNIT 10.16) or SDS-PAGE (see Support Protocol 4 and UNIT 10.2) 1. Microcentrifuge the equivalent of 100 µg rabbit muscle enolase 5 min at maximum speed, 4°C. Discard the supernatant. 2. Add 10 µl of 1 mM DTT/50 mM HEPES, pH 7.0, to the pellet, mix thoroughly, and incubate 30 to 60 min on ice. 3. Add 10 µl glycerol, mix, and then keep on ice if assays are to be performed the same day. This preparation can also be stored at −70°C until required.
4. Immediately prior to assay, add 20 µl of 100 mM acetic acid to the enolase solution and mix thoroughly. Incubate this mixture 5 min in a 30°C water bath, and then keep on ice until the reaction mixture is prepared. Do not store acid-denatured enolase on ice in this state for >1 hr.
5. For each reaction, add the following to a 1.5-ml microcentrifuge tube: 4 µl 5× tyrosine kinase reaction buffer 1 µl acid-denatured enolase (step 4; 2.5 µg enolase total) 1 µl [γ-32P]ATP (to give 5 µCi/µl and 5 µM ATP final) 0 to 14 µl H2O. Cap the tube and warm the mixture 10 min in a 30°C water bath. The amount of water required depends on the amount of enzyme sample to be added; the total reaction volume should be 20 ìl. Sufficient reaction mix can be prepared in a single tube for all the reactions by multiplying the quantities for a single reaction by the total number of reactions + 1. Then the total amount of reaction mix minus enzyme for one reaction can be added to each tube. This is often very convenient and reduces the potential for pipetting errors.
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6. Start the reactions by adding 1 to 14 µl enzyme sample containing tyrosine kinase activity. The volume of enzyme used depends on the amount of activity in the enzyme sample. In a preliminary experiment, the maximum indicated amount can be used to gauge the extent of the phosphotransfer reaction. The volume can be reduced as appropriate in order to allow for linear incorporation of phosphate into the substrate during the assay. For enzyme samples that are immunoprecipitates adsorbed onto Sepharose beads (UNIT 10.16), prepare the reaction mix minus enzyme source first, warm it, then dispense it into the immunoprecipitate. For immunoprecipitates, it is advisable to scale up the reaction for a total volume of 100 ìl: add 75 ìl reaction mixture to 25 ìl immunoprecipitate bound to beads.
7. Incubate the tube 10 min in a 30°C water bath. 8. Stop the reaction by adding 20 µl ice-cold 2× SDS-PAGE sample buffer. Mix thoroughly and heat the tube 3 min in a boiling water bath. Analyze 20 µl of the reaction by SDS-PAGE (UNIT 10.2). Most tyrosine kinases autophosphorylate on a number of tyrosine residues and in in vitro assays are promiscuous with respect to substrate. Backgrounds are often very high so TCA precipitation and adsorption to P81 phosphocellular paper are not very useful for analysis of results. SDS-PAGE (see Support Protocol 4) gives more specific and cleaner results. BASIC PROTOCOL 6
IN-GEL PROTEIN KINASE ASSAYS In-gel protein kinase assays (Kameshita and Fujisawa, 1989) involve the copolymerization of a kinase substrate in the separating gel layer of an SDS-PAGE gel. Protein samples are run on the modified gel as usual (UNIT 10.2). After electrophoresis is complete, the separated proteins are allowed to refold by various gel treatments. The gel is then incubated with [γ-32P]ATP at 30°C for enough time to allow the ATP to penetrate the entire gel. After further washes to remove unincorporated [γ-32P]ATP, the gel is stained and fixed as normal. 32P-containing proteins are revealed by autoradiography (APPENDIX 3A). Although in general these assays are not particularly sensitive, they are sometimes very useful for identifying kinases in complex mixtures. Materials 10 mg/ml kinase substrate: e.g., myelin basic protein Enzyme sample containing kinase activity (see Support Protocol 1), kept on ice 20% (v/v) 2-propanol/50 mM Tris⋅Cl (pH 8.0 at room temperature; APPENDIX 2) 1 mM DTT/50 mM Tris⋅Cl (pH 8.0 at room temperature) 6 M guanidine⋅HCl/1 mM DTT/50 mM Tris⋅Cl (pH 8.0 at room temperature) or 8 M urea/1 mM DTT/50 mM Tris⋅Cl (pH 8.0 at room temperature) 1 mM DTT/0.05% (v/v) Tween 20/50 mM Tris⋅Cl (pH 8.0 at 4°C) Appropriate kinase reaction buffer (see recipes) 10 mM Mg/ATP solution (see recipe) 10 mCi/ml [γ-32P]ATP (3000 Ci/mmol; Amersham, DuPont NEN, or ICN Biomedicals) 5% (w/v) trichloroacetic acid (TCA) 1% (w/v) sodium pyrophosphate/5% (w/v) TCA Container for radioactive incubation: e.g., small tray with tight cover, or heat-sealable polyethylene bag (Seal-a-Meal) Seal-a-Meal apparatus (optional)
Assays of Protein Kinases Using Exogenous Substrates
Additional reagents and equipment for SDS-PAGE (UNIT 10.2) and autoradiography (APPENDIX 3A)
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CAUTION: Because the samples are radioactive, great care should be exercised in sample preparation and loading. Perform the reactions and subsequent manipulations in screwcap microcentrifuge tubes to minimize the risk of 32P contamination. Run gels until the dye front passes out of the gel, as all residual [γ-32P]ATP will migrate with the dye front. This means that the SDS-PAGE running buffer will be contaminated with 32P, and it should be disposed of as radioactive waste according to safety regulations. See APPENDIX 1F for proper handling and disposal. Separate proteins on modified gel 1. Prepare the mixture for a polyacrylamide gel of the appropriate %T (see UNIT 10.2). Add 10 mg/ml kinase substrate to give a final concentration of 1 mg/ml in the gel mixture. Polymerize as usual in a cassette prepared for casting the gel. Myelin basic protein has been used with some success for assays of the MAP kinase family of enzymes. However, any protein is suitable if it is a substrate for the kinase of interest. This assay can also be used to identify potential substrates.
2. Prepare the enzyme samples containing kinase activity for SDS-PAGE in the usual way and carry out electrophoresis (see UNIT 10.2). 3. Once separation is complete, place the gel in 200 ml of 20% 2-propanol/50 mM Tris⋅Cl, pH 8.0. Wash the gel for 20 min, change to fresh buffer, and repeat twice for a total of three 20-min washes. Washing with propanol removes SDS from the gel.
4. Wash the gel with 200 ml of 1 mM DTT/50 mM Tris⋅Cl (pH 8.0) three times, 20 min each. 5. Wash the gel with 250 ml of either 6 M guanidine⋅HCl/1 mM DTT/50 mM Tris⋅Cl (pH 8.0), or 8 M urea/1 mM DTT/50 mM Tris⋅Cl (pH 8.0) twice, 30 min each. Either guanidine⋅HCl or urea can be used to denature the proteins. Depending on the kinase, one or the other may be more successful; guanidine⋅HCl should be tried first, because it seems to work best for most kinases.
6. Wash the gel eight to ten times at 4°C over a period of 18 hr with 250 ml of 1 mM DTT/0.05% Tween 20/50 mM Tris⋅Cl (pH 8.0), to renature proteins. Assay for kinase activity 7. Incubate the gel with 250 ml of an appropriate kinase reaction buffer containing 10 mM MgCl2, for 20 min at 30°C. 8. Remove all traces of buffer and place the gel in a small tray or a Seal-a-Meal bag (the container is to be used for radioactive incubation). Add as small an amount of kinase reaction buffer as possible to just cover the gel. Add 1/4 of the reaction buffer volume of 10 mM Mg/ATP solution to give a final concentration of 50 µM ATP. Add 20 µCi/ml [γ-32P]ATP. 9. Incubate 1 hr at 30°C. The duration of this incubation can be adjusted after viewing the results of the initial experiment.
10. Incubate the gel in 250 ml 5% TCA twice, 15 min each. Wash 10 min in 500 ml of 1% sodium pyrophosphate/5% TCA. Repeat this wash until the solution contains little or no radioactivity. 11. Dry the gel onto filter paper and autoradiograph (APPENDIX 3A).
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SUPPORT PROTOCOL 1
PREPARING A CELL LYSATE FOR KINASE ASSAYS This protocol describes a method for detergent-induced cell lysis to prepare a crude extract containing kinase enzyme activity. Other methods of cell lysis may be appropriate (see UNIT 12.1). Hypotonic lysis and isolation of P100 and S100 fractions can also provide useful data on the recovery of a protein kinase as a soluble or membrane-bound activity. Materials Cultured cells: adherent cells at ∼70% confluence in 100-mm tissue culture dishes or suspension cells at 106 cells/ml PBS (APPENDIX 2), ice-cold Lysis buffer (see recipe) Protease inhibitor stock solutions (see recipe) Microcentrifuge, 4°C 1. Wash the cultured cells twice in ice-cold PBS. Completely aspirate the final wash solution. 2. Add 0.75 ml lysis buffer to ∼2 × 107 cells; for adherent cells check that the monolayer is completely covered. Add protease inhibitors from stock solutions to the appropriate final concentration: 1 mM PMSF, 100 µM benzamidine, 5 µg/ml leupeptin, 5 µg/ml pepstatin A, and 5 µg/ml antipain. A 0.75-ìl lysate should provide sufficient material for immunoprecipitation (use 25% to 50% of the lysate for an initial experiment and scale up or down depending on the results) or simple purification. If the purification procedure involves more than one chromatography step, prepare a larger lysate because only 10% to 20% of the activity is recovered from each chromatography step.
3. Incubate the cells 10 min at 4°C. Scrape the cells from the dish and transfer the lysate to a labeled microcentrifuge tube. 4. Microcentrifuge the lysate 10 min at maximum speed, 4°C. Carefully remove the supernatant and transfer it to a clean microcentrifuge tube. Lysates should be used immediately until proper storage conditions (e.g., −70°C, liquid nitrogen) can determined experimentally. For some kinases, it may not be possible to store the lysate at all. SUPPORT PROTOCOL 2
Assays of Protein Kinases Using Exogenous Substrates
TCA PRECIPITATION TO DETERMINE INCORPORATION OF RADIOACTIVITY One of the classical methods for separating a reaction product from the reactants is differential precipitation. In the case of protein kinase assays using a protein substrate and [γ-32P]ATP, it is very easy to precipitate the protein and leave the [γ-32P]ATP in the soluble fraction. Most proteins are quantitatively precipitated by trichloroacetic acid (TCA), so TCA can be used to precipitate proteins phosphorylated during a kinase assay. Protein precipitate can be captured by filtration and the filter can be washed with TCA. TCA precipitation is a quick and reproducible way to determine the extent of [32P]phosphate transfer to protein substrates. However, peptides are not precipitated by TCA, so adsorption of labeled peptides to P81 phosphocellulose paper (see Support Protocol 3) is a more suitable method for analyzing the results of kinase assays that use a peptide as substrate. Materials Assay samples (see Basic Protocols 1 to 5 and Alternate Protocol) 5% and 10% (w/v) trichloroacetic acid (TCA), ice-cold 95% ethanol Diethyl ether
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Whatman GF-C glass-fiber filters Vacuum manifold (e.g., Fisher) 20-ml scintillation vials Scintillation counter 1. Stop the reaction by adding 20 µl ice-cold 10% TCA. Mix thoroughly and incubate 10 min on ice to precipitate protein. 2. Pipet sample onto Whatman GF-C glass-fiber filter held in a vacuum manifold. Allow the sample to pass through the filter. Rinse the tube with 500 µl ice-cold 5% TCA. Add this wash to the appropriate filter. 3. Wash each filter with 5 ml ice-cold 5% TCA solution four times. Wash once with 10 ml of 95% ethanol, then three times with 10 ml diethyl ether. Allow each filter to dry for a few minutes. CAUTION: Diethyl ether is extremely flammable and washes should be performed in a fume hood away from any flame or heat source. Ether washes should be disposed of in an appropriate manner.
4. Place the dry filters in a 20-ml scintillation vial and count in a scintillation counter by detection of Cerenkov radiation. Alternatively, if dpm quantitation is desired, place dry filter in 10 ml scintillation fluid and count. The filter can also be counted by Cerenkov radiation after the washes with 95% ethanol. If the filters are to be counted in scintillant, however, the diethyl ether washes are essential, because TCA (which is removed by the ether) causes significant chemiluminescence when placed in scintillation fluid, making the counts produced by the scintillation counter meaningless.
ADSORPTION ONTO P81 PHOSPHOCELLULOSE PAPER One of the main methods to separate [32P]-labeled proteins or peptides from [γ-32P]ATP after a protein kinase assay reaction is by adsorbing the protein or peptide to P81 phosphocellulose paper. P81 paper is an ion-exchange matrix with net negative charge at most pHs. At low pH (such as in 75 mM orthophosphoric acid, which is used to wash the paper in this protocol), the excess [γ-32P]ATP left after a kinase assay will not bind. Under the same conditions, however, the phosphorylated peptide will bind to the P81 paper if it carries a net positive charge at low pH. In practice, this is usually achieved by tagging a peptide with two or three arginine or lysine residues at the N- or C-terminus. At the pH of 75 mM orthophosphoric acid the arginyl or lysyl side chains are positively charged and will bind to P81 paper. It is necessary, of course, that the activity of the kinase in question is not affected by the addition of basic residues to the substrate. Usually five or more residues are placed between the hydroxy-amino acid phosphorylation site and the basic residues required for binding to P81 paper.
SUPPORT PROTOCOL 3
When proteins are used as substrates for kinases, the net charge of the protein is also a consideration for adsorption to P81 paper. As for peptides, adsorption to P81 paper is more suitable for basic proteins than for acidic proteins. Under the conditions described in this protocol, histones bind very well to P81 paper because they are highly basic, but casein binds very poorly or not at all because it is a very acidic protein.
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fold here label here in pencil P81 paper
board of nonabsorbent material
Figure 18.7.1 Addition of sample to P81 phosphocellulose paper. The 2 × 2–cm squares of P81 paper are labeled using a pencil (ink is dissolved by the acetone wash), and 50% to 75% of the reaction product is applied to the paper.
Materials Assay samples (see Basic Protocols 1 to 5 and Alternate Protocol) 75 mM orthophosphoric acid (stored up to 6 months at room temperature) Acetone 2 × 2–cm squares of P81 phosphocellulose paper (Whatman) 500-ml plastic beaker with the bottom replaced with solvent-resistant plastic or wire mesh (see Fig. 18.7.2) 20-ml scintillation vial Scintillation counter 1. Cut 2 × 2–cm squares of P81 paper, fold the end over, and label the folded end with pencil as shown in Figure 18.7.1. Place each square on a large piece of nonabsorbent material, such as plastic or acrylic sheeting or an aluminum foil–covered fiberboard. Use a soft pencil to label the squares of P81 paper, as the final acetone wash will remove or destroy ink labels.
Assays of Protein Kinases Using Exogenous Substrates
2. Pipet 15 or 30 µl of the contents of each assay tube quickly onto the surface of the prepared P81 paper squares. Immediately place the paper squares in a 500-ml plastic beaker with the bottom replaced with solvent-resistant plastic or wire mesh (Fig. 18.7.2). Usually 50% to 75% of the final reaction mixture is spotted onto PB1 paper to ensure that there is sufficient signal to be detected.
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Figure 18.7.2 Setup for washing P81 phosphocellulose paper with reaction samples. The paper squares are washed five times for 5 min each in 75 mM orthophosphoric acid and once in acetone. They are then air dried and counted in a scintillation counter.
P81
solventresistant mesh
stir bar
magnetic stirrer
If the plastic mesh used in the beaker is not solvent-resistant, the final acetone washes will dissolve it!
3. Place this beaker into a larger beaker containing 75 mM orthophosphoric acid and incubate 5 min with stirring. 4. Pour the used orthophosphoric acid wash into a container suitable for 32P-containing liquid waste. Refill the large beaker with 75 mM orthophosphoric acid and incubate 5 min more. Repeat for a total of five 5-min washes. 5. Fill the large beaker with acetone and wash the P81 papers for 5 min. Discard the acetone and allow the papers to dry. 6. Place each square of P81 paper in a 20-ml scintillation vial and count by detection of Cerenkov radiation. Alternatively, if the absolute dpm value is desired and a 32P quenching curve is available for the scintillation counter, add scintillation fluid to the vials and count.
ELECTROPHORETIC ANALYSIS OF PHOSPHORYLATION Electrophoretic analysis of phosphorylation allows a more specific determination of protein substrate phosphorylation, because other phosphoproteins are separated by electrophoresis. It is advantageous to use the resolving power of SDS-PAGE to assay relatively crude enzyme samples; this method of analysis may also give useful information on kinase autophosphorylation.
SUPPORT PROTOCOL 4
Stop the reaction (see Basic Protocols 1 to 5 or Alternate Protocol) by adding 20 µl ice-cold 2× SDS-PAGE sample buffer (UNIT 10.2). Mix thoroughly, heat the sample 3 min in a boiling water bath, then analyze by SDS-PAGE (UNIT 10.2). Stain, fix, dry, and autoradiograph (APPENDIX 3A) the gel.
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REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
[γ-32P]ATP solution Dilute 10 mM Mg/ATP solution (see recipe) 1/10 in water to give a 1 mM solution. For the assay, add 10 µl of 1 mM Mg/ATP to 80 µl water and then add 10 µl of 10 mCi/ml [γ-32P]ATP (3.3 pmol ATP). The final ATP concentration is ∼100 ìM (plus 33 nM from the [γ-32P]ATP, which is negligible), and the 100 ìCi (∼2.2 × 108 dpm) in 100 ìl gives a specific activity of 1 ìCi/ìl or ∼2.2 × 107 dpm/nmol. Suppliers for [γ-32P]ATP include Amersham, NEN, and ICN; it is usually available at a variety of specific activities. A good specific activity to use is 3000 Ci/mmol, normally provided by the manufacturer at a concentration of 10 mCi/ml. Amersham’s Redivue is a convenient [γ-32P]ATP solution to use as it is stored at 4°C instead of −20°C. As 1 Ci is ≅ 2.2 × 1012 dpm, then for a specific activity of 3000 Ci/mmol = 3 Ci/ìmol = 6.6 × 1012 dpm/ìmol = 6.6 × 106 dpm/pmol = 2.2 × 106 dpm ≅ 1 ìCi ≅ 0.33 pmol. The stock Mg/ATP solution prepared earlier can be diluted to a working ATP solution of the desired concentration (in this example, 0.1 mM).
CaM kinase reaction buffer, 5× 100 mM Tris⋅Cl, pH 7.5 (APPENDIX 2) 25 mM MgCl2 1 mM CaCl2 Store up to 6 months at −20°C Casein kinase reaction buffer, 5× 100 mM Tris⋅Cl, pH 7.5 at 30°C (APPENDIX 2) 25 mM MgCl2 2.5 mM DTT 750 mM KCl Store up to 6 months at −20°C Cyclic nucleotide–dependent protein kinase reaction buffer, 5× 250 mM Tris⋅Cl, pH 7.5 at 30°C (APPENDIX 2) 25 mM MgCl2 Store up to 6 months at −20°C Lysis buffer 50 mM HEPES (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid), pH 7.4 100 mM NaCl 50 mM sodium fluoride 5 mM β-glycerophosphate 2 mM EDTA 2 mM EGTA 1 mM sodium vanadate (Na3VO4) 1% (v/v) Nonidet P-40 (NP-40) or Triton X-100 Store up to 6 months at −20°C Assays of Protein Kinases Using Exogenous Substrates
To stabilize kinase activity, specific kinase inhibitors, protease inhibitors, and reducing agents may be added to the lysis buffer (see Strategic Planning, Enzyme Sources).
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Mg/ATP solution, 10 mM Stock solutions: 20 mM MgCl2 or MgSO4 20 mM Na2ATP Working solution: Mix equal volumes of the two stock solutions and measure the pH. Adjust the pH slowly to pH 7.4 with 100 mM HCl or 100 mM NaOH. Store up to 1 year in small aliquots at −20°C. If a precipitate forms, the solution should be stirred continually and the pH maintained at 7.4. Adenosine trisphosphate is normally purchased as a sodium salt. Most kinases require ATP to be complexed with magnesium for its efficient utilization as a substrate. In some cases manganese can substitute for Mg2+.
PKC reaction buffer, 5× Solvent: 100 mM Tris⋅Cl, pH 7.5 at 30°C (APPENDIX 2) 25 mM MgCl2 1 mM CaCl2 Store up to 6 months at −20°C Lipids: Just before the buffer is to be used, transfer enough 5 µg/ml phosphatidylserine stock solution in chloroform to provide 100 µg phosphatidylserine/ml buffer (final concentration) to a clean tube. Add sufficient 0.5 µg/ml Diolein stock solution (Sigma) in chloroform to provide 10 µg Diolein/ml buffer (final concentration) to the same tube and mix. Dry the lipids under a stream of nitrogen. Working solution: Add the required volume solvent (see above) and place on ice for 10 min. Sonicate the mixture at medium power for 1 min while the sample is on ice. Let the sample sit on ice 2 min. Repeat sonication 4 more times for a total of 5 min. Discard any unused buffer. Diolein is the old name for various preparations that contain mixed isomers of diacylglycerol, the cofactor for PKC. Diolein is a suitable source for the cofactor in these assays, and it is less expensive than pure preparations of 1,2-diacylglycerol.
Protease inhibitor stock solutions 100 mM phenylmethylsulfonyl fluoride (PMSF) in 100% ethanol 100 mM benzamidine 1 mg/ml leupeptin 1 mg/ml pepstatin A 1 mg/ml antipain Store up to 6 months at −20°C Synthetic peptide substrate solution Prepare 10 mM solutions of each peptide (highest quality available) in Milli-Q water and adjust pH to 7.4 before storage at −70°C or use. If the peptide is insoluble, check the sequence—a peptide with a high proportion of acidic residues may be more soluble at alkaline pH, and one with a high proportion of basic residues may be more soluble at acidic pH. Hydrophobic peptides may be solubilized by adding up to 20% (v/v) acetonitrile. Whatever is added must not compromise the assay conditions: e.g., if a peptide must be dissolved at pH 10, adding the solution to the reaction mix must not change the pH of the reaction. Most synthetic peptides are quite stable when stored at −70°C; the stability of individual peptide substrates should be determined for the storage conditions used, however. If peptides are custom synthesized they should ideally be purified by HPLC before use.
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Tyrosine kinase reaction buffer, 5× 100 mM HEPES (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid), pH 7.4 5 mM MnCl2 5 mM DTT 500 µM sodium vanadate (Na3VO4) Store up to 6 months at −20°C COMMENTARY Background Information
Assays of Protein Kinases Using Exogenous Substrates
In recent years there has been an explosion of interest in protein kinases. It is now obvious that this class of enzyme plays many essential roles in a large array of biological processes, including the cellular responses to hormones, control of most stages of the cell cycle, and development of the nervous system in systems as diverse as fruit fly and man. Because of this interest, a very large number of protein kinase and protein phosphatase genes have been identified; current estimates are that there are well over 2000 genes encoding these enzymes in mammalian genomes. Obviously the study of protein kinases and phosphatases can provide fundamental information regarding the regulation of many diverse systems; the protocols provided in this unit include enough information to enable researchers new to this type of experiment to embark on the study of protein kinases. Protein kinases can be assayed using two types of substrate, intact proteins or synthetic peptides, and there are advantages and disadvantages to the use of each. A number of intact proteins may be used as general protein kinase substrates, and those in common use are available in relatively pure form from a number of manufacturers at reasonable prices. The main disadvantage to the use of proteins as substrates is the lack of specificity, sometimes making interpretation of experimental data difficult. Intact proteins such as myelin basic protein (MBP) contain many different phosphorylation sites. MBP can be phosphorylated by MAP kinase, some protein kinase C isoforms, and Ca2+/calmodulin kinase II, each at one or more sites. Hence, it is often impossible to differentiate which kinase is responsible for phosphorylating MBP in a complex mixture like a whole cell lysate. Some proteins cause headaches because they cannot be used with simple experimental protocols, e.g., casein. The use of synthetic peptides improves the specificity of the reaction by providing a single, well-defined phosphorylation site. They also allow experiments to define the concensus se-
quence for phosphorylation by a particular protein kinase and make it possible to design a substrate that has excellent kinetic properties for the kinase of interest. Peptides are often easier to use than their protein counterparts as in the case of casein kinases, where peptides can be designed that allow the use of phosphocellulose paper rather than other more involved methods for assessment of experimental results. Use of peptides is not without disadvantages though, the most significant being the cost of producing reasonable quantities of large numbers of peptides. When this is coupled with the observation that sometimes peptides are not recognized by the kinase because some structural element is missing, using peptide substrates can be a frustrating and expensive business. However, this is the exception rather than the rule, and the use of specific peptides to assay protein kinases is ultimately the best choice. One unit of kinase activity is usually defined as the amount of enzyme required to transfer 1 µmol of phosphate to a substrate from ATP per minute. From the following example it is possible to calculate a number of the reaction parameters: enzyme + [γ-32P]ATP + substrate → enzyme + ADP + [32P]product For this example, 1 mg protein (based on protein assay; see UNIT 10.1A) was used for the assay. The [γ-32P]ATP in the assay mixture had a specific activity of 106 dpm/nmol. After the 10-min reaction the 32P-labeled product had a specific activity of 105 dpm; therefore, there were (105/106) nmol of 32P-labeled product, and the concentration was 10−1 nmol or 100 pmol. Hence, the enzyme transferred 10 pmol of phosphate per minute, and represents 0.01 U of enzyme activity when measured with this substrate. Put another way, this represents an enzyme specific activity of 10 pmol/min/mg of protein. This latter expression of enzyme activity is a useful measure when comparing different preparations of a kinase or, during purification of a kinase, as a measure of the extent of purification through different steps.
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Critical Parameters Although it may seem obvious, the most critical element in performing kinase assays is using the correct controls. This is especially true when performing assays on tissue samples or cell extracts. Controls should always include addition of no substrate, addition of no enzyme source, addition of heat-denatured enzyme source, and, for reactions that require activator or cofactor, addition of any activator or cofactor and addition of no activator or cofactor. Another important aspect is the concentration of ATP in the assay. The ATP concentration can vary depending on the type of experiment to be performed. For derivation of kinetic data, the concentrations of both ATP and peptide or protein substrate are critical. If kinase detection during purification is required, assay linearity is not as important as long as the right controls are present. For mapping in vitro phosphorylation sites on a kinase substrate, very often the ATP concentration is kept extremely low but the specific activity very high. When designing potential peptide substrates for individual protein kinases, there are a number of considerations to remember. For example, phosphorylated peptide will bind to the P81 phosphocellulose paper. To be able to analyze the assay with this paper, it is necessary that the peptide carry a net positive charge at low pH. In practice this is usually achieved by tagging a peptide with two or three arginine or lysine residues at the N- or C-terminus. At the pH of 75 mM orthophosphoric acid, the arginyl or lysyl side chains are positively charged and will bind to the P81 paper. It is important to determine that the activity of the kinase of interest is relatively unaffected by the addition of basic residues to its peptide substrates in this way. To help ensure this is the case, it is a good idea to place five or more residues between the hydroxy-amino acid to be phosphorylated and the basic residues required for binding to P81 paper. Net charge considerations also pertain to some extent to proteins used as substrates for protein kinases. Under the conditions described in Support Protocol 3, histones will bind very well to P81 paper, because they are highly basic, but casein, which is very acidic, binds very poorly or not at all. In these cases, the P81 paper assay is very good when histones are used to assay kinase activity, but worthless when casein is used. When crude enzyme sources, such as cell lysates, are analyzed, many other proteins are present and phosphorylation of multiple proteins in the crude mixture is possible, as is
autophosphorylation of the protein kinase. Because many proteins will be precipitated by trichloroacetic acid (TCA) or will bind to P81 paper, there can be elevated backgrounds and severe overestimates of peptide phosphorylation when Support Protocols 2 and 3 are used. When performing kinetic studies, this overestimation of phosphorylation can be very misleading, as substrates other than the peptide under study will undoubtedly be phosphorylated and with different kinetics.
Troubleshooting Any troubleshooting of kinase assay procedures should be very straightforward. If the assay does not work, verify that both the [γ-32P]ATP and the peptide or protein substrate have been added at the correct concentrations. It is also very important to keep the enzyme source on ice before use and to store it appropriately between experiments. Very often when this type of experiment does not work it is because of a labile enzyme, poor storage of the enzyme, or a combination. It is important to know how long a particular enzyme is stable under the conditions used and the rate of inactivation or denaturation, if relevant. It is also essential to know if the enzyme activity is comparable between different preparations, especially for purified proteins.
Anticipated Results Depending on the requirements, information can be obtained about the specific activity of a particular enzyme preparation, kinetic parameters for the reaction catalyzed, or whether a particular kinase has been identified or substantially purified.
Time Considerations Kinase assays are quite simple to set up and perform. If the samples for assay are available, it is a matter of only ∼3 to 4 hr to go from assay set up to data interpretation when either TCA precipitation (Support Protocol 2) or adsorption to phosphocellulose paper (Support Protocol 3) is used to separate phosphorylated protein from contaminating [γ-32P]ATP. With SDS-PAGE analysis (Support Protocol 4), the assays can be set up, performed, and the samples run on a SDS-PAGE gel in ∼4 to 5 hr. Gel drying and autoradiography can take from 2 hr to a number of days depending on the level of protein phosphorylation. In general, if individual protein bands have ∼20,000 cpm of 32P associated with them, they will be visible with 1 hr of autoradiography.
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Literature Cited Hanson, P.I., Kapiloff, M.S., Lou, L.L., Rosenfeld, M.G., and Shulman, H. 1989. Expression of a multifunctional Ca2+/calmodulin–dependent protein kinase and mutational analysis of its autoregulation. Neuron 3:59-70. Kameshita, I. and Fujisawa, H. 1989. A sensitive method for detection of calmodulin-dependent protein kinase II activity in sodium dodecyl sulfate polyacrylamide gel. Anal. Biochem. 183:139-143. Pearson, R.B., and Kemp, B.E. 1991. Protein kinase phosphorylation site sequences and consensus specificity motifs. Methods Enzymol. 200:62-81. Songyang, Z., Carraway, K.L. III, Eck, M.J., Harrison, S.C., Feldman, R.A., Mohammadi, M.,
Schlessinger, J., Hubbard, S.R., Smith, D.P., Eng, C., Lorenzo, M.J., Ponder, B.A.J., Mayer, B.J., and Cantley, L.C. 1995. Catalytic specificity of protein tyrosine kinases is critical for selective signalling. Nature 373:536-539. Takai, Y., Kishimoto, A., Kikkawa, U., Mori, T., and Nishizuka, Y. 1979. Unsaturated diacylglycerol as a possible messenger for the activation of calcium-activated, phospholipid-dependent protein kinase system. Biochem. Biophys. Res. Commun. 91:1218-1224.
Contributed by A. Nigel Carter The Salk Institute for Biological Studies La Jolla, California
Assays of Protein Kinases Using Exogenous Substrates
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Permeabilization Strategies to Study Protein Phosphorylation
UNIT 18.8
This unit deals with the use of nucleotide triphosphates to label proteins in vitro in permeabilized cells and isolated cellular fractions. These experiments generally utilize [γ-32P]ATP as an exogenously added phosphate donor, although [γ-32P]GTP can be used in specific cases. The method is very straightforward, although numerous considerations must be made before applying it to each new system. describes labeling of phosphoproteins in intact cells with 32P-labeled inorganic phosphate (32Pi). After ascertaining that the protein of interest is a phosphoprotein using the protocols detailed in that unit, the phosphorylation event itself may be studied either in cell-free systems or in a more intact cellular system. In vitro, most protein kinases are capable of phosphorylating many substrates in addition to those phosphorylated in vivo, thus making it desirable to identify and study a protein kinase–catalyzed phosphorylation reaction in a more intact system than a cell lysate or homogenate. Permeabilization of intact cells provides a powerful experimental approach for accomplishing this. Brief permeabilization of cells is possible using a variety of agents. This procedure gives access to the intracellular milieu but still allows signal transduction events to be initiated by hormone stimulation of the cells via cell-surface receptors. UNIT 18.2
Brief chemical permeabilization of cells allows the introduction of cell-impermeable reagents into the intracellular compartment in order to study particular signal-transduction processes. In this type of experiment, relatively large molecules such as the Fab′ fragments of specific antibodies may be introduced into the permeabilized cell, although the kinetics of entry may be too slow for the study of some processes. This technique is often more useful when introducing relatively small molecules into the permeabilized cell, for example non-cell-permeable activators (e.g., hydrophilic or lipophobic small molecules that cannot partition into the membrane) or inhibitors of protein kinases or phosphatases (see, e.g., Calbiochem catalog). The kinetics of entry into permeabilized cells for small molecules such as [γ-32P]ATP are fast, and the system equilibrates the cellular ATP and [γ-32P]ATP with intracellular processes within 2 to 3 min. Figure 18.8.1 shows the 40,000
20,000
32
P (cpm)
30,000
10,000 A 0 0
5
10
15 20 Time (min)
25
30
Figure 18.8.1 Equilibration of the monoester phosphates of the lipid phosphatidylinositol-4,5bisphosphate with the ATP pool after permeabilization of human platelets. The letter A marks the point of addition of [γ-32P]ATP. Contributed by A. Nigel Carter Current Protocols in Molecular Biology (1997) 18.8.1-18.8.19 Copyright © 1997 by John Wiley & Sons, Inc.
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equilibration of the monoester phosphates of the lipid phosphatidylinositol-4,5-bisphosphate with the ATP pool after permeabilization of human platelets. As these monoester phosphates are known to equilibrate with the ATP pool extremely quickly, this experiment provides a good indication of the speed of equilibration of cellular ATP pools with intracellular processes after addition of [γ-32P]ATP to permeabilized cells. Procedures are outlined for performing a protein phosphorylation experiment using permeabilized cells (see Basic Protocol 1) and isolated intracellular organelles (see Basic Protocol 2). Both of these procedures result in lysates from which the protein of interest may be easily immunoprecipitated; however alternative techniques are described for preparing the final lysate from either Basic Protocol 1 or Basic Protocol 2 for electrophoretic analysis (see Alternate Protocols 1, 2, 3, and 4). A related procedure that does not involve permeabilization is outlined for direct analysis of cytosolic or membranebound kinases (see Alternate Protocol 5). Two different methods for determining the specific radioactivity of 32P-containing compounds are also included (see Support Protocols 1 and 2). CAUTION: It cannot be emphasized enough that each investigator should be fully familiarized with their local regulations regarding the safe handling and disposal of 32P and also have the appropriate equipment for dealing with the containment of 32P. See APPENDIX 1F for more information on the safe use of radioisotopes. NOTE: Milli-Q purified water or its equivalent should be used to prepare all solutions used throughout this unit. BASIC PROTOCOL 1
Permeabilizatin Strategies to Study Protein Phosphorylation
ANALYSIS OF PROTEIN PHOSPHORYLATION IN PERMEABILIZED CELLS When setting out to study a protein phosphorylation event via a permeabilization protocol, the general procedure is as follows. First, cells are incubated in a buffer which has been designed to mimic the composition of the intracellular compartment and which contains a permeabilizing agent. The preferred permeabilization agent of choice is streptolysin O, as this agent allows many signal-transduction systems to remain intact for a number of minutes after its addition (Stephens et al., 1994). This is certainly not true of all permeabilization agents or protocols; if another agent (e.g., saponin) is to be tried, the methods detailed below should be used as a general guideline only. Depending on the experiment to be performed, the permeabilization buffer may also contain [γ32-P]ATP and the particular reagents under study. The [32γ-P]ATP provides the “hot” phosphate, and will equilibrate with the intracellular processes utilizing ATP within 2 to 3 min (A.N. Carter, unpub. observ.). After a brief time the experimental reagents, such as inhibitors, can be introduced into the permeabilization medium and hence into the inside of the cell. At this point the cells may be stimulated with the appropriate agent in the same way as for intact cell preparations. The phosphorylation of the protein under study is initially investigated by immunoprecipitation as described in UNIT 10.16 and UNIT 18.2, followed by phosphopeptide mapping as described in UNIT 18.3. Materials Cell culture to be labeled Cell culture medium appropriate for cells being studied, buffered with 20 mM HEPES, pH 7.2, prewarmed to 37°C Cells under study (normally cultured cell lines or isolated primary cells in culture) Extracellular buffer (see recipe), 37°C Permeabilization buffer (see recipe), 37°C 60 U/ml streptolysin O working solution (see recipe), freshly prepared 10 mM cold ATP stock solution (see recipe) 10 mCi/ml [γ-32P]ATP (3000 Ci/mmol; e.g., DuPont NEN)
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Pharmacological agents, peptides, or Fab′ fragments to be studied Appropriate receptor agonists 2× lysis buffer for immunoprecipitation (see recipe), ice-cold Protease inhibitor stock solutions (in absolute ethanol; store up to 6 months at −20°C): 100 mM phenylmethylsulfonyl fluoride (PMSF) 100 mM benzamidine 1 mg/ml pepstatin A 1 mg/ml leupeptin 1 mg/ml antipain 100 mM DTT Dry ice/ethanol bath Noncirculating 37°C water bath containing a flat shelf (preferably made of wire mesh) with space for the number of cell plates in the experiment 50-ml centrifuge tubes Tabletop centrifuge Cell scrapers Screw-cap microcentrifuge tubes Permeabilize cells For monolayer cultures 1a. Approximately 1 hr before starting the experiment, change the medium in the cell culture plates to be labeled to equivalent medium containing 20 mM HEPES. Return cells to incubator and allow the cells to equilibrate with the new medium before use. The number of cells used for any experiment of this type is totally dependent on how easy it is to detect the end product being studied. In some cases, a 60-mm dish of cells that has just reached confluency, with ∼1 × 106 cells, is adequate; in others a 100-mm dish of cells with ∼1 × 107 cells is required. It is advisable to start with ∼1 × 106 cells and see if the signal is detectable. Use of a 60-mm dish allows a smaller volume of permeabilization medium (normally of the order of 0.5 to 2 ml) to be used. Standard cell culture medium is buffered by bicarbonate ions in the medium and the CO2 in the atmosphere of the incubator. Outside the incubator this buffering system is ineffective and the pH will quickly rise if the culture is left in the normal atmosphere. The pH of HEPES buffers are minimally affected by temperature (unlike Tris buffers), hence they provide good pH control for cell culture medium. One drawback is that HEPES may be toxic to some cells above a concentration of ∼20 mM, so it is important to check possible cellular toxicity before the use of HEPES-buffered culture media.
2a. Place the required number of plates containing cells in the 37°C water bath and allow 10 min for the medium to reequilibrate. The cells are stable in this state for an hour or so, depending on the rate of evaporation of the culture medium.
3a. Aspirate the culture medium from each plate and quickly rinse the cells twice, each time with 10 ml of 37°C extracellular buffer. Aspirate the extracellular buffer, then add 0.5 to 2 ml permeabilization buffer to each plate. Be careful when rinsing, as some cell lines are less adherent than others. Usually, most epithelial or mesenchymal lines adhere well and will not slough off the dish unless extreme force is used. Transformed cells usually adhere poorly and can easily be rinsed off the dish if care is not taken.
4a. Add streptolysin O (from the 60 U/ml working solution) to each plate to a final concentration of 0.6 U/ml. Add cold ATP (from the 10 mM stock solution) to a final concentration of 100 µM.
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For suspension cultures 1b. Approximately 1 hr before starting the experiment, transfer cells to sterile 50-ml centrifuge tubes and centrifuge 5 min at 1000 × g, room temperature. Remove the supernatant, then rinse cells twice, each time by adding ∼40 ml of 37°C extracellular buffer, centrifuging 5 min at 1000 × g, room temperature, and removing the supernatant. The cell suspension may be split into aliquots, if desired, after the addition of the final rinse of extracellular medium. To do this, add the final (40-ml) rinse of extracellular medium, resuspend cells gently, and dispense aliquots into fresh tubes. Centrifuge the aliquots and remove the supernatants to complete the final rinse before proceeding to step 2b.
2b. Resuspend the cells in 37°C extracellular buffer at a density of 1 × 107 cells/ml. Allow the cells to equilibrate 15 to 30 min at 37°C with gentle swirling. This step may have to be modified if a cell is used that could be activated by this procedure, (e.g., platelets). If a sensitive cell is used, equilibration time may be kept down to 5 to 10 min and the agitation may be omitted.
3b. Centrifuge cells again for 5 min at 1000 × g, room temperature, and aspirate the buffer. 4b. Add streptolysin O (from the 60 U/ml working solution) to fresh 37°C permeabilization buffer to a final concentration of 0.6 U/ml. Add cold ATP (from the 10 mM stock solution) to a final concentration of 100 µM. Add this permeabilization solution containing streptolysin O and ATP to the cells and mix gently but thoroughly. Addition of cold ATP at the same time as streptolysin O allows cellular ATP levels to be maintained and will therefore preserve cell viability. One important consideration is whether the cells under study possess cell-surface receptors for ATP. The ATP acts as a full agonist for some types of purinergic receptors, and this can obviously cause confusion when analyzing signal-transduction mechanisms. Prior to performing permeabilization studies, one should first determine, using intact cells, whether extracellular ATP activates signal-transduction mechanisms that interfere with the system under investigation. Stephens et al. (1994) have developed a way of inactivating extracellular receptors for ATP in neutrophils prior to permeabilization. The utility of this method for other cell types is not known to date, although it may be used as a guideline if required. Readers are referred to the original paper for full discussions of this method.
Permeabilize and label cells 5. Permeabilize cells by incubating 1 to 5 min at 37°C. The time of incubation depends very much on the particular system under study and the cell type, and must be determined empirically. Permeabilization for >5 min usually reduces hormone responsiveness of cells dramatically. This is probably due to a number of processes, including the loss of cytosolic components into the extracellular pearmeabilization buffer and physical uncoupling of receptors from signal-transduction processes.
6. Add 10 mCi/ml [γ-32P]ATP to a final concentration of 50 to 500 µCi/ml. [γ-32P]ATP is usually purchased at a specific activity of ∼3000 Ci/mmol, and a concentration of 10 mCi/ml. Hence 5 ìl of the stock is 50 ìCi [γ-32P]ATP. The label may be used at this concentration. If the stock [γ-32P]ATP is to be diluted into another solution it is better to do this directly before use. The criterion for the amount of radiolabeled ATP to add (between 50 and 500 ìCi/ml) is the detection limit. Initial experiments must be performed to see how much label must be added. Permeabilizatin Strategies to Study Protein Phosphorylation
The alternative to this separate labeling step is to add the [γ-32P]ATP at the same time as the streptolysin O and cold ATP in step 4a or b. The type of experiment to be performed will determine at which point the [γ-32P]ATP is added to the cells.
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7. Add the pharmacological agents, peptides, or Fab′ fragments under study and allow time for their action as required. Many pharmacological agents amenable to testing by this protocol are listed in the Calbiochem catalog. For example, a peptide containing residues 280 to 305 of the CaM kinase II α subunit can be a potent inhibitor of CaM kinase II and is non-cell-permeable. There are other peptides modeled on pseudosubstrate inhibitors of a variety of protein kinases that can be used as specific inhibitors, all of which are non-cell-permeable. Any Fab′ fragment may be tested by this protocol as long as it has an effect on enzyme activity, either positive or negative. Similarly, if an antibody stops an enzyme from reaching its site of action, it may also be tested via this procedure. The time for a particular agent to act as required should be determined experimentally as part of the initial experimental protocols. As with the [γ-32P]ATP addition, these agents may also be added at the same time as streptolysin O and cold ATP.
8. Add receptor agonists as required. The same considerations are true for receptor agonist additions as for inhibitor and [γ-32P]ATP additions to the cells. One must perform preliminary experiments to empirically determine the timings for these additions for each cell type under study. The remaining steps of this protocol are to prepare the cells for analysis by immunoprecipitation. If electrophoretic analysis is to be performed, see Alternate Protocols 1 and 2.
Prepare cell lysates for immunoprecipitation 9a. For monolayer cultures: Stop the reactions by rapidly aspirating the permeabilization buffer and adding 0.75 ml ice-cold 2× lysis buffer for immunoprecipitation to each plate. Add protease inhibitors and DTT (as stock solutions) to the following final concentrations: PMSF to 1 mM final Benzamidine to 1 mM Pepstatin A to 5 µg/ml final Leupeptin to 5 µg/ml final Antipain to 5 µg/ml final DTT to 1 mM final. Place plates on ice for 10 min, then scrape the cell debris from the bottoms of the plates into the buffer using a separate cell scraper for each plate. 9b. For suspension cultures: Stop the reactions by adding an equal volume of ice-cold 2× lysis buffer for immunoprecipitation. Add protease inhibitors and DTT to the final concentrations in step 9a, then place the tubes of cells on ice for 10 min. 10. Transfer lysates to screw-cap microcentrifuge tubes (one plate or cell aliquot/tube). Microcentrifuge lysates at maximum speed, 4°C, then transfer the supernatants to fresh screw-cap microcentrifuge tubes and freeze immediately in a dry ice/ethanol bath. Store at −70°C until needed. The lysates are now ready for immunoprecipitation (UNIT 10.16). CAUTION: The permeabilization buffer in the latter steps of this protocol contains [γ-32P]ATP, and great care should be taken to remove the medium safely and dispose of it correctly as liquid radioactive waste. The lysates and cell scrapers are also very radioactive at this point. Screw-cap tubes should be used at all subsequent stages of processing the cell lysates as these provide the best containment of the 32P-containing samples. The same microcentrifuge should be used for all centrifugation steps; after use it will undoubtedly be contaminated with 32P. It can then be set aside for future use with 32P-containing samples with the appropriate safety precautions.
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INTACT CELL SAMPLE PREPARATION FOR ELECTROPHORETIC ANALYSIS OF PROTEIN PHOSPHORYLATION The preceding protocol (see Basic Protocol 1) is set up to allow immunoprecipitation of specific proteins after the permeabilization experiment. Two alternatives to this approach, described below, involve using the separating power of polyacrylamide gel electrophoresis (PAGE) to fractionate proteins, instead of immunoprecipitation. Fractionation can be either by one-dimensional SDS-PAGE (UNIT 10.2), or by two-dimensional PAGE in which isoelectric focusing is performed as the first dimension, followed by SDS-PAGE (UNIT 10.3). In each case, sample loading must be optimized so that best resolution of the protein of interest is achieved. These protocols are written for both monolayer cells and suspension cells. To perform the procedure with suspension cells, the cells must be centrifuged down as quickly as possible after reacting with the appropriate pharmacological agents/receptor agonists, then treated as described for monolayer cultures. Obviously, the timing of the reactions is less precise this way, but experiments can be done successfully and reproducibly if reaction and centrifugation times are kept exactly the same. CAUTION: Samples for SDS-PAGE or isoelectric focusing will still contain a great deal 32 P at the latter stages of the protocol. Great care should be exercised in sample preparation. In particular do not use a probe sonicator to shear DNA, as 32P-containing aerosols will be produced. In the case of SDS-PAGE, the gels should be run until the dye front passes out of the gel. This means that the SDS-PAGE running buffer will be also be contaminated with 32P. In the case of isoelectric focusing the 32P will focus out of the gel during electrophoresis; hence the running buffer will be contaminated with 32P. These solutions should be disposed of as radioactive waste as local regulations dictate. ALTERNATE PROTOCOL 1
Intact Cell Sample Preparation for SDS-PAGE Additional Materials (also see Basic Protocol 1) 2× SDS-PAGE sample buffer (see recipe), 4°C Bath sonicator Boiling water bath 1. Prepare cells and perform permeabilization and labeling (see Basic Protocol 1, steps 1 through 8). 2a. For monolayer cultures: Stop the reactions by rapidly aspirating the permeabilization buffer and adding 100 to 250 µl of 4°C 2× SDS-PAGE sample buffer to each plate. Quickly scrape the cells into the buffer using a separate cell scraper for each plate and quantitatively transfer the sample to a screw-cap microcentrifuge tube. 2b. For suspension cultures: Microcentrifuge the reaction tubes 1 min at maximum speed, room temperature. Quickly aspirate the supernatants, taking care not to disturb the cell pellets. Add 100 to 250 µl of 2× SDS-PAGE sample buffer to each tube and vortex thoroughly. When performing this step for suspension cultures it is very important to use exactly the same times for centrifugations if reproducible data are to be obtained.
3. If samples are very viscous as a result of DNA being released from cell nuclei, shear the DNA by placing the capped sample tubes in a bath sonicator filled with ice water and sonicating 5 min. Permeabilizatin Strategies to Study Protein Phosphorylation
4. Boil samples 3 min in a boiling water bath. To perform the analysis, carefully apply up to 50 ìl of the samples to the sample wells of a precast SDS-PAGE gel. Proceed as described in UNIT 10.2.
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Intact Cell Sample Preparation for Isoelectric Focusing
ALTERNATE PROTOCOL 2
Additional Materials (also see Basic Protocol 1) Two-dimensional-PAGE lysis buffer (see recipe), 4°C Two-dimensional-PAGE sample buffer (see recipe) Bath sonicator Dry ice/ethanol bath Lyophilizer 1. Prepare cells and perform permeabilization and labeling (see Basic Protocol 1, steps 1 through 8). 2a. For monolayer cultures: Stop the reactions by rapidly aspirating the permeabilization buffer and adding 100 to 250 µl of 4°C two-dimensional-PAGE lysis buffer. Scrape the cells into the buffer using a separate cell scraper for each plate and transfer the sample to a screw-cap microcentrifuge tube. 2b. For suspension cultures: Microcentrifuge the reaction tubes 1 min at maximum speed, room temperature. Quickly aspirate the supernatants, taking care not to disturb the cell pellets. Add 100 to 250 µl of two-dimensional PAGE lysis buffer to each tube and vortex thoroughly. When performing this step for suspension cultures it is very important to use exactly the same times for centrifugations if reproducible data are to be obtained.
3. If samples are very viscous as a result of DNA being released from cell nuclei, shear the DNA by placing the samples in a bath sonicator filled with ice-water and sonicating 5 min. Freeze samples in a dry ice/ethanol bath, then lyophilize. 4. Redissolve the lyophilized samples in 50 to 100 ml of two-dimensional-PAGE sample buffer. Warm the samples to 37°C for 3 to 5 min. To perform the analysis, apply 10 to 20 ìl of each sample to a prepared isoelectric focusing gel of the required pH gradient (see UNIT 10.3). Samples for isoelectric focusing must have a low salt concentration because a high salt concentration often causes band broadening during isolectric focusing.
ANALYSIS OF PROTEIN PHOSPHORYLATION USING ISOLATED SUBCELLULAR FRACTIONS
BASIC PROTOCOL 2
When the protein under study exists in a particular organelle and is phosphorylated in intact cells, one consideration when studying the phosphorylation event in vitro in isolated organelles is whether the protein is accessible to exogenously added kinases or phosphatases, or whether it is accessible to endogenous kinases or phosphatases. For instance, if it is a protein of the endoplasmic reticulum, is its location transmembrane or intraluminal? If transmembrane, is it normally phosphorylated on the cytosolic side or luminal side of the membrane? Permeabilization protocols can also be used with intact intracellular organelles as well as intact cells. This is obviously an important consideration when dealing with proteins found inside the organelle in question. Another consideration here is whether the organelle can be isolated in an intact condition, or whether its contents will be partially or completely lost during purification. In the case of mitochondria this does not pose a problem, but for more fragile organelles such as the Golgi apparatus this may be more problematic. The study of protein phosphorylation in isolated organelles (see Castle, 1995 for isolation procedures) can be a straightforward extension of the permeabilization protocols de-
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scribed above (see Basic Protocol 1 and Alternate Protocols 1 and 2). The isolated organelle is treated in the same way as one would treat cultured cells in suspension. However, as there is little or no requirement for the maintenance of signal-transduction mechanisms in isolated organelles, the permeabilization procedure can be more prolonged than for intact cells. This allows the introduction of larger molecules such as purified protein kinases into the organelles under study. Materials Suspension of isolated intracellular organelles Intracellular buffer (see recipe), 4°C Permeabilization buffer (see recipe), 37°C 60 U/ml streptolysin O working solution (see recipe) 10 mM cold ATP stock solution (see recipe) 10 mCi/ml [γ-32P]ATP (3000 Ci/mmol; e.g., DuPont NEN) Reagents under study: e.g., kinase/phosphatase inhibitors, protein kinases, or phosphatases 2× lysis buffer for immunoprecipitation (see recipe), 4°C Screw-cap microcentrifuge tubes Benchtop ultracentrifuge (e.g., Beckman TL-100 or Airfuge) 37°C circulating water bath 1. Wash the organelle preparation three times with 4°C intracellular buffer, each time by ultracentrifuging 10 min at ≥100,000 × g. Transfer the equivalent of 1 mg of protein to individual screw-cap microcentrifuge tubes and keep on ice. The best way to wash the organelle preparations is to use a Beckman TL-100 benchtop ultracentrifuge or the older Beckman Airfuge. Both of these instruments will produce g-forces >100,000 × g. Because of the difficulty in centrifuging some organelles (e.g., plasma membrane preparations) it is essential to use ultracentrifugation for all organelles except nuclei, which can be recovered by microcentrifuging 30 sec at maximum speed. See Castle (1995) for details of organelle isolation.
2. Ultracentrifuge the organelle preparation 10 min at ≥100,000 × g, 4°C, and aspirate the supernatant. Add 200 ml 37°C permeabilization buffer, mix gently, and transfer to the 37°C circulating water bath. Work quickly at this point to minimize any proteolytic and other unwanted side reactions.
3. Add streptolysin O (from the 60 U/ml working solution) to each tube to a final concentration of 0.6 U/ml. Add cold ATP (from the 10 mM stock solution) to a final concentration of 100 µM. Exactly the same considerations apply when performing permeabilization of organelles as when performing permeabilization of intact cells (see Basic Protocol 1). However, because there is no intact signal-transduction machinery in isolated organelle preparations, the range of possible experiments is fewer, so the system is usually more robust than for intact-cell experiments.
4. Add 10 mCi/ml [γ-32P]ATP to a final concentration of 50 to 500 µCi/ml. The criterion for the amount of radiolabeled ATP to add (between 50 and 500 ìCi/ml) is the detection limit. Initial experiments must be performed to see how much label must be added.
Permeabilizatin Strategies to Study Protein Phosphorylation
5. Add the reagents under study—e.g., kinase/phosphatase inhibitors, protein kinases, or phosphatases.
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6. Add 200 ml of 4°C 2× lysis buffer to each tube to lyse the organelles. Mix well and place on ice for 5 min. 7. Microcentrifuge the samples 10 min at maximum speed, 4°C. Store lysates at −70°C. The lysates are now ready for immunoprecipitation (UNIT 10.16).
ORGANELLE SAMPLE PREPARATION FOR ELECTROPHORETIC ANALYSIS OF PROTEIN PHOSPHORYLATION Exactly the same considerations apply to the electrophoretic analysis of protein phosphorylation in isolated organelles as apply to permeabilized cells (see Alternate Protocols 1 and 2). Organelle preparations must first be ultracentrifuged and the supernatant removed before adding the appropriate buffer for electrophoresis. For this, the use of a Beckman TL-100 benchtop ultracentrifuge, which can provide g-forces >300,000 × g, is recommended. Complete removal of the supernatant is especially important if two-dimensionalPAGE (Alternate Protocol 4) is used, as the salt concentration must be kept low. CAUTION: Samples for SDS-PAGE or isoelectric focusing will contain a great deal 32P at the latter stages of the protocol. Great care should be exercised in sample preparation. In the case of SDS-PAGE, the gels should be run until the dye front passes out of the gel. This means that the SDS-PAGE running buffer will be also be contaminated with 32P. In the case of isoelectric focusing, the 32P will focus out of the gel during electrophoresis; hence the running buffer will be contaminated with 32P. These solutions should be disposed of as radioactive waste as local regulations dictate. Organelle Sample Preparation for SDS-PAGE
ALTERNATE PROTOCOL 3
Additional Materials (also see Basic Protocol 2) 2× SDS-PAGE sample buffer (see recipe) Boiling water bath 1. Prepare organelles and perform permeabilization and labeling (see Basic Protocol 2, steps 1 through 5). 2. Ultracentrifuge samples 10 min at 100,000 × g, 4°C, and aspirate the supernatant. Add 50 µl of 2× SDS-PAGE sample buffer and mix well. 3. Boil the samples 3 min in a boiling water bath. The samples are now ready for analysis by SDS-PAGE (UNIT 10.2).
Organelle Sample Preparation for Isoelectric Focusing
ALTERNATE PROTOCOL 4
Additional Materials (also see Basic Protocol 2) Two-dimensional-PAGE lysis buffer (see recipe) Lyophilizer 1. Prepare organelles and perform permeabilization and labeling (see Basic Protocol 2, steps 1 through 5). 2. Ultracentrifuge samples 10 min at >100,000 × g, 4°C, and aspirate the supernatant. Add 200 µl of two-dimensional-PAGE lysis buffer and mix well. 3. Lyophilize the samples overnight. The samples are now ready for analysis by isoelectric focusing (UNIT 10.3). Samples for isoelectric focusing must have a low salt concentration because a high salt concentration often causes band broadening during the isoelectric focusing dimension.
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ALTERNATE PROTOCOL 5
DIRECT ANALYSIS OF CYTOSOLIC OR MEMBRANE-BOUND KINASES This protocol utilizes isolated subcellular fractions in straightforward kinase assays by adding exogenous, purified protein kinases to the reaction mixture containing the organelle in question. This works only if the protein under study is phosphorylated by a cytosolic or membrane-bound kinase and its substrate is on the cytosolic face of the organelle. One can then determine the phosphorylation of proteins by analyzing the reaction mixture by SDS-PAGE. It is a good idea to perform the reactions and subsequent manipulations in screw-cap microcentrifuge tubes to minimize the risk of [32P] contamination. This protocol may be used to extend findings based on the use of the permeabilization strategies and assays with isolated organelles detailed elsewhere in this unit. Hence, if a protein of interest is phosphorylated, it can be useful to determine the subcellular localizations of the kinase(s) responsible, as this may make it possible to infer clues as to the identity of the kinase(s). Materials Appropriate kinase assay buffer (UNIT 18.7), 4°C [γ-32P]ATP solution (see recipe in UNIT 18.7) Suspension of isolated intracellular organelles, 4°C Purified protein kinase of interest (keep on ice) 2× SDS-PAGE sample buffer (see recipe), ice-cold Screw-cap microcentrifuge tubes 30°C and boiling water baths 1. Prepare the following reaction mixture in a screw-cap microcentrifuge tube at 4°C: 10 µl 5× kinase assay buffer 5 µl [γ-32P]ATP solution (5 µCi/5 µM ATP final) 1 to 35 µl organelle suspension (i.e., volume equivalent to 1 mg protein) H2O to 50 to 100 µl. After reaction mix has been assembled, warm to 30°C for 5 min. 2. Start reaction by addition of the purified protein kinase of interest. Incubate 15 min at 30°C. In the case of immunoprecipitated enzymes adsorbed onto beads, the assay mix minus the enzyme source can be made up complete first. This can then be dispensed, prewarmed, onto the immunoprecipitates. Incubation time can be adjusted as necessary depending on the extent of protein phosphorylation observed and the linearity of the reaction.
3. Stop the reactions by the addition of 100 µl of ice-cold 2× SDS-PAGE sample buffer. After thorough mixing, heat the samples 3 min in a boiling water bath. The samples are then ready for resolution by SDS-PAGE (UNIT 10.2). Samples for SDS-PAGE contain all the 32P originally in the assays. Great care should be exercised in sample preparation and loading. The gels should be run until the dye front passes out of the gel. This means that the SDS-PAGE running buffer will be contaminated with 32P, and this should be disposed of as radioactive waste as local regulations dictate.
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DETERMINATION OF SPECIFIC RADIOACTIVITY The specific radioactivity (specific activity) of any radioactive compound is defined as the number of atoms of the radioactive isotope in a molecule as a proportion of the total number of atoms of the same type. The structure of ATP is shown in Figure 18.8.2, and the form used in the study of protein kinases is [γ-32P]ATP, indicating that only the γ phosphate group has the radioactive 32P atom. There are, however, two other atoms of 31P. Hence, to attain the maximum possible specific activity, one should replace 100% of all phosphorus atoms with the 32P isotope. This is obviously not necessary in the case of kinase phosphotransfer reactions, as only the γ phosphate is transferred to a substrate; therefore in practice it is irrelevant whether the α and β phosphate groups have the 32P label. The units used by vendors of [γ-32P]ATP are curies per mole (Ci/mol), even though the curie is no longer the international unit of radioactivity.
SUPPORT PROTOCOL 1
NH2 N
N O H
O
P HO
γ
O O
P OH
β
O O
P
O
CH2
N
N O
OH
α HO
OH
Figure 18.8.2 Structure of ATP highlighting the α, β, and γ phosphates/phosphorus atoms.
It is important to know the specific radioactivity of the ATP in a kinase assay, as important data can be derived from this information. Consequently this can be determined as required, although one can make assumptions based on the volume of [γ-32P]ATP added to cold ATP prior to the assay. Obviously, the specific activity of [γ-32P]ATP cannot be assumed when adding [γ-32P]ATP to permeabilized cells or after labeling intact cells with 32 Pi. In these cases, one can determine the specific activity experimentally as described below. The specific radioactivity of [γ-32P]ATP can be determined by a number of methods, but the following are reliable and relatively simple. Methodologies for the extraction of ATP from either 32Pi-labeled cells or from cell lysates are detailed below (Stephens and Downes, 1990). The basis for the extraction is the same in each case, but slightly different alternative steps are required. Protein or cell debris is first precipitated with perchloric acid and removed by centrifugation. The supernatants are then neutralized with a mixture of tri-n-octylamine and 1,1,2-trichlorotrifluoroethane (Freon). The aqueous phase from this step can then be directly analyzed for ATP concentration and 32P content. ATP may be quantified using anion-exchange HPLC to separate nucleotides and a UV detector to detect nucleotides via UV absorption at 254 nm. The peak areas associated with injection of different amounts of ATP onto the column are then calculated and a standard curve constructed. The radioactivity associated with the ATP peak is easily determined by scintillation counting. Analysis of Protein Phosphorylation
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Materials 32 P-labeled cells or cell lysates Phosphate-buffered saline (PBS; APPENDIX 2) Standard ATP samples of known concentration in 4% perchloric acid 4% or 8% (v/v) perchloric acid, ice-cold Tri-n-octylamine (Sigma) 1,1,2-trichlorotrifluoroethane (Freon; e.g., Aldrich, Sigma) Linear gradient consisting of: Solution A: Milli-Q water filtered through 0.2-mm filter Solution B: 1.25 M NH4H2PO4, pH 3.8 (adjust pH with H3PO4; filter through 0.2-mm filter) Nucleotide standard mix: e.g., 1 mM each of AMP, ADP, ATP, GMP, GDP, and GTP Cell scrapers Polypropylene screw-cap microcentrifuge tubes or 5-ml polypropylene tubes with very tight-fitting caps Tabletop centrifuge HPLC system consisting of: Whatman Partisphere SAX HPLC column and guard column Apparatus for producing linear gradient UV detector set at 254 nm Fraction collector Whatman low-dead-volume 0.2-mm syringe filters Additional reagents and equipment for HPLC (UNITS 10.12-10.14) Extract ATP from cells or cell lysates For labeled cells 1a. Aspirate the medium from the labeled cells and rinse twice with PBS. Aspirate the final wash thoroughly and place the cells on ice. Quickly add 0.5 ml of ice-cold 4% perchloric acid and mix gently to lyse the cells. Incubate 5 min on ice. 2a. Scrape the cells into the acid solution using a cell scraper and quantitatively transfer the lysate to a screw-cap microcentrifuge tube. Microcentrifuge 5 min at maximum speed, 4°C, to bring down the cell debris. 3a. Transfer the supernatant to a fresh tube and keep on ice. Freshly prepare a mixture of 1.2 parts tri-n-octylamine to 1 part Freon (on a v/v basis) and mix well. Add 0.6 ml of this mixture to the perchloric acid supernatants, cap the tubes tightly, and vortex for at least 1 min. Extract four to six standard ATP samples of known concentration in parallel with the test supernatants. It is very important that the samples be mixed extremely thoroughly at this point to ensure complete neutralization.
For labeled cell lysates 1b. In a 5-ml polypropylene tube add an equal volume of ice-cold 8% perchloric to the amount of labeled cell lysate required and place on ice. Mix thoroughly and incubate 5 min on ice. 2b. Centrifuge 5 min (or until protein is completely pelleted) at 3800 × g, 4°C. Permeabilizatin Strategies to Study Protein Phosphorylation
3b. Transfer the supernatant to a fresh tube and keep on ice. Freshly prepare a mixture of 1.2 parts tri-n-octylamine to 1 part Freon (on a v/v basis) and mix well. Add 1.2 vol of this mixture to the volume of sample remaining after protein precipitation and
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vortex extremely well. Extract four to six standard ATP samples of known concentration in parallel with the test supernatants. It is very important that the samples be mixed extremely thoroughly at this point to ensure complete neutralization.
Recover neutralized aqueous sample 4. After complete mixing, centrifuge 5 min at 3800 × g, room temperature. Three layers should be evident following centrifugation. The bottom layer is excess tri-n-octylamine/Freon, the central layer is octylamine perchlorate (which is often yellow in color), and the upper layer is the neutralized aqueous sample.
5. Transfer the upper layer to a fresh tube and measure the pH using pH paper. The pH should be pH 6 to 7. If the pH of the sample is 90% of the radioactivity from the cellulose.
12. Remove the elution tip from each of the microcentrifuge tubes, being careful to leave all the eluate in the tube (some may cling to the sides of the tip as drops, which should be removed and added back to the contents of the tube). Save the elution tip. If eluting more than one spot, keep track of which tip was used for which peptide. 13. Clarify the eluate(s) by microcentrifuging 5 min at full speed (a small cellulose pellet will be visible after centrifugation; its size will depend on how snugly the sintered disk fits into the elution tip). Transfer the supernatant to a fresh microcentrifuge tube. It is very important to remove all traces of cellulose at this point, as contamination of the phosphopeptide with cellulose can ruin further analyses.
14. Count both the eluate and the “empty” elution tips by Cerenkov counting. ∼90% of the radioactivity should be in the eluates, with little remaining in the cellulose left in the tips. Given the pain and frustration involved in their manufacture, a good elution tip should be saved and reused. To clean these tips, apply a vacuum to the small end of the tip and suck the cellulose out (into a vacuum flask) while aspirating ∼10 ml elution buffer or deionized water through the tip to rinse it. Dry and then count the tips on a scintillation counter before reusing them.
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15. Lyophilize the eluates in a SpeedVac, then count them by Cerenkov counting. The counts here should be slightly higher than those of the liquid eluate. The number of cpm in this final sample of eluted peptide will often determine how it can be analyzed further. BASIC PROTOCOL 2
DETERMINATION OF THE POSITION OF THE PHOSPHORYLATED AMINO ACID IN THE PEPTIDE BY MANUAL EDMAN DEGRADATION If insufficient material is available for direct sequencing, a manual Edman degradation of the peptide can be performed to determine at which position the phosphorylated amino acid is present in the peptide. During each cycle of Edman degradation, the most amino-terminal amino acid residue is released from the peptide, and a sample from the reaction mixture is taken after each cycle. Phosphoserine or phosphothreonine is released as a derivative of serine or threonine and free phosphate; in contrast, phosphotyrosine is released as the anilinothiazolinone derivative of phosphotyrosine. Free phosphate and the PTH derivative of phosphotyrosine can be separated from the peptide by electrophoresis on a TLC plate. This approach indicates at which cycle the radioactivity and thus the phosphorylated amino acid is released from the peptide. Materials Eluted phosphopeptide (see Support Protocol 1) 5% (v/v) phenylisothiocyanate (PITC) in pyridine 10:1 (v/v) heptane/ethyl acetate—mix 10 parts heptane with 1 part ethyl acetate 2:1 (v/v) heptane/ethyl acetate—mix 2 parts heptane with 1 part ethyl acetate 100% (w/v) trifluoroacetic acid (TFA) Electrophoresis buffer, pH 1.9 (see recipe) 200 to 500 cpm 32P (prepared by diluting 32P orthophosphate with deionized water) or 2 mg/ml PTH-phosphotyrosine (see recipe) Microcentrifuge tubes (Myriad Industries) 45°C water bath Scintillation counter appropriate for Cerenkov counting Glass-backed TLC plates (20 × 20 cm, 100 µm cellulose; EM Science) 65°C drying oven or fan Additional reagents and equipment for electrophoresis of peptides on a TLC plate (see Basic Protocol 1 and Figure 18.9.3) and autoradiography (APPENDIX 3A) Determine experimental parameters 1. Decide the number of cycles to be run based on the list of candidate peptides. The number of cycles is designated as X. The starting volume for each cycle will be 20 ìl.
2. Dissolve the eluted peptide in 20 µl deionized water in what will be called the reaction tube (a microcentrifuge tube). 3. Remove a sample equal to 20/(X + 1) µl to a new tube; this is the starting material sample. Store this at 4°C. This sample will be lyophilized with the other cycle fractions at a later point.
Phosphopeptide Mapping and Identification of Phosphorylation Sites
Perform the Edman reactions 4. Add enough deionized water to the reaction tube to restore the volume to 20 µl. Count the sample at this point: a. to insure that the expected number of cpm have in fact been removed from the initial sample (as the starting material sample);
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b. to check the cpm at the start of each given cycle. 5. Add 20 µl of 5% phenylisothiocyanate in pyridine to each reaction tube, vortex well, spin briefly in a microcentrifuge to collect the sample at the bottom, and incubate at 45°C for 30 min. 6. Add 200 µl of 10:1 heptane/ethyl acetate to each reaction tube and vortex for 15 sec. Microcentrifuge 1 min at full speed to separate the two phases. The pyridine will partition into the (upper) organic phase.
7. Carefully remove the upper organic phase using a plastic transfer pipet. Reextract the (bottom) aqueous phase a second time with 10:1 heptane/ethyl acetate as in step 6. 8. Extract the aqueous phase two more times as in step 6, this time using 2:1 heptane/ethyl acetate. 9. Freeze the aqueous phase on dry ice and lyophilize in a SpeedVac evaporator. 10. Dissolve the dried sample in 50 µl of 100% trifluoroacetic acid (TFA) and incubate this at 45°C for 10 min. 11. Lyophilize the sample in a SpeedVac evaporator. 12. Count the sample by Cerenkov counting. There should be the same number of cpm as at the beginning of the cycle (i.e., at step 4 in this case).
13. Add 20 µl deionized water to the reaction tube, vortex, and microcentrifuge briefly. Remove a portion for analysis of the first-cycle products that is equal to 20/X. Store this at 4°C with the starting material sample. 14. Add deionized water to restore the sample volume to 20 µl to start the second cycle. Repeat steps 5 to 12. 15. After the second cycle, add 20 µl deionized water, resuspend the remaining sample, and remove 20/(X−1) µl to a new tube for analysis of the second-cycle products. Repeat steps 4 to 12. 16. Continue repeating steps 4 to 12 until the desired number of cycles have been run. For each new cycle, the amount of the sample to be removed is 20/X−Y where Y equals the cycle number minus 1.
Analyze the Edman products 17. Lyophilize all samples to dryness in a SpeedVac evaporator. Count all final samples by Cerenkov counting, 18. If lyophilized, dissolve the samples in 5 µl of pH 1.9 electrophoresis buffer or deionized water. Microcentrifuge 2 min at maximum speed to bring down any insoluble material. Alternatively, if the sample volumes removed after each cycle are small enough, skip steps 17 and 18 and load the samples directly onto the TLC plate.
19. Spot all samples from the analysis of a given phosphopeptide at least 1 cm apart on a line of origins running vertically through the center of the TLC plate (Fig. 18.9.5). As a marker, depending on the phosphoamino acid content of the peptide under investigation, spot 50 to 200 cpm of [32P]phosphate or 1 to 2 µg PTH-phosphotyrosine (0.5 to 1.0 µl of 2 mg/ml PTH-phosphotyrosine) at an origin on that same vertical
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A
B marker origin cycle 6 cycle 5 cycle 4
4.5 cm
2 cm
15 cm
cycle 3 cycle 2
15.5 cm
cycle 1 starting material 3 cm 10 cm
5.0 cm
10 cm 12.5 cm
12.5 cm
Figure 18.9.5 Sample and dye origins and blotter dimensions for analysis of manual Edman degradation products at pH 1.9. (A) Location of the sample and standard origins. To mark a TLC plate, the plate is placed on top of a life-size template on top of a light box and the origins are marked on the cellulose side using a very blunt extra-soft pencil. (B) Dimensions of the blotter and the location of the slot that fits over multiple sample and marker origins. The blotter is soaked in electrophoresis buffer, blotted with a sheet of Whatman paper to remove most of the buffer, and placed on top of the TLC plate so that the origins are in the middle of the slot.
line. Load 1⁄3 to 1⁄2 µl of sample at a time, air drying the sample between applications (see Basic Protocol 1, step 25). 20. Wet the plate as described in Figure 18.9.5. 21. Prepare the HTLE 7000 apparatus and electrophorese the samples for 25 min at 1.0 kV in pH 1.9 electrophoresis buffer (see Basic Protocol 1 and Figure 18.9.3). 22. After drying the plate (either in a 65°C oven or with a fan) mark it appropriately with radioactive or fluorescent markers and expose it to presensitized film with an intensifying screen at −70°C (autoradiography; APPENDIX 3A). BASIC PROTOCOL 3
DIAGNOSTIC SECONDARY DIGESTS TO TEST FOR THE PRESENCE OF SPECIFIC AMINO ACIDS IN THE PHOSPHOPEPTIDE Further information about a phosphopeptide of interest can be obtained by digestion with sequence-specific proteases or cleavage by site-specific chemicals. After incubation with a protease or chemical, the peptide is analyzed by separation in two dimensions on a TLC plate. A change in mobility upon treatment with a particular reagent indicates that the peptide was susceptible to cleavage, and consequently that the amino acid or amino acid sequence that confers susceptibility to cleavage by this reagent must be present in the peptide.
Phosphopeptide Mapping and Identification of Phosphorylation Sites
Materials Eluted phosphopeptide (see Support Protocol 1) Enzyme to be used for digestion and appropriate buffer (see Table 18.9.1) 2-mercaptoethanol
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Table 18.9.1
Specificities and Digestion Conditions for Enzymes and Other Cleavage Reagents
Enzyme or reagent Specificitya
Digestion conditions
Comments
TPCK-trypsin
K—X; R—X
pH 8.0-8.3
α-Chymotrypsin Thermolysin
F—X; W—X; Y—X X—L; X—I; X—V
pH 8.3 pH 8.0, 1 mM CaCl2, 55°C
Does not cut K/R-P; cuts inefficiently at K/R-XP.Ser/P.Thr and K/R-D/E; cuts wells at K/RP.Ser/P.Thr; cuts X-K/R-K/R-K/R incompletely Does not cleave F/W/Y-P or P.Tyr-X Will recognize most apolar residues to some extent; CaCl2 may affect the electrophoretic mobility —
Proline-specific P—X endopeptidase Cyanogen bromide M—X (CNBr)
pH 7.6
V8 protease
E—X
50 mg/ml CNBr in 70% formic acid, 90 min, 23°C pH 7.6
Endoproteinase Asp-N Formic acid
X—CSO3H; X—D
pH 7.6
D—P
70% formic acid, 37°C, 24-48 hr
CNBr is toxic; will only cleave unoxidized methionine V8 will not cleave at every E in whole proteins; does give a consistent pattern Will cleave X-E at high enzyme concentrations —
aThe dash indicates the cleavage site. See APPENDIX 1C for definitions of the one-letter abbreviations for amino acids.
Electrophoresis buffer of appropriate pH (see recipe) Water bath at appropriate temperature for enzyme digestion Glass-backed TLC plates (20 × 20 cm, 100 µm cellulose; EM Science) Additional reagents and equipment for chromatography and electrophoresis of phosphopeptides (see Basic Protocol, steps 24 to 31) 1. Dissolve the eluted phosphopeptide in 50 µl of the appropriate buffer in a microcentrifuge tube and microcentrifuge briefly to collect all solution at the bottom of the tube. Check the pH of the peptide solution by spotting 1 µl on a piece of pH paper to be sure that this final pH will allow enzyme activity. If the buffer’s pH has been altered dramatically by addition of the peptide, adjust it before adding enzyme. 2. Remove a portion of the sample (usually 50%) to be run both as an undigested control and as a mix with a portion of the digested sample. 3. Add 1 to 2 µg enzyme to the portion of the sample to be digested, vortex, and concentrate the sample in the bottom of the tube by microcentrifuging briefly. 4. Incubate all tube(s) in a water bath at the appropriate temperature for at least 1 hr. 5. Add another aliquot of enzyme and continue the incubation step for an additional hour. 6. Add 1 µl of 2-mercaptoethanol to each sample and boil 5 min to inactivate the enzyme Do this to all samples to ensure uniformity of sample preparation. It is necessary to completely inactivate the enzyme prior to loading the sample on the plate when analyzing a mix of digested and undigested peptide, since the undigested sample may be rapidly digested when the two samples are mixed.
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7. Lyophilize the samples in a SpeedVac evaporator. 8. Resuspend the samples by vortexing vigorously in 6 µl of electrophoresis buffer of the appropriate pH. Microcentrifuge at full speed to bring down insoluble material. 9. Load half of the undigested sample on a single TLC plate. Load half of the digested sample on each of two TLC plates; on one of these load the other half of the corresponding undigested sample as a mix. 10. Perform electrophoresis and chromatography on the plate as described above in Basic Protocol 1, steps 24 to 31. Based on the position where the particular phosphopeptide being analyzed ran in the original map, choose a pH and running time that will allow good separation of the peptide from its potential cleavage products but will ensure retention of the smaller cleavage products on the plate. SUPPORT PROTOCOL 2
PREPARATION OF PHOSPHOPEPTIDES FOR MICROSEQUENCE DETERMINATION OR MASS SPECTROMETRY The following is a general protocol and list of considerations for generating enough material for analysis by mass spectrometry or microsequencing starting with either intact cells or an in vitro system. 1. Optimize 32P labeling of the protein of interest. If the site of interest is seen only in stimulated cells, a time course of phosphorylation following stimulation may be helpful, as would determination of the optimal concentration of agonist. If an in vitro system is being employed, determine the optimum conditions (time, and ratio of kinase and substrate concentrations) for the kinase reaction. Include 1 mM cold ATP in the reactions to maximize the stoichiometry. UNIT 18.7, which deals with in vitro phosphorylation reactions, provides a detailed discussion of these parameters and how to manipulate them.
2. Calculate the number of cells or amount of substrate needed to isolate 10 pmol phosphorylated material. Even under optimal conditions, it is often not possible to achieve more than 25% stoichiometry of phosphorylation in vitro. In intact cells, the stoichiometry may be even less. It cannot hurt to overestimate the amount of starting material required, as the losses taken during the isolation procedures will always exceed expectation.
3. When calculating how to scale up the reactions, consider the following points.
Phosphopeptide Mapping and Identification of Phosphorylation Sites
a. The radioactivity of these samples is only used for visualization purposes—i.e., to determine which gel band to isolate, which phosphopeptide to isolate from the TLC plate, and which HPLC fraction(s) to use for final analysis. Thus, the majority of the material can be unlabeled, as only ∼1000 cpm per map spot are necessary for analysis at the time when the preparative HPLC is run. When isolating overexpressed protein from cells, labeling only 2 or 3 dishes of the 20 needed to generate enough material may be sufficient. When using an in vitro system, perform only one reaction using γ [32P]-ATP (include only the very minimum amount of cold ATP necessary for kinase activity) to generate the labeled material. To generate enough material for further analysis, perform an additional kinase reaction with unlabeled ATP only. For visualization, mix the labeled and unlabeled samples before resolving them by SDS-PAGE.
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b. The efficiency of protein elution decreases as the amount of gel increases, so try to keep the number of lanes on the preparative gel(s) to a minimum. About four lanes/slices of acrylamide gel can be successfully extracted per tube. c. If ∼20 µg substrate protein is present in each elution sample, it will not be necessary to add carrier protein at the TCA precipitation step (see Basic Protocol 1 step 11). This will result in a cleaner sample, as the tryptic fragments of the carrier protein will be eliminated from the mix of fragments run on the TLC plate. d. The 20 µg trypsin used to digest map samples in Basic Protocol 1 is in vast excess. While it is important that the digestion go as far to completion as possible, it is probably not necessary to scale up the amount of trypsin used. Instead, consider pooling several like samples at the performic acid digestion step (at the end of the 60 min incubation in order to give the protein the maximum time to dissolve). Adding 1 to 5 µg (total) trypsin to even 50 µg of protein for digestion is not unreasonable. Minimizing the amount of trypsin used will in turn minimize the amount of “extra” protein loaded on each TLC plate and ensure that the sample does not streak due to overloading. e. Determine the number of TLC plates to be run based on the amount of total protein to be analyzed—total protein includes the amount of trypsin and the amount (if any) of carrier protein used as well as the amount of the protein of interest. Even though the capacity of the TLC plates is ∼100 µg, to ensure good separation, no more than 60 µg of protein should be run on each plate. REAGENTS AND SOLUTIONS Use deionized, distilled water in all recipes and protocol steps. For common stock solutions, see APPENDIX 2; for suppliers, see APPENDIX 4.
Chromatography buffers Phosphochromatography buffer: 750 ml n-butanol 500 ml pyridine 150 ml glacial acetic acid 600 ml deionized water Store at room temperature Isobutyric acid buffer: 1250 ml isobutyric acid 38 ml n-butanol 96 ml pyridine 58 ml acetic acid 558 ml deionized water Regular chromatography buffer: 785 ml n-butanol 607 ml pyridine 122 ml glacial acetic acid 486 ml deionized water Store all of the above buffers up to 6 months at room temperature.
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Electrophoresis buffers For each of these buffers, mix well and check the pH. Record the pH and the date on the bottle; if the pH is more than a tenth of a unit off, remake the solution. Do not adjust the pH. Store all buffers at room temperature. pH 1.9 buffer: 50 ml formic acid (88% w/v) 156 ml glacial acetic acid 1794 ml deionized water pH 3.5 buffer 100 ml glacial acetic acid 10 ml pyridine 1890 ml deionized water pH 4.72 buffer 100 ml n-butanol 50 ml pyridine 50 ml glacial acetic acid 1800 ml deionized water pH 6.5 buffer 8 ml glacial acetic acid 200 ml pyridine 1792 ml deionized water pH 8.9 buffer 20 g ammonium carbonate 2000 ml deionized water Green marker dye Prepare a solution containing 5 mg/ml ε-dinitrophenyllysine (yellow) and 1 mg/ml xylene cyanol FF (blue) in pH 4.72 electrophoresis buffer (see recipe) diluted 1:1 with deionized water. Store up to 1 year at room temperature. PTH-phosphotyrosine, 2 mg/ml Combine 20 µl of 100 mg/ml phosphotyrosine with 20 µl of 5% (v/v) phenylisothiocyanate in pyridine. Incubate 30 min at 45°C. Extract twice with 200 µl of 10:1 (v/v) heptane/ethyl acetate, then twice with 200 µl of 2:1 (v/v) heptane/ethyl acetate (see Basic Protocol 2, steps 6 to 8, for extraction technique). Freeze the aqueous phase and lyophilize in a SpeedVac evaporator. Dissolve the sample in 0.1 N HCl, incubate 20 min at 80°C, and lyophilize again in a SpeedVac evaporator. Dissolve in 1 ml pH 1.9 buffer (see recipe for electrophoresis buffers). PVP-360 in 100 mM acetic acid 0.5 g PVP-360 (Sigma) 575 µl glacial acetic acid 99.4 ml deionized water Store up to 1 year at room temperature
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COMMENTARY Background Information Phosphopeptide mapping is a very sensitive technique that can help the investigator answer a variety of questions about a protein of interest. For some, phosphopeptide mapping is a tool to find out whether a particular protein is phosphorylated on one or more sites. This question can be answered by simply running a phosphopeptide map of the protein labeled in living cells. Other investigators want to know whether the increase in phosphorylation seen when cells are treated with a particular agent is restricted to one or more specific sites or whether it is evenly distributed over all phosphorylation sites present in the protein. Finally, detailed analysis of phosphopeptides isolated from a TLC plate can be used to identify the residues that are phosphorylated in a protein of interest. Several different strategies may be followed to identify the phosphorylation site represented by a particular spot on a phosphopeptide map. This is most definitively accomplished by eluting phosphopeptides from cellulose plates for either direct sequencing or for analysis by mass spectrometry (see Support Protocol; Fischer et al., 1991; Wang et al., 1993; Mitchelhill et al., 1997). While these two techniques require expensive instruments and expertise not found in most laboratories, such analysis can often be arranged by collaboration. However, sometimes it is not possible to take advantage of these techniques, since they require 1 to 10 pmole of material for analysis. For a 50-kDa protein one would need 0.1 to 1.0 µg starting material, assuming 50% recovery and 100% stoichiometry of phosphorylation at the site of interest. Use of an in vitro phosphorylation system that mimics the situation in intact cells will simplify matters greatly. Further considerations and strategies for preparation of samples for these two techniques are discussed at the end of this chapter in Support Protocol 2. Another approach to phosphorylation-site identification is to make an educated guess as to the identity of the site. Clues to a site’s identity include phosphoamino acid analysis of the individual phosphopeptide (UNIT 18.3); the result of manual Edman degradation of a phosphopeptide providing the cycle at which the phosphate is released and thus the position of the phosphorylated residue in the peptide (see Basic Protocol 2); and secondary enzymatic digests of the phosphopeptide that can be used in a diagnostic sense to determine the presence of other specific amino acids in the peptide (see
Basic Protocol 3). All three of these techniques are easily accomplished in a laboratory that is already set up for phosphopeptide mapping. The first step in all three is the isolation of the phosphopeptide from the cellulose plate (see Support Protocol 1). The validity of one’s guess can be tested by phosphopeptide mapping of a mutant protein lacking a phosphate acceptor at the site in question. Alternatively the guess can be substantiated by synthesizing the tryptic phosphopeptide and testing it for comigration with the phosphopeptide isolated from the peptide map.
Critical Parameters and Troubleshooting Generating phosphopeptide maps Keep in mind that the sort of analyses presented throughout this unit will give information regarding only the acid-stable forms of phosphoamino acids (i.e., phosphoserine, phosphothreonine, and phosphotyrosine) and will essentially ignore other forms such as phosphohistidine and phosphoaspartate, should they be present. Carrier Protein. The authors prefer to use RNase as carrier protein during TCA precipitation, particularly when analyzing proteins labeled in intact cells, because it degrades 32P-labeled RNA species that may have copurified with the protein of interest. The nucleotides generated by the degradation of RNA do not precipitate in TCA. Cleaving the protein. In order to generate a phosphopeptide map, the 32P-labeled protein needs to be cleaved into smaller fragments that can be separated by electrophoresis and chromatography on TLC plates. To do this requires an enzyme or chemical agent that cleaves reproducably and with a certain frequency. If not enough cleavage sites are present, the fragments generated will be too large and will not be separated easily by electrophoresis and chromatography on TLC plates. In addition, large fragments may contain multiple phosphorylation sites. This leads to maps that are less informative and more difficult to analyze. The authors routinely use trypsin and chymotrypsin. Other reagents are available (Table 18.9.1), but most of them cut much less frequently and some of them do not work very efficiently on full-length proteins. Removing ammonium bicarbonate. Following digestion, repeated cycles of lyophilization
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are carried out to evaporate all ammonium bicarbonate. The presence of salts in the sample will interfere with the electrophoretic separation of the peptides. After lyophilization, the protein digest appears as an invisible film at the bottom of the tube. The presence of any crystalline material indicates the presence of salts, most likely ammonium bicarbonate that can be removed by additional rounds of lyophilization. Controlling oxidation. Both cysteine and methionine can give rise to several oxidized derivatives. The oxidation state of these amino acids affects the mobility of peptides during chromatography, resulting in separation of oxidation state isomers. This complicates the interpretation of the phosphopeptide map. To prevent this, the protein or peptides are oxidized by incubation in performic acid at 4°C. Incubation at higher temperatures may give rise to unwanted side reactions and should be avoided. Elution and TCA precipitation or transfer to a membrane? In Basic Protocol 1, the 32P-labeled protein is isolated from a small slice of a dried polyacrylamide gel by rehydrating and grinding up the gel followed by elution in a buffer containing SDS and 2-mercaptoethanol. The protein is subsequently TCA precipitated, oxidized, and digested with trypsin. This is a time-consuming and laborious procedure. Yields are variable and usually not better than 50%. The alternative is to transfer the protein to a PVDF membrane; any unoccupied proteinbinding sites on the strips of membrane containing the protein of interest are blocked by incubation with PVP-360 in acetic acid before incubation with trypsin. Most peptides dislodge from the membrane during the digestion. This protocol is much faster and less laborious, and does not require the use of additional carrier proteins that may lead to overloading of the TLC plate and to streaky maps. Obviously this method is a poor choice for proteins that transfer poorly from the polyacrylamide gel to a membrane. In addition, it is possible that certain peptides that are generated during protease digestion retain a high affinity for the membrane and therefore fail to elute. If those peptides contain a phosphorylation site, this site will not be represented on the peptide map. This can lead to misinterpretations of the results. It is therefore advisable to first compare maps generated with Basic Protocol 1 and the Alternate Protocol. If these maps are identical, and if the protein transfers well from the gel to the membrane, the Alternate Protocol should be the protocol of choice.
Amount of sample. The authors like to load at least 1000 cpm on a plate for a peptide map. If the final sample has many more than 1000 cpm and a “pretty-looking” map is desired, it may be better to load only a fraction of the sample. Remember that overloading can lead to streaky maps. If a preparative map from which a particular peptide will be isolated is being run, it may be best to run the entire sample on two (or more) separate plates. Theoretically, it should be possible to separate 100 µg of material on a single TLC plate; this is often not the case in practice. Check the rate at which the first drop spotted sinks into the cellulose; as more sample is spotted, this rate will decrease. If, while spotting, one gets to a point where the sample drop just sits on the origin and does not spread into the cellulose, stop loading. Peptide diffusion. Peptides diffuse during the electrophoresis and chromatography, and this leads to a reduction in resolution and sensitivity. To counteract this, the authors try to keep the area on the TLC plate onto which the sample is spotted as small as possible by spotting only a small amount at a time (i.e., less than 1 µl) and drying the sample origin between spottings. In addition, the sample is concentrated by wetting the TLC plates with electrophoresis buffer using a blotter with holes cut out around the origin (Figs. 18.9.1 and 18.9.3). Pressing the edges of the hole onto the plate results in buffer moving from the blotter towards the center of the hole. This concentrates the sample on the origin. For this process to work well, the origin has to be precisely in the center of the hole. In addition, the buffer has to move with similar speed from the entire circumference towards the origin. The sample will inevitably streak if the buffer takes a long time to wet the spot, or moves unevenly through the spot. Electrophoresis system. In the authors’ laboratories the HTLE 7000 electrophoresis system (Fig. 18.9.2) is used. This system features water cooling and an inflatable airbag that presses the TLC plate against the cooling plate. Water cooling prevents overheating during electrophoresis. The inflatable airbag presses excess buffer from the TLC plate; this limits diffusion of the peptides and improves resolution. Buffers. Three different buffers are typically used for electrophoresis: pH 1.9, pH 4.72, and pH 8.9. Less often used are pH 3.5 and pH 6.5 electrophoresis buffer. To find out which buffer gives the best separation of peptides generated from a particular protein, all three buffers should be tested. If possible, the authors prefer
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to work with pH 1.9 buffer. Most peptides dissolve well at this pH. In addition, use of this buffer results less often in streaky maps. The authors usually spot the digest on the origins as marked in Figures 18.9.1 and 18.9.3. For optimal separation of the phosphopeptides generated from a particular protein, the position of the origin and the electrophoresis time may need to be changed. We prefer to change the time of electrophoresis rather than changing the voltage. Chromatographic process. Chromatography usually takes 12 to 15 hr, but the exact time may vary depending on the age of the chromatography buffer, the batch of plates, the buffer system, the quality of reagents used in the buffer, and the temperature in the room. Three different chromatography buffers are commonly used: isobutyric acid buffer, regular chromatography buffer, or phosphochromatography buffer (see Reagents and Solutions). Pyridine, which is present in all three chromatography buffers, oxidizes and turns yellow over time. Do not use oxidized pyridine to make up chromatography buffers. To find out which buffer gives the best separation of the peptides generated from a particular protein, all three chromatography buffers should be compared. Most investigators prefer not to use isobutyric acid buffer because it is particularly foul smelling. During the chromatography run, the air space in the tank saturates with buffer and this makes it possible for the volatile chromatography buffer to run all the way to the top of the TLC plate. When the chromatography tank is opened, most of the buffer-saturated air will escape from the tank. This makes it counterproductive to extend the run after the tank has been opened. Therefore do not open the tank when chromatography is in progress, and take all plates out when the chromatography tank is opened. Separation of the yellow and blue dye functions as a control for successful electrophoresis and allows one to follow the progress during chromatography. The dyes can also be used as standards relative to which the mobility of phosphopeptides of interest can be described, and can be used as markers for the comparison of one plate to another. The yellow compound is neutral at pH 4.72 and pH 8.9 and defines the position to which neutral peptides migrate; at p H 1.9 ε-dinitrophenyllysine is positively charged.
Phosphopeptide identification After running several phosphopeptide maps, it may become apparent that particular phosphopeptides present on the map change in intensity upon treatment of the cells with specific reagents. Such observations often lead to the next question—what is the identity of peptide “A” that becomes phosphorylated following treatment of the cells with factor “B”? If approximately 1 to 10 pmole of phosphorylated peptide can be generated, the peptide is isolated from the TLC plate, purified by HPLC, and identified by mass spectrometry or microsequencing. In many cases, it is not possible to obtain a phosphorylated peptide in such quantities. The investigator is then forced to learn as much about the phosphopeptide as possible before making an educated guess. We find it useful to make a list of all possible candidate peptides including some of their properties. The next step is to eliminate as many candidates as possible using mobility predictions and the results of relatively simple experiments that can be performed on the minute amounts of labeled peptides isolated from TLC plates. Making a list of candidate peptides and eliminating the first candidates Make a list of all possible phosphopeptides that could be generated from the protein of interest given the enzyme used in the primary digest; be sure to include partial cleavage products on this list. This list of peptides should include the nature and position of amino acids that can be phosphorylated and the peptides’ susceptibility to further cleavage by proteases or chemicals (for an example, see van der Geer and Hunter, 1990) After making such a list, first calculate and then plot the predicted mobilities of all candidate phosphopeptides. See Table 18.9.2 for values that can be used to do this. To calculate electrophoretic mobility. The mobility of a peptide in the electrophoresis dimension is dependent on its charge (e) to mass (M) ratio, as Mr = keM−2/3. When calculating relative mobilities (Mr) the simplified equation Mr = eM−1 can be used with good success. The net charge on a peptide is calculated by summing the charges of the N and C termini and those of the side chains of its amino acids at a given pH, and dividing by either the actual mass of the peptide or simply by the number of amino acids in it. Approximate charge values at the specific pHs commonly
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Table 18.9.2 Approximate Charge Values at Specific pHs Commonly Used for Electrophoresisa
Amino-terminal NH2 Carboxy-terminal COOH Arginine Aspartate Cysteine (oxidized) Histidine Glutamate Lysine Phosphoserine Phosphothreonine Phosphotyrosine
pH 1.9
pH 3.5
pH 4.7
pH 6.5
pH 8.9
+1 N +1 N −1 +1 N +1 −1 −1 −1
+1 −0.5 +1 N −1 +1 N +1 −1 −1 −1
+1 −1 +1 −0.7 −1 +1 −0.5 +1 −1 −1 −1
+1 −1 +1 −1 −1 +0.5 −1 +1 −1.3 −1.3 −1.3
+0.5 −1 +1 −1 −1 N −1 +1 −2 −2 −2
aN indicates neutral.
Phosphopeptide Mapping and Identification of Phosphorylation Sites
used for electrophoresis are given in Table 18.9.2. To calculate chromatographic mobility. A peptide’s mobility in the chromatographic dimension is dependent on its hydrophobicity, and thus on its amino acid sequence. The order of the amino acids will also change the peptide’s mobility; thus, two peptides of identical sequence which are phosphorylated at one or the other of two possible sites may migrate different distances in the second map dimension, although they migrate identically in the first dimension since their charge:mass ratio is the same. While it is not possible to exactly predict chromatographic mobilities, relative mobilities can be plotted with some success by calculating an average mobility for the peptide based on migratory values of its constituent amino acids. This is not ideal, since the calculated Rf values of individual amino acids are significantly influenced by the presence of their charged amino and carboxy termini, which of course are noncontributory in the context of a peptide. This accounts in part for the compression of the calculated maps compared with the observed peptide migrations. Values for chromatographic mobilities of amino acids have been published in (Boyle et al., 1991); these were determined for each individual amino acid relative to the ε-DNP-lysine (yellow) marker using cellulose plates available twenty years ago. The quality of the cellulose used in such plates has changed markedly over the years; contact Ned Lamb for values that have been empirically determined more recently (http://www.genestream.org).
Bear in mind that these calculations can also be accomplished using a computer program. Ned Lamb (CNRS, Paris) has constructed a Web site for analysis of phosphopeptide maps, which may be found at http://www.genestream. org. Phospepsort 4, the program that he has developed based on an earlier version which originated at the Salk Institute, gives the biophysical characteristics as well as the electrophoretic and chromatographic mobilities of each proteolytic fragment. Alternatively, predicted peptide mobilities can be visualized using the graphical interface to Phospepsort 4: Mobility. In addition, Ned Lamb is working on a program that fits the calculated mobility values to the actual values for peptides of known composition. The Resolve program then reads the position of a spot on the actual map and calculates which peptide(s) derived from the protein being mapped could have the mobility of that spot. It is imperative to note that to date there is no program that accurately predicts the mobility of all peptides of a protein. This may be explained by the fact that mobilities are calculated using values established for single amino acids rather than for peptides. Plotting predicted phosphopeptide mobilities on a graph using linear axes results in a greatly compressed map as compared to that observed in one’s autoradiograms, especially in the chromatographic dimension. Therefore, do not despair if the predicted map of all phosphopeptides in the protein of interest looks nothing like the actual map that was generated experimentally. The value of such predictions comes from the fact that the relative positions of phosphopep-
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tides are predicted with great accuracy by such programs/calculations. Thus, if the predicted mobility of a hypothetical phosphopeptide places it on the anode side of a cluster of phosphopeptide candidates, while the phosphopeptide of interest on the actual map is present on the cathode side of the cluster, the hypothetical phosphopeptide may be eliminated from further consideration. This example illustrates how careful use of predicted peptide mobility maps may lead to elimination of candidate peptides. Isolating peptides from TLC plates Phosphopeptides isolated by elution from cellulose, as described above, can be used without further purification for certain types of analysis. What many people overlook, however, is that this sample is by no means necessarily pure. It includes, in addition to the radioactive phosphopeptide in question, any unlabeled tryptic fragments that may have comigrated with it on the cellulose plate—these peptides are generated from trypsin itself and from the carrier protein used in the TCA precipitation during sample preparation. For this reason, the sample is usually further purified by HPLC to clean it up before analysis by mass spectrometry or microsequencing; the column fractions are counted in a scintillation counter to determine which ones to use for further analysis. However, for manual Edman degradation (see Basic Protocol 2), secondary cleavage (see Basic Protocol 3), and phosphoamino acid analysis (UNIT 18.3) no further purification is necessary, as the interpretation of the results relies solely on the visualization of the resultant 32P-containing reaction products. The presence of unlabeled contaminants does not interfere with the interpretation of the results. Phosphoamino acid analysis Perhaps the most obvious step to take with an unidentified phosphopeptide in hand is to determine the phosphoamino acid content of the peptide of interest. This will eliminate many candidate peptides from consideration; this step is obviously not necessary if a long exposure of the phosphoamino acid analysis of the labeled protein in question indicates that only one species of phosphorylated amino acid is present. For phosphoamino acid analysis only 50 cpm of purified phosphopeptide is needed (though in peptide mapping and related protocols one can never have enough cpm). The details of such analysis have been discussed in detail in UNIT 18.3. Briefly, the phosphopeptide
eluted from the TLC plate is hydrolyzed by incubation for 60 min at 110°C in 30 µl of 6 N HCl. The appearance of yellow to brown color in the sample during hydrolysis indicates that some cellulose remained despite efforts to clarify the phosphopeptide eluate. This sometimes causes the sample to streak. After the sample is lyophilized, it is resolved with stainable standards in two dimensions by electrophoresis as described (UNIT 18.3). The phosphoamino acid composition is determined by matching the resultant spot(s) on the autoradiogram with the ninhydrin-stained standards on the cellulose plate. Manual Edman degradation At pH 8 to 9, phenylisothiocyanate reacts with the free amino group(s) of a peptide to form a corresponding phenylthiocarbamyl peptide. Treatment of these PTC-peptides with acid (TFA) results in the cleavage of the derivatized amino-terminal amino acid and its release as an anilinothiazolinone molecule. This latter species is not stable, and will cyclize to yield the phenylthiohydantoin (PTH) derivative of the amino acid in aqueous acid. If a phosphoserine or phosphothreonine residue is present, a βelimination during the cyclization releases free phosphate. Phosphotyrosine, however, is simply released as the anilinothiazolinone derivative. This may be converted to the phenylhydantoin form for analysis by incubating it in 0.1 N HCl at 80°C for 20 min. PTH-phosphotyrosine to use as a marker is easily synthesized by reacting phosphotyrosine with phenylisothiocyanate and then heating it in acid (see Basic Protocol 2, step 5); it can be visualized as a dark spot using a hand-held UV light. While the protocol is relatively simple, each cycle takes ∼2 hr to complete and requires at least 100 cpm for an unambiguous result. It is important to run a portion of the starting material out on the TLC plate to show how much, if any, free phosphate is there, since some hydrolysis of the peptide may have occurred during its isolation. Because at each cycle the reaction may not go to completion, one should plan to do at least one more cycle than is predicted to be necessary to release the phosphate (i.e., if all the candidate peptides are phosphorylated at or before the third residue from the N terminus, at least 4 cycles should be run). Thus, how many cpm are in the eluted map spot may determine how many cycles are run. In any case, we generally do not attempt to perform more than 5 or 6 cycles. If no phosphate is released during the course of these
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cycles, this conclusion in itself will usually greatly reduce the number of candidate sites. In addition to the position of the phosphorylated residue, clues to a peptide’s sequence may also be gleaned from Edman degradation. The way in which the residual peptide shifts its electrophoretic position on the plate after each cycle will indicate whether an acidic or basic amino acid has just been removed. If the tryptic peptide’s carboxy-terminal residue is a lysine, the lysine’s ε-amino group will be derivatized in the first cycle, and so a positive charge will be lost in that instance as well. If an automated peptide sequencer is available, 20 or more Edman cycles may be analyzed by coupling the phosphopeptide via carboxyl groups to a Sequelon membrane (Millipore) and letting the machine do the work. At the end, the fractions are counted to see where the radioactivity is released. This method obviously requires far fewer cpm in a phosphopeptide sample than does the manual Edman protocol, since one only analyzes the released material for radioactivity rather than a portion of the whole sample at the end of each cycle. A method for adapting an automated sequencer for such a purpose is discussed in Mitchelhill et al. (1997). An adaptation of the protocol presented above is found in Fischer et al. (1997). This protocol uses a volatile isothiocyanate (trifluoroethyl isothiocyanate) and volatile buffers; as a consequence the extraction steps can be eliminated and this results in shorter cycle times (∼45 min).
Phosphopeptide Mapping and Identification of Phosphorylation Sites
Secondary digests In the past, secondary digests of tryptic phosphopeptides represented a large part of the further analysis of these peptides. The utility of these digests, however, is totally dependent on the sequence of the protein in question. A list of enzymes commonly used in such digests is found in Table 18.9.1, along with their cleavage site(s) and optimal pH and temperature. As is true for trypsin, these enzymes sometimes cleave inefficiently when encountering a cleavage site in a certain sequence context—some of these problematic sites are listed as well. In our experience, the enzymes/reagents that cleave only one amino acid (i.e., proline-specific endopeptidase, V8, or cyanogen bromide) tend to be more useful and give less ambiguous results. Do not expect that the results of one enzyme digest will eliminate more than a few candidate sites. As it is most likely that the enzyme in question will not cleave the phosphopeptide, it
is imperative that a positive control be included in the digest—i.e., a peptide whose sequence is known and which the enzyme will cleave. If such a peptide is not available, a portion of the primary digest might be used as a positive control—with luck this will contain at least one peptide whose migration will be changed by this secondary digestion. Running out the mix of undigested and digested peptides is very important, since failure to comigrate with the original peptide will unequivocally demonstrate a change in the phosphopeptide’s mobility. A sample treated exactly as the digested sample, but without the cleavage reagent, should be analyzed in parallel. This is to ensure that changes in mobility seen in the digested peptide are truly due to the presence of the cleavage reagent. When deciding where to spot the samples and what conditions to use to run the plates, keep in mind that if the peptide did not migrate very far in the original map, it may be possible to load two sample on a single plate. Also keep in mind, however, that free phosphate may be released during the enzymatic digestion (due to elevated temperatures and resulting hydrolysis). Free 32P-phosphate originating from the sample loaded to the right may complicate the interpretation of the sample loaded on the left (anode) side of the plate. It should be noted that additional information about the other amino acids in a phosphopeptide may be gleaned just by running a tryptic digest in the electrophoretic dimension at another pH—identification of the different peptides by their mobility in the chromatography dimension will allow one to compare the mobility of peptides when run at different pHs and decide if a particular spot has changed its migration in the first dimension. Such a change in migration would be affected only if the phosphopeptide contained an amino acid whose charge was changed at the second pH used. Comigration of a synthesized phosphopeptide with a phosphopeptide isolated from a peptide map After eliminating all but a few of the candidate peptides, comigration of a synthetic phosphopeptide with the 32P-labeled peptide generated by digesting a protein from labeled cells may provide convincing evidence as to the latter’s identity. Obviously, care must be taken that the synthetic peptide be quite pure (generating only one spot on the cellulose plate) and that, while enough is loaded to be easily visualized, the plate not be overloaded (which
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would cause streaking and therefore render the comigration ambiguous). If the peptide is synthesized as a phosphopeptide, ∼5 to 25 µg of pure peptide is needed for each comigration, as it will need to be ninhydrin stained for visualization. This is by far the easier approach—starting with an unphosphorylated peptide entails not only finding a kinase that will phosphorylate it, but also purifying the phosphopeptide first before running the comigration. In either case, synthesis of peptides, and in particular those with special residues such as phosphorylated amino acids, is not inexpensive, and so this type of experiment is usually attempted after one has accumulated several other clues regarding a site’s identity. The alternative is to mutate the remaining candidate phosphorylation sites. An epitopetagged version of the mutant protein can then be expressed in cells and mapped to check whether the spot in question disappears from the map. This approach of course assumes that the sequence of the protein in question is known and that one has a clone in hand for mutagenesis. While satisfying to many, this mutagenesis approach is not definitive, as it can be argued that the mutated form of the protein may not fold correctly and therefore may not be phosphorylated correctly, resulting in a misleading loss of map spots. Another potential problem is that, if the phosphorylation of the site in question is an ordered event, dependent on another site’s state of phosphorylation, mutation of this other site could lead to the erroneous conclusion that the disappearance of a particular spot is the direct result of the mutation introduced, rather than an event of secondary consequence. Mutagenesis then should be taken as a supporting argument for the presumed identity of a phosphorylation site, rather than as definitive proof. Taken in conjunction with other evidence, it can nevertheless be quite convincing. Scaling up phosphopeptide mapping to isolate peptides for mass spectrometry and microsequencing The most definitive methods for determining the site(s) of phosphorylation in a protein are the determination of the amino acid sequence or mass of the phosphopeptides. While the sensitivity of the instruments used in both these techniques has improved over the past 5 years, both methods require on the order of 1 to 10 pmole of material for analysis. When analyzing the phosphorylation of a receptor protein-tyrosine kinase of which ∼10,000
molecules are present on the cell surface, one would need to grow 60 10-cm dishes of cells to isolate the 6 × 1011 molecules (1 pmol) required for a successful analysis. In this calculation, we have assumed 100% recovery and 100% stoichiometry of phosphorylation at the sites of interest. However, since it is very possible that not every molecule is phosphorylated at the sites in question, and that one is likely to take at least a 50% loss of material over the entire protocol, it would be best to start then with at least 240 dishes of cells. The use of overexpressed protein in cells would obviously facilitate the accumulation of sufficient amounts of protein for analysis. Isolation of enough material for analysis becomes less difficult if an in vitro system can be used to generate the phosphopeptides in question. Protein kinases are notoriously promiscuous in vitro, so it may be possible to generate the sites of interest using a kinase that is actually not the one responsible for the phosphorylation in vivo. The protein of interest produced in bacteria makes a good substrate, as it is most likely not phosphorylated to start with. It is first necessary to show that the phosphopeptides generated by incubation of recombinant protein with a purified kinase in vitro comigrate exactly with those purified from labeled cells. If there is any doubt about this, it is reassuring to run the maps for a longer time than usual in the electrophoresis dimension, or at a different pH to further demonstrate comigration. Given the specificity of the two-dimensional separation on these maps, it is unlikely that two phosphopeptides that truly comigrate will not be identical.
Anticipated Results Upon developing the first autoradiogram of an initial peptide map, depending on the number of spots seen, the researcher will most likely be left wondering what it all means. There are several things to keep in mind when studying the pattern of spots on a peptide map. First, migration in the electrophoresis dimension is a function of the charge to mass ratio of a peptide. Migration in the chromatography dimension is related to the hydrophobicity of a peptide. The more hydrophobic a peptide, the further it migrates in the chromatographic dimension. In general, most phosphopeptides of the protein studied will be represented by one spot in the map. It is possible then to determine the relative stoichiometry of phosphorylation at different sites by comparing the intensity of each spot on the autoradiogram. This, however, relies on the
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Figure 18.9.6 An example of a tryptic phosphopeptide map based on that of human Nck-alpha. For the first (horizontal) dimension, electrophoresis was run at pH 1.9 for 25 min at 1.0 kV; the anode is at the left. Ascending chromatography was run for 15 hr in phosphochromo buffer. The sites represented by spots 1 to 7, with the exception of spot 2, have been identified. Spot 1 is an 11-amino-acid, phosphotyrosine-containing peptide. While spot 2 also contains phosphotyrosine, and runs in a position likely to be the doubly phosphorylated version of this peptide, it turns out to be unrelated to spot 1. Spot 3 represents a 5-amino-acid, phosphoserine-containing peptide; this same peptide with an amino-terminal arginine runs as spot 4; thus it can be seen that in this case the tryptic cleavage is largely incomplete. Spot 5 represents a 20-amino-acid phosphoserine-containing peptide which, with an amino-terminal lysine, runs as spot 6. Spot 7 represents a peptide that is unrelated to 5 and 6. Spot 8 represents free phosphate, released during sample preparation.
Phosphopeptide Mapping and Identification of Phosphorylation Sites
assumption that all tryptic peptides are recovered during the the entire protocol with similar efficiencies. There are several cases, however, in which this one phosphopeptide:one spot rule does not hold. One such case is when the enzyme used to digest the protein of interest has not worked to completion, yielding both partial and complete digestion products seen in the map. Multiple digestion products are also generated at sites where a run of basic residues is present. Trypsin works very efficiently as an endopeptidase, hydrolysing peptide bonds following basic residues. In contrast, trypsin works poorly as an exopeptidase. Trypsin will
cleave randomly within the run of basic residues and is unable to take off additional basic residues that may have been left. This results in a series of digestion products differing by the number of basic residues at their amino- or carboxy-terminus. Addition of such a basic residue to a phosphopeptide changes its migration in both the electrophoresis and the chromatography dimension (it is more positively charged and runs further towards the cathode, it is also more hydrophilic and as a consequence runs less far relative to the buffer front in phosphochromo buffer). Similarly, a given peptide will migrate differently if phosphorylated at two positions
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rather than one. Addition of another negative charge will again make the peptide more hydrophilic and thus it will migrate less far in the chromatography dimension. This time, however, it will migrate further towards the anode in the electrophoresis dimension, giving a diagonal pattern descending in the opposite direction to that seen with the addition of a lysine or an arginine. It is important to remember that the electrophoretic mobility of a peptide is dependent on its mass. For larger peptides, the slope of the diagonal seen with the addition of either positive or negative charges will be steeper, since the addition of another charge when divided by the mass will make less of a difference to the distance the peptide travels. While trypsin has been traditionally used in peptide mapping, it may not be the enzyme of choice for proteins phosphorylated by PKA, PKC, or other protein kinases whose recognition sequence involves multiple arginines or lysines, as trypsin often fails to cleave after all such residues when they are present in runs. In addition, trypsin cleaves inefficiently at arginines or lysines two residues amino-terminal of a phosphoserine or phosphothreonine (i.e., R/K-X-P.Ser). Sometimes an individual peptide may appear to have an electrophoretic partner that migrates directly above or below it in the chromatographic dimension. This sort of pattern may be observed as the result of two different scenarios: (1) it may be the result of incomplete oxidation of the peptide if it contains a methionine residue (in which case the lower spot is the oxidized form), or (2) it may be the result of methylation of the peptide running in the lower position. Such a methylation may occur during the performic acid oxidation and is dependent on the 1.5-ml microcentrifuge tubes being used. Historically we have found that certain tubes are more apt than others to produce such unwanted side reactions; for this reason it is advisable to stock certain lots of tubes that do not produce such artifacts in the final maps. Similarly, brands and batches of tubes appear to differ in the extent to which peptides “stick” to them during the final steps of the protocol. An exemplary tryptic phosphopeptide map based on that of a real protein (Nck) is shown in Figure 18.9.6. This map illustrates the points mentioned above. Perhaps most importantly, it also illustrates the fact that just because two spots appear to be on a diagonal it is not a foregone conclusion that they are related. Although peptides 1 and 2 appear to represent the
singly and doubly phosphorylated forms of a single tryptic peptide, in this case it turned out that they represent two completely different peptides. Peptides 3 and 4 and 5 and 6, respectively, represent two sets of peptides that are related and differ only by the addition of a basic residue. Spot 8 represents free phosphate, liberated by hydrolysis of phosphoester bonds that has occurred during sample preparation. It is useful to both compare the amount of free phosphate generated in different samples and to use the phosphate spot as another standard marker when comparing peptide mobilities on different plates.
Time Considerations To generate a two-dimensional phosphopeptide map, at least 9 days will elapse from the time the 32P label is added to the cells until the autoradiogram of the map is in hand. The typical researcher, intrigued by one or more particular spots that appear or disappear from such maps depending on how the cells or samples were treated, may rush to attempt to identify the phosphorylation site represented by such spot(s). Please be advised that this will take at least 4 months of hard work and effort, assuming that everything goes well. There are several different strategies to follow, which are outlined throughout the unit (especially see Background Information). Choice of a particular course will depend on the reagents and the equipment available for analysis.
Literature Cited Boyle, W.J., van der Geer P., and Hunter, T. 1991. Phosphopeptide mapping and phosphoamino acid analysis by two-dimensional separation on thin-layer cellulose plates. Methods Enzymol. 201:110-148. Fischer, W.H., Karr, D., Jackson, B., Park, M., and Vale, W. 1991. Microsequence analysis of proteins purified by gel electrophoresis. Methods Neurosci. 6:69-84. Fischer, W.H., Hoeger, C.A., Meisenhelder, J. Hunter, T., and Craig, A.G. 1997. Determination of phosphorylation sites in peptides and proteins employing a volatile Edman reagent. J. Protein Chem. 16:329-333. Mitchelhill, K.I., Michell, B.J., House, C.M., Stapleton, D., Dyck, J., Gamble, J., Ullrich, C., Witters, L.A., and Kemp, B.E. 1997. Posttranslational modifications of the 5′-AMP-activated protein kinase β1 subunit. J. Biol. Chem. 272:24475-24479. van der Geer, P. and Hunter, T. 1990. Identification of tyrosine 706 in the kinase insert as the major colony-stimulated factor 1 (CSF-1)–stimulated autophosphorylation site in the CSF-1 receptor
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in a murine macrophage cell line. Mol. Cell. Biol. 10:2991-3002. Wang, Y.K., Liao, P.-C., Allison, J., Gage, D.A., Andrews, P.C., Lubman, D.M., Hanash, S.M., and Strahler, J.R. 1993. Phorbol 12-myristate 13-acetate-induced phosphorylation of op18 in Jurkat T cells. J. Biol. Chem. 268:14269-14277.
Internet Resources http://www.genestream.org This Web site contains a program for calculating the mobility of a peptides of known composition and a program that reads the position of a spot on the actual map and calculates which peptide(s) derived from the protein being mapped could have the mobility of that spot.
Key References Boyle et al., 1991. See above. van der Geer, P., Luo, K. Sefton, B.M., and Hunter, T. 1993. Phosphopeptide mapping and phosphoamino acid analysis on cellulose thin-layer plates. In Protein Phosphorylation; a Practical Approach (D.G. Hardie, ed.) pp. 31-59, IRL Press, Oxford. Both of these papers discuss many of the protocols described in this unit.
Contributed by Jill Meisenhelder and Tony Hunter The Salk Institute for Biological Studies La Jolla, California Peter van der Geer University of California, San Diego La Jolla, California
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Use of Protein Phosphatase Inhibitors
UNIT 18.10
Reversible protein phosphorylation is recognized as a major mechanism regulating the physiology of plant and animal cells. Virtually every biochemical process within eukaryotic cells is controlled by the covalent modification of key regulatory proteins. This in turn dictates the cellular response to a variety of physiological and environmental stimuli; errors in signals transduced by phosphoproteins contribute to many human diseases. Thus, defining protein phosphorylation events, and specifically, the phosphoproteins involved, is crucial for obtaining a better understanding of the physiological events that distinguish normal and diseased states. In studying protein phosphorylation, two common experimental problems arise that mandate the use of protein phosphatase inhibitors. The first problem arises when the goal of the experiment is to decipher physiological events regulated by reversible protein phosphorylation but the hormonal stimuli or signaling pathways involved are not known. This situation is further complicated by the fact that many protein phosphorylations are rapid and transient, thereby eluding investigation. One solution is to employ cell-permeable compounds that inhibit cellular protein phosphatases in order to amplify the basal activity of protein kinases. Inhibition of protein phosphatases also prevents the turnover of protein-bound phosphate, thereby enhancing the detection of phosphoproteins by standard techniques such as SDS-PAGE (UNIT 10.2A) and autoradiography (APPENDIX 3A) or phosphorimaging. In other situations, the aim may be to analyze the impact of hormones and other physiological stimuli on the function of a specific phosphoprotein. Such detailed biochemical analysis is only made possible after the protein of interest has been separated from other cellular components that potentially interfere with these studies. In this circumstance, the cells or tissues exposed to hormones must be homogenized in the presence of protein phosphatase inhibitors to preserve the cellular phosphorylation events promoted by the hormones. The goal is to utilize the most effective inhibitors to abolish the activity of protein phosphatases that may dephosphorylate the proteins of interest during the lengthy protein isolation procedures. This unit describes the general or basic protocols for inhibiting the cellular PP1/PP2A activity with okadaic acid (see Basic Protocol 1) as well as in vitro with microcystin-LR (see Basic Protocol 2). Basic Protocols 3 and 4 describe approaches recommended for inhibiting cellular and in vitro activity, respectively, of PP2B/calcineurin. Finally, Basic Protocol 5 outlines a widely utilized strategy for inhibiting protein tyrosine phosphatases. The reader should be aware that these Basic Protocols are deliberately designed to cover the widest range of experimental situations and therefore represent prototypic procedures that can be readily used with many phosphatase inhibitors that are not specifically mentioned in this unit. A variety of very effective inhibitors, particularly those targeting protein serine/threonine phosphatases, are currently commercially available. Some of these compounds are cell-permeable and yet others are better utilized in vitro to suppress phosphatase activity in lysates and/or cell fractions. This unit describes methods for defining physiologically relevant protein phosphorylation events. Thus, in some cases, cells in culture are treated with cell-permeable phosphatase inhibitors to detect and establish the in vivo phosphorylation events. In other cases, the cells are stimulated with known physiological stimuli that modify cellular protein phosphorylation. In either situation, subsequent analyses require that the cells be homogenized in buffers containing phosphatase inhibitors that preserve the protein modifications through the steps of protein purification and analysis. This in turn facilitates the identification of the particular amino acids modified and sets Contributed by Douglas C. Weiser and Shirish Shenolikar Current Protocols in Molecular Biology (2003) 18.10.1-18.10.13 Copyright © 2003 by John Wiley & Sons, Inc.
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the stage for more defined molecular approaches, such as site-directed mutagenesis, that will allow the investigator to establish the functional role of individual protein phosphorylation events. PROTEIN SERINE/THREONINE PHOSPHATASES Serines and threonines constitute the major sites of protein phosphorylation in all eukaryotic cells. Varying estimates suggest that even in cells transformed using oncogenic viral protein tyrosine kinases, >97% of the protein-bound phosphate appears on serines and threonines. Thus, it is not surprising that numerous microorganisms produce toxins and natural products that elicit their deleterious effects by inhibiting cellular protein serine/threonine phosphatases (McCluskey et al., 2002). Many of these compounds show some degree of selectivity for subgroups of protein serine/threonine phosphatases and may provide insight into the cellular phosphatases that catalyze protein dephosphorylation events. The specificity of some of these compounds has been determined in vitro as differences in their concentration-response curves for the inhibition of purified protein phosphatases. When used in vivo, the chemical properties of the compounds, many of which are hydrophobic, do not allow accurate estimates of cellular concentration or predict their subcellular distribution. Thus, the in vitro specificity of the phosphatase inhibitors may be lost and other more direct molecular approaches may be required to define the protein phosphatases that regulate specific cellular events. The principal use of phosphatase inhibitors, as described in this unit, is to complement the in vitro use of purified phosphatases (UNIT 18.5) to define the regulatory importance of reversible protein phosphorylation, rather than to identify the physiologically relevant protein phosphatases. Early biochemical studies classified eukaryotic protein serine/threonine phosphatases into two major groups, termed type-1 (PP1) and type-2 protein phosphatases (Shenolikar and Nairn, 1991). The type-2 phosphatases are further separated into three distinct groups, PP2A, PP2B, and PP2C, showing very different profiles of inhibition by the available compounds (Table 18.10.1). Molecular cloning of mammalian phosphatase catalytic subunits further expanded this family of enzymes, which to date includes up to five additional phosphatase catalytic subunits numbered PP3 to PP7. Of these five, all but PP7 share a highly conserved catalytic site, and therefore are similarly inhibited by various compounds. Yet others, such as PP2C or PP7, show complete insensitivity to the known phosphatase inhibitors and there are no currently available tools to inhibit these phosphatases. Table 18.10.1
Commercially Available PP1/PP2A Inhibitorsa
Compound
Phosphatases
Recommended use
Okadaic acid Calyculin A Tautomycin Cantharidin A Microcystin-LR Nodularin Fostreicin
PP2A > PP1 PP1 ≥ PP2A PP1 > PP2A PP2A > PP1 PP1 = PP2A PP1 = PP2A PP2A>>PP1
In vivo In vivo In vivo In vivo In vitro In vitro In vitro
Supplier(s) Alexis Biochemicals, Sigma Alexis Biochemicals, Sigma Biomol Research Laboratories — Alexis Biochemicals, Sigma — A.B. Scientific, Sigma, Alexis Biochemicals
aAll compounds listed in this table also inhibit PP3, PP4, PP5, and PP6. Their shared sensitivity to these
Use of Protein Phosphatase Inhibitors
compounds results in their inclusion in the category of PP2A-like enzymes. In contrast, these compounds show no significant inhibitory activity towards PP2B, PP2C, and PP7. As the compounds possess little structural homology, several different inhibitors may be used to validate the physiological effects of inhibiting their shared targets, serine/threonine phosphatases.
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Table 18.10.2
Compound Cyclosporin Ab FK506 Cypermethrinc Deltamethrin
Commercially Available PP2B (Calcineurin)a Inhibitors
Phosphatases
Recommended use
PP2B PP2B PP2B PP2B
In vivo In vivo In vivo In vitro
Suppliers Alexis Biochemicals, Sigma Alexis Biochemicals Alexis Biochemicals Alexis Biochemicals
aPP2B (calcineurin) is a Ca2+/calmodulin-activated protein serine/threonine phosphatase and, like many Ca2+/calmodulin-activated enzymes, is inhibited by the calcium chelator, EGTA. Inhibition of PP2B activity using
EGTA is not a very viable approach in cells and the compounds listed in this table more effectively suppress cellular PP2B activity. bA number of inactive and partially active compounds related to cyclosporin are available. Rapamycin, which shares common target proteins with FK506, namely FK506-binding proteins or FKBPs, does not inhibit PP2B activity and is used as control for FK506. cCompounds related to cypermethrin, such as resmethrin, show weak or no activity as PP2B inhibitors and can function controls for the pyrethroid inhibitors.
The compounds listed in Table 18.10.1 inhibit several classes of protein serine/threonine phosphatases. Enzymes such as PP3, PP4, PP5, and PP6 share structural homology to PP2A and therefore show similar sensitivity to these compounds. In contrast, PP2B is inhibited by a distinct group of compounds (Table 18.10.2) that show no activity against either PP1 or the PP2A-like phosphatases. PP1/PP2A Classes of Protein Serine/Threonine Phosphatases PP1 and PP2A together account for >90% of protein serine/threonine phosphatase activity in most eukaryotic cells. With the exception of mammalian skeletal muscle, where PP1 predominates, most cells and tissues possess equivalent amounts of PP1 and PP2A. This has made okadaic acid and calyculin A, both potent cell-permeable PP1/PP2A inhibitors, the most widely utilized compounds for studying cellular protein phosphorylation. INHIBITION OF CELLULAR PP1/PP2A ACTIVITY WITH OKADAIC ACID Okadaic acid (OA), a polyether produced by marine dinoflagellates, is concentrated by shellfish and is a common cause of diarrhetic shellfish poisoning in humans. OA is commercially available as a free acid or as a potassium, sodium, or ammonium salt. Prevailing evidence favors the sodium salt of OA as the most readily permeable across cell membranes.
BASIC PROTOCOL 1
Biochemical studies demonstrate that OA inhibits the purified PP2A catalytic subunit with an IC50 value of 0.2 to 1.0 nM (Cohen et al., 1989). This concentration of OA, however, has little effect on PP2A activity in cell lysates, which typically exhibit 30- to 150-fold higher IC50 values than the purified enzyme. Extensive dilution of cell extracts enhances the efficacy of OA as a PP2A inhibitor. This may suggest that OA binds, albeit with low affinity, to other cellular components in cell extracts. This may be more of an issue in intact cells where significantly higher OA concentrations (10 to 100 nM) are required for effective PP2A inhibition. PP1, unlike PP2A, has a higher IC50 value for OA (10 to 100 nM; Cohen et al., 1989), whether assayed with purified enzyme or cell lysates, and the sensitivity of PP1 activity to OA is not modified by dilution of cell lysates. In some cellular studies, OA concentrations between 10 and 50 nM resulted in significant enhancement of protein phosphorylation, presumably reflecting the inhibition of PP2A and other PP2A-like enzymes. In contrast, OA concentrations >10 µM are needed to effectively inhibit cellular PP1 activity.
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This difference in affinity can provide an opportunity to discriminate between the physiological functions of the two major classes of cellular protein serine/threonine phosphatases, namely PP1 and PP2A. Numerous studies suggest that the predominant site of OA accumulation is the lipid bilayer of the plasma membrane (Suganuma et al., 1989; Nam et al., 1990; Namboodiripad and Jennings, 1996). OA transfers slowly from this site to other compartments and this may account for the slow onset of phosphatase inhibition by OA in most cells (up to 90 min) as well as the requirement of 10- to 100-fold higher concentrations of OA to inhibit PP2A activity in intact cells compared to cell lysates. Thus, it is surprising to find that OA can also be washed out of cells, albeit thorough washing can take hours. This reversible behavior may allow investigators to analyze the reversal of phosphorylation events triggered by OA. Duration of OA treatment to elicit maximal changes in cellular protein phosphorylation is largely empirically determined and must be defined for each individual cell type. In general, cells display a maximal response to micromolar concentrations of OA within 10 to 30 min. Lower OA concentrations require significantly longer exposure times (up to 2 hr). Due to their remarkable ability to activate multiple cellular signaling pathways, OA and other cell-permeable phosphatase inhibitors display some cytotoxicity. This is partially alleviated by lowering OA concentrations and reducing exposure times. Thus, OA treatments for several hours or days are not recommended. If longer times are necessary, calyculin A (CA), which inhibits PP1 and PP2A with near equal potency, may be a better substitute. While the literature on calyculin A is less extensive, this compound can be used at much lower concentrations (0.1 to 1.0 µM) than OA to yield similar physiological effects. OA induces errors in DNA repair that are not mediated by phosphatase inhibition, (Nakagama et al., 1997) which suggests the presence of additional targets for OA. This behavior mandates that appropriate controls be used in cellular studies. In this regard, the treatment of cells with vehicle (DMSO) is commonly used as a control for OA, CA, and other phosphatase inhibitors that are dissolved in this solvent. There are, however, a number of commercially available inactive OA analogs that may serve as better controls. The best of these is 1-norokadaone, whose structure closely matches that of OA and maintains some of the non-phosphatase-mediated effects elicited by OA. Structure-function studies that established a critical role for the acidic side-chain of OA in phosphatase inhibition also generated a number of inactive OA analogs in which the critical carboxylate group is esterified (Nashiwaki et al., 1991). Such compounds, like methyl okadaate, show low or no activity in vitro as PP1/PP2A inhibitors. However, these compounds may not be effective negative controls, as they are slowly de-esterified in cells, yielding the active OA. Finally, OA, CA, and several other phosphatase inhibitors show significant activity as tumor promoters in mouse skin and gut (Fujuki et al., 1988; Suganuma et al., 1990). Thus, it is important that the investigator takes extreme care in handling and disposing of solutions containing these compounds.
Use of Protein Phosphatase Inhibitors
Materials Cultured mammalian cells grown to 1 hr, as very few physiological events respond to this drug in periods under 30 min.
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IN VITRO INHIBITION OF PP2B/CALCINEURIN ACTIVITY WITH CYPERMETHRIN
BASIC PROTOCOL 4
Cypermethrin and other type II pyrethroids are synthetic insecticides that show activity as PP2B inhibitors and are available from many commercial sources. In addition, a number of pyrethroid analogs, resmethrin, allethrin, and permethrin, show reduced or no activity as PP2B inhibitors and can be used as controls. This is particularly important for cellular studies using cypermethrin, as many of the type-II pyrethroids have a wide range of physiological effects including oxidative stress, cytotoxicity, and tumorigenesis. Only a few of these are likely to be mediated by PP2B inhibition. Given the availability of cyclosporin A, a better characterized and more potent PP2B inhibitor, the value of using pyrethroids in mammalian cells is questionable. Materials 1.0 mM cypermethrin (Alexis Biochemicals) in DMSO and store in dark glass containers pretreated with 1% polyethylene glycol (PEG) Sample cell extracts or subcellular fractions NOTE: Cypermethrin and other type II pyrethroids are sold in both solid and liquid form. The liquid form represents cypermethrin dissolved in organic solvents, such as DMSO, acetone, or ethanol. Hydrophobicity of these compounds means that long-term storage in plastic or polyethylene containers should be avoided because these compounds adhere to such surfaces and may be slowly extracted from solution. Many pyrethroids are also light-sensitive. If stored in dark glass containers pretreated with 1% (w/v) PEG to prevent surface adherence, stock solutions of cypermethrin are stable for many months. For cellular studies using cypermethrin, the compound should be added directly to the culture medium as its PP2B inhibitory activity is reduced when made up in fresh medium. While the underlying basis for a time-dependent loss of cypermethrin activity is not clear, this provides a strong argument for using cypermethrin and the pyrethroid inhibitors solely for in vitro studies of PP2B function. 1. Add 1.0 mM cypermethrin (10 to 20 µM final concentration) to cell extracts or subcellular fractions and analyze phosphoproteins within a period not exceeding 60 min. Cypermethrin addition in vitro should result in rapid PP2B inhibition that is evident within minutes. Prolonged incubations with this compound are not recommended.
INHIBITION OF PROTEIN TYROSINE PHOSPHATASES WITH SODIUM ORTHOVANADATE
BASIC PROTOCOL 5
Protein tyrosine phosphatases (PTPases) represent a larger family of enzymes derived from a common ancestral gene and share no structural homology with the protein serine/threonine phosphatases (Sefton and Shenolikar, 1995). Thus, PTPases are not sensitive to any of the compounds described above. PTPases share a common catalytic mechanism that involves the formation of an enzyme phospho-intermediate with the modification of a conserved cysteine within the catalytic cleft. While cysteine-modifying reagents, such as phenyl arsene oxide, and metals such as zinc chloride, potently inhibit PTPases, these reagents are non-specific and likely target many proteins other than PTPases and therefore, will not be further discussed here. Currently, the most useful reagent for inhibiting PTPases in vivo and in vitro is vanadate, a phosphate analog. One might predict that vanadate would also be non-specific and inhibit many phosphotransferases and phosphohydrolases, including protein serine/threonine phosphatases. However, the remarkable chemistry of the vanadium metal
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ion (Gordon, 1991) can be exploited to produce a preferential and effective PTPase inhibitor. Materials Sodium orthovanadate (Sigma) 0.1 M NaOH Sample cells Boiling water bath or heating block Additional reagents and equipment for metabolic labeling (Sefton, 1997) or immunoblotting with an anti-phosphotyrosine antibody (DiGiovanna et al., 1996) 1. Dissolve sodium orthovanadate (200 mM final concentration) in sterile, distilled water (often a yellowish solution) and adjust pH of the solution to 10.0 with 0.1 NaOH. Place solution in a boiling water bath until the solution clarifies or becomes translucent. Readjust pH to 10.0 and store the stock solution in a flint glass container at room temperature or dispense into aliquots in plastic containers and store at −20°C. Sodium orthovanadate, whether solid or in solution at neutral pH, slowly oxidizes and acquires a yellow color. The presence of thiols, often used to maintain protein structure and function following the homogenization of tissues or cells, promotes a rapid (100 µM fostreicin is needed to inhibit the structurally related serine/threonine phosphatase, PP1. Use of fostreicin is, however, limited by its propensity to be inactivated by oxidation and thus must be stored in the presence of a large excess of antioxidants. Commercially available fostreicin, therefore, contains numerous additives, making it very difficult to use as a specific reagent in cellular studies. Nevertheless, it is likely that selective PP1 and PP2A inhibitors can and will be developed (see Key References). Compared to most
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current serine/threonine phosphatase inhibitors, which display cytotoxicity and tumor promoter activity, clinical trials established fostreicin as a potential anti-neoplastic drug. This also hints at the possibility that more selective PP1 or PP2A inhibitors may be better experimental tools than currently available compounds whose cytotoxicity may be a result of inhibiting many different protein serine/threonine phosphatases. On the other hand, it is the remarkable chemistry of the vanadium metal ion that makes the current compounds such effective tools for analyzing highly labile and difficult-to-detect cellular phosphorylation events. Finally, a number of broad-acting or nonspecific reagents have been used to inhibit protein phosphatases. These include sodium fluoride (50 mM) and sodium pyrophosphate or glycerophosphate (10 mM). These compounds suppress protein dephosphorylations in cell lysates and fractions, although not as effectively as other inhibitors. They can serve as alternatives for inhibiting phosphatases in vitro but show no utility in phosphoprotein analysis in intact cells.
Critical Parameters
Use of Protein Phosphatase Inhibitors
Treatment of cells with the cell-permeable phosphatase inhibitors described in this unit is often so effective that it saturates and occludes the physiological signaling pathways, making studies that utilize combinations of hormones and phosphatase inhibitors difficult to interpret. Many mammalian proteins are regulated both positively and negatively by phosphorylation, generally occurring at distinct sites. Thus, the treatment of cells with phosphatase inhibitors may facilitate both types of modifications and yield a compound functional readout, which may bear little or no relationship to the physiological mechanisms that orchestrate the orderly control of protein function. Even when utilizing phosphatase inhibitors in vitro, artifactual or post-homogenization phosphorylations can result from inappropriate exposure of proteins to kinases normally located in distinct subcellular compartments and therefore are not normally physiological regulators of these proteins. Thus, this unit stresses the detection of cellular protein phosphorylation and events bearing on the functional role of such modifications. The pharmacological approach to studying cellular protein phosphorylation as described in this unit can be employed by investigators from many backgrounds. However, further op-
timization of the outlined experimental strategy may be necessary for individual cell types, where permeability of the compounds and the array of phosphatases inhibited may dictate unique physiological consequences. Thus, the investigator is encouraged to undertake pilot studies that determine the optimal concentrations of the compound and the duration of cell exposure required to obtain a measurable and meaningful cellular response. Also, as emphasized above, while the cell-permeable phosphatase inhibitors display some target specificity, they cannot unequivocally define the phosphatases that regulate specific phosphoproteins or physiological events. Finally, the investigator should be aware that many of the compounds described above display some capacity to bind proteins other than phosphatases albeit with much lower affinity. Thus, control experiments should utilize compounds, such as the inactive okadaic acid or cyclosporin analogs, that fail to inhibit the target phosphatases. These controls should provide a more compelling argument supporting the biological effects uniquely observed with the active compounds as reflecting the inhibition of the target protein phosphatases. To define the specific phosphatases, the investigator must turn to more molecular approaches that may involve the overexpression of phosphatases (currently a very challenging task), or the endogenous protein inhibitors that show exquisite selectivity for individual phosphatases (Oliver and Shenolikar, 1998). An alternate to overexpression may be the microinjection of phosphatases and the inhibitor proteins, an approach that has been successful in modulating cellular phosphatase activity (Alberts et al., 1993; Mulkey et al., 1994; Morishita et al., 2001). The difficulty of introducing protein inhibitors into cells, whether by overexpression or microinjection, is that most, if not all, of these proteins are tightly regulated by covalent modification. The functions of various inhibitors may be enhanced or dampened by reversible protein phosphorylation, making it difficult to predict their effects on phosphatases. Yet another approach is the use of antisense oligonucleotides or interference RNAs to “knock-down” expression of specific protein phosphatases. While this approach harbors tremendous specificity, it is difficult to control the effectiveness of these inhibitors which lower phosphatase expression, and elicit appropriate physiological responses that cannot be readily predicted. The use of dominantnegative reagents, including inactive phos-
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phatase catalytic subunits or fragments of phosphatases, has shown some promise in suppressing phosphatase functions, but these reagents most likely rely on their competition with endogenous phosphatases for regulators rather than substrates, thereby making it difficult to predict the physiological outcome. However, catalytically inactive phosphatases have proven very useful in deciphering the functions of PTPases. These inactive phosphatases function as substrate traps and form stable complexes with cellular phosphoproteins, allowing identification of the potential physiological substrates of the PTPase. Even in this situation, there are serious concerns that overexpression of inactive PTPases may distort the natural specificity of these enzymes and yield spurious or unreliable data. Protein serine/threonine phosphatases themselves also occasionally form stable complexes with cellular substrates and such complexes can be isolated using affinity chromatography based on phosphatase inhibitors, such as microcystin-LR, immobilized on Sepharose or other dextran surfaces. Such affinity matrices are not commercially available but a number of publications describe the synthesis and use of such resins. The reader may refer to Campos et al. (1996) or Damer et al. (1998). However, this approach has thus far been more successful in identifying phosphatase regulators than substrates.
Troubleshooting The use of phosphatase inhibitors as described in this unit is associated with several problematic issues regarding compound stability and permeability. First and foremost, many of these compounds are highly hydrophobic. Some are light-sensitive and others difficult to store for long periods. These factors contribute to the problem of defining their precise concentration in culture media and buffers and predicting their effects on target phosphatases. In cellular studies, there are additional problems in that external inhibitor concentration is not an accurate predictor of the intracellular concentration of compounds, which are not evenly distributed throughout the cell. Thus, the literature reports widely varying IC50 values for inhibition of purified phosphatases and the sensitivity of known PP1 and/or PP2A-regulated events in cells. Thus, the protocols described in this unit deliberately err on the side of using excess phosphatase inhibitors. While this ensures effective phosphatase inhibition, additional problems may arise in terms of the cytotoxicity elicited by these reagents, which varies
widely among different cell types. Quiescent, confluent and highly differentiated cells appear more susceptible to programmed cell death in response to phosphatase inhibitors such as OA and CA, although the prolonged overactivation of multiple signaling pathways by these inhibitors may also contribute to the cytotoxicity observed in actively growing cells. Thus, investigators are encouraged to determine the lowest concentrations of inhibitors and shortest duration of cell exposure necessary for a consistent and measurable physiological response.
Anticipated Results Phosphatase inhibitors that inhibit the major mammalian protein serine/threonine or tyrosine phosphatases unmask an array of kinase cascades and thereby enhance the phosphorylation of a wide range of proteins. This in turn permits the investigator to detect and analyze novel phosphorylation events. By selecting specific phosphatase inhibitors and restricting their concentrations, the investigator can also inhibit a subset of cellular phosphatases yielding more defined results by not only limiting the panel of phosphoproteins visualized but also the sites modified on the given phosphoprotein. For an example of a specific phosphatase inhibition experiment, refer to Haystead et al. (1989). Functional effects associated with the rapid phosphorylation-dephosphorylation of cellular proteins are more readily visualized in the presence of phosphatase inhibitors. These experiments can establish the importance of protein phosphorylation in physiological events where the precise protein target or phosphorylation event has not been identified. However, readouts such as gene expression that result from increased phosphorylation of nuclear proteins often require long exposure of cells to phosphatase inhibitors and thereby risk the potentially confounding effects of cytotoxicity. Here, a compromise must be reached and inhibitor concentrations lowered to levels that maintain cell viability and still promote the transcription activation of target genes.
Time Considerations A common scenario for the experiments described above is that cells are metabolically labeled using 32P-orthophosphate for periods between 90 and 120 min (Sefton, 1997). Phosphatase inhibitors are then added to the media and the cells incubated for a further 30 min. Following homogenization, the phosphoproteins are analyzed using SDS-PAGE (UNIT 10.2A)
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and autoradiography or phosphorimaging, which adds an additional 2 to 3 hr. Thus, the total time required may be minimally estimated at 6 hr. In the absence of metabolic labeling, SDSPAGE may be followed by electrophoretic transfer and analysis of changes in protein mobility reflecting increased phosphorylation or the modification of specific proteins. Phosphorylation sites may be identified by western immunoblotting using phosphospecific antibodies. These approaches provide different pieces of information but carry similar total time considerations.
Literature Cited Alberts, A.S., Thorburn, A.M., Shenolikar, S., Mumby, M.C., and Feramisco, J.R. 1993. Regulation of cell cycle progression and nuclear affinity of the retinoblastoma protein by protein phosphatases. Proc. Natl. Acad. Sci. U.S.A. 90:388-392. Campos, M., Fadden, P., Alms, G., Qian, Z., and Haystead, T.A. 1996. Identification of protein phosphatase-1-binding proteins by microcystinbiotin affinity chromatography. J. Biol. Chem. 271:28478-28484. Cohen, P.T.W. 1997. Novel protein serine/threonine phosphatases: Variety is the spice of life. Trends Biochem. Sci. 22:245-251. Cohen, P.T. 2002. Protein phosphatase 1- targeted in many directions. J. Cell Sci. 115:241-256. Cohen, P., Klumpp, S., and Schelling, D.L. 1989. An improved procedure for identifying and quantitating protein phosphatases in mammalian tissues. FEBS Lett. 250:596-600. Damer, C.K., Partridge, J., Pearson, W.R., and Haystead, T.A. 1998. Rapid identification of protein phosphatase 1-binding proteins by mixed peptide sequencing and data base searching. Characterization of a novel holoenzymic form of protein phosphatase 1. J. Biol. Chem. 273:2439624405. DiGiovanna, M.P., Roussel, R.R., and Stern, D.F. 1996. Production of antibodies that recognize specific tyrosine-phoshorylated peptides. In Current Protocols in Protein Science (J.E. Coligan, B.M. Dunn, D.W. Specher, and P.T. Wingfield, eds.), pp. 13.6.1-13.6.13. John Wiley & Sons, New York. Fischer, E.H., Charbonneau, H., and Tonks, N.K. 1991. Protein tyrosine phosphatases: A diverse family of intracellular and transmembrane enzymes. Science 253:401-406. Fujuki, H., Suganuma, M., Suguri, H., Yoshizawa, S., Takagi, K., Uda, N., Wakanatsu, K., Yamada, K., Murata, M., and Yasumoto, T. 1988. Diarrhetic shellfish toxin, dinophysistoxin-1, is a potent tumor promoter on mouse skin. Jap. J. Cancer Res. 79:1089-1093. Use of Protein Phosphatase Inhibitors
Goldstein, B.J. 2001. Protein-tyrosine phosphatase 1B (PTP1B): A novel therapeutic target for type
2 diabetes mellitus, obesity and related states of insulin resistance. Curr. Drug Target–Immun. Endocrin. Metab. Disorders 1:265-275. Gordon, J.A. 1991. Use of vanadate as protein-phosphotyrosine phosphatase inhibitor. Methods Enzymol. 201:477-482. Haystead, T.A., Sim, A.T., Carling, D., Honnor, R.C., Tsukitani, Y., Cohen, P., and Hardie, D.G. 1989. Effects of the tumour promoter okadaic acid on intracellular protein phosphorylation and metabolism. Nature 337:78-81. Kiguchi, K., Glesne, D., Chubb, C.H., Fujiki, H., and Huberman, E. 1994. Differential induction of apoptosis in human breast tumor cells by okadaic acid and related inhibitors of protein phosphatases 1 and 2A. Cell Growth & Differentiation 5: 995-1004. Maehama, T., Taylor, G.S., and Dixon, J.E. 2001. PTEN and myotubularin: Novel phosphoinositide phosphatases. Annu. Rev. Biochem. 70:24779. McCluskey, A., Sim, A.T.R., and Sakoff, J.A. 2002. Serine-threonine protein phosphatase inhibitors: Development of potential therapeutic strategies. J. Medicinal Chem. 45:1151-1175. Morimoto, Y., Ohba, T., Kobayashi, S., and Haneli, T. 1997. The protein phosphatase inhibitors okadaic acid and calyculin A induce apoptosis in human osteoblastic cells. Exp. Cell Res. 230:181-186. Morishita, W., Connor, J.H., Xia, H., Quinlan, E.M., Shenolikar, S., and Malenka, R.C. 2001. Regulation of synaptic strength by protein phosphatase 1. Neuron 32:1133-1148. Mulkey, R.M., Endo, S., Shenolikar, S., and Malenka, R.C. 1994. Involvement of a calcineurin/inhibitor-1 phosphatase cascade in hippocampal long-term depression. Nature 369:486-488. Nakagama, H., Kaneko, S., Shima, H., Fukuda, H., Kominami, R., Sugimura, T., and Nagao, M. 1997. Induction of minisatellite mutation in NIH3T3 cells by treatment with the tumor promoter okadaic acid. Proc. Natl. Acad. Sci. U.S.A. 94:10813-10816. Nam, K.Y., Hiro, M., Kimura, S., Fujiki, H., and Imanishi, Y. 1990. Permeability of a non-TPAtype tumor promoter, okadaic acid, through lipid bilayer memebrane. Carcinogenesis 11:11711174. Namboodiripad, A.N. and Jennings, M.L. 1996. Permeability characteristics of erythrocyte membrane to okadaic acid and calyculin A. Am. J. Physiol. 270:C449- C456. Nashiwaki, S., Fujiki, H., Suganuma, M., FuruyaSuguri, H., Matsushima, R., Iida, Y., Ojika, M., Yamada, K., Uemura, D., and Yasumoto, T. 1991. Structure-activity relationship within a series of okadaic acid derivatives. Carcinogenesis 11:1837-1841. Oliver, C.J. and Shenolikar, S. 1998. Physiologic importance of protein phosphatase inhibitors. Frontiers in Biosci. 3:D961-D972.
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Schonthal, A.H. 1998. Analyzing gene expression with the use of serine/threonine phosphatase inhibitors. Methods Mol. Biol. 93:35-40. Sefton, B.M. 1997. Labeling cultured cells with 32Pi and preparing cell lysates for immunoprecipitation. In Current Protocols in Protein Science (J.E. Coligan, B.M. Dunn, D.W. Specher, and P.T. Wingfield, eds.), pp. 13.2.1-13.2.8. John Wiley & Sons, New York. Sefton, B.M and Shenolikar, S. 1995. Overview of protein phosphorylatyion. In Current Protocols in Protein Science (J.E. Coligan, B.M. Dunn, D.W. Specher, and P.T. Wingfield, eds.), pp. 13.1.1-13.1.5. John Wiley & Sons, New York. Shenolikar, S. and Nairn, A.C. 1991. Protein phosphatases: Recent progress. Adv. Second Messenger Phosphoprotein Res. 23:1-121. Shenolikar, S. and Brautigan, D.L. 2000. Targeting protein phosphatases: Medicines for the new millenium. Science (STKE) Nov. 2000. http://stke.sciencemag.org/. Suganuma, M., Suttajit, M., Suguri, H., Ojika, M., Yamada, K., and Fujiki, H. 1989. Specific bind-
ing of okadaic acid, a new tumor promoter in mouse skin. FEBS Lett. 250:615-618. Suganuma, M., Fujiki, H., Furuya-Suguri, H., Yoshizawa, S., Yasumoto, S., Kato, Y., Fusetani, N., and Sugimura, T. 1990. Calyculin A, an inhibitor of protein phosphatases, is a potent tumor promoter on CD-1 mouse skin. Cancer Res. 15:3521-3525.
Key References Current Medicinal Chemistry. November, 2002, vol. 9. A special “hot topics” issue, which is devoted to the synthesis of serine/threonine phosphatase inhibitors.
Contributed by Douglas C. Weiser and Shirish Shenolikar Duke University Medical Center Durham, North Carolina
Analysis of Protein Phosphorylation
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Design and Use of Analog-Sensitive Protein Kinases
UNIT 18.11
Many protein kinases can be engineered to accept analogs of ATP that are not efficiently used by wild-type kinases. These engineered kinases, which are referred to as “analogsensitive” or “–as” alleles, are also often sensitive to protein kinase inhibitor variants that do not block the activity of nonmutant kinases. Selective in vitro use of radiolabeled ATP analogs by –as kinases can be exploited to identify the direct phosphorylation targets of individual kinases in complex extracts. In organisms in which it is practical to replace wild-type kinase genes with engineered alleles, the in vivo activity of an –as kinase can be reversibly blocked with an allele-specific inhibitor. Thus, analog-sensitive kinases can be effective tools for discovery of the cellular functions and phosphorylation targets of individual enzymes. A theoretical background for the design and use of these alleles is discussed in the Commentary section at the end of this unit, as are strategies for construction of candidate –as alleles of any kinase. In vivo use of analog-sensitive kinase alleles requires access to kinase inhibitor analogs. This unit describes protocols for synthesis of 1NM-PP1 and 1NA-PP1 (see Basic Protocols 1 and 2), which are among the most effective allele-specific inhibitors of –as kinases. These protocols assume knowledge of organic synthesis, and it is advisable to work with a chemist that can provide assistance if one is not familiar with organic chemistry methods. Under no circumstances should one attempt these protocols if not comfortable with safety procedures fundamental to synthetic organic chemistry. Much of the equipment required for these procedures is uncommon in molecular biology groups but readily accessible in chemistry laboratories. The authors have provided information relevant to analysis of NMR spectra for researchers familiar with this technique. A detailed description of product analysis is, however, beyond the scope of this section. The authors have also supplied a protocol for production of γ-32P-labeled N6(benzyl)ATP, an ATP analog that is commonly selectively used by –as kinases, from the ADP analog (see Basic Protocol 3 and Support Protocols 1 and 2). This procedure employs enzymatic phosphorylation of N6(benzyl)ADP by immobilized nucleoside diphosphate kinase (NDPK), and can be used to produce other γ-labeled ATP analogs from ADP analogs (Mourad and Parks, 1966; Kraybill et al., 2002). Commercial availability of N6(benzyl)ADP, as well as other ADP analogs, is still somewhat limited. A method for synthesis of N6(benzyl)ADP has been published (Shah et al., 1997; Shah and Shokat, 2002). Labeling of ADP analogs by the procedure provided here requires moderately large quantities (∼2.5 mCi) of [γ-32P]ATP, and it is therefore advisable to ensure that this raises no issues with one’s license to use radioactive materials. A final protocol (see Basic Protocol 4) describes the assays for testing inhibition of analog-sensitive kinases in yeast. NOTE: Although the protocols described in this unit are directly derived from published material, it is important to note that the use of analog-sensitive kinases, analog-sensitive kinase inhibitors, and ATP analogs is protected by U.S. patents 6,610,483; 6,521,417; 6,390,821 and 6,383,790, as well as patents in other countries, which are owned by Princeton University and exclusively licensed to Cellular Genomics Incorporated (CGI) of Branford, Connecticut. Information regarding accessing rights to make and use the technology can be obtained by contacting CGI. Analysis of Protein Phosphorylation Contributed by Justin Blethrow, Chao Zhang, Kevan M. Shokat, and Eric L. Weiss Current Protocols in Molecular Biology (2004) 18.11.1-18.11.19 Copyright © 2004 by John Wiley & Sons, Inc.
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BASIC PROTOCOL 1
SYNTHESIS OF 1NM-PP1 This protocol and the one that follows for 1NA-PP1 (see Basic Protocol 2) are based on previously published methods (Bishop et al., 1998; Bishop et al., 1999). The synthetic schemes are diagrammed in Figure 18.11.1. A representative NMR spectrum is presented in Figure 18.11.2. Materials 1-naphthylacetic acid (Acros Organics) Hexane N,N-dimethylformamide (DMF; Aldrich) Oxalyl chloride (Aldrich) Tetrahydrofuran (THF; Aldrich) Sodium hydride (Aldrich) Malononitrile (Aldrich) 1 N H2SO4 Ethyl acetate MgSO4, anhydrous 1,4-dioxane (Aldrich) Sodium bicarbonate (Aldrich) Dimethyl sulfate (Aldrich) Diethyl ether (Et2O) Silica Gel 60 (pore size 0.040 to 0.063 mm; Merck) Ethanol (Aldrich) Triethylamine (Aldrich) tert-butylhydrazine hydrochloride (Aldrich) Chloroform (CHCl3) 1% (v/v) methanol in CHCl3 Formamide (Aldrich) Activated charcoal Celite Büchi Rotavapor Model R-200 or equivalent rotary evaporator Ice bath Separatory funnel Oil baths, 80°, 100°C, 180°C 10-in. (25.4-cm) length × 2-in. (5.0-cm) i.d. chromatography column TLC plates (Silica Gel F254; EM Science) and tank Reflux condenser Filter paper Buchner funnel Synthesize 1-naphthylacetylmalononitrile 1. Dissolve 3.72 g 1-naphthylacetic acid (20.0 mmol) in 100 ml hexane. 2. Add 146 mg N,N-dimethylformamide (DMF; 2.0 mmol). 3. Add, dropwise, 12.7 g oxalyl chloride (100 mmol). 4. Wait for gas evolution to stop (∼30 min), then remove solvent under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator. 5. Redissolve the residue (containing the acid chloride) in 30 ml tetrahydrofuran (THF).
Design and Use of Analog-Sensitive Protein Kinases
6. Suspend 1.06 g sodium hydride (44.0 mmol) in 10 ml THF.
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CN O
O
OH
O
CI ii
i
CN O
CN
CN
iii
iv
NH2
NC H2N
vi
N N
N N
N N
1NM-PP1
O
O CI
NH2
O CN iii
ii
CN
CN
v
NC N N
H2N
CN
vi
N N
N N
1NA-PP1
Figure 18.11.1 Synthetic schemes of 1NM-PP1 and 1NA-PP1. Conditions: (i) 5 eq oxalyl chloride, 0.1 eq DMF, room temperature, 1 hr; (ii) 2 eq NaH, 1 eq malononitrile, THF, 0°C to room temperature, 1 hr; (iii) 8 eq NaHCO3, 5 eq dimethyl sulfate, dioxane/water (6:1), reflux, 1 hr; (iv) 2 eq triethylamine, 1 eq tert-butylhydrazine hydrochloride, EtOH, reflux, 1 hr; (v) 1 eq tert-butylhydrazine, DMF, room temperature, 1 hr; (vi) formamide, 180°C, 10 hr.
9
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Figure 18.11.2 NMR data for 1-tert-butyl-3-naphthalen-1ylmethyl-1H-pyrazolo[3,4-d]pyrimidin-4-ylamine (1NMPP1). Compound is white powder; 1H NMR (CDCl3, 400 Mhz) δ 1.82 (s, 9H), 4.73 (s, 2H), 4.87 (br s, 2H), 7.17 (d, J = 7 Hz, 1H), 7.37 (t, J = 8 Hz, 1H), 7.52 (m, 2H), 7.78 (d, J = 8 Hz, 1H), 7.87 (m, 1H), 8.19 (m, 1H), 8.23 (s, 1H); 13C NMR (CDCl3, 100 Mhz) δ 29.2, 32.7, 60.0, 101.1, 123.5, 125.6, 125.8, 126.2, 126.6, 128.2, 128.9, 131.9, 133.9, 134.0, 140.5, 154.5, 154.7, 157.6, HRMS (EI) molecular ion calculated for C20H21N5 331.17993, found 331.17951.
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7. Dissolve 1.45 g malononitrile (22.0 mmol) in 10 ml THF and add dropwise to the NaH/THF suspension (step 6) on an ice bath. 8. Add the THF solution of acid chloride from step 5 dropwise to the malononitrile solution with vigorous stirring. Stir for 1 hr. 9. Add ∼15 ml of 1 N H2SO4 and 50 ml water. 10. Extract three times, each time with 100 ml ethyl acetate, in a separatory funnel. The ethyl acetate phase is the top phase.
11. Combine ethyl acetate extracts and dry by addition of ~5 g anhydrous MgSO4. 12. Evaporate the solvent under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator, to yield 1-naphthylacetylmalononitrile, a yellow solid. Yield should be ∼70%.
Synthesize 1-naphthylmethyl(methoxy)methylidenemalononitrile 13. Dissolve 2.23 g 1-naphthylacetylmalononitrile (from step 12; 10.0 mmol) in a mixture of 18 ml dioxane and 3 ml water. 14. Add 6.8 g sodium bicarbonate. 15. Add 4.9 ml of dimethyl sulfate with vigorous stirring. Heat the reaction mixture at 100°C on an oil bath for 2 hr. 16. Cool the reaction mixture to room temperature. 17. Dilute with 70 ml water and extract three times, each time with 100 ml Et2O, in a separatory funnel. 18. Combine the Et2O extracts and dry over anhydrous MgSO4. 19. Evaporate the Et2O under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator to give an oil. 20. Prepare a 10-in. (25.4-cm) × 2-in. (5.0-cm) column containing 200 g Silica Gel 60, saturated with 1:1 Et2O/hexane (the mobile phase). 21. Dissolve the oil from step 19 in a small amount of mobile phase and apply to column. Elute with mobile phase. Sample the eluate as it emerges from the column and analyze by thin-layer chromatography (TLC) using 1:1 Et2O/hexanes as the mobile phase. Pool fractions containing a species with Rf of ∼0.24. 22. Evaporate solvent under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator to produce a white crystalline solid 1-naphthylmethyl(methoxy)methylidenemalononitrile. Yield should be ∼75%.
Synthesize 5-amino-1-tert-butyl-3-(1′-naphthylmethyl)-4-cyano-1H-pyrazole 23. Dissolve 1.74 g of 1-naphthylmethyl(methoxy)methylidenemalononitrile (from step 22; 7.0 mmol) in 50 ml ethanol. 24. Add 1.51 g triethylamine (15.0 mmol) and 0.92 g tert-butylhydrazine hydrochloride (7.4 mmol). Reflux at 80°C on an oil bath for 1 hr. 25. Cool the mixture and evaporate the solvent under reduced pressure. 26. Suspend the residue in water and extract three times, each time with with 50 ml CHCl3, in a separatory funnel. Design and Use of Analog-Sensitive Protein Kinases
The CHCl3 phase is the top phase.
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27. Combine the CHCl3 extracts, dry over anhydrous MgSO4, and evaporate the solvent under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator, to give a yellow solid. 28. Prepare a 10-in. (25.4-cm) × 2-in. (5.0-cm) column containing 100 g Silica Gel 60, saturated with CHCl3 (the mobile phase). 29. Dissolve the yellow solid in a minimal volume of CHCl3 and apply to the column. Elute with 1% methanol in CHCl3. Sample the eluate as it emerges from the column and analyze by thin-layer chromatography (TLC) using 1% methanol in CHCl3 as the mobile phase. Pool fractions containing a species with Rf of ∼0.1. 30. Evaporate solvent to obtain a white solid, 5-amino-1-tert-butyl-3-(1′-naphthylmethyl)-4-cyano-1H-pyrazole. Yield should be ∼75%.
Synthesize 4-amino-1-tert-butyl-3-(1′-naphthylmethyl)-1H-pyrazolo[3,4-d]pyrimidine (1NM-PP1) 31. Suspend 0.9 g 5-amino-1-tert-butyl-3-(1′-naphthylmethyl)-4-cyano-1H-pyrazole (from step 30; 3.4 mmol) in 20 ml of formamide. 32. Heat this suspension at 180°C on an oil bath, stirring with a reflux condenser for 10 hr. 33. Cool reaction mixture and dilute with 80 ml water. 34. Collect precipitate by filtration on paper in a Buchner funnel. 35. Dissolve precipitate in 40 ml room temperature ethanol. 36. Add 1 g powdered activated charcoal. Boil at 80°C for 10 min. 37. Pour a celite pad on a Buchner funnel using a slurry of celite in ethanol. Filter the hot mixture through this celite pad. Collect the filtrate. 38. Evaporate the solvent to obtain 1NM-PP1 as a white powder. Expect yields of ∼75%.
SYNTHESIS OF 1NA-PP1 1NA-PP1 can be synthesized in the same manner as 1NM-PP1, with two differences. First, the acid chloride (1-naphthoyl chloride) is directly used in the first step of the synthesis, since it is commercially available. Second, a different condition is used in the reaction that forms the pyrazole ring, giving much better yields than the original procedure. A representative NMR spectrum is presented in Figure 18.11.3. Materials 1-naphthoyl chloride (Aldrich) Tetrahydrofuran (THF; Aldrich) Sodium hydride (Aldrich) Malononitrile (Aldrich) 1 N H2SO4 Ethyl acetate MgSO4, anhydrous 1,4-dioxane (Aldrich) Sodium bicarbonate (Aldrich) Dimethyl sulfate (Aldrich)
BASIC PROTOCOL 2
Analysis of Protein Phosphorylation
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9
8
7
6
5
4
3
2
1
ppm
Figure 18.11.3 NMR data for 1-tert-butyl-3-naphthalen-1-yl-1H-pyrazolo[3,4-d]pyrimidin-4-ylamine (1NA-PP1). Compound is white powder; 1H NMR (CDCl3, 400 Mhz) δ 1.87 (s, 9H), 5.04 (br s, 2H), 7.05 (m, 2H), 7.58 (t, J = 8 Hz, 1H), 7.64 (d, J = 7 Hz, 1H), 7.92 (m, 2H), 7.95 (d, J = 8 Hz, 1H), 8.36 (s, 1H); 13C NMR (CDCl3, 100 Mhz) δ 29.3, 60.6, 101.5, 125.5, 125.6, 126.5, 127.1, 128.4, 128.4, 129.6, 130.6, 131.8, 134.0, 140.4, 153.9, 154.7, 157.7, HRMS (EI) molecular ion calculated for C19H19N5 317.16427, found 317.16247.
Diethyl ether (Et2O) Silica Gel 60 (pore size 0.040 to 0.063 mm; Merck) Sodium ethoxide (Acros Organics) tert-butylhydrazine hydrochloride (Aldrich) N,N-dimethylformamide (DMF; Aldrich) Chloroform (CHCl3) 1% (v/v) methanol in CHCl3 Formamide (Aldrich) Ethanol (Aldrich) Activated charcoal Celite
Design and Use of Analog-Sensitive Protein Kinases
Separatory funnel Oil baths, 80°, 100°, and 180°C 10-in. (25.4-cm) length × 2-in. (5.0-cm) i.d. and 8-in. (20.3-cm) length × 1.5-in. (3.8-cm) i.d. chromatography columns TLC plates (Silica Gel 60 F254; EM Science) and tank Reflux condenser Filter paper Buchner funnel
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Synthesize 1-naphthoylmalononitrile 1. Dissolve 3.80 g (20 mmol) 1-naphthoyl chloride in 30 ml tetrahydrofuran (THF). 2. Dissolve 1.06 g sodium hydride (44.0 mmol) in 10 ml THF. 3. Dissolve 1.45 g malononitrile (22.0 mmol) in 10 ml THF and add dropwise to the NaH/THF suspension on an ice bath. 4. Add the THF solution of 1-naphthoyl chloride from step 1 dropwise to the malononitrile solution with vigorous stirring. Stir for 1 hr. 5. Add ∼15 ml of 1 N H2SO4 and 50 ml water. 6. Extract three times, each time with 100 ml ethyl acetate in separatory funnel. The ethyl acetate phase is the top phase.
7. Combine ethyl acetate extracts and dry by addition of anhydrous MgSO4. 8. Evaporate the solvent to yield 1-naphthoylmalononitrile, a yellow solid. Yield should be approximately 70%.
Synthesize 1-naphthyl(methoxy)methylidenemalononitrile 9. Dissolve 2.20 g 1-naphthoylmalononitrile (from step 8; 10.0 mmol) in a mixture of 18 ml dioxane and 3 ml water. 10. Add 6.8 g sodium bicarbonate. 11. Add 4.9 ml of dimethyl sulfate with vigorous stirring. Heat the reaction mixture at 100°C on an oil bath for 2 hr. 12. Cool the reaction mixture to room temperature. 13. Dilute with 70 ml water and extract three times, each time with 100 ml Et2O. 14. Combine the Et2O extracts and dry over anhydrous MgSO4. 15. Evaporate the Et2O under reduced pressure to give an oil. 16. Prepare a 10-in. (25.4-cm) × 2-in. (5.0-cm) column containing 200 g Silica Gel 60, saturated with 1:1 Et2O/hexane (the mobile phase). 17. Dissolve the oil from step 15 in a small amount of mobile phase and apply to column. Elute with mobile phase. Sample the eluate as it emerges from the column and analyze by thin-layer chromatography (TLC) using 1:1 Et2O/hexanes as the mobile phase. Pool fractions containing a species with Rf of ∼0.22. 18. Evaporate solvent under reduced pressure in a Büchi Rotavapor or equivalent rotary evaporator, to produce a white crystalline solid, 1-naphthyl(methoxy)methylidenemalononitrile. Yield should be ∼75%.
Synthesize 5-amino-1-tert-butyl-3-(1′-naphthyl)-4-cyano-1H-pyrazole 19. Add 0.38 g sodium ethoxide (5.5 mmol) and 0.68 g tert-butylhydrazine hydrochloride (5.5 mmol) to 10 ml DMF. Stir mixture at room temperature for 0.5 hr. 20. Filter the mixture. Save the filtrate, which contains tert-butylhydrazine. 21. Dissolve 1.17 g 1-naphthyl(methoxy)methylidenemalononitrile (from step 18; 5.0 mmol) in 20 ml DMF. Add the tert-butylhydrazine-containing filtrate from step 20. Stir the reaction mixture at room temperature for 1 hr. 22. Evaporate the solvent under reduced pressure.
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23. Suspend the residue in water and extract three times, each time with 50 ml CHCl3, in a separatory funnel. 24. Combine the CHCl3 extracts, dry over anhydrous MgSO4, and evaporate the solvent to give a yellow solid. 25. Prepare an 80-in. (20.3-cm) × 1.5-in. (3.8-cm) column containing 100 g Silica Gel 60, saturated with CHCl3 (the mobile phase). 26. Dissolve the yellow solid in a minimal volume of CHCl3 and apply to the column. Elute with 1% methanol in CHCl3. Sample the eluate as it emerges from the column and analyze by thin-layer chromatography (TLC) using 1% methanol in CHCl3 as the mobile phase. Pool fractions containing a species with Rf of ∼0.1. 27. Evaporate solvent to produce a white solid, 5-amino-1-tert-butyl-3-(1′-naphthyl)-4cyano-1H-pyrazole. Yield should be ∼75%.
Synthesize 4-amino-1-tert-butyl-3-(1′-naphthyl)-1H-pyrazolo[3,4-d]pyrimidine (1NA-PP1) 28. Suspend 0.99 g 5-amino-1-tert-butyl-3-(1′-naphthyl)-4-cyano-1H-pyrazole (3.4 mmol) in 20 ml of formamide. 29. Heat this suspension at 180°C on an oil bath, stirring with a reflux condenser for 10 hr. 30. Cool the reaction mixture and dilute with 80 ml water. 31. Collect the precipitate by filtration on paper in a Buchner funnel. 32. Dissolve precipitate in 40 ml room temperature ethanol. Add 1 g powdered activated charcoal. Boil at 80°C for 10 min. 33. Pour a celite pad on a Buchner funnel using a slurry of celite in ethanol. Filter the hot mixture through this celite pad. Collect the filtrate. 34. Evaporate the solvent to obtain 1NA-PP1 as a white powder. Expect yields of ∼75%. BASIC PROTOCOL 3
PREPARATION OF γ-32P-LABELED N6(BENZYL)ATP NDPK is a ubiquitous enzyme that catalyzes the exchange of γ phosphate between nucleoside triphosphate species and nucleoside diphosphate species by means of a bi-bi-ping-pong mechanism involving a phospho-enzyme intermediate (Weaver, 1962; Mourad and Parks, 1966). In this protocol, 6× His–tagged NDPK is immobilized in the solid phase by metal affinity chromatography, and the column-bound NDPK is equilibrated with [γ -32P]ATP in low magnesium to generate the labeled phospho-enzyme. The column is then washed to remove residual ATP, and a ten-fold molar excess of N6(benzyl)ADP is added in the presence of larger amounts of magnesium. Eluate from this stage contains a mixture of [γ -32P]N6(benzyl)ATP and N6(benzyl) ADP, and can be used directly in kinase reactions. This protocol can also be used to generate γ-35S-labeled nucleoside triphosphates.
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Materials 1:1 slurry of cobalt affinity resin (IDA-Co2+-Sepharose) in HBS/0.02% sodium azide (see Support Protocol 1) HEPES buffered saline (HBS): 150 mM NaCl/100 mM HEPES, pH 7.4 Purified NDPK-6×His (see Support Protocol 2) Phosphate-buffered saline (PBS): 150 M NaCl/100 mM sodium phosphate pH 7.4 (see APPENDIX 2 for sodium phosphate buffer) [γ -32P]ATP (∼3000 Ci/mmol or ∼7000 Ci/mmol) 1 mM N 6(benzyl)ADP (Shah et al., 1997; Shah and Shokat, 2002) PBS (see above) containing 5 mM MgCl2 2-mm glass beads (VWR) 1-ml disposable pipet tip Stand and clamp to accommodate 1-ml disposable pipet tip NOTE: All steps are performed at room temperature. Prepare cobalt affinity column 1. Place a 2-mm glass bead inside a 1-ml disposable pipet tip and flick the tip to make sure the bead is firmly lodged near the end. Use scissors to remove the end of the tip where it extends beyond the glass bead. Mount this miniature column in a clamp on a stand. 2. Add 200 µl of 1:1 slurry of cobalt affinity resin in HBS/0.02% sodium azide (prepared as in Support Protocol 1) to the column prepared in step 1. 3. Add 1 ml HBS and allow the buffer to drain until level with the top of the beads. If desired, a 1-ml pipet can be used to drive fluid flow through the beads. If this is done, be careful to avoid introducing a negative relative pressure in the column when removing the pipet, as this can disturb the beads.
Add NDPK-6×His and radiolabeled ATP to column 4. Add ∼165 µg purified NDPK-6×His. If the NDPK is in a buffer other than the recommended HKG buffer (see Support Protocol 2), it must be freed of any chelators or reducing agents, including EGTA, before addition to the column.
5. When this has flowed into the column, follow with 1 ml PBS. If desired, retain the eluate to quantify protein retention. PBS is used in place of HBS in all steps subsequent to addition of NDPK. IDA-Co2+ appears to have a weak affinity for the various nucleoside phosphate species; PBS competitively blocks these interactions. A slight lavender color may be noticed on the initial addition of PBS. This is insoluble cobaltous phosphate. It has not been found to interfere with any subsequent steps, nor has the use of PBS caused any significant loss of enzyme from the column. NDPK-6×His has an A280 extinction coefficient of 1.31 for a 1 mg/ml solution, as calculated using the Gill equation.
6. Dilute 833 pmol [γ -32P]ATP (2.5 mCi at 3000 Ci/mmol, 5.8 mCi at 7000 Ci/mmol) with PBS to contain no greater than 1 mM reducing agent. Commercial [γ -32P]ATP may contain an appreciable quantity of DTT or other reducing agent. This should be diluted so that enzyme is not lost from the column due to reduction of cobalt. The reactions can be scaled for use of smaller quantities of label. Analysis of Protein Phosphorylation
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7. Add the radioisotope to the column and allow it to flow through, driving the flow if necessary. A moderate darkening of the column due to a low level of metal reduction may be observed, and a small amount of reduced metal may be present in the eluate. This should not normally be cause for concern. Small samples of the load and eluate may be taken to quantify label retention on the column by liquid scintillation counting.
8. Wash the column at least twice, each time with 1 ml PBS. The eluate should be sampled to quantify label release in these steps, to ensure that residual [γ -32P]ATP has been cleared from the column.
Add N6(benzyl)ADP to column and elute N6(benzyl)ATP 9. Add 8 µl of 1 mM N6(benzyl)ADP (8 nmol) to 32 µl PBS containing 5 mM MgCl2. This ADP analog and others can be synthesized using published procedures (Shah et al., 1997; Shah and Shokat, 2002).
10. Carefully add this to the column, and then gently force it into the beads until the liquid is again level with the top of the beads. 11. Add 250 µl PBS containing 5 mM MgCl2 and gently force it through the beads until level, collecting the eluate. A small sample of the eluate should be taken for liquid scintillation counting to determine yield. SUPPORT PROTOCOL 1
PREPARATION OF COBALT AFFINITY RESIN (IDA-Co2+-SEPHAROSE) Materials Iminodiacetic acid (IDA)–Sepharose slurry (Sigma) 200 mM cobalt chloride HEPES-buffered saline (HBS): 150 mM NaCl/100 mM HEPES, pH 7.4 HBS (see above) containing 0.02% sodium azide 15-ml conical tubes 1. Place ∼3 ml of IDA-Sepharose bead slurry in a 15-ml conical tube. 2. Wash several times with water, each time by filling the 15-ml conical tube and then decanting. 3. Wash once with 200 mM cobalt chloride. The binding is essentially instantaneous.
4. Wash four times with HBS. 5. Store beads at 4°C in a bead volume equivalent of HBS containing 0.02% sodium azide. SUPPORT PROTOCOL 2
Design and Use of Analog-Sensitive Protein Kinases
EXPRESSION AND PURIFICATION OF NDPK A 6× His–tagged NDPK from S. cerevisiae is expressed in BL21(DE3) E. coli and purified by metal-affinity chromatography. Materials pJDB1 expression plasmid: available from the Shokat Laboratory (
[email protected]) or Weiss Laboratory (
[email protected]) E. coli strain BL21(DE3) (Novagen) IPTG Cobalt affinity resin (IDA-Co2+-Sepharose; see Support Protocol 1) HIK200 buffer (see recipe), 4°C
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HEK10 buffer (see recipe), 4°C HKG buffer (see recipe), 4°C Liquid nitrogen 30°C shaking incubator 5-ml chromatography column MWCO 15,000 dialysis membrane Additional reagents and equipment for transformation of E. coli (UNIT 1.8), growing bacterial cultures (UNIT 1.2), preparation of bacterial lysates (UNIT 1.7), dialysis (APPENDIX 3C), and determination of protein concentration (UNIT 10.1A) Grow transformed bacteria 1. Transform (UNIT 1.8) the pJDB1 expression plasmid into E. coli strain BL21(DE3). This plasmid is a derivative of pET19b containing the S. cerevisiae gene YNK1 with a C-terminal 6×-His tag. This plasmid confers resistance to kanamycin. Cells carrying this plasmid should be cultured in medium containing 30 ìg/ml kanamycin for the transformation procedure (see UNIT 1.2).
2. Grow transformed strain to mid-log phase (OD600 = 0.5 to 1.0) at 37°C (UNIT 1.2). 3. Add IPTG to the culture for a final concentration of 0.4 mM. 4. Incubate with shaking at 37°C for 3 hr. Prepare bacterial lysate The following steps are performed at 4°C, using chilled buffers. 5. Isolate E. coli cells by centrifugation (UNIT 1.7). 6. Produce clarified lysate by method of choice (UNIT 1.7), ensuring that the salt concentration is not less than 150 mM in the final extract. 7. Prepare a 5-ml column of IDA-Co2+-Sepharose. A 5-ml column is appropriate for a 1-liter-scale expression.
Purify lysate by cobalt affinity chromatography and dialysis 8. Apply the lysate (from step 7) to the column. 9. Wash with 4 column volumes HIK200 buffer. 10. Elute with 2.5 column volumes HEK10 buffer. 11. Dialyze (APPENDIX 3C) against three changes of HKG buffer, each time for 2 hr, using a MWCO 15,000 dialysis membrane. 12. Determine protein concentration by measuring A280 (UNIT 10.1A), divide into aliquots, and freeze using liquid nitrogen. Store at −80°C. NDPK-6×His has an A280 extinction coefficient of 1.31 for a 1 mg/ml solution, as calculated using the Gill equation (also see UNIT 10.1A). The purified protein has a tendency to precipitate in buffers containing quit 221-You have transferred 230379 bytes in 1 files. 221-Total traffic for this session was 231859 bytes in 1 transfers. 221-Thank you for using the FTP service on iubio.bio.indiana.edu. 221 Goodbye.
Figure 19.1.4 Using UNIX FTP to download a file. An anonymous FTP session is established with the molecular biology FTP server at the University of Indiana to download the ClustalW alignment program. The user inputs are shown in boldface.
Although FTP occurs within the UNIX environment, Macintosh and PC users can employ programs that utilize graphical user interfaces (GUI, pronounced “gooey”) to navigate through the UNIX directories on the FTP server. Users need not have any knowledge of UNIX commands to download files; they can instead rely on pop-up menus and the ability to point-and-click their way through the UNIX file structure. The most popular FTP program on the Macintosh platform for FTP sessions is Fetch. Internet Basics for Biologists
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Figure 19.1.5 Using Fetch to download a file. An anonymous FTP session is established with the molecular biology FTP server at the University of Indiana (top) to download the ClustalW alignment program (bottom). Notice the difference between this GUI-based program and the UNIX equivalent illustrated in Figure A.3J.4.
A sample Fetch window is shown in Figure 19.1.5 to illustrate the difference between using a GUI-based FTP program and the equivalent UNIX FTP in Figure 19.1.4. In the figure, notice that the Automatic radio button (near the bottom of the second window under the Get File button) is selected, meaning that Fetch will determine the appropriate type of file transfer to perform. This may be manually overridden by selecting either Text or Binary, depending on the nature of the file being transferred. As a rule, text files should be transferred as Text, programs or executables as Binary, and graphic format files such as PICT and TIFF files as Raw Data. THE WORLD WIDE WEB While FTP is of tremendous use in the transfer of files from one computer to another, it does suffer from some limitations. When working with FTP, once a user enters a particular directory, they can only see the names of the directories or files. In order to actually view
Informatics
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what is within the files, it is necessary to physically download them onto one’s own computer. This inherent drawback led to the development of a number of distributed document delivery systems (DDDS), interactive client-server applications that allowed information to be viewed without having to perform a download. The first generation of DDDS development led to programs like Gopher, which allowed plain text to be viewed directly through a client-server application. From this evolved the most widely known and widely used DDDS, namely the World Wide Web. The Web is an outgrowth of research performed at the European Nuclear Research Council (CERN) in 1989 that was aimed at sharing research data between several locations. That work led to a medium through which text, images, sounds, and videos could be delivered to users on demand, anywhere in the world. Navigation on the World Wide Web Navigation on the Web does not require advance knowledge of the location of the information being sought. Instead, users can navigate by clicking on specific text, buttons, or pictures. These clickable items are collectively known as hyperlinks. Once one of these hyperlinks is clicked, the user is taken to another Web location, which could be at the same site or halfway around the world. Each document displayed on the Web is called a Web page, and all of the related Web pages on a particular server are collectively called a Web site. Navigation strictly through the use of hyperlinks has been nicknamed “Web surfing.” Users can take a more direct approach to finding information by entering a specific address. One of the strengths of the Web is that the programs used to view Web pages (appropriately termed browsers) can be used to visit Gopher and FTP sites as well, somewhat obviating the need for separate Gopher or FTP applications. As such, a unified naming convention was introduced to indicate to the browser program both the location of the remote site and, more importantly, the type of information at that remote location so that the browser could properly display the data. This standard-form address is known as a uniform resource locator (URL), and takes the general form protocol://computer. domain, where protocol specifies the type of site and computer.domain specifies the location (Table 19.1.2). The http used for the protocol in World Wide Web URLs stands for hypertext transfer protocol, the method used in transferring Web files from the host computer to the client. Browsers Browsers, which are used to look at Web pages, are client-server applications that connect to a remote site, download the requested information at that site, display the information on the user’s monitor, then disconnect from the remote host. The information retrieved from the remote host is in a platform-independent format called hypertext markup language (HTML). HTML code is strictly text-based, and any associated graphics or sound for that document exist as separate files in a common format. For example, images Table 19.1.2 Uniform Resource Locator (URL) Format for Each Type of Transfer Protocol
Internet Basics for Biologists
Site
URL format (example)
General form
protocol://computer.domain
FTP site Gopher site
ftp://ftp.ncbi.nlm.nih.gov gopher://gopher.iubio.indiana.edu
Web site
http://www.nhgri.nih.gov
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may be stored and transferred in GIF format, developed by CompuServe for the quick and efficient transfer of graphics; while GIF format is most commonly used for graphics, other formats such as JPEG and BMP may also be used. Because of this, a browser can display any Web page on any type of computer, whether it be a Macintosh, IBM-compatible, Linux, or UNIX machine. The text is usually displayed first, then the remaining elements are placed on the page as they are downloaded. With minor exceptions, a given Web page will look the same when the same browser is used on any of the above platforms. The two major players in the area of browser software are Netscape, with their Communicator product, and Microsoft, with Internet Explorer. As with many other areas where multiple software products are available, the choice between Netscape and Internet Explorer comes down to one of personal preference. While the computer literati will debate the fine points of difference between these two packages, for the average user, both packages perform equally well and offer the same types of features, adequately addressing the Web-browser needs of most users. It is worth mentioning that, while the Web is by definition a visually based medium, it is also possible to travel through Web space and view documents without the associated graphics. For users limited to line-by-line terminals, a browser called Lynx is available. Developed at the University of Kansas, Lynx allows users to use their keyboard arrow keys to highlight and select hyperlinks, using their return key the same way that Netscape and Internet Explorer users would click their mouse. Internet versus Intranet The World Wide Web is normally thought of as a way to communicate with people at a distance, but the same infrastructure can be used to connect people within an organization. Such intranets provide an easily accessible repository of relevant information, capitalizing on the simplicity of the Web interface. It also provides another channel for broadcast or confidential communication within the organization. Having an intranet is of particular value when members of an organization are physically separated, whether it be in different buildings or different cities. Intranets are protected in such a way that people who are not on the organization’s network are prohibited from accessing the internal Web pages; additional protections through the use of passwords are also common. Finding Information on the World Wide Web Most people find information on the Web the old fashioned way—by word of mouth either by using lists such as Table 19.1.3 or by simply following hyperlinks put in place by Web authors. Continuously clicking from page to page can be a highly ineffective way of finding information, especially when the information sought is of a very focused nature. One way of finding interesting and relevant Web sites is to consult virtual libraries, which are curated lists of Web resources arranged by subject. Virtual libraries of special interest to biologists include the WWW Virtual Library, maintained by Keith Robison at Harvard and the EBI BioCatalog, based at the European Bioinformatics Institutes. The URLs for these sites can be found in Table 19.1.3. It is also possible to directly search the Web by using search engines such as Alta Vista and Excite, among others. A search engine is simply a specialized program that can perform full-text or keyword searches on databases that catalog Web content. The result of a search is a hyperlinked list of Web sites fitting the search criteria from which the user can visit any or all of the found sites. However, search engines use slightly different methods in compiling their databases. One variation is the attempt to capture most or all of the text of every Web page that the search engine is able to find and catalog (“Web crawling”). Another technique is to catalog only the title of each Web page rather than its
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Table 19.1.3
World Wide Web Sites of Interest
Site
URL
Domain names gTLD-MOU Internet Software Consortium
http://www.gtld-mou.org http://www.isc.org
Electronic mail and newsgroups BIOSCI Newsgroups http://www.bio.net/docs/biosci.FAQ.html Eudora http://www.eudora.com Microsoft Exchange http://www.microsoft.com/exchange/ NewsWatcher ftp://ftp.acns.nwu.edu/pub/newswatcher/ File Transfer Protocol Fetch 3.0/Mac FTP Voyager
http://www.dartmouth.edu/pages/softdev/fetch.html http://ftpvoyager.deerfield.com
Internet access America Online AT&T Bell Atlantic Bell Canada CompuServe MCI Ricochet Telus
http://www.aol.com http://www.att.com/worldnet http://www.bellatlantic.net http://www.bell.ca http://www.compuserve.com http://www.mci.com http://www.ricochet.net http://telus.com
Virtual libraries EBI BioCatalog Amos’ WWW Links Page NAR Database Collection WWW Virtual Library
http://www.ebi.ac.uk/biocat/biocat.html http://www.expasy.ch/alinks.html http://www3.oup.co.uk/nar/Volume_28/Issue_01/introduction/ http://mcb.harvard.edu/BioLinks.html
World Wide Web browsers Internet Explorer http://www.microsoft.com/insider/ie5/default.htm Lynx ftp://ftp2.cc.ukans.edu/pub/lynx Netscape Navigator http://home.netscape.com World Wide Web search engines AltaVista http://www.altavista.com Excite http://www.excite.com HotBot http://hotbot.lycos.com Infoseek http://infoseek.go.com Lycos http://www.lycos.com Northern Light http://www.northernlight.com World Wide Web meta-search engines MetaCrawler http://www.metracrawler.com Savvy Search http://www.savvysearch.com
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Table 19.1.4 Number of Hits Returned for Four Defined Search Queries on Some of the More Popular Search and Meta-Search Engines
Search term genetic mapping human genome positional cloning prostate cancer
Search Engine
Meta-search engine
HotBot
Excite
Infoseek
Lycos
MetaCrawler SavvySearch
478 13,231 279 14,044
1,040 34,760 735 53,940
4,326 15,980 1,143 24,376
9,395 19,536 666 33,538
62 42 40 60
58 54 52 57
entire text. A third is to consider words that must appear next to each other or only relatively close to one another. Because of these differences in search-engine algorithms, the results returned by issuing the same query to a number of different search engines can produce wildly different results (Table 19.1.4). It may also be noted from Table 19.1.4 that most of the numbers are exceedingly large, reflecting the overall size of the World Wide Web. Unless a particular search engine ranks its results by relevance (for example, by scoring words in a title higher than words in the body of the Web page), the results obtained may not be particularly useful. It is also necessary to keep in mind that, depending on the indexing scheme that the search engine is using, the found pages may actually no longer exist, leading the user to the dreaded “404 Not Found” error. Compounding this problem is the issue of coverage—the number of Web pages that any given search engine is actually able to survey and analyze. A comprehensive study by Lawrence and Giles (1998) indicates that the coverage provided by any of the search engines studied is both small and highly variable. For example, the HotBot engine produced 57.5% coverage of what was estimated to be the size of the “indexable Web,” while Lycos had only 4.41% coverage, a full order of magnitude less than HotBot. The most important conclusion from this study was the extent of coverage increased as the number of search engines are increased and the results from those individual searches are combined. Combining the results obtained from the six search engines examined in this study produced coverage approaching 100%. To address this point, a new class of search engines called meta-search engines have been developed. These programs will take the user’s query and poll anywhere from five to ten of the “traditional” search engines. The meta-search engine will then collect the results, filter out duplicates, and return a single, annotated list to the user. One big advantage is that the meta-search engines take relevance statistics into account, returning much smaller lists of results. Even though the hit list is substantially smaller, it is much more likely to contain sites that directly address the original query. Since the programs must poll a number of different search engines, searches conducted this way obviously take longer to perform, but the higher degree of confidence in the compiled results for a given query outweighs the extra few minutes (sometimes only seconds) of search time. Reliable and easy-to-use meta-search engines include SavvySearch and MetaCrawler (see Table 19.1.3). LITERATURE CITED Froehlich, F. and Kent, A. 1991. ARPANET, the Defense Data Network, and Internet. In Encyclopedia of Communications. Marcel Dekker, New York. Krol, E. and Klopfenstein, B.C. 1996. The Whole Internet User’s Guide and Catalog. O’Reilly and Associates, Sebastopol, Calif. Kennedy, A.J. 1999. The Internet: Rough Guide 2000. Rough Guides, London.
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Lawrence, S. and Giles, C.L. 1998. Searching the World Wide Web. Science 280:98-100. Rankin, B. 1996. Dr. Bob’s Painless Guide to the Internet and Amazing Things You Can Do with E-mail. No Starch Press, San Francisco. Quarterman, J. 1990. The Matrix: Computer Networks and Conferencing Systems Worldwide. Digital Press, Bedford, Mass.
Contributed by Andreas D. Baxevanis National Human Genome Research Institute, NIH Bethesda, Maryland B.F. Francis Ouellette Centre for Molecular Medicine and Therapeutics University of British Columbia Vancouver, British Columbia
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Sequence Databases: Integrated Information Retrieval and Data Submission
UNIT 19.2
UNIT 19.1 discusses general tools for electronic information retrieval, giving only brief glimpses of the databases of interest to molecular biologists. This unit provides a more thorough overview of biomedical information resources, focusing on sequence data, structure information, and the associated literature—and also discusses how nucleotide sequence data gets into the databases in the first place—an often neglected but nevertheless important topic. UNIT 19.3 covers sequence homology searching in detail.
First it is important to appreciate not only the variety of available data but also how rapidly it is expanding (Fig. 19.2.1A). The biomedical literature, as represented by MEDLINE, is growing at a rate of 400,000 articles per year. MEDLINE, produced by the U.S. National Library of Medicine (NLM), currently contains bibliographic information and journal abstracts from more than 11,000,000 publications indexed from ∼4,300 biomedical journals. Genetic and physical mapping data are expanding rapidly, as are protein crystal and NMR structures; DNA and protein sequence data have been growing exponentially for years (Fig. 19.2.1B to D). Even faster growth rates are anticipated for cDNA and concerted genome sequencing as technology advances. In mid-1997 complete sequences of Saccharomyces cerevisiae and seven bacterial genomes were available and the sequencing phase of the Human Genome Project was initiated. By January 2000, there were 24 bacterial and archaeal genomes completed and another 70 in progress. Drosophila melanogaster and Caenorhabditis elegans were both essentially done. Other eukaryotes for which substantial genomic sequencing had been done included human, mouse, Arabidopsis thaliana, rice, corn and the malaria parasite (Plasmodium falciparum). GenBank is a comprehensive repository of sequence data and associated annotation, built and distributed by the National Center for Biotechnology Information (NCBI) at the NLM in Bethesda, Maryland. GenBank is part of an international collaboration with the DNA Database of Japan (DDBJ) in Mishima and the European Molecular Biology Laboratory (EMBL) Data Library at the European Bioinformatics Institute near Cambridge, United Kingdom. Because these partners exchange data daily, what we refer to here as simply “GenBank” actually contains sequences from GenBank, EMBL, and DDBJ. In the 1990s, new types of sequence data became a major part of the database entries. For example, “expressed sequence tags” or ESTs are partial sequences that are derived from automated, “single-pass” sequencing on clones that are randomly selected from cDNA libraries, usually with the intent of surveying expressed genes (Adams et al., 1991; Kahn et al., 1992; Okubo et al., 1992; Waterston et al., 1992). These data present certain analytic challenges (Boguski et al., 1993) that are dealt with in UNIT 19.3, and they also require special procedures for batch submission to sequence databases (see discussion of submitting expressed sequence tags (EST), sequence tag site (STS), or genome survey sequence (GSS) data). Short genomic sequences, including those derived from “exon trapping” or “exon amplification” and genomic survey sequences (Church et al., 1993; Smith et al., 1994) present analytical challenges similar to those for ESTs. There are special procedures for submission of these “GSS” entries, similar to the procedures for EST submission. Large-scale sequencing of complete genomes has required additional methods for submission and has spurred development of new ways to access and display the data. Informatics for Molecular Biologists Contributed by Jane M. Weisemann, Mark S. Boguski, and B.F. Francis Ouellette Current Protocols in Molecular Biology (2000) 19.2.1-19.2.20 Copyright © 2004 by John Wiley & Sons, Inc.
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A
12 Articles (×106)
10
C
6 4 2 0
Protein sequences (× 105) Nucleotide sequences (× 106)
B
8
'65
'68
'72
'76
'80
'84
'88
'92
'96 2000
'65
'68
'72
'76
'80
'84
'88
'92
'96 2000
'65
'68
'72
'76
'80
'84
'88
'92
'96 2000
'65
'68
'72
'76
'80 '84 Years
'88
'92
'96 2000
6 5 4 3 2 1 0 6 5 4 3 2 1
D
Structure entries (× 103)
0 14 12 10 8 6 4 2
Figure 19.2.1 The cumulative growth of biomedical research data. (A) Growth of MEDLINE. (B) Growth in the number of nucleotide sequences in GenBank (Benson et al., 2000). (C) Growth in the number of protein sequences in the “nonredundant” set of proteins in GenBank. ( D) Growth of protein and nucleic acid three-dimensional structures represented in the Brookhaven Protein Data Bank, April 2000 (http://www.rcsb.org/pdb/). These figures occasionally induce panic or despair in individuals who fear that it is impossible to make sense of so much data; however, modern information retrieval systems such as Entrez (Fig. 19.2.2) have succeeded in integrating this data, reducing redundancy, and providing powerful and convenient user interfaces.
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Retrieval of records by electronic mail and anonymous FTP are covered in UNIT 19.1, as well as access to databases via the World Wide Web. Database homology searching is described in detail in UNIT 19.3. The present unit focuses on the wide variety of sequence, structure, and bibliographic databases—as well as how they have been integrated (Fig. 19.2.2) into a single, easy-to-use retrieval environment called Entrez, which provides users with “one stop shopping” over the Internet. INTRODUCTION TO ENTREZ Entrez (Fig. 19.2.2) ties together a diverse set of information resources: it accesses nucleotide and protein sequences from a number of databases as well as genome data from the NCBI Genomes division, three-dimensional structures from MMDB (Molecular Modeling Database of NCBI), human disease data from OMIM (Online Mendelian Inheritance in Man), genetic locus data from LocusLink, and bibliographic citations from PubMed (discussed below). Entrez is available through the World Wide Web (http://www.ncbi.nlm.nih.gov/Entrez/) and in a client/server version (available by FTP, see http://ncbi.nlm.nih.gov/Entrez/Network/nentrez.overview.html or ftp://ncbi.nlm.nih. gov/entrez/README_1). In the case of the client/server application, Network Entrez (NetEntrez), the client runs on the user’s local machine and interacts with the server at NCBI. Versions of the client program are available for Macintosh, Microsoft Windows, Unix/X-windows and some other systems. Entrez for the World Wide Web (WebEntrez) works through standard WWW browsers. NetEntrez and WebEntrez have similar func-
OMIM
GenBank EMBL DDBJ dbEST dbSTS dbGSS patent
maps and genomes
PubMed
full-text electronic journals
GenBank nucleotide sequences
3-dimensional structures
protein sequences
taxonomy
Swiss-Prot PIR PRF PDB GenBank EMBL DDBJ patent
Figure 19.2.2 Data sources and links that constitute the Entrez system.
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tionality, but there are differences in the way that they are used. WebEntrez has the advantage of additional links—e.g., to WebBLAST outputs and to full-text articles for some journals. The NetEntrez interface is more intuitive and is often easier to use for complex queries. Entrez can be used in many ways and some common scenarios are presented below. Suppose we have just done a similarity search and have observed a number of promising matches on the “hit list.” The information on this list is often insufficient to allow us to reach even preliminary conclusions about the biological implications of our discovery. Thus, we want to find the publication describing this sequence, which meant, until recently, that we had to use a separate retrieval system to obtain the database record corresponding to the sequence. In this database record we would find the literature citation, and then we would have to take a trip down to the library to actually pull the printed copy of the journal. The “activation energy” for this process is high enough that one sometimes decides it is not worth it for “borderline” matches. Even if our motivation is high enough, perhaps there is no literature citation in the database record, or it involves a journal to which our library does not subscribe. In addition to finding the corresponding publication to confirm our search results, we may also want to retrieve some of the matching sequences and perform additional searches and alignments with these. All together, this is quite an involved process. Entrez substantially lowers the activation energy by combining all of these activities in an easy-to-use system with a mouse-driven, graphical user interface. Entrez establishes all of the requisite links between DNA sequences, the proteins they encode, and the literature that describes their biology. The literature component of Entrez comes from the PubMed database, which includes abstracts. While an abstract is not a substitute for the full text of a paper, one can frequently go an amazingly long way in the analysis and interpretation of sequence homology results by the information contained in an abstract. For an increasing list of journals, the Web version of Entrez also has links to full-text articles. For many human disease genes, there are also links from PubMed records to the description of the disease in the Online Mendelian Inheritance in Man (OMIM) database. The PubMed database of bibliographic information contains citations from MEDLINE and PreMEDLINE (preliminary citations that eventually become MEDLINE records). Entrez not only contains explicit links between different data sources, but also implicit or computed links within the data itself. Daily, the entire set of DNA and protein sequence databases are compared among one another and all significant homologies are computed and stored in the system. Thus, it is possible to instantly retrieve the homologs of any sequence in Entrez with the click of a mouse, without having to manually perform a new database search. Thus Entrez contains within it thousands of answers to questions that have not yet been asked. In one interesting case, this automated system actually “discovered” a significant homology prior to its recognition by a human biologist (Boguski and McEntyre, 1994).
Sequence Databases
In addition to precomputed sequence homologies, Entrez performs a similar operation on PubMed records. It is possible to compute a statistical relationship between two articles based on their frequency of use of significant terms. To indicate what constitutes a significant term, consider two negative examples: i.e., the terms “novel” and “important” are not significant terms because they appear in most scientific papers and thus are not of use in distinguishing one paper from another in terms of specific content. The set of “related articles” (sometimes referred to as “neighbors”) is recomputed daily. This constitutes a very powerful information resource. You can enter Entrez via an author name or keyword and retrieve not only a specific paper but an entire bibliography of related material. You can then branch out (or “neighbor out”) to other areas of the information
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space and effortlessly go back and forth between the literature and the sequences. Specific examples have appeared in reviews (Baxevanis et al., 1997; Cockerill, 1994; Harper, 1994; Schuler et al., 1996). Further examples can also be found on the NCBI Coffee Break web site (http://www.ncbi.nlm.nih.gov/Coffeebreak/). These pages describe recent biological discoveries and include tutorials demonstrating the linkage between various components of Entrez and other bioinformatics tools. Finding information on sequence homologs using the Web version of Entrez is made even easier by the connections between Entrez and Web-based BLAST output. For example, it is possible to do a BLAST search using WebBLAST, and the hit list will have hypertext links to protein or nucleotide entries in Entrez. Other features of Entrez include taxonomy-based sequence retrieval; from the Web page for the NCBI Taxonomy Browser you can scan through organism names and phylogenetic trees and select the full set of nucleotide or protein sequences for any species. There are now links in Entrez from sequences and the literature to protein three-dimensional structures in NCBI’s Molecular Modeling Database (MMDB). A three-dimensional structure viewer called Cn3D has also been added to the system. Cn3D can be run as a client/server program or as a “Helper” application integrated into a standard Web browser. Another special viewer is available for the Entrez Genomes division. The genome-level views presented by Entrez are built from sequences of complete chromosomes, from composites of sequence fragments, and from integrated genetic and physical maps. Entrez allows the user to visualize the sequence information at varying levels of detail, either graphically or as text. The chromosome views are tightly linked to the other Entrez component databases, allowing the user to jump effortlessly between maps, sequences, and bibliographic components. DATA SUBMISSION: GENERAL CONSIDERATIONS During the late 1970s, when DNA sequencing technology was still new, the Journal of Biological Chemistry required that any submitted manuscript containing sequence data be accompanied by the actual autoradiograms, so that reviewers could check the accuracy of the sequence data. We have come a long way since then. Currently, journals prefer not to even publish sequence data because it takes up a great deal of space and is much more usable in electronic form. Most journals, however, require proof in the form of a database accession number that the authors of a publication have sent their data to GenBank, EMBL, or DDBJ. Accession numbers are unique identifiers (consisting of one letter followed by five digits or two letters followed by six digits) that serve as confirmation that the data has been submitted and allow the scientific community to retrieve it. Making sequences available by depositing them in the public databases is recognized as an important responsibility of researchers to the community. GenBank, the EMBL Data Library, and the DDBJ are partners in the international database collaboration; authors may submit their sequences to whichever database is most convenient without regard to where papers describing those sequences may be published. Sequences should be submitted to a single site; data submitted to one site are exchanged with the others on a daily basis. Sequences by themselves have little meaning; hence, thorough, accurate biological annotation is very important. One important source of annotation is the published article referring to the sequence; indeed, NCBI has made every effort to link citations to the sequences via MEDLINE and Entrez. However, electronic annotation obtained from authors at the time of submission is becoming even more critical as the amount of data
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grows (Fig. 19.2.1B), because the only realistic way of searching through this much data is by computer. The database staff are very helpful in assisting submitters in the proper annotation of their data. Sequences submitted to any of the sequence databases go through a series of validation checks conducted by the database staff. Accession numbers are returned to the authors, usually within 24 hr, or less, if there are no problems with the submission(s). The accession number should be included in any manuscript that deals with the sequence, preferably in a footnote on the first page of the article (or in the manner prescribed by the individual journal). The full text of the provisional record is passed back to the authors for their review prior to being released. Authors may request that the data be kept confidential until publication. See Internet Resources for URLs and e-mail addresses of the sequence databases. SUBMITTING A SEQUENCE TO THE NUCLEOTIDE DATABASE Submission Methods As mentioned above, a sequence can be submitted to any of the three sequence databases (see discussion of Data Submission: General Considerations; also see Internet Resources). A submission to GenBank is described in this section, but readers should keep in mind that the example could apply to DDBJ or EMBL submissions. Researchers should submit their sequences to whichever database is most convenient. The important point is to only submit to one database. If the researcher has access to the World Wide Web, the easiest way to submit data to GenBank is to use the BankIt form on the NCBI home page (or for DDBJ and EMBL submissions, use Sakura or WebIn, respectively; see Internet Resources). BankIt (http://www.ncbi.nlm.nih.gov/BankIt/) allows one to simply copy and paste most of the submission information and data directly out of one’s word processor or sequence-analysis package into an electronic form. BankIt was designed to handle the most common types of GenBank submissions, such as mRNAs or short genomic records. NCBI also has developed a platform-independent submission program called Sequin, which runs stand-alone or over the network. Sequin is suitable for a wide range of sequence lengths and complexities, including traditional (gene-sized) nucleotide sequences, segmented entries (e.g., genomic sequences of a spliced gene where not all of the intronic sequences have been determined), long (genome-sized) sequences with many annotated features, and sets of related sequences (i.e., population, phylogenetic, or mutation studies of a particular gene, region, or viral genome). It also has a number of built-in validation functions for enhanced quality assurance. The validator checks such things as missing organism information, incorrect coding-region lengths (compared to the attached protein sequence), internal stop codons in coding regions, mismatched amino acids, or nonconsensus splice sites. Sequin can present various views of the data, including GenBank-, FASTA-, and EMBL-formatted files, and a graphical view of the sequence including features and alignments. A sequence editor is built into Sequin; this editor automatically adjusts feature intervals as the sequence is edited. Additional capabilities include sequence-analysis functions—e.g., a coding region translator, an ORF finder, and a function that will replace a sequence with the sequence of the complementary strand and simultaneously adjust the positions of the features.
Sequence Databases
Sequin is available from NCBI by anonymous FTP (UNIT 19.1); see http://www.ncbi. nlm.nih.gov/Sequin/index.html for further details on Sequin, including full detailed documentation.
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BankIt is best suited for submitting one or a small number of sequences. Sequin provides more of a data “workbench” environment that can accommodate large as well as small contigs with detailed annotation that can be added over time and which can finally be used to submit data to GenBank. In addition to these methods, NCBI provides streamlined submission procedures for large data sets—e.g., expressed sequence tags (ESTs), sequence-tagged sites (STSs), genome survey sequences (GSSs), and high-throughput genome sequences (HTGSs). NCBI provides a means for interacting with the laboratory information-management systems of genome-sequencing centers to ensure the efficient, timely, and accurate submission of their data. These options are discussed in more detail in subsequent sections (see discussion of Submitting EST, STS, or GSS Data, and Submitting High-Throughput Genome Sequences). Instructions and Tips for Preparing Sequence Submissions The most important item is, of course, the new DNA sequence itself. It should be free of sequencing errors, cloning artifacts, etc.; standard things to check for in this regard are described below. In the special case of “single-pass” sequences such as ESTs, the data often contain ambiguities and sequencing errors; in these cases it is important to note it as “single-pass” data. What does one need to know about a sequence to submit it to GenBank? The sequence record contains references to the article in which the sequence is published, information on the submitters, and information on the source organism. The record also should describe the sequence as fully as possible. Significant regions of the sequence, such as coding region, promoter region, and transcription boundaries, are labeled with defined feature names. These features are detailed in the GenBank Feature Table document, available by anonymous FTP (UNIT 19.1) from ftp://ncbi.nlm.nih.gov in the directory /genbank/docs, or on the Web at http://www.ncbi.nlm.nih.gov/collab/FT/index.html. This list of features (along with their syntax and qualifiers) is somewhat overwhelming, and it is much easier to familiarize oneself with this system by simply looking at some well-done examples of existing GenBank records (see Appendix to this unit). The DEFINITION line (see Appendix to this unit for illustrations of this and other elements of the GenBank record) is a short description of the biological sequence in the record and the organism from which it was derived. The LOCUS field contains the length of the sequence, the type of sequence, and the date that it was released to the public (or re-released following any updates, corrections, or modifications). The ORGANISM field in the SOURCE section contains a complete phylogenetic classification. The REFERENCE section contains one or more citations, the name and address of the submitter, and the date that the sequence was originally submitted to the database. The COMMENT field may be used to include annotations that cannot be accommodated in the FEATURES table. REFERENCEs and COMMENTs that apply to the complete record are referred to as “descriptors.” Whenever possible, annotations should be expressed as “legal” FEATURES, using the proper syntax, because this is the only manner in which they are computer-readable. A more complex example, illustrating detailed annotation of a human breakpoint cluster region, is the GenBank record with the accession number U07000, which can be obtained from Entrez (see Introduction to Entrez, above) or the e-mail server. A few other examples are: accession numbers AF010398-AF010400, a segmented set with three segments (see Appendix to this unit, Example 4); AF018073, a bacterial sequence with multiple genes; and AF015226, a mitochondrial sequence with multiple genes (see Appendix to this unit, Example 3). Informatics for Molecular Biologists
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Further details on submission to the sequence databases can be found on the database Web sites and in Kans and Ouellette (1998). The following are seven important questions that should be asked when preparing a sequence for submission. 1. What organism is the source of this DNA? Taxonomic classification of the organism from which the sequence was isolated is very important when using GenBank to ensure that the correct genetic code is used when translating the DNA to generate the protein product from a coding sequence (CDS) feature (see item 4). If the wrong taxa are used, or if a mitochondrial origin is not indicated, then an erroneous translation will be produced and may go undetected. Correct taxonomic classifications are also important for retrieval purposes via Entrez. Over 60,000 species are represented in GenBank. The ten most common at this time are: Homo sapiens (human), Mus musculus (mouse), Drosophila melanogaster (fruit fly), Caenorhabditis elegans (nematode worm), Arabidopsis thaliana (thale cress), Oryza sativa (rice), Rattus norvegicus (lab rat), Danio rerio (zebrafish), Saccharomyces cerevisiae (baker’s yeast), and Lycopersicon esculentum (tomato). A sequence submission in most cases need only include the genus and species, which the database staff use to obtain the complete classification from NCBI’s Taxonomy Database. If precise information is not available, the researcher should send in the best conjecture as to the genus, species, and/or common name, and the database staff will attempt to determine the proper classification. Because there is great biodiversity among microbial organisms used for research, it is often important to include the strain name or number with the DNA sequence for microbial submissions. These will appear as a qualifier, for example, /strain=“S288c”, and are quite useful when doing sequence comparisons or trying to explain certain sequence conflicts. Strain information is also important in the context of genetic mapping data from the mouse (e.g., /strain=“BALB/c”) and other organisms. 2. Is this a genomic or mRNA (cDNA) sequence? The sequenced molecule is most often DNA, but the annotation should indicate the biological identity of the starting material. For example, cDNA (i.e., an mRNA that was made into a DNA molecule using reverse transcriptase so that it could be cloned) should be annotated in the database as an mRNA. HIV is an interesting case. HIV sequences, as well as those from other retroviruses, may be classified in different ways: as cDNA corresponding to genomic RNA (therefore as “RNA”); as nonintegrated, nonproviral DNA (therefore as single-stranded DNA or “ssDNA”); or as integrated, proviral DNA, (therefore as double-stranded DNA or “dsDNA”). 3. If this is a genomic sequence, are there introns for which the sequence has not been determined?
Sequence Databases
In many cases only parts of a gene have been sequenced—e.g., the exons with only a few nucleotides of flanking intron. In this case it is necessary to create what is known as a “segmented entry.” What this means is that the researcher will create individual submissions for each discrete sequence and indicate the relationships between them—i.e., their linear order. Additionally, if the approximate distances (in base pairs or kilobase pairs) between the segments of a set are known, this information should be included. This will permit the complete gene to be reassembled automatically by software programs. It should be noted that this is an example of a type of entry for which Sequin is the best tool.
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4. Is there a coding sequence (CDS) or other RNA product (rRNA or tRNA) on this DNA? If so, what are their positions in the DNA sequence? If this is an mRNA, or if it is a DNA molecule encoding a protein product, it is important to indicate that a coding region is present and to supply its start and stop codons. RNA products also have boundaries or coordinates that should be indicated. An important check that is done with all submissions is to verify that the coordinates submitted correspond with the indicated feature(s). If there are any discrepancies, these are resolved by the database annotation staff in consultation with the submitter. As described above under question 1, a common problem occurs when the taxonomic information is incorrect or incomplete, leading to the use of an improper genetic code. Fifteen different genetic codes are presently in use, as outlined in the GenBank Feature Table Document (also see information on BLAST parameters in the BLAST unit, see UNIT 19.3.) Any codon usage that is not represented in this list must use the translation exception qualifier (/transl_except) to indicate the discrepancy. Sequin has a built-in tool for analyzing open reading frames. The user can set the minimum size of the open reading frames and the genetic code and the “ORF Finder” will graphically display all open reading frames in a sequence. There is also a separate Web version of the ORF Finder (http://www.ncbi.nlm.nih.gov/gorf/gorf.html). 5. If there is a CDS, what is gene name and the name of the encoded protein? If the gene product is an enzyme, what is the EC number? Putting a protein or gene product name on a CDS is the best way of labeling the protein encoded by the DNA. It is possible to add more than one qualifier, but a /product, /gene, and /EC_number (if the product is an enzyme) are quite sufficient. This is the label that will be carried over to the protein databases—e.g., GenPept or Swiss-Prot. Enzyme Commission (EC) numbers can be obtained or verified using the World Wide Web (UNIT 19.1) at http://www.expasy.ch/enzyme/enzyme-searchec.html. 6. Is there information about your sequence for which there does not seem to be an appropriate place on the forms? There are sometimes problems finding the appropriate place to put a given feature in the BankIt or Sequin forms. In such cases, just attach a note to the submission and the database staff will find the correct place or contact you for more information. 7. Is this sequence ready for submission? Has it been tested for the presence of vector contamination, mitochondrial DNA, ribosomal and tRNAs, and repetitive (e.g., Alu) elements? Potential problems can be detected by using the BLAST programs to compare a new sequence against special data sets that represent common “contaminants.” The network-aware version of Sequin will allow you to run BLASTN against vector and mitochondrial databases. The VecScreen system has recently been developed for quickly finding vector contamination in sequences. VecScreen is automatically run on BankIt sequences and it can be run from within Sequin. It can also be used independently from its own web page (http://www.ncbi.nlm.nih.gov/VecScreen/VecScreen.html). Other validation routines are built into both the Sequin and BankIt submission tools. These routines include a comparison of the conceptual translation of a coding region with the submitted amino acid sequence, a check that actual data length matches the given length, a check for correct use of qualifiers on features, and a check for illegal characters. Although the database staff applies a number of
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validation tests to each new sequence, it is more expeditious for authors themselves to catch any problems prior to submission. SUBMITTING AN UPDATE OR CORRECTION TO AN EXISTING GENBANK ENTRY Authors are encouraged to submit updates or corrections to their sequence records. In fact anyone who notices a problem, error or omission in a database record should bring this to the attention of the database staff. It is not uncommon for the authors to forget to notify the databases that a previously confidential sequence may now be released. If an accession number is seen in a published article and the sequence cannot be retrieved, this is almost certainly the case. The databases will release the data if the complete journal citation is supplied, including the full title of the paper, or if a copy of the title page plus the page showing the accession number is faxed to the databases (see Internet Resources for contact information). Update information consisting of a simple revision, such as a citation change or release of information, can be submitted by sending an e-mail message in paragraph form explaining the change. The message must include the accession number of the sequence to be updated along with all update, correction, or publication information. Updates can also be submitted using BankIt or Sequin. See the BankIt or Sequin Web pages for details (see Submitting a Sequence to the Nucleotide Database, Submission Methods). SUBMITTING EST, STS, OR GSS DATA Expressed-sequence tags (ESTs), sequence-tagged sites (STSs), and genome survey sequences (GSSs) are usually submitted to GenBank and the specialized databases dbEST, dbSTS, or dbGSS as batches of dozens to thousands of entries, with a great deal of redundancy in the citation, submitter, and library information. For these data, there are special procedures designed to improve the efficiency of the submission process. A special “tagged flat file” input format is used. Documents describing the format can be obtained by sending a request to
[email protected]. They are also available from the dbEST, dbSTS, or dbGSS Web pages at: http://www.ncbi.nlm.nih.gov/dbEST/, http://www. ncbi.nlm.nih.gov/dbSTS/, and http://www.ncbi.nlm.nih.gov/dbGSS/, respectively. Once the data are ready to submit, they should be e-mailed to
[email protected]. For large data sets, an account can be set up into which the data can be deposited by FTP. For more information write to
[email protected]. SUBMITTING HIGH-THROUGHPUT GENOME SEQUENCES (HTGS) Some groups may find that their needs are not conveniently met by the standard means for data submission—e.g., a genome center that has its own internal information system or a group assembling a very large contig. The staff of NCBI, EBI, or DDBJ are happy to discuss these issues and make special arrangements with such groups so that their data can be conveniently incorporated into the databases. This includes setting up automatic exchange of data, creation of special FTP accounts, and the generation of tools to ensure data exchange in the most useful format. At NCBI, for example, FTP accounts have been set up for all submitting genome sequencing centers and a variety of tools have been created for accelerating the submission of high-throughput genome sequences (HTGS). Sequence Databases
For large sequencing operations that wish to develop an integrated direct submission system, or for those researchers who have a data submission problem that does not seem
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to be addressed by the standard tools, e-mail can be sent to
[email protected]. The situation should be explained in as much detail as possible and a phone number and postal address should be provided so that the GenBank staff can contact the investigators to discuss their situation. CONCLUSION The Handbook of Nucleic Acid Sequences (Barrell and Clark, 1974) contained all known DNA and RNA sequences available at that time. The book was only about half an inch thick and contained 51 tRNA sequences, 11 5S rRNA sequences, some assorted viral and mRNAs, and 9 DNA sequences—the longest of which was an 89-nucleotide sequence from bacteriophage f1. All together, this “database” amounted to ∼10,000 nucleotides. If the current data in GenBank (5.8 billion nucleotides in 5.6 million entries) were published in a comparable printed form the stack of books would be about the height of Mount McKinley (20,320 feet). The early history of electronic sequence databases is covered in part by Smith (1990). Developments since 1990 can be surveyed by consulting the annual database issues of Nucleic Acids Research (e.g., Benson et al., 2000) and the authors recommend scanning this special issue occasionally to learn about new databases and services. There has been a great proliferation of electronic data resources and a revolution in their delivery to the scientific community via the Internet, as described in UNIT 19.1. This unit has discussed retrieval and submission of sequence data and associated literature as well as access to precomputed homology results via the Entrez system. But the real power of sequences is the possibility of making new and sometimes profound discoveries by computing on the data. The information content of sequences is a fundamental link between genotype and phenotype, and it is possible to infer much about genes and proteins—their functions, structures, evolution, and regulation—by the simple discovery of a sequence homology. For detailed discussion of how to effectively search for homologies in sequence databases, consult the BLAST unit (UNIT 19.3). LITERATURE CITED Adams, M.D., Kelley, J.M., Gocayne, J.D., Dubnick, M., Polymeropoulos, M.H., Xiao, H., Merril, C.R., Wu, A., Olde, B., Moreno, R.F., Kerlavage, A.R., McCombie, W.R., and Venter, J.C. 1991. Complementary DNA sequencing: Expressed sequence tags and human genome project. Science 252:1651-1656. Barrell, B.G. and Clark, B.F.C. 1974. Handbook of Nucleic Acid Sequences. Joynson-Bruvvers, Oxford. Baxevanis, A.D., Boguski, M.S., and Ouellette, B.F.F. 1997. Computational analysis of DNA and protein sequences. In Genome Analysis: A Laboratory Manual (B. Birren, E.D. Green, S. Kapholz, R.M. Myers, and J. Roskams, eds.) pp. 533-586. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. Benson, D.A., Karsch-Mizrachi, I., Lipman, D.J., Ostell, J., Rapp, B.A., Wheeler, D.L. 2000. GenBank. Nucl. Acids. Res. 28:15-18. Boguski, M. and McEntyre, J. 1994. I think therefore I publish. Trends Biochem. Sci. 19:71. Boguski, M.S., Lowe, T.M., and Tolstoshev, C.M. 1993. dbEST: Database for “Expressed Sequence Tags”. Nature Genet. 4:332-333. Church, D.M., Stotler, C.J., Rutter, J.L., Murrell, J.R., Trofatter, J.A., and Buckler, A.J. 1993. Isolation of genes from complex sources of mammalian genomic DNA using exon amplification. Nature Genet. 6:98-105. Cockerill, M. 1994. A versatile tool for retrieving molecular sequences. Trends Biochem. Sci. 19:94-96. Harper, R. 1994. Access to DNA and protein databases on the Internet. Current Opin. Biotechnol. 5:4-18. Kahn, A.S., Wilcox, A.S., Polymeropoulos, M.H., Hopkins, J.A., Stevens, T.J., Robinson, M., Orpana, A.K., and Sikela, J.M. 1992. Single pass sequencing and physical and genetic mapping of human brain cDNAs. Nature Genet. 2:180-185.
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Kans, J.A. and Ouellette, B.F.F. 1998. Submitting DNA sequences to the databases. In Bioinformatics: A Practical Guide to the Analysis of Genes and Proteins (A.D. Baxevanis and B.F.F. Ouellette, eds.) pp. 319-353. John Wiley & Sons, New York. Okubo, K., Hori, N., Matoba, R., Niiyama, T., Fukushima, A., Kojima, Y., and Matsubara, K. 1992. Large scale cDNA sequencing for analysis of quantitative and qualitative aspects of gene expression. Nature Genet. 2:173-179. Schuler, G.D., Epstein, J.A., Ohkawa, H., and Kans, J.A. 1996. Entrez: Molecular biology database and retrieval system. Methods Enzymol. 266:141-162. Smith, M.W., Holmsen, A.L., Wei, Y.H., Peterson, M., and Evans, G.A. 1994. Genomic sequence sampling: A strategy to high resolution sequence-based physical mapping of complex genomes. Nature Genet. 7:40-47. Smith, T.F. 1990. The history of the genetic sequence databases. Genomics 6:702-707. Waterston, R., Martin, C., Craxton, M., Huynh, C., Coulson, A., Hillier, L., Durbin, R., Green, P., Shownkeen, R., Halloran, N., Metzstein, M., Hawkins, T., Wilson, R., Berks, M., Du, Z., Thierry-Mieg, J., and Sulston, J. 1992. A survey of expressed genes in Caenorhabditis elegans. Nature Genet. 1:114-123.
INTERNET RESOURCES DNA Data Bank of Japan (DDBJ; Center for Information Biology, National Institute of Genetics), 1111 Yata, Mishima, Shiznoka 411, Japan; Fax 81-559-81-6849. e-mail submissions:
[email protected] updates: ddbjupdt@ ddbj.nig.ac.jp information: ddbj@ ddbj.nig.ac.jp home page: http://www.ddbj.nig.ac.jp/ WWW submissions: http://sakura.ddbj.nig.ac.jp/ European Molecular Biology Laboratory (EMBL), EMBL Outstation, European Bioinformatics Institutes (EBI), Welcome Trust Genome Campus, Hinxton, Cambridge, CB10 1SD, United Kingdom. e-mail submissions:
[email protected] updates:
[email protected] information:
[email protected] home page: http://www.ebi.ac.uk WWW submissions: http://www.ebi.ac.uk/Submissions/index.html WebIn: http://www.ebi.ac.uk/embl/Submission/webin.html National Center for Biotechnology Information (NCBI), National Library of Medicine (NLM), Bldg. 38A, Room 8N-803, 8600 Rockville Pike, Bethesda, Maryland 20894; Telephone: 301-496-2475; Fax: 301480-9241. e-mail submissions:
[email protected] EST/GSS/STS:
[email protected] updates:
[email protected] information:
[email protected] home page: http://www.ncbi.nlm.nih.gov/ WWW submissions: http://www.ncbi.nlm.nih.gov/Genbank/index.html BankIt: http://www.ncbi.nlm.nih.gov/BankIt/
Contributed by Jane M. Weisemann and Mark S. Boguski National Center for Biotechnology Information Bethesda, Maryland B.F. Francis Ouellette Centre for Molecular Medicine and Therapeutics University of British Columbia Vancouver, Canada
Sequence Databases
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APPENDIX: SAMPLE GENBANK RECORDS Note that these examples present data in a format called the “GenBank report.” This file format is available with Sequin or Entrez, but other formats for viewing the data are also available, including the “EMBL” format, FASTA format (see UNIT 19.3), and a graphical view. Example 1: A Human mRNA Sequence Record with the Feature Labels and Other Landmarks Underlined This is a typical record for an mRNA sequence with a coding region (CDS) and gene features. The CDS feature includes /gene, standard_name, and /function qualifiers, which are a few of many qualifiers that could be used. There is also the /translation qualifier with the amino acid sequence for the coding region. The position of the coding region on the nucleotide sequence is given by the interval 731..3616. The record also has a source feature. This feature is required for all GenBank entries. A source feature must at a minimum include an /organism qualifier. There are two citations associated with the record. REFERENCE 1 is the publication in which the sequence is described and the accession number is published. REFERENCE 2 shows the name and address of the submitter of the record and the date of submission. The ORGANISM field contains the name of the organism (the same as that given in the /organism qualifier) and the phylogenetic classification. The DEFINITION gives a brief summary of the identity of the sequence. The typical basic DEFINITION consists of the organism name, gene product, gene symbol in parentheses, molecule type (e.g., mRNA), and a statement about the completeness of the coding region. The LOCUS field gives the length of the sequence in the record, the molecule type (mRNA, DNA, RNA, etc.), and the date the current version of the record was released to the public. Note that GenBank report qualifiers are underlined in Example 1 for ease of identification, but not in actual GenBank reports. LOCUS DEFINITION ACCESSION VERSION KEYWORDS SOURCE ORGANISM
HSU11690 4266 bp mRNA PRI 10-DEC-1994 Human faciogenital dysplasia (FGD1) mRNA, complete cds. U11690 U11690.1 GI:595424 . human. Homo sapiens Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Mammalia; Eutheria; Primates; Catarrhini; Hominidae; Homo. REFERENCE 1 (bases 1 to 4266) AUTHORS Pasteris,G.N., Cadle,A., Logie,L.J., Porteous,M.E.M., Schwartz,C.E., Stevenson,R.E., Glover,T.W., Wilroy,R.S. and Gorski,J.L. TITLE Isolation and characterization of the faciogenital dysplasia (Aarskog-Scott syndrome) gene: a putative rho/rac guanine nucleotide exchange factor JOURNAL Cell 79, 669-678 (1994) MEDLINE 95042764 REFERENCE 2 (bases 1 to 4266) AUTHORS Pasteris,N.G. TITLE Direct Submission JOURNAL Submitted (29-JUN-1994) Noe G. Pasteris, Pediatric Genetics, University of Michigan, 1150 W. Medical Center Dr., Ann Arbor, MI 48109, USA FEATURES Location/Qualifiers source 1..4266 /organism="Homo sapiens" /db_xref="taxon:9606" /chromosome="X" /map="Xp11.21" /clone="pFCF1.1,pFCF3.85"
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gene CDS
/tissue_type="craniofacial" /clone_lib="lambda ZAP II 16 week human fetal craniofacial cDNA library" /dev_stage="fetal" 731..3616 /gene="FGD1" 731..3616 /gene="FGD1" /standard_name="faciogenital dysplasia" /function="putative guanine nucleotide exchange factor" /codon_start=1 /protein_id="AAA57004.1" /db_xref="GI:595425" /translation="MHGHRAPGGRRAFGARTPGHEPAGAAPPACADSDPGASEPGLLA RRGSGSALGGPLDPQFVGPSDTSLGAAPGHRVLPCGPSPQHHRALRFSYHLEGSQPRP —————sequence omitted for brevity—————————— GAPQDVKAQRSLPLIGFEVGPPEAGERPDRRHVFKITQSHLSWYFSPETEELQRRWMA VLGRAGRGDTFCPGPTLSEDREMEEAPVAALGATAEPPESPQTRDKT" 858 a 1442 c 1203 g 763 t
BASE COUNT ORIGIN 1 cagggctcct tcccgcccgc cgccgccgag acccaggctg aagctgggga ggacggtgga 61 gcccggccca ggccgttctc ctgcccccgc ggctggagca tcgtctggga ctactggctg ———————sequence omitted for brevity———————————4201 ttttttgtgt ttttttgtgt gttttttttt ttttaagaaa aatacagttt atttcaggct 4261 ttacag //
Example 2: A Bacterial Genomic Sequence Record This bacterial genomic sequence contains several genes. The record has three CDS features for two complete protein coding regions (mutS and mutL) and one partial coding region (cotE). There are additional features in the record: ribosome binding sites (RBS), promoter regions (-10_signal)and stem_loop regions. The DEFINITION includes the name of all three genes and indication of the completeness of the coding regions. REFERENCE 2 is the main citation for this entry. REFERENCE 1 cites a paper that pertains only to nucleotides 1 to 81 of the sequence. LOCUS DEFINITION
ACCESSIO VERSION KEYWORDS . SOURCE ORGANISM
REFERENCE AUTHORS TITLE JOURNAL MEDLINE REFERENCE AUTHORS TITLE
Sequence Databases
JOURNAL MEDLINE REFERENCE AUTHORS TITLE
BSU27343 Bacillus mismatch complete U27343 U27343.1
4740 bp DNA BCT 21-NOV-1996 subtilis spore coat protein (cotE) gene, partial cds, and repair recognition proteins (mutS) and (mutL) genes, cds. GI:1002518
Bacillus subtilis. Bacillus subtilis Bacteria; Firmicutes; Bacillus/Clostridium group; Bacillus/Staphylococcus group; Bacillus. 1 (bases 1 to 81) Zheng,L.B., Donovan,W.P., Fitz-James,P.C. and Losick,R. Gene encoding a morphogenic protein required in the assembly of the outer coat of the Bacillus subtilis endospore Genes Dev. 2 (8), 1047-1054 (1988) 89006215 2 (bases 1 to 4740) Ginetti,F., Perego,M., Albertini,A.M. and Galizzi,A. Bacillus subtilis mutS mutL operon: identification, nucleotide sequence and mutagenesis Microbiology 142 (Pt 8), 2021-2029 (1996) 96349107 3 (bases 1 to 4740) Albertini,A.M. Direct Submission
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JOURNAL Submitted (22-MAY-1995) Alessandra M. Albertini, Dipartimento di Genetica e Microbiologia, University of Pavia, 207 via Abbiategrasso, Pavia, 27100, Italy FEATURES Location/Qualifiers source 1..4740 /organism="Bacillus subtilis" /strain="168" /db_xref="taxon:1423" /map="170 degrees" /clone="pFG2752, pFG2764, pFG2767" gene 1..57 /gene="cotE" CDS
