The Structural. Basis of Muscular Contraction

Transcription

1 The Structural Basis of Muscular Contraction

2 The Structural Basis of Muscular Contraction John Squire Imperial College of Science and Technology London, England PLENUM PRESS. NEW YORK AND LONDON

3 Library of Congress Cataloging in Publication Data Main entry under title: The Structural basis of muscular contraction. Bibliography: p. Includes index. 1. Muscle contraction. 2. Ultrastructure (Biology) 1. Squire, John, [DNLM: 1. Muscle contraction. WE 500 S927) QP321.S ' ISBN-13: : / e-isbn-13: AACR Plenum Press, New York Softcover reprint of the hardcover 1st edition 1981 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N.Y All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

4 Preface Muscular contraction provides one of the most fascinating topics for a biophysicist to study. Although muscle comprises a molecular machine whereby chemical energy is converted to mechanical work, its action in producing force is something that is readily observable in everyday life, a feature that does not apply to most other structures of biophysical interest. In addition, muscle is so beautifully organized at the microscopic level that those important structural probes, electron microscopy (with the associated image analysis methods) and X-ray diffraction, have provided a wealth of information about the arrangements of the constituent proteins in a variety of muscle types. But, despite all this, the answer to the question "How does muscle work?" is still uncertain, especially with regard to the molecular events by which force is actually generated, and the question remains one of the major unsolved problems in biology. With this problem in mind, this book has been written to collect together the available evidence on the structures of the muscle filaments and on their arrangements in different muscle cells, to extract the common structural features of these cells, and thus to attempt to define a possible series of mechanical steps that will describe at molecular resolution the process by which force is generated. The book cannot be considered to be an introductory text; in fact, it presents a very detailed account of muscle structure as gleaned mainly from electron microscopy and X-ray diffraction. But neither is it written on the assumption that the reader is an expert on the structural methods used. An attempt has been made throughout to present the often complicated structural arguments in such a way that those not familiar with diffraction methods need not feel out of their depth. To promote this, Chapters 2 and 3 on techniques have not been written for the experts v

5 VI PREFACE (for whom there are already several excellent mathematical presentations, e.g., Fraser and MacRae, 1973); rather they are presented as a liberally illustrated qualitative treatment that, it is hoped, those readers without any background in diffraction methods will find valuable. One of the advantages of a structure such as muscle is that it is amenable to a great variety of experimental approaches (e.g., biochemical, physiological, and structural). But, in order for real progress to be made, researchers with quite different training need to meet on common ground. Each needs to understand at least the basic ideas behind the techniques and results of his colleagues. Chapters 2 and 3 are an attempt to allow physiologists and biochemists with no formal training in diffraction to find common ground with the structuralists. A little study and application of these chapters by such people and by students new to muscle research should make the rest of the book palatable and meaningful. But the book is not written just for nonstructuralists. It is, indeed, written very much with structuralists in mind. Each chapter is a review of what is known about particular aspects of muscle structure at the time of writing (early 1980). But, as other structuralists will be well aware, there is a great deal of controversy in this subject. Where this is so, both sides of the argument are stated, but if there seems to be good reason for preferring one interpretation to another, then the reasons for the preference are given. So, in many cases where controversies exist, a definite point of view is clearly stated in the belief that the conclusions are the most reasonable deductions from the data at the present time. However, there are one or two areas (e.g., the contractile machinery of vertebrate smooth muscle) where there really is an enormous amount of uncertainty but in which there seems to be a common preference for a particular view without there being strong enough evidence on which to form an opinion. In such cases, a "devil's advocate" approach has been adopted in order to provoke discussion. Even so, there are bound to be areas in which the informed reader disagrees with what is said. Such things are healthy and help to improve our understanding. However, a sincere effort has been made to make sure that nowhere are the clear results of a particular piece of research in any way misrepresented. The main object of the book is to provide stimulus for thought and so to further the progress of muscle research. An attempt has been made to separate what has actually been proved from what is commonly assumed to occur during muscular contraction. For this reason, it will be found that many new ideas are presented as are many new interpretations of data that have been available for some time. Although the reference list is very extensive, no attempt has been made to provide encyclopedic coverage of every paper that has ever been written on muscle

6 PREFACE Vll structure. Rather, the book provides an account of the present state of the art, and it draws on those papers that have been most instrumental in establishing particular features of muscle. It is hoped that the book will therefore provide a useful handbook on muscle structure for the 1980s. Finally, there are several people, in addition to those listed in the Acknowledgments section, to whom the Author owes a great debt of gratitude that will be very hard to repay. Expert opinions on various chapters have been given by Drs. Arthur Elliott, Gerald Offer, Pauline Bennett, John Kendrick-Jones, and Ed O'Brien, and their very valuable comments have much improved the text. Needless to say any errors that remain are the fault of the Author. I also acknowledge gladly the help of the members of my research group (the Biopolymer Group) here at Imperial College. These are Drs. Pradeep Luther and Alan Freundlich, Mr. Peter Munro, and Mrs. Elizabeth Jelinek. Apart from providing some of the figures used in the book, they have all helped in its preparation and thus eased the burden on the Author. But I would like to put in writing here my indebtedness to my own family: to my parents, who made all things possible and who have never failed in their encouragement; and to my wife, Melanie, and children, Deborah, Emily, Katherine, and Elizabeth, who have sustained me throughout and who have given up nearly four years of normal family life so that this book could be written. It is to them that the book is dedicated. London John M. Squire

7 Acknowledgments I am happy to acknowledge here the contribution that many individuals and publishers have made to this book; without their help it could not have been produced. Many individuals very readily sent copies of their original plates and diagrams for inclusion in the book. These include: Dr. Doreen Ashhurst, Dr. Carolyn Cohen, Dr. Roger Craig, Dr. Arthur Elliott, Dr. Marshall Elzinga, Dr. Bruce Fraser, Dr. Alan Freundlich, Dr. Margaret Ann Goldstein, Dr. John Haselgrove, Professor Sir Andrew Huxley, Dr. Aaron Klug, Dr. Jack Lowy, Dr. Pradeep Luther, Dr. Peter Munro, Dr. Gerald Offer, Dr. David Parry, Professor Frank Pepe, Professor Mike Reedy, Dr. Michael Sjostrom, Dr. Vic Small, Dr. Apolinary Sobieszek, Professor Andrew Somlyo, Dr. Peter Vibert, and Dr. John Wray. My thanks are due to them for their generosity. I am also indebted to the following publishers for their permission to reproduce copyrighted material here: Academic Press, Annual Reviews, Inc., Cold Spring Harbor Laboratory, Journal of Physiology, Longmans, Macmillan Journals, Ltd., North-Holland Press, Palo Alto Medical Research Foundation, Pergamon Press, Prentice Hall, Inc., Rockefeller University Press, Science, and The Royal Society. All figures in the book are original work unless stated otherwise in the caption. Much of the original work reported here was part of the research of the Biopolymer Group at Imperial College which has been generously supported by Project Grants from the Science Research Council, the Medical Research Council, and also from the Muscular Dystrophy Association of America. I am pleased to be able to acknowledge the help of these grant-giving bodies in supporting the Biopolymer Group. ix

8 x ACKNOWLEDGMENTS Finally, there are a few people who have helped me enormously by carrying out many of the time-consuming clerical aspects of producing a book such as this. In particular, I acknowledge with thanks the two typists, Mrs. Eileen Williams and Mrs. Shelagh Vaughan-Davies, who transformed my original hand-written notes into a presentable manuscript, and also Mrs. Elizabeth Jelinek, Mr. Jeff Harford, and my wife, Melanie, for their invaluable help in compiling reference lists and other related material. JMS

9 Contents 1. Introduction 1.1. Introduction: Muscles and Movement Classification of Muscle Types Vertebrate Skeletal Muscle Introduction The Sarcomere The Sliding Filament Model Force Generation Introduction to Muscle Physiology l.4.l. Contractile Response in Vertebrate Skeletal Muscles Comparative Innervation and Response in Different Muscles Excitation-Contraction Coupling l.4.4. The Energy Supply Classification of Vertebrate Fiber Types The Molecular Biophysicist's Approach to Muscle X-Ray Diffraction Methods in Muscle Research 2.1. Introduction Principles of Diffraction Interference of Waves Diffraction from Periodic Arrays Diffraction from Two-Dimensional Arrays xi

10 XlI CONTENTS Diffraction from Three-dimensional Arrays: Crystals Diffraction from Helical Structures Importance of Helices The Continuous Helix... ; The Discontinuous Helix Complex Helical Molecules Three-Dimensional Arrays of Helical Molecules Summary Multistranded Helices The Jargon of X-Ray Crystallography Practical X-Ray Diffraction Methods Introduction Focusing X-Ray Cameras Specimen Mounting for X-Ray Diffraction Methods of Recording the Diffraction Pattern X-Ray Generators Muscle Preparation, Electron Microscopy, and Image Analysis 3.1. Introduction Muscle Dissection and Initial Treatment Dissection Adjustment of Sarcomere Length Glycerol-Extracted Muscle Single Fibers Preparative Methods in Biological Electron Microscopy Embedding Methods Negative Staining and Shadowing of Isolated Particles Cryosectioning and Other Freezing Methods Biological Electron Microscopy Introduction Basic Electron Microscope Design Image Formation in the Electron Microscope Contrast Enhancement in Biological Specimens Specimen Deterioration in the Electron Microscope III 3.5. Methods of Image Analysis Introduction Photographic Methods Automatic Image-Averaging Methods

11 CONTENTS Xlll Optical Diffraction Image Averaging by Optical Diffraction... : Computer Methods and Three-Dimensional Reconstruction Protein Conformation and Characterization 4.1. Amino Acids, Polypeptides, and Proteins Regular Protein Conformations Basic Ideas The f3 Conformation The a-helix Diffraction from an a-helix Structures of Synthetic a-helical Polypeptides Structure of Fibrous a-proteins Introduction: The Coiled Coil Diffraction from a Coiled-Coil Structure Evaluation of the Coiled-Coil Model Three-Dimensional Packing of Coiled-Coil Molecules Globular Proteins General Description Levels of Structure Structural Influence of Specific Amino Acids Thin Filament Structure and Regulation 5.1. Introduction Actin Characterization of G-Actin F-Actin Formation and Structure Three-Dimensional Reconstruction from Paracrystals of F -Actin Actin Interactions Tropomyosin Preliminary Characterization of Tropomyosin Analysis of Tropomyosin Crystals and Tactoids Amino Acid Sequence and Structure of Tropomyosin Structure of Actin Filaments Containing Tropomyosin

12 xiv CONTENTS 5.4. Troponin... _ Components of the Troponin Complex Properties of the Whole Troponin Complex Location of Troponin on Tropomyosin and in the Thin Filament Thin Filament Structure and Regulation X-Ray Diffraction Evidence for Changes in Thin Filament Structure during Regulation Analysis of the Observed Changes A Model for Thin Filament Regulation Structural Details of the Regulation Scheme Further Aspects of Thin Filament Regulation Structure, Components, and Interactions of the Myosin Molecule 6.1. Introduction Characterization of the Myosin Molecule Studies of the Molecular Size and Shape Proteolytic Fragments of Myosin The Subunit Structure of Myosin Shape and Size of the Myosin Head Flexibility of the Myosin Molecule Aggregation of Myosin and its Subfragments Introduction.. : Formation of Synthetic Myosin Filaments Formation of Myosin Segments Paracrystals of the Myosin Rod and Its Subfragments Studies of Myosin Aggregates in Solution Conclusion Vertebrate Skeletal Muscle 7.1. Introduction: Structure of the Sarcomere Introduction Lateral Order in the Sarcomere Axial Periodicities in the Sarcomere Thick Filament Symmetry and the Transverse Structure of the A-Band Introduction

13 CONTENTS xv Biochemical Evidence on Myosin Content Symmetry Evidence from the Myosin Superlattice Evidence from Electron Microscopy The Nature of the A-Band Superlattice X-Ray Diffraction Evidence on the Myosin Cross-Bridge Arrangement in Relaxed Muscle Discussion Components and Axial Structure of the A-Band A-Band Components Structure of the M-Region Location of C Protein in the A-Band General Structure of the Bridge Region Analysis of the C-Protein Periodicity Structure of the I-Band General Description of the I-Band Structure of the Z-Band The N-Lines The Three-Dimensional Structure of the Sarcomere Comparative Ultra structures of Diverse Muscle Types 8.1. Introduction Arthropod Muscles General Description The Thick Filaments in Insect Flight Muscles I-Band Structure in Insect Flight Muscle Discussion: Details of Other Arthropod Muscles Molluscan Muscles Introduction Structure of Scallop Striated Adductor Muscle Structure of Molluscan Smooth Muscles Structure of Paramyosin Filaments Vertebrate Smooth Muscles Introduction Structure of the Myosin Ribbons Discussion Obliquely Striated Muscles Introduction Myofilament Structure and Arrangement Discussion

14 XVI CONTENTS 9. Molecular Packing in Myosin-Containing Filaments 9.1. Introduction The General Model of Myosin Filament Structure Summary of the Structural Properties of Myosin Molecules Myosin Packing in Uniform Layers Packing in a Planar Sheet Packing in Cylindrical Myosin Filaments Para myosin Filament Structure Subfilament Models of Myosin Packing Introduction Three-Stranded Filaments Multistranded Filaments Results from Crustacean Muscles Detailed Models of Vertebrate Skeletal Muscle Myosin Filaments Myosin Packing in the M-Region and Filament Tip Extra Proteins and Filament Length Determination Models for the Myosin Filaments in Vertebrate Smooth Muscle Discussion Summary Models with Nonequivalent Myosin Molecules Structure of the Myosin Molecule Conclusion Structural Evidence on the Contractile Event Introduction Background The Sliding Filament Model, Independent Force Generators, and Cycling Cross Bridges Biochemical Kinetics of the Actomyosin ATPase Structure of Defined Static States Cross-Bridge Configurations in Relaxed Muscle X-Ray Diffraction Evidence on Rigor Muscle Modeling of the Rigor State: Introduction Modeling of the Rigor State: Insect Flight Muscle Modeling of the Rigor State: Vertebrate Muscle

15 CONTENTS xvii Evidence for Structural Changes during Contraction Vertebrate Striated Muscles Insect Flight Muscle Modeling: Changes in Myosin Filament Structure Modeling: The Meridional Pattern and the Observed Spacing Changes Modeling: The Equatorial Diffraction Data Changes in the Thin Filaments Artificially Modified Muscle Structures Introduction The Effects of Different ATP Analogues S-l Labeling Studies Scallop Muscle Summary Discussion: Modeling the Contractile Event Introduction Evidence from Mechanical Experiments Early Experiments A. F. Huxley'S 1957 Model... '" Podolsky's Model A. F. Huxley and Simmons' Model Insect Flight Muscle Equatorial X-Ray Diffraction Evidence on Cross-Bridge Kinetics Evidence on the Number of Attached Bridges Interpretation of Equatorial Diffraction Data The Time Course of Cross-Bridge Movement Scenarios for the Cross-Bridge Cycle Location of the Instantaneous Cross-Bridge Elasticity Structural Steps in the Cross-Bridge Cycle Conclusion: Future Prospects References Suggested Further Reading 685 Index