The Structural Basis of Muscular Contraction

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 inter­ est. 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 pro­ vided 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 fila­ ments 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 resolu­ tion 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.

1. Introduction.- 1.1. Introduction: Muscles and Movement.- 1.2. Classification of Muscle Types.- 1.3. Vertebrate Skeletal Muscle.- 1.3.1. Introduction.- 1.3.2. The Sarcomere.- 1.3.3. The Sliding Filament Model.- 1.3.4. Force Generation.- 1.4. Introduction to Muscle Physiology.- 1.4.1. Contractile Response in Vertebrate Skeletal Muscle.- 1.4.2. Comparative Innervation and Response in Different Muscles.- 1.4.3. Excitation-Contraction Coupling.- 1.4.4. The Energy Supply.- 1.4.5. Classification of Vertebrate Fiber Types.- 1.5. The Molecular Biophysicist’s Approach to Muscle.- 2. X-Ray Diffraction Methods in Muscle Research.- 2.1. Introduction.- 2.2. Principles of Diffraction.- 2.2.1. Interference of Waves.- 2.2.2. Diffraction from Periodic Arrays.- 2.2.3. Diffraction from Two-Dimensional Arrays.- 2.2.4. Diffraction from Three-dimensional Arrays: Crystals.- 2.3. Diffraction from Helical Structures.- 2.3.1. Importance of Helices.- 2.3.2. The Continuous Helix.- 2.3.3. The Discontinuous Helix.- 2.3.4. Complex Helical Molecules.- 2.3.5. Three-Dimensional Arrays of Helical Molecules.- 2.3.6. Summary.- 2.3.7. Multistranded Helices.- 2.4. The Jargon of X-Ray Crystallography.- 2.5. Practical X-Ray Diffraction Methods.- 2.5.1. Introduction.- 2.5.2. Focusing X-Ray Cameras.- 2.5.3. Specimen Mounting for X-Ray Diffraction.- 2.5.4. Methods of Recording the Diffraction Pattern.- 2.5.5. X-Ray Generators.- 3. Muscle Preparation, Electron Microscopy, and Image Analysis.- 3.1. Introduction.- 3.2. Muscle Dissection and Initial Treatment.- 3.2.1. Dissection.- 3.2.2. Adjustment of Sarcomere Length.- 3.2.3. Glycerol-Extracted Muscle.- 3.2.4. Single Fibers.- 3.3. Preparative Methods in Biological Electron Microscopy.- 3.3.1. Embedding Methods.- 3.3.2. Negative Staining and Shadowing of Isolated Particles.- 3.3.3. Cryosectioning and Other Freezing Methods.- 3.4. Biological Electron Microscopy.- 3.4.1. Introduction.- 3.4.2. Basic Electron Microscope Design.- 3.4.3. Image Formation in the Electron Microscope.- 3.4.4. Contrast Enhancement in Biological Specimens.- 3.4.5. Specimen Deterioration in the Electron Microscope.- 3.5. Methods of Image Analysis.- 3.5.1. Introduction.- 3.5.2. Photographic Methods.- 3.5.3. Automatic Image-Averaging Methods.- 3.5.4. Optical Diffraction.- 3.5.5. Image Averaging by Optical Diffraction.- 3.5.6. Computer Methods and Three-Dimensional Reconstruction.- 4. Protein Conformation and Characterization.- 4.1. Amino Acids, Polypeptides, and Proteins.- 4.2. Regular Protein Conformations.- 4.2.1. Basic Ideas.- 4.2.2. The ? Conformation.- 4.2.3. The a?-Helix.- 4.2.4. Diffraction from an ?-Helix.- 4.2.5. Structures of Synthetic ?-Helical Polypeptides.- 4.3. Structure of Fibrous ?-Proteins.- 4.3.1. Introduction: The Coiled Coil.- 4.3.2. Diffraction from a Coiled-Coil Structure.- 4.3.3. Evaluation of the Coiled-Coil Model.- 4.3.4. Three-Dimensional Packing of Coiled-Coil Molecules.- 4.4. Globular Proteins.- 4.4.1. General Description.- 4.4.2. Levels of Structure.- 4.4.3. Structural Influence of Specific Amino Acids.- 5. Thin Filament Structure and Regulation.- 5.1. Introduction.- 5.2. Actin.- 5.2.1. Characterization of G-Actin.- 5.2.2. F-Actin Formation and Structure.- 5.2.3. Three-Dimensional Reconstruction from Paracrystals of F-Actin.- 5.2.4. Actin Interactions.- 5.3. Tropomyosin.- 5.3.1. Preliminary Characterization of Tropomyosin.- 5.3.2. Analysis of Tropomyosin Crystals and Tactoids.- 5.3.3. Amino Acid Sequence and Structure of Tropomyosin.- 5.3.4. Structure of Actin Filaments Containing Tropomyosin.- 5.4. Troponin.- 5.4.1. Components of the Troponin Complex.- 5.4.2. Properties of the Whole Troponin Complex.- 5.4.3. Location of Troponin on Tropomyosin and in the Thin Filament.- 5.5. Thin Filament Structure and Regulation.- 5.5.1. X-Ray Diffraction Evidence for Changes in Thin Filament Structure during Regulation.- 5.5.2. Analysis of the Observed Changes.- 5.5.3. A Model for Thin Filament Regulation.- 5.5.4. Structural Details of the Regulation Scheme.- 5.6. Further Aspects of Thin Filament Regulation.- 6. Structure, Components, and Interactions of the Myosin Molecule.- 6.1. Introduction.- 6.2. Characterization of the Myosin Molecule.- 6.2.1. Studies of the Molecular Size and Shape.- 6.2.2. Proteolytic Fragments of Myosin.- 6.2.3. The Subunit Structure of Myosin.- 6.2.4. Shape and Size of the Myosin Head.- 6.2.5. Flexibility of the Myosin Molecule.- 6.3. Aggregation of Myosin and its Subfragments.- 6.3.1. Introduction.- 6.3.2. Formation of Synthetic Myosin Filaments.- 6.3.3. Formation of Myosin Segments.- 6.3.4. Paracrystals of the Myosin Rod and Its Subfragments.- 6.3.5. Studies of Myosin Aggregates in Solution.- 6.4. Conclusion.- 7. Vertebrate Skeletal Muscle.- 7.1. Introduction: Structure of the Sarcomere.- 7.1.1. Introduction.- 7.1.2. Lateral Order in the Sarcomere.- 7.1.3. Axial Periodicities in the Sarcomere.- 7.2. Thick Filament Symmetry and the Transverse Structure of the A-Band.- 7.2.1. Introduction.- 7.2.2. Biochemical Evidence on Myosin Content.- 7.2.3. Symmetry Evidence from the Myosin Superlattice.- 7.2.4. Evidence from Electron Microscopy.- 7.2.5. The Nature of the A-Band Superlattice.- 7.2.6. X-Ray Diffraction Evidence on the Myosin Cross-Bridge Arrangement in Relaxed Muscle.- 7.2.7. Discussion.- 7.3. Components and Axial Structure of the A-Band.- 7.3.1. A-Band Components.- 7.3.2. Structure of the M-Region.- 7.3.3. Location of C Protein in the A-Band.- 7.3.4. General Structure of the Bridge Region.- 7.3.5. Analysis of the C-Protein Periodicity.- 7.4. Structure of the I-Band.- 7.4.1. General Description of the I-Band.- 7.4.2. Structure of the Z-Band.- 7.4.3. The N-Lines.- 7.5. The Three-Dimensional Structure of the Sarcomere.- 8. Comparative Ultrastructures of Diverse Muscle Types.- 8.1. Introduction.- 8.2. Arthropod Muscles.- 8.2.1. General Description.- 8.2.2. The Thick Filaments in Insect Flight Muscles.- 8.2.3. I-Band Structure in Insect Flight Muscle.- 8.2.4. Discussion: Details of Other Arthropod Muscles.- 8.3. Molluscan Muscles.- 8.3.1. Introduction.- 8.3.2. Structure of Scallop Striated Adductor Muscle.- 8.3.3. Structure of Molluscan Smooth Muscles.- 8.3.4. Structure of Paramyosin Filaments.- 8.4. Vertebrate Smooth Muscles.- 8.4.1. Introduction.- 8.4.2. Structure of the Myosin Ribbons.- 8.4.3. Discussion.- 8.5. Obliquely Striated Muscles.- 8.5.1. Introduction.- 8.5.2. Myofilament Structure and Arrangement.- 8.6. Discussion.- 9. Molecular Packing in Myosin-Containing Filaments.- 9.1. Introduction.- 9.1.1. The General Model of Myosin Filament Structure.- 9.1.2. Summary of the Structural Properties of Myosin Molecules.- 9.2. Myosin Packing in Uniform Layers.- 9.2.1. Packing in a Planar Sheet.- 9.2.2. Packing in Cylindrical Myosin Filaments.- 9.2.3. Paramyosin Filament Structure.- 9.3. Subfilament Models of Myosin Packing.- 9.3.1. Introduction.- 9.3.2. Three-Stranded Filaments.- 9.3.3. Multistranded Filaments.- 9.3.4. Results from Crustacean Muscles.- 9.4. Detailed Models of Vertebrate Skeletal Muscle Myosin Filaments.- 9.4.1. Myosin Packing in the M-Region and Filament Tip.- 9.4.2. Extra Proteins and Filament Length Determination.- 9.5. Models for the Myosin Filaments in Vertebrate Smooth Muscle.- 9.6. Discussion.- 9.6.1. Summary.- 9.6.2. Models with Nonequivalent Myosin Molecules.- 9.6.3. Structure of the Myosin Molecule.- 9.6.4. Conclusion.- 10. Structural Evidence on the Contractile Event.- 10.1. Introduction.- 10.1.1. Background.- 10.1.2. The Sliding Filament Model, Independent Force Generators, and Cycling Cross Bridges.- 10.1.3. Biochemical Kinetics of the Actomyosin ATPase.- 10.2. Structure of Defined Static States.- 10.2.1. Cross-Bridge Configurations in Relaxed Muscle.- 10.2.2. X-Ray Diffraction Evidence on Rigor Muscle.- 10.2.3. Modeling of the Rigor State: Introduction.- 10.2.4. Modeling of the Rigor State: Insect Flight Muscle.- 10.2.5. Modeling of the Rigor State: Vertebrate Muscle.- 10.3. Evidence for Structural Changes during Contraction.- 10.3.1. Vertebrate Striated Muscles.- 10.3.2. Insect Flight Muscle.- 10.3.3. Modeling: Changes in Myosin Filament Structure.- 10.3.4. Modeling: The Meridional Pattern and the Observed Spacing Changes.- 10.3.5. Modeling: The Equatorial Diffraction Data.- 10.3.6. Changes in the Thin Filaments.- 10.4. Artificially Modified Muscle Structures.- 10.4.1. Introduction.- 10.4.2. The Effects of Different ATP Analogues.- 10.4.3. S-1 Labeling Studies.- 10.4.4. Scallop Muscle.- 10.5. Summary.- 11. Discussion: Modeling the Contractile Event.- 11.1. Introduction.- 11.2. Evidence from Mechanical Experiments.- 11.2.1. Early Experiments.- 11.2.2. A. F. Huxley’s 1957 Model.- 11.2.3. Podolsky’s Model.- 11.2.4. A. F. Huxley and Simmons’ Model.- 11.2.5. Insect Flight Muscle.- 11.3. Equatorial X-Ray Diffraction Evidence on Cross-Bridge Kinetics.- 11.3.1. Evidence on the Number of Attached Bridges.- 11.3.2. Interpretation of Equatorial Diffraction Data.- 11.3.3. The Time Course of Cross-Bridge Movement.- 11.4. Scenarios for the Cross-Bridge Cycle.- 11.4.1. Location of the Instantaneous Cross-Bridge Elasticity.- 11.4.2. Structural Steps in the Cross-Bridge Cycle.- 11.5. Conclusion: Future Prospects.- References.- Suggested Further Reading.