This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Muscle contraction requires the cyclic interactions of myosin heads from the thick filaments with actin, the major component of the thin filament. We seek to visualize, in atomic detail, how the myosin motor works by obtaining snapshots of the molecule in different states of contraction. Over the past few years, we have determined the atomic structures of the scallop myosin head in a variety of weak actin-binding states, as well as a piece of the myosin tail that dimerizes the molecule. The myosin motor is switched on and off by calcium ions which bind to troponin/tropomyosin on the thin filament in vertebrate skeletal muscles. A complete atomic description of these structures is sought to understand how motor activity is controlled. We have now determined the atomic structure of nearly 80% of the striated muscle tropomyosin molecule. To accomplish our goals, a complete understanding of the interactions between the component proteins of muscle must be obtained. Our long term aim is to obtain complexes of these proteins that are suitable for X-ray analysis. Currently, we have in hand crystals of specific isoforms of myosin and tropomyosin whose unusual properties are especially suited for understanding actin binding. We have recently crystallized, collected medium resolution X-ray data, and partially refined the structure of the squid myosin head, which for the first time has shown the rigor-like strong actin-binding conformation of a muscle myosin. We have also obtained crystals of a non-muscle tropomyosin isoform that binds actin much tighter than conventional tropomyosins. Obtaining high resolution synchrotron data from these and related crystals will be a critical step towards understanding the features of these molecules necessary for their functions. Many muscle diseases are due to defects in the myosin motor, the actin component, and in the control machinery. Our studies aim to establish information for structure-based drug design.
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