The control of vertebrate muscle contraction is regulated by the binding and unbinding of Ca2+ to the thin-filament protein troponin (Tn). Upon Ca2+ binding, structural changes occur in the thin filament which mediate the interaction of myosin and actin in the presence of ATP. Dr. Mendelson proposes to study these changes and also those that occur when myosin heads are bound to the thin filament. Neutron scattering and diffraction will be used to study filamentous samples in vitro. Selective deuteration of one or more of the members of a complex will allow either the protonated or deuterated member(s) to be rendered "invisible" to neutrons by solvent contrast matching. Structural changes of the visible components can thus be investigated in situ with little ambiguity. The average cross- helix distance between the tropomyosin (Tm) molecules and their ordering in thin filaments will be investigated + Ca2 and with varying amounts of myosin subfragment-one present. The average intra-pair separation between Tn molecules will be studied +Ca2+. Changes in the structure of F-actin in situ will also be investigated +Ca2+. Phase information will be collected on the thin filament with the aim of understanding the entire thin filament structure with and without Ca2+
