Mechanics and models of the myosin motor

In striated muscles, shortening comes about by the sliding movement of thic k filaments, composed mostly of myosin, relative to thin filaments, compose d mostly of actin. This is brought about by cyclic action of 'cross-bridges ' composed of the heads of myosin molecules projecting from a thick filamen t, which attach to an adjacent thin filament, exert force for a limited tim e and detach, and then repeat this cycle further along the filament. Tilt. requisite energy is provided by the hydrolysis of a molecule of adenosine t riphosphate to the diphosphate and inorganic phosphate, the steps of this r eaction being coupled to mechanical events within the cross-bridge. The nat ure of these events is discussed. There is good evidence that one of them i s a change in the angle of tilt of a 'lever arm' relative to the 'catalytic domain' of the myosin head which binds to the actin filament. It is sugges ted here that this event is super posed on a slower, temperature-sensitive change in the orientation of the catalytic domain on the actin filament. Ma ny uncertainties remain.