Electron microscopy of myosin molecules

Myosin is the major contractile protein of muscle, where it exists as a filamentous polymer (about 1.6μm long and 15nm in diameter) containing several hundred myosin molecules. The myosin filaments overlap with filaments of actin (the other main protein of muscle) and contraction is brought about by a sliding of these two filament types past each other. The sliding force is generated by a portion of each myosin molecule (the myosin crossbridge) which projects from the surface of the filament and attaches cyclically to actin in a manner which “rows” the filaments past each other. The energy for rowing comes from the hydrolysis of ATP by the myosin crossbridges, a process that is activated by their interaction with actin. Myosin also occurs in many nonmuscle cells where, together with actin, it generates a variety of motile activities (e.g. cell division) by a mechanism that is probably similar to the sliding mechanism occurring in muscle.