Mechanical properties of tropomyosin and implications for muscle regulation.

Tropomyosin is a protein that controls the interactions of actin and myosin as a part of the regulation of muscle contraction. The 420 A long alpha-helical coiled-coil molecules form long filaments, both in muscle and in crystals. The x-ray diffraction data from tropomyosin crystals have indicated large scale motions of the filaments that can be related to the inherent mechanical properties of the molecule, and by extension, to the role of tropomyosin in the cooperative activation of the thin filaments of muscle. Diffuse scattering analysis has provided information about the amplitudes of the motions that has been used to calculate the intrinsic flexibility of the molecule. It can then be shown that each tropomyosin molecule by itself can only mediate interactions of the nearest-neighboring tropomyosin molecules along the filament. The repeating nature of the thin filament, however, allows the entire filament to activate cooperatively.