actin a major protein constituent of the thin filaments of muscle and of the microfilaments found in practically all eukaryotic cells – it comprises 5–10% of the protein of such cells (see actin filament). In solutions of low ionic strength, actin is a globular 42 kDa monomer, termed G-actin. At physiological ionic strengths, G-actin polymerizes into a fibrous form, termed F-actin, which resembles two strings of beads wound round each other. F-actin is a helix of actin monomers, with a helix diameter of about 7 nm, the structure repeating at intervals of 36 nm along the helix axis. When a solution of actin is mixed with a solution of the muscle protein myosin, a complex called actomyosin is formed, and the viscosity of the solution increases markedly. This increase in viscosity is reversed by the addition of ATP, which acts to dissociate the actomyosin complex. It is thought that the force of muscle contraction arises from an interaction of actin, myosin, and ATP.

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