/ak"teuhn/, n. Biochem.a globulin that is present in muscle plasma and that in connection with myosin plays an important role in muscle contraction.[1940-45; perh. ACT + -IN2]
* * *One of two proteins responsible for contraction of muscle cells and the motility of other cells.It occurs as a monomer, G-actin, a globular protein, and in living cells as a polymer, F-actin, which resembles two strings of beads twisted around each other into thin filaments. The filaments occur in regular structures, alternated and interwoven with thick filaments that contain myosin, the other major muscle protein. The thick and thin filaments slide past each other, under the control of calcium ions, resulting in contraction (shortening) and relaxation (lengthening) of the muscle cells.
* * *protein that is an important contributor to the contractile property of muscle and other cells. It exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin), the form involved in muscle contraction. In muscle, two long strands of beadlike actin molecules are twisted together to form a thin filament, bundles of which alternate and interdigitate with bundles of thick filaments formed of myosin, the most abundant protein found in muscle. Two other muscle proteins, tropomyosin and troponin, regulate the temporary fusion of actin and myosin; this fusion results in the contraction of muscle.Actin and myosin have been found in dozens of other (nonmuscle) cells and are believed to be responsible for the contractile properties of animal cells generally.
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