actin A contractile protein found in muscle tissue, in which it occurs in the form of filaments (called thin filaments). Each thin filament consists of two chains of globular actin molecules, around which is twisted a strand of tropomyosin and interspersed troponin. Units of muscle fibre (see sarcomere) consist of actin and myosin filaments, which interact to bring about muscle contraction (see also sliding filament theory). Actin is also found in the microfilaments that form part of the cytoskeleton of all cells.

actin A globular protein of relative molecular mass 60 000 which is a major component of microfilaments generally, and which is especially important in the myofibrils of striated muscle cell.

actin (ak-tin) n. a protein, found in muscle, that plays an important role in the process of contraction. See striated muscle.