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Thin Filaments in Skeletal Muscle Fibers

From the Z discs, the thin filaments extend inward toward the center of a sarcomere, where they partially overlap with the thick filaments. Approximately 300-400 G actin (= globular actin) proteins make up most of a thin filament. The G actins attach end to end (= polymerize) to form two twisted strands. Together, the two strands are referred to as F actin (= fibrous actin). Large nebulin proteins span the length of the F actins and help direct their assembly. The nebulins seem to function as “measuring sticks” that make sure the F actins extend to the proper length. After the F actins are constructed, capping proteins prevent them from depolymerizing or unraveling. The medial end of an F actin is presumably capped by tropomodulin and the lateral end by Cap Z. The Cap Z proteins are attached to the Z disc by actinin proteins. Each of the G actins has a binding site for a myosin cross-bridge or head. When a muscle fiber is at rest, the myosin binding sites are covered by a series of tropomyosin proteins. A troponin molecule is associated with each tropomyosin. When bound to calcium, the troponins move the tropomyosins so contraction can take place.