Role of Tropomyosin in the Regulation of Contraction in Smooth Muscle

Smooth muscle contraction is due to the interaction of myosin filaments with thin filaments. Thin filaments are composed of actin, tropomyosin, caldesmon and calmodulin in ratios 14:2:1:1. Tissue specific isoforms of alpha and beta tropomyosin are expressed in smooth muscle. Compared with skeletal muscle tropomyosin, the cooperative activation of actomyosin is enhanced by smooth muscle tropomyosin: cooperative unit size is 10 and the equilibrium between on and off states is shifted towards the on state. The smooth muscle-specific actin-binding protein caldesmon, together with calmodulin regulates the activity of the thin filament in response to Ca2+. Caldesmon and calmodulin control the tropomyosin-mediated transition between on and off activity states.