Second Messenger Effects on the Myosin Phosphorylation System in Smooth Muscle

Abstract

All smooth muscle cells contain the contractile proteins actin, myosin, and tropomyosin (Hartshorne, 1987). The enzyme myosin is the primary protein of the thick filament in smooth muscle and is composed of two high molecular weight subunits, or heavy chains, and two each of two types of low molecular weight subunits, or light chains. The molecular mass of each heavy chain subunit is about 200 kDa whereas the light chain subunits are 20 and 17 kDa respectively. The native hexameric form of myosin is configured as an intertwined coiled-tail region of two heavy chains embedded in the thick filament and two globular head regions that protrude at regular intervals to form cross bridges. These head regions contain the actin-binding domain, the catalytic site for ATP hydrolysis, and the associated light chain subunits. According to the sliding filament theory of muscle contraction, thick (myosin) and thin (actin and tropomyosin) filaments move past one another. This process is related to the binding of cross bridges to actin and to the hydrolysis of ATP. The sliding filament theory has been developed primarily from detailed investigations of skeletal muscle, but the general organization of thin and thick filaments in smooth muscle is consistent with a similar mechanism of contraction.