Phosphorylation of myosin as a regulatory mechanism in smooth muscle.

Contractile activity of the smooth muscle cell is regulated by the concentration of intracellular Ca2+. The Ca2+ transients are sensed by the target protein, calmodulin, and via activation of myosin light chain kinase (by Ca2(+)-calmodulin) transmitted to the contractile apparatus. Phosphorylation of myosin increases its actin-activated ATPase activity and in smooth muscle fibers is thought to initiate contraction. The effects of phosphorylation on the conformation of myosin are not established, but at least two areas of the molecule are influenced by phosphorylation of the two light chains. These are at the actin-binding site and at the head-neck junction. The latter site is important in regulating ATPase activity and a working hypothesis is that phosphorylation increases flexibility at this site and facilitates cross-bridge cycling. The phosphorylation theory has extensive experimental support, and is accepted as a major regulatory component in smooth muscle. However, the simplest interpretation of this scheme cannot adequately account for the varied physiological responses. Either there are aspects of the phosphorylation theory that are not considered, or an additional regulatory mechanism is involved. Both possibilities are discussed.