The Mechanism of Regulation of Smooth Muscle Myosin by Phosphorylation

Abstract

This chapter focuses on the mechanism of regulation of smooth muscle myosin by phosphorylation. The contraction of smooth muscle and all other types of muscle is brought about by the cyclic interaction of myosin with actin. These two proteins also exist in virtually all eukaryotic cells and are thought to be responsible for a wide variety of motile functions such as ameboid motion, organelle translocation, endocytosis, phagocytosis, cytokinesis, and capping. The purified myosin exhibits several properties in vitro that are predicted by its in vivo characteristics. It can self-associate at physiological ionic strength through its rod portion to form filaments very similar to those seen inside the muscle. There are several models that can be proposed for regulation of smooth muscle myosin by phosphorylation. One of these is that phosphorylation directly affects the rate of ATP hydrolysis. The rate of these steps can be measured by monitoring the tryptophan fluorescence of myosin. It has been shown that there is an increase in the intrinsic tryptophan fluorescence of some myosins, including turkey gizzard smooth muscle myosin, that accompanies the hydrolysis step.