Myosin light chain kinases and myosin phosphorylation in skeletal muscle.

Myosin light chain kinases appear to exist as a family of tissue- and species-specific isozymes. The skeletal muscle kinases, although differing widely in molecular weight among vertebrate species, are catalytically similar and antigenically related. The smooth muscle kinases are catalytically and antigenically distinct from the skeletal muscle kinases. The functional basis for the existence of myosin light chain kinase isozymes has not been determined. Phosphorylation of fast-twitch skeletal muscle myosin P-light chain occurs at physiologically relevant contraction frequencies and durations, and the extent of P-light chain phosphorylation correlates with enhancement of isometric twitch tension in fast-twitch muscle under a variety of experimental conditions. Phosphorylation of myosin P-light chain in vertebrate fast-twitch skeletal muscle may play a modulatory role in calcium regulation of muscle contractility.