Mary H. Nunnally

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.

4 Calmodulin-Dependent Protein Kinases

Publisher Summary This chapter discusses the broad specificity of calmodulin-dependent protein kinases in relation to protein substrates. Myosin light chain kinases are asymmetric, monomeric enzymes that vary considerably in mass depending upon tissue source and animal species. They have a calmodulin-binding domain that is distinct from the catalytic domain. In addition, the smooth-muscle myosin light chain kinases contain two sites that are phosphorylated by cyclic AMP-dependent protein kinase. These sites, one of which effects calmodulin activation of the enzyme, are not in the catalytic or calmodulin-binding domains. Limited information is available on the catalytic mechanism of myosin light chain kinases or characterization of specific chemical and physical interactions between enzyme and substrates. Analyses of the catalytic properties indicate a sequential kinetic mechanism for skeletal-muscle myosin light chain kinase. Evidence has been presented in kinetic studies with rabbit skeletal-muscle myosin light chain kinase for a rapid equilibrium bi-bi kinetic mechanism. In addition, it appears that the kinase can form a dead-end complex with ADP and unphosphorylated myosin light chain.