THE RELATIONSHIP BETWEEN CALMODULIN BINDING AND PHOSPHORYLATION OF SMOOTH-MUSCLE MYOSIN KINASE BY THE CATALYTIC SUBUNIT OF 3'-5' CAMP-DEPENDENT PROTEIN-KINASE

Abstract

Smooth muscle myosin light chain kinase, a calmodulin-dependent enzyme, binds 1 mol of calmodulin/mol of kinase in the presence of Ca. This enzyme is a substrate for cAMP-dependent protein kinase whether or not calmodulin is bound. When calmodulin is not bound to myosin kinase, protein kinase incorporates phosphate into 2 sites in myosin kinase. Under these circumstances, phosphorylation markedly lowers the rate of myosin kinase activity. The decrease in myosin kinase activity is due to a 10- to 20-fold increase in the amount of calmodulin necessary for 50% activation of kinase activity. The effect of phosphorylation on the activity of myosin kinase can be reversed by dephosphorylation using a purified phosphatase isolated from smooth muscle. When calmodulin is bound to myosin kinase, phosphate is incorporated into a single site with no effect on myosin kinase activity. The presence of at least 2 sites that can be phosphorylated in myosin kinase was confirmed by tryptic digestion of denatured myosin kinase.