The relationship between calmodulin binding and phosphorylation of smooth muscle myosin kinase by the catalytic subunit of 3‘:5‘ cAMP-dependent protein kinase.

Smooth muscle myosin light chain kinase, a calmodulin-dependent enzyme, binds 1 mol of calmodulin/mol of kinase in the presence of calcium (Adelstein, R. S., and Klee, C. B. (1981) J. Biol. Chem. 256, in press. 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 two 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 10-20-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 (Pato, M. D., and Adelstein, R. S. (1980) J. Biol. Chem. 255, 6535-6538) 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 two sites that can be phosphorylated in myosin kinase was confirmed by tryptic digestion of denatured myosin kinase.