Regulation of myosin light chain kinase by reversible phosphorylation and calcium-calmodulin.

1. Myosin light chain kinases from smooth muscle and platelets can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase. 2) Phosphorylation of both kinases, in the absence of calmodulin, markedly decreases kinase activity. 3) The decrease in smooth muscle myosin kinase activity is due to a decreased affinity of the phosphorylated kinase for calmodulin. 4) Dephosphorylation of the smooth muscle kinase by a phosphatase isolated from smooth muscle restores the affinity of the kinase for calmodulin.