INHIBITION OF ACANTHAMOEBA MYOSIN-I HEAVY-CHAIN KINASE BY CA2+-CALMODULIN

The actin-activated Mg2+-ATPase activity of Acanthamoeba myosins I depends on phosphorylation of their single heavy chains by myosin I heavy chain kinase. Kinase activity is enhanced >50-fold by autophosphorylation at multiple sites. The rate of kinase autophosphorylation is increased approximately 20-fold by acidic phospholipids independent of the presence of Ca2+ and diglycerides. We show in this paper that Ca2+-calmodulin inhibits phospholipid-stimulated autophosphorylation of myosin I heavy chain kinase and hence also inhibits the catalytic activity of unphosphorylated kinase in the presence of phospholipid. Ca2+-calmodulin does not inhibit kinase activity in the absence of phospholipid. Micromolar Ca2+-calmodulin also inhibits binding of myosin I heavy chain kinase to phospholipid vesicles and purified plasma membranes. Proteolytic removal of a 7-kDa NH2-terminal segment from the 97-kDa kinase prevents binding of both calmodulin and phospholipid; therefore, we propose that they bind to the same or overlapping sites. These data provide a mechanism by which Ca2+ could inhibit the actin-activated Mg2+-ATPase activity of the myosin I isozymes in vivo and thus regulate myosin I-dependent motile activities.