AUTOPHOSPHORYLATION-INDEPENDENT ACTIVATION OF ACANTHAMOEBA MYOSIN-I HEAVY-CHAIN KINASE BY PLASMA-MEMBRANES

The three isoforms of Acanthamoeba myosin I (non-filamentous myosin wi th only a single heavy chain) express actin-activated Mg2+-ATPase acti vity only when phosphorylated at a single site by myosin I heavy chain kinase. The kinase is activated by autophosphorylation that is greatl y stimulated by acidic phospholipids. Substantial fractions of the thr ee myosins I and the kinase are associated in situ with membranes, and all four enzymes bind to purified membranes in vitro. We now report t hat when kinase and myosin I are incubated together with phosphatidyls erine vesicles not only does the kinase autophosphorylate more rapidly than soluble kinase in the absence of phosphatidylserine but that, pr obably as a result, the kinase phosphorylates myosin I more rapidly th an soluble kinase phosphorylates soluble myosin I. Similarly, plasma m embrane-bound kinase phosphorylates membrane-bound myosin I and activa tes its actin-activated Mg2+-ATPase activity more rapidly than soluble kinase phosphorylates and activates soluble myosin I in the absence o f membranes. However, the enhanced activity of membrane-bound kinase ( which is comparable to the activity of kinase in the presence of phosp hatidylserine) is not due to autophosphorylation of the membrane-bound kinase, which is very much slower than for kinase activated by phosph atidylserine vesicles.