Regulation of the actin-activated ATPase activity of Acanthamoeba myosin I by cross-linking actin filaments

The actin-activated Mg²⁺-ATPase activity of phosphorylated Acanthamoeba myosin I was previously shown to be cooperatively dependent on the myosin concentration (Albanesi, J.P., Fujisaki, H., and Korn, E.D. (1985) J. Biol. Chem. 260, 11174-11179). This observation was rationalized by assuming that myosin I contains a high-affinity and a low-affinity F-actin-binding site and that binding at the low-affinity site is responsible for the actin-activated ATPase activity. Therefore, enzymatic activity would correlate with the cross-linking of actin filaments by myosin I, and the cooperative increase in specific activity at high myosin:actin ratios would result from the fact that cross-linking by one myosin molecule would increase the effective F-actin concentration for neighboring myosin molecules. This model predicts that high specific activity should occur at myosin:actin ratios below that required for cooperative interactions if the actin filaments are cross-linked by catalytically inert cross-linking proteins. This prediction has been confirmed by cross-linking actin filaments with either of three gelation factors isolated from Acanthamoeba, one of which has not been previously described, or by enzymatically inactive unphosphorylated Acanthamoeba myosin I.