title: Effect of actin filament length and filament number concentration on the actin-activated ATPase activity of Acanthamoeba myosin I.
abstract: The actin-activated Mg2+-ATPase activities of phosphorylated Acanthamoeba myosins IA and IB were previously found to have a highly cooperative dependence on myosin concentration (Albanesi, J. P., Fujisaki, H., and Korn, E. D. (1985) J. Biol. Chem. 260, 11174-11179). This behavior is reflected in the requirement for a higher concentration of F-actin for half-maximal activation of the myosin Mg2+-ATPase at low ratios of myosin:actin (noncooperative phase) than at high ratios of myosin:actin (cooperative phase). These phenomena could be explained by a model in which each molecule of the nonfilamentous myosins IA and IB contains two F-actin-binding sites of different affinities with binding of the lower affinity site being required for expression of actin-activated ATPase activity. Thus, enzymatic activity would coincide with cross-linking of actin filaments by myosin. This theoretical model predicts that shortening the actin filaments and increasing their number concentration at constant total F-actin should increase the myosin concentration required to obtain the cooperative increase in activity and should decrease the F-actin concentration required to reach half-maximal activity at low myosin:actin ratios. These predictions have been experimentally confirmed by shortening actin filaments by addition of plasma gelsolin, an F-actin capping/severing protein. In addition, we have found that actin "filaments" as short as the 1:2 gelsolin-actin complex can significantly activate Acanthamoeba myosin I.
mesh_terms: D000199:Actins / Q000494:pharmacology*; D000205:Actomyosin / Q000378:metabolism; D000656:Amoeba / Q000201:enzymology*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D018260:Gelsolin; D046911:Macromolecular Substances; D008840:Microfilament Proteins / Q000494:pharmacology; D008854:Microscopy, Electron; D009218:Myosins / Q000378:metabolism*; D013329:Structure-Activity Relationship
chemical_list: D000199:Actins; D002135:Calcium-Binding Proteins; D018260:Gelsolin; D046911:Macromolecular Substances; D008840:Microfilament Proteins; D000205:Actomyosin; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins