title: A kinetic model for the molecular basis of the contractile activity of Acanthamoeba myosins IA and IB.
abstract: Acanthamoeba myosins IA and IB are single-headed, monomeric molecules consisting of one heavy chain and one light chain. Both have high actin-activated Mg2+-ATPase activity, when the heavy chain is phosphorylated, but neither seems to be able to form the bipolar filaments that are generally thought to be required for actomyosin-dependent contractility. In this paper, we show that, at fixed F-actin concentration, the actin-activated Mg2+-ATPase activities of myosins IA and IB increase about 5-fold in specific activity in a cooperative manner as the myosin concentration is increased. The myosin concentration range over which this cooperative change occurs depends on the actin concentration. More myosin I is required for the cooperative increase in activity at high concentrations of F-actin. The cooperative increase in specific activity at limiting actin concentrations is caused by a decrease in the KATPase for F-actin. The high and low KATPase states of the myosin have about the same Vmax at infinite actin concentration. Both myosins are completely bound to the F-actin long before the Vmax values are reached. Therefore, much of the actin activation must be the result of interactions between F-actin and actomyosin. These kinetic data can be explained by a model in which the cooperative shift of myosin I from the high KATPase to the low KATPase state results from the cross-linking of actin filaments by myosin I. Cross-linking might occur either through two actin-binding sites on a single molecule or by dimers or oligomers of myosin I induced to form by the interaction of myosin I monomers with the actin filaments. The ability of Acanthamoeba myosins IA and IB to cross-link actin filaments is demonstrated in the accompanying paper (Fujisaki, H., Albanesi, J.P., and Korn, E.D. (1985) J. Biol. Chem. 260, 11183-11189).
authors.forename: J P
authors.initials: JP
authors.lastname: Albanesi
authors.forename: H
authors.initials: H
authors.lastname: Fujisaki
authors.forename: E D
authors.initials: ED
authors.lastname: Korn
mesh_terms: D000199:Actins / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000656:Amoeba / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D004789:Enzyme Activation; D007700:Kinetics; D046911:Macromolecular Substances; D009132:Muscles / Q000378:metabolism; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D011817:Rabbits; D013816:Thermodynamics
chemical_list: D000199:Actins; D046911:Macromolecular Substances; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins