The chemical mechanism of myosin-I: implications for actin-based motility and the evolution of the myosin family of motor proteins.
The Acanthamoeba myosin-IA and myosin-IB molecular motors bind to membranes, so they may produce the force to move organelles and membranes along actin filaments. We have determined the rate constants for the actin-activated myosin-I ATPase by pre-steady state kinetic analysis. ATP binds rapidly to myosin-I and dissociates the enzyme from actin filaments at a rate > 500 s-1. Myosin-I hydrolyzes ATP to ADP and inorganic phosphate (Pi) at 20-50 s-1. Phosphate dissociation is the rate limiting step in the ATPase cycle, 0.01 s-1 for myosin-I alone and at 10 s-1 when myosin-I is bound to actin filaments. ADP dissociation is rapid. Phosphorylation controls the ATPase cycle by increasing the rate of phosphate release from myosin-I bound to actin. At steady state the major species are myosin-ATP and myosin-ADP-Pi, which rapidly bind to and dissociate from actin filaments. During the ATPase cycle myosin-I binds so weakly to actin filaments that it cannot support processive movement like kinesin, unless several motors cluster together on a membrane or actin filament. These properties of the enzyme emphasize the importance of characterizing mechanisms that promote the self-association of myosin-I isoforms at specific binding sites in cells.
Pollard
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Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA. pollard@salk.edu
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D000048:Acanthamoeba; D000199:Actins / Q000737:chemistry*; D000818:Animals; D002465:Cell Movement / Q000502:physiology*; D019143:Evolution, Molecular; D018995:Myosin Heavy Chains / Q000502:physiology; D009218:Myosins / Q000737:chemistry* / Q000502:physiology; D010766:Phosphorylation
D000199:Actins; D018995:Myosin Heavy Chains; D009218:Myosins