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Papers In Press, published online ahead of print June 16, 2005
Department of Physiology, UMASS Medical School, Worcester, MA 01655-0127
Corresponding Author: Kazuaki.Homma{at}umassmed.edu
Myosin X is expressed in a variety of cell types and plays a role in cargo movement and filopodia extension, but its mechanoenzymatic characteristics are not fully understood. Here we analyzed the kinetic mechanism of the ATP hydrolysis cycle of acto-myosin X using a single-headed construct (M10IQ1). Myosin X was unique for the weak “strong actin binding state” (AMD) with Kd of 1.6 µM due to the large dissociation rate constant (2.1 s-1). Vmax and KATPase of the actin-activated ATPase activity of M10IQ1 were 13.5 s-1 and 17.4 µM, respectively. The ATP hydrolysis rate (>100 s-1) and the phosphate release rate from acto-myosin X (>100 s-1) were much faster than the entire ATPase cycle rate and, thus, not rate limiting. The ADP off rate from acto-myosin X was 23 s-1, which was two times larger than the Vmax. Pi-burst size was low (0.46 mol/mol) indicating that the equilibrium is significantly shifted toward the pre-hydrolysis intermediate. The steady-state ATPase rate can be explained by combination of the unfavorable equilibrium constant of the hydrolysis step and the relatively slow ADP off rate. The duty ratio calculated from our kinetic model, 0.6, was consistent with the duty ratio, 0.7, obtained from comparison of Km ATPase and Km motility. Our results suggest that myosin X is a high duty ratio motor.
J. Biol. Chem, 10.1074/jbc.M504779200
Submitted on May 2, 2005
Revised on June 7, 2005
Accepted on June 15, 2005
Myosin X is a high duty ratio motor
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