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J. Biol. Chem., Vol. 280, Issue 32, 29381-29391, August 12, 2005

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Myosin X Is a High Duty Ratio Motor^*

Kazuaki Homma and Mitsuo Ikebe

From the Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655

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 a K_d of 1.6 µM attributed to the large dissociation rate constant (2.1 s^-1). V_max and K_ATPase 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 V_max. The P_i-burst size was low (0.46 mol/mol), indicating that the equilibrium is significantly shifted toward the prehydrolysis intermediate. The steady-state ATPase rate can be explained by a 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 K_{m ATPase} and K_{m motility}. Our results suggest that myosin X is a high duty ratio motor.

Received for publication, May 2, 2005 , and in revised form, June 7, 2005.

^* This work was supported by National Institutes of Health Grants AR 048526, AR 048898, and AR 41653. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 508-856-1954; Fax: 508-856-4600; E-mail: Mitsuo.Ikebe{at}umassmed.edu.

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