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J. Biol. Chem., Vol. 283, Issue 16, 10581-10592, April 18, 2008

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Human Myosin Vc Is a Low Duty Ratio Nonprocessive Motor^*

Shinya Watanabe¹, Tomonobu M. Watanabe, Osamu Sato, Junya Awata, Kazuaki Homma, Nobuhisa Umeki, Hideo Higuchi, Reiko Ikebe, and Mitsuo Ikebe²

From the Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655 and Biomedical Research Organization, Tohoku University, Sendai 981, Japan

There are three distinct members of the myosin V family in vertebrates, and each isoform is involved in different membrane trafficking pathways. Both myosin Va and Vb have demonstrated that they are high duty ratio motors that are consistent with the processive nature of these motors. Here we report that the ATPase cycle mechanism of the single-headed construct of myosin Vc is quite different from those of other vertebrate myosin V isoforms. K_ATPase of the actin-activated ATPase was 62 µM, which is much higher than that of myosin Va (1 µM). The rate of ADP release from actomyosin Vc was 12.7 s^-1, which was 2 times greater than the entire ATPase cycle rate, 6.5 s^-1. P_i burst size was 0.31, indicating that the equilibrium of the ATP hydrolysis step is shifted to the prehydrolysis form. Our kinetic model, based on all kinetic data we determined in this study, suggests that myosin Vc spends the majority of the ATPase cycle time in the weak actin binding state in contrast to myosin Va and Vb. Consistently, the two-headed myosin Vc construct did not show processive movement in total internal reflection fluorescence microscope analysis, demonstrating that myosin Vc is a nonprocessive motor. Our findings suggest that myosin Vc fulfills its function as a cargo transporter by different mechanisms from other myosin V isoforms.

Received for publication, September 12, 2007 , and in revised form, December 3, 2007.

^* This work was supported by National Institutes of Health Grants DC006103, 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.

¹ Present address: Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, MA 01605.

² To whom correspondence should be addressed: Dept. of Physiology, University of Massachusetts Medical School, 55 Lake Ave. N., Worcester, MA 01655. Tel.: 508-856-1954; Fax: 508-856-4600; E-mail: Mitsuo.Ikebe{at}umassmed.edu.

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