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J. Biol. Chem., Vol. 281, Issue 8, 4949-4957, February 24, 2006

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A Unique ATP Hydrolysis Mechanism of Single-headed Processive Myosin, Myosin IX^*

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

Recent studies have revealed that myosin IX is a single-headed processive myosin, yet it is unclear how myosin IX can achieve the processive movement. Here we studied the mechanism of ATP hydrolysis cycle of actomyosin IXb. We found that myosin IXb has a rate-limiting ATP hydrolysis step unlike other known myosins, thus populating the prehydrolysis intermediate (M·ATP). M·ATP has a high affinity for actin, and, unlike other myosins, the dissociation of M·ATP from actin was extremely slow, thus preventing myosin from dissociating away from actin. The ADP dissociation step was 10-fold faster than the overall ATP hydrolysis cycle rate and thus not rate-limiting. We propose the following model for single-headed processive myosin. Upon the formation of the M·ATP intermediate, the tight binding of actomyosin IX at the interface is weakened. However, the head is kept in close proximity to actin due to the tethering role of loop 2/large unique insertion of myosin IX. There is enough freedom for the myosin head to find the next location of the binding site along with the actin filament before complete dissociation from the filament. After ATP hydrolysis, P_i is quickly released to form a strong actin binding form, and a power stroke takes place.

Received for publication, August 18, 2005 , and in revised form, December 2, 2005.

^* This work was supported by National Institute 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.

¹ To whom correspondence should be addressed: Dept. of Physiology, University of Massachusetts Medical School, 55 Lake Ave. N., Worcester, MA 01655. Tel.: 508-856-6698; Fax: 508-856-4997; E-mail: mitsuo.ikebe{at}umassmed.edu.

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