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J. Biol. Chem., Vol. 278, Issue 24, 21352-21360, June 13, 2003

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Determination of Human Myosin III as a Motor Protein Having a Protein Kinase Activity^*

Shigeru Komaba   ¶, Akira Inoue  ¶, Shinsaku Maruta ||, Hiroshi Hosoya  and Mitsuo Ikebe  **

From the Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655-0127, Department of Biological Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan, ^|| Department of Bioengineering, Faculty of Engineering, Soka University, Hachioji, 192-8577, Japan

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with K_actin of 112 µM and V_max of 0.34 s^–1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 ± 0.05 µm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

Received for publication, January 23, 2003 , and in revised form, March 31, 2003.

^* This work was supported by National Institutes of Health Grants GM55834 and AR41653. 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.

^¶ These authors contributed equally to this work.

^** 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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