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J. Biol. Chem., Vol. 281, Issue 42, 31987-31994, October 20, 2006

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Regulation of Myosin V Processivity by Calcium at the Single Molecule Level^*

From the Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont 05405

Calcium can affect myosin V (myoV) function in at least two ways. The full-length molecule, which adopts a folded inhibited conformation in EGTA, becomes extended and active in the presence of calcium. Calcium also dissociates one or more calmodulin molecules from the extended neck. Here we investigated at the single molecule level how calcium regulates the processive run length of full-length myosin V (dFull) and a truncated dimeric construct (dHMM), which cannot adopt the folded conformation. The processivity of dFull and dHMM is tightly controlled by the calcium and calmodulin concentration, with shorter runs occurring at higher calcium concentration. The data indicate that a calcium-dependent dissociation of calmodulin from the neck region of myoV terminates its processive run. dFull showed unexpected processive movement in EGTA, suggesting that a small population of extended, active molecules are in equilibrium with the inhibited, folded form. Single turnover assays showed that the ATPase activity of the folded full-length molecule is inhibited by more than 50-fold compared with the extended molecule. The results imply that activation and termination of the processive runs of myoV can be accomplished by multiple mechanisms.

Received for publication, May 30, 2006 , and in revised form, August 11, 2006.

^* This work was supported by National Institutes of Health Grant HL38113 (to K. M. T.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental material including Table 1s and Fig. 1s.

¹ To whom correspondence should be addressed: Dept. of Molecular Physiology and Biophysics, University of Vermont, 149 Beaumont Ave., Burlington, VT 05405. Tel.: 802-656-8750; Fax: 802-656-0747; E-mail: kathleen.trybus{at}uvm.edu.

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