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J. Biol. Chem., Vol. 278, Issue 31, 28533-28539, August 1, 2003

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The Unique Properties of Tonic Smooth Muscle Emerge from Intrinsic as Well as Intermolecular Behaviors of Myosin Molecules^*

Josh E. Baker, Christine Brosseau, Patty Fagnant and David M. Warshaw 

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

To better understand the molecular basis for some of the unique mechanical properties of tonic smooth muscle, we use a laser trap to assay the mechanochemistry of single smooth muscle heavy meromyosin molecules lacking a seven-amino acid insert in the nucleotide binding loop (minus insert). We measured a second-order ATP-induced actin dissociation rate, k_T, of 2.2 x 10⁶ M^–1 s^–1, an ADP release rate, k_–D, of 19 s^–1, a second-order ADP binding rate, k_D, of 60 x 10⁵ M^–1 s^–1, and an ADP affinity, K_D, of 3.2 µM, which is more than 100-fold greater than that measured for skeletal muscle myosin. By performing in vitro motility studies under nearly identical conditions, we show that the relatively slow actin velocity generated by minus-insert heavy meromyosin is significantly influenced, but not limited, by k_–D. Our results support a model in which two separate intermediate steps in the actin-myosin catalyzed ATP hydrolysis reaction are energetically coupled through mechanical interactions, and we discuss this model in the context of the ability of tonic muscle to maintain high forces at low energetic cost (latch).

Received for publication, April 7, 2003 , and in revised form, May 14, 2003.

^* This work was supported in part by National Institutes of Health Grants HL07647 (to J. E. B.) and AR47906 and HL59408 (to D. M. W.) and by the Totman Fund for Cerebrovascular Research (to D. M. W.). 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.: 802-656-4300; Fax: 802-656-0747; E-mail: warshaw{at}physiology.med.uvm.edu.

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