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J. Biol. Chem., Vol. 280, Issue 6, 4135-4143, February 11, 2005

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Effect of Caldesmon on the Position and Myosin-induced Movement of Smooth Muscle Tropomyosin Bound to Actin^*

Philip Graceffa and Andrew Mazurkie

From the Boston Biomedical Research Institute, Watertown, Massachusetts 02472

It is known that the actin-binding protein caldesmon inhibits actomyosin ATPase activity and might in this way take part in the thin filament regulation of smooth muscle contraction. Although the molecular mechanism of this inhibition is unknown, it is clear that the presence of actin-bound tropomyosin is necessary for full inhibition. Recent evidence also suggests that the myosin-induced movement of tropomyosin plays a key role in regulation. In this work, fluorescence studies provide evidence to show that caldesmon interacts with and alters the position of tropomyosin in a reconstituted actin thin filament and thereby limits the ability of myosin heads to move tropomyosin. Caldesmon interacts with the Cys-190 region in the COOH-terminal half of tropomyosin, resulting in the movement of this part of tropomyosin to a new position on actin. Additionally, this constrains the myosin-induced movement of this region of tropomyosin. On the other hand, caldesmon does not appear to interact with the Cys-36 region in the NH₂-terminal half of tropomyosin and neither alters the position of nor significantly constrains the myosin-induced movement of this part of tropomyosin. The ability of caldesmon to limit the myosin-induced movement of tropomyosin provides a possible molecular basis for the inhibitory function of caldesmon. The different movements of the two halves of tropomyosin indicate that actin-bound tropomyosin moves as a flexible molecule and not as a rigid rod. Interestingly, caldesmon, which inhibits tropomyosin's potentiation of actomyosin ATPase activity, moves tropomyosin in one direction, whereas myosin heads, which enhance potentiation, move tropomyosin in the opposite direction.

Received for publication, September 9, 2004 , and in revised form, October 19, 2004.

^* This work was supported by National Institutes of Health Grants HL66219 and AR41637. 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: Boston Biomedical Research Institute, 64 Grove St., Watertown, MA 02472. Tel.: 617-658-7813; Fax: 617-972-1753; E-mail: graceffa{at}bbri.org.

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