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Rotation of the Lever Arm of Myosin in Contracting Skeletal Muscle Fiber Measured by Two-Photon Anisotropy

J. Borejdo ^*, A. Shepard ^*, I. Akopova ^*, W. Grudzinski  and J. Malicka 

^* Department of Molecular Biology and Immunology, University of North Texas, Fort Worth, Texas 76107; and Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201

Correspondence: Address reprint requests to Julian Borejdo, Dept. of Molecular Biology and Immunology, University of North Texas, 3500 Camp Bowie Blvd., Fort Worth, TX 76107. Tel: 817-735-2106; E-mail: jborejdo{at}hsc.unt.edu.

The rotation of the lever arm of myosin cross-bridges is believed to be responsible for muscle contraction. To resolve details of this rotation, it is necessary to observe a single cross-bridge. It is still impossible to do so in muscle fiber, but it is possible to investigate a small population of cross-bridges by simultaneously activating myosin in a femtoliter volume by rapid release of caged ATP. In earlier work, in which the number of observed cross-bridges was limited to 600 by confocal microscopy, we were able to measure the rates of cross-bridge detachment and rebinding. However, we were unable to resolve the power stroke. We speculated that the reason for this was that the number of observed cross-bridges was too large. In an attempt to decrease this number, we used two-photon microscopy which permitted observation of 1/2 as many cross-bridges as before with the same signal/noise ratio. With the two-photon excitation, the number of cross-bridges was small enough to resolve the beginning of the power stroke. The results indicated that the power stroke begins 170 ms after the rigor cross-bridge first binds ATP.

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