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Structural Changes of Actin-Bound Myosin Heads after a Quick Length Change in Frog Skeletal Muscle

Japan Synchrotron Radiation Research Institute, SPring-8, Kouto, Sayo-gun, Hyogo, Japan

Correspondence: Address reprint requests to N. Yagi, Tel.: 81-791-58-0908; E-mail: yagi{at}spring8.or.jp.

Changes in the x-ray diffraction pattern from a frog skeletal muscle were recorded after a quick release or stretch, which was completed within one millisecond, at a time resolution of 0.53 ms using the high-flux beamline at the SPring-8 third-generation synchrotron radiation facility. Reversibility of the effects of the length changes was checked by quickly restoring the muscle length. Intensities of seven reflections were measured. A large, instantaneous intensity drop of a layer line at an axial spacing of 1/10.3 nm^–1 after a quick release and stretch, and its partial recovery by reversal of the length change, indicate a conformational change of myosin heads that are attached to actin. Intensity changes on the 14.5-nm myosin layer line suggest that the attached heads alter their radial mass distribution upon filament sliding. Intensity changes of the myosin reflections at 1/21.5 and 1/7.2 nm^–1 are not readily explained by a simple axial swing of cross-bridges. Intensity changes of the actin-based layer lines at 1/36 and 1/5.9 nm^–1 are not explained by it either, suggesting a structural change in actin molecules.

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