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The Conformation of Myosin Head Domains in Rigor Muscle Determined by X-Ray Interference

M. Reconditi ^*, N. Koubassova , M. Linari ^*, I. Dobbie , T. Narayanan , O. Diat , G. Piazzesi ^*, V. Lombardi ^* and M. Irving 

^* Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, University of Florence, Florence, Italy; Institute of Mechanics, University of Moscow, Moscow, Russia; School of Biomedical Sciences, New Hunt's House, King's College London, Guy's Campus, London, United Kingdom; and European Synchrotron Radiation Facility, Grenoble, France

Correspondence: Address reprint requests to Malcolm Irving, School of Biomedical Sciences, New Hunt's House, King's College London, Guy's Campus, London SE1 1UL, UK. Tel.: +44-207-848-6431; Fax: +44-207-848-6435; E-mail: malcolm.irving{at}kcl.ac.uk.

In the absence of adenosine triphosphate, the head domains of myosin cross-bridges in muscle bind to actin filaments in a rigor conformation that is expected to mimic that following the working stroke during active contraction. We used x-ray interference between the two head arrays in opposite halves of each myosin filament to determine the rigor head conformation in single fibers from frog skeletal muscle. During isometric contraction (force T₀), the interference effect splits the M3 x-ray reflection from the axial repeat of the heads into two peaks with relative intensity (higher angle/lower angle peak) 0.76. In demembranated fibers in rigor at low force (<0.05 T₀), the relative intensity was 4.0, showing that the center of mass of the heads had moved 4.5 nm closer to the midpoint of the myosin filament. When rigor fibers were stretched, increasing the force to 0.55 T₀, the heads' center of mass moved back by 1.1–1.6 nm. These motions can be explained by tilting of the light chain domain of the head so that the mean angle between the Cys⁷⁰⁷–Lys⁸⁴³ vector and the filament axis increases by 36° between isometric contraction and low-force rigor, and decreases by 7–10° when the rigor fiber is stretched to 0.55 T₀.

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