Structural Changes in the Myosin Motors During Activation of Skeletal Muscle

Abstract

Structural changes in the myosin motors during the transition from the resting state to the plateau of isometric contraction were investigated by X-ray interference from single fibers of frog skeletal muscle. Isolated intact fibers (2.1μm sarcomere length, 4°C) were mounted vertically at beamline ID2 of the ESRF synchrotron (Grenoble, France) between a loudspeaker motor and a capacitance force transducer. 2D diffraction patterns were collected on a CCD detector 10 m from the preparation with 5 ms time resolution. During the development of the isometric tetanus, the intensity of the M3 reflection, originating from the axial repeat of the myosin motors, first decreases to 30% (at 50 ms) of its resting value, then increases to a steady value 70% of that at rest. The M3 reflection has a major peak with spacing 14.34 nm at rest and two peaks with mean spacing 14.57 nm at the tetanus plateau (Linari et al., Proc. Natl. Acad. Sci. USA 97:7226, 2000). The changes in the fine structure of the M3 reflection during activation were best fit by a structural model in which (1) all thick filaments have the same mean spacing at a given time during activation (2) the number of active motors increases in proportion to the isometric force (Brunello et al., J. Physiol. 577:971 2006), (3) the conformation of the active motors is independent of the level of force and strain in the thick filament.Supported by MiUR Italy, MRC UK, CNISM, EMBO, ESRF, EMBL.