Myosin Heads Contribute to the Maintenance of Filament Order in Relaxed Rabbit Muscle

Raising the temperature of rabbit skeletal muscle from ∼0°C to ∼20°C has been shown to enhance the helical organization of the myosin heads and to change the intensities of the 10 and 11 equatorial reflections. We show here by time-resolved x-ray diffraction combined with temperature jump that the movement of the heads to enhance the organized myosin helix occurs at the same fast rate as the change in the intensities of the equatorial reflections. However, model calculations indicate that the change in the equatorials cannot be explained simply in terms of the movement of myosin heads. Analysis of electron micrographs of transverse sections of relaxed muscle fibers cryofixed at ∼5°C and ∼35°C shows that in addition to the reorganization of the heads the thin and thick filaments are less constrained to their positions in the hexagonal filament lattice in the warm muscle than in the cold. Incorporating the changes in filament order in model calculations reconciles these with the observed changes in equatorial reflections. We suggest the thin filaments in the cold muscle are boxed into their positions by the thermal movement of the disordered myosin heads. In the warmer muscle, the packed-down heads leave the thin filaments more room to diffuse laterally.