Muscle force arises by actin filament rotation and torque in the Z-filaments

Actin filament rotation in skeletal muscle is studied by a mechanical model that simulates structure and tension. The four anchoring Z-filaments are twisted around and change the structure of the Z-lattice. The "small square" without twist represents the resting stage of muscle. Torque causes contraction by clockwise rotation (as seen from the Z-band), drilling into the A-band and transition of the "small square" to "basket weave" by increasing the twist and decreasing the torque. Release decreases the torque ("force-depression") by passive clockwise rotation. Stretch causes increased torque ("stretch activation") by passive counterclockwise rotation. Torque arises during Ca(2+)-activation by a conformational change in the highly charged coiled-coils: The four alpha-actinin Z-filaments generate strong torque for the isometric tension. Quick release experiments show that less than one rotation reduces this torque to zero. The 5-12 rotations necessary for isotonic shortening result from torque-generation in the two long tropomyosin coiled-coils. Myosin controls the velocity of active and passive rotations.