Elastic distortion of myosin heads and repriming of the working stroke in muscle

Abstract

MUSCLE contraction is driven by a cyclical interaction between the globular head domain of myosin and the actin filaments^1-6. We used quick stretches of 5 nm per half sarcomere to synchronize the movements of myosin heads in active single muscle fibres^5,7,8. The intensity of the 14.5 nm X-ray reflection decreased during the stretch, showing that the instantaneous elasticity of muscle^1,5 involves distortion of myosin heads. Head movement continued at about 1,500s^-1 after the stretch, accompanied by partial force recovery. This indicates a reversal of the force-generating 'working stroke' in the myosin heads^5,8,9 that is smaller and faster than assumed previously^5,10,11. By 50 ms after the stretch, myosin heads have regained both their original conformation and the ability to execute a normal working stroke. This 'repriming' process is slower than that following shortening¹² but much faster than the ATP turnover rate per myosin head.