Acto-myosin cross-bridge stiffness depends on the nucleotide state of the myosin II

How various myosin isoforms fulfill the diverse physiological requirements of distinct muscle types remains unclear. Myosin II isoforms expressed in skeletal muscles determines the mechanical performance of the specific muscles as fast movers, or slow movers but efficient force holders. Here, we employed a single-molecule optical trapping method and compared the chemo-mechanical properties of slow and fast muscle myosin II isoforms. Stiffness of the myosin motor is key to its force-generating ability during muscle contraction. We found that acto-myosin (AM) cross-bridge stiffness depends on its nucleotide state as the myosin progress through the ATPase cycle. The strong actin bound ‘AM.ADP’ state exhibited > 2 fold lower stiffness than ‘AM rigor’ state. The two myosin isoforms displayed similar ‘rigor’ stiffness. We conclude that the time-averaged stiffness of the slow myosin is lower due to prolonged duration of the AM.ADP state, which determines the force-generating potential and contraction speed of the muscle, elucidating the basis for functional diversity among myosins.