Key result
The strong actin-bound 'AM.ADP' state of slow muscle myosin II exhibited more than 2-fold lower stiffness than the 'AM rigor' state, while both fast and slow myosin isoforms displayed similar rigor stiffness.
Why the study?
How various myosin isoforms fulfill the diverse physiological requirements of distinct muscle types remains unclear.
The time-averaged stiffness of slow myosin is lower due to a prolonged AM.ADP state, elucidating the biophysical basis for functional diversity among muscle types.
Myosin stiffness findings remain preclinical; leaves open translation to cardiac myosin-targeted therapies.
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.
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Wang et al. (2020) studied this question. Slow muscle myosin II (MHC-1 / SolM-II) vs. Fast muscle myosin II (MHC-2 / PsoM-II) was evaluated on Acto-myosin cross-bridge stiffness and detachment rate. The strong actin-bound 'AM.ADP' state of slow muscle myosin II exhibited more than 2-fold lower stiffness than the 'AM rigor' state, while both fast and slow myosin isoforms displayed similar rigor stiffness.
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