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 slow and fast 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 study demonstrates that acto-myosin cross-bridge stiffness depends on the nucleotide state, explaining the functional diversity and force-generating potential of different muscle types.
May inform isoform-specific force generation in skeletal muscle; leaves open extension to cardiac myosin function and clinical translation.
How various myosin isoforms fulfill the diverse physiological requirements of distinct muscle types remain unclear. Myosin II isoforms expressed in skeletal muscles determine the mechanical performance of the specific muscles. Here, we employed a single-molecule optical trapping method and compared the chemomechanical 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 progresses 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 slow and fast myosin isoforms displayed similar 'rigor' stiffness.
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