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

Abstract

<jats:title>Abstract</jats:title><jats:p>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 &gt; 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.</jats:p>

Cardiac ventricular myosin and slow skeletal myosin exhibit dissimilar chemo-mechanical properties despite the same myosin heavy chain isoform

“…AM binding events measured at 5 µM ATP concentration yielded the average stroke size of 5.32 nm for M-II. Consistent with our previous studies ( 23), we measured total stroke size of about 6 nm for SolM-II (23).…”

“…Extent of the noise amplitude reduction of both the trapped beads during AM interaction was used to derive the stiffness of the interacting motor head. Our recent study and other reports (23,28,29) have established that a single motor head interacts with the actin filament during single association-dissociation events. In Figure 4A, stiffness measurements performed at two different ATP concentrations, i.e., 1 and 50 µM ATP are shown.…”

“…The mechanical parameters such as powerstroke size and the motor stiffness were comparable. Remarkably, the recently found feature for SolM-II (23), i.e., the change of crossbridge stiffness during actomyosin ATPase cycle, was ascertained for the βM-II as well. Accordingly, AM.ADP state displayed lower stiffness than the rigor stiffness for βM-II.…”

“…The change of AM cross-bridge stiffness during the ATPase cycle is rather a very recent observation (23). Interestingly, for βM-II we made similar observation.…”

“…We estimated high cross-bridge stiffness of about 1.83 ± 0.09 pN/nm at 1 µM ATP, while it reduced to about 0.88 ± 0.08 pN/nm at 50 µM ATP. We demonstrated recently for slow SolM-II, that the ADP bound AM state has lower stiffness than the AM rigor state (23). For SolM-II, stiffness of 1.75 ± 0.1 pN/nm and 0.5 ± 0.04 pN/nm was determined at 10 and 500 µM ATP, respectively (Figure 4B).…”