Key result
Stochastic effects significantly impact myosin II motor ensembles smaller than 15.
Why the study?
Forces generated by small groups of nonprocessive myosin II motors involve stochastic effects that are highly relevant and need to be characterized.
A one-step master equation models stochastic force generation by small ensembles of myosin II motors, revealing that stochastic effects are significant for ensembles smaller than 15.
May inform models of cardiac contractility; leaves open empirical testing in cardiomyocytes.
Forces in the actin cytoskeleton are generated by small groups of nonprocessive myosin II motors for which stochastic effects are highly relevant. Using a cross-bridge model with the assumptions of fast power-stroke kinetics and equal load sharing between equivalent states, we derive a one-step master equation for the activity of a finite-sized ensemble of mechanically coupled myosin II motors. For constant external load, this approach yields analytical results for duty ratio and force-velocity relation as a function of ensemble size. We find that stochastic effects cannot be neglected for ensemble sizes below 15. The one-step master equation can be used also for efficient computer simulations with linear elastic external load and reveals the sequence of buildup of force and ensemble rupture that is characteristic for reconstituted actomyosin contractility.
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Erdmann et al. (2012) studied this question. Small ensembles of myosin II motors was evaluated. Stochastic effects cannot be neglected for myosin II motor ensemble sizes below 15, which corresponds to the typical size of cytoskeletal mini-filaments.
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