Stochastic force generation by small ensembles of myosin II motors

TL;DR

This paper models the stochastic force generation of small myosin II motor ensembles, deriving analytical and simulation tools to understand their behavior under load, especially for ensembles smaller than 15 motors.

Contribution

It introduces a one-step master equation for finite-sized myosin II ensembles, enabling analytical and efficient simulation of their stochastic force generation.

Findings

Stochastic effects are significant for ensembles smaller than 15 motors.

Derived analytical expressions for duty ratio and force-velocity relation.

Revealed force build-up and rupture sequences in actomyosin contractility.

Abstract

Forces in the actin cytoskeleton are generated by small groups of non-processive myosin II motors for which stochastic effects are highly relevant. Using a crossbridge model with the assumptions of fast powerstroke 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 build-up of force and ensemble rupture that is characteristic for reconstituted actomyosin contractility.