  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.
