Effect of fluctuation in step size on actin-myosin sliding motion

It is possible that the step size, or power stroke, of a skeletal muscle myosin is not constant; rather, it fluctuates for each force generation. The estimated widths of the fluctuation are as large as the estimated values of the step size. Although such non-negligible fluctuation is presumed to affect the sliding motion, these effects remain unclear. We examined a system driven by a single myosin molecule sliding along an actin filament to reveal its basic effects. First, we calculated the sliding velocity of the system for each fluctuation width and found that the mean velocity increased with the fluctuation width. We also found that the estimated fluctuation widths satisfied the conditions for maximizing the sliding velocity. Next, we examined the sliding motion along a heterogeneous filament, on which binding sites for myosins were distributed randomly. We found that the loss in sliding velocity that was attributable to heterogeneity of the filament became small when fluctuation in the step size existed. This finding implied that the fluctuation stabilized velocity sliding along possible heterogeneous filaments. These benefits of fluctuation in step size might be used in biological systems, such as a muscle system, and are applicable to fabricated micromachines.