Myosin step size: Estimation from slow sliding movement of actin over low densities of heavy meromyosin

We have estimated the step size of the myosin cross-bridge ( d, displacement of an actin filament per one ATP hydrolysis) in an in vitro motility assay system by measuring the velocity of slowly moving actin filaments over low densities of heavy meromyosin on a nitrocellulose surface. In previous studies, only filaments greater than a minimum length were observed to undergo continuous sliding movement. These filaments moved at the maximum speed ( v o), while shorter filaments dissociated from the surface. We have now modified the assay system by including 0.8% methylcellulose in the ATP solution. Under these conditions, filaments shorter than the previous minimum length move, but significantly slower than v o, as they are propelled by a limited number of myosin heads. These data are consistent with a model that predicts that the sliding velocity ( v) of slowly moving filaments is determined by the product of v o and the fraction of time when at least one myosin head is propelling the filament, that is, v = v o {1-(1- t s t c ) N }, where t s is the time the head is strongly bound to actin, t c is the cycle time of ATP hydrolysis, and N is the average number of myosin heads that can interact with the filament. Using this equation, the optimum value of t s t c to fit the measured relationship between v and N was calculated to be 0.050. Assuming d = v o t s, the step size was then calculated to be between 10 nm and 28 nm per ATP hydrolyzed, the latter value representing the upper limit. This range is within that of geometric constraint for conformational change imposed by the size of the myosin head, and therefore is not inconsistent with the swinging cross-bridge model tightly coupled with ATP hydrolysis.