Switch 1 Mutation S217A Converts Myosin V into a Low Duty Ratio Motor

We have determined the kinetic mechanism and motile properties of the switch 1 mutant S217A of myosin Va. Phosphate dissociation from myosin V-ADP-P-i (inorganic phosphate) and actomyosin V-ADP-P-i and the rate of the hydrolysis step ( myosin V-ATP -> myosin V-ADP-P-i) were all similar to 10-fold slower in the S217A mutant than in wild type (WT) myosin V, resulting in a slower steady-state rate of basal and filamentous actin (actin)-activated ATP hydrolysis. Substrate binding and ADP dissociation kinetics were all similar to or slightly faster in S217A than in WT myosin V and mechanochemical gating of the rates of dissociation of ADP between trail and lead heads is maintained. The reduction in the rate constants of the hydrolysis and phosphate dissociation steps reduces the duty ratio from similar to 0.85 in WT myosin V to similar to 0.25 in S217A and produces a motor in which the average run length on actin at physiological concentrations of ATP is reduced 10-fold. Thus we demonstrate that, by mutational perturbation of the switch 1 structure, myosin V can be converted into a low duty ratio motor that is processive only at low substrate concentrations.