IN-VITRO ACTIN FILAMENT SLIDING VELOCITIES PRODUCED BY MIXTURES OF DIFFERENT TYPES OF MYOSIN

Using in vitro motility assays, we examined the sliding velocity of ac tin filaments generated by pairwise mixings of six different types of actively cycling myosins. In isolation, the six myosins translocated a ctin filaments at differing velocities. We found that only small propo rtions of a more slowly translating myosin type could significantly in hibit the sliding velocity generated by a myosin type that translocate d filaments rapidly. In other experiments, the addition of noncycling, unphosphorylated smooth and nonmuscle myosin to actively translating myosin also inhibited the rapid sliding velocity, but to a significant ly reduced extent. The data were analyzed in terms of a model derived from the original working cross-bridge model of A. F. Huxley. We found that the inhibition of rapidly translating myosins by slowly cycling was primarily dependent upon only a single parameter, the cross-bridge detachment rate at the end of the working powerstroke. In contrast, t he inhibition induced by the presence of noncycling, unphosphorylated myosins required a change in another parameter, the transition rate fr om the weakly attached actomyosin state to the strongly attached state at the beginning of the cross-bridge power stroke.