Roles of the hydrophobic triplet in the motor domain of myosin in the interaction between myosin and actin.

Myosin is a molecular motor and a member of a protein family comprising at least 18 classes. There is an about 1,000-fold difference in the in vitro sliding velocity between the fastest myosin and the slowest one. Previous studies revealed that the hydrophobic triplet in the motor domain (Val534, Phe535, and Pro536 in Dictyostelium myosin) is important for the strong binding of myosin to actin. We studied the role of the triplet in the sliding motion of myosin by means of site directed mutagenesis because the sliding velocity is determined by the time that myosin interacts with actin strongly. We produced mutant Dictyostelium myosins and subfragment-1s that have the triplet sequences of various classes of myosin with different sliding velocities. The V(max) and K(actin) values of the actin-activated ATPase for all these mutant subfragment-1s were lower than those of the wild-type Dictyostelium myosin. The mutant myosins exhibited much lower sliding velocities than the wild type. The time that the mutant subfragment-1s are in the strongly bound state did not correlate well with the sliding velocity. Our results suggested that (i) the hydrophobic triplet alone does not determine the sliding velocity of myosin, (ii) the size of the amino acid side chain in the triplet is crucial for the ATPase activity and the motility of myosin, and (iii) the hydrophobic triplet is important not only for strong binding to actin but also for the structural change of the myosin motor domain during the power stroke.