  Myosin V and myosin VI are two classes of two-headed molecular motors of the
myosin superfamily that move processively along helical actin filaments in
opposite directions. Here we present a hand-over-hand model for their
processive movements. In the model, the moving direction of a dimeric molecular
motor is automatically determined by the relative orientation between its two
heads at free state and its head's binding orientation on track filament. This
determines that myosin V moves toward the barbed end and myosin VI moves toward
the pointed end of actin. During the moving period in one step, one head
remains bound to actin for myosin V whereas two heads are detached for myosin
VI: The moving manner is determined by the length of neck domain. This
naturally explains the similar dynamic behaviors but opposite moving directions
of myosin VI and mutant myosin V (the neck of which is truncated to only
one-sixth of the native length). Because of different moving manners, myosin VI
and mutant myosin V exhibit significantly broader step-size distribution than
native myosin V. However, all three motors give the same mean step size of 36
nm (the pseudo-repeat of actin helix). Using the model we study the dynamics of
myosin V quantitatively, with theoretical results in agreement with previous
experimental ones.
