Fig. 1.

Aspects of molecular walking under load. (A) Force-feedback single-bead optical trap. A single myosin-V molecule is attached to a plastic bead, and the bead is trapped by a focused infrared laser. Defined levels of forward or backward force are maintained by monitoring bead position and automatically stepping the stage to compensate for steps taken by the motor. (B) Ratchet-and-pawl. The gray block can be dragged easily to the left, because the pawl flips up and can slip along the toothed track. By contrast, the mechanism blocks movement to the right, because the pawl flips down and engages. Bicycle freewheels work this way. (C) A possible scheme for force-induced backstepping. A backstepping sequence (states 0, −1, −2, −0) can be driven by applying backward force, but a forward stepping sequence (states 0, 1, 2, 0) cannot. Forward stepping has an absolute requirement for ATP. The structure of the binding interface between the motor and its track means that backward force is able to unpeel the myosin head from its track, whereas forward force is not.