Vesicle movements and microtubule-based motors.

The movements of many cytoplasmic vesicles follow the paths of microtubules, some moving in one direction and others moving in the opposite direction on the same microtubule. Recently we have isolated one cytoplasmic motor, kinesin, and defined another, the axoplasmic retrograde factor, both of which are capable of powering anionic latex beads in both directions along polar microtubule arrays. Evidence summarized here supports but does not prove the hypothesis that kinesin and the retrograde motors are indeed responsible for powering vesicle movements.