Cytoplasmic dynein is a microtubule-based molecular motor that participates in a multitude of cell activities, from cell division to organelle transport. Unlike
kinesin and
myosin, where different tasks are performed by highly specialized members of these superfamilies, a single form of the
dynein heavy chain is utilized for different functions. This versatility demands an extensive regulation of motor function. Using an improved application of an optical trap, we were now able to demonstrate that
cytoplasmic dynein can generate a discrete power
stroke as well as a processive walk in either direction; i.e., towards the plus- or towards the minus-end of a microtubule. Thus,
dynein's motor functions can be described by four basic modes of motion: processive and nonprocessive movement, and movement in the forward and reverse directions. Importantly, these four modes of movement can be controlled by two switches. One switch, based on
phosphate, determines the directionality of movement. The second switch, depending on
magnesium, converts
cytoplasmic dynein from a nonprocessive to a processive motor. The two switches can be triggered separately or jointly by changing concentrations of
phosphate and
magnesium in the local environment. The control of four modes of movement by two switches has major implications for our understanding of the cellular functions and regulation of
cytoplasmic dynein. Based on recent studies of
dynein's structure we are able to draw new conclusions on
cytoplasmic dynein's stepping mechanism.