Shining Light on the Mechanism of Nature’s Nanomachines

Within cells, materials such as membranes and proteins must travel many micrometers to reach their cellular sites of action. Such transport processes cannot occur efficiently via passive diffusion over cellular length scales. To solve this transport problem, nature enlists molecular motor proteins to shuttle cellular cargoes along the cytoskeleton, a network of intracellular tracks. These motors are ‘two-legged’ molecular walkers and need to be able to move continuously (or processively) along their tracks to achieve long-distance transport. In this talk, I will present our recent study to understand how cytoplasmic dynein, a molecular motor that moves on tracks called microtubules, achieves processive motion. We developed a versatile method for labeling each of the two moving parts within dynein with a different colored fluorescent dye, by producing them separately and then pairing them using chemically attached DNA strands. We then used single-molecule, high-precision fluorescence microscopy to directly visualize the movements of dynein’s two motor domains simultaneously in real time. Our results show that dynein has a highly variable stepping pattern that is distinct from all other processive cytoskeleton-based motors. Surprisingly, dynein stepping is stochastic when its two motor domains are close together. However, a tension-based coordination mechanism emerges to govern dynein stepping as the distance between motor domains increases. This dual-mode stepping behavior may allow tuning of dynein for its diverse cellular functions. My future research will focus on the biophysics of molecular motor-based transport phenomena such as the regulation of dynein by different cellular factors and the mechanism of a special class of kinesin motors, another group of microtubule-based motors, that move in the same direction as dynein.