Molecular motors in living cells are involved in muscle contraction, cellular maintenance, developmental shape changes, and invasion of cellular pathogens. A major thrust is underway to understand how motors interact and are regulated within the cellular milieu. We have taken a unique approach to this problem by developing methods for non-invasively measuring motor biophysics, coordination, and regulation inside the cell from outside the cell.
Chlamydomonas, a unicellular alga, is an ideal organism for studying motor function. Kinesin-2 and dynein-2, the motors for anterograde and retrograde transport in Chlamydomonas respectively, are responsible for both IFT and the flagellar membrane transport of extracellular cargo. Kinesin and dynein bind to a flagellar transmembrane protein, which in turn binds to extracellular cargo, allowing for the movement of extracellular cargos (e.g. microbeads, bacteria, or accumulated debris) along the length of the flagellum. We capture microspheres in a laser trap, present them to immobilized Chlamydomonas flagella, and through flagellar membrane transport record molecular motor protein function as the extracellular bead is moved as cargo. Our early work suggested that oppositely directed molecular motors are reciprocally coordinated rather than operating in a tug-of-war fashion. Further, many motors (> 10) can engage simultaneously to move the extracellular cargo. Our recent work shows that the velocities of transport are quantal (multiples of a fundamental velocity), routinely travel at much higher velocities than those expected or allowed by our current understanding of processive motors, and that forces driving transport grow non-linearly with cargo size. These data suggest motor dynamics and mechanics that are unique to the intracellular environment.