A torque component present in mitotic kinesin Eg5 revealed by three-dimensional tracking

Mitotic kinesin Eg5 is a homotetrameric molecular motor that cross-links and slides microtubules. The extent to which Eg5 moves processively is not clear. Here we use three-dimensional tracking of a quantum dot attached to the microtubule in a motility assay to directly visualize the corkscrew motion of a sliding microtubule. We show that the rotational pitch of microtubule sliding conveniently reports on the processivity of the driving motors, confirming that two-headed Eg5 is much less processive than two-headed kinesin-1.