Distinguishing Inchworm and Hand-Over-Hand Processive Kinesin Movement by Neck Rotation Measurements

Abstract

The motor enzyme kinesin makes hundreds of unidirectional 8-nanometer steps without detaching from or freely sliding along the microtubule on which it moves. We investigated the kinesin stepping mechanism by immobilizing a Drosophila kinesin derivative through the carboxyl-terminal end of the neck coiled-coil domain and measuring orientations of microtubules moved by single enzyme molecules at submicromolar adenosine triphosphate concentrations. The kinesin-mediated microtubule-surface linkage was sufficiently torsionally stiff (≥2.0 ± 0.9 × 10 ⁻²⁰ Newton meters per radian ² ) that stepping by the hypothesized symmetric hand-over-hand mechanism would produce 180° rotations of the microtubule relative to the immobilized kinesin neck. In fact, there were no rotations, a finding that is inconsistent with symmetric hand-over-hand movement. An alternative “inchworm” mechanism is consistent with our experimental results.