Kinesin is a double-headed motor protein that moves along microtubules in 8-nm steps. Two broad
classes of model have been invoked to explain kinesin movement: hand-over-hand and inchworm.
In hand-over-hand models, the heads exchange leading and trailing roles with every step, whereas no
such exchange is postulated for inchworm models, where one head always leads. By measuring the stepwise
motion of individual enzymes, we find that some kinesin molecules exhibit a striking alternation in the
dwell times between sequential steps, causing these motors to "limp" along the microtubule. Limping
implies that kinesin molecules strictly alternate between two different conformations as they step,
indicative of an asymmetric, hand-over-hand mechanism.
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Steven M. Block
Prof. of Applied Physics & Biological Sciences