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Mechanics of the kinesin step


Kinesin is a molecular walking machine that organizes cells by hauling packets of components directionally along microtubules. The physical mechanism that impels directional stepping is uncertain. We show here that, under very high backward loads, the intrinsic directional bias in kinesin stepping can be reversed such that the motor walks sustainedly backwards in a previously undescribed mode of ATP-dependent backward processivity. We find that both forward and backward 8-nm steps occur on the microsecond timescale and that both occur without mechanical substeps on this timescale. The data suggest an underlying mechanism in which, once ATP has bound to the microtubule-attached head, the other head undergoes a diffusional search for its next site, the outcome of which can be biased by an applied load.

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Figure 1: Example optical trapping records.
Figure 2: Stepping behaviour.
Figure 3: Average time course of forward and backward steps.
Figure 4: Model.

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We thank J. Howard for the gift of the Drososphila kinesin, the reviewers of this manuscript for careful and constructive criticism, and Marie Curie Cancer Care for unswerving support.

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Correspondence to R. A. Cross.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Figure S1

Simulated steps added to bead position noise. Testing the detection limits for steps by adding various simulated steps to real measured noise and trying to detect the steps using our algorithm. (PDF 2001 kb)

Supplementary Figure S2

Dwell time distributions at various loads and ATP concentrations. (PDF 764 kb)

Supplementary Figure Legends

Legends to accompany Supplementary Figures S1 and S2. (DOC 21 kb)

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Carter, N., Cross, R. Mechanics of the kinesin step. Nature 435, 308–312 (2005).

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