Kinesin is a processive motor that takes 8.3-nm center-of-mass steps along microtubules for each adenosine triphosphate hydrolyzed. Whether kinesin moves by a “hand-over-hand” or an “inchworm” model has been controversial. We have labeled a single head of the kinesin dimer with a Cy3 fluorophore and localized the position of the dye to within 2 nm before and after a step. We observed that single kinesin heads take steps of 17.3 ± 3.3 nm. A kinetic analysis of the dwell times between steps shows that the 17-nm steps alternate with 0-nm steps. These results strongly support a hand-over-hand mechanism, and not an inchworm mechanism. In addition, our results suggest that kinesin is bound by both heads to the microtubule while it waits for adenosine triphosphate in between steps.
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References and Notes

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We thank Y. Oono for helpful discussion of one-versus two-headed binding and the use of fig. S2 and A. Carter for preparing Fig. 1B. Supported by NIH grant nos. AR44420 (P.R.S.) and AR42895 (R.D.V.).

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Volume 303 | Issue 5658
30 January 2004

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Received: 17 November 2003
Accepted: 10 December 2003
Published in print: 30 January 2004


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Ahmet Yildiz
Center for Biophysics and Computational Biology, University of Illinois, Urbana-Champaign, IL 61801, USA.
Michio Tomishige*
Howard Hughes Medical Institute and the Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94107, USA.
Ronald D. Vale
Howard Hughes Medical Institute and the Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94107, USA.
Paul R. Selvin
Center for Biophysics and Computational Biology, University of Illinois, Urbana-Champaign, IL 61801, USA.
Physics Department, University of Illinois, Urbana-Champaign, IL 61801, USA.


† To whom correspondence should be addressed. E-mail: [email protected]

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