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Kinesin Steps Do Not Alternate in Size

Adrian N. Fehr ^*, Charles L. Asbury  and Steven M. Block ^* 

Departments of ^* Applied Physics and Biological Sciences, Stanford University, Stanford, California 94305; and Department of Physiology & Biophysics, University of Washington, Seattle, Washington 98195

Correspondence: Address reprint requests and inquiries to Steven M. Block, Tel.: 650-724-4046; Fax: 650-723-6132; E-mail: sblock{at}stanford.edu.

Kinesin is a two-headed motor protein that transports cargo inside cells by moving stepwise on microtubules. Its exact trajectory along the microtubule is unknown: alternative pathway models predict either uniform 8-nm steps or alternating 7- and 9-nm steps. By analyzing single-molecule stepping traces from "limping" kinesin molecules, we were able to distinguish alternate fast- and slow-phase steps and thereby to calculate the step sizes associated with the motions of each of the two heads. We also compiled step distances from nonlimping kinesin molecules and compared these distributions against models predicting uniform or alternating step sizes. In both cases, we find that kinesin takes uniform 8-nm steps, a result that strongly constrains the allowed models.