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Kinesin Steps Do Not Alternate in Size Adrian N. Fehr,* Charles L. Asbury,z

Summary: Kinesin Steps Do Not Alternate in Size
Adrian N. Fehr,* Charles L. Asbury,z
and Steven M. Block*y
Departments of *Applied Physics and y
Biological Sciences, Stanford University, Stanford, California 94305; and z
Department of
Physiology & Biophysics, University of Washington, Seattle, Washington 98195
ABSTRACT 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.
Received for publication 16 October 2007 and in final form 7 December 2007.
Address reprint requests and inquiries to Steven M. Block, Tel.: 650-724-4046; Fax: 650-723-6132;
E-mail: sblock@stanford.edu.
Conventional kinesin is a homodimeric motor protein with
two microtubule-binding head domains linked to a common,
coiled-coil stalk. It moves processively, taking up to
hundreds of steps along microtubules before dissociating


Source: Asbury, Chip - Department of Physiology and Biophysics, University of Washington at Seattle


Collections: Biology and Medicine