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J. Cell Biol.,
Volume 140, Number 6, March 23, 1998 1395-1405
Department of Physiology and Biophysics, University of Washington, Seattle, Washington 98195-7290
A single kinesin molecule can move for hundreds of steps along a microtubule without dissociating.
One hypothesis to account for this processive movement is that the binding of kinesin's two heads is coordinated so that at least one head is always bound to the
microtubule. To test this hypothesis, the motility of a full-length single-headed kinesin heterodimer was examined in the in vitro microtubule gliding assay. As the
surface density of single-headed kinesin was lowered,
there was a steep fall both in the rate at which microtubules landed and moved over the surface, and in the
distance that microtubules moved, indicating that individual single-headed kinesin motors are not processive
and that some four to six single-headed kinesin molecules are necessary and sufficient to move a microtubule continuously. At high ATP concentration, individual single-headed kinesin molecules detached from
microtubules very slowly (at a rate less than one per
second), 100-fold slower than the detachment during
two-headed motility. This slow detachment directly
supports a coordinated, hand-over-hand model in
which the rapid detachment of one head in the dimer is
contingent on the binding of the second head.
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