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A more recent version of this article appeared on October 1, 2007
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Submitted on May 8, 2007
Revised on July 12, 2007
Accepted on July 20, 2007
Department of Biology and Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, Amherst, MA 01003
Monitoring Editor: Kerry Bloom
In higher eukaryotic cells, microtubules within metaphase and anaphase spindles undergo poleward flux, the slow, poleward movement of tubulin subunits through the spindle microtubule lattice. Although a number of studies have documented this phenomenon across a wide range of model systems, the possibility of poleward flux before nuclear envelope breakdown (NEB) has not been examined. Using a mammalian cell line expressing photoactivatable GFP-tubulin, we observe microtubule motion, both toward and away from centrosomes, at a wide range of rates (0.5 µm/min – 4.5 µm/min) in prophase cells. Rapid microtubule motion in both directions is dynein-dependent. In contrast, slow microtubule motion, which occurs at rates consistent with metaphase flux, is insensitive to inhibition of dynein, but sensitive to perturbation of Eg5 and Kif2a, two proteins with previously documented roles in flux. Our results demonstrate that microtubules in prophase cells are unexpectedly dynamic and that a subpopulation of these microtubules shows motion that is consistent with flux. We propose that the marked reduction in rate and directionality of microtubule motion from prophase to metaphase results from changes in microtubule organization during spindle formation.