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Biophys J, July 2001, p. 66-78, Vol. 81, No. 1
Department of Biology, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27599-3280 USA
Whether polarized treadmilling is an intrinsic property
of microtubules assembled from pure tubulin has been controversial. We
have tested this possibility by imaging the polymerization dynamics of
individual microtubules in samples assembled to steady-state in vitro
from porcine brain tubulin, using a 2% glycerol buffer to reduce
dynamic instability. Fluorescence speckled microtubules were bound to
the cover-glass surface by kinesin motors, and the assembly dynamics of
plus and minus ends were recorded with a spinning-disk confocal
fluorescence microscopy system. At steady-state assembly, 19% of the
observed microtubules (n = 89) achieved
treadmilling in a plus-to-minus direction, 34% in a minus-to-plus
direction, 37% grew at both ends, and 10% just shortened. For the
population of measured microtubules, the distribution of lengths
remained unchanged while a 20% loss of original and 27% gain
of new polymer occurred over the 20-min period of observation. The lack
of polarity in the observed treadmilling indicates that stochastic
differences in dynamic instability between plus and minus ends are
responsible for polymer turnover at steady-state assembly, not
unidirectional treadmilling. A Monte Carlo simulation of plus and minus
end dynamics using measured dynamic instability parameters reproduces
our experimental results and the amount of steady-state polymer
turnover reported by previous biochemical assays.
Biophys J, July 2001, p. 66-78, Vol. 81, No. 1
© 2001 by the Biophysical Society 0006-3495/01/07/66/13 $2.00
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