The Journal of Cell Biology, Vol 71, 749-767, Copyright © 1976 by The Rockefeller University Press
A quantitative analysis of microtubule elongation
J Bryan
Methods have been developed for differentially inhibiting microtubule
nucleation and elongation in vitro. By use of polyanions, assembly-
competent tubulin solutions of several milligrams/milliliter can be
prepared which do not exhibit appreciable spontaneous assembly during the
time-course of an experiment. Microtubule elongation can be initiated by
the addition of known numbers of microtubule fragments. A detailed analysis
of the resulting process demonstrates that: (a) rings are not obligatory
intermediates in the nucleation sequence, and neither rings nor
protofilament sheets are obligatory intermediates in the elongation
reaction. (b) The end of an elongating microtubule often has a short region
of open protofilament sheet or "C-microtubule" similar to that observed in
vivo. (c) The development of turbidity follows a simple exponential
approach to an equilibrium value. (d) The final equilibrium values are
independent of the number of added nucleating fragments, while the initial
growth rates and half-times to reach equilibrium are dependent on the
number of added nuclei. (e) The final lengths of the microtubules at
equilibrium are inversely proportional to the number of added fragments.
(f) The equilibrium constants are independent of microtubule length. (g)
The number of assembly and disassembly sites per microtubule is not a
function of microtubule length. (h) The forward rate constants, the final
polymer concentrations, and growth rates of microtubules are dependent upon
the concentration of polyanion present. These results are strongly
supportive of the idea that microtubule assembly is a "condensation-
polymerization" and provide basic information on the kinetics and length
distributions of the elongation in vitro.
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