Differentiation between dynamic instability and end-to-end annealing models for length changes of steady-state microtubules

Short microtubules can be formed by shearing a sample at polymerization steady state of microtubules formed by glycerol-induced assembly of pure tubulin dimer. Such short microtubules show a rapid increase in mean length. The rate of this increase is too fast to be accounted for by statistical redistribution of subunits between microtubules. We propose that the fast length changes are a result of the end-to-end annealing of microtubules demonstrated by Rothwell et al. (Rothwell, S. W., Grasser, W. A., and Murphy, D. B. (1986) J. Cell Biol. 102, 619-627). This proposal has been tested by measuring the rate of annealing of free microtubules to Tetrahymena axonemes under conditions identical to those used for the lengthening of sheared microtubules. That free microtubules anneal to axonemal microtubules is indicated by the following observations. Axonemes elongate at both ends in the presence of steady state microtubules, as predicted for a symmetrical annealing process; under conditions where the microtubule number concentration is greater than that for axonemes, the initial rate of axoneme elongation is more rapid with a low concentration of long microtubules at steady state than with a high number concentration of short microtubules at steady state. These observations are inconsistent with the predictions of a model based on microtubule dynamic instability (Mitchison, T., and Kirschner, M. (1984) Nature 312, 237-242). The annealing rate observed with axonemes can account for the rate of elongation of sheared steady state microtubules.