We have re-examined in vitro the most basic features of microtubule assembly and disassembly, in a simple system in which GTP-tubulin complexes successively go through an assembly phase and a disassembly phase, in the absence of excess of free GTP. We found that microtubule nucleation persists at a constant rate during most of microtubule assembly being completely insensitive to the free GTP-tubulin concentration, in contrast with previous hypothesis [Caudron et al., 2000], (PhD thesis of Nicolas Caudron, 1999-2001, directed by Odile Valiron and Didier Job).
Nicolas Caudron.Chemical instability in microtubule solutions: study and search for effectors.
[Abstract]
We have proposed that microtubule nucleate from a pool of tubulin oligomers that are present in tubulin solutions at the beginning of assembly. Such stable tubulin oligomers were isolated and we showed that they could combine to form microtubule seeds [Caudron et al., 2002]. Further, we described that subsequent microtubule elongation proceeds at a constant rate, independently of the free tubulin-GTP concentration. The rate at which tubulin sheets close to form tubes may be the limiting step during microtubule elongation. In contrast, we found that the rate of microtubules catastrophe depends very strongly on the initial free GTP-tubulin concentration and not on the instantaneous GTP-tubulin concentration. Very recently, we showed that this difference is related to the proportion of GDP-tubulin that appears in solution during tubulin assembly. Surprisingly, GDP-tubulin is efficiently incorporated into microtubules and microtubules built, in part, from GDP-tubulin are extremely stable, compared to polymers assembled from GTP-tubulin only, despite absence of detectable structural differences. Thus, GDP-tubulin seems to be a potent regulator of MTs dynamics.