An oscillatory mode for microtubule assembly.

Depending upon the conditions under which polymerization takes place, pure tubulin can assemble into microtubules following either the usual monotonic kinetics or a more complex oscillatory mechanism. When present, these oscillations involve large cyclic changes in the extent of polymer formed before a steady-state is reached. Analysis of the microtubules formed at different times shows that these oscillations involve marked redistribution in both the length and number of microtubules. No significant difference is found between two populations of microtubules corresponding to the same level of assembly, one for which the extent of polymerization will remain stable with time and one for which it will decrease by as much as 90% in the next oscillation. The amplitude of these oscillations is sensitive to changes in the concentrations of protein, nucleotide (GTP, GDP or GMPpNp), magnesium ion or GTP regenerating system. A complete shift from an oscillatory to a monotonic polymerization can be induced by a minor increase in the concentration of free nucleotide, GTP or GDP.