Microtubule formation and the initial association of tubulin dimers

Microtubule protein could be prepared in high yield, and could form copious microtubules, in solutions containing glutamate but not in solutions containing only phosphate ions. Correspondingly, tubulin after isolation showed an association equilibrium in the presence of glutamate (or other zwitterions), but not in phosphate buffers. The correlation suggests that this association to tetramers is probably the initial step in the mechanism of microtubule formation.