Monomeric gamma-tubulin nucleates microtubules

gamma-Tubulin is required for nucleation and polarized organization of micr otubules in vivo. The mechanism of microtubule nucleation by gamma-tubulin and the role of associated proteins is not understood. Here we show that in vitro translated monomeric gamma-tubulin nucleates microtubules by lowerin g the size of the nucleus from seven to three tubulin subunits, In capping the minus end with high affinity (10(10) M-1) and a binding stoichiometry o f one molecule of gamma-tubulin/microtubule, gamma-tubulin establishes the critical concentration of the plus end in the medium and prevents minus end growth. gamma-Tubulin interacts strongly with beta-tubulin. A structural m odel accounts for these results.