Monomeric γ-tubulin nucleates microtubules

Abstract

γ-Tubulin is required for nucleation and polarized organization of microtubules in vivo. The mechanism of microtubule nucleation by γ-tubulin and the role of associated proteins is not understood. Here we show that in vitro translated monomeric γ-tubulin nucleates microtubules by lowering the size of the nucleus from seven to three tubulin subunits. In capping the minus end with high affinity (1010 M-1) and a binding stoichiometry of one molecule of γ-tubulin/microtubule, γ-tubulin establishes the critical concentration of the plus end in the medium and prevents minus end growth. γ-Tubulin interacts strongly with β-tubulin. A structural model accounts for these results.