Insights into microtubule nucleation from the crystal structure of human γ-tubulin

Tubulin in the groove Tubulin proteins have a central role in the life of eukaryotic cells. αβ-Tubulin polymerizes to form the microtubules required for chromosome segregation and organelle positioning. γ-Tubulin initiates microtubule assembly in vivo and is part of a multimeric complex at the centrosome. The crystal structure of human γ-tubulin bound to GTPγS is reported this week, the highest resolution (2.7 Å) structure of any tubulin to date. The structure suggests new ways of thinking about the roles of conformational change and nucleotide binding in microtubule assembly. The antitumour drug vinblastine is known to target tubulin. Its actual binding site and mechanism of action are unknown but now the X-ray structure of vinblastine bound in a tubulin/protein complex has been determined. Vinblastine introduces a wedge at the junction of two tubulin molecules, thereby interfering with microtubule production and promoting self-association of tubulin molecules into spiral aggregates. A hydrophobic groove on the α-tubulin surface acts both as a binding site for vinblastine and as a point of intermolecular contact in microtubules, so may be an attractive candidate for new microtubule depolymerizing drugs.