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

Microtubules are hollow polymers of alpha beta-tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires gamma-tubulin, organized as an oligomer within the 2.2-MDa gamma-tubulin ring complex (gamma-TuRC) of higher eukaryotes(1-3). Structural insight is lacking regarding gamma-tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-angstrom crystal structure of human gamma-tubulin bound to GTP-gamma S ( a non-hydrolysable GTP analogue). We observe a 'curved' conformation for gamma-tubulin -GTP gamma S, similar to that seen for GDP-bound, unpolymerized alpha beta-tubulin(4). Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in gamma-tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between alpha- and beta-tubulins in the microtubule(5), and this association probably forms the basis for gamma-tubulin oligomerization within the gamma-TuRC. Laterally associated gamma-tubulins in the gamma-TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between alpha beta-tubulin heterodimers. Because they are dimeric, alpha beta-tubulins cannot form microtubule-like lateral associations in the curved conformation(5). The lateral array of gamma-tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.