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

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Abstract
Microtubules are hollow polymers of αβ-tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires γ-tubulin, organized as an oligomer within the 2.2-MDa γ-tubulin ring complex (γ-TuRC) of higher eukaryotes1,2,3. Structural insight is lacking regarding γ-tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-Å crystal structure of human γ-tubulin bound to GTP-γS (a non-hydrolysable GTP analogue). We observe a ‘curved’ conformation for γ-tubulin–GTPγS, similar to that seen for GDP-bound, unpolymerized αβ-tubulin4. Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in γ-tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between α- and β-tubulins in the microtubule5, and this association probably forms the basis for γ-tubulin oligomerization within the γ-TuRC. Laterally associated γ-tubulins in the γ-TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between αβ-tubulin heterodimers. Because they are dimeric, αβ-tubulins cannot form microtubule-like lateral associations in the curved conformation5. The lateral array of γ-tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.