The structure of the γ-tubulin small complex: Implications of its architecture and flexibility for microtubule nucleation

The γ-tubulin small complex (γ-TuSC) is an evolutionarily conserved heterotetramer essential for microtubule nucleation. We have determined the structure of the Saccharomyces cerevisiae γ-TuSC at 25-Å resolution by electron microscopy. γ-TuSC is Y-shaped, with an elongated body connected to two arms. Gold labeling showed that the two γ-tubulins are located in lobes at the ends of the arms, and the relative orientations of the other γ-TuSC components were determined by in vivo FRET. The structures of different subpopulations of γ-TuSC indicate flexibility in the connection between a mobile arm and the rest of the complex, resulting in variation of the relative positions and orientations of the γ-tubulins. In all of the structures, the γ-tubulins are distinctly separated, a configuration incompatible with the microtubule lattice. The separation of the γ-tubulins in isolated γ-TuSC likely plays a role in suppressing its intrinsic microtubule-nucleating activity, which is relatively weak until the γ-TuSC is incorporated into higher order complexes or localized to microtubule-organizing centers. We propose that further movement of the mobile arm is required to bring the γ-tubulins together in microtubule-like interactions, and provide a template for microtubule growth.