The microtubule cytoskeleton is essential in mediating a number of critical cellular processes, affecting cell shape, transport, organelle organization, and chromosomal segregation during mitosis. Microtubule network dynamics are controlled by many factors including the efficiency and localization of the nucleation machinery. Microtubule nucleation is dependent on the universally conserved γ-tubulin small complex (γTuSC), a 300 kDa heterotetramer composed of two copies of γ-tubulin and one each of accessory proteins GCP2 and GCP3. In yeast, nucleation is mediated by a heptameric ring of γTuSC, which presents 13 γ-tubulins to form a template for microtubule nucleation. We have obtained single-particle structures of the γTuSC as a monomer and dimer at resolutions of 3.6-4.6Å, allowing us to build an atomic model for this important complex. By comparison with a crystal structure of isolated γ-tubulin, it is clear that γ-tubulin is activated upon assembly into the γTuSC, in a manner analogous to the bent to straight transition in αβ-tubulin upon assembly into the microtubule lattice. Our structures allow us to map phosphorylation sites, revealing several at key interfaces, highly suggestive of their role in regulating spindle pole body attachment and assembly into rings. When combined with previous lower resolution structures of helical assemblies, we observe that additional conformational changes occur during ring assembly and activation.
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