A structural model for the autoinhibition of the γ-TuRC–activating CM1 domain

Microtubule nucleation by γ-tubulin ring complexes (γ-TuRCs) must be spatiotemporally controlled to avoid major cell division errors. γ-TuRCs are activated by the centrosome-specific binding of a highly conserved centrosomin motif 1 (CM1) domain, but how binding is prevented in the cytosol remains unclear. We provide the first structural explanation for CM1 autoinhibition, revealing that stable intramolecular interactions mediate cooperative folding to block γ-TuRC–binding interfaces. Using various structural approaches, we show that CM1 forms a conserved dimeric coiled-coil hairpin that sequesters a key phenylalanine residue essential for γ-TuRC binding. This appears sufficient for autoinhibition in humans, but in flies autoinhibition requires the association of an upstream CM1 autoinhibition (CAI) domain, which forms a 4-helix dimer that covers the opposite phenylalanine residue of the hairpin dimer. Collectively, the data provide the first structural model for CM1 autoinhibition and reveal evolutionary differences across species.