biostudies-literatureWurtz MDeutsche Forschungsgemeinschaft473https://www.ebi.ac.uk/biostudies/studies/S-EPMC8789826biostudies-literatureUnknown13(1)The gamma-tubulin ring complex (γ-TuRC) is the principal microtubule nucleation template in vertebrates. Recent cryo-EM reconstructions visualized the intricate quaternary structure of the γ-TuRC, containing more than thirty subunits, raising fundamental questions about γ-TuRC assembly and the role of actin as an integral part of the complex. Here, we reveal the structural mechanism underlying modular γ-TuRC assembly and identify a functional role of actin in microtubule nucleation. During γ-TuRC assembly, a GCP6-stabilized core comprising GCP2-3-4-5-4-6 is expanded by stepwise recruitment, selective stabilization and conformational locking of four pre-formed GCP2-GCP3 units. Formation of the lumenal bridge specifies incorporation of the terminal GCP2-GCP3 unit and thereby leads to closureNature communicationsModular assembly of the principal microtubule nucleator γ-TuRC.PMC8789826DFG Schi 295/4-4DFG PF 963/1-4Rahadian ASAtorino ESPfeffer SWurtz MBohler ASchiebel EZupa EEustermann SNeuner AVermeulen BJATonon GfalseModular assembly of the principal microtubule nucleator γ-TuRC.The gamma-tubulin ring complex (γ-TuRC) is the principal microtubule nucleation template in vertebrates. Recent cryo-EM reconstructions visualized the intricate quaternary structure of the γ-TuRC, containing more than thirty subunits, raising fundamental questions about γ-TuRC assembly and the role of actin as an integral part of the complex. Here, we reveal the structural mechanism underlying modular γ-TuRC assembly and identify a functional role of actin in microtubule nucleation. During γ-TuRC assembly, a GCP6-stabilized core comprising GCP2-3-4-5-4-6 is expanded by stepwise recruitment, selective stabilization and conformational locking of four pre-formed GCP2-GCP3 units. Formation of the lumenal bridge specifies incorporation of the terminal GCP2-GCP3 unit and thereby leads to closure2022-01-01T00:00:00Z2022 Jan2026-05-30T20:56:20.209Z2025-04-04T10:02:11.28ZS-EPMC87898263507898310.1038/s41467-022-28079-0