An architectural switch in the evolution of the γ-tubulin ring complex

Abstract

Canonical microtubules contain 13-protofilaments and are templated by the γ-tubulin ring complex (γ-TuRC). However, some eukaryotes assemble non-canonical microtubules, like the 11-protofilament structures found in Caenorhabditis elegans. How γ-TuRCs adapt to template alternative microtubule geometries is unclear. Here, we present the cryo-electron microscopy structure of the C. elegans γ-TuRC (γ-TuRCCe), revealing a cone-shaped assembly consistent with an 11-protofilament template. While the complex incorporates the conserved subunits actin, GCP2 and GCP3, γ-TuRCCe replaces GCP4-6 with a divergent 4-spoked assembly containing additional copies of GCP2 and the nematode-specific proteins GTAP-1 and GTAP-2. Structures of nucleotide-free γ-TuRCCe subcomplexes reveal partial γ-tubulin unfolding, suggesting nucleotide binding stabilizes eukaryotic tubulins. Remarkably, reconstituted 4-spoked assemblies can multimerize into ∼13-fold symmetric microtubule nucleation templates in vitro, contrasting with the native complex’s 11-protofilament architecture. Our work defines the structural blueprint of an 11-protofilament microtubule template and shows how divergent γ-tubulin components are repurposed to accommodate non-canonical microtubule lattices.