Asymmetric molecular architecture of the human γ-tubulin ring complex

Abstract

The{gamma} -tubulin ring complex ({gamma}-TuRC) is an essential regulator of centrosomal and acentrosomal microtubule formation 1-4. Metazoan{gamma} -TuRCs isolate as [~]2 MDa complexes containing the conserved proteins{gamma} -tubulin, GCP2 and GCP3, as well as the expanded subunits GCP4, GCP5, and GCP6 3,5,6. However, in current structural models,{gamma} -TuRCs assemble solely from subcomplexes of{gamma} -tubulin, GCP2 and GCP3 7. The role of the metazoan-specific subunits in{gamma} -TuRC assembly and architecture are not currently known, due to a lack of high resolution structural data for the native complex. Here, we present a cryo-EM structure of the native human{gamma} -TuRC at 3.8[A] resolution. Our reconstruction reveals an asymmetric, single helical-turn and cone-shaped structure built from at least 34 polypeptides. Pseudo-atomic models indicate that GCP4, GCP5 and GCP6 form distinct Y-shaped assemblies that structurally mimic GCP2/GCP3 subcomplexes and are distal to the{gamma} -TuRC \"seam\". Evolutionary expansion in metazoan-specific subunits diversifies the{gamma} -TuRC by introducing large (>100,000 [A]2) surfaces that could interact with different regulatory factors. We also identify an unanticipated structural bridge that includes an actin-like protein and spans the{gamma} -TuRC lumen. Despite its asymmetric composition and architecture, the human{gamma} -TuRC arranges{gamma} -tubulins into a helical geometry poised to nucleate microtubules. The observed compositional complexity of the{gamma} -TuRC could self-regulate its assembly into a cone-shaped structure to control microtubule formation across diverse contexts, e.g. within biological condensates 8 or alongside existing filaments 9.