Asymmetric molecular architecture of the human γ -tubulin ring complex

Abstract

SUMMARY: The γ -tubulin ring complex ( γ -TuRC) is an essential regulator of centrosomal and acentrosomal microtubule formation 1–4 . Metazoan γ -TuRCs isolate as ∼2 MDa complexes containing the conserved proteins γ -tubulin, GCP2 and GCP3, as well as the expanded subunits GCP4, GCP5, and GCP6 3,5,6 . However, in current structural models, γ -TuRCs assemble solely from subcomplexes of γ -tubulin, GCP2 and GCP3 7 . The role of the metazoan-specific subunits in γ -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 γ -TuRC at 3.8Å 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 γ -TuRC “seam”. Evolutionary expansion in metazoan-specific subunits diversifies the γ -TuRC by introducing large (>100,000 Å 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 γ -TuRC lumen. Despite its asymmetric composition and architecture, the human γ -TuRC arranges γ -tubulins into a helical geometry poised to nucleate microtubules. The observed compositional complexity of the γ -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 .