MZT Proteins Form Multi-Faceted Structural Modules in the γ-Tubulin Ring Complex

Microtubule organization depends on the gamma-tubulin ring complex (gamma-TuRC), a similar to 2.3-MDa nucleation factor comprising an asymmetric assembly of gamma-tubulin and GCP2-GCP6. However, it is currently unclear how the gamma-TuRC-associated microproteins MZT1 and MZT2 contribute to the structure and regulation of the holocomplex. Here, we report cryo-EM structures of MZT1 and MZT2 in the context of the native human gamma-TURC. MZT1 forms two subcomplexes with the N-terminal alpha-helical domains of GCP3 or GCP6 (GCP-NHDs) within the gamma-TuRC "lumenal bridge." We determine the X-ray structure of recombinant MZT1/GCP6-NHD and find it is similar to that within the native gamma-TuRC. We identify two additional MZT/GCP-NHD-like subcomplexes, one of which is located on the outer face of the gamma-TuRC and comprises MZT2 and GCP2-NHD in complex with a centrosomin motif 1 (CM1)-containing peptide. Our data reveal how MZT1 and MZT2 establish multi-faceted, structurally mimetic "modules" that can expand structural and regulatory interfaces in the gamma-TuRC.