γ-Tubulin ring complexes (γTuRCs) initiate microtubule growth and mediate microtubule attachment at microtubule-organizing centers, such as centrosomes and the Golgi complex. However, the mechanisms that control γTuRC-mediated microtubule nucleation have remained mostly unknown. Here, we show that the DNA polymerase δ catalytic subunit (PolD1) binds directly to γTuRCs and potently inhibits γTuRC-mediated microtubule nucleation. Whereas PolD1 depletion through RNA interference does not influence centrosome-based microtubule growth, the depletion augments microtubule nucleation at the Golgi complex. Conversely, PolD1 overexpression inhibits Golgi-based microtubule nucleation. Golgi-derived microtubules are required for the assembly and maintenance of the proper Golgi structure, and we found that alteration of PolD1 levels affects Golgi structural organization. Moreover, suppression of PolD1 expression impairs Golgi reassembly after nocodazole-induced disassembly and causes defects in Golgi reorientation and directional cell migration. Collectively, these results reveal a mechanism that controls noncentrosomal γTuRC activity and regulates the organization of Golgi-derived microtubules.

Microtubule nucleation is mainly mediated by the γ‐tubulin ring complex (γTuRC), whose core components are γ‐tubulin and γ‐tubulin complex proteins GCP2–6. A substantial fraction of γ‐tubulin also exists with GCP2 and GCP3 in a tetramer called the γ‐tubulin small complex (γTuSC). To date, the mechanisms underlying the turnover of γ‐tubulin and GCPs have remained unclear. Here, we show that γ‐tubulin, GCP2, and GCP3 are proteolyzed by the ubiquitin‐proteasome system, and we identify cullin 1, cullin 4A, and cullin 4B as the E3 ligases that mediate the ubiquitination and, consequently, the degradation of γ‐tubulin. Notably, we found that γTuSC disassembly promotes the degradation of γ‐tubulin, GCP2, and GCP3, which indicates a role for γTuSCs in the stabilization of its components.

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