Multifaceted modes of γ-tubulin complex recruitment and microtubule nucleation at mitotic centrosomes

Microtubule nucleation is mediated by multi-protein ©-tubulin ring complexes (γ-TuRCs). In most eukaryotes, a GCP4/5/4/6 “core” complex promotes γ-tubulin small complex (γ-TuSC) association to generate γ-TuRCs within the cytosol. However, the importance of this core complex is uncertain, as its components are non-essential in various species. In Drosophila, Spindle defective-2 (Spd-2) and Centrosomin (Cnn) redundantly recruit γ-tubulin complexes to centrosomes during mitosis, but it remains unclear how. Here we show that Spd-2 recruits γ-TuRCs formed via the GCP4/5/4/6 core, but that Cnn can recruit γ-TuSCs independently of the GCP4/5/4/6 core via its well-conserved CM1 domain. Moreover, by selectively abolishing γ-tubulin complex recruitment, we show that mitotic centrosomes can nucleate microtubules independently of γ-tubulin complexes and that this depends on the TOG domain protein Mini-spindles (Msps). Collectively, our data help explain the dispensability of the GCP4/5/4/6 core for Drosophila development and show why centrosomes are such robust microtubule organising centres.