During nucleation – an essential early step in de novo formation of microtubules – the γ-tubulin complex (γ-TuC) is recruited to prospective microtubule-organising centres. Relatively little is known about how this occurs, although it is thought that – in fission yeast – the interacting proteins Mto1 and Mto2 recruit γ-TuC. Ken Sawin and colleagues previously explored how knocking out mto1 or mto2 affected nucleation; now, on , they use site-directed mutagenesis of mto1 to further investigate the association of Mto1 and Mto2 with γ-TuC. The authors first show that, similar to Δmto1 mutants, cytoplasmic microtubule nucleation is abolished in fission yeast carrying a mutant mto1 with a disrupted centrosomin 1 motif (CM1) region; moreover, the mutant Mto1 does not interact with γ-TuC, although it localises normally and interacts with Mto2. By contrast, mutations outside the CM1 region of Mto1 phenocopy Δmto2 yeast – limited microtubule nucleation does occur, but binding of Mto1 to Mto2 or γ-TuC is inhibited. The authors next show that Mto1 and Mto2 form a γ-TuC-independent complex, and that each protein binds only weakly to γ-TuC in the absence of the other. They conclude that Mto1 and Mto2 act cooperatively to promote the association of the Mto1/2 complex with γ-TuC.
