Microtubules are essential structures that control various cellular processes by facilitating the movement of cargo across the cell. They are nucleated at microtubule-organising centres (MTOCs), which contain a core complex called the γ-tubulin small complex (γ-TuSC). The γ-TuSC may associate with additional proteins to form the γ-tubulin ring complex (γ-TuRC), which increases its microtubule nucleation capacity. Now, Reinhard Fischer and colleagues (Gao et al., 2019) compare the composition of two different types of MTOC in Aspergillus nidulans – spindle pole bodies (SPBs) and septal MTOCs (sMTOCs). They find that MztA, a homologue of the mammalian MOZART proteins (which regulate spindle and interphase microtubule organisation), localises to both SPBs and sMTOCs. mztA deletion strains (ΔmztA) display abnormal spindles in mitosis, suggesting that MztA promotes accurate spindle assembly. Furthermore, MtzA interacts with the γ-TuSC component GcpC and recruits the γ-TuRC to sMTOCs. Microtubule nucleation at sMTOCs is, consequently, impaired in ΔmztA strains. Moreover, the authors show that MztA localises to the inner, but not the outer plaque of SPBs, where it recruits GcpD to assemble the γ-TuRC. Using super-resolution microscopy, they further demonstrate that the number of γ-TuSC components at the outer and γ-TuRC components at the inner plaque of the SPBs changes during the cell cycle. Thus, this work reveals that the composition of MTOCs varies within one cell and throughout the cell cycle, presumably to perform distinct cellular and cell cycle-related functions.
