Microtubule-organizing centers of Aspergillus nidulans are anchored at septa by a disordered protein

Microtubule-organizing centers (MTOCs) are large, multi-subunit protein complexes. Schizosaccharomyces pombe harbors MTOCs at spindle pole bodies, transient MTOCs in the division plane (eMTOCs) and nuclear-envelope associated MTOCs in interphase cells (iMTOCs). In the filamentous fungus Aspergillus nidulans SPBs and septum-associated MTOCs were described. Although comparable to S. pombe eMTOCs, A. nidulans sMTOCS are permanent septum-associated structures. The composition of sMTOCs is poorly understood and how they are targeted to septa was unknown. Here, we show that in A. nidulans several SPB outer plaque proteins also locate to sMTOCs while other SPB proteins do not, including SfiA, a protein required for SPB duplication in Saccharomyces cerevisiae and S. pombe and PcpA, the anchor for -TuSCs at the SPB inner plaque. The A. nidulans disordered protein Spa18(Mto2) and the centrosomin-domain containing protein ApsB(Mto1) were required for recruiting the -TuRC component GcpC to sMTOCs and for seeding MT formation from septa. Testing different septum-associated proteins for a role in sMTOC function, Spa10 was identified. It forms a septal pore disc structure, recruits Spa18 and ApsB to septa and is required for sMTOC activity. This is the first evidence for a septum-specific protein, Spa10, as anchor for a specific class of MTOCs.