The spindle pole body of Aspergillus nidulans is asymmetrically composed with changing numbers of gamma-tubulin complexes

Centrosomes are important microtubule-organizing centers (MTOC) in animal cells. In addition, non-centrosomal MTOCs (ncMTOCs) are found in many cell types. Their composition and structure are only poorly understood. Here we analyzed nuclear MTOCs (spindle-pole bodies, SPBs) and septal MTOCs in Aspergillus nidulans. They both contain γ-tubulin along with a family of γ-tubulin complex proteins (GCPs). Our data suggest that SPBs consist of γ-tubulin small complexes (γ-TuSC) at the outer and larger γ-tubulin ring complexes (γ-TuRC) at the inner plaque. We show that an orthologue of the human MOZART protein interacted with the inner plaque receptor PcpAPcp1 at SPBs, while no interaction nor co-localization was detected between MztA and the outer plaque receptor ApsBMto1. Septal MTOCs consist of γ-TuRCs including MztA but are anchored through AspB and Spa18Mto2. MztA was not essential although abnormal spindles were observed frequently. Quantitative PALM imaging revealed unexpected dynamics of the protein composition of SPBs with changing numbers of γ-tubulin complexes over time during interphase and constant numbers during mitosis.