Genetic evidence for a microtubule-capture mechanism during polar growth of Aspergillus nidulans

Abstract

The cellular switch from symmetry to polarity in eukaryotes depends on the microtubule and actin cytoskeletons. In fungi such as Schizosaccharomyces pombe or Aspergillus nidulans, the microtubule cytoskeleton determines the sites of actin polymerization through cortical cell-end marker proteins. Here we describe A. nidulans MigA (microtubule guidance) as the first orthologue of the karyogamy protein Kar9 from Saccharomyces cerevisiae in filamentous fungi. A. nidulans MigA interacts with the cortical ApsA protein and is involved in spindle positioning during mitosis. MigA is also associated with septal and nuclear microtubule organizing centers (MTOCs). PALM super-resolution microscopy revealed that MigA is recruited to assembling and retracting microtubule plus-ends in an EbA-dependent manner. MigA is required for microtubule convergence in hyphal tips and plays a role in correct localization of the cell-end markers TeaA and TeaR. In addition, MigA interacts with a class V myosin, suggesting an active microtubule capture and pulling mechanism of microtubule ends along actin filaments. Hence, the organization of microtubules and actin depend on each other, and positive feedback loops ensure robust polar growth.