Dimorphic switching is a characteristic aspect of many pathogenic fungi. During its life cycle, the phytopathogenic fungus Ustilago maydis undergoes a morphogenetic transition between budding and filamentous growth. This switch is controlled by the multiallelic b mating-type locus, which encodes a pair of homeodomain transcription factors, named bE and bW. While many genes have been identified that are regulated by this master control locus, the molecular mechanism by which the b mating-type genes induce filamentous growth is still unknown. We are studying the function of the Rho family GTP binding protein Rac1, which acts as molecular switch during filamentous growth. We discovered that b-induced filament formation involves stimulation of the activity of the Rac1-specific Rho-GEF Cdc24. Rho-GEF dependent recruitment of active Rac1 into a Bem1-scaffolded ternary complex triggers destruction of Cdc24 and thus constitutes a negative feedback loop. We will analyze the role of this autoregulation for establishment and maintenance of hyphal tip growth. Furthermore, the dynamic localization of active Rac1 during filament formation and the role of Rac1 recycling by the guanine nucleotide dissociation inhibitor Rdi1 will be studied. In addition, we will analyze whether Ras1, which interacts with Cdc24, stimulates the GEF activity of Cdc24 and thus may participate in dimorphic switching.
Schematic model for the Rho-GEF dependent recruitment of active Rac1-GTP into a Bem1-scaffolded complex. The inactive GDP-bound form of Rac1 is indicated by orange color, the active GTP-bound form by green color. Interaction of Cla4 with Rac1-GTP results in activation of Cla4 kinase activity indicated by alteration of shape. Cla4 dependent phosphorylation, indicated by the red spot, triggers destruction of Cdc24 presumably via proteasomal degradation.