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The morphogenetic transition from yeast to filamentous growth is a characteristic feature of many pathogenic fungi. The corn pathogen Ustilago maydis serves as an excellent model system to study the molecular mechanism of polarized growth. Dimorphic switching is part of its sexual life and requires the small GTP-binding protein Rac1 together with its downstream effector, the p21-activated kinase Cla4. Small GTP-binding proteins of the Rho-family are activated by guanine nucleotide exchange factors (GEFs) and inactivated by GTPase activating proteins (GAPs). Guanine nucleotide dissociation inhibitors (GDIs) extract the GTPases from the membrane and sequester the inactive form in the cytosol.
In this study it is shown that dimorphic switching involves b mating-type dependent stimulation of the Rac1-specific GEF Cdc24. During polarized growth active Cdc24 recruits Rac1 into a Bem1-scaffolded complex which is located at the hyphal tip. Remarkably, ternary complex formation triggers destruction of Cdc24, most presumably by Cla4-dependent phosphorylation of Cdc24. Expression of stabilised Cdc24 mutants interfered with filamentous growth and plant infection indicating an important role for Cla4-induced destruction of Cdc24 during the maintenance of polarized growth. This allows the conclusion that degradation of Cdc24 ensures dynamic localization of active Cla4 kinase at the apical growth zone. This negative feedback regulation requires the ability of Rac1 to pass through its GDP-bound state. Therefore, the distinct functions of Rac1-specific GAPs and the Rho-GDI Rdi1 were analysed during hyphal tip growth. Evidence is provided that recycling of inactive Rac1 from the membrane depends on Rdi1 and endocytosis.
The closely related Rho-GTPases Cdc42 and Rac1 are highly conserved regulators of the actin cytoskeleton and have specific cellular functions in U. maydis. This study demonstrates that Rac1 can be substituted in the complex with Bem1 and Cla4 by mutant Cdc42F56W, which displays high affinity to Cdc24. This indicates that GEF-dependent recruitment of GTPases into higher-order complexes plays a major role for their signalling specificity.