Die Rolle von Ras1 bei der Regulation des polaren Wachstums in Ustilago maydis

Small G-Proteins of the Ras-family act as molecular switches that cycle between the active GTP-bound state and the inactive GDP-bound state. In their GTP-bound form Ras-GTPases activate effector proteins that are necessary for the regulation of various cellular processes. Ustilago maydis, a dimorphic phytopathogenic fungus, is an excellent model organism to study the role of Ras-GTPases throughout its life cycle. Ras1 is an essential protein in U. maydis, it plays a role in the regulation of filamentous growth, septum formation, vesicle morphology and organization of the cytoskeleton. Additionally, Ras1 can interact and activate the MAPK-cascade that is involved in pheromone signal transduction. The Rho-GTPase Rac1 could be identified as an effector of Ras1 at the cytoplasmic membrane, most likely via activation of its cognate GEF Cdc24. Two GEFs could be identified, that activate Ras1 during the stimulation of the MAPK-cascade and the Rac1-signalingmodule, Sql2 and Tfh1. The intracellular localization of Ras1 depends on a palmitoylation motive. Palmitoylation of Ras1 is not essential, but necessary for Ras1 induced filament formation and activation of Rac1via Ras1.