The role of small GTPases for exocyst localization and regulation of polar growth in Ustilago maydis

二态转换是许多病原真菌的特征。在其生命周期中，植物致病真菌黑穗病菌经历了萌芽和丝状生长之间的形态发生转变。这种开关由多等位基因b交配型位点控制，该位点编码一对同源结构域转录因子，称为bE和bW。虽然已经确定了许多受这个主控制位点调控的基因，但b交配型基因诱导丝状生长的分子机制仍然未知。我们正在研究Rho家族GTP结合蛋白Rac1的功能，它在丝状生长过程中起着分子开关的作用。我们发现b诱导的细丝形成涉及刺激rac1特异性Rho-GEF Cdc24的活性。Rho-GEF依赖的活性Rac1募集到bem1支架的三元复合物中触发Cdc24的破坏，从而构成一个负反馈回路。我们将分析这种自动调节在菌丝尖端生长的建立和维持中的作用。此外，还将研究活性Rac1在纤维形成过程中的动态定位以及Rac1被鸟嘌呤核苷酸解离抑制剂Rdi1回收的作用。此外，我们将分析与Cdc24相互作用的Ras1是否刺激Cdc24的GEF活性，从而可能参与二态转换。

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.