The Small GTP-Binding Proteins AgRho2 and AgRho5 Regulate Tip-Branching, Maintenance of the Growth Axis and Actin-Ring-Integrity in the Filamentous Fungus Ashbya gossypii

Abstract

GTPases of the Rho family are important molecular switches that regulate many basic cellular processes. The function of the Rho2 and Rho5 proteins from Saccharomyces cerevisiae and of their homologs in other species is poorly understood. Here, we report on the analysis of the AgRho2 and AgRho5 proteins of the filamentous fungus Ashbya gossypii. In contrast to S. cerevisiae mutants of both encoding genes displayed a strong morphological phenotype. The Agrho2 mutants showed defects in tip-branching, while Agrho5 mutants had a significantly decreased growth rate and failed to maintain their growth axis. In addition, the Agrho5 mutants had highly defective actin rings at septation sites. We also found that a deletion mutant of a putative GDP-GTP-exchange factor (GEF) that was homologous to a Rac-GEF from other species phenocopied the Agrho5 mutant, suggesting that both proteins act in the same pathway, but the AgRho5 protein has acquired functions that are fulfilled by Rac-proteins in other species.

Identification of Dck1 and Lmo1 as upstream regulators of the small GTPase Rho5 in Saccharomyces cerevisiae

“…Using Ashbya gossypii as another fungal model system (which is more closely related to S. cerevisiae than C. albicans but which shows exclusively filamentous growth), we found that deletion mutants devoid of the DCK1 or RHO5 homologs are indeed severely affected in hyphal growth and Rho5 did not relocate to mitochondria under oxidative stress (Nordmann et al ., ). This difference could be explained by the basically respiratory metabolism observed in A. gossypii , and its concurrent stronger dependence on mitochondrial functions, as opposed to S. cerevisiae , a yeast specialized in fermentation and devoid of a Pasteur effect (Lagunas et al ., ; van Dijken et al ., ).…”

The Small GTPases in Fungal Signaling Conservation and Function

“…Furthermore, the Rho3 protein works in polarity maintenance [ 90 ]. In A. gossypii , the Ag Rho2 regulates tip-branching, whereas Ag Rho5 modulates acting rings at septation sites, mycelial growth, and growth axis [ 91 ]. Furthermore, the GEF orthologous to a Rac-GEF from other organisms shows a similar phenotype as Agrho5 lacking mutant, which indicates that both proteins participate in the same pathway [ 91 ].…”

“…In A. gossypii , the Ag Rho2 regulates tip-branching, whereas Ag Rho5 modulates acting rings at septation sites, mycelial growth, and growth axis [ 91 ]. Furthermore, the GEF orthologous to a Rac-GEF from other organisms shows a similar phenotype as Agrho5 lacking mutant, which indicates that both proteins participate in the same pathway [ 91 ]. In this fungus, the Ras-like GTPase Rsr1p/Bud1p is localized at the hyphae tip and plays a role in apical polarization of the actin cytoskeleton, a decisive factor of growth direction ( Figure 7 ).…”

The Small Yeast GTPase Rho5 and Its Dimeric GEF Dck1/Lmo1 Respond to Glucose Starvation

“…Moreover, Dck1 is occasionally associated with the yeast bud neck, indicating that it could interact with the actomyosin ring during cytokinesis ( Figure 1 B). While such a function has been postulated for the Rho5 homolog in the related filamentous fungus Ashbya gossypii [ 39 ], the lack of morphological defects in the yeast rho5 deletion suggests that such an interaction, if it exists, would only have a weak physiological effect. Interestingly, dck1 and lmo1 deletions were slightly more resistant, rather than more sensitive, towards Latrunculin A.…”