Ras1 and Ras2 play antagonistic roles in regulating cellular cAMP level, stationary-phase entry and stress response in Candida albicans

The GTPase Ras1 activates the yeast-to-hypha transition in Candida albicans by activating cAMP synthesis. Here, we have characterized Ras2. Ras2 belongs to a group of atypical Ras proteins in some fungal species that share poor identity with other Ras GTPases with many variations in conserved motifs thought to be crucial for Ras-associated activities. We find that recombinant Ras2 is enzymatically as active as Ras1. However, only RAS1 can rescue the lethality of the Saccharomyces cerevisiae ras1 ras2 mutant, suggesting functional divergence of the two genes. ras2Δ is normal in hyphal growth, but deleting RAS2 in the ras1Δ background greatly aggravates the hyphal defect, indicating that Ras2 also has a role in hyphal development. Strikingly, while RAS1 deletion causes a ~20-fold decrease in cellular cAMP, further deletion of RAS2 restores it to ~30% of the wild-type level. Consistently, while the ras1Δ mutant enters the stationary phase prematurely, the double mutant does so normally. Moreover, ras1Δ cells exhibit increased resistance to H₂O₂ and higher sensitivity to the heavy metal Co²⁺, whereas ras2Δ cells show the opposite phenotypes. Together, our data reveal a novel regulatory mechanism by which two antagonizing Ras GTPases balance each other in regulating multiple cellular processes in C. albicans.