Phosphate is the third nutrient monitored by TOR in <i>Candida albicans</i> and provides a target for fungal-specific indirect TOR inhibition.

Abstract

The Target of Rapamycin (TOR) pathway regulates morphogenesis and responses to host cells in the fungal pathogen <i>Candida albicans</i> Eukaryotic Target of Rapamycin complex 1 (TORC1) induces growth and proliferation in response to nitrogen and carbon source availability. Our unbiased genetic approach seeking unknown components of TORC1 signaling in <i>C. albicans</i> revealed that the phosphate transporter Pho84 is required for normal TORC1 activity. We found that mutants in <i>PHO84</i> are hypersensitive to rapamycin and in response to phosphate feeding, generate less phosphorylated ribosomal protein S6 (P-S6) than the WT. The small GTPase Gtr1, a component of the TORC1-activating EGO complex, links Pho84 to TORC1. Mutants in Gtr1 but not in another TORC1-activating GTPase, Rhb1, are defective in the P-S6 response to phosphate. Overexpression of Gtr1 and a constitutively active Gtr1^Q67L mutant suppresses TORC1-related defects. In <i>Saccharomyces cerevisiae pho84</i> mutants, constitutively active Gtr1 suppresses a TORC1 signaling defect but does not rescue rapamycin hypersensitivity. Hence, connections from phosphate homeostasis (PHO) to TORC1 may differ between <i>C. albicans</i> and <i>S. cerevisiae</i> The converse direction of signaling from TORC1 to the PHO regulon previously observed in <i>S. cerevisiae</i> was genetically shown in <i>C. albicans</i> using conditional <i>TOR1</i> alleles. A small molecule inhibitor of Pho84, a Food and Drug Administration-approved drug, inhibits TORC1 signaling and potentiates the activity of the antifungals amphotericin B and micafungin. Anabolic TORC1-dependent processes require significant amounts of phosphate. Our study shows that phosphate availability is monitored and also controlled by TORC1 and that TORC1 can be indirectly targeted by inhibiting Pho84.