Two Functionally Redundant FK506-Binding Proteins Regulate Multidrug Resistance Gene Expression and Govern Azole Antifungal Resistance

Abstract

Increasing resistance to antifungal therapy is an impediment to effective treatment of fungal infections. Candida glabrata is an opportunistic human fungal pathogen which is inherently less susceptible to cost-effective azole antifungals. Gain-of-function mutations in the Zn-finger pleiotropic drug resistance transcriptional activator-encoding gene, CgPDR1, are the most prevalent cause of azole resistance in clinical settings. CgPDR1 is also transcriptionally activated upon azole exposure, however, factors governing CgPDR1 gene expression are not yet fully understood. Here, we have uncovered a novel role for two FK506-binding proteins, CgFpr3 and CgFpr4, in regulation of the CgPDR1 regulon. We show that CgFpr3 and CgFpr4 possess peptidyl-prolyl isomerase domain, and act redundantly to control CgPDR1 expression, as Cgfpr3Δ4Δ mutant displayed elevated expression of CgPDR1 gene, along with overexpression of its target genes, CgCDR1, CgCDR2 and CgSNQ2, that code for ATP-binding cassette multidrug transporters. Further, CgFpr3 and CgFpr4 are required for maintenance of histone H3 and H4 protein levels, and fluconazole exposure leads to elevated H3 and H4 protein levels. Consistent with a role of histone proteins in azole resistance, disruption of genes coding for the histone demethylase CgRph1 and histone H3K36-specific methyltransferase CgSet2 leads to increased and decreased susceptibility to fluconazole, respectively, with Cgrph1Δ mutant displaying significantly lower basal expression of CgPDR1 and CgCDR1 genes. These data underscore a hitherto unknown role of histone methylation in modulating the most common azole antifungal resistance mechanism. Altogether, our findings establish a link between CgFpr-mediated histone homeostasis and CgPDR1 gene expression, and implicate CgFpr in virulence of C. glabrata.