The role of Ire1 in azole sensitivity in Saccharomyces cerevisiae
Abstract
In eukaryotes, the Unfolded Protein Response (UPR) maintains proteostasis in the endoplasmic reticulum (ER). In yeast, the UPR is activated by the ER-resident kinase/RNase sensor protein; Inositol requiring enzyme 1 (Ire1). During ER stress, Ire1 oligomerizes and splices the premature HAC1 mRNA. The Hac1 transcription factor which binds to the unfolded protein response element (UPRE) and upregulate genes to mitigate the ER stress. Although poorly understood, the UPR is thought to play an essential role in antifungal resistance of pathogenic species. Therefore, with the highly characterized Saccharomyces cerevisiae model organism, I characterize the function of Ire1 upon azole treatment. For the first time in S. cerevisiae, I found that while the Ire1 is required for azole resistance, the UPR transcriptional response is dispensable as HAC1 splicing and Ire1 oligomerization upon azole treatment do not occur. This suggests a requirement for Ire1 in azole resistance which is Hac1 and UPR independent. Moreover, I have found that the kinase and luminal domains of Ire1 are dispensable during azole resistance. I also show that the upregulation of the UPRE-regulated ERAD proteins; KAR2 and HRD1 are dispensable, suggesting the absence of proteotoxic stress upon azole treatment. Interestingly, I found that in the absence of Ire1, ergosterol synthesis genes which are regulated independently of the UPR are downregulated, which may explain the increased azole resistance in ∆ire1 mutants. Overall, our data suggest that the S. cerevisiae Ire1 has a UPR independent function in the regulation of ergosterol synthesis genes which confers greater resistance to azoles.