Role of the unfolded protein response pathway in regulation of INO1 and in the sec14 bypass mechanism in Saccharomyces cerevisiae.

INO1, encoding inositol 1-phosphate synthase, is the most highly regulated of a class of genes containing the repeated element, UAS(INO), in their promoters. Transcription of UAS(INO)-containing genes is modulated by the availability of exogenous inositol and by signals generated by alteration of phospholipid metabolism. The unfolded protein response (UPR) pathway also is involved in INO1 expression and the ire1Delta and hac1Delta mutants are inositol auxotrophs. We examined the role of the UPR in transmitting a signal generated in response to inositol deprivation and to alteration of phospholipid biosynthesis created in the sec14(ts) cki1Delta genetic background. We report that the UPR is required for sustained high-level INO1 expression in wild-type strains, but not for transient derepression in response to inositol deprivation. Moreover, the UPR is not required for expression or regulation of INO1 in response to the change in lipid metabolism that occurs in the sec14(ts) cki1Delta genetic background. Thus, the UPR signal transduction pathway is not involved directly in transcriptional regulation of INO1 and other UAS(INO)-containing genes. However, we discovered that inactivation of Sec14p leads to activation of the UPR, and that sec14 cki1 strains exhibit defective vacuolar morphology, suggesting that the mechanism by which the cki1Delta mutation suppresses the growth and secretory defect of sec14 does not fully restore wild-type morphology. Finally, synthetic lethality involving sec14 and UPR mutations suggests that the UPR plays an essential role in survival of sec14 cki1 strains.