To deal with accumulation of unfolded or misfolded protein caused by ER stress, UPR is activated to alter transcriptional programs through IRE1 and ATF6 (Fig. 2). In yeast, ER stress induces Ire1p homodimerization and trans-autophosphorylation to activate its RNase activity to initiate removal of a 252-base intron from the mRNA encoding the transcription factor Hac1p (19). Splicing of HAC1 mRNA increases its translational efficiency and alters the C-terminal sequence of Hac1p to generate a potent transcriptional activator. The protein encoded by spliced HAC1 mRNA binds and activates transcription from the UPR element (UPRE, minimal motif TGACGTG(C/A)) upstream of many UPR target genes (2, 20). Analysis of the promoter regions of UPR-inducible genes in mammals, such as BiP, GRP94, and calreticulin, identified a mammalian ER stress response element (ERSE, minimal motif: CCAATN9CCACG) that is necessary and sufficient for UPR gene activation (21). By using an ERSE as a probe in a yeast one-hybrid screen, two UPR-specific b-ZIP transcription factors, the X-box DNA-binding protein (XBP1) and ATF6, were isolated. XBP1 was identified as a homologue of yeast Hac1p that is a substrate for mammalian IRE1 RNase activity (22–24). On activation of the UPR, IRE1 RNase cleaves XBP1 mRNA to remove a 26-nucleotide intron, generating a translational frameshift. As the precedent of HAC1 regulation in yeast, the spliced XBP1 mRNA encodes a protein with a novel C terminus that acts as a potent transcription activator for many UPR target genes.
ATF6 is a UPR transducer that can bind ERSE elements in the promoters of UPR-responsive genes (21). There are two forms of ATF6, ATF6 (90 kDa) and ATF6 (110 kDa, also known as CREB-RP), both of which require the presence of the transcription factor NF-Y to bind to an ERSE (14, 25, 26). On activation of the UPR, ATF6 is transported to Golgi where it is cleaved by site-1 protease (S1P) and site-2 protease (S2P) to generate a 50-kDa cytosolic b-ZIP-containing fragment that migrates to the nucleus to activate transcription of UPR target genes (17, 27) (Fig. 2). Notably, S1P and S2P are also the processing enzymes that cleave the ER-associated transmembrane sterol-response element-binding protein upon cholesterol deprivation. The cytosolic fragment of cleaved sterol-response element-binding protein migrates to the nucleus to activate transcription of genes required for sterol biosynthesis (27).
ATF6 regulates a group of genes encoding ER-resident molecular chaperones and genes encoding folding enzymes, whereas XBP1 regulates a subset of ER-resident chaperone genes that are essential for protein folding, maturation, and degradation in the ER (28, 29). It was previously proposed that XBP1 mRNA is induced by ATF6 in response to ER stress to generate more substrate XBP1 mRNA for IRE1-mediated splicing (22, 25, 30). However, UPR induction of XBP1 transcripts and proteins was not altered in the cells having defective or reduced ATF6 cleavage (29, 30). Induction of ATF6 mRNA upon ER stress was partially compromised in the absence of XBP1; therefore it was proposed that ATF6 lies downstream of XBP1 in some cases (29). These results suggest that XBP1 and ATF6 are situated largely in parallel pathways and may interact with each other upon ER stress.