In yeast, the transmembrane protein kinase/endoribonuclease Ire1p activated by endoplasmic reticulum stress cleaves HAC1 mRNA, leading to production of the transcription factor Hac1p that activates the unfolded protein response (UPR). In mammals, no Hac1p counterpart has yet been discovered despite the presence of Ire1p homologs in the endoplasmic reticulum. Instead, the transcription factor ATF6 specific to the mammalian UPR is regulated by intramembrane proteolysis. Here, we identified the transcription factor XBP1, a target of ATF6, as a mammalian substrate of such an unconventional mRNA splicing system and showed that only the spliced form of XBP1 can activate the UPR efficiently. Our results reveal features of the UPR conserved during evolution and clarify the relationship between IRE1- and ATF6-dependent pathways.

The unfolded protein response (UPR) controls the levels of molecular chaperones and enzymes involved in protein folding in the endoplasmic reticulum (ER). We recently isolated ATF6 as a candidate for mammalian UPR-specific transcription factor. We report here that ATF6 constitutively expressed as a 90-kDa protein (p90ATF6) is directly converted to a 50-kDa protein (p50ATF6) in ER-stressed cells. Furthermore, we showed that the most important consequence of this conversion was altered subcellular localization; p90ATF6 is embedded in the ER, whereas p50ATF6 is a nuclear protein. p90ATF6 is a type II transmembrane glycoprotein with a hydrophobic stretch in the middle of the molecule. Thus, the N-terminal half containing a basic leucine zipper motif is oriented facing the cytoplasm. Full-length ATF6 as well as its C-terminal deletion mutant carrying the transmembrane domain is localized in the ER when transfected. In contrast, mutant ATF6 representing the cytoplasmic region translocates into the nucleus and activates transcription of the endogenous GRP78/BiP gene. We propose that ER stress-induced proteolysis of membrane-bound p90ATF6 releases soluble p50ATF6, leading to induced transcription in the nucleus. Unlike yeast UPR, mammalian UPR appears to use a system similar to that reported for cholesterol homeostasis.

When unfolded proteins accumulate in the endoplasmic reticulum (ER), transcription of glucose-regulated proteins (GRPs) representing ER-resident molecular chaperones is markedly induced via the unfolded protein response (UPR) pathway. In contrast to recent progress in the analysis of yeast UPR, both cis-acting elements and transactivators responsible for mammalian UPR have remained obscure. Here, we analyzed the promoter regions of human GRP78, GRP94, and calreticulin genes and identified a novel element designated the ER stress response element (ERSE). ERSE, with a consensus of CCAATN 9 CCACG, was shown to be necessary and sufficient for induction of these GRPs. Using yeast onehybrid screening, we isolated a human cDNA encoding a basic leucine zipper (bZIP) protein, ATF6, as a putative ERSE-binding protein. When overexpressed in HeLa cells, ATF6 enhanced transcription of GRP genes in an ERSE-dependent manner, whereas CREB-RP, another bZIP protein closely related to ATF6, specifically inhibited GRP induction. Endogenous ATF6 constitutively expressed as a 90-kDa protein was converted to a 50-kDa protein in ER-stressed cells, which appeared to be important for the cellular response to ER stress. These results suggest that, as in yeast, bZIP proteins are involved in mammalian UPR, acting through newly defined ERSE.

Metazoans express three unfolded protein response transducers (IRE1, PERK, and ATF6) ubiquitously to cope with endoplasmic reticulum (ER) stress. ATF6 is an ER membrane-bound transcription factor activated by ER stress-induced proteolysis and has been duplicated in mammals. Here, we generated ATF6alpha- and ATF6beta-knockout mice, which developed normally, and then found that their double knockout caused embryonic lethality. Analysis of mouse embryonic fibroblasts (MEFs) deficient in ATF6alpha or ATF6beta revealed that ATF6alpha is solely responsible for transcriptional induction of ER chaperones and that ATF6alpha heterodimerizes with XBP1 for the induction of ER-associated degradation components. ATF6alpha(-/-) MEFs are sensitive to ER stress. Unaltered responses observed in ATF6beta(-/-) MEFs indicate that ATF6beta is not a negative regulator of ATF6alpha. These results demonstrate that ATF6alpha functions as a critical regulator of ER quality control proteins in mammalian cells, in marked contrast to worm and fly cells in which IRE1 is responsible.

Proteins synthesized in the endoplasmic reticulum (ER) are properly folded with the assistance of ER chaperones. Malfolded proteins are disposed of by ER‐associated protein degradation (ERAD). When the amount of unfolded protein exceeds the folding capacity of the ER, human cells activate a defense mechanism called the ER stress response, which induces expression of ER chaperones and ERAD components and transiently attenuates protein synthesis to decrease the burden on the ER. It has been revealed that three independent response pathways separately regulate induction of the expression of chaperones, ERAD components, and translational attenuation. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and bipolar disorder, which are collectively known as ‘conformational diseases’. In this review, I will summarize recent progress in this field. Molecules that regulate the ER stress response would be potential candidates for drug targets in various conformational diseases.

All eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) by signaling an adaptive pathway termed the unfolded protein response (UPR). In yeast, a type-I ER transmembrane protein kinase, Ire1p, is the proximal sensor of unfolded proteins in the ER lumen that initiates an unconventional splicing reaction on HAC1 mRNA. Hac1p is a transcription factor required for induction of UPR genes. In higher eukaryotic cells, the UPR also induces site-2 protease (S2P)-mediated cleavage of ER-localized ATF6 to generate an N-terminal fragment that activates transcription of UPR genes. To elucidate the requirements for IRE1␣ and ATF6 for signaling the mammalian UPR, we identified a UPR reporter gene that was defective for induction in IRE1␣-null mouse embryonic fibroblasts and S2P-deficient Chinese hamster ovary (CHO) cells. We show that the endoribonuclease activity of IRE1␣ is required to splice XBP1 (X-box binding protein) mRNA to generate a new C terminus, thereby converting it into a potent UPR transcriptional activator. IRE1␣ was not required for ATF6 cleavage, nuclear translocation, or transcriptional activation. However, ATF6 cleavage was required for IRE1␣-dependent induction of UPR transcription. We propose that nuclear-localized IRE1␣ and cytoplasmic-localized ATF6 signaling pathways merge through regulation of XBP1 activity to induce downstream gene expression. Whereas ATF6 increases the amount of XBP1 mRNA, IRE1␣ removes an unconventional 26-nucleotide intron that increases XBP1 transactivation potential. Both processing of ATF6 and IRE1␣-mediated splicing of XBP1 mRNA are required for full activation of the UPR.

Transcription of genes encoding molecular chaperones and folding enzymes in the endoplasmic reticulum (ER) is induced by accumulation of unfolded proteins in the ER. This intracellular signaling, known as the unfolded protein response (UPR), is mediated by the cis-acting ER stress response element (ERSE) in mammals. In addition to ER chaperones, the mammalian transcription factor CHOP (also called GADD153) is induced by ER stress. We report here that the transcription factor XBP-1 (also called TREB5) is also induced by ER stress and that induction of CHOP and XBP-1 is mediated by ERSE. The ERSE consensus sequence is CCAAT-N 9 -CCACG. As the general transcription factor NF-Y (also known as CBF) binds to CCAAT, CCACG is considered to provide specificity in the mammalian UPR. We recently found that the basic leucine zipper protein ATF6 isolated as a CCACG-binding protein is synthesized as a transmembrane protein in the ER, and ER stress-induced proteolysis produces a soluble form of ATF6 that translocates into the nucleus. We report here that overexpression of soluble ATF6 activates transcription of the CHOP and XBP-1 genes as well as of ER chaperone genes constitutively, whereas overexpression of a dominant negative mutant of ATF6 blocks the induction by ER stress. Furthermore, we demonstrated that soluble ATF6 binds directly to CCACG only when CCAAT exactly 9 bp upstream of CCACG is bound to NF-Y. Based on these and other findings, we concluded that specific and direct interactions between ATF6 and ERSE are critical for transcriptional induction not only of ER chaperones but also of CHOP and XBP-1.Secretory and transmembrane proteins must fold properly in the endoplasmic reticulum (ER) prior to subsequent transport to subcellular compartments that reside in the secretory pathway (14,19). This productive folding process, however, can be perturbed by a variety of physiological and environmental stress conditions that cause accumulation of unfolded proteins in the ER. Under such ER stress conditions, homeostasis of protein folding in the ER is maintained by interorganelle signaling from the ER to the nucleus, a process called the unfolded protein response (UPR) (20,30). Thus, from yeast to humans, transcription of genes encoding molecular chaperones and folding enzymes in the ER is induced in the nucleus in response to unfolding in the ER. Mammalian ER stress-inducible proteins include molecular chaperones such as GRP78 (also known as BiP), GRP94, GRP170 (also known as ORP150), and calreticulin as well as folding enzymes such as peptidyl-prolylcis-trans-isomerase FKBP13, protein disulfide isomerase, and protein disulfide isomerase-like proteins ERp72, ERp61 (also known as ERp57 or GRP58), and ERp29 (references 13 and 20 and references therein), indicating that synthesis of the majority of proteins assisting or facilitating protein folding in the ER is coregulated. Thus, the cell can adjust the folding capacity in the ER quite effectively by simply controlling cellular UPR activity.The mechanism of the UPR has been v...

The unfolded protein response (UPR) is a transcriptional and translational intracellular signaling pathway activated by the accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER). We have used C. elegans as a genetic model system to dissect UPR signaling in a multicellular organism. C. elegans requires ire-1-mediated splicing of xbp-1 mRNA for UPR gene transcription and survival upon ER stress. In addition, ire-1/xbp-1 acts with pek-1, a protein kinase that mediates translation attenuation, in complementary pathways that are essential for worm development and survival. We propose that UPR transcriptional activation by ire-1 as well as translational attenuation by pek-1 maintain ER homeostasis. The results demonstrate that the UPR and ER homeostasis are essential for metazoan development.