title: IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response.
abstract: 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 IRE1alpha and ATF6 for signaling the mammalian UPR, we identified a UPR reporter gene that was defective for induction in IRE1alpha-null mouse embryonic fibroblasts and S2P-deficient Chinese hamster ovary (CHO) cells. We show that the endoribonuclease activity of IRE1alpha 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. IRE1alpha was not required for ATF6 cleavage, nuclear translocation, or transcriptional activation. However, ATF6 cleavage was required for IRE1alpha-dependent induction of UPR transcription. We propose that nuclear-localized IRE1alpha 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, IRE1alpha removes an unconventional 26-nucleotide intron that increases XBP1 transactivation potential. Both processing of ATF6 and IRE1alpha-mediated splicing of XBP1 mRNA are required for full activation of the UPR.
authors.affiliation: Howard Hughes Medical Institute, University of Michigan Medical Center, Ann Arbor, Michigan 48109, USA.
authors.forename: Kyungho
authors.initials: K
authors.lastname: Lee
authors.forename: Witoon
authors.initials: W
authors.lastname: Tirasophon
authors.forename: Xiaohua
authors.initials: X
authors.lastname: Shen
authors.forename: Marek
authors.initials: M
authors.lastname: Michalak
authors.forename: Ron
authors.initials: R
authors.lastname: Prywes
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Okada
authors.forename: Hiderou
authors.initials: H
authors.lastname: Yoshida
authors.forename: Kazutoshi
authors.initials: K
authors.lastname: Mori
authors.forename: Randal J
authors.initials: RJ
authors.lastname: Kaufman
mesh_terms: D051702:Activating Transcription Factor 6; D000818:Animals; D016466:CHO Cells; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D016384:Consensus Sequence; D006224:Cricetinae; D003412:Cricetulus; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000502:physiology*; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D005347:Fibroblasts; D005786:Gene Expression Regulation / Q000502:physiology*; D017930:Genes, Reporter; D007438:Introns; D008565:Membrane Proteins / Q000502:physiology*; D008666:Metalloendopeptidases / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D008957:Models, Genetic; D009685:Nuclear Envelope / Q000378:metabolism; D009690:Nucleic Acid Conformation; D017510:Protein Folding; D017346:Protein Serine-Threonine Kinases / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D012326:RNA Splicing; D000071701:Regulatory Factor X Transcription Factors; D015398:Signal Transduction; D014157:Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology*; D014162:Transfection; D000071717:X-Box Binding Protein 1
chemical_list: D051702:Activating Transcription Factor 6; C497112:Atf6 protein, mouse; D004268:DNA-Binding Proteins; D008565:Membrane Proteins; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605543:XBP1 protein, human; C000605544:Xbp1 protein, mouse; C486556:Ern2 protein, mouse; D017346:Protein Serine-Threonine Kinases; D008666:Metalloendopeptidases; C120654:MBTPS2 protein, human