IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response.
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.
Lee
Kyungho
K
Howard Hughes Medical Institute, University of Michigan Medical Center, Ann Arbor, Michigan 48109, USA.
Tirasophon
Witoon
W
Shen
Xiaohua
X
Michalak
Marek
M
Prywes
Ron
R
Okada
Tetsuya
T
Yoshida
Hiderou
H
Mori
Kazutoshi
K
Kaufman
Randal J
RJ
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
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