IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response (original) (raw)
- Kyungho Lee1,
- Witoon Tirasophon2,6,
- Xiaohua Shen2,
- Marek Michalak3,
- Ron Prywes4,
- Tetsuya Okada5,
- Hiderou Yoshida5,
- Kazutoshi Mori5, and
- Randal J. Kaufman1,2,7
- 1Howard Hughes Medical Institute and 2Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan 48109, USA; 3CIHR Group in Molecular Biology of Membrane Proteins, Department of Biochemistry, University of Alberta, Edmonton, Alberta Canada T6G 2H7; 4Department of Biological Sciences, Columbia University, New York, New York 10027, USA; 5Graduate School of Biostudies, Kyoto University, Kyoto 606-8304, Japan
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 IRE1α and ATF6 for signaling the mammalian UPR, we identified a UPR reporter gene that was defective for induction in _IRE1α_-null mouse embryonic fibroblasts and S2P-deficient Chinese hamster ovary (CHO) cells. We show that the endoribonuclease activity of IRE1α is required to splice XBP1 (X-box bindingprotein) mRNA to generate a new C terminus, thereby converting it into a potent UPR transcriptional activator. IRE1α was not required for ATF6 cleavage, nuclear translocation, or transcriptional activation. However, ATF6 cleavage was required for IRE1α-dependent induction of UPR transcription. We propose that nuclear-localized IRE1α 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, IRE1α removes an unconventional 26-nucleotide intron that increases XBP1 transactivation potential. Both processing of ATF6 and IRE1α-mediated splicing of XBP1 mRNA are required for full activation of the UPR.
- Endoplasmic reticulum
- stress response
- transcription factors
- signal transduction
- posttranscriptional regulation
Footnotes
↵6 Present address: Institute of Molecular Biology and Genetics, Mahidol University, Nakorn Prathom 73107, Thailand.
↵7 Corresponding author.
E-MAIL kaufmanr{at}umich.edu; FAX (734) 763-9323.
Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/gad.964702.
- Received November 26, 2001.
- Accepted December 31, 2001.
Cold Spring Harbor Laboratory Press