Dragged by a Newborn Peptide

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Science Signaling  28 Apr 2009:
Vol. 2, Issue 68, pp. ec145
DOI: 10.1126/scisignal.268ec145

The transmembrane endoplasmic reticulum (ER) endoribonuclease IRE1α is activated by ER stress in mammals to cleave the mRNA transcript encoding X-box binding protein 1 (XBP1), enabling the cytosolic production of XBP1s (spliced), which, unlike the unspliced XBP1u isoform, stimulates the transcription of genes that encode proteins involved in maintaining ER homeostasis (see Ron, who also discusses the somewhat different scenario in yeast described by Aragón et al.). Whereas conventional mRNA splicing takes place in the nucleus, XBP1 mRNA is membrane-bound (consistent with localization of IRE1α to the ER membrane), leading Yanagitani et al. to investigate the mechanism of XBP1 mRNA targeting and the possible role of location in its splicing. After using digitonin to permeabilize the plasma membranes of nonstressed human embryonic kidney (HEK) 293T cells, the authors used a second detergent, dodecyl-maltoside, to identify cytosolic and membrane fractions and found substantial localization of XBP1 mRNA (present predominantly as XBP1u) in the membrane fraction. Pharmacological induction of ER stress led to redistribution of XBP1 mRNA to the cytosolic fraction, and reverse transcription polymerase chain reaction analysis revealed that the cytosolic XBP1 was predominantly XBP1s. Mutational analysis revealed that membrane recruitment of XBP1u and its efficient splicing depended on XBP1u’s in-frame translation sequence (lost in XBP1s through a splicing-dependent frameshift) rather than the spliced sequences per se. Moreover, the XBP1u, but not the XBP1s, protein associated with membrane fractions and synthetic liposomes. Membrane localization of XBP1u mRNA was inhibited by puromycin, which releases nascent polypeptides from the ribosome, and membrane localization of both XBP1u mRNA and protein depended on a particular XBP1u hydrophobic region. In contrast, recruitment was not diminished by IRE1α knockout. The authors propose that recruitment of XBP1u mRNA to the ER membrane by the nascent XBP1u polypeptide as part of the mRNA-ribosome-nascent chain complex facilitates its cleavage by IRE1α and the production of the nonmembrane-associated XBP1s.

K. Yanagitani, Y. Imagawa, T. Iwawaki, A. Hosoda, M. Saito, Y. Kimata, K. Kohno, Cotranslational targeting of XBP1 protein to the membrane promotes cytoplasmic splicing of its own mRNA. Mol. Cell 34, 191–200 (2009). [Online Journal]

D. Ron, Targeting of mRNAs to their sites of unconventional splicing in the unfolded protein response. Mol. Cell 34, 133–134 (2009). [Online Journal]

T. Aragón, E. van Anken, D. Pincus, I. M. Serafimova, A. V. Korennykh, C. A. Rubio, P. Walter, Messenger RNA targeting to endoplasmic reticulum stress signalling sites. Nature 457, 736–740 (2009). [PubMed]

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