The transmembrane kinase Ire1p is a site-specific endonuclease that initiates mRNA splicing in the unfolded protein response

The endoplasmic reticulum (ER) communicates with the nucleus through the unfolded protein response (UPR), which senses accumulation of unfolded proteins in the ER lumen and leads to increased transcription of genes encoding ER-resident chaperones. As a key regulatory step in this signaling pathway, the mRNA encoding the UPR-specific transcription factor Hac1p becomes spliced by a unique mechanism that requires tRNA ligase but not the spliceosome. Splicing is initiated upon activation of Ire1p, a transmembrane kinase that lies in the ER and/or inner nuclear membrane. We show that Ire1p is a bifunctional enzyme: in addition to being a kinase, it is a site-specific endoribonuclease that cleaves HAC1 mRNA specifically at both splice junctions. The addition of purified tRNA ligase completes splicing; we therefore have reconstituted HAC1 mRNA splicing in vitro from purified components.

tRNA ligase is required for regulated mRNA splicing in the unfolded protein response

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers an intracellular signaling pathway, the unfolded protein response (UPR), that leads to increased transcription of genes encoding ER-resident proteins. Transcriptional activation is mediated by a dedicated transcription factor, Hac1p, whose activity is controlled by regulated splicing of its mRNA. We have identified a mutation in tRNA ligase that disrupts the UPR in the yeast Saccharomyces cerevisiae. In this mutant, splicing of HAC1 mRNA, but not tRNA, is blocked. In contrast, HAC1 mRNA splicing is not impaired in cells that are blocked in spliceosome-mediated mRNA splicing. Furthermore, the splice junctions of HAC1 mRNA do not conform to the consensus sequences of other yeast pre-mRNAs. Our results suggest that the regulated splicing of HAC1 mRNA occurs by a novel pathway, involving tRNA ligase and bypassing the spliceosome.

The unfolded protein response: an intracellular signalling pathway with many surprising features

The unfolded protein response (UPR) is an intracellular signalling pathway--originating in the endoplasmic reticulum (ER) and leading to the cell nucleus--that controls transcription of genes encoding ER-resident proteins. Recent developments in this field show that this pathway utilizes unique regulatory mechanisms, including translational attenuation and a regulated mRNA splicing step catalysed by a bifunctional transmembrane kinase/endoribonuclease and tRNA ligase. This review describes the characterization of the UPR signalling pathway, focusing on the novel regulatory mechanisms that it has revealed.

A role for presenilin-1 in nuclear accumulation of Ire1 fragments and induction of the mammalian unfolded protein response

The unfolded protein response (UPR) mediates signaling from the endoplasmic reticulum to the nucleus. In yeast, a key regulatory step in the UPR is the spliceosome-independent splicing of HAC1 mRNA encoding a UPR-specific transcription factor, which is initiated by the transmembrane kinase/endoribonuclease Ire1. We show that yeast HAC1 mRNA is correctly spliced in mammalian cells upon UPR induction and that mammalian Ire1 can precisely cleave both splice junctions. Surprisingly, UPR induction leads to proteolytic cleavage of Ire1, releasing fragments containing the kinase and nuclease domains that accumulate in the nucleus. Nuclear localization and UPR induction are reduced in presenilin-1 knockout cells. These results suggest that the salient features of the UPR are conserved among eukaryotic cells and that presenilin-1 controls Ire1 proteolysis in mammalian cells.

Intracellular signaling from the endoplasmic reticulum to the nucleus

Cells respond to an accumulation of unfolded proteins in the endoplasmic reticulum (ER) by increasing transcription of genes encoding ER resident proteins. The information is transmitted from the ER lumen to the nucleus by an intracellular signaling pathway called the unfolded protein response (UPR). Recent work has shown that this signaling pathway utilizes several novel mechanisms, including translational attenuation and a regulated mRNA splicing step. In this review we aim to integrate these recent advances with current knowledge about maintenance of ER composition and abundance.

Mechanism of non-spliceosomal mRNA splicing in the unfolded protein response pathway

The unfolded protein response is an intracellular signaling pathway that, in response to accumulation of misfolded proteins in the lumen of the endoplasmic reticulum (ER), upregulates transcription of ER resident chaperones. A key step in this pathway is the non-conventional, regulated splicing of the mRNA encoding the positive transcriptional regulator Hac1p. In the yeast Saccharomyces cerevisiae, the bifunctional transmembrane kinase/endoribonuclease Ire1p cleaves HAC1 mRNA at both splice junctions and tRNA ligase joins the two exons together. We have reconstituted HAC1 mRNA splicing in an efficient in vitro reaction and show that, in many ways, the mechanism of HAC1 mRNA splicing resembles that of pre-tRNA splicing. In particular, Ire1p endonucleolytic cleavage leaves 2', 3'-cyclic phosphates, the excised exons remain associated by base pairing, and exon ligation by tRNA ligase follows the same chemical steps as for pre-tRNA splicing. To date, this mechanism of RNA processing is unprecedented for a messenger RNA. In contrast to the striking similarities to tRNA splicing, the structural features of the splice junctions recognized by Ire1p differ from those recognized by tRNA endonuclease. We show that small stem-loop structures predicted to form at both splice junctions of HAC1 mRNA are required and sufficient for Ire1p cleavage.

Ornithine decarboxylase from Leishmania mexicana promastigotes: interaction with pyridoxal 5'-phosphate and alpha-difluoromethylornithine

The catalytic properties of ornithine decarboxylase (ODC) from Leishmania mexicana as well as the interaction with its cofactor pyridoxal 5'-phosphate (PLP) and the irreversible inhibitor alpha-difluoromethylornithine (DFMO) have been studied using partially purified preparations of the enzyme obtained from parasite promastigotes. Leishmania extracts prepared in the presence of saturating concentrations of PLP yielded an enzyme considerably more resistant to heat inactivation and with a three-fold higher activity than the ODC obtained without the addition of cofactor. The complete removal of PLP by treatment with hydroxylamine yielded the apoenzyme which shows an absolute requirement for PLP to recover its enzymatic activity. The Km values for L-ornithine and PLP were 0.7 mM and 25 microM, respectively, while Ki for DFMO was 0.2 mM. The restoration of ODC activity from apoenzyme and cofactor seems to involve time and temperature-dependent activation processes. L. mexicana ODC has an apparent molecular mass of 240 +/- 20 kDa.