Differences in HAC1 mRNA processing and translation between yeast and mammalian cells indicate divergence of the eukaryotic ER stress response.
Perturbation of normal endoplasmic reticulum (ER) function induces a stress response found throughout eukaryotes, sometimes termed the unfolded protein response (UPR). In yeast, auxotrophic mutants have identified two genes, IRE1 and HAC1, whose products are key components. Normally HAC1 mRNA is not translated owing to a 252-nt "intron." Disruption of ER function activates Ire1p to remove this intron through endogenous endoribonuclease activity. Together with tRNA ligase, cleavage and splicing produces a translatable HAC1 mRNA to give Hac1p, a transcription factor that upregulates the expression of genes responsive to ER stress. No Hac1p homologue has been identified in mammalian cells, but Ire1p homologues exist with endoribonuclease activity required for a fully functional UPR, raising the possibility that the key features of the yeast UPR might be conserved in higher eukaryotic cells. To address this, we expressed yeast HAC1 in HeLa and HEK 293T human cell lines, both on its own and as fusions with yellow fluorescent protein (YFP) to investigate its processing and translation. HAC1 mRNA was not processed, but efficiently translated irrespective of whether the cells were subjected to ER stress. Expression of exogenous HAC1 mRNA constructs in yeast showed UPR-induced splicing required the presence of its 3' UTR. These results suggest that the mammalian ER stress response has diverged from the yeast UPR.
Bowring
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Department of Medical Biochemistry, University of Wales College of Medicine, Heath Park, Cardiff, Wales, CF14 4XN, United Kingdom.
Llewellyn
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D020413:3' Untranslated Regions; D001426:Bacterial Proteins / Q000378:metabolism; D050976:Basic-Leucine Zipper Transcription Factors; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D007438:Introns; D008164:Luminescent Proteins / Q000378:metabolism; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D010957:Plasmids / Q000378:metabolism; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000378:metabolism*; D012343:RNA, Transfer / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics* / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D014162:Transfection; D015854:Up-Regulation
D020413:3' Untranslated Regions; D001426:Bacterial Proteins; D050976:Basic-Leucine Zipper Transcription Factors; D005656:Fungal Proteins; C091341:HAC1 protein, S cerevisiae; D008164:Luminescent Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; C054562:yellow fluorescent protein, Bacteria; D012343:RNA, Transfer