Browsing by Author "Kimmig, Philipp"

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The unfolded protein response in fission yeast : Ire1 modulates stability of select mRNAs to maintain protein homeostasis
(2013) Kimmig, Philipp; Wolf, Dieter (Prof. Dr.)
Virtually all proteins that eukaryotic cells display on their surface or that are secreted into the extracellular spare are first folded and assembled in the membrane-surrounded organelle endoplasmic reticulum (ER). Only properly folded and assembled proteins depart from the ER to the cell surface. If the ER does not have enough capacity to fold, a condition termed “ER stress”, a signal pathway called the “Unfolded Protein Response” (UPR), is switched on to increase the protein folding capacity, to expand the surface area and volume of the compartment. All eukaryotic cells, from unicellular yeasts to mammalian cells, contain a highly conserved protein-folding sensor Ire1. In all species analyzed to date, Ire1, an ER membrane-resident kinase/endoribonuclease, is known to activate the UPR through an unconventional messenger RNA (mRNA) splicing mechanism to induce translation of a potent transcription factor Hac1 (in yeast) or XBP1 (in metazoans). This unique splicing event provides the switch that drives a comprehensive gene expression program in which the production of ER components is increased to boost the protein folding capacity of the compartment. In this thesis an organism is identified, the yeast Schizosaccharomyces pombe, in which the UPR does not involve mRNA splicing or the initiation of a gene expression program to increase the folding capacity of the ER. Rather Schizosaccharomyces pombe lacking, a Hac1/XBP1 ortholog, utilizes Ire1 RNase activity to an entirely different end. We found that activation of Ire1 in S. pombe leads to the decay of a specific class of mRNAs that all encode proteins entering the ER. Interestingly, the set of down-regulated mRNA targets are particularly enriched for those encoding proteins involved in sterol metabolism, suggesting a potential qualitative change of the physiology of the cell. The deletion of the cytosolic mRNA degradation pathway shows an accumulation of RNA cleavage fragments of the down-regulated mRNA targets upon ER stress, an event by means that the Ire1 endonuclease directly cleaves these mRNAs. Intriguingly, the down-regulated mRNAs contain a short three-nucleotide base UG/C consensus at the Ire1 cut sites where cleavage occurs after G. Thus, rather than increasing the protein folding capacity of the ER when faced with an increased protein folding load, S. pombe cells correct the imbalance by decreasing the load via mRNA cleavage. Besides decreasing the ER load, a single mRNA—the mRNA that encodes the molecular chaperone BiP, which is one of the major protein-folding components in the ER—uniquely escapes this decay. Rather then being degraded, Ire1 truncates Bip1 mRNA in its 3’ UTR, which—counter-intuitively stabilizes the non-polyadenylated 5’ fragment and results in increased Bip1 translation. Decreasing the protein load by selective mRNA degradation and the single up-regulation of the major chaperone in the ER illustrate how a universally conserved machinery has been invented to maintain ER homeostasis in fission yeast.