Gbp2 and Hrb1 continue their mRNA quality control in the cytoplasm and take part in Nonsense Mediated Decay
von Sebastian Grosse
Datum der mündl. Prüfung:2019-08-27
Erschienen:2019-09-02
Betreuer:Prof. Dr. Heike Krebber
Gutachter:Prof. Dr. Heike Krebber
Gutachter:Prof. Dr. Reinhard Lührmann
Zum Verlinken/Zitieren:
http://dx.doi.org/10.53846/goediss-7627
Dateien
Name:Dissertation_Sebastian_Grosse_2019_no_CV.pdf
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Zusammenfassung
Englisch
The eukaryotic mRNA life cycle is orchestrated by many pathways. Every aspect of this life cycle is associated with quality control factors. This tight coupling of mRNA transcription, processing, export and translation with different parts of the quality control machinery ensures the production of correct polypeptides. Surveillance of mRNA maturation involves several different pathways. Nuclear quality control checks mRNA processing: capping, splicing and polyadenylation. In S. cerevisiae the nuclear guard proteins Npl3, Gbp2, Hrb1 and Nab2 promote mRNA export only when these processing steps have occurred correctly. An aberrant mRNA is not only retained in the nucleus, it is also targeted for rapid RNA decay. An mRNA that passes the nuclear quality control checks and reaches the cytoplasm is subject to three different cytoplasmic surveillance pathways, which monitor if an mRNA is translated correctly. The cytoplasmic quality control inhibits translation and degrades the mRNA and the newly synthesised polypeptide. One of these pathways is Nonsense Mediated Decay (NMD), which targets mRNAs that undergo premature translation termination. The yeast SR-like proteins Gbp2 and Hrb1 are nuclear guard proteins that monitor splicing. They recruit the degradation machinery to remove mRNAs that fail to be spliced correctly. While it was known, that Gbp2 and Hrb1 accompany mRNA into the cytoplasm until translation, it remained unknown why they stay on the mRNA. Here we show that Gbp2 and Hrb1 continue their quality control in the cytoplasm and are part of the NMD pathway. Aggregation of Hsp104 shows that cytoplasmic quality control is defective without Gbp2 and Hrb1. In their absence both NMD induced mRNA decay and translation inhibition are impaired. Our data suggest that the proteins are directly involved in the NMD pathway. If the degradation is impaired, ongoing NMD stalls Gbp2 and Hrb1 in the cytoplasm. This agrees with other data showing physical interactions with the main NMD effectors Upf1, Upf2 and Upf3. We gathered evidence that Upf1 mediates the direct recruitment of degradation factors to NMD targets in vivo. Both Gbp2 and Hrb1 interact with the cytoplasmic degradation machinery. Strikingly, Upf1 requires Hrb1 for the correct recruitment of the decapping enzyme, while it requires Gbp2 for effective translation inhibition. Gbp2, as an RGG motif protein, has the potential to be a general translation inhibitor. However, our results suggest, that it is NMD specific. Further, Gbp2 and Hrb1, which preferentially associate with the 5’ ends of mRNAs, might be structural elements of cytoplasmic mRNPs. In their absence, wild typical mRNAs are more susceptible to degradation by the 5’-3’ exonuclease Xrn1 in vivo. The association of Gbp2 and Hrb1 with the cytoplasmic degradation machinery is a new facet in understanding the intricate machinery of mRNA quality control and might offers a new perspective in unravelling the functions of SR-proteins in eukaryotes.
Keywords: Nonsense Mediated Decay; NMD; SR Proteins; S. cerevisiae; Yeast
