Nuclear mRNA quality control factors Gbp2 and Hrb1 take part in cytoplasmic mRNA surveillance through nonsense-mediated decay
by Yen-Yun Lu
Date of Examination:2021-09-23
Date of issue:2022-01-27
Advisor:Prof. Dr. Heike Krebber
Referee:Prof. Dr. Heike Krebber
Referee:Prof. Dr. Jörg Großhans
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Abstract
English
Yeast SR-like proteins Gbp2 and Hrb1 shuttle between the nucleus and the cytoplasm, and serve as quality control factors for pre-mRNA splicing. They are loaded onto the RNA co-transcriptionally, and upon correct splicing, recruit mRNA export receptors to facilitate transport through the nuclear pore complex. When errors in splicing occur, they recruit instead the nuclear exosome cofactor TRAMP complex, thereby retaining the faulty transcripts in the nucleus and supporting their degradation. In the cytoplasm, Gbp2 and Hrb1 were found to co-purify with polysome fractions and therefore conceivably serve cytoplasmic functions related to translation. Their exact role therein was however unknown. Nonsense-mediated mRNA decay (NMD) is a cytoplasmic quality control pathway primarily targeting transcripts that contain premature termination codons. Considering the surveillance activity of Gbp2 and Hrb1 in splicing, and the fact that splicing defects are a major source of substrates for NMD, it was investigated whether the two proteins are involved in NMD. Despite a great number of studies, the complete list of participating factors and the exact mechanisms of NMD have not been fully defined. The intimate link between NMD and numerous human diseases calls for our greater understanding of this conserved quality control pathway. The results of this work demonstrate that Gbp2 and Hrb1 are part of the NMD mRNP, where they ensure the efficient removal of the target transcript. They seem not to be essential for substrate recognition and NMD activation, but actively participate in subsequent events. Presumably, they mediate mRNP remodeling that may support a direct connection between Upf1, the main factor of NMD, and the 5’ end of the transcript, the site where downstream processes majorly occur. They bind to the 5’-associating translation initiation factor eIF4G to repress translation of the target RNA. Further, they help to recruit decay machineries to promote proper elimination of the defective transcript. Identification of these novel roles of Gbp2 and Hrb1 in the cytoplasm demonstrates a molecular link between the nuclear and cytoplasmic mRNA surveillance systems, which likely contributes to a collective maintenance of transcriptome integrity. Importantly, the functions of Gbp2 and Hrb1 suggest conserved characteristics with the mammalian exon junction complex and its associating SR proteins, presenting a framework for future exploration of so far unregarded functions of human SR proteins in both cytoplasmic and nuclear mRNA quality control. Continued research on yeast SR-like proteins will bring further insights and provide exciting possibilities to unravel the more complex mechanisms in human cells.
Keywords: SR-like proteins; nonsense-mediated mRNA decay; RNA surveillance