Nuclear mRNA Quality Control and Cytoplasmic NMD Are Linked by the Guard Proteins Gbp2 and Hrb1

Pre-mRNA splicing is critical for cells, as defects in this process can lead to altered open reading frames and defective proteins, potentially causing neurodegenerative diseases and cancer. Introns are removed in the nucleus and splicing is documented by the addition of exon-junction-complexes (EJCs) at exon-exon boundaries. This "memory" of splicing events is important for the ribosome, which translates the RNAs in the cytoplasm. In case a stop codon was detected before an EJC, translation is blocked and the RNA is eliminated by the nonsense-mediated decay (NMD). In the model organism Saccharomyces cerevisiae, two guard proteins, Gbp2 and Hrb1, have been identified as nuclear quality control factors for splicing. In their absence, intron-containing mRNAs leak into the cytoplasm. Their presence retains transcripts until the process is completed and they release the mRNAs by recruitment of the export factor Mex67. On transcripts that experience splicing problems, these guard proteins recruit the nuclear RNA degradation machinery. Interestingly, they continue their quality control function on exported transcripts. They support NMD by inhibiting translation and recruiting the cytoplasmic degradation factors. In this way, they link the nuclear and cytoplasmic quality control systems. These discoveries are also intriguing for humans, as homologues of these guard proteins are present also in multicellular organisms. Here, we provide an overview of the quality control mechanisms of pre-mRNA splicing, and present Gbp2 and Hrb1, as well as their human counterparts, as important players in these pathways.