Accurate translation requires correct mRNAs with intact open reading frames. Cells eliminate defective transcripts to prevent mistranslation by three cytoplasmic mRNA quality control events termed nonsense-mediated decay (NMD), no-go decay (NGD) and non-stop decay (NSD). Translation termination on correct transcripts requires Dbp5 (human DDX19), which delivers eRF1 to the ribosomes and prevents an early contact of eRF1 with eRF3, precluding the immediate dissociation of both release factors and subsequent termination readthrough. Here, we report evidence for an influence of Dbp5 on NMD, as it delivers eRF1 also to PTC-containing transcripts. In contrast to regular translation termination and NMD, functional NGD and NSD require the eRF1-eRF3-like proteins Dom34-Hbs1. We suggest that Dbp5 delivers Dom34 to NGD and NSD substrates as well. However, in contrast to regular termination, it does not prevent an Hbs1 contact, but allows formation of a ternary Dom34-Hbs1-Dbp5 complex. The Dbp5-mediated delivery of Dom34-Hbs1 in NGD and NSD might rather shield and position the complex to prevent a premature contact of Dom34 and Rli1 to prevent inefficient splitting of the ribosomal subunits. Together, we have gathered evidence suggesting an important role of Dbp5 in cytoplasmic mRNA quality control.
Cell viability largely depends on the surveillance of mRNA export and translation. Upon pre-mRNA processing and nuclear quality control, mature mRNAs are exported into the cytoplasm via Mex67-Mtr2 attachment. At the cytoplasmic site of the nuclear pore complex, the export receptor is displaced by the action of the DEAD-box RNA helicase Dbp5. Subsequent quality control of the open reading frame requires translation. Our studies suggest an involvement of Dbp5 in cytoplasmic no-go-and non-stop decay. Most importantly, we have also identified a key function for Dbp5 in translation termination, which identifies this helicase as a master regulator of mRNA expression.
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