The role of Upf proteins in modulating the translation read-through of nonsense-containing transcripts

The yeast UPF1, UPF2 and UPF3 genes encode trans-acting factors of the nonsense-mediated mRNA decay pathway. In addition, the upf1Delta strain demonstrates a nonsense suppression phenotype and Upf1p has been shown to interact with the release factors eRF1 and eRF3. In this report, we show that both upf2Delta and upf3Delta strains demonstrate a nonsense suppression phenotype independent of their effect on mRNA turnover. We also demonstrate that Upf2p and Upf3p interact with eRF3, and that their ability to bind eRF3 correlates with their ability to complement the nonsense suppression phenotype. In vitro experiments demonstrate that Upf2p, Upf3p and eRF1 compete with each other for interacting with eRF3. Con versely, Upf1p binds to a different region of eRF3 and can form a complex with these factors. These results suggest a sequential surveillance complex assembly pathway, which occurs during the premature translation termination process. We propose that the observed nonsense suppression phenotype in the upfDelta strains can be attributed to a defect in the surveillance complex assembly.