The nonsense-mediated mRNA decay pathway is an example of an evolutionarily conserved surveillance pathway that rids the cell of transcripts that contain nonsense mutations. The product of the UPF1 gene is a necessary component of the putative surveillance complex that recognizes and degrades aberrant mRNAs. Recent results indicate that the Upf1p also enhances translation termination at a nonsense codon. The results presented here demonstrate that the yeast and human forms of the Upf1p interact with both eukaryotic translation termination factors eRF1 and eRF3. Consistent with Upf1p interacting with the eRFs, the Upf1p is found in the prion-like aggregates that contain eRF1 and eRF3 observed in yeast [PSI + ] strains. These results suggest that interaction of the Upf1p with the peptidyl release factors may be a key event in the assembly of the putative surveillance complex that enhances translation termination, monitors whether termination has occurred prematurely, and promotes degradation of aberrant transcripts.[Key Words: mRNA decay; translation termination; release factors; nonsense mutation; ribosome; mRNA surveillance]Received January 21, 1998; revised version accepted April 1, 1998. Recent studies have demonstrated that cells have evolved elaborate mechanisms to rid themselves of aberrant proteins and transcripts that can dominantly interfere with their normal functioning (He et

All eukaryotic cells analyzed have developed mechanisms to eliminate the production of mRNAs that prematurely terminate translation. The mechanisms are thought to exist to protect cells from the deleterious effects of in-frame nonsense codons that are generated by routine inefficiencies and inaccuracies in RNA metabolism such as pre-mRNA splicing. Depending on the particular mRNA and how it is produced, nonsense codons can mediate a reduction in mRNA abundance either (i) before its release from an association with nuclei into the cytoplasm, presumably but not certainly while the mRNA is being exported to the cytoplasm and translated by cytoplasmic ribosomes, or (ii) in the cytoplasm. Here, we provide evidence for a factor that functions to eliminate the production of nonsensecontaining RNAs in mammalian cells. The factor, variously referred to as Rent1 (regulator of nonsense transcripts) or HUPF1 (human Upf1 protein), was identified by isolating cDNA for a human homologue to Saccharomyces cerevisiae Upf1p, which is a group I RNA helicase that functions in the nonsensemediated decay of mRNA in yeast. Using monkey COS cells and human HeLa cells, we demonstrate that expression of human Upf1 protein harboring an arginine-to-cysteine mutation at residue 844 within the RNA helicase domain acts in a dominantnegative fashion to abrogate the decay of nonsense-containing mRNA that takes place (i) in association with nuclei or (ii) in the cytoplasm. These findings provide evidence that nonsensemediated mRNA decay is related mechanistically in yeast and in mammalian cells, regardless of the cellular site of decay.All eukaryotic cells are thought to be characterized by pathways that eliminate the production of mRNAs that encode truncated proteins (reviewed in refs. 1-4). Truncated proteins can arise as the consequence of inefficiencies or inaccuracies in RNA metabolism. In fact, errors in transcription initiation and splicing can result in the production of either nonfunctional protein or protein that acts in a dominant-negative or gain-of-function fashion and may have provided the selective pressure under which the nonsense-mediated decay pathway evolved (reviewed in ref.2). The nonsense-mediated decay pathway also eliminates nonsensecontaining mRNAs for Igs and T cell receptors that arise as a consequence of nonproductive gene rearrangements and hypermutations (reviewed in ref. 5), certain selenoprotein mRNAs when a UGA codon is recognized as a premature termination codon rather than a selenocysteine codon (6), and nonsensecontaining mRNAs that arise as the consequence of random germ-line or somatic defects (reviewed in ref.2).Trans-acting factors required for nonsense-mediated mRNA decay in S. cerevisiae have been defined by genetic analyses to include Upf1p͞Isf2p͞Sal2p͞Nam7p, Upf2p͞Nmd2p͞Sua1p͞Ifs1p, Upf3p͞Sua6p, Xrn1p, which is a 5Ј 3 3Ј exonuclease, and Dcp1p, which is a decapping enzyme (7-19). Evidence that the three Upf factors function in a common pathway that leads to the accelerated decay of nonsense-cont...

All eukaryotes that have been studied to date possess the ability to detect and degrade transcripts that contain a premature signal for the termination of translation. This process of nonsense-mediated RNA decay has been most comprehensively studied