A MUTATED HUMAN HOMOLOG TO YEAST UPF1 PROTEIN HAS A DOMINANT-NEGATIVEEFFECT ON THE DECAY OF NONSENSE-CONTAINING MESSENGER-RNAS IN MAMMALIAN-CELLS

All eukaryotic cells analyzed have developed mechanisms to eliminate t he production of mRNAs that prematurely terminate translation. The mec hanisms are thought to exist to protect cells from the deleterious eff ects of in-frame nonsense codons that are generated by routine ineffic iencies and inaccuracies in RNA metabolism such as pre-mRNA splicing. Depending on the particular mRNA and how it is produced, nonsense codo ns can mediate a reduction in mRNA abundance either (i) before its rel ease from an association with nuclei into the cytoplasm, presumably bu t not certainly while the mRNA is being exported to the cytoplasm and translated by cytoplasmic ribosomes, or (ii) in the cytoplasm, Here, w e provide evidence for a factor that functions to eliminate the produc tion of nonsense-containing RNAs in mammalian cells. The factor, vario usly referred to as Rent1 (regulator of nonsense transcripts) or HUPF1 (human Upf1 protein), was identified by isolating cDNA for a human ho mologue to Saccharomyces cerevisiae Upf1p, which is a group I RNA heli case that functions in the nonsense-mediated decay of mRNA in yeast. U sing monkey COS cells and human HeLa cells, we demonstrate that expres sion of human Upf1 protein harboring an arginine-to-cysteine mutation at residue 844 within the RNA helicase domain acts in a dominant-negat ive fashion to abrogate the decay of nonsense-containing mRNA that tak es place (i) in association with nuclei or (ii) in the cytoplasm. Thes e findings provide evidence that nonsense-mediated mRNA decay is relat ed mechanistically in yeast and in mammalian cells, regardless of the cellular site of decay.