CLONING AND CHARACTERIZATION OF HUPF1, A HUMAN HOMOLOG OF THE SACCHAROMYCES-CEREVISIAE NONSENSE MESSENGER-RNA-REDUCING UPF1 PROTEIN

Levels of most nonsense mRNAs are normally reduced in prokaryotes and eukaryotes when compared with that of corresponding functional mRNAs, Genes encoding polypeptides that selectively reduce levels of nonsense mRNA have so far only been identified in simple eukaryotes, We have n ow cloned a human cDNA whose deduced amino acid sequence shows the hig hest degree of homology to that of UPF1, a bona fide Saccharomyces cer evisiae group I RNA helicase required for accelerated degradation of n onsense mRNA, Based on the total sequence of the shorter yeast UPF1 pr otein, the overall identity between the human protein and UPF1 is 51%, Besides NTPase and other RNA helicase Consensus motifs, UPF1 and its human homolog also share similar putative zinc finger motifs that are absent in other group I RNA helicases, Northern blot analysis with the human cDNA probe revealed two transcripts in several human cell lines . Further, antibodies raised against a synthetic peptide of the human polypeptide detected a single 130 kDa polypeptide on Western blots fro m human and mouse cells, Finally, immunofluorescence and Western blot analyses revealed that the human and mouse polypeptides, like yeast UP F1, are expressed in the cytoplasm, but not in the nucleus, We have th us identified the first mammalian homolog of yeast UPF1, a protein tha t regulates levels of nonsense mRNA, and we tentatively name this prot ein human HUPF1 (for human homolog of UPF1).