REF, an evolutionarily conserved family of hnRNP-like proteins, interacts with TAP/Mex67p and participates in mRNA nuclear export

Vertebrate TAP and its yeast ortholog Mex67p are involved in the export of messenger RNAs from the nucleus. TAP has also been implicated in the export of simian type D viral RNAs bearing the constitutive transport element (CT E). Although TAP directly interacts with CTE-bearing RNAs, the mode of inte raction of TAP/Mex67p with cellular mRNAs is different from that with the C TE RNA and is likely to be mediated by protein-protein interactions. Here w e show that Mex67p directly interacts with Yra1p, an essential yeast hnRNP- like protein. This interaction is evolutionarily conserved as Yra1p also in teracts with TAP. Conditional expression in yeast cells implicates Yra1p in the export of cellular mRNAs. Database searches revealed that Yra1p belong s to an evolutionarily conserved family of hnRNP-like proteins having more than one member in Mus musculus, Xenopus laevis, Caenorhabditis elegans, an d Schizosaccharomyces pombe and at least one member in several species incl uding plants. The murine members of the family directly interact with TAP. Because members of this protein family are characterized by the presence of one RNP-motif RNA-binding domain and exhibit RNA-binding activity, we call ed these proteins REF-bps for RNA and export factor binding proteins. Thus, Yra1p and members of the REF family of hnRNP-like proteins may facilitate the interaction of TAP/Mex67p with cellular mRNAs.