Recent evidence supports the idea that pre-mRNA splicing and mRNA export are mechanistically coupled. In metazoans, this process appears to be mediated by a multicomponent complex, which associates with the spliced RNA upstream of the exon–exon junction. One of these components (Aly/REF) has a homolog in the budding yeast Saccharomyces cerevisiae known as Yra1p. The YRA1 gene is essential for growth and required for mRNA export. Notably, YRA1 is one of the only ∼5% intron-containing genes in yeast. Moreover, the YRA1 intron has several unusual features and is conserved in other budding yeast species. Previously, overexpression of intronless YRA1 was shown to be toxic. We show here that overexpression of the intron-containing gene results in increased levels of unspliced pre-mRNA but normal levels of Yra1 protein; conversely, expression of the cDNA results in increased levels of protein and accumulation of nuclear poly(A)+ RNA. Two additional lines of evidence suggest that expression of Yra1p is autoregulated: First, expression of excess Yra1p from a plasmid reduces the level of tagged, chromosomal Yra1p, and, second, this effect requires wild-type protein. Replacement of the YRA1 intron with that of other S. cerevisiae genes cannot rescue the dominant-negative growth defect of intronless YRA1. We conclude that the level of Yra1p is negatively autoregulated by a mechanism that involves splicing of its unusual intron. Tight control of the levels of Yra1p might be necessary to couple the rates of pre-mRNA splicing and mRNA 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 (CTE). Although TAP directly interacts with CTE-bearing RNAs, the mode of interaction of TAP/Mex67p with cellular mRNAs is different from that with the CTE RNA and is likely to be mediated by protein–protein interactions. Here we show that Mex67p directly interacts with Yra1p, an essential yeast hnRNP-like protein. This interaction is evolutionarily conserved as Yra1p also interacts with TAP. Conditional expression in yeast cells implicates Yra1p in the export of cellular mRNAs. Database searches revealed that Yra1p belongs to an evolutionarily conserved family of hnRNP-like proteins having more than one member in Mus musculus, Xenopus laevis, Caenorhabditis elegans, and Schizosaccharomyces pombe and at least one member in several species including 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 called 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.