REF proteins mediate the export of spliced and unspliced mRNAs from the nucleus

The REF family of evolutionarily conserved heterogeneous ribonucleoprotein (hnRNP)-like proteins consists of one central RNP-type RNA binding domain f lanked by Arg-Gly-rich regions of variable length. Members of this protein family bind directly to RNA and the mRNA export factor TAP/Mex67p, and it h as been suggested that they facilitate the recruitment of TAP/Mex67p to cel lular mRNPs, We show that the variable regions are necessary for binding of REFs to RNA and to TAP. Antibodies specific to REFs prevent their interact ion with RNA in vitro. After microinjection into Xenopus oocytes, these ant ibodies inhibit mRNA nuclear export. This inhibition of export is observed whether or not the mRNAs are generated by splicing. The antibodies do not i nterfere with pre-mRNA splicing or with the nuclear export of constitutive transport: element (CTE)-containing RNAs (directly mediated by TAP), so REF proteins must play a critical role in mRNA nuclear export, acting downstre am of splicing and upstream of TAP/Mex67p. We also show that recombinant RE Fs stimulate directly the export of mRNAs that are otherwise exported ineff iciently. Together, our data indicate that REFs are directly implicated in the export of mRNAs from the nucleus. More generally, we show that spliced and unspliced mRNAs use common export factors to reach the cytoplasm.