SummaryMessenger RNA (mRNA) export adaptors play an important role in the transport of mRNA from the nucleus to the cytoplasm. They couple early mRNA processing events such as 5′ capping and 3′ end formation with loading of the TAP/NXF1 export receptor onto mRNA. The canonical adaptor REF/ALY/Yra1 is recruited to mRNA via UAP56 and subsequently delivers the mRNA to NXF1 [1]. Knockdown of UAP56 [2, 3] and NXF1 [4–7] in higher eukaryotes efficiently blocks mRNA export, whereas knockdown of REF only causes a modest reduction, suggesting the existence of additional adaptors [8–10]. Here we identify a new UAP56-interacting factor, UIF, which functions as an export adaptor, binding NXF1 and delivering mRNA to the nuclear pore. REF and UIF are simultaneously found on the same mRNA molecules, and both proteins are required for efficient export of mRNA. We show that the histone chaperone FACT specifically binds UIF, but not REF, via the SSRP1 subunit, and this interaction is required for recruitment of UIF to mRNA. Together the results indicate that REF and UIF represent key human adaptors for the export of cellular mRNAs via the UAP56-NXF1 pathway.

REF/Yra1p interacts with TAP/Mex67p, and in yeast this interaction leads to displacement of Sub2p from Yra1p (7). TAP heterodimerizes with p15 and binds nucleoporins through central and C-terminal domains (8), directing the mRNP to the nuclear pore and promoting transport to the cytoplasm. On the cytoplasmic side of the nuclear pore, Dbp5p triggers displacement of Mex67p from mRNA. Yra1p binds mRNA early during its nuclear maturation but is no longer bound once it reaches the nuclear periphery (9). Consistent with this finding, analysis of Balbiani ring pre-mRNPs shows that UAP56 and REF accompany the mRNP to the nuclear periphery where UAP56 and then REF dissociate during translocation through the pore (10).Although Yra1p is essential for yeast mRNA export, depletion of REF in higher eukaryotes does not block bulk mRNA export (11), suggesting that other proteins can fulfill this role and that there may be functional redundancy between export adaptors. The shuttling SR proteins 9G8, SRp20, and SF2/ASF directly bind TAP by short arginine-rich peptides (12, 13) and can function as export factors (14). Even in yeast, other proteins can recruit Mex67p to the mRNP, including Yra2p (15) and Npl3p.The fact that TAP binds RNA weakly in vitro led to the idea that export factors such as REF, which bind RNA avidly, bridge the interaction between TAP and mRNA, leading to the term mRNA export adaptors (15). However, both TAP and Mex67p are readily UV-cross-linked to mRNA in vivo (16)(17)(18), suggesting a direct stable interaction at some point during export. Here, we show that mRNA is handed over from export adaptors to TAP and that at least in vitro, export adaptors have the ability to enhance the RNA-binding activity of TAP. ResultsThe RNA-and TAP-Binding Sites on REF Overlap. In Saccharomyces cerevisiae, Mex67p binding to Yra1p triggers displacement of Sub2p (7), so we established whether this is the case for the mammalian orthologs. We examined whether UAP56 coimmunoprecipitated (Co-IP) with REF2-I (REF) in the presence of increasing amounts of TAP. This analysis revealed that TAP triggered dissociation of UAP56 from REF (Fig. 1A, lanes 5 and 6).We analyzed organization of the resulting REF-TAP-RNA ternary complex by examining how REF binds RNA. NMR

No abstract