A CAP-BINDING PROTEIN COMPLEX MEDIATING U SNRNA EXPORT

CAP structures are added cotranscriptionally to all RNA polymerase II transcripts(1,2). They affect several processes including RNA stabilit y(3-5), pre-messenger RNA splicing(6-11), RNA export from the nucleus( 12-14) and translation initiation(15-17). The effect of the cap on tra nslation is mediated by the initiation factor eIF-4F (refs 15-19), whe reas the effect on pre-mRNA splicing involves a nuclear complex (CBC) composed of two cap binding proteins, CBP80 and CBP20(11). A role for CBC in the nuclear export of capped RNAs has also been proposed(11,13) , We report here the characterization of human and Xenopus CBP20s. Ant ibodies against recombinant CBP20 prevent interaction of CBC with capp ed RNAs in vitro. Following microinjection into Xenopus oocytes, the a ntibodies inhibit both pre-mRNA splicing and export of U small nuclear RNAs to the cytoplasm, These results demonstrate that CBC mediates th e effect of the cap structure in U snRNA export, and provide direct ev idence for the involvement of a cellular RNA-binding factor in the tra nsport of RNA to the cytoplasm.