Importin provides a link between nuclear protein import and U snRNA export

Importin-α mediates nuclear protein import by binding nuclear localization signals and importin-β. We find approximately 30% of SRPlp, the yeast importin-α, in a nuclear complex with the Saccharomyces cerevisiae nuclear cap-binding protein complex (CBC). Similarly, a large fraction of Xenopus CBC is associated with importin-α in the nucleus. CBC promotes nuclear export of capped U snRNAs and shuffles between nucleus and cytoplasm. The CBC-importin-α complex binds specifically to capped RNA, suggesting that CBC might shuttle while bound to importin-α. Strikingly, importin-β binding displaces the RNA from the CBC-importin-α complex. Thus, the commitment of CBC for nuclear reentry triggers the release of the export substrate into the cytoplasm. We provide evidence for a mechanism that ensures that importin- mediated RNA release is a specifically cytoplasmic event.