IMPORTIN PROVIDES A LINK BETWEEN NUCLEAR-PROTEIN AND U-SNRNA EXPORT

Importin-alpha mediates nuclear protein import by binding nuclear loca lization signals and importin-beta. We find approximately 30% of SRP1p , the yeast importin-alpha, in a nuclear complex with the Saccharomyce s cerevisiae nuclear cap-binding protein complex (CBC). Similarly, a l arge fraction of Xenopus CBC is associated with importin-alpha in the nucleus. CBC promotes nuclear export of capped U snRNAs and shuttles b etween nucleus and cytoplasm. The CBC-importin-alpha complex binds spe cifically to capped RNA, suggesting that CBC might shuttle while bound to importin-alpha. Strikingly, importin-beta binding displaces the RN A from the CBC-importin-alpha 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 impo rtin-mediated RNA release is a specifically cytoplasmic event.