title: Importin provides a link between nuclear protein import and U snRNA export.
abstract: Importin-alpha mediates nuclear protein import by binding nuclear localization signals and importin-beta. We find approximately 30% of SRP1p, the yeast importin-alpha, 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-alpha in the nucleus. CBC promotes nuclear export of capped U snRNAs and shuttles between nucleus and cytoplasm. The CBC-importin-alpha complex binds specifically to capped RNA, suggesting that CBC might shuttle while bound to importin-alpha. Strikingly, importin-beta binding displaces the RNA 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 importin-mediated RNA release is a specifically cytoplasmic event.
authors.affiliation: Zentrum für Molekulare Biologie, Universität Heidelberg, Federal Republic of Germany.
authors.forename: D
authors.initials: D
authors.lastname: Görlich
authors.forename: R
authors.initials: R
authors.lastname: Kraft
authors.forename: S
authors.initials: S
authors.lastname: Kostka
authors.forename: F
authors.initials: F
authors.lastname: Vogel
authors.forename: E
authors.initials: E
authors.lastname: Hartmann
authors.forename: R A
authors.initials: RA
authors.lastname: Laskey
authors.forename: I W
authors.initials: IW
authors.lastname: Mattaj
authors.forename: E
authors.initials: E
authors.lastname: Izaurralde
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D002467:Cell Nucleus / Q000737:chemistry / Q000378:metabolism; D003593:Cytoplasm / Q000737:chemistry; D005800:Genes, Fungal / Q000235:genetics; D006160:Guanosine Triphosphate / Q000378:metabolism; D028884:Karyopherins; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000032:analysis / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism; D011485:Protein Binding; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps / Q000378:metabolism; D012342:RNA, Small Nuclear / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D014981:Xenopus / Q000378:metabolism; D028901:alpha Karyopherins; D020931:ran GTP-Binding Protein
chemical_list: D028884:Karyopherins; D009687:Nuclear Proteins; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps; D012342:RNA, Small Nuclear; D016601:RNA-Binding Proteins; D028901:alpha Karyopherins; D006160:Guanosine Triphosphate; D020931:ran GTP-Binding Protein