title: Distinct functions for the two importin subunits in nuclear protein import.
abstract: The import of nuclear proteins proceeds through the nuclear pore complex and requires nuclear localization signals (NLSs), energy and soluble factors, namely importin-alpha (M(r) 60K), importin-beta (90K) and Ran. Importin-alpha is primarily responsible for NLS recognition and is a member of a protein family that includes the essential yeast nuclear pore protein SRP1p (ref. 16). As the first event, the complex of importin-alpha and importin-beta binds the import substrate in the cytosol. Here we show that this nuclear pore targeting complex initially docks as a single entity to the nuclear pore via importin-beta. Then the energy-dependent, Ran-mediated translocation through the pore results in the accumulation of import substrate and importin-alpha in the nucleus. In contrast, importin-beta accumulates at the nuclear envelope, but not in the nucleoplasm. Immunoelectron microscopy detects importin-beta on both sides of the nuclear pore. This suggests that the nuclear pore targeting complex might move as a single entity from its initial docking site through the central part of the nuclear pore before it disassembles on the nucleoplasmic side.
authors.affiliation: Wellcome/CRC Institute, Cambridge, UK.
authors.forename: D
authors.initials: D
authors.lastname: Görlich
authors.forename: F
authors.initials: F
authors.lastname: Vogel
authors.forename: A D
authors.initials: AD
authors.lastname: Mills
authors.forename: E
authors.initials: E
authors.lastname: Hartmann
authors.forename: R A
authors.initials: RA
authors.lastname: Laskey
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D028884:Karyopherins; D016253:Microscopy, Immunoelectron; D009685:Nuclear Envelope / Q000378:metabolism / Q000648:ultrastructure; D009687:Nuclear Proteins / Q000737:chemistry / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D014981:Xenopus; D020931:ran GTP-Binding Protein
chemical_list: D028884:Karyopherins; D009687:Nuclear Proteins; D021382:Protein Sorting Signals; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate; D020931:ran GTP-Binding Protein