Distinct functions for the two importin subunits in nuclear protein import

Nature. 1995 Sep 21;377(6546):246-8. doi: 10.1038/377246a0.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphate / metabolism
  • Animals
  • Biological Transport
  • Cell Nucleus / metabolism*
  • HeLa Cells
  • Humans
  • Karyopherins
  • Microscopy, Immunoelectron
  • Nuclear Envelope / metabolism
  • Nuclear Envelope / ultrastructure
  • Nuclear Proteins / chemistry
  • Nuclear Proteins / metabolism*
  • Protein Sorting Signals / metabolism
  • Rats
  • Recombinant Proteins / metabolism
  • Xenopus
  • ran GTP-Binding Protein


  • Karyopherins
  • Nuclear Proteins
  • Protein Sorting Signals
  • Recombinant Proteins
  • Adenosine Triphosphate
  • ran GTP-Binding Protein