THE import of nuclear proteins proceeds through the nuclear pore complex and requires nuclear localization signals (NLSs)1,2, energy 3,4 and soluble factors5, namely importin-α(M r 60K)6-12,28, importin-β (90K)8-11,13 and Ran14,15. Importin-α is primarily responsible for NLS recognition6-12,29 and is a member of a protein family that includes the essential yeast nuclear pore protein SRPlp (ref. 16). As the first event, the complex of importin-α and importin-β binds the import substrate in the cytosol8,9. Here we show that this nuclear pore targeting complex initially docks as a single entity to the nuclear pore via importin-β. Then the energy-dependent, Ran-mediated translocation through the pore results in the accumulation of import substrate and importin-α in the nucleus. In contrast, importin-β accumulates at the nuclear envelope, but not in the nucleoplasm. Immunoelectron microscopy detects importin-β 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.
|Number of pages||3|
|Publication status||Published - 01.12.1995|