Distinct functions for the two importin subunits in nuclear protein import.

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.