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)^1,2, energy ^3,4 and soluble factors⁵, namely importin-α(M _r 60K)^6-12,28, importin-β (90K)^8-11,13 and Ran^14,15. Importin-α is primarily responsible for NLS recognition^6-12,29 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 cytosol^8,9. 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.