Npap60/Nup50 Is a Tri-Stable Switch that Stimulates Importin-α:β-Mediated Nuclear Protein Import

Many nuclear-targeted proteins are transported through the nuclear pore complex (NPC) by the importin-α:β receptor. We now show that Npap60 (also called Nup50), a protein previously believed to be a structural component of the NPC, is a Ran binding protein and a cofactor for importin-α:β-mediated import. Npap60 is a tri-stable switch that alternates between binding modes. The C terminus binds importin-β through RanGTP. The N terminus binds the C terminus of importin-α, while a central domain binds importin-β. Npap60:importin-α:β binds cargo and can stimulate nuclear import. Endogenous Npap60 can shuttle and is accessible from the cytoplasmic side of the nuclear envelope. These results identify Npap60 as a cofactor for importin-α:β nuclear import and as a previously unidentified subunit of the importin complex.