Selective disruption of nuclear import by a functional mutant nuclear transport carrier

p10/NTF2 is a nuclear transport carrier that mediates the uptake of cytopla smic RanGDP into the nucleus. We constructed a point mutant of p10, D23A, t hat exhibited unexpected behavior both in digitonin-permeabilized and micro injected mammalian cells. D23A p10 was markedly more efficient than wild-ty pe (wt) p10 at supporting Ran import, but simultaneously acted as a dominan t-negative inhibitor of classical nuclear localization sequence (cNLS)-medi ated nuclear import supported by karyopherins (Kaps) alpha and beta1. Bindi ng studies indicated that these two nuclear transport carriers of different classes, p10 and Kap-beta1,compete for identical and/or overlapping bindin g sites at the nuclear pore complex (NPC) and that D23A p10 has an increase d affinity relative to wt p10 and Kap-beta1 for these shared binding sites. Because of this increased affinity, D23A p10 is able to import its own car go (RanGDP) more efficiently than wt p10, but Kap-beta1 can no longer compe te efficiently for shared NPC docking sites, thus the import of cNLS cargo is inhibited. The competition of different nuclear carriers for shared NPC docking sites observed here predicts a dynamic equilibrium between multiple nuclear transport pathways inside the cell that could be easily shifted by a transient modification of one of the carriers.