Ran binding domains promote the interaction of Ran with p97/beta-karyopherin, linking the docking and translocation steps of nuclear import.

Abstract Nuclear protein import is accomplished by two sequential events: docking at the nuclear pore complex followed by ATP-dependent translocation across the nuclear envelope. Docking of nuclear targeted proteins requires a 56-kDa nuclear localization signal receptor (α-karyopherin, importin-α, SRP1α) and a 97-kDa protein (β-karyopherin, importin-β). Components necessary for translocation include the Ran/TC4 GTPase and NTF2/B-2. The functions of these factors at a molecular level remain unclear. We have now found that a complex of Ran, in the GTP-bound state, with either the Ran binding protein, RanBP1, or an isolated Ran binding domain binds with high affinity and specificity to β-karyopherin to form a ternary complex. We find that a C-terminal truncation mutant of Ran, Δ-DE Ran, also binds to β-karyopherin and that Δ-DE Ran can associate with a cytosolic, multiprotein complex that contains β-karyopherin and another Δ-DE Ran binding protein of 115/120 kDa. These data suggest a physical link between docking and translocation mediated by a Ran GTPase-Ran binding protein complex.