RAN BINDING DOMAINS PROMOTE THE INTERACTION OF RAN WITH P97 BETA-KARYOPHERIN, LINKING THE DOCKING AND TRANSLOCATION STEPS OF NUCLEAR IMPORT/

Nuclear protein import is accomplished by two sequential events: docki ng at the nuclear pore complex followed by ATP-dependent translocation across the nuclear envelope. Docking of nuclear targeted proteins req uires a 56-kDa nuclear localization signal receptor (alpha-karyopherin , importin-alpha, SRP1 alpha) and a 97-kDa protein (beta-karyopherin, importin-beta). Components necessary for translocation include the Ran /TC4 GTPase and NTF2/B-2. The functions of these factors at a molecula r level remain unclear. We have now found that a complex of Ran, in th e GTP-bound state, with either the Ran binding protein, RanBP1, or an isolated Ran binding domain binds with high affinity and specificity t o beta-karyopherin to form a ternary complex, We find that a C-termina l truncation mutant of Ran, Delta-DE Ran, also binds to beta-karyopher in and that Delta-DE Ran can associate with a cytosolic, multiprotein complex that contains beta-karyopherin and another Delta-DE Ran bindin g protein of 115/120 kDa. These data suggest a physical link between d ocking and translocation mediated by a Ran GTPase-Ran binding protein complex.