Fig. 1. The Ran-GTP gradient governs the directionality of nucleocytoplasmic transport mediated by members of the karyopherin family of nuclear transport factors. The key role played by the GTP-bound form of Ran in mediating cargo binding and release by karyopherins functioning in nuclear import (importins) or nuclear export (exportins) is illustrated. See text for more detailed discussion. Imp, importin; Exp, exportin.