Fig. 1. (A) The Ran GTPase cycle. GTP hydrolyis and guanine-nucleotide exchange on Ran occur in the cytoplasm and the nucleus, respectively, owing to the localization of the only known Ran-specific GTPase-activating protein (GAP) and guanine-nucleotide-exchange factor (GEF). Additional Ran-binding proteins, RanBP1 and Mog1, coactivate the reactions as indicated. Nuclear import and export of Ran through the nuclear pore complexes (NPCs, shown in grey) are accomplished by the RanGDP-binding NTF2 protein and the family of RanGTP-binding karyopherins (Imp/Exp), respectively. (B) Dynamic steady-state localization of Ran (Gsp1p) visualized in S. cerevisiae. A low-copy number plasmid encoding GFP-Gsp1p under control of the NOP1 promoter (Lau et al., 2000) was introduced into temperature-sensitive S. cerevisiae strains defective for RanGAP (rna1-1) and RanGEF (prp20-1) and an isogenic wild-type strain. Fluorescence microscopy was performed as described (Lau et al., 2000).