SUMO-1 MODIFICATION AND ITS ROLE IN TARGETING THE RAN GTPASE-ACTIVATING PROTEIN, RANGAP1, TO THE NUCLEAR-PORE COMPLEX

RanGAP1 is the GTPase-activating protein for Ran, a small ras-like GTP ase involved in regulating nucleocytoplasmic transport. In vertebrates , RanGAP1 is present in two forms: one that is cytoplasmic, and anothe r that is concentrated at the cytoplasmic fibers of nuclear pore compl exes (NPCs). The NPC-associated form of RanGAP1 is covalently modified by the small ubiquitin-like protein, SUMO-1, and we have recently pro posed that SUMO-1 modification functions to target RanGAP1 to the NPC. Here, we identify the domain of RanGAP1 that specifies SUMO-1 modific ation and demonstrate that mutations in this domain that inhibit modif ication also inhibit targeting to the NPC, Targeting of a heterologous protein to the NPC depended on determinants specifying SUMO-1 modific ation and also on additional determinants in the COOH-terminal domain of RanGAP1. SUMO-1 modification and these additional determinants were found to specify interaction between the COOH-terminal domain of RanG AP1 and a region of the nucleoporin, Nup358, between Ran-binding domai ns three and four, Together, these findings indicate that SUMO-1 modif ication targets RanGAP1 to the NPC by exposing, or creating, a Nup358 binding site in the COOH-terminal domain of RanGAP1. Surprisingly, the COOH-terminal domain of RanGAP1 was also found to harbor a nuclear lo calization signal. This nuclear localization signal, and the presence of nine leucine-rich nuclear export signal motifs, suggests that RanGA P1 may shuttle between the nucleus and the cytoplasm.