A NOVEL UBIQUITIN-LIKE MODIFICATION MODULATES THE PARTITIONING OF THERAN-GTPASE-ACTIVATING PROTEIN RANGAP1 BETWEEN THE CYTOSOL AND THE NUCLEAR-PORE COMPLEX

Ran is a nuclear Ras-like GTPase that is required for the bidirectiona l transport of proteins and ribonucleoproteins across the nuclear pore complex (NPC). A key regulator of the Ran GTP/GDP cycle is the 70-kD Ran-GTPase-activating protein RanGAP1, Here, we report the identificat ion and localization of a novel form of RanGAP1. Using peptide sequenc e analysis and specific mAbs, RanGAP1 was found to be modified by conj ugation to a ubiquitin-like protein. Immunoblot analysis and immunoloc alization by light and EM demonstrated that the 70-kD unmodified form of RanGAP1 is exclusively cytoplasmic, whereas the 90-kD modified form of RanGAP1 is associated with the cytoplasmic fibers of the NPC. The modified form of RanGAP1 also appeared to associate with the mitotic s pindle apparatus during mitosis. These findings have specific implicat ions for Ran function and broad implications for protein regulation by ubiquitin-like modifications. Moreover, the variety and function of u biquitin-like protein modifications in the cell may be more diverse th an previously realized.