MODIFICATION OF RAN GTPASE-ACTIVATING PROTEIN BY THE SMALL UBIQUITIN-RELATED MODIFIER SUMO-1 REQUIRES UBC9, AN E2-TYPE UBIQUITIN-CONJUGATING ENZYME HOMOLOG

Covalent modification of the Ran GTPase-activating protein RanGAP1 wit h the ubiquitin-related protein SUMO-1 promotes its association with N up358, a component of the cytoplasmic fibrils emanating from the nucle ar pore complex (1, 2), In Xenopus egg extracts, Nup358 can be found i n a complex with Ubc9 (3), a structural homologue of the E2-type ubiqu itin-conjugating enzymes (UBCs). Here we show that a subset of the hum an homologue of Ubc9 (HsUbc9) colocalizes with RanGAP1 at the nuclear envelope, HsUbc9 forms thiolester conjugates with recombinant SUMO-1, but not with recombinant ubiquitin, indicating that it is functionally distinct from EB-type UBCs, Finally, HsUbc9 is required for the modif ication of RanGAP1 by SUMO-1. These results suggest that HsUbc9 is a c omponent of a novel enzymatic cascade that modifies RanGAP1, and possi bly other substrates, with SUMO-1.