The yeast UBC9 gene encodes a protein with homology to the E2 ubiquitin-conjugating enzymes that mediate the attachment of ubiquitin to substrate proteins . Depletion of Ubc9p arrests cells in G2 or early M phase and stabilizes B-type cyclins . p18(Ubc9), the Xenopus homolog of Ubc9p, associates specifically with p88(RanGAP1) and p340(RanBP2) . Ran-binding protein 2 (p340(RanBP2)) is a nuclear pore protein  , and p88(RanGAP1) is a modified form of RanGAP1, a GTPase-activating protein for the small GTPase Ran . It has recently been shown that mammalian RanGAP1 can be conjugated with SUMO-1, a small ubiquitin-related modifier [5-7], and that SUMO-1 conjugation promotes RanGAP1's interaction with RanBP2 [2,5,6]. Here we show that p18(Ubc9) acts as an E2-like enzyme for SUMO-1 conjugation, but not for ubiquitin conjugation. This suggests that the SUMO-1 conjugation pathway is biochemically similar to the ubiquitin conjugation pathway but uses a distinct set of enzymes and regulatory mechanisms. We also show that p18(Ubc9) interacts specifically with the internal repeat domain of RanBP2, which is a substrate for SUMO-1 conjugation in Xenopus egg extracts.