UBC9P AND THE CONJUGATION OF SUMO-1 TO RANGAP1 AND RANBP2

The yeast UBC9 gene encodes a protein with homology to the E2 ubiquiti n-conjugating enzymes that mediate the attachment of ubiquitin to subs trate proteins [1], Depletion of Ubc9p arrests cells in G2 or early M phase and stabilizes B-type cyclins [1]. p18(Ubc9), the Xenopus homolo g of Ubc9p, associates specifically with p88(RanGAP1) and p340(RanBP2) [2]. Ran-binding protein 2 (p340(RanBP2)) is a nuclear pore protein [ 3,4], and p88(RanGAP1) is a modified form of RanGAP1, a GTPase-activat ing protein for the small GTPase Ran [2]. It has recently been shown t hat 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 conju gation. This suggests that the SUMO-1 conjugation pathway is biochemic ally 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, whic h is a substrate for SUMO-1 conjugation in Xenopus egg extracts.