Figure 7 - full size

Figure 7. Figure 7. Model of Ubc9 Target Discrimination via Gain in Affinities The increased affinity can be a result of an additional binding interface mediated by an E3 (right panel) or by direct interaction of the target with Ubc9 (either free, thioester, or conjugated to SUMO) (left panel). (Top) Cartoon of Ubc9 thioester-target interactions, mediated either through the target, conjugated SUMO, or an E3 ligase. (Bottom) Shown are the Ubc9 surfaces involved in these interactions; interaction between the catalytic cleft in Ubc9 with the SUMO consensus motif (light green). Additional binding interfaces (dark green) enhance affinity and hence modification. Sumoylation of Ubc9 increases the interaction of targets with a SIM (Song et al., 2005) at a defined distance as seen for Sp100 in this study (left panel). Alternatively, the target can recruit the SUMO vert, similar Ubc9 thioester (picture according to Reverter and Lima [2005]) in, e.g., Daxx and TDG (Figure S5 and ([Hochstrasser, 2007], [Lin et al., 2006] and [Takahashi et al., 2005]). RanGAP1 itself has two binding interfaces (Bernier-Villamor et al., 2002) (middle panel) and is an unusually efficient SUMO substrate. ELK1 contains a negatively charged amino acid-dependent sumoylation motif (Yang et al., 2006). Similarly, the SP-RING E3 ligases (model based on E2-RING complex in the ubiquitin SCF system [Zheng et al., 2000]) stabilize the interaction between the target and Ubc9, resulting in enhanced modification (right panel).

The above figure is reprinted by permission from Cell Press: Mol Cell (2008, 31, 371-382) copyright 2008.