Small ubiquitin-related modifiers in chains

Post-translational modification of proteins by SUMOs (small ubiquitin-related modifiers) plays an important role in a wide variety of biological processes. The mammalian SUMO family includes three members, SUMO-1, SUMO-2 and SUMO-3. While target proteins are predominantly conjugated to monomeric SUMO, all three SUMO family members are able to multimerize in vitro. In cells, SUMOs have the potential to multimerize via internal consensus sites for SUMOylation that are present in SUMO-2 and SUMO-3. A SUMO-binding motif in Ubc9 (ubiquitin-conjugating enzyme 9) contributes to SUMO chain formation in vitro and SUMO E3 ligases further enhance SUMO polymerization. SUMO chain formation is reversible; SUMO polymers are disassembled by SUMO proteases both in vitro and in vivo. Despite recent progress, the functional relevance of SUMO polymerization is still unclear and little is known about the identity of the endogenous target proteins that are conjugated to SUMO polymers.