SnapShot: The SUMO System

SUMO (small ubiquitin-related modifier) is a highly conserved eukaryotic member of the family of ubiquitin-related proteins. Despite limited sequence similarity, SUMO and ubiquitin have highly homologous structures (i.e., with a root-mean-square deviation [rmsd] difference of 2.1 A). Like ubiquitin, SUMO is covalently attached to other proteins (SUMOylation) and thus functions as a posttranslational modifier. Although a major function of ubiquitination is to promote protein degradation, SUMOylation does not usually trigger proteolyis of the conjugated protein. Instead, a main function of SUMOylation is to foster—and occasionally disrupt—protein-protein interactions.Although less frequent than ubiquitination, SUMOylation regulates numerous processes and has many substrates in the cytosol and the nucleus. Lower eukaryotes possess only one SUMO form, but higher eukaryotes express several, perhaps functionally distinct, SUMO variants. In most organisms, SUMO is essential for viability. This SnapShot depicts the reactions involved in SUMO ligation, the effects of SUMOylation on target proteins, and cellular processes regulated by SUMOylation.