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All SUMO isoforms are synthesized from C-terminally extended precursors that first need to be processed to their active forms, which terminate with a pair of glycine residues. SUMO2 and SUMO3 are nearly identical and will hereafter be referred to as SUMO2/3 when isoform type needs to be specified. Yeast has only one form of SUMO (encoded by SMT3), but most vertebrates express at least three SUMO isoforms, SUMO1, SUMO2, and SUMO3, each encoded by a distinct gene. It may modify a single substrate lysine or multiple lysines (multisumoylation) or form SUMO chains on its substrates (polysumoylation). Like ubiquitin, SUMO is usually ligated to substrate lysine side chains, forming amide (isopeptide) linkages. SUMO attachment to proteins requires ATP and typically involves three enzymes: a heterodimeric SUMO-activating enzyme (E1) that activates the SUMO C-terminal carboxyl group and then forms a thioester with the SUMO C-terminus, a SUMO-conjugating enzyme (E2), and one of several SUMO ligases (E3s) 11. Many cellular proteins bear SUMO-interacting motifs (SIMs) that bind SUMO non-covalently 10. Although SUMO attachment to proteins can have distinct physiological consequences, its primary molecular function is to control interactions of the modified proteins with other proteins. Correspondingly, sumoylation is known to affect numerous cellular processes. The UBL known as small ubiquitin-related modifier (SUMO) is attached to a multitude of proteins in all species that have been analyzed, including Saccharomyces cerevisiae 2– 5, Arabidopsis thaliana 6, 7 and humans 8, 9. Ubiquitin and UBLs share a common fold, known as the β-grasp fold, and a conserved enzymatic strategy for their activation and attachment to substrates 1. Among the many known post-translational protein modifiers are other proteins, most commonly ubiquitin or one of several ubiquitin-like proteins (UBLs). Cellular proteins are often covalently modified following their translation, providing organisms with added control of protein activity, localization or stability.