Ubc9

Attachment of SUMO (small-ubiquitin-related modifier) to a lysine residue(s) in target proteins, a post-translational modification called SUMOylation, can alter protein function through changes in protein stability, transcriptional activity or subcellular localization. Covalent attachment of SUMO to protein substrates requires an E1 activating enzyme and an E2 conjugation enzyme. In 1997, Ubc9 (ubiquitin-conjugating enzyme 9; also known as Ube2i) was identified as the yeast E2 conjugation enzyme for SUMO modification. A functionally conserved human homologue of the yeast Ubc9 was also identified. Ubc9 is required for SUMO conjugation in Saccharomyces cerevisiae, suggesting that it is the only E2 ligase for the SUMO conjugation reaction. Comparison of the Ubc9 crystal structure with that of the ubiquitination E2 enzymes Ubc1 and Ubc4 revealed that Ubc9 shares considerable sequence and tertiary structural similarity with the class I E2-type ubiquitin-conjugating enzyme family. Ubc9, however, has two small sequence insertions which create an exposed β-loop at residues 32-36, and a bulge in the loop at residues 94-102. Although ubiquitination requires an E3 ligase, Ubc9 is sufficient for SUMOylation, as some identified substrates can be specifically modified in vitro using only E1 (Uba2/Aos1), Ubc9, mature SUMO and ATP. However, three distinct types of SUMO E3 ligases have been identified: PIAS (protein inhibitor of activated STAT) family members; the vertebrate nuclear pore protein RanBP2/Nup358; and the polycomb group protein Pc2. E3 ligases strongly enhance SUMOylation of target proteins, which is often marginal in vitro in the presence of only E1 and E2 enzymes, and are likely to contribute to SUMOylation substrate specificity and efficiency.