SUMO: getting it on

Post-translational modification of cellular proteins by the SUMO (small ubiquitin-related modifier) is involved in numerous modes of regulation in widely different biological processes. In contrast with ubiquitination, SUMO conjugation is highly specific in terms of target lysine residues, but many aspects of substrate and lysine selection by the SUMO conjugating machinery are still poorly understood. SUMOylation events usually occur on the PsiKXE SUMO consensus motifs, which mediate binding to Ubc9 (ubiquitin-conjugating enzyme 9), the SUMO E2 conjugating enzyme. Although most, if not all, SUMO conjugations are catalysed by Ubc9, far from all PsiKXE tetrapeptides are modified, demonstrating a need for additional specificity determinants in SUMOylation. Recent results intimately link regulation of SUMOylation to other post-translational modifications, including phosphorylation and acetylation and reveal that certain lysine residues are marked for SUMOylation by negatively charged amino acid residues or phosphorylation events immediately downstream of the consensus site. In the present review, we explore the intriguing role of extended motifs in the regulation of SUMO conjugation.