SUMO-SIM interactions: From structure to biological functions

Post-translational modification by Small Ubiquitin-like Modifier (SUMO) proteins regulates numerous cellular processes. This modification involves the covalent and reversible attachment of SUMO to target proteins through an isopeptide bond, using a cascade of E1, E2 and E3 SUMOylation enzymes. Most functions of SUMO depend on the establishment of non-covalent protein-protein interactions between SUMOylated substrates and their binding partners. The vast majority of these interactions involve a conserved surface in the SUMO protein and a SUMO interacting motif (SIM), a short stretch of hydrophobic amino acids and an acidic region, in the interactor protein. Despite single SUMO-SIM interactions are relatively weak, they can have a huge impact at different levels, altering the activity, localization and stability of proteins, triggering the formation of macromolecular assemblies or inducing phase separation. Moreover, SUMO-SIM interactions are ubiquitous in most enzymes of the SUMO pathway, and play essential roles in SUMO conjugation and deconjugation. Here, we analyze the role of SUMO-SIM contacts in SUMO enzymes and targets and discuss how this humble interaction participates in SUMOylation reactions and mediates the outcome of this essential post-translational modification.