SUMOylation is an important post-translational modification involved in regulating various cellular processes. Similar to ubiquitylation, sumoylation leads to the formation of an isopeptide bond between the C-terminal glycine of SUMO to an acceptor lysine within consensus Ψ-K-x-D/E motif on the target protein. Unlike SUMO1, SUMO2/3 carries a sumoylation motif; thus, similar to ubiquitin, SUMO2/3 can form a poly-SUMO chain. SUMO1 may incorporate in such a chain but as an end cap to terminate further elongation.
Recently, a novel non-covalent interaction mode of SUMO recognition has been identified by means of SIM (SUMO interaction motif), which has a consensus sequence of V/I-x-V/I-V/I. The SIM interacts with SUMO by binding to the groove between second β-strand and α-helix. Interestingly, the E3 ubiquitin-protein ligase RNF4 contains four tandem SIM repeats for selective interaction with poly-SUMO modified proteins, which it targets for ubiquitin-mediated proteasome degradation. Here, a multifaceted biophysical approach, including the usage of NMR, X-Ray crystallography, SAXS and a knowledge-based HADDOCK model, was employed to characterise structures of the RNF4-SIMs domain and tetra-SUMO2 chain to elucidate the interaction between them.

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