Insights into the Microscopic Structure of RNF4-SIM-SUMO Complexes from MD Simulations.

Post-translational modification with one of the isoforms of the small ubiquitin-like modifier (SUMO) affects thousands of proteins in the human proteome. The binding of SUMO to SUMO interacting motifs (SIMs) can translate the SUMOylation event into functional consequences. The E3 ubiquitin ligase RNF4 contains multiple SIMs and connects SUMOylation to the ubiquitin pathway. SIM2 and SIM3 of RNF4 were shown to be the most important motifs to recognize SUMO chains. However, the study of SIM-SUMO complexes is complicated by their typically low affinity and variable binding of the SIMs in parallel and antiparallel orientations. We investigated properties of complexes formed by SUMO3 with peptides containing either SIM2 or SIM3 using molecular dynamics simulations. The affinities of the complexes were determined using a state-of-the-art free energy protocol and were found to be in good agreement with experimental data, thus corroborating our method. Long unrestrained simulations allowed a new interpretation of experimental results regarding the structure of the SIM-SUMO interface. We show that both SIM2 and SIM3 bind SUMO3 in parallel and antiparallel orientations and identified main interaction sites for acidic residues flanking the SIM. We noticed unusual SIM-SUMO interfaces in a previously reported NMR structure (PDB: 2mp2) of a complex formed by a SUMO3 dimer with the bivalent SIM2-SIM3 peptide. Computational determination of the individual SIM-SUMO affinities based on these structural arrangements yielded significantly higher dissociation constants. To our knowledge, our approach adds new opportunities to characterize individual SIM-SUMO complexes and suggests that further studies will be necessary to understand these interactions when occurring in multivalent form.