A Molecular dynamics simulation of UBE2S (green)~Ub (yellow) reveals electrostatic interactions between the positive surface of Ub and UBE2S E139 (cutout). Rosetta model of APC2 (gray mesh, from PBD: 5L9T) docked on UBE2S~Ub would sterically clash with this extended conformation (Brown et al., 2016). 