NMR studies of the SUMO-activating enzyme in complex with Ubc9 (Wang et al., 2007, this issue of Molecular Cell) complement a recent crystal structure of Ubc12 bound to the NEDD8-activating enzyme ternary complex (Huang et al., 2007), elucidating details of the first steps in the conjugation of ubiquitin and ubiquitin-like proteins.