The proteasome substrate receptor hRpn10 C-terminal RAZUL (blue) is intrinsically disordered (left), but folds to form a 4-helix bundle with the AZUL domain of E3 ligase E6AP (green). (PDB 6U19) (Buel, G., Chen, X., et al. bioRxiv doi:https://doi.org/10.1101/797027, 2019).