Prior to crystallisation, the SBC was buffer exchanged into 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, and 1.8 mM KH2PO4 (1x PBS) buffer with 0.5 mM TCEP. For soaking experiments SBC apo crystals were obtained by crystallising 17 mg/ml SBC in 100 mM tris(hydroxymethyl)aminomethane (TRIS)-N, N-Bis(2-hydroxyethyl) glycine (BICINE) pH 6.5-7, 9-14% polyethylene glycol (PEG) 8K, 26-20% ethylene glycol at 4 °C using the sittingdrop method (0.4 μl of protein + 0.35 μl reservoir solutions + 0.05 μl of seed stock of disrupted crystals). Ligands were soaked overnight into crystals that were obtained after approximately 5-14 days. Co-crystal complexes of SBC-ligand were obtained by incubating 17mg/ml SBC with 0.5mM compound 13/ MN551 at 4 °C overnight prior to crystallisation in 17.5% PEG 3350, 0.1M BTP pH6.5, 0.178M sodium citrate tribasic dihydrate at 4 °C using the sitting-drop method (0.4 μl of protein + 0.4 μl reservoir solutions). Soaked SBC crystals were 8lash frozen without any additional cryoprotectant whereas the co-crystals were frozen in presence of 20% ethylene glycol as cryoprotectant. Diffraction data were collected at Diamond Light Source beamline i03/i04 or The European Synchrotron Radiation Facility ID30-A1 and images were processed with Xia2 Dials/ autoPROC . The structure was solved by molecular replacement using SBC crystal structure (PDB entry 2C9W) as a search model. Subsequent iterative model building, and re8inement was done according to standard protocols using COOT and Phenix. Ligand restraints were generated using the Phenix eLBOW/PRODRG server . Data collection and re8inement statistics for all structures are described in Supplementary Table .