Crystal structure of H-2K b /S598 and H-2K b /S598 Q600Y complexes Crystals of H-2K b S598-Aba were grown at 21uC in 0.1 M cacodylate pH 6.6, 16% PEG (polyethylene glycol) 8,000, 0.2 M Ca(OAc) 2 , using a protein concentration of 9 mg/ml. Crystals were cryoprotected by stepwise equilibration against mother liquor containing 5, 10 and 15% glycerol and flash frozen by placing in a nitrogen stream. A 1.8 Å resolution dataset was collected on an inhouse X-ray source. Crystals of H-2K b S598 Q600Y -Aba were grown at 21uC in 0.1 M cacodylate pH 6.5, 13% PEG 8,000, 0.2 M Ca(OAc) 2 , using a protein concentration of 6 mg/ml. Crystals were cryoprotected by gradual equilibration against mother liquor containing 20% PEG 8,000 and 5% glycerol before flash freezing. A 2.8 Å resolution dataset was collected on an in-house source. The WT data were integrated in MOSFLM [52] and scaled/merged using SCALA [53] . The Q600Y variant data were processed using HKL2000. Both structures were solved by molecular replacement in PHASER [54] , against previously solved H-2K b complexes (PDBid's: 1G7Q and 1RJY, respectively). The resultant models underwent iterative cycles of refinement in PHENIX [55] and, REFMAC5 [56] (restrained and TLS refinement) followed by model building/ correction in COOT [57] . The solvent structures were built using ARP/wARP [58] and COOT. A summary of the processing and refinement statistics is presented in Table S1 .