Structural and functional consequences of altering a peptide MHC anchor residue

To better understand TCR discrimination of multiple ligands, we have analyzed the crystal structures of two Db peptide/I-E-k complexes that differ by only a single amino acid substitution at the P6 anchor position within the peptide (E73D), Detailed comparison of multiple independently determined structures at 1.9 Angstrom resolution reveals that removal of a single buried methylene group can alter a critical portion of the TCR recognition surface, Significant variance was observed in the peptide P5-P8 main chain as well as a rotamer difference at LeuP8, similar to 10 Angstrom distal from the substitution, No significant variations were observed in the conformation of the two MHC class II molecules. The ligand alteration results in two peptide/MHC complexes that generate bulk T cell responses that are distinct and essentially nonoverlapping. For the Hb-specific T cell 3.L2, substitution reduces the potency of the ligand 1000-fold. Soluble 3.L2 TCR binds the two peptide/MHC complexes with similar affinity, although with faster kinetics. These results highlight the role of subtle variations in MHC Ag presentation on T cell activation and signaling.