While the Major Histocompatibility Complex Class I (MHC-I) molecules typically bind short peptide (p) fragments (8-10 amino acids in length), longer, bulged, peptides are often be presented by MHC-I. Such bulged pMHC-I complexes represent challenges for T-cell receptor (TCR) ligation, although the general principles underscoring the interaction between TCRs and bulged pMHC-I complexes are unclear. To address this, we have explored the energetic basis of how an immunodominant TCR (termed SB27) binds to a 13 amino acid viral peptide (LPEPLPQGQLTAY) complexed to Human Leukocyte Antigen (HLA) B*3508. Using the crystal structure of the SB27 TCR-HLA B*3508LPEP complex as a guide, we undertook a comprehensive alanine-scanning mutagenesis approach at the TCR-pMHC-I interface and examined the effect of the mutations by biophysical (affinity measurements) and cellular approaches (tetramer staining). While the structural footprint on the HLA B*3508 was small, the energetic footprint was even smaller in that only two HLA B*3508 residues were critical for the TCR interaction. Instead, the energetic basis of this TCR-pMHC-I interaction was attributed to peptide-mediated interactions in which the complementarity determining region (CDR) 3alpha and germline encoded CDR1beta loops of the SB27 TCR played the principal role. Our findings highlight the peptide-centricity of TCR ligation towards a bulged pMHC-I complex.
Liu, Y. C., Chen, Z., Burrows, S. R., Purcell, A. W., McCluskey, J., Rossjohn, J., & Gras, S. (2012). The energetic basis underpinning T-cell receptor recognition of a super-bulged peptide bound to a major histocompatibility complex class I molecule. The Journal of Biological Chemistry, 287(15), 12267 - 12276. https://doi.org/10.1074/jbc.M112.344689