Identification of a crucial energetic footprint on the al helix of human histocompatibility leukocyte antigen (HLA)-A2 that provides functional interactions for recognition by tax peptide/HLA-A2-specifrc T cell receptors

Structural studies have shown that class I major histocompatibility complex (MHC)-restricted peptide-specific T cell receptor (TCR)-alpha/betas make m ultiple contacts with the alpha1 and alpha2 helices of the MHC, but it is u nclear which or how many of these interactions contribute to functional bin ding. We have addressed this question by performing single amino acid mutag enesis of the 15 TCR contact sites on the human histocompatibility leukocyt e antigen (HLA)-A2 molecule recognized by the A6 TCR specific for the Tax p eptide presented by HLA-A2. The results demonstrate that mutagenesis of onl y three amino acids (R65, K66, and A69) that are clustered on the alpha1 he lix affected T cell recognition of the Tax/HLA-A2 complex. At least one of these three mutants affected T cell recognition by every member of a large panel of Tax/HLA-A2-specific T cell lines. Biacore measurements showed that these three HLA-A2 mutations also altered A6 TCR binding kinetics, reducin g binding affinity. These results show that for Tax/HLA-A2-specific TCRs, t here is a location on the central portion of the alpha1 helix that provides interactions crucial to their function with the MHC molecule.