[Analysis of the allele specific Ag-binding site on murine class II MHC]

Residues 46 and 54 on pigeon cytochrome c 43-58 (p43-58) analogues function as agretopes (sites bound to MHC molecules). Phenylalanine (F) and alanine (A) at positions 46 and 54 on p43-58 respectively bind to I-Ab. Aspartic acid (D) and alanine at positions 46 and 54 respectively bind to I-Ak. To determine the allele specific binding sites (desetope (s)) on class II molecules that are correspondent to the agretopes of peptide antigen (Ag), we analyzed directly binding capacity of p43-58 analogues with glutamic acid (E) at the epitopic position 50 (50E) to L cell transfectants expressing recombinant I-A molecules between b and k types. An Ak binding peptide, 46D50E54A, bound to transfectant possessing amino acid sequence of k type on N-terminal half of alpha-helix of A alpha-chain irrespective of the b type sequence on the other part, whereas an Ab binding peptide, 46F50E54A, did not bind to these transfects. Thus, agretopic residue 46 of 46D50E54A peptides appeared to bind to N-terminal half of alpha-helix of A alpha-chain. To define critical residues for the allele specific peptide binding, we then analyzed peptide binding capacity of Ak mutants substituted one of four polymorphic residues between Ak and Ab molecules. An Ak mutant, Ak alpha(56A), where arginine (R) at position 56 of the Ak alpha-chains was substituted with alanine located at the same position 56 of the Ab alpha-chains hardly bound 46D50E54A. By contrast, the Ak alpha(56A) bound 46F50E54A. Furthermore, Ak restricted T cell hybridomas responded to 46F50E54A but not to 46D50E54A in the presence of the Ak alpha(56A) APC. Thus, an amino acid on the position 56 of A alpha-chain determines critically specificity of the allele specific peptide binding (desetope).