Simultaneous involvement of all six predicted antigen binding loops of the T cell receptor in recognition of the MHC/antigenic peptide complex.

The TCR is predicted to resemble the Fab fragment of an Ig molecule and by analogy to possess six Ag binding loops that contact MHC proteins bound with antigenic peptides. We have identified residues in the predicted Ag binding loops (beta 1, beta 2, and beta 3) on a TCR beta-chain that are important in the recognition of the MHC/antigenic peptide complex. Using site-directed mutagenesis, we altered the residues forming the predicted Ag binding site on the beta-chain expressed by the T lymphocyte clone D5, which specifically recognizes p-azobenzenearsonate-conjugated peptides presented by the class II MHC molecule I-Ad. Amino acid substitution of individual residues in each loop affected Ag recognition, demonstrating that all three putative Ag binding loops of the D5 TCR beta-chain are important in interaction with I-Ad/arsonate-conjugated Ag. Taken together with our previous work on the D5 TCR alpha-chain (Nalefski et al., J. Exp. Med. 175:1553), these results suggest that all six Ag binding loops of the D5 TCR alpha- and beta-chains interact simultaneously with the MHC/peptide complex. Consequently, the area of interaction between the TCR and the MHC/antigenic peptide complex is predicted to be extensive.