SIMULTANEOUS INVOLVEMENT OF ALL 6 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 an d by analogy to possess six Ag binding loops that contact MHC proteins bound with antigenic peptides. We have identified residues in the pre dicted Ag binding loops (beta1, beta2, and beta3) on a TCR beta-chain that are important in the recognition of the MHC/antigenic peptide com plex. Using site-directed mutagenesis, we altered the residues forming the predicted Ag binding site on the beta-chain expressed by the T ly mphocyte clone D5, which specifically recognizes p-azobenzenearsonate- conjugated peptides presented by the class II MHC molecule I-A(d). Ami no acid substitution of individual residues in each loop affected Ag r ecognition, demonstrating that all three putative Ag binding loops of the D5 TCR beta-chain are important in interaction with I-A(d)/arsonat e-conjugated Ag. Taken together with our previous work on the D5 TCR a lpha-chain (Nalefski et al., J. Exp. Med. 175:1553), these results sug gest that all six Ag binding loops of the D5 TCR alpha- and beta-chain s 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.