Functional and molecular characterization of I-A_β^k mutants is consistent with the predicted three dimensional structure of class II MHC molecules

Class II MHC (Ia) molecules have been shown to be critical as restriction elements in the T helper/inducer cell recognition of antigen. Efforts to determine the role of allelic variation in MHC restricted antigen presentation have included the use of serologically selected mutants to correlate structural variations in Class II molecules with changes in the antigen presenting function of Ia bearing cells. Such studies have revealed that serologically selected mutations tend to occur in a single immunodominant region and that even a single amino acid substitution can alter T cell recognition of Ia molecules. We report here the characterization of two more serologically selected Class II A_β chain mutations. Each is due to a single base change which alters a single amino acid. One of these mutations is in the third hypervariable region (amino acid 64-glutamine to arginine) and alters the antigen presenting function. The second mutation at amino acid 48, though a relatively non-conservative change (arginine to cysteine), has no effect on APC phenotype. Such a result would be predicted based on comparisons made with the proposed three dimensional crystallographic structure of Class I molecules and models proposed for Class II molecules based on Class I structure. The amino acid change at position 48 is in a portion of the molecule that is most likely unavailable to bind antigen or interact with T cell receptor whereas the mutation at amino acid 64 is on an exposed face of the alpha helix, a region which could affect interaction with either antigen and/or the T cell receptor.