Tryptic peptide comparison of Ia antigen alpha and beta polypeptides from the I-A mutant B6.C-H-2bm12 and its congenic parental strain B6.

Previous studies of the B5.C-H-2 bm12 (bm12) strain have demonstrated the presence of a mutation localized to the I-A subregion of the mouse H-2 major histocompatibility complex. This mutation has been shown to be responsible for defects in Ir-gene function in Ia and MLR determinants. A comparison of the molecular size of the bm12 mutant and the parental B6 Ia-antigen component polypeptides failed to demonstrate any differences in the alpha and beta polypeptides. Thus, no major structural additions for deletions are present in the Ia alpha and beta chain polypeptide or carbohydrate structure. A significant decrease in the amount of invariant (31K) polypeptide was, however, consistently observed in the bm12 Ia antigen preparations. Tryptic peptide comparisons of 14C B6 and 3H bm 12 alpha and beta polypeptides demonstrated a limited number of peptide differences in the bm 12 beta polypeptide but none in the bm12 alpha polypeptide. The significance of these biochemical mutations and altered biological phenomena are discussed in relation to a model of the immunological interaction sites on Ia antigens.