Previous studies of the B6.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 and 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 α and β polypeptides. Thus, no major structural additions or deletions are present in the Ia α and β 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 α and β polypeptides demonstrated a limited number of peptide differences in the bm12 β polypeptide but none in the bm12 α 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.
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