Structural studies on the murine IA alloantigens. II. Molecular weight characterization of the products of the I-A and I-E/C subregions

Murine splenocytes were radiolabeled with ³H- and ¹⁴C-amino acids; the Ia alloantigens encoded by the I-A and I-E/C subregions were isolated by immunoprecipitation and analyzed for structural variation by poly-acrylamide gel electrophoresis. The I-A subregion products (k, d, and b haplotypes) are composed of two polypeptides, α and β, with m.w. of 34,000 and 26,000 daltons, respectively. Haplotype-associated differences in m.w. were detected in the I-E/C products of the k, r, p, and d haplotypes. The α and β chains of E/C(k) and E/C(r) are 34,000 and 28,000 daltons, respectively; E/C(p) and E/C(d) molecules are composed of 31,000 and 29,000 dalton polypeptides. Thus, there is both subregion (I-A vs I-E/C) and haplotype (E/C(k), E/C(r) vs E/C(d), E/C(p)) associated variation in the m.w. of the Ia alloantigens. Additionally, the covalent vs noncovalent association of the Ia subunits was examined and it was found that the α and β chains of both I-A and I-E/C are not covalently associated. However, the I-A α and β chains tend to associate through disulfide bonds during detergent lysis; the presence of alkylating agents during cell lysis prevents this association, and only free α and β chains are observed under nonreducing conditions.