The murine Ia alloantigens encoded by the I-A and I-E/C subregions were examined by SDS-polyacrylamide gel electrophoresis and microsequence analysis. By sensitive double label experiments (14C and 3H), the alpha (α) and beta (β) polypeptides of the A and E/C subregions were analyzed for molecular weight (mw) variation. The α and β chains of the A subregion (k, b, and d haplotypes) electrophoresed at 34,000 and 26,000 daltons, respectively. Haplotype associated differences in mw were detected for the E/C subregion products — k and r haplotype subunit mw were 34,000 (α) and 28,000 (β) daltons, whereas the α and β chains of p and d haplotypes were 31,000 and 29,000 daltons, respectively.
The α and β chains of both the A and E/C antigens do not exist on the cell surface as disulfide bonded α plus β forms.