Terry L . Delovitch and Brian

The I region of the mouse H-2 complex is divided into subregions, I-A, I-B, I-J, I-E, and I-C, that are marked by the Ia-1, Ia-2, Ia-4, Ia-5, and Ia-3 loci, respectively (1, 2). The Ia-1, Ia-5, and Ia-3 locus structural products are Ia antigens, a group of polymorphic, cell-surface glycoproteins that consist of two noncovalently associated polypeptide chains of mol wt =30,000-35,000 (a-subunit) and 25,000-30,000 (8subunit) (3, 4). Sequential precipitation analyses have previously shown that an Ia-5 locus product is independently precipitable from a second, distinct hi molecule determined by either the Ia-5 or Ia-3 locus (5). More recently, it was reported (6) that Ia.22 and Ia.23, private specificities of the I-E subregion of the H-2 k and H-2 a haplotypes, respectively, are coprecipitable with Ia.7, formerly considered to be a public specificity of the I-C subregion. To explain this finding, it was proposed that Ia.7 be assigned to the I-E subregion; and that Ia.6, which is difficult to detect serologically, is the only specificity determined by the I-C subregion. It is implied in the latter studies that only one Ia molecule controlled by a locus mapping between the I-J and S regions is detectable by immunoprecipitation. To resolve this apparent discrepancy, we have performed sequential precipitation, two-dimensional (2D) 1 gel electrophoresis, and peptide mapping analyses of I-EC 2 subregion products. We demonstrate here the presence of two Ia molecules in t h e / EC subregion that are identical in molecular size and charge, but differ by 220% in their peptides obtained by partial digestion with Staphylococcus aureus protease V8.