Two-dimensional polyacrylamide gel electrophoresis of Ia antigens obtained from 35S-methionine-labeled lymphocytes has revealed that the expression on the cell surface of an antigen coded for by a locus in the I-A subregion may be controlled by an I-E subregion product. Three distinct polypeptide chains (Aa, Ab, and Ae) coded for by loci in I-A have been identified in gels of immunoprecipitated Ia antigens; one of these (Ae) is coprecipitated with an I-E antigen by anti-I-E antibodies and probably is complexed to the I-E chain in the cell.
Mice of haplotypes which do not have serologically-detectable I-E antigens (such as b and s haplotypes) appear not to synthesize and I-E product. These mice also do not express Ae on their cell surfaces, although precursors of surface Ae can be detected in the cytoplasm. Some haplotypes, however, contain a locus mapping between Ia.4 in I-J and H-2D which can complement I-A in either the cis or trans position to allow the cell surface expression of Ae.