Biochemical characterization of Ia antigens. Characterization of a 22,000-dalton A delta d polypeptide expressed by I-Ad haplotype mice.

Small amounts of a 22,000-dalton (22Kd) Ia polypeptide were immunoprecipitated with monoclonal anti-Ia antibodies from murine B lymphocyte tumor cells and from splenocytes that express the I-Ad subregion. HPLC peptide map comparisons of A alpha d, A beta d and the 22Kd polypeptides demonstrated that the 22Kd polypeptide has extensive homology with A alpha d. Amino terminal sequence analysis of the 3H-tyrosine 22Kd polypeptide indicates this polypeptide has tyrosines at residues 16, 17, 30, and 32. Unexpectedly, this amino terminal 22Kd sequence is homologous with A beta d rather than A alpha d. These results suggest that the 22Kd polypeptide is a hybrid molecule containing both A alpha d and A beta d-like sequences. The 22Kd polypeptide has provisionally been designated A delta d.