Structural characterization of H-2 antigens purified from mouse liver

Papain solubilized H-2a histocompatibility antigens (H-2Kk plus H-2Dd) have been purified by a large-scale procedure that can routinely provide 2-3 mg of heavy chain from 1 kg of mouse liver. The heavy chains were homogeneous by sodium dodecyl sulfate electrophoresis. Disc gel electrophoresis resolved two protein bands that were identified as H-2Dd and H-2Kd by immune complex formation and autoradiography. Comparative amino acid composition and NH2-terminal sequence analyses of unfractionated H-2a, H-2Kk, and HLA suggested close structural relationships. However, the following observations suggest that papain cleaves these membrane bound antigens at different positions with respect to the COOH terminus: the molecular weight of the peptide portion of papain solubilized HLA is smaller than that of H-2 (30,000 versus 33,700); the COOH-terminal sequences are different; and, finally, papain-solubilized H-2 contains a free cysteine residue in addition to the two disulfide bridges that are present in both H-2 and HLA.