Evolutionary relationship between immunoglobulins and transplantation antigens

The major human and murine histocompatibility antigens are tetrameric molecules with an apparent molecular weight of about 130,000. They are composed of two types of polypeptide chains. The two light chains, previously identified as beta2-microglobulins, are bound to the two heavy, alloantigenic HL-A or H-2 polypeptide chains by noncovalent interactions only. The heavy chains are held together by disulfide bridge(s) located in the part of the molecule that is attached to the cell membrane. By limited proteolysis of the histocompatibility antigens evidence was obtained suggesting that the heavy chain may consist of three compact domains connected by more extended stretches of polypeptide chain. Each domain appeared to contain a single disulfide bride encompassing about 60 to 70 amino-acid residues. Staphylococcus aureus protein A is known to bind exclusively to the Fe region of immunoglobulin G. It was, however, observed that protein A interacts in a similar way with the H-2 antigen heavy chain. This observation, together withthe homology of the primary structure of beta2-microglobulin to immunoglobulin G, the tetrameric structure of the alloantigens, the ogranizations of the heavy polypeptide chain into compact domains, and the presence of a single, immunoglobulin-like disulfide loop in each domain, establishes a close similarity in structure between histocompatibility antigens and immunoglobulins. The similarity in structural features suggests a common evolutionary origin of the two types of molecules.

Highly purified papain-solubilized HL-A antigens contain beta2-microglobulin

HL-A antigens comprising 11 different antigenic specificities were isolated after papain solubilization of spleen-cell membrane constituents. During the entire purification procedure, beta(2)-microglobulin appeared together with the HL-A antigens. The highly purified antigens were composed of two polypeptide chains. The large subunit carried the antigenic specificity whereas the small polypeptide chain was very similar, if not identical, to beta(2)-microglobulin. The two HL-A antigen polypeptide chains were held together by noncovalent interactions only, and beta(2)-microglobulin, isolated from urine, could replace the small subunit in forming a complex with the large polypeptide chain. The topographical relationship in the cell membrane between beta(2)-microglobulin and the large HL-A antigen polypeptide chain is unknown. The two polypeptide chains may be fortuitously bound as a result of the solubilization procedure.