Summary: | Human histocompatbility (HLA) antigens are expressed on the surfaces of almost all cell types and according to various criteria are integral membrane glycoproteins. They are composed of equimolar amounts of two dissimilar polypeptide chains which are non covalently linked and which have molecular weights of about 43,000 and 12,000 as judged by polyacrylamide gel electrophoresis in sodium dodecyl sulphate (SDS). The larger chain carries the polymorphic allogeneic specificities and all of the carbohydrate. The smaller chain represents β2 microglobulin, whose amino acid sequence shows marked homology to the C(H)3 domain of the γ 1 chain of immunoglobulin G. This homology and other arguments have led to the proposals that HLA antigens and immunoglobulins have a common evolutionary origin, and that HLA antigens have an immunoglobulin like structure in which two basic units of one each of the larger and smaller polypeptide chains are linked either non covalently by a disulphide bridge to give a 4 chain dimer of molecular weight 110,000. In contrast to the latter proposal, some results presented here suggest that HLA antigens do not have a dimeric structure.
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