title: Structural and evolutionary analysis of HLA-D-region products.
abstract: The major histocompatibility complex (MHC)--HLA in man and H-2 in mouse--encodes two classes of cell-surface antigens involved in the immune response. The amino acid sequences have been determined for a number of these molecules. Class I antigens, typified by the HLA-ABC antigens, are composed of a 43,000-molecular weight (MW) glycosylated transmembrane polypeptide with three external domains (alpha 1, alpha 2 and alpha 3), of which the one nearest the membrane (alpha 3) is associated with a 12,000-MW nonglycosylated polypeptide, beta 2-microglobulin. The HLA-D-region or class II antigens, DR, DC and SB, are composed of two glycosylated transmembrane polypeptides, of MWs 34,000 (alpha-chain) and 28,000 (beta-chain). Both chains have two external domains which presumably associate with each other, alpha 2, beta 2 being membrane proximal and alpha 1, beta 1 N-terminal and membrane distal. All four membrane-proximal domains (class I alpha 3, beta 2-microglobulin, class II alpha 2 and beta 2) have amino acid sequences that show significant similarities with immunoglobulin constant-region domains. This, together with the similarly placed internal disulphide bonds, suggests they might have an immunoglobulin-like structure (Fig. 1). We have now used computer graphics techniques to predict a detailed three-dimensional structure for the membrane-proximal domains of the class II antigens (alpha 2 and beta 2) based on the known coordinates of immunoglobulin constant domains (Fig. 2). The transmembrane regions of class II antigens have been modelled as two alpha-helices packed together. The proposed structure accounts for conservation of amino acids and leads to evolutionary predictions.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D006680:HLA Antigens; D015793:HLA-B7 Antigen; D006682:HLA-DP Antigens; D006683:HLA-DQ Antigens; D006684:HLA-DR Antigens; D000949:Histocompatibility Antigens Class II; D006801:Humans; D007127:Immunoglobulin Constant Regions; D007140:Immunoglobulin Fab Fragments; D046911:Macromolecular Substances; D008565:Membrane Proteins; D051379:Mice; D011487:Protein Conformation; D001613:beta 2-Microglobulin
chemical_list: D006680:HLA Antigens; D015793:HLA-B7 Antigen; D006682:HLA-DP Antigens; D006683:HLA-DQ Antigens; D006684:HLA-DR Antigens; D000949:Histocompatibility Antigens Class II; D007127:Immunoglobulin Constant Regions; D007140:Immunoglobulin Fab Fragments; D046911:Macromolecular Substances; D008565:Membrane Proteins; D001613:beta 2-Microglobulin