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Molecular Characterization of ABHand Colton Blood Group Antigens

Blood group antigens are structural variants in surface carbohydrate or amino acid polymorphisms on extracellular domains of membrane proteins. The red cell water channelforming integral protein (Aquaporin CHIP) is a homotetramer with only one N-glycosylated subunit, however no CHIP-associated blood group antigens have yet been identified. Immunoblotting, monosaccharide composition analysis, and selective glycosidase digestions revealed that the CHIP-associated oligosaccharide contains ABH determinants and resembles a band 3-type glycan that cannot be cleaved from intact membranes by Peptide:N-glycosidase F. The molecular structure of the Colton antigens was previously unknown, but CHIP was selectively immunoprecipitated with anti-Coa or anti-Cob. The DNAsequence from Colton-typed individuals predicted that residue 45 is alanine in the Co(a+b-) phenotype and valine in the Co(a-b+) phenotype. The nucleotide polymorphism corresponds to a PfiMI endonuclease digestion site in the DNA from Co(a-b+) individuals. These studies have defined antigens within two blood group systems on CHIP: (a) an ABHbearing polylactosaminoglycan attached to a poorly accessible site in the native membrane; and (b) the Colton antigen polymorphism which may permit the identification of rare individuals with defective water channel expression. (J. Clin. Invest. 1994. 94:1043-1049.)