HUMAN RED-CELL AQUAPORIN CHIP .1. MOLECULAR CHARACTERIZATION OF ABH AND COLTON BLOOD-GROUP ANTIGENS

Blood group antigens are structural variants in surface carbohydrate o r amino acid polymorphisms on extracellular domains of membrane protei ns. The red cell water channel-forming integral protein (Aquaporin CHI P) is a homotetramer with only one N-glycosylated subunit, however no CHIP-associated blood group antigens have yet been identified. Immunob lotting, monosaccharide composition analysis, and selective glycosidas e digestions revealed that the CHIP-associated oligosaccharide contain s ABH determinants and resembles a band 3-type glycan that cannot be c leaved 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-Co-a or anti-Co-b. The DNA s equence from Colton-typed individuals predicted that residue 45 is ala nine in the Co(a+b-) phenotype and valine in the Co(a-b+) phenotype. T he nucleotide polymorphism corresponds to a PflMI: endonuclease digest ion site in the DNA from Co(a-b+) individuals. These studies have defi ned antigens within two blood group systems on CHIP: (a) an ABH-bearin g polylactosaminoglycan attached to a poorly accessible site in the na tive membrane; and (b) the Colton antigen polymorphism which may permi t the identification of rare individuals with defective water channel expression.