Molecular mapping of human band 3 aging antigenic sites and active amino acids using synthetic peptides.

An aging antigen, senescent cell antigen appears on old cells and marks them for death by initiating the binding of IgG autoantibody and subsequent removal by phagocytes. This antigen is derived from the major anion transport protein, protein band 3, that is involved in respiration and acid base balance. We use synthetic peptides from the transmembrane, anion transport segment of band 3 to "walk" band 3 to identify potential aging antigenic sites. A competitive inhibition assay with affinity purified IgG autoantibody from senescent red cells was used. Results indicate that: aging antigenic sites reside on human band 3 residues 538-554, 593-601, and 812-830; and that the smallest residues which act as aging antigenic sites are 593-601 and 813-818. The contribution of lysine and/or arginine to antigenicity is examined by synthesizing peptide analogs in which glycines or arginines are substituted for lysines or arginines. Substitution of neutral glycine for the positively charged amino acids arginine or lysine or both arginine and lysine did not result in a significant difference in antigenicity between the analog and the native band 3 peptide. Substitution of the positively charged arginine for the positively charged lysine resulted in a significant reduction in antigenicity. The chicken sequence of band 3 peptides 538-554 and 812-827 differs from that of the human peptides at several sites. Antigenicity of these chicken "analogs" were tested and compared to the human peptides. The data suggest that the three-dimensional configuration of band 3 segments plays a dominant role in defining the antigenic determinants reactive with senescent cell IgG autoantibodies.