C8 binding protein bears I antigenic determinants.

C8 binding protein (C8bp) is an integral membrane glycoprotein of peripheral blood cells, which inhibits the C5b-9-mediated lysis in a homologous system. In the present study, we analyzed the carbohydrate portion of the C8bp. We found that C8bp is associated with I antigenic determinant, a sugar sequence found on human erythrocytes of adults. To assess whether or not the sugar residues are essential for the C8bp function, I determinant was cleaved off from the isolated C8bp by endo-beta-galactosidase (E.C. 3.2.2.103) that hydrolyses internal beta-galactosidic-linked-N-acetyllactosamine residues. Enzyme treatment removed I-antigen, the inhibitory function of C8bp, however, was not affected. When intact erythrocytes were treated with endo-beta-galactosidase, I-antigen was lost and the lytic insusceptibility of human erythrocytes to homologous C5b-9 could not be abolished. Thus, I-antigen is associated with the C8bp, but its presence is not required for the homologous species restriction.