High-frequency antigens of human erythrocyte membrane sialoglycoproteins, IV. Molecular properties of the U antigen.

The nature of the common erythrocyte antigen U, that is absent from S-s-U-cells, which lack glycophorin B (Ss sialoglycoprotein), was investigated using six different antisera. The molecular features of a U-like antigen (Duclos), detected by a hitherto unique serum, were also studied. The U and Duclos antigens are complex in that they exhibit relationships with the MNSs and Rh blood group systems. Various fractionation, cleavage, or modification products of normal erythrocyte membranes were used in hemagglutination inhibition assays. Both, the U and Duclos antigens were found to represent labile structures that require lipids, at least for optimum expression of antigen activity. The antigens could be solubilized using conditions of Triton X-100 extraction that release glycophorin B, but solubilize the Rh antigens only to a small extent. Anti-U and anti-Duclos were also inhibited, albeit weakly, by glycophorin B-containing fractions obtained by chromatographic separation of Triton X-100 extracts. The residues approx. 33-39 of glycophorin B represent essential parts of the U antigen, as judged from proteolytic digestion and chemical modification. Conversely, the expression of Duclos activity seems to require a region of glycophorin B (C-terminal of the positions approx. 34-36) that could not be cleaved by various proteinases. Data obtained with anti-Duclos have to be interpreted with caution, since there is evidence that this serum might contain a mixture of antibodies.