Binding of glycophorins to Plasmodium falciparum merozoites

Plasmodium falciparum merozoites recognize and attach to glycophorins, the surface sialoglycoproteins of human erythrocytes. The structural requirements for a merozoite binding site were studied with the use of two methods. In the first, certain glycophorins and their tryptic fragments were added directly to isolated merozoites prior to their addition to erythrocytes. Low concentrations (50 micrograms ml-1) of glycophorin A inhibited merozoite invasion. At higher concentrations a mixture of glycophorins A, B and C (GPS) (100 micrograms ml-1) and glycophorin B (200 micrograms ml-1) also inhibited invasion. GPS from Tn erythrocytes which lack both sialic acid and galactose residues was almost as effective as normal GPS in blocking invasion. None of the monosaccharides present on glycophorin, including N-acetylneuraminic acid, inhibited merozoite invasion. Erythrocytes treated with lectins were only partially resistant to invasion. These results indicated that the oligosaccharide side chains are not the major structural determinant of the merozoite binding site. Glycophorin A was cleaved by trypsin and the separated fragments added to merozoites. Only the external N-terminal tryptic fragment T1 and the trypsin resistant hydrophobic core, T6, showed some, but considerably less, inhibitory activity than the intact molecule. In the second approach, the binding of 125I-labeled GPS to isolated merozoites was determined. 125I-GPS binding was saturated at 0.23 micrograms for 10(9) merozoites and was competitively inhibited by unlabeled GPS but not by free sugars. Desialylated GPS bound almost to the same extent as the intact molecule.