Role of internal domains of glycophorin in Plasmodium falciparum invasion of human erythrocytes

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Abstract
Human erythrocyte glycophorin, a putative receptor to Plasmodium falciparum malaria parasites, was studied in terms of its structural domains involved in mediating invasion. These domains were isolated from purified glycophorin A and from supernatants and membranes obtained from protease-treated erythrocytes. They were tested for invasion blocking capacity by using an in vitro assay system. The role of carbohydrate-rich domains was assessed with the following compounds: (i) sialoglycopeptides released by proteases either from whole cells or isolated glycophorin A; (ii) the sialoglycoproteins fetuin and alpha 1 acid glycoprotein and the N-acetylglucosamine-rich ovomucoid; and (iii) the saccharides N-acetylneuraminlactose, N-acetylglucosamine, and free sialic acid. With the exception of N-acetylglucosamine, all of the compounds failed to block invasion. The role of carbohydrate-poor domains of glycophorin was assessed with peptides isolated from membranes of proteolyzed cells and with the hydrophobic fragment of glycophorin A. Glycophorin and the derived hydrophobic peptides formed high-molecular-weight aggregates in physiological solutions. They all inhibited invasion to a comparable extent. The inhibitory potency of glycophorin A increased by sixfold after reconstitution into egg lecithin vesicles. The observations reported here underscore the role played by the hydrophobic domain in the glycophorin-mediated blockage of invasion. They also suggest that in the interactions between P. falciparum merozoites and the erythrocyte membrane, the exposed glycosylated domains of glycophorins provide the initial but rather weak binding sites, whereas the internal domains of the molecules provide the more stable attachment sites for merozoites.