A protein on Plasmodium falciparum-infected erythrocytes functions as a transferrin receptor

Abstract

Several observations suggest that iron is essential for the development of malaria parasites^1–6 but there is evidence that the parasites in erythrocytes do not obtain iron from haemoglobin. The total haemin level in parasitized erythrocytes does not vary during parasite development⁷, indicating that the iron-containing moiety of haemoglobin is not detectably metabolized. Although parasite proteases can degrade the protein part of haemoglobin in red cells^8,9, no parasite enzymes that degrade haemin have been identified. In mammalian cells, haemin is degraded to carbon monoxide and bilirubin by the enzyme haeme oxygenase¹⁰. This enzyme has not been found in malaria parasites¹¹. In fact haemin has been found to be toxic to parasite carbohydrate metabolism¹². Thus, iron apparently cannot be liberated from haemin and instead is sequestered in infected red cells as haemozoin, the characteristic pigment associated with malarial infection^13,14. If iron bound to transferrin is the source of ferric ions for malaria parasites within mature erythrocytes, then the parasite must synthesize its own transferrin receptor and localize it on the surface of the infected cell, because the receptors for transferrin are lost during erythrocyte maturation^5,16. Our results here suggest that Plasmodium falciparum synthesizes its own transferrin receptors enabling it to take up iron from transferrin by receptor-mediated endocytosis.