Laura H. Derick

Plasmodium falciparum erythrocyte membrane protein 1 is anchored to the actin-spectrin junction and knob-associated histidine-rich protein in the erythrocyte skeleton.

A distinctive pathological feature of Plasmodium falciparum malaria is the endothelial attachment of erythrocytes infected with mature asexual-stage parasites in microvessels of the major organs. Electron-dense protrusions described as knobs are displayed on the surface of parasitized erythrocytes and act as attachment points in cytoadherence. Parasite-encoded knob-associated histidine-rich protein (KAHRP) is a major component of knobs found on the cytoplasmic side of the host cell membrane. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is a family of parasite-encoded cytoadherence receptors localized to knobs on the surface of parasitized erythrocytes. Despite its high antigenic diversity, PfEMP1 has a remarkably conserved cytoplasmic domain. We demonstrate in this study that the cytoplasmic domain of PfEMP1 (VAR(CD)) binds to host spectrin and actin and to full-length KAHRP in vitro. Apparent dissociation constants determined for VAR(CD)/F-actin and VAR(CD)/KAHRP interactions are 44.9+/-6.4 and 10. 7+/-2.2 nM, respectively. Further, we provide evidence that KAHRP polypeptides self-associate in solution to form structures similar to knobs and show binding of self-associated KAHRP clusters to spectrin-actin-protein 4.1 complexes. Findings in this study suggest that PfEMP1 is localized to the knob in P. falciparum-infected erythrocytes by binding to the host spectrin-actin junction and to self-associated KAHRP through its conserved cytoplasmic domain.

Isolation of skeleton-associated knobs from human red blood cells infected with malaria parasite Plasmodium falciparum

In falciparum malaria, only ring-stage parasites are detected in the peripheral circulation whereas mature parasites (trophozoites and schizonts) sequester in tissue capillaries of the human host [1,2]. The attachment of infected erythrocytes to capillary endothelial cells is mediated by knobs as visualized by scanning electron microscopy [3-51. By transmission electron microscopy, surface knobs appear to contain an electron-dense, submembranous cup-shaped structure which suggests their close physical association with the underlying erythrocyte membrane skeleton [2,5,6]. The complete protein composition of knobs and the mechanism of knob formation are not known. Several new techniques for ultrastructural visualization of protein interactions in the human erythrocyte membrane or the spread membrane skeleton have been developed recently [7-111. These techniques were employed in the present work to visualize membrane protein interactions of human red