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The Apicomplexan parasite responsible for the most virulent form of malaria, Plasmodium falciparum, invades human erythrocytes through multiple ligand-receptor interactions. Some strains of P. falciparum are sensitive to neuraminidase treatment of the host erythrocyte and these parasites have been termed sialic acid-dependent as they utilize receptors containing sialic acid. In contrast, other strains can efficiently invade neuraminidase-treated erythrocytes and hence are sialic acid-independent. The molecular interactions that allow P. falciparum to differentially utilize receptors for merozoite invasion are not understood. The P. falciparum reticulocyte-binding protein homologue (PfRh or PfRBL) family have been implicated in the invasion process but their exact role is unknown. PfRh1, a member of this protein family, appears to be expressed in all parasite lines analysed but there are marked differences in the level of expression between different strains. We have used targeted gene disruption of the PfRh1 gene in P. falciparum to show that the encoded protein is required for sialic acid-dependent invasion of human erythrocytes. The DeltaPfRh1 parasites are able to invade normally; however, they utilize a pattern of ligand-receptor interactions that are more neuraminidase-resistant. Current data suggest a strategy based on the differential function of specific PfRh proteins has evolved to allow P. falciparum parasites to utilize alternative receptors on the erythrocyte surface for evasion of receptor polymorphisms and the host immune system.

To establish infection in the host, malaria parasites export remodeling and virulence proteins into the erythrocyte. These proteins can traverse a series of membranes, including the parasite membrane, the parasitophorous vacuole membrane, and the erythrocyte membrane. We show that a conserved pentameric sequence plays a central role in protein export into the host cell and predict the exported proteome in Plasmodium falciparum. We identified 400 putative erythrocyte-targeted proteins corresponding to approximately 8% of all predicted genes, with 225 virulence proteins and a further 160 proteins likely to be involved in remodeling of the host erythrocyte. The conservation of this signal across Plasmodium species has implications for the development of new antimalarials.

The apicoplast and mitochondrion of the malaria parasite Plasmodium falciparum are important intracellular organelles and targets of several anti-malarial drugs. In recent years, our group and others have begun to piece together the metabolic pathways of these organelles, with a view to understanding their functions and identifying further anti-malarial targets. This has involved localization of putative organellar proteins using fluorescent reporter proteins such as green fluorescent protein (GFP). A major limitation to such an approach is the difficulties associated with using existing plasmids to genetically modify P. falciparum. In this paper, we present a novel series of P. falciparum transfection vectors based around the Gateway recombinatorial cloning system. Our system makes it considerably easier to construct fluorescent reporter fusion proteins, as well as allowing the use of two selectable markers. Using this approach, we localize proteins involved in isoprenoid biosynthesis and the posttranslational processing of apicoplast-encoded proteins to the apicoplast, and a protein putatively involved in the citric acid cycle to the mitochondrion. To confirm the localization of these proteins, we have developed a new immunofluorescence assay (IFA) protocol using antibodies specific to the apicoplast and mitochondrion. In comparison with published IFA methods, we find that ours maintains considerably better structural preservation, while still allowing sufficient antibody binding as well as preserving reporter protein fluorescence. In summary, we present two important new tools that have enabled us to characterize some of the functions of the apicoplast and mitochondrion, and which will be of use to the wider malaria research community in elucidating the localization of other P. falciparum proteins.

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.