Investigation of the Plasmodium falciparum food vacuole through inducible expression of the chloroquine resistance transporter (PfCRT).

Haemoglobin degradation during the erythrocytic life stages is the major function of the food vacuole (FV) of Plasmodium falciparum and the target of several anti-malarial drugs that interfere with this metabolic pathway, killing the parasite. Two multi-spanning food vacuole membrane proteins are known, the multidrug resistance protein 1 (PfMDR1) and Chloroquine Resistance Transporter (PfCRT). Both modulate resistance to drugs that act in the food vacuole. To investigate the formation and behaviour of the food vacuole membrane we have generated inducible GFP fusions of chloroquine sensitive and resistant forms of the PfCRT protein. The inducible expression system allowed us to follow newly-induced fusion proteins, and corroborated a previous report of a direct trafficking route from the ER/Golgi to the food vacuole membrane. These parasites also allowed the definition of a food vacuole compartment in ring stage parasites well before haemozoin crystals were apparent, as well as the elucidation of secondary PfCRT-labelled compartments adjacent to the food vacuole in late stage parasites. We demonstrated that in addition to previously demonstrated Brefeldin A sensitivity, the trafficking of PfCRT is disrupted by Dynasore, a non competitive inhibitor of dynamin-mediated vesicle formation. Chloroquine sensitivity was not altered in parasites over-expressing chloroquine resistant or sensitive forms of the PfCRT fused to GFP, suggesting that the PfCRT does not mediate chloroquine transport as a GFP fusion protein.

Plasmodium falciparum heterochromatin protein 1 marks genomic loci linked to phenotypic variation of exported virulence factors.

Epigenetic processes are the main conductors of phenotypic variation in eukaryotes. The malaria parasite Plasmodium falciparum employs antigenic variation of the major surface antigen PfEMP1, encoded by 60 var genes, to evade acquired immune responses. Antigenic variation of PfEMP1 occurs through in situ switches in mono-allelic var gene transcription, which is PfSIR2-dependent and associated with the presence of repressive H3K9me3 marks at silenced loci. Here, we show that P. falciparum heterochromatin protein 1 (PfHP1) binds specifically to H3K9me3 but not to other repressive histone methyl marks. Based on nuclear fractionation and detailed immuno-localization assays, PfHP1 constitutes a major component of heterochromatin in perinuclear chromosome end clusters. High-resolution genome-wide chromatin immuno-precipitation demonstrates the striking association of PfHP1 with virulence gene arrays in subtelomeric and chromosome-internal islands and a high correlation with previously mapped H3K9me3 marks. These include not only var genes, but also the majority of P. falciparum lineage-specific gene families coding for exported proteins involved in host-parasite interactions. In addition, we identified a number of PfHP1-bound genes that were not enriched in H3K9me3, many of which code for proteins expressed during invasion or at different life cycle stages. Interestingly, PfHP1 is absent from centromeric regions, implying important differences in centromere biology between P. falciparum and its human host. Over-expression of PfHP1 results in an enhancement of variegated expression and highlights the presence of well-defined heterochromatic boundaries. In summary, we identify PfHP1 as a major effector of virulence gene silencing and phenotypic variation. Our results are instrumental for our understanding of this widely used survival strategy in unicellular pathogens.

Reticulocyte-binding protein homologue 5 - an essential adhesin involved in invasion of human erythrocytes by Plasmodium falciparum.

Invasion of erythrocytes is a prerequisite in the life history of the malaria parasite. Members of the reticulocyte-binding homologue family (PfRh) have been implicated in the invasion process and in some cases have been shown to act as adhesins, binding to specific receptors on the erythrocyte surface. We have identified a further, putatively essential, PfRh family member in the most virulent human malaria Plasmodium falciparum, called PfRh5, which binds to an unknown class of glycosylated receptors on the erythrocyte surface. This protein is an atypical PfRh family member, being much smaller than others and lacking a transmembrane and cytosolic region at the C-terminus. This suggests it may be part of a functional protein complex. PfRh5 localises to the rhoptries in merozoites and follows the tight junction during the process of erythrocyte invasion. Furthermore, rabbit immune serum raised against a portion of the ecto-domain, inhibits parasite invasion in vitro. We hypothesise an essential role for the PfRh5 adhesin in erythrocyte selection and commitment to invasion. Given its small size, we believe PfRh5 may prove to be a valuable candidate for inclusion in a multi-component anti-malarial vaccine.