Abstract. Resistance to chloroquine in Plasmodium falciparum bears a striking similarity to the multi-drug resistance (MDR) phenotype of mammalian tumor cells which is mediated by overexpression of P-glycoprotein. We show here that the P. falciparum homologue of the P-glycoprotein (Pghl) is a 160,000-D protein that is expressed throughout the asexual erythrocytic life cycle of the parasite. Quantitative immunoblotting analysis has shown that the protein is expressed at approximately equal levels in chloroquine resistant and sensitive isolates suggesting that overexpression of Pghl is not essential for chloroquine resistance. The chloroquine-resistant cloned line FAC8 however, does express approximately threefold more Pghl protein than other isolates which is most likely because of the increased pfmdrl gene copy number present in this isolate. Immunofluorescence and immunoelectron microscopy has demonstrated that Pghl is localized on the membrane of the digestive vacuole of mature parasites. This subcellular localization suggests that Pghl may modulate intracellular chloroquine concentrations and has important implications for the normal physiological function of this protein.

SummaryPlasmodium falciparum is predicted to transport over 300 proteins to the cytosol of its chosen host cell, the mature human erythrocyte, including 19 members of the Hsp40 family. Here, we have generated transfectant lines expressing GFP-or HA-Strep-tagged versions of these proteins, and used these to investigate both localization and other properties of these Hsp40 co-chaperones. These fusion proteins labelled punctate structures within the infected erythrocyte, initially suggestive of a Maurer's clefts localization. Further experiments demonstrated that these structures were distinct from the Maurer's clefts in protein composition. Transmission electron microscopy verifies a non-cleft localization for HA-Strep-tagged versions of these proteins. We were not able to label these structures with BODIPY-ceramide, suggesting a lower size and/or different lipid composition compared with the Maurer's clefts. Solubility studies revealed that the Hsp40-GFP fusion proteins appear to be tightly associated with membranes, but could be released from the bilayer under conditions affecting membrane cholesterol content or organization, suggesting interaction with a binding partner localized to cholesterol-rich domains. These novel structures are highly mobile in the infected erythrocyte, but based on velocity calculations, can be distinguished from the 'highly mobile vesicles' previously described. Our study identifies a further extra-parasitic structure in the P. falciparum-infected erythrocyte, which we name 'J-dots' (as their defining characteristic so far is the content of J-proteins). We suggest that these J-dots are involved in trafficking of parasiteencoded proteins through the cytosol of the infected erythrocyte.

No abstract

No abstract