PfATP2 is an essential flippase on the Plasmodium falciparum surface that influences parasite sensitivity to antiplasmodial compounds

Antimalarials play a crucial role in the fight against malaria. However, resistance of the most virulent malaria parasite, Plasmodium falciparum, to front-line antimalarials is spreading. To identify new antimalarials, millions of compounds have been screened for their ability to inhibit the growth of blood-stage P. falciparum parasites. To gain insight into the mode of action of novel compounds and the ease by which parasites can acquire resistance to them, many have been tested in in vitro evolution experiments, in which parasites are exposed to the compound for a prolonged period of time. In a recent study, parasite resistance to two compounds, MMV007224 and MMV665852, was associated with amplification of the pfatp2 gene, implicating PfATP2, a putative phospholipid flippase, as a parasite drug target or resistance determinant. These two compounds, along with MMV665794 (which is structurally related to MMV007224) had previously been reported to dysregulate pH in parasites. Here, we show that PfATP2 localises to the parasite surface and is essential for parasite growth. We demonstrate that parasites genetically engineered to overexpress PfATP2 display reduced sensitivity to MMV665794, MMV007224 and MMV665852 compared to parasites with a normal expression level of the protein, and that parasites in which PfATP2 is knocked down become hypersensitive to the three compounds. We show that PfATP2 expression level does not affect the cytosolic pH of parasites, or the potency by which MMV665794 or MMV007224 dysregulate parasite pH. We show that PfATP2-overexpressing parasites internalise a fluorescent phosphatidylserine analogue (NBD-PS) at a greater rate than parasites with a normal expression level of PfATP2, and that parasites in which PfATP2 is knocked down have a reduced rate of NBD-PS uptake. Further, we provide evidence that MMV665794 and MMV007224 inhibit both ATP-dependent and ATP-independent NBD-PS internalisation mechanisms, the former at lower concentrations. Together, our data are consistent with PfATP2 serving as a major ATP-dependent phosphatidylserine internalisation mechanism at the parasite plasma membrane, and being a target of MMV665794 and MMV007224.