Artemisinins are extracted from sweet wormwood (Artemisia annua) and are the most potent antimalarials available1, rapidly killing all asexual stages of Plasmodium falciparum2. Artemisinins are sesquiterpene lactones widely used to treat multidrug-resistant malaria1, a disease that annually claims 1 million lives. Despite extensive clinical and laboratory experience3,4,5 their molecular target is not yet identified. Activated artemisinins form adducts with a variety of biological macromolecules, including haem, translationally controlled tumour protein (TCTP) and other higher-molecular-weight proteins6. Here we show that artemisinins, but not quinine or chloroquine, inhibit the SERCA orthologue (PfATP6) of Plasmodium falciparum in Xenopus oocytes with similar potency to thapsigargin (another sesquiterpene lactone and highly specific SERCA inhibitor). As predicted, thapsigargin also antagonizes the parasiticidal activity of artemisinin. Desoxyartemisinin lacks an endoperoxide bridge and is ineffective both as an inhibitor of PfATP6 and as an antimalarial. Chelation of iron by desferrioxamine abrogates the antiparasitic activity of artemisinins and correspondingly attenuates inhibition of PfATP6. Imaging of parasites with BODIPY-thapsigargin labels the cytosolic compartment and is competed by artemisinin. Fluorescent artemisinin labels parasites similarly and irreversibly in an Fe2+-dependent manner. These data provide compelling evidence that artemisinins act by inhibiting PfATP6 outside the food vacuole after activation by iron.
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We thank T. Joët and A.-C. Uhlemann for discussions, P. Wünnenberg for technical assistance, A. Craig for a P. falciparum cDNA library and K. Tanabe for the P. falciparum genomic clone 3L6. We thank the charity Hope (Wessex Medical Trust) for financial support of J.M. East, and T. Bolton for access to confocal microscopy. U. E.-L. was funded by the Deutsche Forschungsgemeinschaft, S.A.W. and P.G.B. are funded by the Wellcome Trust and S.K. holds an MRC (UK) grant. This paper is dedicated to the memory of G. Cowan.
S.K. acted as a paid scientific advisor to the Medicines for Malaria initiative in the development of artemisins as antimalarials; however, that advisory role has no bearing on this work, and those advised have not influenced this work. The other authors have no competing financial interests.
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