Tryptophanyl-tRNA synthetases as drug targets in the malaria parasite Plasmodium falciparum

© 2016 Dr Charisse Flerida PasajeIncreasing resistance to first-line antimalarials has a strong impact on the health and economic burden of the disease, highlighting the need for new drugs with novel modes of action. The malaria parasite Plasmodium falciparum relies on efficient protein translation, so the loss of function of factors involved in protein biosynthesis could be detrimental to the parasites. Aminoacyl-tRNA synthetases (aaRS) are enzymes that are key to the production of substrates for protein translation, an event that occurs in three cellular compartments of Plasmodium: the cytosol, the mitochondrion, and a remnant chloroplast called the apicoplast. This work explores the tryptophanyl-tRNA synthetase (TrpRS), which charges tRNATrp, as a promising antimalarial target owing to its specificity within the parasite and its essential role in protein production. This work identified two isoforms of TrpRS in Plasmodium; one eukaryotic type that localises to the cytosol and a bacterial type that localises to the apicoplast. Using in vitro biochemical assays, the cytosolic TrpRS was found to preferentially aminoacylate tRNATrp from a eukaryotic source while the apicoplast TrpRS efficiently charges tRNATrp from a bacterial source. A structural analogue of tryptophan and an inhibitor of bacterial TrpRSs, indolmycin, specifically inhibits aminoacylation by the apicoplast TrpRS in vitro, and inhibits intraerythrocytic stage Plasmodium parasite growth, killing parasites with a delayed death effect characteristic of apicoplast inhibitors. Indolmycin treatment inhibits apicoplast inheritance and is rescuable by addition of the apicoplast metabolite isopentenyl pyrophosphate (IPP). These data establish that inhibition of an apicoplast housekeeping enzyme leads to loss of the apicoplast and this is sufficient for delayed death. Furthermore, repression of protein translation through the control of translation initiation, which is activated in response to various stressors, was explored as the mechanism of resistance not only to indolmycin but also to another potent aaRS inhibitor, borrelidin. Taken together, these findings identified the apicoplast TrpRS as an essential component of protein translation and a promising antimalarial target