Aminoacyl-tRNA synthetases as drug targets in eukaryotic parasites.

Aminoacyl-tRNA synthetases are central enzymes in protein translation, providing the charged tRNAs needed for appropriate construction of peptide chains. These enzymes have long been pursued as drug targets in bacteria and fungi, but the past decade has seen considerable research on aminoacyl-tRNA synthetases in eukaryotic parasites. Existing inhibitors of bacterial tRNA synthetases have been adapted for parasite use, novel inhibitors have been developed against parasite enzymes, and tRNA synthetases have been identified as the targets for compounds in use or development as antiparasitic drugs. Crystal structures have now been solved for many parasite tRNA synthetases, and opportunities for selective inhibition are becoming apparent. For different biological reasons, tRNA synthetases appear to be promising drug targets against parasites as diverse as Plasmodium (causative agent of malaria), Brugia (causative agent of lymphatic filariasis), and Trypanosoma (causative agents of Chagas disease and human African trypanosomiasis). Here we review recent developments in drug discovery and target characterisation for parasite aminoacyl-tRNA synthetases.

A dual-targeted aminoacyl-tRNA synthetase in Plasmodium falciparum charges cytosolic and apicoplast tRNACys.

Plasmodium parasites possess two endosymbiotic organelles: a mitochondrion and a relict plastid called the apicoplast. To accommodate the translational requirements of these organelles in addition to its cytosolic translation apparatus, the parasite must maintain a supply of charged tRNA molecules in each of these compartments. In the present study we investigate how the parasite manages these translational requirements for charged tRNACys with only a single gene for CysRS (cysteinyl-tRNA synthetase). We demonstrate that the single PfCysRS (Plasmodium falciparum CysRS) transcript is alternatively spliced, and, using a combination of endogenous and heterologous tagging experiments in both P. falciparum and Toxoplasma gondii, we show that CysRS isoforms traffic to the cytosol and apicoplast. PfCysRS can recognize and charge the eukaryotic tRNACys encoded by the Plasmodium nucleus as well as the bacterial-type tRNA encoded by the apicoplast genome, albeit with a preference for the eukaryotic type cytosolic tRNA. The results of the present study indicate that apicomplexan parasites have lost their original plastidic cysteinyl-tRNA synthetase, and have replaced it with a dual-targeted eukaryotic type CysRS that recognizes plastid and nuclear tRNACys. Inhibitors of the Plasmodium dual-targeted CysRS would potentially offer a therapy capable of the desirable immediate effects on parasite growth as well as the irreversibility of inhibitors that disrupt apicoplast inheritance.

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.

Dual targeting of aminoacyl-tRNA synthetases to the apicoplast and cytosol in Plasmodium falciparum.

The causative agent of malaria, Plasmodium, possesses three translationally active compartments: the cytosol, the mitochondrion and a relic plastid called the apicoplast. Aminoacyl-tRNA synthetases to charge tRNA are thus required for all three compartments. However, the Plasmodiumfalciparum genome encodes too few tRNA synthetases to supply a unique enzyme for each amino acid in all three compartments. We have investigated the subcellular localisation of three tRNA synthetases (AlaRS, GlyRS and ThrRS), which occur only once in the nuclear genome, and we show that each of these enzymes is dually localised to the P. falciparum cytosol and the apicoplast. No mitochondrial fraction is apparent for these three enzymes, which suggests that the Plasmodium mitochondrion lacks at least these three tRNA synthetases. The unique Plasmodium ThrRS is the presumed target of the antimalarial compound borrelidin. Borrelidin kills P. falciparum parasites quickly without the delayed death effect typical of apicoplast translation inhibitors and without an observable effect on apicoplast morphology. By contrast, mupirocin, an inhibitor of the apicoplast IleRS, kills with a delayed death effect that inhibits apicoplast growth and division. Because inhibition of dual targeted tRNA synthetases should arrest translation in all compartments of the parasite, these enzymes deserve further investigation as potential targets for antimalarial drug development.

Protein translation in Plasmodium parasites.

The protein translation machinery of the parasite Plasmodium is the target of important anti-malarial drugs, and encompasses many promising targets for future drugs. Plasmodium parasites have three subcellular compartments that house genomes; the nucleus, mitochondrion and apicoplast, and each requires its own compartmentalized transcription and translation apparatus for survival. Despite the availability of the complete genome sequence that should reveal the requisite elements for all three compartments, our understanding of the translation machineries is patchy. We review what is known about cytosolic and organellar translation in Plasmodium and discuss the molecules that have been identified through genome sequencing and post-genomic analysis. Some translation components are yet to be found in Plasmodium, whereas others appear to be shared between translationally active organelles.