The Apicomplexan Molecular Physiology Section continued its studies of the plasmodial surface anion channel (PSAC) and made two significant contributions.? ? First, we identified and characterized the first PSAC mutant (PNAS 104:1063-1068, 2007). This mutant was generated by in vitro selection with blasticidin S, a toxin that reaches its intracellular target via permeation through PSAC. Because blasticidin S resistance correlated with reduced permeability to multiple solutes, we predicted there may be marked changes in PSAC. Single-channel and whole-cell electrophysiology confirmed this prediction by revealing altered channel gating, selectivity, pharmacology, and functional copy number/infected cell. Mutant parasites cultured without blasticidin S reverted to the wild-type channel phenotype and exhibited restored susceptibility to killing by blasticidin S. These findings 1) confirm that PSAC is the primary mechanism of organic solute uptake after infection because changes in PSAC affected the permeability of each solute, 2) implicate parasite genes in the expression of PSAC because human erythrocytes lack heritable genetic material, 3) provide a new approach to cloning PSACs gene, 4) reveal a new drug resistance mechanism in malaria parasites, and 5) suggest that PSAC serves an essential role for the intracellular parasite because a fitness cost was associated with blasticidin S resistance.? ? Second, we identified solute-inhibitor interactions within PSACs pore (Mol. Pharmacol. 71:1241-50, 2007). In this study, we found that phenyltrimethyl ammonium and isoleucine transport through PSAC were less effectively inhibited by known PSAC antagonists than the uptake of sorbitol and alanine. This observation was unexpected because all four solutes were thought to share a single transport mechanism. We excluded uptake via unrelated channels because specific PSAC inhibitors also exhibited solute-dependent affinities. Mixtures of permeating solutes, whole-cell electrophysiology, and temperature-dependence studies suggested that a single ion channel with two separate routes for permeating solutes is the most conservative explanation for our findings. Such a model may permit fine-tuning of PSACs unusual selectivity and allow the parasite to acquire a diverse collection of nutrients. These findings also suggest that PSAC antagonists must effectively block both routes to be suitable lead compounds for antimalarial drug development.
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