Growth of the malaria parasite within human red blood cells (RBC) is accompanied by increased uptake of many solutes including anions, sugars, purines, amino acids, and organic cations. While this increased uptake has been known for more than 20 years, its precise mechanism and the site of uptake within the infected RBC have been strongly debated. In the past year, we resolved these important problems with the whole-cell and single channel patch-clamp methods. With these methods, uninfected RBCs had ohmic whole-cell conductances of < 100 pS, consistent with their low tracer permeabilities. In contrast, trophozoite-infected RBCs exhibited voltage-dependent, non-saturating currents which are 150-fold larger, predominantly carried by anions, and abruptly abolished by channel blockers. Patch-clamp measurements and spectral analysis confirm that a small (<10 pS) ion channel on the infected RBC surface, present at ~1000 copies/cell, is responsible for these currents. This channel (called the plasmodial surface anion channel, PSAC) fully accounts for the increased uptake of small nutrients in infected RBCs. The surface location of this new channel and its permeability to organic solutes needed for parasite growth suggest a primary role in a sequential diffusive pathway for parasite nutrient acquisition.At present, our laboratory is focused on cloning the gene responsible for PSAC. We recently developed a biochemical approach for isolating and purifying parasite proteins on the surface of infected cells; these proteins will be sequenced with state-of-the-art mass spectroscopy methods and should lead to cloning PSAC?s gene.We are also working on identifying high-affinity blockers of PSAC with a high-throughput screen of 250,000 candidate channel blockers. This screen will use a spectrophotometric assay for channel activity developed in our laboratory.
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