Extracellular lysines on the plasmodial surface anion channel involved in Na+ exclusion

The human malaria parasite, Plasmodium falciparum, induces an unusual ion channel, the plasmodial surface anion channel (PSAC), on its host red blood cell (RBC) membrane. PSAC has a broad selectivity with permeability to anions, sugars, amino acids, purines, and certain vitamins, suggesting a role in nutrient acquisition by the intracellular parasite. Permeating solutes cover a range of molecular sizes and may be either neutral or carry a net negative or positive charge. Despite this broad selectivity, PSAC must efficiently exclude Na+ to avoid osmotic lysis of infected RBCs in the bloodstream. Here, we used amine-reactive N-hydroxysulfosuccinimide esters to probe PSAC's unusual selectivity. PSAC permeation rates, measured with both a kinetic osmotic lysis assay and single-channel patch-clamp, irreversibly decrease after treatment with these reagents. Sequential labelings with different esters and the effects of their chain length suggest that PSAC has multiple lysine residues near its extracellular pore mouth and that inhibition occurs via steric hindrance of its pore by the amide-linked side chain. When combined with the effects of pH on permeation, these findings implicate a combination of cation repulsion by pore mouth charges and a weak binding site for permeant solutes in PSAC's broad selectivity yet effective exclusion of Na+.