Biochemical Characterization and Chemical Inhibition of PfATP4-associated Na-ATPase Activity in Membranes

The antimalarial activity of chemically diverse compounds, including the clinical candidate cipargamin, has been linked to the ATPase PfATP4 in the malaria-causing parasite The characterization of PfATP4 has been hampered by the inability thus far to achieve its functional expression in a heterologous system. Here, we optimized a membrane ATPase assay to probe the function of PfATP4 and its chemical sensitivity. We found that cipargamin inhibited the Na-dependent ATPase activity present in membranes from WT parasites and that its potency was reduced in cipargamin-resistant PfATP4-mutant parasites. The cipargamin-sensitive fraction of membrane ATPase activity was inhibited by all 28 of the compounds in the "Malaria Box" shown previously to disrupt ion regulation in in a cipargamin-like manner. This is consistent with PfATP4 being the direct target of these compounds. Characterization of the cipargamin-sensitive ATPase activity yielded data consistent with PfATP4 being a Na transporter that is sensitive to physiologically relevant perturbations of pH, but not of [K] or [Ca]. With an apparent for ATP of 0.2 mm and an apparent for Na of 16-17 mm, the protein is predicted to operate at below its half-maximal rate under normal physiological conditions, allowing the rate of Na efflux to increase in response to an increase in cytosolic [Na]. In membranes from a cipargamin-resistant PfATP4-mutant line, the apparent for Na is slightly elevated. Our study provides new insights into the biochemical properties and chemical sensitivity of an important new antimalarial drug target.