Expression and functional characterization of a Plasmodium falciparum Ca2+-ATPase (PfATP4) belonging to a subclass unique to apicomplexan organisms

We have obtained a full-length P type ATPase sequence (PfATP4) encoded by Plasmodium falciparum and expressed PfATP4 in Xenopus laevis oocytes to study its function. Comparison of the hitherto incomplete open reading frame with other Ca2+-ATPase sequences reveals that PfATP4 differs significantly from previously defined categories. The Ca2+-dependent ATPase activity of PfATP4 is stimulated by a much broader: range of [Ca2+](free) (3.2-320 muM) than are an avian SERCA1 pump or rabbit SERCA 1a (maximal activity < 10 <mu>M). The activity of PfATP4 is resistant to inhibition by ouabain (200 muM) or thapsigargin (0.8 muM) but is inhibited by vanadate (1 mM) or cyclopiazonic acid (1 muM). We used a quantitative polymerase chain reaction to assay expression of mRNA encoding PfATP4 relative to that for beta -tubulin in synchronized asexual stages and found variable expression throughout the life cycle with a maximal 5-fold increase in meronts compared with ring stages. This analysis suggests that PfATP4 defines a novel subclass of Ca2+-ATPases unique to apicomplexan organisms and therefore offers potential as a drug target.