The plasmodium receptor for activated C kinase protein inhibits Ca2+ signaling in mammalian cells

Plasmodium falciparum, the most lethal malarial parasite, expresses an ortholog for the protein kinase C (PKC) activator RACK1. However, PKC has not been identified in this parasite, and the mammalian RACK1 can interact with the inositol 1,4,5-trisphosphate receptor (InsP3R). Therefore we investigated whether the Plasmodium ortholog PfRACK also can affect InsP3R-mediated Ca2+ signaling in mammalian cells. GFP-tagged PfRACK and endogenous RACK1 were expressed in a similar distribution within cells. PfRACK inhibited agonist-induced Ca2+ signals in cells expressing each isoform of the InsP3R, and this effect persisted when expression of endogenous RACK1 was reduced by siRNA. PfRACK also inhibited Ca2+ signals induced by photorelease of caged InsP3. These findings provide evidence that PfRACK directly inhibits InsP3-mediated Ca2+ signaling in mammalian cells. Interference with host cell signaling pathways to subvert the host intracellular milieu may be an important mechanism for parasite survival.