Abstract
Plasmodium sporozoites, injected by mosquitoes into the skin of the host, traverse cells during their migration to hepatocytes where they continue their life cycle. The mechanisms used by the parasite to rupture the plasma membrane of the host cells are not known. Here we report the presence of a phospholipase on the surface of Plasmodium berghei sporozoites (P. berghei phospholipase; Pb PL) and demonstrate that it is involved in the establishment of a malaria infection in vivo. Pb PL is highly conserved among the Plasmodium species. The protein is about 750 amino acids, with a predicted signal sequence and a carboxyl terminus that is 32% identical to the vertebrate lecithin:cholesterol acyltransferase, a secreted phospholipase. Pb PL contains a motif characteristic of lipases and a catalytic triad of a serine, aspartate, and histidine that is found in several phospholipases. We have verified its lipase and membrane lytic activity in vitro, using recombinant baculovirus-expressed protein. To study its role in vivo, we have disrupted the P. berghei PL open reading frame and generated mutants in its active site. During an infection through mosquito bite, the infectivity of the knock-out parasites in the liver is decreased by ∼90%. The prepatent period of the resulting blood infection is 1 day longer as compared with wild type. Further, the mutant sporozoites are impaired in their ability to cross epithelial cell layers. Thus, the Pb PL functions as a lipase to damage cell membranes and facilitates sporozoite passage through cells during their migration from the skin to the bloodstream.