The role of Plasmodium falciparum food vacuole plasmepsins

Plasmepsins (PMs) are thought to have an important function in hemoglobin degradation in the malarial parasite Plasmodium falciparum and have generated interest as antimalarial drug targets. Four paralogous plasmepsins reside in the food vacuole of P. falciparum. Targeted gene disruption by double crossover homologous recombination has been employed to study food vacuole plasmepsin function in cultured parasites. Parasite clones with deletions in each of the individual PM I, PM II, and HAP genes as well as clones with a double PM IV/PM I disruption have been generated. All of these clones lack the corresponding PMs, are viable, and appear morphologically normal. PM II and PM IV/I disruptions have longer doubling times than the 3D7 parental line in rich RPMI medium. This appears to be because of a decreased level of productive progeny rather than an increased cell cycle time. In amino acid-limited medium, all four knockouts exhibit slower growth than the parental strain. Compared with 3D7, knock-out clone sensitivity to aspartic and cysteine protease inhibitors is changed minimally. These results suggest substantial functional redundancy and have important implications for the design of antimalarial drugs. The slow growth phenotype may explain why P. falciparum has maintained four plasmepsin genes with overlapping functions.