Plasmepsins I, II, IV, and HAP comprise a novel family of hemoglobin-degrading proteases in the malaria parasite, Plasmodium falciparum

Intraerythrocytic stages of the malaria parasite, Plasmodium falciparum rely on red cell hemoglobin as a nutrient source, consuming vast amounts of it within a short time. Hemoglobin degradation occurs in the parasite's food vacuole (FV), a highly specialized, acidic organelle. Four proteases have been purified from the FV and shown to act in a semi-ordered pathway in vitro to degrade the hemoglobin tetramer. Degradation is initiated by aspartic proteases plasmepsins I and II (PM1 and II) which have been extensively characterized. Additional aspartic proteases exist in P. falciparum, including histo-aspartic protease (HAP) and plasmepsin IV (PM IV), that share high sequence identity with PM I and II. This thesis describes the characterization of HAP and PM IV in terms of localization and expression in parasites, enzymatic activity, and proteolytic processing. PM I, II, HAP and IV lie in a cluster on chromosome 14 and all localize to the FV. Despite containing a histidine in palce of a catalytic aspartic acid, HAP is an active enzyme with a unique inhibitor sensitivity. Both HAP and PM IV prefer to cleave denatured globin, suggesting they act after native hemoglobin has been denatured by PM I and II action. The processing of PM I-IV is identical in many regard and a single, novel maturase appears to activate them all. PM I-IV comprise a novel family of hemoglobin degrading aspartic proteases in P. falciparum and may be excellent targets for antimalarial drug development.