Francis S E, Banerjee R, Goldberg D E
Howard Hughes Medical Institute, Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Biol Chem. 1997 Jun 6;272(23):14961-8. doi: 10.1074/jbc.272.23.14961.
During the intraerythrocytic stage of infection, the malaria parasite Plasmodium falciparum digests most of the host cell hemoglobin. Hemoglobin degradation occurs in the acidic digestive vacuole and is essential for the survival of the parasite. Two aspartic proteases, plasmepsins I and II, have been isolated from the vacuole and shown to make the initial cleavages in the hemoglobin molecule. We have studied the biosynthesis of these two enzymes. Plasmepsin I is synthesized and processed to the mature form soon after the parasite invades the red blood cell, while plasmepsin II synthesis is delayed until later in development. Otherwise, biosynthesis of the plasmepsins is identical. The proplasmepsins are type II integral membrane proteins that are transported through the secretory pathway before cleavage to the soluble form. They are not glycosylated in vivo, despite the presence of several potential glycosylation sites. Proplasmepsin maturation appears to require acidic conditions and is reversibly inhibited by the tripeptide aldehydes N-acetyl-L-leucyl-L-leucyl-norleucinal and N-acetyl-L-leucyl-L-leucyl-methional. These compounds are known to inhibit cysteine proteases and the chymotryptic activity of proteasomes but not aspartic proteases. However, proplasmepsin processing is not blocked by other cysteine protease inhibitors, nor by the proteasome inhibitor lactacystin. Processing is also not blocked by aspartic protease inhibitors. This inhibitor profile suggests that unlike most other aspartic proteases, proplasmepsin maturation may not be autocatalytic in vivo, but instead could require the action of an unusual processing enzyme. Compounds that block processing are expected to be potent antimalarials.
在感染的红细胞内期，疟原虫恶性疟原虫会消化大部分宿主细胞血红蛋白。血红蛋白降解发生在酸性消化泡中，对疟原虫的存活至关重要。已从该泡中分离出两种天冬氨酸蛋白酶，即疟原虫天冬氨酸蛋白酶I和II，并显示它们在血红蛋白分子中进行初始切割。我们研究了这两种酶的生物合成。疟原虫天冬氨酸蛋白酶I在疟原虫侵入红细胞后不久就被合成并加工成成熟形式，而疟原虫天冬氨酸蛋白酶II的合成则延迟到发育后期。否则，疟原虫天冬氨酸蛋白酶的生物合成是相同的。前疟原虫天冬氨酸蛋白酶是II型整合膜蛋白，在切割成可溶性形式之前通过分泌途径运输。尽管存在几个潜在的糖基化位点，但它们在体内并未糖基化。前疟原虫天冬氨酸蛋白酶的成熟似乎需要酸性条件，并且被三肽醛N-乙酰-L-亮氨酰-L-亮氨酰-正亮氨醛和N-乙酰-L-亮氨酰-L-亮氨酰-甲硫醛可逆抑制。已知这些化合物可抑制半胱氨酸蛋白酶和蛋白酶体的糜蛋白酶活性，但不抑制天冬氨酸蛋白酶。然而，前疟原虫天冬氨酸蛋白酶的加工不会被其他半胱氨酸蛋白酶抑制剂阻断，也不会被蛋白酶体抑制剂乳胞素阻断。加工也不会被天冬氨酸蛋白酶抑制剂阻断。这种抑制剂谱表明，与大多数其他天冬氨酸蛋白酶不同，前疟原虫天冬氨酸蛋白酶的成熟在体内可能不是自催化的，而是可能需要一种特殊加工酶的作用。预计阻断加工的化合物将是有效的抗疟药。
