page 47, Klemba et al. show how the parasite brings together this food source with its own enzymes that will degrade it. The results offer insight into how to treat this deadly disease.
The parasite breaks down hemoglobin for nutrients in its specialized organelle called the food vacuole, using proteases such as plasmepsin II (PM II). Hemoglobin gets to the food vacuole in vesicles that pinch off from cytostomes—openings through the parasite plasma membrane that lead to the blood cell cytoplasm. But how the endogenous protease reaches the food vacuole to meet its substrate had not been defined, leading some researchers to wonder whether a direct targeting pathway exists from the ER to the food vacuole. The new results suggest this is not the case.
Instead, the group sees that an inactive precursor form of the protease hitches a ride with hemoglobin in cytostome-derived vesicles. GFP fusions of the PM II precursor were seen to be secreted from the ER to cytostomes. From there, the fusions traveled in vesicles shared by hemoglobin to the food vacuole, where the precursor was processed to its active form.
It is not clear whether PM II is sent directly to cytostomes or simply diffuses laterally once at the plasma membrane. If the former is true, a cytostome-targeted trafficking pathway might be a pathogen-specific target for drug treatments. ▪