Files
Abstract
The endoplasmic reticulum (ER) is thought to play an essential role during egress of malaria parasites. The assumed roles of the ER include the biogenesis and secretion of egress-related organelles, and trafficking of proteins to these organelles. However, no proteins localized to the parasite ER have been shown to play a role in egress of malaria parasites. In this study, we generated conditional mutants of the Plasmodium falciparum Endoplasmic Reticulum-resident Calcium-binding protein (PfERC), a member of the CREC family of proteins. Knockdown of PfERC showed that this gene is essential for asexual growth and further analysis revealed that PfERC is required for parasite egress. As the ER is assumed to have several roles in egress, we tested the role of PfERC in ER biology. We show that, although PfERC can bind Ca2+ and has potential Ca2+-dependent conformational changes, it was not required for calcium storage or calcium signaling in the ER. Furthermore, we show that PfERC was not required for protein trafficking to or biogenesis of egress-related organelles. Rather we found that PfERC knockdown prevented the rupture of the parasitophorous vacuole membrane (PVM). The failure to rupture this membrane was due to inhibition of the proteolytic maturation of the secreted subtilisin like serine protease, SUB1. Using double mutant parasites, we further show that PfERC is required for the proteolytic maturation of the essential aspartic protease, Plasmepsin X, which is required for SUB1 maturation. Lastly, we show that processing of substrates downstream of the proteolytic cascade is inhibited by PfERC knockdown. Thus, these data establish the ER-resident CREC family protein, PfERC, as a key early regulator of the egress proteolytic cascade of malaria parasites. This work presents a new avenue of research of determining the regulation of egress via non-enzymatic proteins localized in the secretory pathway.