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Abstract

Plasmodium falciparum is the causative agent of the most devastating human malaria worldwide. The disease is transmitted when a female Anopheles mosquito injects sporozoites into human skin, which migrate and infect the liver. Upon completion of the liver stage, the parasite enters the bloodstream and infects circulating red blood cells (RBCs), thereby starting the intra-erythrocytic cycle. The environment within RBCs represents a challenging milieu for the parasite to develop and propagate normally. Therefore, to ensure its survival, P. falciparum exports over 400 proteins to the host cell involved in several host cell modifications. These parasite-induced host cell renovations are responsible for much of the pathology associated with malaria. Despite the intensive and continuous research work over the years, some fundamental questions remain unanswered, especially the role of many exported proteins. Elucidating their function is of utmost importance as a better understanding of parasite biology is needed to prioritize targets. Functional analysis of such proteins has been hampered in the past due to the lack of adequate genetic systems in this model organism. However, the recent advent of genetic tools such as the selection linked integration targeted gene disruption (SLI-TGD) strategy now allows rapid gene disruption in the P. falciparum system. Additionally, the glucosamine-6-phosphate activated ribozyme (glmS) system was developed to conditionally knockdown the expression of essential proteins, thereby enabling their functional characterization. In this project, we aimed to identify and characterize exported proteins essential for the survival and propagation of the parasite during the intra-erythrocytic development. For this purpose, we used a bioinformatics pipeline approach to prioritize our targets and select 15 genes encoding for exported proteins. Subsequently, these genes were subjected to a screening using the SLI-TGD approach. Furthermore, the glmS ribozyme system was used to analyze and characterize essential genes that could not be disrupted in the SLI-TGD screening. Of the 15 gene candidates screened in this project, 14 genes could not be disrupted, but only the pfj23 gene could be knocked out. Analyses of parasites depleted of Pfj23 revealed aberrant SBP1 distribution and segmented Maurer's clefts architecture. Also, infected erythrocytes with disrupted Pfj23 displayed deformed and worm-like elongated knobs morphologies. Moreover, the binding of infected RBCs to chondroitin sulfate A was significantly reduced upon Pfj23 inactivation. Among the 14 genes that could not be disrupted, the PF3D7_0301800 gene was selected to generate a regulatable copy of its protein using the glmS ribozyme system. Characterization of PF3D7_0301800 revealed aberrant KAHRP distribution upon its downregulation. Additionally, knockdown of PF3D7_0301800 displayed iRBCs with smooth surface without knobs referred to as the "knobless" phenotype. Further analyses could reveal the role of PF3D7_0301800 in KAHRP trafficking and knobs formation. Hence, understanding the function of PF3D7_0301800 and Pfj23 could provide essential insights into the parasite's biology.