Autophagy-Related Protein ATG18 Regulates Apicoplast Biogenesis in Apicomplexan Parasites

Abstract

Mechanisms by which 3′-phosphorylated phosphoinositides (3′-PIPs) regulate the development of apicomplexan parasites Plasmodium falciparum and Toxoplasma gondii are poorly understood. The catabolic process of autophagy, which is dependent on autophagy-related proteins (ATGs), is one of the major targets of 3′-PIPs in yeast and mammals. In the present study, we identified autophagy-related protein ATG18 as an effector of 3′-PIPs in these parasites. P. falciparum ATG18 (PfATG18) and T. gondii ATG18 (TgATG18) interact with 3′-PIPs but exhibited differences in their specificity of interaction with the ligand PIP. The conditional knockdown of T. gondii or P. falciparum ATG18 (Tg/PfATG18) impaired replication of parasites and resulted in their delayed death. Intriguingly, ATG18 depletion resulted in the loss of the apicomplexan parasite-specific nonphotosynthetic plastid-like organelle apicoplast, which harbors the machinery for biosynthesis of key metabolites, and the interaction of ATG18 to phosphatidylinositol 3-phosphate (PI3P) was critical for apicoplast inheritance. Furthermore, ATG18 regulates membrane association and apicoplast localization of ATG8. These findings provide insights into a novel noncanonical role of ATG18 in apicoplast inheritance. This function of ATG18 in organelle biogenesis is unprecedented in any organism and may be conserved across most apicomplexan parasites.

Characterisation of two Toxoplasma PROPPINs homologous to Atg18/WIPI suggests they have evolved distinct specialised functions

“…Besides, when checking for the presence of the apicoplast 48 hours after transfection, about 30% of the vacuoles had parasites lacking the organelle. This is in accordance with a recently published study describing an essential function for TgPROP2 (simply called ‘TgATG18’ in the aforementioned study) for apicoplast maintenance and parasite viability [38].…”

“…The lipid-biding properties of PROPPINs are thought to be key for exerting their function [40,41], We demonstrated here that the lipid-binding motif of TgPROPs, and TgPI3k (but not TgPIKfyve) activity were important for vesicular localisation of these proteins. Bansal et al also found that recombinant TgATG18/TgPROP2 is able to bind PtdIns3P through the lipid binding motif present in the WD-40 domain of the protein, and showed this is important for the function they describe in apicoplast homeostasis [38]. Interestingly, PtdIns3P has been known for some time to be a critical player in maintaining apicoplast homeostasis [28].…”

Identification of a Golgi apparatus protein complex important for the asexual erythrocytic cycle of the malaria parasite Plasmodium falciparum

“…Autophagy is a process whereby cellular material such as misfolded proteins are degraded inside the lysosome (Yang & Klionsky, ). In P. falciparum , various roles have been suggested for autophagy proteins (Atg) such as trafficking to the food vacuole, organelle expulsion and protein secretion (Hain & Bosch, ), and apicoplast inheritance (Bansal, Tripathi, Thakur, Mohmmed, & Sharma, ). Despite the recent interest in Plasmodium autophagy, several Plasmodium Atg homologues remain to be identified.…”

“…In yeast, Atg2 establishes a complex with Atg9 (a 6 transmembrane domains protein) and Atg18 (a phosphatidylinositol triphosphate binding protein) and is involved in the formation of pre‐autophagosomal structure during the preliminary step of autophagy (Tanida, , Hain & Bosch, ). A homologue of Atg18 has been identified in P. falciparum (PF3D7_1012900), and it has recently been shown to be critical for the inheritance of the apicoplast (Bansal et al, ; Wang et al, ). Of note, PfAtg18 was not found in our list of GP1 interacting proteins.…”

A pan‐apicomplexan phosphoinositide‐binding protein acts in malarial microneme exocytosis

“…Intriguingly, despite representing only a minor fraction of the total lipids of eukaryotic membranes, phosphoinositides (PIPs) are critical components involved in a variety of cellular processes and recent work has shown that it was also the case for apicomplexan parasites (reviewed in [33]). More specifically for P. falciparum, roles for PIPs have been shown in cytokinesis and merozoite formation [34], apicoplast biogenesis and inheritance [35][36][37], hemoglobin endocytosis [38], merozoite egress [17], gametocyte activation [39][40][41], and ookinete motility [17], the latter two occurring in the mosquito vector, and finally in resistance to artemisinin [42,43]. In line with these varied functions, the determination of the subcellular distribution of several species of PIPs in the malaria parasite asexual erythrocytic stages showed localizations to structures such as the Golgi apparatus, the plasma membrane, the food vacuole membrane, the endoplasmic reticulum, and the apicoplast [34,35,44,45].…”