A member of the ferlin calcium sensor family is essential forToxoplasma gondiirhoptry secretion

Abstract

23Invasion of host cells by apicomplexan parasites such as Toxoplasma gondii is critical for 24 their infectivity and pathogenesis. In Toxoplasma, secretion of essential egress, motility 25 and invasion-related proteins from microneme organelles is regulated by oscillations of 26 intracellular Ca 2+ . Later stages of invasion are considered Ca 2+ -independent, including 27 the secretion of proteins required for host cell entry and remodeling from the parasite's 28 rhoptries. We identified a family of three Toxoplasma proteins with homology to the 29 ferlin family of double C2 domain-containing Ca 2+ sensors. In humans and model 30 organisms such Ca 2+ sensors orchestrate Ca 2+ -dependent exocytic membrane fusion with 31 the plasma membrane. One ferlin that is conserved across the Apicomplexa, TgFER2, 32 localizes to the parasite's cortical membrane skeleton, apical end, and rhoptries. 33Unexpectedly, conditionally TgFER2-depleted parasites secreted their micronemes 34 normally and were completely motile. However, these parasites were unable to invade 35 host cells and were therefore not viable. Specifically, knockdown of TgFER2 prevented 36 rhoptry secretion and these parasites failed to form the moving junction on the parasite-37 host interface necessary for host cell invasion. Collectively, these data demonstrate that 38 the putative Ca 2+ sensor TgFER2 is required for the secretion of rhoptries. These findings 39 provide the first regulatory and mechanistic insights into this critical yet poorly 40 understood aspect of apicomplexan host cell invasion. 41 42 3 Graphical abstract 43 44 45

Biogenesis and discharge of the rhoptries: Key organelles for entry and hijack of host cells by the Apicomplexa

“…TgFER1 and TgFER2 are conserved throughout the entire phylum, whereas TgFER3 is only found in Coccidia (Coleman et al, 2018).…”

“…Ferlins are known to be involved in membrane trafficking and vesicle fusion and can bind calcium and phospholipids via their multiple C2 domains. TgFER1 and TgFER2 are conserved throughout the entire phylum, whereas TgFER3 is only found in Coccidia (Coleman et al., 2018). Conditional depletion of TgFER2 causes a severe defect in invasion due to a defect in rhoptry secretion but the role of Ca 2+ in TgFER2 function is still uncertain (Figure 3) (Coleman et al., 2018).…”

“…TgFER1 and TgFER2 are conserved throughout the entire phylum, whereas TgFER3 is only found in Coccidia (Coleman et al., 2018). Conditional depletion of TgFER2 causes a severe defect in invasion due to a defect in rhoptry secretion but the role of Ca 2+ in TgFER2 function is still uncertain (Figure 3) (Coleman et al., 2018). TgFER2 was also found to interact with TgNd6 and TgNd9 (Aquilini et al., 2020).…”

“…Conditional depletion of TgFER2 causes a severe defect in invasion due to a defect in rhoptry secretion but the role of Ca 2+ in TgFER2 function is still uncertain (Figure 3) (Coleman et al, 2018).…”

Evolution, Composition, Assembly, and Function of the Conoid in Apicomplexa

“…At the surface of the rhoptries, another acetylated protein called ARO (armadillo repeat only protein) ensures the positioning of these organelles to the apical pole of the parasite and their organization as a bundle [51][52][53]. In addition, a protein of the ferlin calcium sensor family (FER2) was shown to be critical for rhoptry secretion [54]. Recently, a lipid-binding rhoptry protein called RASP2 was described as the most direct mediator of rhoptry discharge in Toxoplasma and Plasmodium [47].…”