A member of the ferlin calcium sensor family is essential for Toxoplasma gondii rhoptry secretion

Invasion of host cells by apicomplexan parasites such as Toxoplasma gondii is critical for their infectivity and pathogenesis. In Toxoplasma, secretion of essential egress, motility and invasion-related proteins from microneme organelles is regulated by oscillations of intracellular Ca2+. Later stages of invasion are considered Ca2+-independent, including the secretion of proteins required for host cell entry and remodeling from the parasites rhoptries. We identified a family of three Toxoplasma proteins with homology to the ferlin family of double C2 domain-containing Ca2+ sensors. In humans and model organisms such Ca2+ sensors orchestrate Ca2+-dependent exocytic membrane fusion with the plasma membrane. One ferlin that is conserved across the Apicomplexa, TgFER2, localizes to the parasites cortical membrane skeleton, apical end, and rhoptries. Unexpectedly, conditionally TgFER2-depleted parasites secreted their micronemes normally and were completely motile. However, these parasites were unable to invade host cells and were therefore not viable. Specifically, knockdown of TgFER2 prevented rhoptry secretion and these parasites failed to form the moving junction on the parasite-host interface necessary for host cell invasion. Collectively, these data demonstrate that the putative Ca2+ sensor TgFER2 is required for the secretion of rhoptries. These findings provide the first regulatory and mechanistic insights into this critical yet poorly understood aspect of apicomplexan host cell invasion.\n\nGraphical abstract\n\nO_FIG O_LINKSMALLFIG WIDTH=199 HEIGHT=200 SRC=\"FIGDIR/small/304048_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (58K):\norg.highwire.dtl.DTLVardef@505202org.highwire.dtl.DTLVardef@138400aorg.highwire.dtl.DTLVardef@1f468beorg.highwire.dtl.DTLVardef@1089d34_HPS_FORMAT_FIGEXP M_FIG C_FIG