A forward genetic screen identifies a negative regulator of rapid Ca2+-dependent cell egress (MS1) in the intracellular parasite Toxoplasma gondii

Toxoplasma gondii , like all apicomplexan parasites, uses Ca 2+ signaling pathways to activate gliding motility to power tissue dissemination and host cell invasion and egress. A group of "plant-like" Ca 2+ -dependent protein kinases (CDPKs) transduces cytosolic Ca 2+ flux into enzymatic activity, but how they function is poorly understood. To investigate how Ca 2+ signaling activates egress through CDPKs, we performed a forward genetic screen to isolate gain-of-function mutants from an egress-deficient cdpk3 knockout strain. We recovered mutants that regained the ability to egress from host cells that harbored mutations in the gene Suppressor of Ca 2+ -dependent Egress 1 (SCE1). Global phosphoproteomic analysis showed that SCE1 deletion restored many Δ cdpk3 -dependent phosphorylation events to near wild-type levels. We also show that CDPK3-dependent SCE1 phosphorylation is required to relieve its suppressive activity to potentiate egress. In summary, our work has uncovered a novel component and suppressor of Ca 2+ -dependent cell egress during Toxoplasma lytic growth.