A forward genetic screen identifies a negative regulator of rapid Ca-dependent cell egress (MS1) in the intracellular parasite .

, like all apicomplexan parasites, uses Ca signaling pathways to activate gliding motility to power tissue dissemination and host cell invasion and egress. A group of "plant-like" Ca-dependent protein kinases (CDPKs) transduces cytosolic Ca flux into enzymatic activity, but how they function is poorly understood. To investigate how Ca signaling activates egress through CDPKs, we performed a forward genetic screen to isolate gain-of-function mutants from an egress-deficient knockout strain. We recovered mutants that regained the ability to egress from host cells that harbored mutations in the gene (SCE1). Global phosphoproteomic analysis showed that SCE1 deletion restored many Δ-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-dependent cell egress during lytic growth.