The critical role of Toxoplasma gondii GRA1 in nutrient salvage

Toxoplasma gondii is an obligate intracellular pathogen that infects humans and many animals. It harbors unique secretory organelles to facilitate its parasitic lifestyle, including dense granules that secrete diverse proteins to different destinations. Yet the biological functions of these secretory proteins are mostly unknown. Here, we examined the roles of GRA1, the first dense granule protein discovered. GRA1 is secreted to the parasitophorous vacuole (PV), and its conditional depletion led to growth arrest of tachyzoites, suggesting a crucial role of GRA1 for parasite propagation during acute infection. Furthermore, GRA1 inactivation resulted in severe metabolic defects, including reduced glycolysis and tricarboxylic acid cycle activities and a drop in cellular ATP levels. GRA1 depletion also disrupted the integrity of the mitochondrion and apicoplasts. Mechanistically, disruption of GRA1 altered the structures of membranous tubules in the PV, called the intravacuolar network (IVN), which are thought to mediate the transport of molecules in and out of the parasites. Consistently, GRA1 inactivation impaired the secretion of various GRA proteins to the PV membrane (PVM) or host cells, including GRA17 that is involved in nutrient uptake from host cells. In agreement with the mislocalization of GRA proteins, mutants lacking GRA1 had a strong defect in absorbing nutrients like glucose and pantothenate from host cells. Consequently, the parasites displayed slower growth and a higher tendency to differentiate into bradyzoites. Together, these results suggest that GRA1 plays a vital role in maintaining the structural integrity of the IVN, thus contributing to the scavenging of host nutrients.