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. IMPORTANCE: Toxoplasma gondii critically relies on host nutrients for growth, but the underlying mechanisms are largely unknown. Here, we discovered that the secretory protein GRA1 in Toxoplasma played a crucial role in scavenging host nutrients to support parasite proliferation. GRA1 is the first reported GRA protein, but its function remains enigmatic since its discovery in 1989. GRA1 is secreted to the PV from the dense granules, which harbor many proteins that traffic to PV, PVM, and even host cells to perform diverse functions, including forming channels on the PVM to take up host nutrients to establish parasitism. GRA1 disruption impaired the structure of IVN and abolished the targeting of other GRA proteins to their destinations, which compromised the parasite's ability to import nutrients from the hosts. These findings reveal the functional mode of GRA1 in T. gondii and highlight its potential as a target for developing new interventions against toxoplasmosis.