A GCN2-Like Eukaryotic Initiation Factor 2 Kinase Increases the Viability of Extracellular Toxoplasma gondii Parasites

Toxoplasmosis is a significant opportunistic infection caused by the protozoan parasite Toxoplasma gondii, an obligate intracellular pathogen that relies on host cell nutrients for parasite proliferation. Toxoplasma parasites divide until they rupture the host cell, at which point the extracellular parasites must survive until they find a new host cell. Recent studies have indicated that phosphorylation of Toxoplasma eukaryotic translation initiation factor 2-alpha (TgIF2α) plays a key role in promoting parasite viability during times of extracellular stress. Here we report the cloning and characterization of a TgIF2α kinase designated TgIF2K-D that is related to GCN2, a eukaryotic initiation factor 2α (eIF2α) kinase known to respond to nutrient starvation in other organisms. TgIF2K-D is present in the cytosol of both intra- and extracellular Toxoplasma parasites and facilitates translational control through TgIF2α phosphorylation in extracellular parasites. We generated a TgIF2K-D knockout parasite and demonstrated that loss of this eIF2α kinase leads to a significant fitness defect that stems from an inability of the parasite to adequately adapt to the environment outside host cells. This phenotype is consistent with that reported for our nonphosphorylatable TgIF2α mutant (S71A substitution), establishing that TgIF2K-D is the primary eIF2α kinase responsible for promoting extracellular viability of Toxoplasma. These studies suggest that eIF2α phosphorylation and translational control are an important mechanism by which vulnerable extracellular parasites protect themselves while searching for a new host cell. Additionally, TgIF2α is phosphorylated when intracellular parasites are deprived of nutrients, but this can occur independently of TgIF2K-D, indicating that this activity can be mediated by a different TgIF2K.