The Vacuolar Zinc Transporter TgZnT Protects Toxoplasma gondii from Zinc Toxicity

Zinc (Zn²⁺) is the most abundant biological metal ion aside from iron and is an essential element in numerous biological systems, acting as a cofactor for a large number of enzymes and regulatory proteins. Zn²⁺ must be tightly regulated, as both the deficiency and overabundance of intracellular free Zn²⁺ are harmful to cells. Zn²⁺ transporters (ZnTs) play important functions in cells by reducing intracellular Zn²⁺ levels by transporting the ion out of the cytoplasm. We characterized a Toxoplasma gondii gene (TgGT1_251630, TgZnT), which is annotated as the only ZnT family Zn²⁺ transporter in T. gondii. TgZnT localizes to novel vesicles that fuse with the plant-like vacuole (PLV), an endosome-like organelle. Mutant parasites lacking TgZnT exhibit reduced viability in in vitro assays. This phenotype was exacerbated by increasing zinc concentrations in the extracellular media and was rescued by media with reduced zinc. Heterologous expression of TgZnT in a Zn²⁺-sensitive Saccharomyces cerevisiae yeast strain partially restored growth in media with higher Zn²⁺ concentrations. These results suggest that TgZnT transports Zn²⁺ into the PLV and plays an important role in the Zn²⁺tolerance of T. gondii extracellular tachyzoites.

IMPORTANCE Toxoplasma gondii is an intracellular pathogen of human and animals. T. gondii pathogenesis is associated with its lytic cycle, which involves invasion, replication, egress out of the host cell, and invasion of a new one. T. gondii must be able to tolerate abrupt changes in the composition of the surrounding milieu as it progresses through its lytic cycle. We report the characterization of a Zn²⁺ transporter of T. gondii (TgZnT) that is important for parasite growth. TgZnT restored Zn²⁺ tolerance in yeast mutants that were unable to grow in media with high concentrations of Zn²⁺. We propose that TgZnT plays a role in Zn²⁺ homeostasis during the T. gondii lytic cycle.

Nathan M. Chasen, Andrew J. Stasic, Beejan Asady, Isabelle Coppens, Silvia N. J. Moreno. 2019. mSphere.; 4(3). pii: e00086-19. doi: 10.1128/mSphere.00086-19.