The Toxoplasma protein phosphatase 6 catalytic subunit (TgPP6C) is essential for cell cycle progression and virulence.

Protein phosphatases are post-translational regulators of Toxoplasma gondii proliferation, tachyzoite-bradyzoite differentiation and pathogenesis. Here, we identify the putative protein phosphatase 6 (TgPP6) subunits of T. gondii and elucidate their role in the parasite lytic cycle. The putative catalytic subunit TgPP6C and regulatory subunit TgPP6R likely form a complex whereas the predicted structural subunit TgPP6S, with low homology to the human PP6 structural subunit, does not coassemble with TgPP6C and TgPP6R. Functional studies showed that TgPP6C and TgPP6R are essential for parasite growth and replication. The ablation of TgPP6C significantly reduced the synchronous division of the parasite's daughter cells during endodyogeny, resulting in disordered rosettes. Moreover, the six conserved motifs of TgPP6C were required for efficient endodyogeny. Phosphoproteomic analysis revealed that ablation of TgPP6C predominately altered the phosphorylation status of proteins involved in the regulation of the parasite cell cycle. Deletion of TgPP6C significantly attenuated the parasite virulence in mice. Immunization of mice with TgPP6C-deficient type I RH strain induced protective immunity against challenge with a lethal dose of RH or PYS tachyzoites and Pru cysts. Taken together, the results show that TgPP6C contributes to the cell division, replication and pathogenicity in T. gondii. Author summary: Toxoplasma gondii is a protozoan parasite characterized by a highly spatially and temporally coordinated replication process. Some protein phosphatases are conserved among apicomplexan protozoa and regulate numerous cellular and biological processes, such as parasite proliferation, tachyzoite-bradyzoite differentiation and virulence. Here, we identify the role of the putative protein phosphatase 6 of T. gondii (TgPP6), and found two of its subunits, TgPP6C and TgPP6R, to be vital for the parasite replication and virulence. RHΔpp6c and RHΔpp6r exhibited various degrees of asynchronous division and morphological deformation during endodyogeny compared to wild-type parasites. TgPP6C affected the phosphorylation status of proteins involved in the parasite cell division. We also demonstrated the protective efficacy of immunization of mice using RHΔpp6c against acute and chronic infection by wild-type RH, PYS and Pru strains, respectively. These findings reveal novel roles of TgPP6C in the replication and virulence of T. gondii. [ABSTRACT FROM AUTHOR]