A Glycosylphosphatidylinositol-Anchored Carbonic Anhydrase-Related Protein of Toxoplasma gondii Is Important for Rhoptry Biogenesis and Virulence

Carbonic anhydrase-related proteins (CARPs) have previously been described as catalytically inactive proteins closely related to alpha-carbonic anhydrases (alpha-CAs). These CARPs are found in animals (both vertebrates and invertebrates) and viruses as either independent proteins or domains of other proteins. We report here the identification of a new CARP (TgCA_ RP) in the unicellular organism Toxoplasma gondii that is related to the recently described eta-class CA found in Plasmodium falciparum. TgCA_ RP is posttranslationally modified at its C terminus with a glycosylphosphatidylinositol anchor that is important for its localization in intracellular tachyzoites. The protein localizes throughout the rhoptry bulbs of mature tachyzoites and to the outer membrane of nascent rhoptries in dividing tachyzoites, as demonstrated by immunofluorescence and immunoelectron microscopy using specific antibodies. T. gondii mutant tachyzoites lacking TgCA_ RP display a growth and invasion phenotype in vitro and have atypical rhoptry morphology. The mutants also exhibit reduced virulence in a mouse model. Our results show that TgCA_ RP plays an important role in the biogenesis of rhoptries. IMPORTANCE Toxoplasma gondii is an intracellular pathogen that infects humans and animals. The pathogenesis of T. gondii is linked to its lytic cycle, which starts when tachyzoites invade host cells and secrete proteins from specialized organelles. Once inside the host cell, the parasite creates a parasitophorous vacuole (PV) where it divides. Rhoptries are specialized secretory organelles that contain proteins, many of which are secreted during invasion. These proteins have important roles not only during the initial interaction between parasite and host but also in the formation of the PV and in the modification of the host cell. We report here the identification of a new T. gondii carbonic anhydrase-related protein (TgCA_ RP), which localizes to rhoptries of mature tachyzoites. TgCA_ RP is important for the morphology of rhoptries and for invasion and growth of parasites. TgCA_ RP is also critical for parasite virulence. We propose that TgCA_ RP plays a role in the biogenesis of rhoptries.