ROP18 Is a Rhoptry Kinase Controlling the Intracellular Proliferation of Toxoplasma gondii

Abstract

Toxoplasma gondii is an obligate intracellular parasite for which the discharge of apical organelles named rhoptries is a key event in host cell invasion. Among rhoptry proteins, ROP2, which is the prototype of a large protein family, is translocated in the parasitophorous vacuole membrane during invasion. The ROP2 family members are related to protein-kinases, but only some of them are predicted to be catalytically active, and none of the latter has been characterized so far. We show here that ROP18, a member of the ROP2 family, is located in the rhoptries and re-localises at the parasitophorous vacuole membrane during invasion. We demonstrate that a recombinant ROP18 catalytic domain (amino acids 243–539) possesses a protein-kinase activity and phosphorylate parasitic substrates, especially a 70-kDa protein of tachyzoites. Furthermore, we show that overexpression of ROP18 in transgenic parasites causes a dramatic increase in intra-vacuolar parasite multiplication rate, which is correlated with kinase activity. Therefore, we demonstrate, to our knowledge for the first time, that rhoptries can discharge active protein-kinases upon host cell invasion, which can exert a long-lasting effect on intracellular parasite development and virulence. Apicomplexa are unicellular eukaryotes that cause a number of diseases, including malaria. Most of them are obligate intracellular parasites, developing in a parasitophorous vacuole (PV) within their host cell. PV formation during invasion is associated with the exocytosis of parasite secretory organelles named rhoptries, whose role is unknown. Toxoplasma gondii is a model Apicomplexa responsible for toxoplasmosis, a fatal congenital or opportunistic infection in humans and animals. We have studied a novel rhoptry protein dubbed ROP18, which is translocated to the PV membrane upon invasion. ROP18 belongs to a family of rhoptry proteins that share homologies with serine-threonine kinases, but those described so far lack residues critical for enzyme activity. We show that ROP18 possesses all the features needed to be active, and we experimentally demonstrate this activity, which phosphorylates at least one parasite protein. We show that overexpression of ROP18 causes a dramatic increase in parasite multiplication rate that is correlated with kinase activity, and likely dependent on a PV membrane modification. We therefore demonstrate that rhoptries can discharge active protein-kinases upon invasion, which can exert a long-lasting effect on intracellular parasite development and virulence.