Structure of the Toxoplasma gondii ROP18 Kinase Domain Reveals a Second Ligand Binding Pocket Required for Acute Virulence

Background: ROP18 is a Toxoplasma secreted Ser/Thr protein kinase important for acute virulence. Results: The crystal structure of the unphosphorylated ROP18 kinase domain was determined in complex with an ATP analog. Conclusion: The structure is inconsistent with a previously proposed model of autoinhibition and identifies an additional ligand binding site important for virulence. Significance: Structure-function studies of ROP18 will aid development of novel drugs against toxoplasmosis.At least a third of the human population is infected with the intracellular parasite Toxoplasma gondii, which contributes significantly to the disease burden in immunocompromised and neutropenic hosts and causes serious congenital complications when vertically transmitted to the fetus. Genetic analyses have identified the Toxoplasma ROP18 Ser/Thr protein kinase as a major factor mediating acute virulence in mice. ROP18 is secreted into the host cell during the invasion process, and its catalytic activity is required for the acute virulence phenotype. However, its precise molecular function and regulation are not fully understood. We have determined the crystal structure of the ROP18 kinase domain, which is inconsistent with a previously proposed autoinhibitory mechanism of regulation. Furthermore, a sucrose molecule bound to our structure identifies an additional ligand-binding pocket outside of the active site cleft. Mutational analysis confirms an important role for this pocket in virulence.