Genome-Wide Bimolecular Fluorescence Complementation-Based Proteomic Analysis of ROP18's Human Interactome Shows Its Key Role in Regulation of Cell Immunity and Apoptosis.

Toxoplasma gondii rhoptry protein ROP18 (TgROP18) is a key virulence factor secreted into the host cell during invasion, where it modulates the host cell response by interacting with its host targets. However, only a few TgROP18 targets have been identified. In this study, we applied a high-throughput protein-protein interaction (PPI) screening in human cells using bimolecular fluorescence complementation (BiFC) to identify the targets of Type I strain ROP18 (ROP18I) and Type II strain ROP18 (ROP18II). From a pool of more than 18,000 human proteins, 492 and 141 proteins were identified as the targets of ROP18I and ROP18II, respectively. Gene ontology, search tool for the retrieval of interacting genes/proteins PPI network, and Ingenuity pathway analyses revealed that the majority of these proteins were associated with immune response and apoptosis. This indicates a key role of TgROP18 in manipulating host's immunity and cell apoptosis, which might contribute to the immune escape and successful parasitism of the parasite. Among the proteins identified, the immunity-related proteins N-myc and STAT interactor, IL20RB, IL21, ubiquitin C, and vimentin and the apoptosis-related protein P2RX1 were further verified as ROP18I targets by sensitized emission-fluorescence resonance energy transfer (SE-FRET) and co-immunoprecipitation. Our study substantially contributes to the current limited knowledge on human targets of TgROP18 and provides a novel tool to investigate the function of parasite effectors in human cells.