Abstract
The p47 immunity-related GTPase (IRG) Irgb6 plays a pioneering role in host defense against Toxoplasma gondii infection. Irgb6 is recruited to the parasitophorous vacuole membrane (PVM) formed by T. gondii and disrupts it. Despite the importance of this process, the molecular mechanisms accounting for PVM recognition by Irgb6 remain elusive because of lack of structural information on Irgb6. Here we report the crystal structures of mouse Irgb6 in the GTP-bound and nucleotide-free forms. Irgb6 exhibits a similar overall architecture to other IRGs in which GTP binding induces conformational changes in both the dimerization interface and the membrane-binding interface. The membrane-binding interface of Irgb6 assumes a unique conformation, composed of N- and C-terminal helical regions forming a phospholipid binding site. In silico docking of phospholipids further revealed membrane-binding residues that were validated through mutagenesis and cell-based assays. Collectively, these data demonstrate a novel structural basis for Irgb6 to recognize T. gondii PVM in a manner distinct from other IRGs.
Highlights
Infection by intracellular pathogens stimulates innate and acquired immune systems to produce IFN
Recent studies demonstrate that host defense during acute toxoplamosis requires IFN-γ-inducible GTPases that localize at a T. gondii-forming vacuole called the parasitophorous vacuole (PV) inside infected cells and destroy the structure, leading to parasite killing
We previously reported that Irgb6 binds to phosphoinositide 5P (PI5P) and PS, which are both components of the T. gondii parasitophorous vacuole membrane (PVM) (Lee et al, 2020)
Summary
Infection by intracellular pathogens stimulates innate and acquired immune systems to produce IFN. A number of IFN-γ–inducible products play pivotal and pleiotropic roles in cell-autonomous immunity against various intracellular pathogens such as viruses, bacteria and protozoan parasites (MacMicking, 2012). Recent studies demonstrate that host defense during acute toxoplamosis requires IFN-γ-inducible GTPases that localize at a T. gondii-forming vacuole called the parasitophorous vacuole (PV) inside infected cells and destroy the structure, leading to parasite killing. Sequential and hierarchical recruitment of IRGs and GBPs leads to efficient PVM disruption and pathogen clearance (Khaminets et al, 2010). The IRG Irgb has been shown to be localized at the PVM soon after T. gondii invasion to host cells and acts as a pioneer for the recruitment of other IRGs and GBPs (Khaminets et al, 2010; Lee et al, 2020). Genetic ablation of Irgb results in severely impaired accumulation of other IRGs and GBPs, reflecting the pioneering role of Irgb to induce host defense (Lee et al, 2020)
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