Structural basis of Irgb6 inactivation by Toxoplasma gondii through the phosphorylation of switch I

Irgb6 is a priming immune-related GTPase (IRG) that counteracts Toxoplasma gondii. It is known to be recruited to the low virulent type II T. gondii parasitophorous vacuole (PV), initiating cell-autonomous immunity. However, the molecular mechanism by which immunity-related GTPases become inactivated after the parasite infection remains obscure. Here, we found that Thr95 of Irgb6 is prominently phosphorylated in response to low virulent type II T. gondii infection. We observed that a phosphomimetic T95D mutation in Irgb6 impaired its localization to the PV and exhibited reduced GTPase activity in vitro. Structural analysis unveiled an atypical conformation of nucleotide-free Irgb6-T95D, resulting from a conformational change in the G-domain that allosterically modified the PV membrane-binding interface. In silico docking corroborated the disruption of the physiological membrane binding site. These findings provide novel insights into a T. gondii-induced allosteric inactivation mechanism of Irgb6.

Automated summary

Irgb6 is an immune-related GTPase that plays a role in counteracting Toxoplasma gondii. It is recruited to the parasitophorous vacuole (PV) and activates cell-autonomous immunity. This study found that Thr95 of Irgb6 is phosphorylated in response to T. gondii infection, and a mutation that mimics phosphorylation impairs the localization and GTPase activity of Irgb6. Structural analysis revealed a conformational change in the G-domain that disrupts the PV membrane-binding interface. These findings provide insights into the mechanism of T. gondii-induced inactivation of Irgb6.