mTOR Dysregulation by Vaccinia Virus F17 Controls Multiple Processes with Varying Roles in Infection.

Despite producing enormous amounts of cytoplasmic DNA, poxviruses continue to replicate efficiently by deploying an armory of proteins that counter host antiviral responses at multiple levels. Among these, poxvirus protein F17 dysregulates the host kinase mammalian target of rapamycin (mTOR) to prevent the activation of stimulator of interferon genes (STING) expression and impair the production of interferon-stimulated genes (ISGs). However, the host DNA sensor(s) involved and their impact on infection in the absence of F17 remain unknown. Here, we show that cyclic-di-GMP-AMP (cGAMP) synthase (cGAS) is the primary sensor that mediates interferon response factor (IRF) activation and ISG responses to vaccinia virus lacking F17 in both macrophages and lung fibroblasts, although additional sensors also operate in the latter cell type. Despite this, ablation of ISG responses through cGAS or STING knockout did not rescue defects in late-viral-protein production, and the experimental data pointed to other functions of mTOR in this regard. mTOR adjusts both autophagic and protein-synthetic processes to cellular demands. No significant differences in autophagic responses to wild-type or F17 mutant viruses could be detected, with autophagic activity differing across cell types or states and exhibiting no correlations with defects in viral-protein accumulation. In contrast, results using transformed cells or altered growth conditions suggested that late-stage defects in protein accumulation reflect failure of the F17 mutant to deregulate mTOR and stimulate protein production. Finally, rescue approaches suggest that phosphorylation may partition F17's functions as a structural protein and mTOR regulator. Our findings reveal the complex multifunctionality of F17 during infection.