Protein-S-nitrosylation of adenovirus-5 E1A and human papillomavirus 16 E7 limits their ability to inhibit STING activity

All viruses that establish successful infections express proteins that inhibit innate anti-viral pathways such as the stimulator of interferon genes (STING) pathway. In response, cells have evolved mechanisms to limit viruses by modifying these viral proteins via post-translational modifications (PTMs). One potent PTM, protein-S-nitrosylation, inhibits the ability of human cytomegalovirus (HCMV) to undermine the establishment of an anti-viral state. The direct nitrosylation of HCMV tegument protein pp71 at a central cysteine within its pRB binding domain reduces pp71's ability to limit the activity of STING. Two different proteins encoded by unrelated DNA viruses, adenovirus (AdV) E1A and human papillomavirus (HPV) E7, also contain a pRB binding domain and inhibit STING like pp71. Herein, we report that E1A and E7 are both protein-S-nitrosylated like pp71. Stable cell lines expressing a WT, or mutants in which the predicted modified cysteine was changed to the closely related serine amino acid, thus blocking protein-S-nitrosylation, revealed that E1A and E7 are both protein-S-nitrosylated. Furthermore, induction of the STING pathway promoted IFN-β1 transcript production and the phosphorylation of IRF3, which was limited in E1A and E7 stable cell lines. Mutant stable cell lines exhibited a stronger inhibition of IFN-β1 transcription and reduced IRF3 phosphorylation, suggesting that the PTM limits WT viral protein inhibition of STING. Furthermore, both E1a and E7 can complement the replication of a HCMV that lacks pp71 during times of STING activation. These observations support a model in which protein-S-nitrosylation of viral virulence factors may function as an anti-viral mechanism in DNA virus infections. IMPORTANCE DNA viruses, such as HCMV, AdV, and HPV, have the capacity to cause significant disease. Infection with AdV can cause severe lower respiratory and liver disease in children, and HPV infection is persistent and is a causative agent of cancer. Thus, these infections can be a severe health risk. Host cells have adapted innate responses like protein S-nitrosylation to limit viral replication. Our previous work reported that direct nitrosylation of two HCMV viral proteins, pp65 and pp71, limits their ability to undermine host anti-viral responses. Herein, we investigated whether protein-S-nitrosylation of AdV and HPV proteins inhibits their functions, suggesting that this PTM is an anti-viral mechanism. This may provide insight into the development of broad anti-viral therapeutics for persistent viral infections.