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

## TL;DR

Cells use protein-S-nitrosylation to limit the ability of viral proteins from adenovirus and HPV to block the STING anti-viral pathway.

## Contribution

The study shows that protein-S-nitrosylation inhibits the function of adenovirus E1A and HPV E7 proteins, similar to its effect on HCMV proteins.

## Key findings

- E1A and E7 are protein-S-nitrosylated at a conserved cysteine residue.
- Nitrosylation reduces the ability of E1A and E7 to inhibit STING activity.
- Mutant proteins lacking nitrosylation show stronger inhibition of IFN-β1 and IRF3 phosphorylation.

## Abstract

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.

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.