CHOP Researchers Identify Protein that Restricts Herpes Simplex Virus Infection

Using a novel proteomics approach, a team of researchers led by Children’s Hospital of Philadelphia (CHOP) has identified a host protein that restricts herpes simplex virus 1 (HSV-1) infection. The findings were published today in Nature Microbiology.

When infected by a virus, host cells possess innate defenses against pathogens that can limit their replication and spread. However, viruses have evolved to subvert these defenses, often by binding to these defensive proteins and tagging them for degradation. Although numerous studies have uncovered these mechanisms and targets for RNA viruses, less is known about the intrinsic cellular defenses against DNA viruses.

To identify such cellular defense mechanisms against DNA viruses, the researchers focused on HSV-1, which expresses the E3 ubiquitin-protein ligase ICP0 (ICP0), an enzyme required to promote infection. This enzyme adds the ubiquitin protein to its substrates – including host proteins that would suppress viral infection – which tags the antiviral proteins for degradation by the host’s cellular machinery. Although earlier research has identified ICP0 substrates as barriers to infection, the mechanism for inhibiting viral gene expression is not fully understood.

In order to uncover substrates of ICP0, and thereby discover antiviral host factors that limit HSV-1 infection, the researchers used a technique called Isolation of Proteins on Nascent DNA (iPOND) combined with mass spectrometry to identify proteins bound to the viral DNA (vDNA) HSV-1 genome during infection with wild-type virus or a mutant lacking functional ICP0.

Matthew Weitzman, PhD Using this novel technique, the researchers compared proteomes associated with vDNA in the presence and absence of ICP0. Doing so revealed that cellular protein Schlafen family member 5 (SLFN5) binds vDNA during HSV-1 infection when ICP0 is knocked out. The researchers showed that ICP0 mediates ubiquitination of SLFN5, adding the ubiquitin protein to it and marking it for degradation. In the absence of ICP0, SLFN5 binds vDNA to repress HSV-1 transcription.

“The Schlafen family of proteins was originally identified in immune cells where the proteins retard cell growth, and our findings now suggest they may also act to put viruses ‘to sleep’ unless the antiviral activity is removed by viral countermeasures,” said Matthew Weitzman, PhD, a Professor in the Department of Pathology and Laboratory Medicine at CHOP and senior author of the paper. “These results highlight how comparative proteomics of proteins associated with viral genomes can identify host restriction factors. This approach could be extended to other DNA viruses where targets of viral proteins that promote infection are unknown.”