A 77 Amino Acid Region in the N-Terminal Half of the HSV-1 E3 Ubiquitin Ligase ICP0 Contributes to Counteracting an Established Type 1 Interferon Response

Herpes simplex virus 1 (HSV-1) is a human pathogen capable of establishing lifelong latent infections that can reactivate under stress conditions. A viral immediate early protein that plays important roles in the HSV-1 lytic and latent infections is the viral E3 ubiquitin ligase, ICP0. ICP0 transactivates all temporal classes of HSV-1 genes and facilitates viral gene expression. ICP0 also impairs the antiviral effects of interferon (IFN)-β, a component of host innate defenses known to limit viral replication. To begin to understand how ICP0 allows HSV-1 to disarm the IFN-β response, we performed genetic analyses using a series of ICP0 truncation mutants in the absence and presence of IFN-β in cell culture. We observed that IFN-β pretreatment of cells significantly impaired the replication of the ICP0 truncation mutants, 212 and 312, which code for the first 211 and 311 amino acids of ICP0, respectively; this effect of IFN-β correlated with decreased HSV-1 early and late gene expression. This increased sensitivity to IFN-β was not as apparent with the ICP0 mutant, 389. Our mapping studies indicate that loss of 77 amino acids from residues 312 to 388 in the N-terminal half of ICP0 resulted in a virus that was significantly more sensitive to cells pre-exposed to IFN-β. This 77 amino acid region contains a phospho-SUMO-interacting motif or -SIM, which we propose participates in ICP0's ability to counteract the antiviral response established by IFN-β. Interferons (IFNs) are secreted cellular factors that are induced by viral infection and limit replication. HSV-1 is largely refractory to the antiviral effects of type 1 IFNs, which are synthesized shortly after viral infection, in part through the activities of the viral regulatory protein, ICP0. To understand how ICP0 impedes the antiviral effects of type 1 IFNs, we used a series of HSV-1 ICP0 mutants and examined their viral replication and gene expression levels in cells stimulated with IFN-β (a type 1 IFN). Our mapping data identifies a discrete 77 amino acid region in the N-terminal half of ICP0 that facilitates HSV-1 resistance to IFN-β. This region of ICP0 is modified by phosphorylation and binds to the posttranslational modification SUMO, suggesting that HSV, and potentially other viruses, may counteract type 1 IFN signaling by altering SUMO and/or SUMO modified cellular proteins.