Interferon-stimulated gene 15 (ISG15) encodes an ubiquitin-like protein that covalently conjugates protein. Protein modification by ISG15 (ISGylation) is known to inhibit the replication of many viruses. However, studies on the viral targets and viral strategies to regulate ISGylation-mediated antiviral responses are limited. In this study, we show that human cytomegalovirus (HCMV) replication is inhibited by ISGylation, but the virus has evolved multiple countermeasures. HCMV-induced ISG15 expression was mitigated by IE1, a viral inhibitor of interferon signaling, however, ISGylation was still strongly upregulated during virus infection. RNA interference of UBE1L (E1), UbcH8 (E2), Herc5 (E3), and UBP43 (ISG15 protease) revealed that ISGylation inhibits HCMV growth by downregulating viral gene expression and virion release in a manner that is more prominent at low multiplicity of infection. A viral regulator pUL26 was found to interact with ISG15, UBE1L, and Herc5, and be ISGylated. ISGylation of pUL26 regulated its stability and inhibited its activities to suppress NF-κB signaling and complement the growth of UL26-null mutant virus. Moreover, pUL26 reciprocally suppressed virus-induced ISGylation independent of its own ISGylation. Consistently, ISGylation was more pronounced in infections with the UL26-deleted mutant virus, whose growth was more sensitive to IFNβ treatment than that of the wild-type virus. Therefore, pUL26 is a viral ISG15 target that also counteracts ISGylation. Our results demonstrate that ISGylation inhibits HCMV growth at multiple steps and that HCMV has evolved countermeasures to suppress ISG15 transcription and protein ISGylation, highlighting the importance of the interplay between virus and ISGylation in productive viral infection.

DNA-dependent activator of interferon regulatory factor (DAI) acts as a cytosolic B-form DNA sensor that induces type I interferons. However, DAI is not required for DNA sensing in certain cell types due to redundancy of the DNA sensing system. Here, we investigated the effect of DAI on herpes simplex virus 1 (HSV-1) infection in HepG2 hepatocellular carcinoma cells. DAI transcription was induced after gamma interferon (IFN-␥) treatment or HSV-1 infection. HSV-1 replication was enhanced by DAI knockdown, and ectopic DAI expression repressed viral replication in a manner requiring the Z␤ and D3 domains, but not the Z␣ domain. This activity of DAI was more prominent at low multiplicity of infection (MOI) and correlated with the reduced expression of viral immediate-early genes. Consistently, DAI repressed the activation of ICP0 promoter in reporter gene assays. DNA-dependent activator of interferon (IFN) regulatory factor (DAI), which is also referred to as Z-DNA binding protein 1 (ZBP1) or DLM-1, was initially identified as a highly upregulated protein in mouse tumor stromal cells and in macrophages treated by gamma IFN (IFN-␥) or lipopolysaccharide (1). Structural analyses have revealed that DAI/ZBP1/DLM-1 (referred to as DAI hereafter) contains the amino-terminal Z-form DNA-binding domains, Z␣ and Z␤, which are homologous to those of adenosine deaminase that acts on RNA (ADAR1), an RNA editing enzyme (2-5). Since Z-DNA is located near the transcription start sites of certain genes in the genome, a role of DAI in transcriptional regulation has been suggested (6, 7). Induction of DAI was also observed in mouse hepatocytes infected with hepatitis B virus (HBV) (8) and in mouse embryonic fibroblasts (MEFs) stimulated by B-form DNA (9).Recently, DAI was shown to act as a cytosolic B-form DNA sensor that initiates IFN responses via activation of the nuclear factor-B (NF-B) and interferon regulatory transcription factor 3 (IRF3) pathways in mice (10). In addition to the Z-DNA-binding domains, a region termed the D3 domain was demonstrated to primarily contribute to the recognition of B-DNA in vitro (10). However, all of the Z␣, Z␤, and D3 domains were required for efficient B-DNA binding in vivo and DAI was suggested to undergo DNA-mediated multimerization to evoke activation of IFN responses (11). The carboxyl-terminal region of DAI was responsible for recruitment of both IRF3 and TANK-binding kinase 1 (TBK1), an IB kinase that activates IRF3 (10). The mechanism by which DAI activates the NF-B pathway was shown to involve recruitment of receptor-interacting protein kinase 1 (RIP1) and RIP3 through a RIP homotypic interaction motif (RHIM)-dependent interaction with DAI (12, 13). Recently, the binding of DAI with RIP3 was shown to mediate virus-induced programmed necrosis (14).The requirement of DAI in induction of IFN response by cytosolic stimulation of B-DNA is dependent on cell type. DAI played a role in the DNA-mediated IFN production in mouse fibroblast L929 (10, 12, 15) and mouse SVEC4-10 endothelial cells (12)...

Several viruses have been found to encode a deubiquitinating protease (DUB). These viral DUBs are proposed to play a role in regulating innate immune or inflammatory signaling. In human cytomegalovirus (HCMV), the largest tegument protein encoded by UL48 contains DUB activity, but its cellular targets are not known. Here, we show that UL48 and UL45, an HCMV-encoded inactive homolog of cellular ribonucleotide reductase (RNR) large subunit (R1), target receptor-interacting protein kinase 1 (RIP1) to inhibit NF-κB signaling. Transfection assays showed that UL48 and UL45, which binds to UL48, interact with RIP1 and that UL48 DUB activity and UL45 cooperatively suppress RIP1-mediated NF-κB activation. The growth of UL45-null mutant virus was slightly impaired with showing reduced accumulation of viral late proteins. Analysis of a recombinant virus expressing HA-UL45 showed that UL45 interacts with both UL48 and RIP1 during virus infection. Infection with the mutant viruses also revealed that UL48 DUB activity and UL45 inhibit TNFα-induced NF-κB activation at late times of infection. UL48 cleaved both K48- and K63-linked polyubiquitin chains of RIP1. Although UL45 alone did not affect RIP1 ubiquitination, it could enhance the UL48 activity to cleave RIP1 polyubiquitin chains. Consistently, UL45-null virus infection showed higher ubiquitination level of endogenous RIP1 than HA-UL45 virus infection at late times. Moreover, UL45 promoted the UL48-RIP1 interaction and re-localization of RIP1 to the UL48-containing virion assembly complex. The mouse cytomegalovirus (MCMV)-encoded DUB, M48, interacted with mouse RIP1 and M45, an MCMV homolog of UL45. Collectively, our data demonstrate that cytomegalovirus-encoded DUB and inactive R1 homolog target RIP1 and cooperatively inhibit RIP1-mediated NF-κB signaling at the late stages of HCMV infection.