Functions of Herpesvirus-Encoded Homologs of the Cellular Ribonucleotide Reductase Large Subunit

Deoxyribonucleotides (dNTPs) are important for the efficient growth of DNA viruses. Therefore, many DNA viruses have strategies for the upregulation of cellular dNTP levels. Both α- and γ-herpesviruses encode functional homologs of cellular dNTP anabolic enzymes, including the class I ribonucleotide reductase (RNR) large (R1) and small (R2) subunits, whereas β-herpesviruses modulate host cells to induce genes that increase dNTP levels. Interestingly, β-herpesviruses still express the nonfunctional RNR R1 subunit. However, it is not clear why β-herpesviruses still carry inactive R1 homologs. Recently, the R1 homologs of herpesviruses have been shown to inhibit innate immune signaling pathways. In particular, both functional and nonfunctional R1 homologs target receptor-interacting protein kinase 1 (RIP1) and inhibit RIP1-mediated signaling pathways to promote viral replication. Here, we summarize recent findings on the activity of herpesviral R1 homologs and discuss their roles in the regulation of innate immune signaling pathways.

Differential Regulation of NF-kappaB Signaling during Human Cytomegalovirus Infection

NF-kappaB transcription factors are key regulators of immune and stress responses, apoptosis, and differentiation. Human cytomegalovirus (HCMV) activates or represses NF-kappaB signaling at different times during infection. An initial increase in NF-kappaB activity occurs within a few hours of infection. The virus appears to adapt to this change since initial viral gene expression is promoted by the elevated NF-kappaB activity. Because NF-kappaB upregulates innate immune responses and inflammation, it has also been suggested that HCMV needs to downregulate NF-kappaB signaling. Recent studies have shown that HCMV has various mechanisms that inhibit NF-kappaB signaling. HCMV reduces cell surface expression of tumor necrosis factor receptor 1 (TNFR1) and blocks the DNA binding activity of NF-kappaB. Furthermore, some HCMV tegument proteins antagonize NF-kappaB activation by targeting the key components of NF-kappaB signaling at late stages of infection. In this review, we summarize the recent findings on the relationship between HCMV and NF-kappaB signaling, focusing, in particular, on the viral mechanisms by which the NF-kappaB signaling pathway is inhibited.

Herpesvirus-encoded Deubiquitinating Proteases and Their Roles in Regulating Immune Signaling Pathways

Viruses interact with the host ubiquitination system in a variety of ways. Viral proteins are often a substrate for ubiquitination, which leads to proteasomal degradation. Viruses also have functions to modify the cellular ubiquitination machinery. Recently, deubiquitinating protease (DUB) activity has been found in many viral proteins. In herpesviruses, the DUB domain is found within the large tegument protein, which is conserved in all members of the herpesvirus family. Although a limited number of viral and cellular targets have been identified to date, accumulating evidence shows that herpesviral DUBs may primarily target key cellular regulators of immune signaling pathways to promote viral replication. In this review, we summarize the recent findings on viral DUBs. In particular, we focus on the herpesviral DUBs and their targets, and discuss their potential roles in the regulation of immune signaling pathways.

Possible Roles of UL112-113 Proteins in Human Cytomegalovirus DNA Replication

DNA replication of human cytomegalovirus (HCMV) is a highly regulated process that requires specific interactions between cis-acting lytic origin of replication (oriLyt) and trans-acting viral proteins. Formation of the replication initiation complex is also regulated by specific interactions among viral replication proteins. HCMV replication proteins include origin-binding proteins, core proteins that work in replication forks, and regulatory proteins that modulate host cell functions. This letter describes intriguing questions regarding how HCMV origin-binding proteins interact with oriLyt to initiate DNA replication and how the regulatory UL112-113 proteins, which are found only in beta-herpesviruses, function to promote viral DNA replication.

PIAS1 interacts with the KRAB zinc finger protein, ZNF133, via zinc finger motifs and regulates its transcriptional activity

Zinc finger protein 133 (ZNF133) is composed of a Kruppel-associated box (KRAB) domain and 14 contiguous zinc finger motifs. ZNF133 is regarded as a transcriptional repressor because the KRAB domain has potent repressor activity and the zinc finger motifs usually act in binding to DNA. However, we found that the zinc finger motifs of ZNF133 also possessed transcriptional repressor activity. By two-hybrid screening assay, we found that the zinc finger motifs of ZNF133 interacted with protein inhibitor of activated STAT1 (PIAS1). PIAS1 enhanced the transcriptional repression activity of ZNF133 through the zinc finger motifs. This effect of PIAS1 was relieved by an inhibitor of the histone deacetylases (HDACs). These results demonstrate that the transcriptional repressor activity of ZNF133 is regulated by both the KRAB domain and the zinc finger motifs, and that the repressive effect by zinc finger motifs is mediated by PIAS1.