Murine gammaherpesvirus 68 ORF20 induces cell-cycle arrest in G2 by inhibiting the Cdc2-cyclin B complex

RNA-seq reveals role of cell-cycle regulating genes in the pathogenicity of a field very virulent infectious bursal disease virus

Infectious bursal disease virus (IBDV) infection causes highly contagious and immunosuppressive disease in poultry. The thymus, serving as the primary organ for T cell maturation and differentiation, plays an important role in the pathogenicity of IBDV in the infected chickens. However, there are no reports on the molecular pathogenesis of IBDV in the thymus currently. The aim of the study was to elucidate the molecular mechanisms underlying the pathogenicity of a field very virulent (vv) IBDV strain NN1172 in the thymus of SPF chickens using integrative transcriptomic and proteomic analyses. Our results showed that a total of 4,972 Differentially expressed genes (DEGs) in the thymus of NN1172-infected chickens by transcriptomic analysis, with 2,796 up-regulated and 2,176 down-regulated. Meanwhile, the proteomic analysis identified 726 differentially expressed proteins (DEPs) in the infected thymus, with 289 up-regulated and 437 down-regulated. Overall, a total of 359 genes exhibited differentially expression at both mRNA and protein levels, with 134 consistently up-regulated and 198 genes consistently down-regulated, as confirmed through a comparison of the RNA-seq and the proteomic datasets. The gene ontology (GO) analysis unveiled the involvement of both DEGs and DEPs in diverse categories encompassing cellular components, biological processes, and molecular functions in the pathological changes in IBDV-infected thymus. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that the host mainly displayed severely disruption of cell survival/repair, proliferation and metabolism pathway, meanwhile, the infection triggers antiviral immune activation with a potential emphasis on the MDA5 pathway. Network inference analysis identified seven core hub genes, which include CDK1, TYMS, MCM5, KIF11, CCNB2, MAD2L1, and MCM4. These genes are all associated with cell-cycle regulating pathway and are likely key mediators in the pathogenesis induced by NN1172 infection in the thymus. This study discovered dominant pathways and genes which enhanced our understanding of the molecular mechanisms underlying IBDV pathogenesis in the thymus.

Mechanism of herpesvirus UL24 protein regulating viral immune escape and virulence

Herpesviruses have evolved a series of abilities involved in the process of host infection that are conducive to virus survival and adaptation to the host, such as immune escape, latent infection, and induction of programmed cell death for sustainable infection. The herpesvirus gene UL24 encodes a highly conserved core protein that plays an important role in effective viral infection. The UL24 protein can inhibit the innate immune response of the host by acting on multiple immune signaling pathways during virus infection, and it also plays a key role in the proliferation and pathogenicity of the virus in the later stage of infection. This article reviews the mechanism by which the UL24 protein mediates herpesvirus immune escape and its effects on viral proliferation and virulence by influencing syncytial formation, DNA damage and the cell cycle. Reviewing these studies will enhance our understanding of the pathogenesis of herpesvirus infection and provide evidence for new strategies to combat against viral infection.

Integrative systems immunology uncovers molecular networks of the cell cycle that stratify COVID-19 severity

Several perturbations in the number of peripheral blood leukocytes, such as neutrophilia and lymphopenia associated with Coronavirus disease 2019 (COVID-19) severity, point to systemic molecular cell cycle alterations during severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection. However, the landscape of cell cycle alterations in COVID-19 remains primarily unexplored. Here, we performed an integrative systems immunology analysis of publicly available proteome and transcriptome data to characterize global changes in the cell cycle signature of COVID-19 patients. We found significantly enriched cell cycle-associated gene co-expression modules and an interconnected network of cell cycle-associated differentially expressed proteins (DEPs) and genes (DEGs) by integrating the molecular data of 1469 individuals (981 SARS-CoV-2 infected patients and 488 controls [either healthy controls or individuals with other respiratory illnesses]). Among these DEPs and DEGs are several cyclins, cell division cycles, cyclin-dependent kinases, and mini-chromosome maintenance proteins. COVID-19 patients partially shared the expression pattern of some cell cycle-associated genes with other respiratory illnesses but exhibited some specific differential features. Notably, the cell cycle signature predominated in the patients' blood leukocytes (B, T, and natural killer cells) and was associated with COVID-19 severity and disease trajectories. These results provide a unique global understanding of distinct alterations in cell cycle-associated molecules in COVID-19 patients, suggesting new putative pathways for therapeutic intervention.

The KSHV ORF20 Protein Interacts with the Viral Processivity Factor ORF59 and Promotes Viral Reactivation

Upon Kaposi's Sarcoma-associated herpesvirus (KSHV) lytic reactivation, rapid and widespread amplification of viral DNA (vDNA) triggers significant nuclear reorganization. As part of this striking shift in nuclear architecture, viral replication compartments are formed as sites of lytic vDNA production along with remarkable spatial remodeling and the relocalization of cellular and viral proteins. These viral replication compartments house several lytic gene products that coordinate viral gene expression, vDNA replication, and nucleocapsid assembly. The viral proteins and mechanisms that regulate this overhaul of the nuclear landscape during KSHV replication remain largely unknown. KSHV's ORF20 is a widely conserved lytic gene among all herpesviruses, suggesting it may have a fundamental contribution to the progression of herpesviral infection. Here, we utilized a promiscuous biotin ligase proximity labeling method to identify the proximal interactome of ORF20, which includes several replication-associated viral proteins, one of which is ORF59, the KSHV DNA processivity factor. Using coimmunoprecipitation and immunofluorescence assays, we confirmed the interaction between ORF20 and ORF59 and tracked the localization of both proteins to KSHV replication compartments. To further characterize the function of ORF20, we generated an ORF20-deficient KSHV and compared its replicative fitness to that of wild-type virus. Virion production was significantly diminished in the ORF20-deficient virus as observed by supernatant transfer assays. Additionally, we tied this defect in viable virion formation to a reduction in viral late gene expression. Lastly, we observed an overall reduction in vDNA replication in the ORF20-deficient virus, implying a key role for ORF20 in the regulation of lytic replication. Taken together, these results capture the essential role of KSHV ORF20 in progressing viral lytic infection by regulating vDNA replication alongside other crucial lytic proteins within KSHV replication compartments. IMPORTANCE Kaposi's Sarcoma-associated herpesvirus (KSHV) is a herpesvirus that induces lifelong infection, and as such, its lytic replication is carefully controlled to allow for efficient dissemination from its long-term reservoir and for the spread of the virus to new hosts. Viral DNA replication involves many host and viral proteins, coordinating both in time and space to successfully progress through the viral life cycle. Yet, this process is still not fully understood. We investigated the role of the poorly characterized viral protein ORF20, and through proximity labeling, we found that ORF20 interacts with ORF59 in replication compartments and affects DNA replication and subsequent steps of the late viral life cycle. Collectively, these results provide insights into the possible contribution of ORF20 to the complex lytic DNA replication process and suggest that this highly conserved protein may be an important modulator of this key viral mechanism.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

The interferon-stimulated gene product oligoadenylate synthetase-like protein enhances replication of Kaposi's sarcoma-associated herpesvirus (KSHV) and interacts with the KSHV ORF20 protein

Kaposi’s sarcoma-associated herpesvirus (KSHV) is one of the few oncogenic human viruses known to date. Its large genome encodes more than 85 proteins and includes both unique viral proteins as well as proteins conserved amongst herpesviruses. KSHV ORF20 is a member of the herpesviral core UL24 family, but the function of ORF20 and its role in the viral life cycle is not well understood. ORF20 encodes three largely uncharacterized isoforms, which we found were localized predominantly in the nuclei and nucleoli. Quantitative affinity purification coupled to mass spectrometry (q-AP-MS) identified numerous specific interacting partners of ORF20, including ribosomal proteins and the interferon-stimulated gene product (ISG) oligoadenylate synthetase-like protein (OASL). Both endogenous and transiently transfected OASL co-immunoprecipitated with ORF20, and this interaction was conserved among all ORF20 isoforms and multiple ORF20 homologs of the UL24 family in other herpesviruses. Characterization of OASL interacting partners by q-AP-MS identified a very similar interactome to that of ORF20. Both ORF20 and OASL copurified with 40S and 60S ribosomal subunits, and when they were co-expressed, they associated with polysomes. Although ORF20 did not have a global effect on translation, ORF20 enhanced RIG-I induced expression of endogenous OASL in an IRF3-dependent but IFNAR-independent manner. OASL has been characterized as an ISG with antiviral activity against some viruses, but its role for gammaherpesviruses was unknown. We show that OASL and ORF20 mRNA expression were induced early after reactivation of latently infected HuARLT-rKSHV.219 cells. Intriguingly, we found that OASL enhanced infection of KSHV. During infection with a KSHV ORF20stop mutant, however, OASL-dependent enhancement of infectivity was lost. Our data have characterized the interaction of ORF20 with OASL and suggest ORF20 usurps the function of OASL to benefit KSHV infection.

The herpesviruses are a family of large double-stranded DNA viruses that cause a variety of illnesses from chicken pox to cancer. Kaposi’s sarcoma-associated herpesvirus (KSHV) is a cancer-causing herpesvirus and can lead to development of Kaposi’s sarcoma, a major form of cancer in HIV-positive patients. As for all herpesviruses, infection with KSHV is lifelong. Exactly how KSHV initiates and maintains its infection is still not well understood, but it must manipulate the host cell to establish favorable conditions. Likewise, the host has developed a complicated system to fight off invaders, which includes the production of interferon-stimulated gene products. We have now found that KSHV exploits one such host cell protein, the oligoadenylate synthetase-like protein (OASL). Rather than OASL acting as an antiviral protein as it does during many other viral infections, KSHV appears to have found a way to utilize OASL for its own benefit. The KSHV protein ORF20 interacts with OASL, they co-localize in nucleoli, and both ORF20 and OASL associate and purify with components of the cellular translational machinery. This may help viral infection by selectively controlling protein production.

Duck enteritis virus (DEV) UL54 protein, a novel partner, interacts with DEV UL24 protein

Background

UL24 is a multifunctional protein that is conserved among alphaherpesviruses and is believed to play an important role in viral infection and replication.

Results

In this paper, to investigate putative UL24-binding proteins and to explore the functional mechanisms of DEV UL24, yeast two-hybrid (Y2H) was carried out, and further verified the interaction between UL24 and partners by co-immunoprecipitation and fluorescence microscopy experiments. Interaction partners of UL24 protein were screened by yeast two-hybrid (Y2H) with the cDNA library of DEV-CHv strain post-infection DEF cells. A novel partner, DEV UL54 protein, was discovered by Y2H screening and bioinformatic. Co-immunoprecipitation experiments suggested that DEV UL24 interacted with UL54 proteins. And distribution of a part of UL54 protein was changed from nucleus to cytoplasm in DF-1 cells of co-subcellular localization experiments which also showed that DEV UL24 interacted with UL54 proteins.

Conclusions

The interaction between the DEV UL24 and UL54 proteins was discovered for the first time. Thus, DEV UL54 protein as a novel partner interacted with DEV UL24 protein.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0830-5) contains supplementary material, which is available to authorized users.

Unconserved C terminal of human cytomegalovirus tegument protein pUL76 elicits nuclear aggresome formation and induces DNA damage in transfected cells

BACKGROUND

The HCMV UL76 gene is a member of UL24 family in herpes virus and encodes a highly conserved herpes virus protein. Inherited from common ancestor, members of Herpes_UL24 family encode proteins with a conserved N terminal and varied in C terminal region. To define which region (conserved N terminal or unconserved C terminal) of UL76 was responsible for its ability to induce DNA damage and aggresome formation, the wild-type UL76 gene and two deletion mutants were transfected to cells and analyzed by immunofluorescent staining, Western blotting and comet assay.

RESULTS

We report that the EGFP-fusion proteins present as globular aggresomes and colocalize with γ-H2AX in cells transfected with either pEGFP-UL76 or pEGFP-UL76C. The relative expression level of γ-H2AX and percentage of cells with comet tails were elevated in pEGFP-UL76 or pEGFP-UL76C transfection groups compared to the control.

CONCLUSION

Our findings suggest that the unconserved C terminal (not the conserved N terminal) of pUL76 was sufficient to induce DNA damage and aggresome formation in transfected cells.

Cell cycle regulation during viral infection

To replicate their genomes in cells and generate new progeny, viruses typically require factors provided by the cells that they have infected. Subversion of the cellular machinery that controls replication of the infected host cell is a common activity of many viruses. Viruses employ different strategies to deregulate cell cycle checkpoint controls and modulate cell proliferation pathways. A number of DNA and RNA viruses encode proteins that target critical cell cycle regulators to achieve cellular conditions that are beneficial for viral replication. Many DNA viruses induce quiescent cells to enter the cell cycle; this is thought to increase pools of deoxynucleotides and thus, facilitate viral replication. In contrast, some viruses can arrest cells in a particular phase of the cell cycle that is favorable for replication of the specific virus. Cell cycle arrest may inhibit early cell death of infected cells, allow the cells to evade immune defenses, or help promote virus assembly. Although beneficial for the viral life cycle, virus-mediated alterations in normal cell cycle control mechanisms could have detrimental effects on cellular physiology and may ultimately contribute to pathologies associated with the viral infection, including cell transformation and cancer progression and maintenance. In this chapter, we summarize various strategies employed by DNA and RNA viruses to modulate the replication cycle of the virus-infected cell. When known, we describe how these virus-associated effects influence replication of the virus and contribute to diseases associated with infection by that specific virus.

Human cytomegalovirus gene UL76 induces IL-8 expression through activation of the DNA damage response

Human cytomegalovirus (HCMV), a β-herpesvirus, has evolved many strategies to subvert both innate and adaptive host immunity in order to ensure its survival and propagation within the host. Induction of IL-8 is particularly important during HCMV infection as neutrophils, primarily attracted by IL-8, play a key role in virus dissemination. Moreover, IL-8 has a positive effect in the replication of HCMV. This work has identified an HCMV gene (UL76), with the relevant property of inducing IL-8 expression at both transcriptional and protein levels. Up-regulation of IL-8 by UL76 results from activation of the NF-kB pathway as inhibition of both IKK-β activity or degradation of Ikβα abolishes the IL-8 induction and, concomitantly, expression of UL76 is associated with the translocation of p65 to the nucleus where it binds to the IL-8 promoter. Furthermore, the UL76-mediated induction of IL-8 requires ATM and is correlated with the phosphorylation of NEMO on serine 85, indicating that UL76 activates NF-kB pathway by the DNA Damage response, similar to the impact of genotoxic drugs. More importantly, a UL76 deletion mutant virus was significantly less efficient in stimulating IL-8 production than the wild type virus. In addition, there was a significant reduction of IL-8 secretion when ATM -/- cells were infected with wild type HCMV, thus, indicating that ATM is also involved in the induction of IL-8 by HCMV.

In conclusion, we demonstrate that expression of UL76 gene induces IL-8 expression as a result of the DNA damage response and that both UL76 and ATM have a role in the mechanism of IL-8 induction during HCMV infection. Hence, this work characterizes a new role of the activation of DNA Damage response in the context of host-pathogen interactions.

The importance of herpesviruses is evident by their prevalence in the human population and the diverse range of diseases that they provoke. Their ability to establish latency provides a compelling example of how herpesviruses successfully evade the immune system and manipulate cellular biology. One promising approach for the development of novel anti-viral strategies is to study viral proteins involved in these host-pathogen interactions. One example is the induction of the pro-inflammatory chemokine IL-8 by HCMV which enhances viral replication and dissemination of the virus by neutrophils. Here, we have identified HCMV UL76 gene, conserved in all herpesviruses, as an inducer of IL-8, and thus with an important impact on HCMV pathogenesis. The induction of IL-8 by UL76 results from activation of the DNA Damage response, which may also explain why UL76 also induces cell cycle arrest. These findings are a clear example of how viruses manipulate intracellular signaling pathways with different outcomes that will be beneficial for viral infection. Finally, the fact that UL76 is a non-homologous gene substantiates the premise that many such pathogen genes without homology may indeed have evolved for host manipulation, and are a repository of potential useful tools for experimental manipulation in health and disease.

Virus manipulation of cell cycle

Viruses depend on host cell resources for replication and access to those resources may be limited to a particular phase of the cell cycle. Thus manipulation of cell cycle is a commonly employed strategy of viruses for achieving a favorable cellular environment. For example, viruses capable of infecting nondividing cells induce S phase in order to activate the host DNA replication machinery and provide the nucleotide triphosphates necessary for viral DNA replication (Flemington in J Virol 75:4475-4481, 2001; Sullivan and Pipas in Microbiol Mol Biol Rev 66:179-202, 2002). Viruses have developed several strategies to subvert the cell cycle by association with cyclin and cyclin-dependent kinase complexes and molecules that regulate their activity. Viruses tend to act on cellular proteins involved in a network of interactions in a way that minimal protein-protein interactions lead to a major effect. The complex and interactive nature of intracellular signaling pathways controlling cell division affords many opportunities for virus manipulation strategies. Taking the maxim "Set a thief to catch a thief" as a counter strategy, however, provides us with the very same virus evasion strategies as "ready-made tools" for the development of novel antivirus therapeutics. The most obvious are attenuated virus vaccines with critical evasion genes deleted. Similarly, vaccines against viruses causing cancer are now being successfully developed. Finally, as viruses have been playing chess with our cell biology and immune responses for millions of years, the study of their evasion strategies will also undoubtedly reveal new control mechanisms and their corresponding cellular intracellular signaling pathways.

Female infertility in PDE3A(-/-) mice: polo-like kinase 1 (Plk1) may be a target of protein kinase A (PKA) and involved in meiotic arrest of oocytes from PDE3A(-/-) mice

Mechanisms of cAMP/PKA-induced meiotic arrest in oocytes are not completely identified. In cultured, G2/M-arrested PDE3A(-/-) murine oocytes, elevated PKA activity was associated with inactivation of Cdc2 and Plk1, and inhibition of phosphorylation of histone H3 (S10) and of dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15). In cultured WT oocytes, PKA activity was transiently reduced and then increased to that observed in PDE3A(-/-) oocytes; Cdc2 and Plk1 were activated, phosphorylation of histone H3 (S10) and dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15) were observed. In WT oocytes, PKAc were rapidly translocated into nucleus, and then to the spindle apparatus, but in PDE3A(-/-) oocytes, PKAc remained in the cytosol. Plk1 was reactivated by incubation of PDE3A(-/-) oocytes with PKA inhibitor, Rp-cAMPS. PDE3A was co-localized with Plk1 in WT oocytes, and co-immunoprecipitated with Plk1 in WT ovary and Hela cells. PKAc phosphorylated rPlk1 and Hela cell Plk1 and inhibited Plk1 activity in vitro. Our results suggest that PKA-induced inhibition of Plk1 may be critical in oocyte meiotic arrest and female infertility in PDE3A(-/-) mice.

MHV-68 Open Reading Frame 20 is a nonessential gene delaying lung viral clearance

Recently, it has been demonstrated that the MHV-68 ORF20-encoded gene product induces cell-cycle arrest at the G2/M phase, followed by apoptosis. To study the role of this conserved gene in vivo, two independent ORF20-deficient MHV-68 viruses and their revertants were constructed. As the replication in vitro of both mutants followed similar kinetics to that of the wild-type and revertant viruses, ORF20 is therefore a nonessential virus gene. No cell cycle arrest could be observed upon infection of cells with wild type MHV-68 or mutant viruses. In addition, no major differences were detected between mock- and virus-infected cells when protein and inactivation levels of the mitotic promoter factor cdc2/cyclinB were analyzed. Following intranasal infection, the recovery of mutant, revertant and wild-type viruses in the lungs was similar. With the ORF20-deficient viruses, however, there was a significant delay of four days in clearance of virus from the lungs. Surprisingly, the magnitude and cell population distribution in the exudates of the lung was essentially similar to mice infected with wild-type, revertant or ORF20-deleted viruses. Subsequent establishment of latency was normal for both mutants, demonstrating that ORF20 does not play a critical role in establishment of a persistent infection. These results indicate that while expression of ORF20 may impact on the pathogenicity of the infection, the observed induction of G2/M arrest in ORF20-expressing cells may not be the primary function of ORF20 in the context of viral infection.

Relocalization of upstream binding factor to viral replication compartments is UL24 independent and follows the onset of herpes simplex virus 1 DNA synthesis

Herpes simplex virus 1 (HSV-1) induces relocalization of several nucleolar proteins. We have found that, as for fibrillarin, the HSV-1-induced redistribution of two RNA polymerase I components, upstream binding factor (UBF) and RPA194, was independent of the viral protein UL24, which affects nucleolin localization. Nevertheless, the kinetics and sites of redistribution for fibrillarin and UBF differed. Interestingly, UBF remained associated with RPA194 during infection. Although UBF is redistributed to viral replication compartments during infection, we did not detect foci of UBF at early sites of viral DNA synthesis, suggesting that it may not be directly involved in this process at early times.

Differential importance of highly conserved residues in UL24 for herpes simplex virus 1 replication in vivo and reactivation

The UL24 gene of herpes simplex virus 1 (HSV-1) is widely conserved among all subfamilies of the Herpesviridae. It is one of only four HSV-1 genes for which mutations have been mapped that confer a syncytial plaque phenotype. In a mouse model of infection, UL24-deficient viruses exhibit reduced titres, particularly in neurons, and an apparent defect in reactivation from latency. There are several highly conserved residues in UL24; however, their importance in the role of UL24 in vivo is unknown. In this study, we compared virus strains with substitution mutations corresponding to the PD-(D/E)XK endonuclease motif of UL24 (vUL24-E99A/K101A) or a substitution of another highly conserved residue (vUL24-G121A). Both mutant viruses cause the formation of syncytial plaques at 39 degrees C; however, we found that the viruses differed dramatically when tested in a mouse model of infection. vUL24-E99A/K101A exhibited titres in the eye that were 10-fold lower than those of the wild-type virus KOS, and titres in trigeminal ganglia (TG) that were more than 2 log10 lower. Clinical signs were barely detectable with vUL24-E99A/K101A. Furthermore, the percentage of TG from which virus reactivated was also significantly lower for this mutant than for KOS. In contrast, vUL24-G121A behaved similarly to the wild-type virus in mice. These results are consistent with the endonuclease motif being important for the role of UL24 in vivo and also imply that the UL24 temperature-dependent syncytial plaque phenotype can be separated genetically from several in vivo phenotypes.

Human cytomegalovirus UL76 induces chromosome aberrations

Background

Human cytomegalovirus (HCMV) is known to induce chromosome aberrations in infected cells, which can lead to congenital abnormalities in infected fetuses. HCMV UL76 belongs to a conserved protein family from herpesviruses. Some reported roles among UL76 family members include involvement in virulence determination, lytic replication, reactivation of latent virus, modulation of gene expression, induction of apoptosis, and perturbation of cell cycle progression, as well as potential nuclease activity. Previously, we have shown that stable expression of UL76 inhibits HCMV replication in glioblastoma cells.

Methods

To examine chromosomal integrity and the DNA damage signal γ-H2AX in cells constitutively expressing UL76, immunofluorescent cell staining and Western blotting were performed. The comet assay was employed to assess DNA breaks in cells transiently expressing UL76.

Results

We report that stably transfected cells expressing UL76 developed chromosome aberrations including micronuclei and misaligned chromosomes, lagging and bridging. In mitotic cells expressing UL76, aberrant spindles were increased compared to control cells. However, cells with supernumerary centrosomes were marginally increased in UL76-expressing cells relative to control cells. We further demonstrated that UL76-expressing cells activated the DNA damage signal γ-H2AX and caused foci formation in nuclei. In addition, the number of cells with DNA breaks increased in proportion to UL76 protein levels.

Conclusion

Our findings suggest that the virus-associated protein UL76 induces DNA damage and the accumulation of chromosome aberrations.

The conserved N-terminal domain of herpes simplex virus 1 UL24 protein is sufficient to induce the spatial redistribution of nucleolin

UL24 is widely conserved among herpesviruses but its function during infection is poorly understood. Previously, we discovered a genetic link between UL24 and the herpes simplex virus 1-induced dispersal of the nucleolar protein nucleolin. Here, we report that in the absence of viral infection, transiently expressed UL24 accumulated in both the nucleus and the Golgi apparatus. In the majority of transfected cells, nuclear staining for UL24 was diffuse, but a minor staining pattern, whereby UL24 was present in nuclear foci corresponding to nucleoli, was also observed. Expression of UL24 correlated with the dispersal of nucleolin. This dispersal did not appear to be a consequence of a general disaggregation of nucleoli, as foci of fibrillarin staining persisted in cells expressing UL24. The conserved N-terminal region of UL24 was sufficient to cause this change in subcellular distribution of nucleolin. Interestingly, a bipartite nuclear localization signal predicted within the C terminus of UL24 was dispensable for nuclear localization. None of the five individual UL24 homology domains was required for nuclear or Golgi localization, but deletion of these domains resulted in the loss of nucleolin-dispersal activity. We determined that a nucleolar-targeting signal was contained within the first 60 aa of UL24. Our results show that the conserved N-terminal domain of UL24 is sufficient to specifically induce dispersal of nucleolin in the absence of other viral proteins or virus-induced cellular modifications. These results suggest that UL24 directly targets cellular factors that affect the composition of nucleoli.