Bovine herpesvirus 1 tegument protein VP22 interacts with histones, and the carboxyl terminus of VP22 is required for nuclear localization

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Mechanism of herpesvirus protein kinase UL13 in immune escape and viral replication

Upon infection, the herpes viruses create a cellular environment suitable for survival, but innate immunity plays a vital role in cellular resistance to viral infection. The UL13 protein of herpesviruses is conserved among all herpesviruses and is a serine/threonine protein kinase, which plays a vital role in escaping innate immunity and promoting viral replication. On the one hand, it can target various immune signaling pathways in vivo, such as the cGAS-STING pathway and the NF-κB pathway. On the other hand, it phosphorylates regulatory many cellular and viral proteins for promoting the lytic cycle. This paper reviews the research progress of the conserved herpesvirus protein kinase UL13 in immune escape and viral replication to provide a basis for elucidating the pathogenic mechanism of herpesviruses, as well as providing insights into the potential means of immune escape and viral replication of other herpesviruses that have not yet resolved the function of it.

VP8, the Major Tegument Protein of Bovine Herpesvirus-1, Is Partially Packaged during Early Tegument Formation in a VP22-Dependent Manner

Bovine herpesvirus-1 (BoHV-1) is a major cause of rhinotracheitis and vulvovaginitis in cattle. VP8, the major tegument protein of BoHV-1, is essential for viral replication in the host. VP8 is phosphorylated by the viral kinase US3, mediating its translocation to the cytoplasm. VP8 remains nuclear when not phosphorylated. Interestingly, VP8 has a significant presence in mature BoHV-1YmVP8, in which the VP8 phosphorylation sites are mutated. This suggests that VP8 might be packaged during primary envelopment of BoHV-1. This was investigated by mass spectrometry and Western blotting, which showed VP8, as well as VP22, to be constituents of the primary enveloped virions. VP8 and VP22 were shown to interact via co-immunoprecipitation experiments, in both BoHV-1-infected and VP8-transfected cells. VP8 and VP22 also co-localised with one another and with nuclear lamin-associated protein 2 in BoHV-1-infected cells, suggesting an interaction between VP8 and VP22 in the perinuclear region. In cells infected with VP22-deleted BoHV-1 (BoHV-1ΔUL49), VP8 was absent from the primary enveloped virions, implying that VP22 might be critical for the early packaging of VP8. In conclusion, a novel VP22-dependent mechanism for packaging of VP8 was identified, which may be responsible for a significant amount of VP8 in the viral particle.

The PirB toxin protein from Vibrio parahaemolyticus induces apoptosis in hemocytes of Penaeus vannamei

Acute hepatopancreatic necrosis disease (AHPND) is a major debilitating disease that causes massive shrimp death resulting in substantial economic losses in shrimp aquaculture. The Pir toxin proteins secreted by a unique strain of Vibrio parahaemolyticus play an essential role in the pathogenesis of AHPND. At present, most studies on the effects of Pir toxin proteins in shrimp focus on digestive tissues or organs such as hepatopancreas, stomach, etc., with none on the immune organs. In the present study, two recombinant Pir toxin proteins (rPirA and rPirB) of V. parahaemolyticus were expressed with rPirB shown to enter shrimp hemocytes. Employing pull-down and LC-MS/MS analysis, GST-rPirB was found to interact with 13 proteins in hemocytes, including histone H3 and histone H4 and among which histone H4 had the highest protein score. Further analysis using GST pull-down and Far-Western blot analysis revealed that rPirB could interact with histone H4. In addition, using the purified nucleosome protein from Drosophila S2 cells, it was found that PirB protein could specifically bind to histones. When flow cytometry was applied, it was observed that the interaction between PirB and histones in shrimp hemocytes induces apoptosis, which results in the dephosphorylation of Serine 10 in histone H3. Collectively, the current study shows that in addition to its effect on the digestive tract of shrimp, the PirB toxin protein interacts with histones to affect the phosphorylation of histone H3-S10, thereby inducing apoptosis.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

Identification of Marek's Disease Virus VP22 Tegument Protein Domains Essential for Virus Cell-to-Cell Spread, Nuclear Localization, Histone Association and Cell-Cycle Arrest

VP22 is a major tegument protein of alphaherpesviruses encoded by the UL49 gene. Two properties of VP22 were discovered by studying Marek's disease virus (MDV), the Mardivirus prototype; it has a major role in virus cell-to-cell spread and in cell cycle modulation. This 249 AA-long protein contains three regions including a conserved central domain. To decipher the functional VP22 domains and their relationships, we generated three series of recombinant MDV genomes harboring a modified UL49 gene and assessed their effect on virus spread. Mutated VP22 were also tested for their ability to arrest the cell cycle, subcellular location and histones copurification after overexpression in cells. We demonstrated that the N-terminus of VP22 associated with its central domain is essential for virus spread and cell cycle modulation. Strikingly, we demonstrated that AAs 174-190 of MDV VP22 containing the end of a putative extended alpha-3 helix are essential for both functions and that AAs 159-162 located in the putative beta-strand of the central domain are mandatory for cell cycle modulation. Despite being non-essential, the 59 C-terminal AAs play a role in virus spread efficiency. Interestingly, a positive correlation was observed between cell cycle modulation and VP22 histones association, but none with MDV spread.

The Involvement of Histone H3 Acetylation in Bovine Herpesvirus 1 Replication in MDBK Cells

During bovine herpesvirus 1 (BoHV-1) productive infection in cell cultures, partial of intranuclear viral DNA is present in nucleosomes, and viral protein VP22 associates with histones and decreases histone H4 acetylation, indicating the involvement of histone H4 acetylation in virus replication. In this study, we demonstrated that BoHV-1 infection at the late stage (at 24 h after infection) dramatically decreased histone H3 acetylation [at residues K9 (H3K9ac) and K18 (H3K18ac)], which was supported by the pronounced depletion of histone acetyltransferases (HATs) including CBP/P300 (CREB binding protein and p300), GCN5L2 (general control of amino acid synthesis yeast homolog like 2) and PCAF (P300/CBP-associated factor). The depletion of GCN5L2 promoted by virus infection was partially mediated by ubiquitin-proteasome pathway. Interestingly, the viral replication was enhanced by HAT (histone acetyltransferase) activator CTPB [N-(4-Chloro-3-trifluoromethylphenyl)-2-ethoxy-6-pentadecylbenzamide], and vice versa, inhibited by HAT inhibitor Anacardic acid (AA), suggesting that BoHV-1 may take advantage of histone acetylation for efficient replication. Taken together, we proposed that the HAT-dependent histone H3 acetylation plays an important role in BoHV-1 replication in MDBK (Madin-Darby bovine kidney) cells.

Intracellular localization of Equine herpesvirus type 1 tegument protein VP22

Intracellular localization of Equine herpesvirus type 1 (EHV-1) tegument protein VP22 was examined by using a plasmid that expressed VP22 fused with an enhanced green fluorescent protein (EGFP). Also a recombinant EHV-1 expressing VP22 fused with a red fluorescent protein (mCherry) was constructed to observe the localization of VP22 in infected cells. When EGFP-fused VP22 was overexpressed in the cells, VP22 localized in the cytoplasm and nucleus. Live cell imaging suggested that the fluorescently tagged VP22 also localized in the cytoplasm and nucleus. These results show that VP22 localizes in the cytoplasm and nucleus independently of other viral proteins. Experiments with truncation mutants of pEGFP-VP22 suggested that 154-188 aa might be the nuclear localization signal of EHV-1 VP22.

Nuclear diacylglycerol lipase-α in rat brain cortical neurons: evidence of 2-arachidonoylglycerol production in concert with phospholipase C-β activity

In this report, we describe the localization of diacylglycerol lipase-α (DAGLα) in nuclei from adult cortical neurons, as assessed by double-immunofluorescence staining of rat brain cortical sections and purified intact nuclei and by western blot analysis of subnuclear fractions. Double-labeling assays using the anti-DAGLα antibody and NeuN combined with Hoechst staining showed that only nuclei of neuronal origin were DAGLα positive. At high resolution, DAGLα-signal displayed a punctate pattern in nuclear subdomains poor in Hoechst's chromatin and lamin B1 staining. In contrast, SC-35- and NeuN-signals (markers of the nuclear speckles) showed a high overlap with DAGLα within specific subdomains of the nuclear matrix. Among the members of the phospholipase C-β (PLCβ) family, PLCβ1, PLCβ2, and PLCβ4 exhibited the same distribution with respect to chromatin, lamin B1, SC-35, and NeuN as that described for DAGLα. Furthermore, by quantifying the basal levels of 2-arachidonoylglycerol (2-AG) by liquid chromatography and mass spectrometry (LC-MS), and by characterizing the pharmacology of its accumulation, we describe the presence of a mechanism for 2-AG production, and its PLCβ/DAGLα-dependent biosynthesis in isolated nuclei. These results extend our knowledge about subcellular distribution of neuronal DAGLα, providing biochemical grounds to hypothesize a role for 2-AG locally produced within the neuronal nucleus.

Cell cycle modulation by Marek's disease virus: the tegument protein VP22 triggers S-phase arrest and DNA damage in proliferating cells

Marek's disease is one of the most common viral diseases of poultry affecting chicken flocks worldwide. The disease is caused by an alphaherpesvirus, the Marek's disease virus (MDV), and is characterized by the rapid onset of multifocal aggressive T-cell lymphoma in the chicken host. Although several viral oncogenes have been identified, the detailed mechanisms underlying MDV-induced lymphomagenesis are still poorly understood. Many viruses modulate cell cycle progression to enhance their replication and persistence in the host cell, in the case of some oncogenic viruses ultimately leading to cellular transformation and oncogenesis. In the present study, we found that MDV, like other viruses, is able to subvert the cell cycle progression by triggering the proliferation of low proliferating chicken cells and a subsequent delay of the cell cycle progression into S-phase. We further identified the tegument protein VP22 (pUL49) as a major MDV-encoded cell cycle regulator, as its vector-driven overexpression in cells lead to a dramatic cell cycle arrest in S-phase. This striking functional feature of VP22 appears to depend on its ability to associate with histones in the nucleus. Finally, we established that VP22 expression triggers the induction of massive and severe DNA damages in cells, which might cause the observed intra S-phase arrest. Taken together, our results provide the first evidence for a hitherto unknown function of the VP22 tegument protein in herpesviral reprogramming of the cell cycle of the host cell and its potential implication in the generation of DNA damages.

Deletion of the herpes simplex virus 1 UL49 gene results in mRNA and protein translation defects that are complemented by secondary mutations in UL41

Herpes simplex virus 1 (HSV-1) virions, like those of all herpesviruses, contain a protein layer termed the tegument localized between the capsid and the envelope. VP22, encoded by the U(L)49 gene, is one of the most abundant tegument proteins in HSV-1 virions. Studies with a U(L)49-null mutant showed that the absence of VP22 resulted in decreased protein synthesis at late times in infection. VP22 is known to form a tripartite complex with VP16 and vhs through direct interactions with VP16. Given that U(L)49-null mutants have been shown to acquire spontaneous secondary mutations in the U(L)41 gene, which encodes vhs, we hypothesized that VP22 and vhs may play antagonistic roles during HSV-1 infections. In the present study, we show that the protein synthesis defect observed in U(L)49-null virus infections was rescued by a secondary, compensatory frameshift mutation in U(L)41. A double mutant bearing a deletion of U(L)49 and a point mutation in vhs previously shown to specifically abrogate vhs's RNase activity also resulted in a rescue of protein synthesis. To determine whether the U(L)49(-) protein synthesis defect, and the rescue by secondary mutations in vhs, occurred at the mRNA and/or translational levels, quantitative reverse transcriptase PCR (qRT-PCR) and polysome analyses were performed. We found that the absence of VP22 caused a small decrease in mRNA levels as well as a defect in polysome assembly that was independent of mRNA abundance. Both defects were complemented by the secondary mutations in vhs, indicating functional interplay between VP22 and vhs in both accumulation and translation of viral mRNAs.

Herpes simplex virus 1 VP22 regulates translocation of multiple viral and cellular proteins and promotes neurovirulence

Herpes simplex virus 1 (HSV-1) protein VP22, encoded by the UL49 gene, is a major virion tegument protein. In the present study, we showed that VP22 was required for efficient redistribution of viral proteins VP16, VP26, ICP0, ICP4, and ICP27 and of cellular protein Hsc-70 to the cytoplasm of infected cells. We found that two dileucine motifs in VP22, at amino acids 235 and 236 and amino acids 251 and 252, were necessary for VP22 regulation of the proper cytoplasmic localization of these viral and cellular proteins. The dileucine motifs were also required for proper cytoplasmic localization of VP22 itself and for optimal expression of viral proteins VP16, VP22, ICP0, UL41, and glycoprotein B. Interestingly, a recombinant mutant virus with alanines substituted for the dileucines at amino acids 251 and 252 had a 50% lethal dose (LD(50)) for neurovirulence in mice following intracerebral inoculation about 10(3)-fold lower than the LD(50) of the repaired virus. Furthermore, the replication and spread of this mutant virus in the brains of mice following intracerebral inoculation were significantly impaired relative to those of the repaired virus. The ability of VP22 to regulate the localization and expression of various viral and cellular proteins, as shown in this study, was correlated with an increase in viral replication and neurovirulence in the experimental murine model. Thus, HSV-1 VP22 is a significant neurovirulence factor in vivo.

Characterization of the nuclear import and export signals, and subcellular transport mechanism of varicella-zoster virus ORF9

Varicella-zoster virus (VZV) open reading frame 9 (ORF9) mRNA is one of the most abundantly expressed messages during VZV infection. However, little is known concerning the function of ORF9 protein. Here, we found that transient expression of ORF9 fused to enhanced yellow fluorescent protein (EYFP) in COS-7 cells showed a predominantly cytoplasmic localization in the absence of other viral proteins. By constructing a series of ORF9 variants fused to EYFP, a bona fide bipartite nuclear localization signal of ORF9 was, for the first time, determined and mapped to aa 16-32 (RRKTTPSYSGQYRTARR). Additionally, the nuclear export signal (NES) was identified and found to be in a leucine-rich region at aa 103-117 (LRHELVEDAVYENPL). Finally, ORF9 was demonstrated to be targeted to the cytoplasm through the functional NES by Ran and the chromosomal region maintenance 1-dependent pathway, and to the nucleus via an importin β-dependent pathway that does not require importin α5.

Herpes simplex virus type 1 tegument protein VP22 is capable of modulating the transcription of viral TK and gC genes via interaction with viral ICP0

VP22, a tegument protein of herpes simplex virus type 1 (HSV-1), is present in many copies in one virion and undergoes different types of post-translational modification. VP22 is believed to have certain functions in viral infection apart from virus assembly. Here we show that VP22 physically interacted with infected cell polypeptide 0 (ICP0) and colocalized in the nucleus, indicating that VP22 could be functionally involved in the modulation of viral transcription through interaction with ICP0. In the HSV-1 infection system and chloramphenicol acetyltransferase (CAT) transcriptional system, VP22-ICP0 interaction was confirmed to play a role in modulating the transcription of some viral genes and could be a factor in viral transcription, which is probably required in the transcriptional control of latent infection.

Intracellular trafficking of VP22 in bovine herpesvirus-1 infected cells

The intracellular trafficking of different VP22-enhanced yellow fluorescent protein (EYFP) fusion proteins expressed by bovine herpesvirus-1 (BHV-1) recombinants was examined by live-cell imaging. Our results demonstrate that (i) the fusion of EYFP to the C terminus of VP22 does not alter the trafficking of the protein in infected cells, (ii) VP22 expressed during BHV-1 infection translocates to the nucleus through three different pathways, namely early mitosis-dependent nuclear translocation, late massive nuclear translocation that follows a prolonged cytoplasmic stage of the protein in non-mitotic cells, and accumulation of a small subset of VP22 in discrete dot-like nuclear domains during its early cytoplasmic stage, (iii) the addition of the SV40 large-T-antigen nuclear localization signal (NLS) to VP22-EYFP abrogates its early cytoplasmic stage, and (iv) the VP22 (131)PRPR(134) NLS is not required for the late massive nuclear translocation of the protein, but this motif is essential for the targeting of VP22 to discrete dot-like nuclear domains during the early cytoplasmic stage. These results show that the amount of VP22 in the nucleus is precisely regulated at different stages of BHV-1 infection and suggest that the early pathways of VP22 nuclear accumulation may be more relevant to the infection process as the late massive nuclear influx starts when most of the viral progeny has already emerged from the cell.

Functional roles of the tegument proteins of herpes simplex virus type 1

Herpes virions consist of four morphologically distinct structures, a DNA core, capsid, tegument, and envelope. Tegument occupies the space between the nucleocapsid (capsid containing DNA core) and the envelope. A combination of genetic, biochemical and proteomic analysis of alphaherpes virions suggest the tegument contains in the order of 20 viral proteins. Historically the tegument has been described as amorphous but increasing evidence suggests there is an ordered addition of tegument during assembly. This review highlights the diverse roles, in addition to structural, that tegument plays during herpes viral replication using as an example herpes simplex virus type 1. Such diverse roles include: capsid transport during entry and egress; targeting of the capsid to the nucleus; regulation of transcription, translation and apoptosis; DNA replication; immune modulation; cytoskeletal assembly; nuclear egress of capsid; and viral assembly and final egress.

White spot syndrome virus protein ICP11: A histone-binding DNA mimic that disrupts nucleosome assembly

White spot syndrome virus (WSSV) is a large ( approximately 300 kbp), double-stranded DNA eukaryotic virus that has caused serious disease in crustaceans worldwide. ICP11 is the most highly expressed WSSV nonstructural gene/protein, which strongly suggests its importance in WSSV infection; but until now, its function has remained obscure. We show here that ICP11 acts as a DNA mimic. In crystal, ICP11 formed a polymer of dimers with 2 rows of negatively charged spots that approximated the duplex arrangement of the phosphate groups in DNA. Functionally, ICP11 prevented DNA from binding to histone proteins H2A, H2B, H3, and H2A.x, and in hemocytes from WSSV-infected shrimp, ICP11 colocalized with histone H3 and activated-H2A.x. These observations together suggest that ICP11 might interfere with nucleosome assembly and prevent H2A.x from fulfilling its critical function of repairing DNA double strand breaks. Therefore, ICP11 possesses a functionality that is unique among the handful of presently known DNA mimic proteins.

The varicella-zoster virus (VZV) ORF9 protein interacts with the IE62 major VZV transactivator

The varicella-zoster virus (VZV) ORF9 protein is a member of the herpesvirus UL49 gene family but shares limited identity and similarity with the UL49 prototype, herpes simplex virus type 1 VP22. ORF9 mRNA is the most abundantly expressed message during VZV infection; however, little is known concerning the functions of the ORF9 protein. We have found that the VZV major transactivator IE62 and the ORF9 protein can be coprecipitated from infected cells. Yeast two-hybrid analysis localized the region of the ORF9 protein required for interaction with IE62 to the middle third of the protein encompassing amino acids 117 to 186. Protein pull-down assays with GST-IE62 fusion proteins containing N-terminal IE62 sequences showed that amino acids 1 to 43 of the acidic transcriptional activation domain of IE62 can bind recombinant ORF9 protein. Confocal microscopy of transiently transfected cells showed that in the absence of other viral proteins, the ORF9 protein was localized in the cytoplasm while IE62 was localized in the nucleus. In VZV-infected cells, the ORF9 protein was localized to the cytoplasm whereas IE62 exhibited both nuclear and cytoplasmic localization. Cotransfection of plasmids expressing ORF9, IE62, and the viral ORF66 kinase resulted in significant colocalization of ORF9 and IE62 in the cytoplasm. Coimmunoprecipitation experiments with antitubulin antibodies indicate the presence of ORF9-IE62-tubulin complexes in infected cells. Colocalization of ORF9 and tubulin in transfected cells was visualized by confocal microscopy. These data suggest a model for ORF9 protein function involving complex formation with IE62 and possibly other tegument proteins in the cytoplasm at late times in infection.

Characterization of a UL49-null mutant: VP22 of herpes simplex virus type 1 facilitates viral spread in cultured cells and the mouse cornea

Herpes simplex virus type 1 (HSV-1) virions, like those of all herpesviruses, contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. The HSV-1 tegument is composed of at least 20 different viral proteins of various stoichiometries. VP22, the product of the U(L)49 gene, is one of the most abundant tegument proteins and is conserved among the alphaherpesviruses. Although a number of interesting biological properties have been attributed to VP22, its role in HSV-1 infection is not well understood. In the present study we have generated both a U(L)49-null virus and its genetic repair and characterized their growth in both cultured cells and the mouse cornea. While single-step growth analyses indicated that VP22 is dispensable for virus replication at high multiplicities of infection (MOIs), analyses of plaque morphology and intra- and extracellular multistep growth identified a role for VP22 in viral spread during HSV-1 infection at low MOIs. Specifically, VP22 was not required for either virion infectivity or cell-cell spread but was required for accumulation of extracellular virus to wild-type levels. We found that the absence of VP22 also affected virion composition. Intracellular virions generated by the U(L)49-null virus contained reduced amounts of ICP0 and glycoproteins E and D compared to those generated by the wild-type and U(L)49-repaired viruses. In addition, viral spread in the mouse cornea was significantly reduced upon infection with the U(L)49-null virus compared to infection with the wild-type and U(L)49-repaired viruses, identifying a role for VP22 in viral spread in vivo as well as in vitro.

Characterization of the nuclear localization and nuclear export signals of bovine herpesvirus 1 VP22

The bovine herpesvirus 1 (BHV-1) tegument protein VP22 is predominantly localized in the nucleus after viral infection. To analyze subcellular localization in the absence of other viral proteins, a plasmid expressing BHV-1 VP22 fused to enhanced yellow fluorescent protein (EYFP) was constructed. The transient expression of VP22 fused to EYFP in COS-7 cells confirmed the predominant nuclear localization of VP22. Analysis of the amino acid sequence of VP22 revealed that it does not have a classical nuclear localization signal (NLS). However, by constructing a series of deletion derivatives, we mapped the nuclear targeting domain of BHV-1 VP22 to amino acids (aa) 121 to 139. Furthermore, a 4-aa motif, 130PRPR133, was able to direct EYFP and an EYFP dimer (dEYFP) or trimer (tEYFP) predominantly into the nucleus, whereas a deletion or mutation of this arginine-rich motif abrogated the nuclear localization property of VP22. Thus, 130PRPR133 is a functional nonclassical NLS. Since we observed that the C-terminal 68 aa of VP22 mediated the cytoplasmic localization of EYFP, an analysis was performed on these C-terminal amino acid sequences, and a leucine-rich motif, 204LDRMLKSAAIRIL216, was detected. Replacement of the leucines in this putative nuclear export signal (NES) with neutral amino acids resulted in an exclusive nuclear localization of VP22. Furthermore, this motif was able to localize EYFP and dEYFP in the cytoplasm, and the nuclear export function of this NES could be blocked by leptomycin B. This demonstrates that this leucine-rich motif is a functional NES. These data represent the first identification of a functional NLS and NES in a herpesvirus VP22 homologue.

Nuclear and Mitochondrial Localization Signals Overlap within Bovine Herpesvirus 1 Tegument Protein VP22

VP22, a tegument protein of bovine herpesvirus 1, accumulates in the nucleus of infected and transiently transfected cells. Previous studies indicated a possible regulatory function of VP22 within nuclei, but how VP22 enters nuclei is unknown. Despite the abundance of basic residues within this protein, no classic nuclear localization signal (NLS) motif has been identified. To identify the signal directing nuclear accumulation, a series of truncations, internal deletions, and point mutations were constructed. Fluorescence microscopy of cells transfected with VP22 constructs indicated that a sequence of 103 residues is necessary and sufficient for nuclear localization. This NLS sequence is conformation-sensitive in contrast to a classical sequential NLS. Energy depletion assays and co-immunoprecipitation suggested that this NLS sequence also binds histone H4, resulting in nuclear retention of VP22. In addition, a mitochondrial targeting sequence was identified at the C-terminal 49 amino acids, which overlapped the sequence required for nuclear targeting. Our findings demonstrate the diversity of VP22 protein to localize within the cell and provide the opportunity for VP22 to direct cargo specifically to different subcellular compartments.

Bovine Herpesvirus VP22 Induces Apoptosis in Neuroblastoma Cells by Upregulating the Expression Ratio of Bax to Bcl-2

Herpesvirus tegument protein VP22 has been shown to have biotherapeutic potential in tumor gene therapy. Some studies indicate that VP22 may enhance the transfer efficiency of therapeutic proteins by delivering them to more cells while trafficking. Our previous study showed that bovine herpesvirus VP22 (BVP22) enhanced equine herpesvirus thymidine kinase-ganciclovir (Etk-GCV) suicide gene therapy by an unknown intracellular effect. In this study, the interaction between BVP22 and host tumor cells was studied in neuroblastoma NXS2 cells. Cell cycle analysis was performed to determine whether BVP22 possesses biotherapeutic potential by altering the cell cycle, making cells more sensitive to therapeutic genes. As a result, the cell cycle was not affected by the transfection of BVP22 into NXS2 cells. However, cytotoxicity induced by BVP22 was observed in NXS2 cells on the second and third days after transient transfection. Further, analyses of caspase-3 activity and apoptosis suggested that BVP22 induces apoptosis in host tumor cells by upregulating the expression ratio of Bax to Bcl-2.

VP16-dependent association of chromatin-modifying coactivators and underrepresentation of histones at immediate-early gene promoters during herpes simplex virus infection

During infection by herpes simplex virus type 1 (HSV-1), the virion protein VP16 activates the transcription of viral immediate-early (IE) genes. Genetic and biochemical assays have shown that the potent transcriptional activation domain of VP16 can associate with general transcription factors and with chromatin-modifying coactivator proteins of several types. The latter interactions are particularly intriguing because previous reports indicate that HSV-1 DNA does not become nucleosomal during lytic infection. In the present work, chemical cross-linking and immunoprecipitation assays were used to probe the presence of activators, general transcription factors, and chromatin-modifying coactivators at IE gene promoters during infection of HeLa cells by wild-type HSV-1 and by RP5, a viral strain lacking the VP16 transcriptional activation domain. The presence of VP16 and Oct-1 at IE promoters did not depend on the activation domain. In contrast, association of RNA polymerase II, TATA-binding protein, histone acetyltransferases (p300 and CBP), and ATP-dependent remodeling proteins (BRG1 and hBRM) with IE gene promoters was observed in wild-type infections but was absent or reduced in cells infected by RP5. In contrast to the previous evidence for nonnucleosomal HSV-1 DNA, histone H3 was found associated with viral DNA at early times of infection. Interestingly, histone H3 was underrepresented on IE promoters in a manner dependent on the VP16 activation domain. Thus, the VP16 activation domain is responsible for recruiting general transcription factors and coactivators to IE promoters and also for dramatically reducing the association of histones with those promoters.

Bovine herpesvirus tegument protein VP22 enhances thymidine kinase/ganciclovir suicide gene therapy for neuroblastomas compared to herpes simplex virus VP22

Herpesvirus tegument protein VP22 can enhance the effect of therapeutic proteins in gene therapy, such as thymidine kinase (tk) and p53; however, the mechanism is unclear or controversial. In this study, mammalian expression vectors carrying bovine herpesvirus 1 (BHV-1) VP22 (BVP22) or herpes simplex virus type 1 (HSV-1) VP22 (HVP22) and equine herpesvirus type 4 (EHV-4) tk (Etk) were constructed in order to evaluate and compare the therapeutic potentials of BVP22 and HVP22 to enhance Etk/ganciclovir (Etk/GCV) suicide gene therapy for neuroblastomas by GCV cytotoxicity assays and noninvasive bioluminescent imaging in vitro and in vivo. BVP22 enhanced Etk/GCV cytotoxicity compared to that with HVP22 both in vitro and in vivo. However, assays utilizing a mixture of parental and stably transfected cells indicated that the enhancement was detected only in transfected cells. Thus, the therapeutic potential of BVP22 and HVP22 in Etk/GCV suicide gene therapy in this tumor system is not due to VP22 delivery of Etk into surrounding cells but rather is likely due to an enhanced intracellular effect.

Herpes simplex virus type 1 tegument protein VP22 interacts with TAF-I proteins and inhibits nucleosome assembly but not regulation of histone acetylation by INHAT

Affinity chromatography was used to identify cellular proteins that interact with the herpes simplex virus (HSV) tegument protein VP22. Among a small set of proteins that bind specifically to VP22, we identified TAF-I (template-activating factor I), a chromatin remodelling protein and close homologue of the histone chaperone protein NAP-1. TAF-I has been shown previously to promote more ordered transfer of histones to naked DNA through a direct interaction with histones. TAF-I, as a subunit of the INHAT (inhibitor of acetyltransferases) protein complex, also binds to histones and masks them from being substrates for the acetyltransferases p300 and PCAF. Using in vitro assays for TAF-I activity in chromatin assembly, we show that VP22 inhibits nucleosome deposition on DNA by binding to TAF-I. We also observed that VP22 binds non-specifically to DNA, an activity that is abolished by TAF-I. However, the presence of VP22 does not affect the property of INHAT in inhibiting the histone acetyltransferase activity of p300 or PCAF in vitro. We speculate that this interaction could be relevant to HSV DNA organization early in infection, for example, by interfering with nucleosomal deposition on the genome. Consistent with this possibility was the observation that overexpression of TAF-I in transfected cells interferes with the progression of HSV-1 infection.

The viral control of cellular acetylation signaling

It is becoming clear that the post-translational modification of histone and non-histone proteins by acetylation is part of an important cellular signaling process controlling a wide variety of functions in both the nucleus and the cytoplasm. Recent investigations designate this signaling pathway as one of the primary targets of viral proteins after infection. Indeed, specific viral proteins have acquired the capacity to interact with cellular acetyltransferases (HATs) and deacetylases (HDACs) and consequently to disrupt normal acetylation signaling pathways, thereby affecting viral and cellular gene expression. Here we review the targeting of cellular HATs and HDACs by viral proteins and highlight different strategies adopted by viruses to control cellular acetylation signaling and to accomplish their life cycle.

Tat-controlled protein acetylation

Human immunodeficiency virus, type 1-encoded transactivator protein Tat is known to be a substrate of and to interact with several nuclear histone acetyltransferases (HATs). Here we show that Tat is a general inhibitor of histone acetylation by cellular HATs and that for at least one of them, the CREB-binding protein (CBP), it induces a substrate selectivity. Indeed, in the presence of Tat, the acetylation of histones by CBP was severely inhibited, while that of p53 and MyoD remained unaffected. The C-terminal domain of Tat, dispensable for the activation of viral transcription, was found to be necessary and sufficient to interfere with histone acetylation. These results demonstrate that Tat is able to selectively modulate cellular protein acetylation by nuclear HATs and therefore to take over this specific signaling system in cells.

Sequential localization of two herpes simplex virus tegument proteins to punctate nuclear dots adjacent to ICP0 domains

The subcellular localization of herpes simplex virus tegument proteins during infection is varied and complex. By using viruses expressing tegument proteins tagged with fluorescent proteins, we previously demonstrated that the major tegument protein VP22 exhibits a cytoplasmic localization, whereas the major tegument protein VP13/14 localizes to nuclear replication compartments and punctate domains. Here, we demonstrate the presence of a second minor population of VP22 in nuclear dots similar in appearance to those formed by VP13/14. We have constructed the first-described doubly fluorescence-tagged virus expressing VP22 and VP13/14 as fusion proteins with cyan fluorescent protein and yellow fluorescent protein, respectively. Visualization of both proteins within the same live infected cells has indicated that these two tegument proteins localize to the same nuclear dots but that VP22 appears there earlier than VP13/14. Further studies have shown that these tegument-specific dots are detectable as phase-dense bodies as early as 2 h after infection and that they are different from the previously described nuclear domains that contain capsid proteins. They are also different from the ICP0 domains formed at cellular nuclear domain 10 sites early in infection but, in almost all cases, are located in juxtaposition to these ICP0 domains. Hence, these tegument proteins join a growing number of proteins that are targeted to discrete nuclear domains in the herpesvirus-infected cell nucleus.

Protein transduction: a novel tool for tissue regeneration

Tissue regeneration in humans is limited and excludes vitals organs like heart and brain. Transformation experiments with oncogenes like T antigen have shown that retrodifferentiation of the respective cells is possible but hard to control. To bypass the risk of cancer formation a protein therapy approach has been developed. The transient delivery of proteins rather than genes could still induce terminally-differentiated cells to reenter the cell cycle. This approach takes advantage of protein-transducing domains that mediate the transfer of cargo proteins into cells. The goal of this brief review is to outline the basics of protein transduction and to discuss potential applications for tissue regeneration.

Herpes simplex virus tegument protein VP22 contains overlapping domains for cytoplasmic localization, microtubule interaction, and chromatin binding

We have previously shown that the 301-amino-acid herpes simplex virus tegument protein VP22 exhibits a range of subcellular localization patterns when expressed in isolation from other virus proteins. By using live-cell analysis of cells expressing green fluorescent protein (GFP)-tagged VP22 we have shown that when VP22 is first expressed in the cell it localizes to the cytoplasm, where, when present at high enough concentrations, it can assemble onto microtubules, causing them to bundle and become highly stabilized. In addition we have shown that when a cell expressing VP22 enters mitosis, the cytoplasmic population of VP22 translocates to the nucleus, where it efficiently binds mitotic chromatin. Here we have investigated the specific regions of the VP22 open reading frame required for these properties. Using GFP-VP22 as our starting molecule, we have constructed a range of N- and C-terminal truncations and analyzed their localization patterns in live cells. We show that the C-terminal 242 residues of VP22 are sufficient to induce microtubule bundling. Within this subregion, the C-terminal 89 residues contain a signal for cytoplasmic localization of the protein, while a larger region comprising the C-terminal 128 residues of the VP22 protein is required for mitotic chromatin binding. Furthermore, a central 100-residue domain of VP22 maintains the ability to bind microtubules without inducing bundling, suggesting that additional regions flanking this microtubule binding domain may be required to alter the microtubule network. Hence, the signals involved in dictating the complex localization patterns of VP22 are present in overlapping regions of the protein.

Improving DNA vaccine potency by linking Marek's disease virus type 1 VP22 to an antigen

We have previously employed an intercellular spreading strategy using herpes simplex virus type 1 (HSV-1) VP22 protein to enhance DNA vaccine potency because DNA vaccines lack the intrinsic ability to amplify in cells. Recently, studies have demonstrated that the protein encoded by UL49 of Marek's disease virus type 1 (MDV-1) exhibits some degree of homology to the HSV-1 VP22 protein and features the property of intercellular transport. We therefore generated a DNA vaccine encoding MDV-1 VP22 linked to a model antigen, human papillomavirus type 16 E7. We demonstrated that compared with mice vaccinated with DNA encoding wild-type E7, mice vaccinated with MDV-1 VP22/E7 DNA exhibited a significant increase in number of gamma-interferon-secreting, E7-specific CD8(+)-T-cell precursors as well as stronger tumor prevention and treatment effects. Furthermore, our data indicated that the antitumor effect was CD8 dependent. These results suggested that the development of vaccines encoding VP22 fused to a target antigen might be a promising strategy for improving DNA vaccine potency.