Transactivation of a viral target gene by herpes simplex virus ICP27 is posttranscriptional and does not require the endogenous promoter or polyadenylation site

Enhancement of HSV-1 cell-free virion release by the envelope protein gC

Glycoprotein C (gC), one of ∼12 HSV-1 envelope glycoproteins, carries out several important functions during infection, including the enhancement of virion attachment by binding to host cell heparan sulfate proteoglycans (HSPG). Here we report that gC can also enhance the release of cell-free progeny virions at the end of the infectious cycle. This activity was observed in multiple cellular contexts including Vero cells and immortalized human keratinocytes. In the absence of gC, progeny virions bound more tightly to infected cells, suggesting that gC promotes the detachment of virions from the infected cell surface. Given this finding, we analyzed the biochemical interactions that tether progeny virions to cells and report evidence for two distinct modes of binding. One is consistent with a direct interaction between gC and HSPG, whereas the other is gC-independent and likely does not involve HSPG. Together, our results i) identify a novel function for a long-studied HSV-1 glycoprotein, and ii) demonstrate that the extracellular release of HSV-1 virions is a dynamic process involving multiple viral and host components.

Herpes Simplex Virus-1 ICP27 Nuclear Export Signal Mutants Exhibit Cell Type-Dependent Deficits in Replication and ICP4 Expression

Herpes simplex virus type-1 (HSV-1) protein ICP27 is an essential immediate early (IE) protein that promotes the expression of viral early (E) and late (L) genes via multiple mechanisms. Our understanding of this complex regulatory protein has been greatly enhanced by the characterization of HSV-1 mutants bearing engineered alterations in the ICP27 gene. However, much of this analysis has been performed in interferon-deficient Vero monkey cells. Here, we assessed the replication of a panel of ICP27 mutants in several other cell types. Our analysis shows that mutants lacking ICP27's amino (N)-terminal nuclear export signal (NES) display a striking cell type-dependent growth phenotype, i.e., they grow semi-permissively in Vero and some other cells but are tightly blocked for replication in primary human fibroblasts and multiple human cell lines. This tight growth defect correlates with a failure of these mutants to replicate viral DNA. We also report that HSV-1 NES mutants are deficient in expressing the IE protein ICP4 at early times postinfection. Analysis of viral RNA levels suggests that this phenotype is due, at least in part, to a defect in the export of ICP4 mRNA to the cytoplasm. In combination, our results (i) show that ICP27's NES is critically important for HSV-1 replication in many human cells, and (ii) suggest that ICP27 plays a heretofore unappreciated role in the expression of ICP4. IMPORTANCE HSV-1 IE proteins drive productive HSV-1 replication. The major paradigm of IE gene induction, developed over many years, involves the parallel activation of the five IE genes by the viral tegument protein VP16, which recruits the host RNA polymerase II (RNAP II) to the IE gene promoters. Here, we provide evidence that ICP27 can enhance ICP4 expression early in infection. Because ICP4 is required for transcription of viral E and L genes, this finding may be relevant to understanding how HSV-1 enters and exits the latent state in neurons.

Release of HSV-1 Cell-Free Virions: Mechanisms, Regulation, and Likely Role in Human-Human Transmission

Herpes simplex virus type 1, or HSV-1, is a widespread human pathogen that replicates in epithelial cells of the body surface and then establishes latent infection in peripheral neurons. When HSV-1 replicates, viral progeny must be efficiently released to spread infection to new target cells. Viral spread occurs via two major routes. In cell-cell spread, progeny virions are delivered directly to cellular junctions, where they infect adjacent cells. In cell-free release, progeny virions are released into the extracellular milieu, potentially allowing the infection of distant cells. Cell-cell spread of HSV-1 has been well studied and is known to be important for in vivo infection and pathogenesis. In contrast, HSV-1 cell-free release has received less attention, and its significance to viral biology is unclear. Here, I review the mechanisms and regulation of HSV-1 cell-free virion release. Based on knowledge accrued in other herpesviral systems, I argue that HSV-1 cell-free release is likely to be tightly regulated in vivo. Specifically, I hypothesize that this process is generally suppressed as the virus replicates within the body, but activated to high levels at sites of viral reactivation, such as the oral mucosa and skin, in order to promote efficient transmission of HSV-1 to new human hosts.

Characterization of a Novel Viral Interleukin 8 (vIL-8) Splice Variant Encoded by Marek's Disease Virus

Marek’s disease virus (MDV) is a highly cell-associated oncogenic alphaherpesvirus that causes lymphomas in various organs in chickens. Like other herpesviruses, MDV has a large and complex double-stranded DNA genome. A number of viral transcripts are generated by alternative splicing, a process that drastically extends the coding capacity of the MDV genome. One of the spliced genes encoded by MDV is the viral interleukin 8 (vIL-8), a CXC chemokine that facilitates the recruitment of MDV target cells and thereby plays an important role in MDV pathogenesis and tumorigenesis. We recently identified a novel vIL-8 exon (vIL-8-E3′) by RNA-seq; however, it remained elusive whether the protein containing the vIL-8-E3′ is expressed and what role it may play in MDV replication and/or pathogenesis. To address these questions, we first generated recombinant MDV harboring a tag that allows identification of the spliced vIL-8-E3′ protein, revealing that it is indeed expressed. We subsequently generated knockout viruses and could demonstrate that the vIL-8-E3′ protein is dispensable for MDV replication as well as secretion of the functional vIL-8 chemokine. Finally, infection of chickens with this vIL-8-E3′ knockout virus revealed that the protein is not important for MDV replication and pathogenesis in vivo. Taken together, our study provides novel insights into the splice forms of the CXC chemokine of this highly oncogenic alphaherpesvirus.

Herpes simplex virus ICP27 regulates alternative pre-mRNA polyadenylation and splicing in a sequence-dependent manner

The herpes simplex virus (HSV) infected cell culture polypeptide 27 (ICP27) protein is essential for virus infection of cells. Recent studies suggested that ICP27 inhibits splicing in a gene-specific manner via an unknown mechanism. Here, RNA-sequencing revealed that ICP27 not only inhibits splicing of certain introns in <1% of cellular genes, but also can promote use of alternative 5' splice sites. In addition, ICP27 induced expression of pre-mRNAs prematurely cleaved and polyadenylated from cryptic polyadenylation signals (PAS) located in intron 1 or 2 of ∼1% of cellular genes. These previously undescribed prematurely cleaved and polyadenylated pre-mRNAs, some of which contain novel ORFs, were typically intronless, <2 Kb in length, expressed early during viral infection, and efficiently exported to cytoplasm. Sequence analysis revealed that ICP27-targeted genes are GC-rich (as are HSV genes), contain cytosine-rich sequences near the 5' splice site, and have suboptimal splice sites in the impacted intron, suggesting that a common mechanism is shared between ICP27-mediated alternative polyadenylation and splicing. Optimization of splice site sequences or mutation of nearby cytosines eliminated ICP27-mediated splicing inhibition, and introduction of C-rich sequences to an ICP27-insensitive splicing reporter conferred this phenotype, supporting the inference that specific gene sequences confer susceptibility to ICP27. Although HSV is the first virus and ICP27 is the first viral protein shown to activate cryptic PASs in introns, we suspect that other viruses and cellular genes also encode this function.

Functional comparison of herpes simplex virus 1 (HSV-1) and HSV-2 ICP27 homologs reveals a role for ICP27 in virion release

Numerous studies have focused on the regulatory functions of ICP27, an immediate-early (IE) protein of herpes simplex virus 1 (HSV-1). However, its homolog in HSV-2, termed ICP27t2, has been little studied. Here, we used two different approaches to functionally compare ICP27t2 and ICP27. In transfection-based assays, ICP27t2 closely resembled ICP27 in its capacity to enhance HSV-1 late gene expression, suppress the splicing of a viral intron, and complement the growth of an HSV-1 ICP27 null mutant. To study ICP27t2 in the context of viral infection, we engineered K2F1, an HSV-1 mutant that encodes ICP27t2 in place of ICP27. In Vero cells, K2F1 replicated with wild-type (WT) kinetics and yields, expressed delayed-early and late proteins normally, and was fully capable of activating several cellular signal transduction pathways that are ICP27 dependent. Thus, we conclude that ICP27t2 and ICP27 are functionally very similar and that ICP27t2 can mediate all ICP27 activities that are required for HSV-1 replication in cell culture. Surprisingly, however, we found that K2F1 forms plaques that are morphologically different from those of WT HSV-1. Investigation of this trait demonstrated that it results from the decreased release of progeny virions into the culture medium. This appears to be due to a reduction in the detachment of K2F1 progeny from the extracellular surface of the infected cell. We identified two HSV-1 ICP27 amino-terminal deletion mutants with a similar release defect. Together, these results demonstrate that ICP27 plays a heretofore-unappreciated role in modulating the efficiency of progeny virion release.

IMPORTANCE

ICP27 is an essential, multifunctional regulatory protein that has a number of critical roles in the HSV-1 life cycle. Although ICP27 homologs are encoded by all known members of the Herpesviridae, previous work with several of these homologs has shown that they cannot substitute for ICP27 in the context of HSV-1-infected cells. Here, we identify ICP27t2 as the first homolog that can efficiently replace ICP27 in HSV-1 infection. Unexpectedly, our results also reveal that the sequence of the ICP27 gene can affect the release of HSV-1 progeny virions from the infected cell. Thus, our comparative study has revealed a novel function for ICP27 in the regulation of virus release.

The herpes simplex virus 1 virion host shutoff protein enhances translation of viral late mRNAs by preventing mRNA overload

We recently demonstrated that the virion host shutoff (vhs) protein, an mRNA-specific endonuclease, is required for efficient herpes simplex virus 1 (HSV-1) replication and translation of viral true-late mRNAs, but not other viral and cellular mRNAs, in many cell types (B. Dauber, J. Pelletier, and J. R. Smiley, J. Virol. 85:5363-5373, 2011, http://dx.doi.org/10.1128/JVI.00115-11). Here, we evaluated whether the structure of true-late mRNAs or the timing of their transcription is responsible for the poor translation efficiency in the absence of vhs. To test whether the highly structured 5' untranslated region (5'UTR) of the true-late gC mRNA is the primary obstacle for translation initiation, we replaced it with the less structured 5'UTR of the γ-actin mRNA. However, this mutation did not restore translation in the context of a vhs-deficient virus. We then examined whether the timing of transcription affects translation efficiency at late times. To this end, we engineered a vhs-deficient virus mutant that transcribes the true-late gene US11 with immediate-early kinetics (IEUS11-ΔSma). Interestingly, IEUS11-ΔSma showed increased translational activity on the US11 transcript at late times postinfection, and US11 protein levels were restored to wild-type levels. These results suggest that mRNAs can maintain translational activity throughout the late stage of infection if they are present before translation factors and/or ribosomes become limiting. Taken together, these results provide evidence that in the absence of the mRNA-destabilizing function of vhs, accumulation of viral mRNAs overwhelms the capacity of the host translational machinery, leading to functional exclusion of the last mRNAs that are made during infection.

IMPORTANCE

The process of mRNA translation accounts for a significant portion of a cell's energy consumption. To ensure efficient use of cellular resources, transcription, translation, and mRNA decay are tightly linked and highly regulated. However, during virus infection, the overall amount of mRNA may increase drastically, possibly overloading the capacity of the translation apparatus. Our results suggest that the HSV-1 vhs protein, an mRNA-specific endoribonuclease, prevents mRNA overload during infection, thereby allowing translation of late viral mRNAs. The requirement for vhs varies between cell types. Further studies of the basis for this difference likely will offer insights into how cells regulate overall mRNA levels and access to the translational apparatus.

Role of immediate early protein ICP27 in the differential sensitivity of herpes simplex viruses 1 and 2 to leptomycin B

Leptomycin B (LMB) is a highly specific inhibitor of CRM1, a cellular karyopherin-β that transports nuclear export signal-containing proteins from the nucleus to the cytoplasm. Previous work has shown that LMB blocks herpes simplex virus 1 (HSV-1) replication in Vero cells and that certain mutations in viral immediate early protein ICP27 can confer LMB resistance. However, little is known of the molecular mechanisms involved. Here we report that HSV-2, a close relative of HSV-1, is naturally resistant to LMB. To see whether the ICP27 gene determines this phenotypic difference, we generated an HSV-1 mutant that expresses the HSV-2 ICP27 instead of the HSV-1 protein. This recombinant was fully sensitive to LMB, indicating that one or more other viral genes must be important in determining HSV-2's LMB-resistant phenotype. In additional work, we report several findings that shed light on how HSV-1 ICP27 mutations can confer LMB resistance. First, we show that LMB treatment of HSV-1-infected cells leads to suppression of late viral protein synthesis and a block to progeny virion release. Second, we identify a novel type of ICP27 mutation that can confer LMB resistance, that being the addition of a 100-residue amino-terminal affinity purification tag. Third, by studying infections where both LMB-sensitive and LMB-resistant forms of ICP27 are present, we show that HSV-1's sensitivity to LMB is dominant to its resistance. Together, our results suggest a model in which the N-terminal portion of ICP27 mediates a nonessential activity that interferes with HSV-1 replication when CRM1 is inactive. We suggest that LMB resistance mutations weaken or abrogate this activity.

Marek's disease virus expresses multiple UL44 (gC) variants through mRNA splicing that are all required for efficient horizontal transmission

Marek's disease (MD) is a devastating oncogenic viral disease of chickens caused by Gallid herpesvirus 2, or MD virus (MDV). MDV glycoprotein C (gC) is encoded by the alphaherpesvirus UL44 homolog and is essential for the horizontal transmission of MDV (K. W. Jarosinski and N. Osterrieder, J. Virol. 84:7911-7916, 2010). Alphaherpesvirus gC proteins are type 1 membrane proteins and are generally anchored in cellular membranes and the virion envelope by a short transmembrane domain. However, the majority of MDV gC is secreted in vitro, although secondary-structure analyses predict a carboxy-terminal transmembrane domain. In this report, two alternative mRNA splice variants were identified by reverse transcription (RT)-PCR analyses, and the encoded proteins were predicted to specify premature stop codons that would lead to gC proteins that lack the transmembrane domain. Based on the size of the intron removed for each UL44 (gC) transcript, they were termed gC104 and gC145. Recombinant MDV viruses were generated in which only full-length viral gC (vgCfull), gC104 (vgC104), or gC145 (vgC145) was expressed. Predictably, gCfull was expressed predominantly as a membrane-associated protein, while both gC104 and gC145 were secreted, suggesting that the dominant gC variants expressed in vitro are the spliced variants. In experimentally infected chickens, the expression of each of the gC variants individually did not alter replication or disease induction. However, horizontal transmission was reduced compared to that of wild-type or revertant viruses when the expression of only a single gC was allowed, indicating that all three forms of gC are required for the efficient transmission of MDV in chickens.

The many roles of the highly interactive HSV protein ICP27, a key regulator of infection

Human herpes viruses cause an array of illnesses ranging from cancers for Epstein?Barr virus and Kaposi?s sarcoma-associated herpes virus, to painful skin lesions, and more rarely, keratitis and encephalitis for HSV. All herpes viruses encode a multifunctional protein, typified by HSV ICP27, which plays essential roles in viral infection. ICP27 functions in all stages of mRNA biogenesis from transcription, RNA processing and export through to translation. ICP27 has also been implicated in nuclear protein quality control, cell cycle control, activation of stress signaling pathways and prevention of apoptosis. ICP27 interacts with many proteins and it binds RNA. This article focuses on how ICP27 performs its many roles and highlights similarities with its homologs, which could be targets for antiviral intervention.

The herpes simplex virus 1 vhs protein enhances translation of viral true late mRNAs and virus production in a cell type-dependent manner

The herpes simplex virus 1 (HSV-1) virion host shutoff protein (vhs) degrades viral and cellular mRNAs. Here, we demonstrate for the first time that vhs also boosts translation of viral true late mRNAs in a cell type-dependent manner and that this effect determines the viral growth phenotype in the respective cell type. Our study was prompted by the detection of stress granules, indicators of stalled translation initiation, in cells infected with vhs mutants but not in wild-type-virus-infected cells. Accumulation of true late-gene products gC and US11 was strongly reduced in the absence of vhs in HeLa cells and several other restrictive cell lines but not in Vero and other permissive cells and was independent of phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). Polysome analysis showed that gC and US11 transcripts were poorly translated in vhs-null-virus-infected HeLa cells, while translation of a cellular mRNA was not affected. Interestingly, hippuristanol, an eIF4A inhibitor, produced a similar phenotype in HeLa cells infected with wild-type HSV-1, while Vero cells were much more resistant to the inhibitor. These results suggest that translation of true late-gene transcripts is particularly sensitive to conditions of limited access to translation factors and that vhs is able either to prevent the limiting conditions or to facilitate translation initiation under these conditions. The varied permissivity of cell lines to vhs-null infection may stem from differences in the resilience of the translation machinery or the ability to control the accumulation of mRNAs.

Phenotypic suppression of a herpes simplex virus 1 ICP27 mutation by enhanced transcription of the mutant gene

ICP27 is an essential herpes simplex virus 1 (HSV-1) regulatory protein that enhances viral gene expression. Although it is predominantly nuclear, it shuttles to the cytoplasm during infection using an N-terminal nuclear export signal (NES). We previously engineered an NES-negative ICP27 mutant, dLeu, that replicates poorly in cultured cells. In this study, we isolated dLeuR, a growth-competent revertant of dLeu. We show that dLeuR possesses one or more extragenic mutations that enhance ICP27 transcription, leading to overexpression of the mutant protein and restoration of viral growth. This work provides evidence of a novel pathway regulating transcription of the ICP27 gene.

Identification and characterization of duck plague virus glycoprotein C gene and gene product

BACKGROUND

Viral envelope proteins have been proposed to play significant roles in the process of viral infection.

RESULTS

In this study, an envelope protein gene, gC (NCBI GenBank accession no. EU076811), was expressed and characterized from duck plague virus (DPV), a member of the family herpesviridae. The gene encodes a protein of 432 amino acids with a predicted molecular mass of 45 kDa. Sequence comparisons, multiple alignments and phylogenetic analysis showed that DPV gC has several features common to other identified herpesvirus gC, and was genetically close to the gallid herpervirus.Antibodies raised in rabbits against the pET32a-gC recombinant protein expressed in Escherichia coli BL21 (DE3) recognized a 45-KDa DPV-specific protein from infected duck embryo fibroblast (DEF) cells. Transcriptional and expression analysis, using real-time fluorescent quantitative PCR (FQ-PCR) and Western blot detection, revealed that the transcripts encoding DPV gC and the protein itself appeared late during infection of DEF cells. Immunofluorescence localization further demonstrated that the gC protein exhibited substantial cytoplasm fluorescence in DPV-infected DEF cells.

CONCLUSIONS

In this work, the DPV gC protein was successfully expressed in a prokaryotic expression system, and we presented the basic properties of the DPV gC product for the first time. These properties of the gC protein provided a prerequisite for further functional analysis of this gene.

The Varicella-Zoster virus IE4 protein: a conserved member of the herpesviral mRNA export factors family and a potential alternative target in antiherpetic therapies

During a viral infection, in addition to cellular mRNAs, amounts of viral mRNAs have to be efficiently transported to the cytoplasm for translation. It is now established that herpesviruses encode a conserved gene family whose proteins act as viral mRNA export factors that mediate nucleocytoplasmic transport of viral transcripts and eventually modulate through this mechanism the antiviral response. This conserved family of proteins contains the IE4 protein of the Varicella-Zoster virus (VZV). Here, we compared the functional characteristics of IE4 with those of its herpesviral homologues and proposed a model by which IE4 would be able to recruit the essential TAP/NXF1 receptor to viral transcripts. Moreover, on the basis of their crucial roles in the infectious cycle, these conserved viral factors should be considered as alternative targets in therapeutic approaches. Here, we discussed the possibility of developing antiherpetic agents targeting IE4 or its herpesviral homologues.

Identification of an ICP27-responsive element in the coding region of a herpes simplex virus type 1 late gene

During productive herpes simplex virus type 1 (HSV-1) infection, a subset of viral delayed-early (DE) and late (L) genes require the immediate-early (IE) protein ICP27 for their expression. However, the cis-acting regulatory sequences in DE and L genes that mediate their specific induction by ICP27 are unknown. One viral L gene that is highly dependent on ICP27 is that encoding glycoprotein C (gC). We previously demonstrated that this gene is posttranscriptionally transactivated by ICP27 in a plasmid cotransfection assay. Based on our past results, we hypothesized that the gC gene possesses a cis-acting inhibitory sequence and that ICP27 overcomes the effects of this sequence to enable efficient gC expression. To test this model, we systematically deleted sequences from the body of the gC gene and tested the resulting constructs for expression. In so doing, we identified a 258-bp "silencing element" (SE) in the 5' portion of the gC coding region. When present, the SE inhibits gC mRNA accumulation from a transiently transfected gC gene, unless ICP27 is present. Moreover, the SE can be transferred to another HSV-1 gene, where it inhibits mRNA accumulation in the absence of ICP27 and confers high-level expression in the presence of ICP27. Thus, for the first time, an ICP27-responsive sequence has been identified in a physiologically relevant ICP27 target gene. To see if the SE functions during viral infection, we engineered HSV-1 recombinants that lack the SE, either in a wild-type (WT) or ICP27-null genetic background. In an ICP27-null background, deletion of the SE led to ICP27-independent expression of the gC gene, demonstrating that the SE functions during viral infection. Surprisingly, the ICP27-independent gC expression seen with the mutant occurred even in the absence of viral DNA synthesis, indicating that the SE helps to regulate the tight DNA replication-dependent expression of gC.

Herpes simplex virus type 1 immediate-early protein ICP22 is required for VICE domain formation during productive viral infection

During productive infection, herpes simplex virus type 1 (HSV-1) induces the formation of discrete nuclear foci containing cellular chaperone proteins, proteasomal components, and ubiquitinated proteins. These structures are known as VICE domains and are hypothesized to play an important role in protein turnover and nuclear remodeling in HSV-1-infected cells. Here we show that VICE domain formation in Vero and other cells requires the HSV-1 immediate-early protein ICP22. Since ICP22 null mutants replicate efficiently in Vero cells despite being unable to induce VICE domain formation, it can be concluded that VICE domain formation is not essential for HSV-1 productive infection. However, our findings do not exclude the possibility that VICE domain formation is required for viral replication in cells that are nonpermissive for ICP22 mutants. Our studies also show that ICP22 itself localizes to VICE domains, suggesting that it could play a role in forming these structures. Consistent with this, we found that ICP22 expression in transfected cells is sufficient to reorganize the VICE domain component Hsc70 into nuclear inclusion bodies that resemble VICE domains. An N-terminal segment of ICP22, corresponding to residues 1 to 146, is critical for VICE domain formation in infected cells and Hsc70 reorganization in transfected cells. We previously found that this portion of the protein is dispensable for ICP22's effects on RNA polymerase II phosphorylation. Thus, ICP22 mediates two distinct regulatory activities that both modify important components of the host cell nucleus.

A study of varicella zoster virus glycoprotein C regulatory region response to viral activators in vitro

A study was conducted to analyze the response of varicella zoster virus (VZV) glycoprotein C gene (ORF14) regulatory sequences downstream as well as upstream of the transcription site to VZV transactivators, IE4 and IE62 and p29, the single-stranded DNA binding protein, in vitro by transiently transfecting a permissive human melanoma cell line (Mewo). This glycoprotein has been shown to be an important factor in VZV pathogenesis and therefore the regulation of its expression has been of much interest. In this study, the promoter region of gC as well as another VZV glycoprotein, gI (as a positive control), was amplified and cloned into a promoter less plasmid expressing the luciferase gene as a reporter. The activities of the regulatory regions from both glycoproteins were assessed by quantifying the luciferase activity. The results show that the luciferase assay is a powerful means of measuring promoter activity; nevertheless, the promoter region and cognate downstream and upstream sequences of the true late gC gene were not responsive to these viral proteins, indicating that other viral/cellular factors and/or viral replication could be involved in gC synthesis during the VZV infection cycle.

Identification of sequences in herpes simplex virus type 1 ICP22 that influence RNA polymerase II modification and viral late gene expression

Previous studies have shown that the herpes simplex virus type 1 (HSV-1) immediate-early protein ICP22 alters the phosphorylation of the host cell RNA polymerase II (Pol II) during viral infection. In this study, we have engineered several ICP22 plasmid and virus mutants in order to map the ICP22 sequences that are involved in this function. We identify a region in the C-terminal half of ICP22 (residues 240 to 340) that is critical for Pol II modification and further show that the N-terminal half of the protein (residues 1 to 239) is not required. However, immunofluorescence analysis indicates that the N-terminal half of ICP22 is needed for its localization to nuclear body structures. These results demonstrate that ICP22's effects on Pol II do not require that it accumulate in nuclear bodies. As ICP22 is known to enhance viral late gene expression during infection of certain cultured cells, including human embryonic lung (HEL) cells, we used our engineered viral mutants to map this function of ICP22. It was found that mutations in both the N- and C-terminal halves of ICP22 result in similar defects in viral late gene expression and growth in HEL cells, despite having distinctly different effects on Pol II. Thus, our results genetically uncouple ICP22's effects on Pol II from its effects on viral late gene expression. This suggests that these two functions of ICP22 may be due to distinct activities of the protein.

Herpes simplex virus type 1 ICP27 regulates expression of a variant, secreted form of glycoprotein C by an intron retention mechanism

We previously showed that herpes simplex virus type 1 (HSV-1) immediate-early (IE) protein ICP27 can posttranscriptionally stimulate mRNA accumulation from a transfected viral late gene encoding glycoprotein C (gC) (K. D. Perkins, J. Gregonis, S. Borge, and S. A. Rice, J. Virol. 77:9872-9884, 2003). We began this study by asking whether ICP27 homologs from other herpesviruses can also mediate this activity. Although the homologs from varicella-zoster virus (VZV) and human cytomegalovirus (HCMV) were inactive, the homolog from bovine herpesvirus 4 (BHV-4), termed HORF1/2, was a very efficient transactivator. Surprisingly, most of the mRNA produced via HORF1/2 transactivation was 225 nucleotides shorter than expected due to the removal of a previously undescribed intron from the gC transcript. We found that the gC mRNA produced in the absence of transactivation was also mostly spliced. In contrast, gC mRNA produced by ICP27 transactivation was predominantly unspliced. Based on these results, we conclude that ICP27 has two distinct effects on the transfected gC gene: it (i) stimulates mRNA accumulation and (ii) promotes the retention of an intron. Interestingly, the spliced transcript encodes a variant of gC that lacks its transmembrane domain and is secreted from transfected cells. As the gC splicing signals are conserved among several HSV-1 strains, we investigated whether the variant gC is expressed during viral infection. We report here that both the spliced transcript and its encoded protein are readily detected in Vero cells infected with three different laboratory strains of wild-type HSV-1. Moreover, the variant gC is efficiently secreted from infected cells. We have designated this alternate form of the protein as gCsec. As the extracellular domain of gC is known to bind heparan sulfate-containing proteoglycans and to inhibit the complement cascade via an interaction with complement component C3b, we speculate that gCsec could function as a secreted virulence factor.

Herpes simplex virus type 1 immediate-early protein ICP27 is required for efficient incorporation of ICP0 and ICP4 into virions

Early in infection, herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins ICP0 and ICP4 localize to the nucleus, where they stimulate viral transcription. Later in infection, ICP0 and to a lesser extent ICP4 accumulate in the cytoplasm, but their biological role there is unknown. Previously, it was shown that the cytoplasmic localization of ICP0/4 requires the multifunctional IE protein ICP27, which is itself an activator of viral gene expression. Here, we identify a viral ICP27 mutant, d3-4, which is unable to efficiently localize ICP0 and ICP4 to the cytoplasm but which otherwise resembles wild-type HSV-1 in its growth and viral gene expression phenotypes. These results genetically separate the function of ICP27 that affects ICP0/4 localization from its other functions, which affect viral growth and gene expression. As both ICP0 and ICP4 are known to be minor virion components, we used d3-4 to test the hypothesis that the cytoplasmic localization of these proteins is required for their incorporation into viral particles. Consistent with this conjecture, d3-4 virions were found to lack ICP0 in their tegument and to have greatly reduced levels of ICP4. Thus, the cytoplasmic localization of ICP0 and ICP4 appears to be a prerequisite for the assembly of these important transcriptional regulatory proteins into viral particles. Furthermore, our results show that ICP27 plays a previously unrecognized role in determining the composition of HSV-1 virions.

Herpes simplex virus immediate-early protein ICP22 triggers loss of serine 2-phosphorylated RNA polymerase II

During eukaryotic mRNA transcription, the synthetic activity and mRNA processing factor interactions of RNA polymerase II (RNAP II) are regulated by phosphorylation of its carboxyl-terminal domain (CTD), with modification occurring primarily on serines 2 and 5 of the CTD. We previously showed that herpes simplex virus type 1 (HSV-1) infection rapidly triggers the loss of RNAP II forms bearing serine 2 phosphorylation (Ser-2P RNAP II). Here we show that the HSV-1 immediate-early (IE) protein ICP22 is responsible for this effect during the IE phase of infection. This activity does not require the viral UL13 protein kinase, which is required for several other regulatory functions of ICP22. Additionally, we show that transient expression of ICP22 can trigger the loss of Ser-2P RNAP II in transfected cells. Thus, the ability of ICP22 to cause the loss of Ser-2 RNAP II does not require other viral factors or the context of the infected cell. Expression of the HSV-1 ICP22-related protein US1.5, which corresponds to residues 147 to 420 of ICP22, also triggers a loss of Ser-2P RNAP II in transfected cells, whereas expression of the varicella-zoster virus ICP22 homolog, ORF63, does not. Our study also provides evidence for a second, viral late gene-dependent pathway that triggers loss of Ser-2P RNAP II in infected cells, consistent with the recent work of Dai-Ju et al. (J. Q. Dai-Ju, L. Li, L. A. Johnson, and R. M. Sandri-Goldin, J. Virol. 80:3567-3581, 2006). Therefore, it appears that HSV-1 has evolved redundant mechanisms for triggering the loss of a specific phosphorylated form of RNAP II.

ICP27-dependent resistance of herpes simplex virus type 1 to leptomycin B is associated with enhanced nuclear localization of ICP4 and ICP0

It was previously shown that herpes simplex virus type 1 (HSV-1) is sensitive to leptomycin B (LMB), an inhibitor of nuclear export factor CRM1, and that a single methionine to threonine change at residue 50 (M50T) of viral immediate-early (IE) protein ICP27 can confer LMB resistance. In this work, we show that deletion of residues 21-63 from ICP27 can also confer LMB resistance. We further show that neither the M50T mutation nor the presence of LMB affects the nuclear shuttling activity of ICP27, suggesting that another function of ICP27 determines LMB resistance. A possible clue to this function emerged when it was discovered that LMB treatment of HSV-1-infected cells dramatically enhances the cytoplasmic accumulation of two other IE proteins, ICP0 and ICP4. This effect is completely dependent on ICP27 and is reversed in cells infected with LMB-resistant mutants. Moreover, LMB-resistant mutations in ICP27 enhance the nuclear localization of ICP0 and ICP4 even in the absence of LMB, and this effect can be discerned in transfected cells. Thus, the same amino (N)-terminal region of ICP27 that determines sensitivity to LMB also enhances ICP27's previously documented ability to promote the cytoplasmic accumulation of ICP4 and ICP0. We speculate that ICP27's effects on ICP4 and ICP0 may contribute to HSV-1 LMB sensitivity.

Herpes simplex virus type 1 infection leads to loss of serine-2 phosphorylation on the carboxyl-terminal domain of RNA polymerase II

Previous studies have shown that herpes simplex virus type 1 (HSV-1) infection alters the phosphorylation of the carboxyl-terminal domain (CTD) of RNA polymerase II (RNAP II), creating a new form of the enzyme known as RNAP II(I). However, the specific phosphorylation changes induced by HSV-1 have not been characterized. In this study, we used phospho-specific anti-CTD antibodies to probe the structure of the postinfection RNAP II. We find that RNAP II(I) is phosphorylated on serine-5 (Ser-5) of the CTD consensus repeat but generally lacks phosphorylation on serine-2 (Ser-2). Since Ser-2 phosphorylation is normally associated with efficient transcriptional elongation and the recruitment of pre-mRNA processing factors, our results suggest that RNAP II(I) may have altered elongation properties and decreased interactions with the mRNA processing machinery. The viral factors responsible for the reduction in Ser-2 CTD phosphorylation were studied. We found that viral immediate-early (IE) gene expression is required and sufficient, in the context of infection, for loss of Ser-2 phosphorylation. However, studies with viral mutants failed to implicate a single IE protein (among ICP0, ICP4, ICP22, and ICP27) in this process. Although most Ser-2-phosphorylated RNAP II is lost after infection, our immunofluorescence analyses identified a small subfraction that escapes loss and relocalizes to splicing antigen-rich nuclear speckles. A similar phenomenon is seen in uninfected cells after various treatments that inhibit RNAP II transcription. We hypothesize that the HSV-1-induced relocalization of residual Ser-2-phosphorylated RNAP II to nuclear speckles reflects a host response to the inhibition of cellular gene transcription.

Isolation of equine herpesvirus-1 lacking glycoprotein C from a dead neonatal foal in Japan

We isolated a variant equine herpesvirus-1 (EHV-1), strain 5089, from the lung of a dead neonatal foal in Japan and characterized the biological nature of the virus. The virus spread in cultured cells mainly by cell-to-cell infection, unlike wild-type EHV-1, which spreads efficiently as a cell-free virus. The virus titer in cultured supernatant and the intracellular virus titer were low compared to those of wild-type EHV-1. Heparin treatment of the virus had no effect on viral infectivity in cell culture. Glycoprotein C (gC) was not detected by Western blotting and fluorescent antibody tests in 5089 virions and 5089-infected cells, respectively. RT-PCR analysis revealed that the expression level of 5089 gC mRNA was reduced considerably compared to that of wild-type EHV-1. Sequencing analysis of the 5089 gC coding region showed a point mutation in the promoter region of the gC open reading frame. However, the mutation did not affect the promoter activity. These results suggested that the lack of gC in 5089 virions might be one of the reasons for spread of the virus by cell-to-cell infection and that gC mRNA expression might not be activated efficiently due to factors other than the mutation in the gC promoter region.

Proteomics of herpes simplex virus infected cell protein 27: association with translation initiation factors

The herpes simplex virus (HSV) immediate early ICP27 protein plays an essential role in stimulating viral early and late gene expression. ICP27 appears to be multifunctional in that it has been reported to stimulate viral late gene transcription, polyadenylation site usage, and RNA export. We report here on proteomic studies involving immunoprecipitation of ICP27 and mass spectrometric identification of co-precipitated proteins. These studies show an association of ICP27 with the cellular translation initiation factors poly A binding protein (PABP), eukaryotic initiation factor 3 (eIF3), and eukaryotic initiation factor 4G (eIF4G) in infected cells. Immunoprecipitation-western blot studies confirmed these associations. Finally, purified MBP-tagged ICP27 (MBP-27) can interact with eIF3 subunits p47 and p116 in vitro. These results suggest that ICP27 may also play a role in stimulating translation of certain viral and host mRNAs and/or in inhibiting host mRNA translation.

The equine herpesvirus 1 EICP27 protein enhances gene expression via an interaction with TATA box-binding protein

The mechanism(s) by which the early EICP27 gene product cooperates with other equine herpesvirus 1 (EHV-1) regulatory proteins to achieve maximal promoter activity remains unknown. Transient transfection assays revealed that deletion of residues 93-140 of the 470-aa EICP27 protein substantially diminished its activation of the immediate-early (IE) promoter, whereas deletion of residues 140-470 that contain a zinc-finger motif abolished this activity. Fluorescence microscopy of cells expressing the full-length EICP27 protein or portions of this protein revealed that an arginine-rich sequence spanning residues 178-185 mediates nuclear entry. Experiments employing the mammalian Gal4 two-plasmid system revealed that the EICP27 protein does not possess an independent trans-activation domain (TAD). Protein-protein interaction assays using purified proteins revealed that residues 124-220 of the EICP27 protein mediate its direct interaction with TATA box-binding protein (TBP). Partial deletion of this TBP-binding domain attenuated the ability of the EICP27 protein to stimulate the IE and early EICP0 promoters by 68% and 71%, respectively, indicating the importance of this protein-protein interaction.

ICP27 selectively regulates the cytoplasmic localization of a subset of viral transcripts in herpes simplex virus type 1-infected cells

Evidence suggests that the herpes simplex virus regulatory protein ICP27 mediates the nuclear export of viral transcripts; however, the extent of this activity during infection is unclear. ICP27 is required for efficient expression of the long, leaky-late UL24 transcripts, but not for that of the short, early UL24 transcripts. We found that infection by an ICP27-null mutant resulted in undetectable UL24 protein expression, which represented at least a 70-fold decrease relative to that of wild-type virus. Because lack of ICP27 had a greater effect on levels of UL24 protein than on transcripts, we examined its effect on subcellular localization of UL24 transcripts. In wild-type-infected cells, both short and long UL24 transcripts fractionated predominantly with the cytoplasm. However, in the absence of ICP27, greater than 50% of long UL24 transcripts were nuclear, while the percentage of short UL24 transcripts that were cytoplasmic was not reduced. These results also imply that the short UL24 transcripts are translated poorly. The effect of ICP27 on cytoplasmic localization of the long UL24 transcripts did not extend to other transcripts with which it shared a common 3' end or to other transcripts tested, including gC and UL42, whose overall expression is highly dependent on ICP27. Thus, the dual effects of ICP27 on mRNA accumulation and cytoplasmic localization are not always linked. These results identify viral transcripts that are dependent on ICP27 for efficient cytoplasmic localization during infection, but they also indicate the existence of ICP27-independent nuclear export pathways that are accessible to many viral transcripts during infection.