Analysis of DNA sequences which regulate the transcription of herpes simplex virus immediate early gene 3: DNA sequences required for enhancer-like activity and response to trans-activation by a virion polypeptide

How an increase in the copy number of HSV-1 during latency can cause Alzheimer's disease: the viral and cellular dynamics according to the microcompetition model

Numerous studies observed a link between the herpes smplex virus-1 (HSV-1) and Alzheimer's disease. However, the exact viral and cellular dynamics that lead from an HSV-1 infection to Alzheimer's disease are unknown. In this paper, we use the microcompetition model to formulate these dynamics by connecting seemingly unconnected observations reported in the literature. We concentrate on four pathologies characteristic of Alzheimer's disease. First, we explain how an increase in the copy number of HSV-1 during latency can decrease the expression of BECN1/Beclin1, the degradative trafficking protein, which, in turn, can cause a dysregulation of autophagy and Alzheimer's disease. Second, we show how an increase in the copy number of the latent HSV-1 can decrease the expression of many genes important for mitochondrial genome metabolism, respiratory chain, and homeostasis, which can lead to oxidative stress and neuronal damage, resulting in Alzheimer's disease. Third, we describe how an increase in this copy number can reduce the concentration of the NMDA receptor subunits NR1 and NR2b (Grin1 and Grin2b genes), and brain derived neurotrophic factor (BDNF), which can cause an impaired synaptic plasticity, Aβ accumulation and eventually Alzheimer's disease. Finally, we show how an increase in the copy number of HSV-1 in neural stem/progenitor cells in the hippocampus during the latent phase can lead to an abnormal quantity and quality of neurogenesis, and the clinical presentation of Alzheimer's disease. Since the current understanding of the dynamics and homeostasis of the HSV-1 reservoir during latency is limited, the proposed model represents only a first step towards a complete understanding of the relationship between the copy number of HSV-1 during latency and Alzheimer's disease.

Targeted Promoter Replacement Reveals That Herpes Simplex Virus Type-1 and 2 Specific VP16 Promoters Direct Distinct Rates of Entry Into the Lytic Program in Sensory Neurons in vivo

Infection and life-long residence in the human nervous system is central to herpes simplex virus (HSV) pathogenesis. Access is gained through innervating axonal projections of sensory neurons. This distinct mode of entry separates the viral genome from tegument proteins, including the potent transactivator of viral IE genes, VP16. This, in turn, promotes a balance between lytic and latent infection which underlies the ability of the virus to invade, disseminate, and set up a large reservoir of latent infections. In the mouse ocular model, TG neurons marked as either “latent” or “lytic” at 48 h postinfection indicated that these programs were selected early and were considered distinct and mutually exclusive. More recently, a temporal analysis of viral program selection revealed a default latent-like state that begins at ~18 h postinfection and in individual neurons, precedes entry into the viral lytic cycle. Studies using refined viral mutants demonstrated that transition out of this latent program depended upon the transactivation function of VP16. Pursuit of the apparent incongruity between the established leaky-late kinetics of VP16 expression with a “preimmediate-early” function led to the discovery of an unrecognized regulatory feature of the HSV-1 VP16 promoter near/downstream of its TATA box. Among three potential sites identified was a putative Egr-1/Sp1 site. Here, we report that a refined mutation of this site, while having no impact on replication in cultured cells or cornea, resulted in ~100-fold reduction in lytic infection in TG in vivo. Notably, the HSV-2 VP16 promoter has 13 direct tandem-repeats upstream of its TATA box forming multiple potential overlapping Egr-1/Sp1 sites. Thus, despite different structures, these promoters might share function in directing the preimmediate-early VP16 protein expression. To test this, the HSV-1 VP16 promoter/5′UTR was replaced by the HSV-2 VP16 promoter/5′UTR in the HSV-1 backbone. Compared to the genomically repaired isolate, the HSV-2 VP16 promoter/5′UTR (1) accelerated the transition into the lytic cycle, and enhanced (2) virulence, and (3) entry into the lytic cycle following a reactivation stressor. These gain-of-function phenotypes support the hypothesis that the VP16 promoter regulates the latent/lytic boundary in neurons and that the HSV-1 and HSV-2 promoter/5′UTRs encode distinct thresholds for this transition.

De Novo Herpes Simplex Virus VP16 Expression Gates a Dynamic Programmatic Transition and Sets the Latent/Lytic Balance during Acute Infection in Trigeminal Ganglia

The life long relationship between herpes simplex virus and its host hinges on the ability of the virus to aggressively replicate in epithelial cells at the site of infection and transport into the nervous system through axons innervating the infection site. Interaction between the virus and the sensory neuron represents a pivot point where largely unknown mechanisms lead to a latent or a lytic infection in the neuron. Regulation at this pivot point is critical for balancing two objectives, efficient widespread seeding of the nervous system and host survival. By combining genetic and in vivo in approaches, our studies reveal that the balance between latent and lytic programs is a process occurring early in the trigeminal ganglion. Unexpectedly, activation of the latent program precedes entry into the lytic program by 12 -14hrs. Importantly, at the individual neuronal level, the lytic program begins as a transition out of this acute stage latent program and this escape from the default latent program is regulated by de novo VP16 expression. Our findings support a model in which regulated de novo VP16 expression in the neuron mediates entry into the lytic cycle during the earliest stages of virus infection in vivo. These findings support the hypothesis that the loose association of VP16 with the viral tegument combined with sensory axon length and transport mechanisms serve to limit arrival of virion associated VP16 into neuronal nuclei favoring latency. Further, our findings point to specialized features of the VP16 promoter that control the de novo expression of VP16 in neurons and this regulation is a key component in setting the balance between lytic and latent infections in the nervous system.

Herpes simplex virus remains a significant human pathogen associated with extensive acute and chronic disease in humans worldwide. The virus invades the peripheral and central nervous systems where it replicates but also establishes life-long latent infections in neurons. Two distinct viral transcriptional programs support these distinct lifestyles, but how entry into either the lytic or latent programs is regulated in the neuron is not understood. This process is fundamentally important to a virus with the capacity to be extremely virulent, in balancing two objectives, efficient widespread seeding of the nervous system and host survival. In this report, we provide new insight into this regulation and data that support a novel model in which virus transported into the neuron from the body surface enters the latent program by default. In a subset of these, there is a transition into the lytic cycle, which requires VP16 transactivation and is gated by a region in the VP16 promoter. Thus, HSV takes advantage of the anatomy and axonal transport systems in sensory neurons so that VP16 is left behind and latency is favored, while features of the VP16 promoter insure adequate virus spread in the nervous system and maximized latent infections.

Targets for Antiviral Chemotherapy: Herpes Simplex Virus Regulatory Protein, Vmw65

The virion protein, Vmw65, of herpes simplex virus selectively induces the transcription of the virus immediate–early genes and is required for normal virus replication and for virulence in animal models. Vmw65 operates by interacting with a host cell transcription factor (Oct-1) and analysis of the structure/function relationship within Vmw65 has facilitated the design of a peptide, corresponding to a local domain of the protein, which interferes with the Vmw65–Oct-1 interaction. The selective interference of protein–protein interactions involved in gene regulation may provide a suitable target for the inhibition of virus replication.

Analysis of herpes simplex virus ICP0 promoter function in sensory neurons during acute infection, establishment of latency, and reactivation in vivo

We have begun an analysis of the functional architecture of the ICP0 promoter in neurons in vivo with the ultimate goal of determining how this gene is regulated during reactivation in vivo. Promoter/reporter mutants in which the Escherichia coli beta-galactosidase (beta-Gal) gene was driven by various permutations of the ICP0 promoter were employed to permit the analysis of promoter function without the added complications that would arise due to inappropriate regulation of ICP0 protein levels. A whole-ganglion immunohistochemical staining procedure (N. M. Sawtell, J. Virol. 77:4127-4138, 2003) was used for direct comparisons of the expression of the promoter/reporter gene to expression of the native protein in the same cell. In this way, the expression of the putative wild-type promoter could be validated and results for mutant promoters could be compared to expression of the native gene. We found that a DNA fragment from bp -562 through the methionine start codon of the ICP0 gene contained all sequences required for properly regulated ICP0 expression in diverse cell types (including sensory neurons of the trigeminal ganglia [TG]) in vitro and in vivo, as indicated by colocalization of ICP0 and beta-Gal. Truncation of the ICP0 promoter to bp -145 or -129 resulted in the loss of immediate-early (alpha) kinetics. The truncated promoters expressed high levels of the reporter gene with leaky late (gamma1) kinetics in vitro and in some cell types in vivo. Unexpectedly, the truncated promoters did not express in TG neurons. Thus, TAATGARAT or other sequences upstream of bp -145 in the ICP0 promoter are required for basal expression of ICP0 in neurons but are not required for basal expression in other cells in vivo. There was a >95% concordance between reporter and native protein expression detected with the 562-bp promoter in neurons during the acute stage. However, this was not the case during reactivation from latency in vivo, as nearly twice as many neurons contained detectable beta-Gal as contained detectable ICP0. This same 562-bp promoter/reporter cassette, when placed in the context of a latency-associated transcript (LAT) null mutant, resulted in >95% concordance of expression of beta-Gal and ICP0 during reactivation in vivo. These last results strongly suggest that there is a posttranscriptional constraint on the expression of ICP0 protein during reactivation from latency and that this constraint is mediated by LAT.

Early expression of herpes simplex virus (HSV) proteins and reactivation of latent infection

During the last decade, new data accumulated describing the early events during herpes simplex virus 1 (HSV-1) replication occurring before capsid formation and virion envelopment. The HSV virion carries its own specific transcription initiation factor (alpha-TIF), which functions together with other components of the cellular transcriptase complex to mediate virus-specific immediate early (IE) transcription. The virus-coded IE proteins are the transactivator and regulatory elements modulating early transcription and subsequent translation of nonstructural virus-coded proteins needed mainly for viral DNA synthesis and for the supply of corresponding nucleoside components. They also cooperate at the late transcription and translation of the virion (capsid, tegument and envelope) proteins. In addition, the transactivator IE proteins down-regulate their own transcription, while others facilitate viral mRNA processing or interfere with the presentation of newly synthesized virus antigens. Establishment of latency is closely related to the transcription of a separate category of transcripts, termed latency-associated (LAT). Formation of LATs occurs mainly in nondividing neurons which are metabolically less active and express lower levels of cellular transcription factors (nonpermissive cells). Expression of the stable non-spliced (2 kb), and especially of stable spliced (1.5 and 1.45 kb) LATs is a prerequisite for HSV reactivation. Different HSV genomes (from various HSV strains) do not undergo IE transcription at the same rate. Restricted IE transcription and the absence of viral DNA synthesis favors LAT formation and persistence of the silenced genome. Uneven levels of LAT expression and differences in the metabolic state of carrier neurons influence the reactivation competence. Under artificial or natural activation conditions, sufficient amounts of IE transactivator proteins and proteins promoting nucleoside metabolism are synthesized even in the absence of the viral alpha-TIF facilitating reactivation.

Cellular transcription factors enhance herpes simplex virus type 1 oriS-dependent DNA replication

The herpes simplex virus type 1 (HSV-1) origin of DNA replication, oriS, contains three binding sites for the viral origin binding protein (OBP) flanked by transcriptional regulatory elements of the immediate-early genes encoding ICP4 and ICP22/47. To assess the role of flanking sequences in oriS function, plasmids containing oriS and either wild-type or mutant flanking sequences were tested in transient DNA replication assays. Although the ICP4 and ICP22/47 regulatory regions were shown to enhance oriS function, most individual elements in these regions, including the VP16-responsive TAATGARAT elements, were found to be dispensable for oriS function. In contrast, two oriS core-adjacent regulatory (Oscar) elements, OscarL and OscarR, at the base of the oriS palindrome were shown to enhance oriS function significantly and additively. Specifically, mutational disruption of either element reduced oriS-dependent DNA replication by 60 to 70%, and disruption of both elements reduced replication by 90%. The properties of protein-DNA complexes formed in gel mobility shift assays using uninfected and HSV-1-infected Vero cell nuclear extracts demonstrated that both OscarL and OscarR are binding sites for cellular proteins. Whereas OscarR does not correspond to the consensus binding site of any known transcription factor, OscarL contains a consensus binding site for the transcription factor Sp1. Gel mobility shift and supershift experiments using antibodies directed against members of the Sp1 family of transcription factors demonstrated the presence of Sp1 and Sp3, but not Sp2 or Sp4, in the protein-DNA complexes formed at OscarL. The abilities of OscarL and OscarR to bind their respective cellular proteins correlated directly with the efficiency of oriS-dependent DNA replication. Cooperative interactions between the Oscar-binding factors and proteins binding to adjacent OBP binding sites were not observed. Notably, Oscar element mutations that impaired oriS-dependent DNA replication had no detectable effect on either basal or induced levels of transcription from the ICP4 and ICP22/47 promoters, as determined by RNase protection assays. The Oscar elements thus appear to provide binding sites for cellular proteins that facilitate oriS-dependent DNA replication but have no effect on transcription of oriS-flanking genes.

The use of herpes simplex virus-based vectors for gene delivery to the nervous system

The ability of herpes simplex virus (HSV) to establish a lifelong, latent infection within neurons has led to much interest in the development of HSV-based vectors for neuronal gene delivery. This review discusses the progress made towards the construction of safe, replication-disabled HSV vectors that are capable of directing long-term transgene expression in latently infected neurons. Such vectors are now being investigated in a variety of animal model systems, with a view to developing gene therapy approaches to a number of metabolic and degenerative neurological diseases.

Truncation of the C-terminal acidic transcriptional activation domain of herpes simplex virus VP16 produces a phenotype similar to that of the in1814 linker insertion mutation

We examined the phenotype of a herpes simplex virus (HSV) type 1 mutant (V422) in which the C-terminal acidic activation domain of the virion transactivator VP16 is truncated at residue 422. The efficiency of plaque formation by V422 on Vero cells was boosted by approximately 100-fold by including hexamethylene bis-acetimide (HMBA) in the growth medium, as previously observed with the in1814 VP16 linker insertion mutant isolated by Preston and colleagues. V422 displayed severely reduced levels of the immediate-early transcripts encoding ICP0 and ICP4 during infection in the presence of cycloheximide, and this defect was partially overcome by the addition of HMBA. The defect in plaque formation exhibited by V422 and in 1814 was efficiently complemented in U2OS osteosarcoma cells, which had previously been shown to complement ICP0 null mutations. Taken in combination, these data confirm the key role of VP16 in triggering the onset of the HSV lytic cycle.

The ability of the inhibitory domain of the POU family transcription factor Oct-2 to interfere with promoter activation by different classes of activation domains is dependent upon the nature of the basal promoter elements

The Oct-2 transcription factor contains an inhibitory domain which is able to repress transcription following DNA binding. Here we show that within the neuronally expressed Oct-2.5 form, the inhibitory domain can strongly inhibit activation by transcription factor activation domains which are either composed predominantly of acidic residues or contain the HOB motif, whereas it has a weaker effect or no effect on proline-rich activation domains and on a glutamine-rich domain. In contrast, the isolated inhibitory domain of Oct-2 can efficiently repress all types of activation domains. This effect is observed however, only on TATA box-containing promoters and not on promoters containing an initiator motif. This widespread inhibition of different activation domains and its dependence on the nature of the basal promoter elements indicate that the inhibitory domain is likely to act by contacting a common downstream target of activation domains within the basal transcriptional complex bound at the TATA box rather than quenching specific activation domains by direct interaction. These effects are discussed in terms of the functional role of the inhibitory domain within Oct-2.5 and the mechanism by which it acts.

The ability of POU family transcription factors to activate or repress gene expression is dependent on the spacing and context of their specific response elements

The different forms of the Oct-2 and Brn-3 POU family transcription factors can have distinct effects on their target promoters involving both the activation and repression of gene expression. To investigate the requirements for these effects we have altered both the context and spacing of the two TAATGARAT binding sites for these factors within the herpes simplex virus immediate-early 3 gene promoter. We show that the activation of this promoter by Brn-3a and its repression by Brn-3b is dependent on the correct spacing of these binding sites. In contrast, repression of the promoter by Oct-2.4 and Oct-2.5 is dependent on both the spacing and context of these sites with the requirements for repression by Oct-2.4 or Oct-2.5 being different. These effects are discussed in the context of the mechanisms by which POU factors activate or repress their target genes.

Transcription factors interacting with herpes simplex virus alpha gene promoters in sensory neurons

Interference with VP16-mediated activation of herpes virus immediate-early (or alpha) genes is thought to be the major cause of establishing viral latency in sensory neurons. This could be brought about by lack of a key activating transcription factor(s) or active repression. In this study we find that sensory neurons express all important components for VP16-mediated alpha gene induction, such as the POU transcription factor Oct-1, host cell factor (HCF) and GABP alpha/beta. However, Oct-1 and GABP alpha/beta are only present at low levels and the VP16-induced complex (VIC) appears different. We do not find protein expression of the transcription factor Oct-2, implicated by others as an alpha gene repressor. The POU factor N-Oct3 (Brn 2 or POU3F2) is also present in sensory neurons and binds viral TAATGARAT motifs with higher affinity than Oct-1, indicating that it may be a candidate repressor for competitive binding to TAATGARAT motifs. When transfected into HeLa cells, where Oct-1 and GABP alpha/beta are highly abundant, N-Oct3 represses model promoters with multimerized TAATGARAT motifs, but fails to repress complete alpha gene promoters. Taken together our findings suggest that modulation of alpha gene promoters could contribute to viral latency when low concentrations of the activating transcription factors Oct-1 and GABP alpha/beta prevail. Our data, however, refute the notion that competing Oct factors are able to block alpha gene transcription to achieve viral latency.

An activity specified by the osteosarcoma line U2OS can substitute functionally for ICP0, a major regulatory protein of herpes simplex virus type 1

Among the five immediate-early regulatory proteins of herpes simplex virus (HSV) type 1, only ICP0 is capable of activating all kinetic classes of viral genes. Consistent with its broad transactivating activity, ICP0 plays a major role in enhancing the reactivation of HSV from latency both in vivo and in vitro. Although not essential for viral replication, ICP0 confers a significant growth advantage on the virus, especially at low multiplicities of infection. In this report we describe the expression of a novel activity by the osteosarcoma cell line U2OS that can substitute functionally for ICP0. Compared with Vero cells, both U2OS cells and cells of the ICP0-expressing line 0-28 significantly enhanced the plating efficiency of an ICP0 null mutant, 7134. In contrast, the plating efficiencies of the wild-type virus in all three cell types were similar. Single-step growth experiments demonstrated that the yield of 7134 in U2OS cells was severalfold higher than that in 0-28 cells and about 100-fold higher than that in Vero cells. In order to identify the viral genes whose expression is enhanced by the activity in U2OS cells, levels of expression of selected viral proteins in extracts of Vero and U2OS cells were compared by Western blot (immunoblot) analysis following low-multiplicity infection. At a multiplicity of 0.1 PFU per cell, the levels of expression of the immediate-early protein ICP4 and the early protein gD in 7134-infected U2OS cells were significantly higher than those in 7134-infected Vero cells. When infections were carried out at a multiplicity of 1 PFU per cell, however, no major differences in the levels of expression of these proteins in U2OS and Vero cells were observed. Cycloheximide reversal experiments demonstrated that the cellular activity expressed in U2OS cells that promotes high-level expression of ICP4 is not synthesized de novo but appears to exist as a preformed protein(s). To confirm this observation and to determine whether, like immediate-early genes, early, delayed-early, and late viral genes are also responsive to the cellular activity, transient-expression assays were performed. The results of these tests demonstrated that basal levels of expression from immediate-early, early, and delayed-early promoters, but not that from a late promoter, were significantly higher in U2OS cells than in Vero cells and that this enhancement occurred in the absence of viral proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Proteins and cis-acting elements associated with transactivation of the varicella-zoster virus (VZV) immediate-early gene 62 promoter by VZV open reading frame 10 protein

Varicella-zoster virus (VZV) open reading frame 10 (ORF10) protein, the homolog of herpes simplex virus type 1 (HSV-1) VP16, is a virion-associated transactivator of the VZV immediate-early (IE) gene 62 (IE62) promoter. VP16 forms a complex with cellular factors (Oct1 and host cell factor [HCF]) and TAATGARAT elements (found in all HSV-1 IE promoter/enhancer sequences) to mediate stimulation of IE transcription. The VZV IE62 promoter also contains three TAATGARAT-like elements. Mutagenesis studies of the VZV IE62 promoter indicated that TAATGARAT-like elements contribute to transactivation of the VZV IE62 promoter by ORF10 protein. Other cis-acting elements such as GA-rich and cyclic AMP-responsive elements were also needed for full transactivation by ORF10 protein. In mobility shift assays, ORF10 protein formed a complex with either of two TAATGARAT-like elements that lack an overlapping octamer-binding motif (octa-/TAATGARAT) but not with a TAATGARAT element with an overlapping octamer-binding motif (octa+/TAATGARAT). In contrast, VP16 formed a high-affinity ternary complex with an octa+/TAATGARAT element and a low-affinity complex with octa-/TAATGARAT elements. Addition of antibodies to ORF10 protein, Oct1, or HCF disrupted the complexes, demonstrating that ORF10 protein interacts with Oct1 and HCF. These results suggest that transactivation of the VZV IE62 gene by ORF10 protein and HSV IE genes by VP16 require similar cellular proteins but distinct cis-acting elements.

Induction of herpes simplex virus type 1 immediate-early gene expression by a cellular activity expressed in Vero and NB41A3 cells after growth arrest-release

Infected cell protein 0 (ICP0), a major immediate-early regulatory protein of herpes simplex virus type 1 (HSV-1), activates expression of all classes of HSV genes as well as a variety of heterologous viral and cellular genes. Previous studies have shown that a cellular activity expressed maximally in Vero cells 8 h after release from growth arrest in the G0/G1 phase of the cell cycle can enhance plaque formation and gene expression of a mutant virus (7134) lacking both copies of the gene encoding ICP0 (W. Cai and P. Schaffer, J. Virol. 65:4078-4090, 1991). This observation suggests that the cellular activity can substitute for ICP0 to activate viral gene expression. To further characterize this cellular activity, Vero and NB41A3 (mouse neuroblastoma) cells were transfected at various times after release from growth arrest with promoter-chloramphenicol acetyltransferase (CAT) constructs containing promoters representing the major kinetic classes of HSV genes, and CAT activity was measured from 2 to 24 h postrelease. The results of these tests demonstrate that CAT expression from immediate-early promoter-CAT plasmids was enhanced 10- and 3-fold when Vero and NB41A3 cells were transfected at 6 and 2 h postrelease, respectively. In contrast, only low levels of immediate-early promoter-driven CAT activity were apparent when cells were transfected at later times postrelease. No significant stimulation of CAT activity was observed from promoter-CAT plasmids containing representative early or late HSV promoters or a heterologous viral (simian virus 40 early) promoter. Differences in the efficiency of uptake of plasmid DNA by cells at various times postrelease did not account for the observed differences in CAT expression. Unlike Vero cells, in which cell division resumed after release from growth arrest, division of NB41A3 cells did not resume. Rather, these cells displayed morphological features suggestive of a differentiated phenotype. Collectively, these findings demonstrate that a cellular activity expressed in Vero and NB41A3 cells after release from growth arrest can activate HSV gene expression by enhancing immediate-early gene expression.

Transcriptional activation by herpes simplex virus type 1 VP16 in vitro and its inhibition by oligopeptides

VP16 is a herpes simplex virus (HSV)-encoded transcriptional activator protein that is essential for efficient viral replication and as such may be a target for novel therapeutic agents directed against viral gene expression. We have reconstituted transcriptional activation by VP16 in an in vitro system that is dependent on DNA sequences from HSV immediate-early gene promoters and on protein-protein interactions between VP16 and Oct-1 that are required for VP16 activation in vivo. Activation increased synergistically with the number of TAATGARAT elements (the cis-acting element for VP16 activation in vivo) upstream of the core promoter, and mutations of this element that reduce Oct-1 or VP16 DNA binding reduced transactivation in vitro. A VP16 insertion mutant unable to interact with Oct-1 was inactive, but, surprisingly, a deletion mutant lacking the activation domain was approximately 65% as active as the full-length protein. The activation domains of Oct-1 were necessary for activation in reactions containing the VP16 deletion mutant, and they contributed significantly to activation by full-length VP16. Addition of a GA-rich element present in many HSV immediate-early gene enhancers synergistically stimulated VP16-activated transcription. Finally, oligopeptides that are derived from a region of VP16 thought to contact a cellular factor known as HCF (host cell factor) and that inhibit efficient VP16 binding to the TAATGARAT element also specifically inhibited VP16-activated, but not basal, transcription. Amino acid substitutions in one of these peptides identified three residues that are absolutely required for inhibition and presumably for interaction of VP16 with HCF.

The herpes simplex virus type 1 particle: structure and molecular functions. Review article

This review is a summary of our present knowledge with respect to the structure of the virion of herpes simplex virus type 1. The virion consists of a capsid into which the DNA is packaged, a tegument and an external envelope. The protein compositions of the structures outside the genome are described as well as the functions of individual proteins. Seven capsid proteins are identified, and two of them are mainly present in precursors of mature DNA-containing capsids. The protein components of the 150 hexamers and 12 pentamers in the icosahedral capsid are known. These capsomers all have a central channel and are connected by Y-shaped triplexes. In contrast to the capsid, the tegument has a less defined structure in which 11 proteins have been identified so far. Most of them are phosphorylated. Eleven virus-encoded glycoproteins are present in the envelope, and there may be a few more membrane proteins not yet identified. Functions of these glycoproteins include attachment to and penetration of the cellular membrane. The structural proteins, their functions, coding genes and localizations are listed in table form.

Transcriptional activation by herpes simplex virus type 1 VP16 in vitro and its inhibition by oligopeptides

VP16 is a herpes simplex virus (HSV)-encoded transcriptional activator protein that is essential for efficient viral replication and as such may be a target for novel therapeutic agents directed against viral gene expression. We have reconstituted transcriptional activation by VP16 in an in vitro system that is dependent on DNA sequences from HSV immediate-early gene promoters and on protein-protein interactions between VP16 and Oct-1 that are required for VP16 activation in vivo. Activation increased synergistically with the number of TAATGARAT elements (the cis-acting element for VP16 activation in vivo) upstream of the core promoter, and mutations of this element that reduce Oct-1 or VP16 DNA binding reduced transactivation in vitro. A VP16 insertion mutant unable to interact with Oct-1 was inactive, but, surprisingly, a deletion mutant lacking the activation domain was approximately 65% as active as the full-length protein. The activation domains of Oct-1 were necessary for activation in reactions containing the VP16 deletion mutant, and they contributed significantly to activation by full-length VP16. Addition of a GA-rich element present in many HSV immediate-early gene enhancers synergistically stimulated VP16-activated transcription. Finally, oligopeptides that are derived from a region of VP16 thought to contact a cellular factor known as HCF (host cell factor) and that inhibit efficient VP16 binding to the TAATGARAT element also specifically inhibited VP16-activated, but not basal, transcription. Amino acid substitutions in one of these peptides identified three residues that are absolutely required for inhibition and presumably for interaction of VP16 with HCF.

Herpes simplex virus VP16 forms a complex with the virion host shutoff protein vhs

Herpes simplex virus (HSV) virions contain at least two regulatory proteins that modulate gene expression in infected cells: the transcriptional activator VP16 and the virion host shutoff protein vhs. VP16 stimulates transcription of the HSV immediate-early genes, and vhs suppresses host protein synthesis and induces accelerated turnover of cellular and viral mRNAs. We report here that vhs binds directly to VP16: vhs and VP16 were coprecipitated from infected cells by an anti-vhs antiserum, and vhs and VP16 protein A fusions each bound intact versions of the other protein in a solid-phase capture assay. In addition, vhs and VP16 interacted in the two-hybrid activator system when coexpressed in Saccharomyces cerevisiae. vhs residues 238 to 344 were sufficient for the interaction, and the VP16 acidic transcriptional activation domain was not required. vhs blocked the ability of VP16 to enter a multiprotein complex on an immediate-early TAATGARATTC consensus sequence, indicating that vhs interacts with one or more regions of VP16 required for promoter recognition. We suggest that this interaction may play a structural role in the assembly of HSV virions and modulate the activity of vhs during infection.

Mutually exclusive binding of two cellular factors within a critical promoter region of the gene for the IE110k protein of herpes simplex virus

We have examined the cis- and trans-acting factors involved in constitutive transcription of the promoter for the IE110k protein of herpes simplex virus type 1. Our results indicate that while the IE110k gene is activated by Vmw65, it also exhibits very efficient constitutive expression approximating that from the simian virus 40 early enhancer-promoter region. We show that despite the presence of multiple copies of the octamer consensus site which mediate Oct-1 binding and subsequent Vmw65 activation, these upstream sequences have a minor effect on constitutive transcription. By progressive exonuclease digestion and subsequent site-directed mutagenesis of the promoter, we have identified a 15-bp region (termed the EC region), from position -89 to -74, which is required for efficient constitutive expression from the IE110k promoter. We demonstrate that two cellular proteins interact with this region and, by competition and methylation interference analyses, show they have distinct but overlapping sequence requirements for binding. One of these proteins is identified as NF-Y, a CCAAT box-binding factor, which binds an inverted CCAAT box located between positions -71 and -75. The second cellular factor, F2, appears to be novel and binds a region with the sequence CGCGCGGC CAT which overlaps the 3' end of the CCAAT box. The terminal AT of the recognition site for F2 represents, on the opposite strand, the terminal AT of the CCAAT box, and these and adjacent bases are critically required for the binding of both factors. These results together with further competition analysis indicate that these factors bind in a mutually exclusive manner to the EC region. The implications of these results for regulation of expression of the IE110k gene are discussed.

Identification of a promoter mapping within the reiterated sequences that flank the herpes simplex virus type 1 UL region

Analysis of the promoter for the herpes simplex virus (HSV) immediate-early (alpha) gene alpha 0 in a short-term transient expression assay revealed that a SacI-to-NcoI fragment from -786 to +148 relative to the cap site directed the synthesis of chloramphenicol acetyltransferase when the fragment was present in either orientation. Although the constitutive levels of promoter activity were similar with either orientation, the reverse-orientation promoter was not induced in response to infection with HSV. Analysis of sequences composing the putative promoter in the opposite orientation revealed the presence of important regulatory elements associated with alpha promoters. These include an alpha-trans-inducing factor (alpha-TIF)-like response element, a high-affinity ICP4-binding site, numerous Sp1-binding sites, and a TATA box. Sequences contained within this region formed specific DNA-protein complexes in extracts from mock-infected and HSV-infected HeLa cells. Transient expression assays revealed that this sequence was positively regulated by the alpha 0 and alpha-TIF genes but negatively regulated by alpha 4. Finally, nuclear run-on transcription assays revealed that this promoter is active in its correct genomic context during the course of virus infection. We suggest that the promoter is a hybrid between an alpha and beta promoter because it exhibits maximal expression at 8 h postinfection and is expressed in the presence of cycloheximide.

Mapping of a major surface-exposed site in herpes simplex virus protein Vmw65 to a region of direct interaction in a transcription complex assembly

The cellular factor Oct-1 is selectively recruited, together with at least one other cellular protein (CFF), into a multicomponent transcription complex whose assembly is directed by the herpes simplex virus regulatory protein Vmw65 (VP16). The acidic carboxy terminus of Vmw65 is not involved in assembly of the complex but is absolutely required for subsequent transcriptional activation. Elucidation of the mechanism of action of Vmw65 is important for an understanding not only of combinatorial control of gene expression by POU- and homeodomain proteins but also of the interaction(s) between activation domains of regulatory proteins and components of the basal transcriptional apparatus. We used a combination of limited proteolysis with a number of site-specific proteases and immunological detection to demonstrate the presence of two main surface-exposed regions in Vmw65. We mapped these sites to within a few amino acids at positions 365-370 408/409. The site at 408/409 is indicative of a flexible exposed linker region between the acidic carboxy-terminal activation domain (residues 430-480) and an N-terminal domain involved in complex formation with the two cellular factors. The site around residues 365-370 is precisely within a region that results from this and other laboratories have shown to be critical for complex formation. Furthermore, we show that this site is selectively protected from proteolysis after complex assembly. Finally, using a series of overlapping peptide encompassing this region, we show that the eight amino acids, R-E-H-A-Y-S-R-A, from positions 360 through 367 are sufficient to inhibit complex formation by intact Vmw65. We propose that these residues contain sufficient information to selectively bind one of the cellular partners involved in complex assembly and that these residues are located in a physical surface-exposed domain of the protein.

Recognition of the surface of a homeo domain protein

Homeo domain proteins exhibit distinct biological functions with specificities that cannot be predicted by their sequence specificities for binding DNA. Recognition of the surface of the Oct-1 POU homeo domain provides a general model for the contribution of selective protein-protein interactions to the functional specificity of the homeo domain family of factors. The assembly of Oct-1 into a multiprotein complex on the herpes simplex virus alpha/IE enhancer is specified by the interactions of its homeo domain with ancillary factors. This complex (C1 complex) is composed of the viral alpha TIF protein (VP16), Oct-1, and one additional cellular component, the C1 factor. Variants of the Oct-1 POU homeo domain were generated by site-directed mutagenesis, which altered the residues predicted to form the exposed surface of the domain-DNA complex. Proteins with single amino acid substitutions on the surface of either helix 1 or 2 of the Oct-1 POU homeo domain had decreased abilities to form the C1 complex. The behavior of these mutants in a cooperative DNA-binding assay with alpha TIF suggested that the Oct-1 POU homeo domain is principally recognized by alpha TIF in the C1 complex. The preferential recognition of Oct-1 over the closely related Oct-2 protein is critically influenced by a single residue on the surface of helix 1 because the introduction of this residue into the Oct-2 POU homeo domain significantly enhanced its ability to form a C1 complex.

Analysis of the gB promoter of herpes simplex virus type 1: high-level expression requires both an 89-base-pair promoter fragment and a nontranslated leader sequence

To investigate the cis-acting sequences involved in regulation of a herpes simplex virus gamma 1 gene, deletion analyses of the glycoprotein B (gB) gene promoter were performed. In transfection assays with gB-chloramphenicol acetyltransferase plasmids, high-level constitutive expression from the gB promoter was found with an 89-bp sequence (-69 to +20). Additional sequences in the 5'-transcribed noncoding leader region (+20 to +136) were required for full stimulation by herpes simplex virus infection. Plasmids with progressive deletions of the gB leader sequence demonstrated that chloramphenicol acetyltransferase expression in infected cells was proportional to the length of the leader region retained. In recombinant viruses containing a gB-gC gene fusion, a similar 83-bp (-60 to +23) region of the gB gene was found to promote accurately initiated gC mRNA from the viral genome with the same kinetics as the wild-type gB gene. Although the kinetics of expression remained the same, RNA abundance was greater with a 298-bp (-260 to +38) promoter than with the 83-bp promoter.

Immediate early and functional AP-1 cis-response elements are involved in the transcriptional regulation of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

Expression from the promoter of the herpes simplex virus type 2 (HSV-2) large subunit of ribonucleotide reductase (ICP10) is stimulated by co-transfection with DNA that encodes the virion protein Vmw65 previously shown to activate in trans the transcription of all IE genes (Wymer et al., 1989). Specific cis response elements involved in ICP10 transcriptional regulation were studied by chloramphenicol acetyltransferase analysis with hybrid ICP10 promoter/CAT structural gene constructions containing wild type or site-directed mutations of the promoter sequences. The data indicate that Vmw65 activation requires an intact TAAT-GARAT motif while complex formation requires an intact Oct-1 element, and the AP-1 consensus elements in the ICP10 promoter are functional in vitro. Thus, expression from the wild type and GA-rich mutant constructions was enhanced 10-20-fold by co-transfection with DNA encoding Vmw65. The GARAT and POU homeobox (PHB) binding motifs were required for Vmw65 mediated activation but the mutant in the POU specific box (PSB) binding motif was activated at higher concentrations of Vmw65 DNA (1.0-3.0 micrograms). The PHB and PSB binding motifs were necessary for complex formation as determined by gel retardation analysis with in vitro synthesized OTF-1 and Vmw65 proteins. The GARAT and GA-rich elements were not required. CAT expression from pICP10-cat was enhanced by co-transfection with jun and fos encoding DNA, and the ICP10 promoter complexed with in vitro synthesized jun protein.

Association of ICP0 but not ICP27 with purified virions of herpes simplex virus type 1

Recent studies have shown that ICP4, one of the major immediate-early proteins of herpes simplex virus type 1 is present within the tegument region of the virion (F. Yao and R. J. Courtney, J. Virol. 63:3338-3344, 1989). With monoclonal antibodies to two additional immediate-early proteins, ICP0 and ICP27, and Western blot (immunoblot) analysis, ICP0, but not ICP27, was also found to be associated with purified virus particles. In an effort to localize the ICP0 within the virion, purified virions were treated with trypsin in the presence and absence of detergent. The data suggest that ICP0 is located within the tegument region of the virion and is not localized in the envelope or within the nucleocapsid. The number of molecules of ICP0 per virion was estimated to be approximately 150.

trans activation of the simian virus 40 late promoter by large T antigen requires binding sites for the cellular transcription factor TEF-1

Simian virus 40 (SV40) T antigen stimulates the level of transcription from several RNA polymerase II promoters, including the SV40 late promoter. The mechanism of trans activation appears to be indirect since binding of T antigen to specific DNA sequences is not required. However, specific promoter elements that respond to T antigen have not previously been defined. We identified DNA sequences from the SV40 late promoter whose ability to stimulate transcription is induced by the expression of T antigen. In particular, the Sph I + II motifs of the SV40 enhancer can confer T-antigen inducibility to the normally uninducible herpes simplex virus thymidine kinase gene promoter when multiple copies of the sequence are inserted 5' of the transcription initiation site and TATA sequence. Binding sites for the cellular transcription factor TEF-1 and octamer binding proteins are contained within the Sph I + II motifs, as well as at other positions in the SV40 promoter. To study the role of individual protein-binding sites in trans activation by T antigen, mutations were constructed in various TEF-1 and octamer protein-binding sites of the SV40 late promoter. These mutations did not significantly affect basal promoter activity. However, mutation of all three TEF-1 sites prevented detectable activation by T antigen. DNase I footprinting of the mutated promoters with purified proteins demonstrated that inducibility by T antigen correlated with binding affinity of TEF-1 for the DNA and not with binding affinity of an octamer binding protein.

Differential dependence of herpes simplex virus immediate-early gene expression on de novo-infected cell protein synthesis

The time course of accumulation of herpes simplex virus immediate-early (IE) mRNA and the requirement for infected cell protein synthesis for mRNA transcription and accumulation were compared. Measurements of transcription in nuclear run-on assays, accumulation of cytoplasmic mRNA by Northern (RNA) blot hybridization, and rates of infected cell protein synthesis by pulse-labeling did not indicate differences among the five IE gene, consistent with previous studies. However, as a result of varying the amount of de novo protein synthesis after infection, at least three patterns of maximal expression of the IE genes were revealed. Addition of the protein synthesis inhibitor anisomycin to cells coincident with infection resulted in maximal rates of transcription and accumulation of functional ICP0 mRNA, while 0.5 h of infected cell protein synthesis prior to addition of the drug was required for maximal expression of ICP22/47 and ICP27 mRNAs. Maximal expression of ICP4 mRNA occurred only when 1 h of de novo protein synthesis occurred prior to the addition of the drug. These results are discussed in the context of alternative mechanisms for regulating IE gene expression.

Identification of Ets- and notch-related subunits in GA binding protein

Recombinant cDNA clones that encode two distinct subunits of the transcription factor GA binding protein (GABP) have been isolated. The predicted amino acid sequence of one subunit, GABP alpha, exhibits similarity to the sequence of the product of the ets-1 protooncogene in a region known to encompass the Ets DNA binding domain. The sequence of the second subunit, GABP beta, contains four 33-amino acid repeats located close to the NH2-terminus of the subunit. The sequences of these repeats are similar to repeats in several transmembrane proteins, including Notch from Drosophila melanogaster and Glp-1 and Lin-12 from Caenorhabditis elegans. Avid, sequence-specific binding to DNA required the presence of both polypeptides, revealing a conceptual convergence of nuclear transforming proteins and membrane-anchored proteins implicated in developmentally regulated signal transduction processes.

The overlapping octamer/TAATGARAT motif is a high-affinity binding site for the cellular transcription factors Oct-1 and Oct-2

The octamer motif in cellular promoters and the related TAATGARAT element in the herpes simplex virus (HSV) immediate-early promoters can both bind cellular octamer-binding proteins. The overlapping octamer/TAATGARAT elements (consensus ATGCTAATGARAT) found in the HSV-1 IE1 promoter thus represent a composite motif, each portion of which can independently bind octamer-binding protein. By comparing the binding characteristics of this composite motif with its individual elements, we show that it binds a single molecule of either Oct-1 or Oct-2 with much higher affinity than does either an octamer or TAATGARAT motif alone. This strong binding allows this element to direct a much higher level of gene expression when linked to a heterologous promoter than that observed with each of its individual components.

Role of alpha-transinducing factor (VP16) in the induction of alpha genes within the context of viral genomes

In herpes simplex virus 1, the five alpha genes are induced by alpha-transinducing factor (alpha TIF; VP16), a virion protein, acting in concert with Oct-1 and other cellular proteins on a cis-acting site in the promoter domain of alpha genes. Because alpha TIF is an essential virion protein, its function as an inducer can best be evaluated only by mutating the cis-acting site. Earlier we reported on a series of 17 mutations in and around the cis-acting site of a 275-bp alpha 27 promoter fused to a reporter gene and recombined into the viral genome. These recombinant viruses were tested in Vero cells in the presence of cycloheximide, and we demonstrated that mutations in the sequence required for Oct-1 binding abolished transactivation whereas mutations in the alpha TIF-dependent GARAT sequence decreased but did not abolish transactivation. We now report that (i) in limited-passage human embryonic lung cells, alpha gene expression from promoters mutated in the GARAT sequences is often higher and more variable than in Vero cells, (ii) in the absence of cycloheximide, the mutant viruses show less significant impairment of reporter gene expression, (iii) Oct-1 can bind either to the overlapping octamer element or to various TAATGARAT sequences with differing degrees of binding strength and these relative binding levels correlate well with levels of gene expression observed in infected cells, (iv) in the cis-acting site upstream of the alpha 4 gene, no degenerate overlapping Oct-1 sequence exists, and therefore in this instance Oct-1 must be binding directly to the TAATGARAT sequence, (v) extension of the alpha 27 promoter by an additional 1,334 bp results in much higher expression of the reporter gene as a result of additional upstream cis-acting sites, and (vi) obliteration of the most proximal Oct-1 binding element within the 275-bp promoter dramatically reduces gene expression even in the presence of the additional upstream cis-acting sites.

Elements in the transcriptional regulatory region flanking herpes simplex virus type 1 oriS stimulate origin function

Like other DNA-containing viruses, the three origins of herpes simplex virus type 1 (HSV-1) DNA replication are flanked by sequences containing transcriptional regulatory elements. In a transient plasmid replication assay, deletion of sequences comprising the transcriptional regulatory elements of ICP4 and ICP22/47, which flank oriS, resulted in a greater than 80-fold decrease in origin function compared with a plasmid, pOS-822, which retains these sequences. In an effort to identify specific cis-acting elements responsible for this effect, we conducted systematic deletion analysis of the flanking region with plasmid pOS-822 and tested the resulting mutant plasmids for origin function. Stimulation by cis-acting elements was shown to be both distance and orientation dependent, as changes in either parameter resulted in a decrease in oriS function. Additional evidence for the stimulatory effect of flanking sequences on origin function was demonstrated by replacement of these sequences with the cytomegalovirus immediate-early promoter, resulting in nearly wild-type levels of oriS function. In competition experiments, cotransfection of cells with the test plasmid, pOS-822, and increasing molar concentrations of a competitor plasmid which contained the ICP4 and ICP22/47 transcriptional regulatory regions but lacked core origin sequences resulted in a significant reduction in the replication efficiency of pOS-822, demonstrating that factors which bind specifically to the oriS-flanking sequences are likely involved as auxiliary proteins in oriS function. Together, these studies demonstrate that trans-acting factors and the sites to which they bind play a critical role in the efficiency of HSV-1 DNA replication from oriS in transient-replication assays.

A cellular factor binds to the herpes simplex virus type 1 transactivator Vmw65 and is required for Vmw65-dependent protein-DNA complex assembly with Oct-1

The herpes simplex virus transactivator Vmw65 assembles into a multicomponent protein-DNA complex along with the octamer binding protein Oct-1. Using affinity chromatography on columns conjugated with purified Vmw65 fusion protein expressed in Escherichia coli, we demonstrate that a cellular factor, distinct from Oct-1, binds to Vmw65 in the absence of target DNA and is necessary for Vmw65-mediated complex assembly with Oct-1.

A cellular factor binds to the herpes simplex virus type 1 transactivator Vmw65 and is required for Vmw65-dependent protein-DNA complex assembly with Oct-1

The herpes simplex virus transactivator Vmw65 assembles into a multicomponent protein-DNA complex along with the octamer binding protein Oct-1. Using affinity chromatography on columns conjugated with purified Vmw65 fusion protein expressed in Escherichia coli, we demonstrate that a cellular factor, distinct from Oct-1, binds to Vmw65 in the absence of target DNA and is necessary for Vmw65-mediated complex assembly with Oct-1.

Differential regulation of endogenous and transduced beta-globin genes during infection of erythroid cells with a herpes simplex virus type 1 recombinant

We infected murine erythroleukemia cells with a nondefective herpes simplex virus (HSV) type 1 recombinant bearing the rabbit beta-globin gene under the control of its own promoter, in order to compare the regulation of a cellular gene residing in the viral genome to that of its active endogenous counterpart. We found that the viral globin gene was activated by HSV immediate-early polypeptides, whereas expression of the endogenous beta-globin gene was strongly suppressed: transcription was greatly inhibited, and beta-globin mRNA was rapidly degraded. Degradation of globin mRNA was induced by a component of the infecting virion and required a functional UL41 gene product. These results demonstrate that HSV products can have opposing effects on the expression of homologous genes located in the cellular and viral genomes and suggest that the preferential expression of HSV genes that occurs during infection is not achieved solely through sequence-specific differentiation between viral and cellular promoters or mRNAs.

Structural requirements in the herpes simplex virus type 1 transactivator Vmw65 for interaction with the cellular octamer-binding protein and target TAATGARAT sequences

Herpes simplex virus type 1 virion protein Vmw65 forms a complex (TRF.C) with TAATGARAT sequences and the cellular transcription factor oct-1, which has been implicated as an intermediate in the activation of gene expression by Vmw65. To examine structural requirements within Vmw65 for this interaction, we analyzed extracts of transfected cells that express mutant Vmw65 proteins by gel retardation assay and identified two regions in the primary sequence of Vmw65 which are necessary for in vitro assembly of TRF.C. The amino-terminal boundary for complex assembly and trans activation mapped between residues 49 and 75. At the carboxyl terminus, deletion as far as residue 388 did not affect in vitro TRF.C assembly, although trans-activating activity was abolished. Deletion beyond residue 388 rapidly impaired the ability of the protein to participate in the TRF.C complex, such that a truncated mutant of 380 residues was completely inactive. These requirements towards the carboxyl terminus overlap a region of strong local sequence similarity between Vmw65 and terminal protein p3 of bacteriophage phi 29. Although substitution of corresponding p3 residues into Vmw65 failed to produce a functional chimera, site-directed mutagenesis within the region of similarity identified a number of single-point mutant proteins which were completely deficient for TRF.C formation. These mutant proteins were also unable to trans activate expression from immediate-early promoters, despite the integrity of the acidic carboxyl terminus. The extreme sensitivity of both TRF.C formation and trans activation to single-residue substitutions within this region of Vmw65 suggests that it is directly involved in the protein-protein or protein-DNA interactions required for assembly of a transcriptional complex containing oct-1.

Enhanced infectivity of herpes simplex virus type 1 viral DNA in a cell line expressing the trans-inducing factor Vmw65

Vmw65 is a structural component of herpes simplex virus (HSV) which is involved in transactivating the expression of the viral immediate-early (IE) genes. To gain further insight into the function of this protein, a cell line, BSV65, was established which expresses biologically active Vmw65 under control of the Moloney leukemia virus long terminal repeat. This cell line was shown to specifically activate IE genes as demonstrated by transient transfection assays with reporter genes linked to HSV IE or delayed-early promoter-regulatory regions. Furthermore, by using mobility shift assays, cell extracts were shown to be capable of forming a Vmw65-containing complex with oligonucleotides that contained a TAATGARAT motif, a conserved cis-acting IE regulatory element which is required for Vmw65-mediated trans induction. BSV65 cells were able to complement HSV type 1 in 1814, a mutant which is unable to trans-induce IE gene expression and whose growth is impaired at low multiplicities of infection. Transfection of purified HSV type 1 viral DNA into BSV65 cells resulted in an approximately 200-fold increase in virus production compared with the parental cell line. In addition, in comparison to wild-type cells, infectious virus production occurred sooner and efficiency of plaque formation was higher in BSV65 cells following transfection of viral DNA but not following infection with virus. Northern (RNA) dot blot analysis of cells transfected with viral DNA showed that transcription of the IE gene Vmw175 was approximately 10-fold greater in BSV65 cells compared with wild-type cells. These results indicate that, in the presence of functional Vmw65, there is a greater probability that transfected viral DNA will lead to a productive infection.

Identification of a domain of the herpes simplex virus trans-activator Vmw65 required for protein-DNA complex formation through the use of protein A fusion proteins

In order to identify structural domains of the herpes simplex virus trans-activator Vmw65 required for protein-DNA complex formation, subfragments of Vmw65 were expressed in Escherichia coli as fusion polypeptides with protein A of Staphylococcus aureus, and the purified hybrids were used in a band shift assay. The results indicate that a region near the amino terminus of Vmw65 between amino acids 141 and 185 is necessary for complex formation.

Herpes simplex virus Vmw65-octamer binding protein interaction: a paradigm for combinatorial control of transcription

The transcriptional status of a given viral or cellular gene is determined both by the availability of functional transcription factors and by the combination and spatial arrangement of the cis-acting elements to which they bind. While differential gene expression can be achieved to some extent by the interaction of different factors with different genes it is clear that in some cases the same factor is required for the regulation of genes that are not coordinately expressed. How a transcription factor achieves selective function in the absence of selective binding is a question that remains largely unanswered. The recent advances in understanding how both protein-protein and protein-DNA interactions are required to mediate the induction of herpes simplex virus immediate early (HSV IE) gene expression have highlighted a novel mechanism for combinatorial control of transcription which has significant implications for the differential control of cellular gene expression.

DNA binding and gene regulation by the herpes simplex virus type 1 protein ICP4 and involvement of the TATA element

We report the results of fine mapping the sequences responsible for negative regulation of immediate-early (IE) gene 3 by its own gene product, ICP4. Affinity-purified ICP4 binds the transcriptional start site of IE gene 3 and protein-protein interactions induce a secondary mobility shift that footprints exactly as the primary complex. Since these DNA-protein complexes contain ICP4, it is likely that the two differ only in stoichiometry of protein. Additional data show that the DNA-binding domain recognized by ICP4 can be embedded as a cassette in foreign DNA and that native ICP4 will recognize and bind the resulting DNA. In two different immediate-early promoters, the ICP4 binding site can be located either 3' or 5' of the TATA box; however, the ICP4 site is rotationally displaced from the transcription factor IID (TFIID) site by a roughly one-half helical turn, suggesting that ICP4 and TFIID are on the opposite helical face when bound at their respective sites. In the IE1 and IE3 promoters, binding of ICP4 causes an alteration in the helical geometry of the minor groove of the TATA region as visualized by copper footprinting. In contrast, TATA hypersensitivity was not detected in the glycoprotein D promoter (an early gene promoter containing the ICP4 site separated from TATA by eight helical turns) or in an artificial IE3 promoter construct in which the TATA-A4 separation was increased from 2.5 to roughly 5 helical turns. Such stereospecific and distance-dependent conformational alterations in the TATA box under the influence of ICP4 binding may be important in the repression of immediate-early genes.

Purification of a set of cellular polypeptides that bind to the purine-rich cis-regulatory element of herpes simplex virus immediate early genes

Expression of herpes simplex virus type 1 (HSV1) immediate early (IE) genes is activated by a polypeptide component of the mature virion termed viral protein 16 (VP16). Stimulation of IE expression by VP16 operates via two cis-regulatory sequences: TAATGARAT, and the purine-rich hexanucleotide sequence GCGGAA. VP16 does not bind directly to either of the IE cis-regulatory sequences. Rather, these elements appear to represent binding sites for host cell proteins. Herein, we report the purification of a host cell factor that binds to the GCGGAA motif. We show further that this factor is capable of binding in vitro to an oligomerized form of the hexanucleotide sequence GAAACG, which is common to a variety of virus- and interferon-inducible genes. The GAAACG repeats of interferon- and virus-inducible genes, and the GA-rich repeats of HSV1 IE genes confer similar functional properties when appended to the promoter of a heterologous gene. These observations raise the possibility that HSV1 may activate its IE genes in a manner that exploits one of the components used by mammalian cells to combat virus infection.

Overlapping octamer and TAATGARAT motifs in the VF65-response elements in herpes simplex virus immediate-early promoters represent independent binding sites for cellular nuclear factor III

Expression of the immediate-early (IE) genes of herpes simplex virus (HSV) is specifically stimulated by a 65-kilodalton virion transcription factor (VF65 or VP16) that is introduced as a component of infecting virions. In both the IE175(ICP4) and IE110(ICP0) promoters, this activation requires an upstream cis-acting target response element that contains a single TAATGARAT consensus element. Furthermore, many HSV IE TAATGARAT elements overlap with ATGCTAAT octamer motifs that are similar to the OTF-1-binding sites found in both immunoglobulin and histone H2b genes and to the nuclear factor III (NFIII)-binding site within the adenovirus type 2 origin of DNA replication. Purified HeLa cell NFIII protein proved to form specific DNA-protein complexes with several upstream regions from both the IE110 and IE175 promoters, and this interaction was subject to efficient competition with an adenovirus type 2 DNA fragment containing an intact NFIII-binding site. Surprisingly, the NFIII protein bound to synthetic oligonucleotides containing only the TAATGARAT consensus elements as well as to those containing the ATGCTAAT octamer sequence, although the former exhibited lower affinity and gave complexes with slightly different electrophoretic mobility. The ATGCTAAT oligonucleotide also competed more efficiently than the TAATGARAT sequence itself for binding to a TAATGARAT probe, indicating that the same protein species binds to both sites. The oligonucleotides also formed novel supershifted complexes with lysed virion proteins, but only in the presence of a crude nuclear extract and not with affinity-purified NFIII alone. We conclude that the cellular NFIII protein can recognize both the ATGCTAAT and TAATGARAT elements independently but that only the interaction with TAATGARAT represents an intermediate step in the transcriptional stimulation of IE genes by the HSV virion factor.

Identification of immediate-early-type cis-response elements in the promoter for the ribonucleotide reductase large subunit from herpes simplex virus type 2

Regulation of the expression of the herpes simplex virus (HSV) type 2 large subunit of ribonucleotide reductase (ICP10) gene was studied directly by immunofluorescence or by chloramphenicol acetyltransferase analysis with hybrid ICP10 promoter constructions. In Vero cells, cotransfection with DNA encoding HSV IE110 or Vmw65 proteins or HCMV IE2 enhanced expression at least 10-fold. In contrast, expression was minimally enhanced by DNA encoding IE175 at low doses and slightly reduced at high doses. IE110-mediated trans-activation was minimal in primary astrocytes and cells from line 293. However, Vmw65 enhanced expression 20-fold in all cell types. cis-Response elements in the ICP10 promoter include a TAATGARAT-like element and other sequences associated with regulation of IE gene expression and potential SP-1, consensus AP-1, and octamer transcription factor 1 binding elements. Factors that bind to the ICP10 promoter were identified in mock and HSV-infected cell extracts. DNA-protein complex formation, presumably involving Vmw65, was demonstrated by gel retardation analysis with mixtures of uninfected cell nuclear extracts and virion lysates. The octamer transcription factor 1 motif (ATGCAAAT) was necessary for optimal Vmw65 binding to the ICP10 promoter as evidenced by competition experiments with oligonucleotides overlapping the consensus IE110 promoter virion response element. The data suggest that ICP10 can be regulated as an immediate-early gene.

Construction and characterization of a herpes simplex virus type 1 mutant unable to transinduce immediate-early gene expression

A herpes simplex virus mutant, in1814, possessing a 12-base-pair insertion in the gene encoding the transinducing factor Vmw65 has been constructed. The insertion abolished the ability of Vmw65 to transinduce immediate-early (IE) gene expression and to form a protein-DNA complex with cell proteins and the IE-specific regulatory element TAATGAGAT. Accumulation of IE RNA 1 and 2 was reduced four- to fivefold in in1814-infected cells, but the level of IE RNA 4 was reduced only by twofold, and IE RNA 3 was unaffected. Mutant in1814 had a high particle/PFU ratio, but many of the particles, although unable to form plaques, were capable of normal participation in the early stages of infection at high multiplicity of infection. The defect of in1814 was overcome partially by transfection of a plasmid encoding the IE protein Vmw110 into cells prior to titration and by prior infection with ultraviolet light-inactivated herpes simplex virus. Mutant in1814 was essentially avirulent when injected into mice. The results demonstrate that transinduction of IE transcription by Vmw65 is important at low multiplicity of infection and in vivo but that at high multiplicity of infection the function is redundant.

Stimulation of estrogen receptor mRNA levels in MCF-7 cells by herpes simplex virus infection

Infection of estrogen-responsive cells (MCF-7) with herpes simplex virus type 1 or 2 stimulates expression of the estrogen receptor message. Experiments on infection with the mutant virus, tsK, together with transfection studies implicate the virion protein, Vmw65, in the response. Cellular protein synthesis is essential for estrogen receptor mRNA expression.