Mutational dissection of the HSV-1 immediate-early protein Vmw175 involved in transcriptional transactivation and repression

Cell Culture Evolution of a Herpes Simplex Virus 1 (HSV-1)/Varicella-Zoster Virus (VZV) UL34/ORF24 Chimeric Virus Reveals Novel Functions for HSV Genes in Capsid Nuclear Egress

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) are both members of the alphaherpesvirus subfamily but belong to different genera. Substitution of the HSV-1 UL34 coding sequence with that of its VZV homolog, open reading frame 24 (ORF24), results in a virus that has defects in viral growth, spread, capsid egress, and nuclear lamina disruption very similar to those seen in a UL34-null virus despite normal interaction between ORF24 protein and HSV pUL31 and proper localization of the nuclear egress complex at the nuclear envelope. Minimal selection for growth in cell culture resulted in viruses that grew and spread much more efficiently that the parental chimeric virus. These viruses varied in their ability to support nuclear lamina disruption, normal nuclear egress complex localization, and capsid de-envelopment. Single mutations that suppress the growth defect were mapped to the coding sequences of ORF24, ICP22, and ICP4, and one virus carried single mutations in each of the ICP22 and US3 coding sequences. The phenotypes of these viruses support a role for ICP22 in nuclear lamina disruption and a completely unexpected role for the major transcriptional regulator, ICP4, in capsid nuclear egress. IMPORTANCE Interactions among virus proteins are critical for assembly and egress of virus particles, and such interactions are attractive targets for antiviral therapy. Identification of critical functional interactions can be slow and tedious. Capsid nuclear egress of herpesviruses is a critical event in the assembly and egress pathway and is mediated by two proteins, pUL31 and pUL34, that are conserved among herpesviruses. Here, we describe a cell culture evolution approach to identify other viral gene products that functionally interact with pUL34.

The herpes viral transcription factor ICP4 forms a novel DNA recognition complex

The transcription factor ICP4 from herpes simplex virus has a central role in regulating the gene expression cascade which controls viral infection. Here we present the crystal structure of the functionally essential ICP4 DNA binding domain in complex with a segment from its own promoter, revealing a novel homo-dimeric fold. We also studied the complex in solution by small angle X-Ray scattering, nuclear magnetic resonance and surface-plasmon resonance which indicated that, in addition to the globular domain, a flanking intrinsically disordered region also recognizes DNA. Together the data provides a rationale for the bi-partite nature of the ICP4 DNA recognition consensus sequence as the globular and disordered regions bind synergistically to adjacent DNA motifs. Therefore in common with its eukaryotic host, the viral transcription factor ICP4 utilizes disordered regions to enhance the affinity and tune the specificity of DNA interactions in tandem with a globular domain.

Functional Characterization of the Serine-Rich Tract of Varicella-Zoster Virus IE62

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV), a major viral trans-activator, initiates the virus life cycle and is a key component of pathogenesis. The IE62 possesses several domains essential for trans-activation, including an acidic trans-activation domain (TAD), a serine-rich tract (SRT), and binding domains for USF, TFIIB, and TATA box binding protein (TBP). Transient-transfection assays showed that the VZV IE62 lacking the SRT trans-activated the early VZV ORF61 promoter at only 16% of the level of the full-length IE62. When the SRT of IE62 was replaced with the SRT of equine herpesvirus 1 (EHV-1) IEP, its trans-activation activity was completely restored. Herpes simplex virus 1 (HSV-1) ICP4 that lacks a TAD very weakly (1.5-fold) trans-activated the ORF61 promoter. An IE62 TAD-ICP4 chimeric protein exhibited trans-activation ability (10.2-fold), indicating that the IE62 TAD functions with the SRT of HSV-1 ICP4 to trans-activate viral promoters. When the serine and acidic residues of the SRT were replaced with Ala, Leu, and Gly, trans-activation activities of the modified IE62 proteins IE62-SRTΔSe and IE62-SRTΔAc were reduced to 46% and 29% of wild-type activity, respectively. Bimolecular complementation assays showed that the TAD of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with Mediator 25 in human melanoma MeWo cells. The SRT of IE62 interacted with the nucleolar-ribosomal protein EAP, which resulted in the formation of globular structures within the nucleus. These results suggest that the SRT plays an important role in VZV viral gene expression and replication.

IMPORTANCE

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV) is a major viral trans-activator and is essential for viral growth. Our data show that the serine-rich tract (SRT) of VZV IE62, which is well conserved within the alphaherpesviruses, is needed for trans-activation mediated by the acidic trans-activation domain (TAD). The TADs of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with cellular Mediator 25 in bimolecular complementation assays. The interaction of the IE62 SRT with nucleolar-ribosomal protein EAP resulted in the formation of globular structures within the nucleus. Understanding the mechanisms by which the TAD and SRT of IE62 contribute to the function of this essential regulatory protein is important in understanding the gene program of this human pathogen.

The intrinsic disorder alphabet. III. Dual personality of serine

Proteins are natural polypeptides consisting of 20 major amino acid residues, content and order of which in a given amino acid sequence defines the ability of a related protein to fold into unique functional state or to stay intrinsically disordered. Amino acid sequences code for both foldable (ordered) proteins/domains and for intrinsically disordered proteins (IDPs) and IDP regions (IDPRs), but these sequence codes are dramatically different. This difference starts with a very general property of the corresponding amino acid sequences, namely, their compositions. IDPs/IDPRs are enriched in specific disorder-promoting residues, whereas amino acid sequences of ordered proteins/domains typically contain more order-promoting residues. Therefore, the relative abundances of various amino acids in ordered and disordered proteins can be used to scale amino acids according to their disorder promoting potentials. This review continues a series of publications on the roles of different amino acids in defining the phenomenon of protein intrinsic disorder and represents serine, which is the third most disorder-promoting residue. Similar to previous publications, this review represents some physico-chemical properties of serine and the roles of this residue in structures and functions of ordered proteins, describes major posttranslational modifications tailored to serine, and finally gives an overview of roles of serine in structure and functions of intrinsically disordered proteins.

Requirement of the N-terminal activation domain of herpes simplex virus ICP4 for viral gene expression

ICP4 is the major activator of herpes simplex virus (HSV) transcription. Previous studies have defined several regions of ICP4 that are important for viral gene expression, including a DNA binding domain and transactivation domains that are contained in the C-terminal and N-terminal 520 and 274 amino acids, respectively. Here we show that the N-terminal 210 amino acids of ICP4 are required for interactions with components of TFIID and mediator and, as a consequence, are necessary for the activation of viral genes. A mutant of ICP4 deleted for amino acids 30 to 210, d3-10, was unable to complement an ICP4 null virus at the level of viral replication. This was the result of a severe deficiency in viral gene and protein expression. The absence of viral gene expression coincided with a defect in the recruitment of RNA polymerase II to a representative early promoter (thymidine kinase [TK]). Affinity purification experiments demonstrated that d3-10 ICP4 was not found in complexes with components of TFIID and mediator, suggesting that the defect in RNA polymerase II (Pol II) recruitment was the result of ablated interactions between d3-10 and TFIID and mediator. Complementation assays suggested that the N-terminal and C-terminal regions of ICP4 cooperate to mediate gene expression. The complementation was the result of the formation of more functional heterodimers, which restored the ability of the d3-10-containing molecules to interact with TFIID. Together, these studies suggest that the N terminus contains a true activation domain, mediating interactions with TFIID, mediator, and perhaps other transcription factors, and that the C terminus of the molecule contains activities that augment the functions of the activation domain.

The potential link between PML NBs and ICP0 in regulating lytic and latent infection of HSV-1

Herpes simplex virus type 1 (HSV-1) is a common human pathogen causing cold sores and even more serious diseases. It can establish a latent stage in sensory ganglia after primary epithelial infections, and reactivate in response to stress or sunlight. Previous studies have demonstrated that viral immediate-early protein ICP0 plays a key role in regulating the balance between lytic and latent infection. Recently, It has been determined that promyelocytic leukemia (PML) nuclear bodies (NBs), small nuclear sub-structures, contribute to the repression of HSV-1 infection in the absence of functional ICP0. In this review, we discuss the fundamentals of the interaction between ICP0 and PML NBs, suggesting a potential link between PML NBs and ICP0 in regulating lytic and latent infection of HSV-1.

The N terminus and C terminus of herpes simplex virus 1 ICP4 cooperate to activate viral gene expression

Infected cell polypeptide 4 (ICP4) activates transcription from most viral promoters. Two transactivation domains, one N-terminal and one C terminal, are largely responsible for the activation functions of ICP4. A mutant ICP4 molecule lacking the C-terminal activation domain (n208) efficiently activates many early genes, whereas late genes are poorly activated, and virus growth is severely impaired. The regions within the N terminus of ICP4 (amino acids 1 to 210) that contribute to activation were investigated by analysis of deletion mutants in the presence or absence of the C-terminal activation domain. The mutants were assessed for their abilities to support viral replication and to regulate gene expression. Several deletions in regions conserved in other alphaherpesviruses resulted in impaired activation and viral growth, without affecting DNA binding. The single small deletion that had the greatest effect on activation in the absence of the C terminus corresponded to a highly conserved stretch of amino acids between 81 and 96, rendering the molecule nonfunctional. However, when the C terminus was present, the same deletion had a minimal effect on activity. The amino terminus of ICP4 was predicted to be relatively disordered compared to the DNA-binding domain and the C-terminal 500 amino acids. Moreover, the amino terminus appears to be in a relatively extended conformation as determined by the hydrodynamic properties of several mutants. The data support a model where the amino terminus is an extended and possibly flexible region of the protein, allowing it to efficiently interact with multiple transcription factors at a distance from where it is bound to DNA, thereby enabling ICP4 to function as a general activator of polymerase II transcription. The C terminus of ICP4 can compensate for some of the mutations in the N terminus, suggesting that it either specifies redundant interactions or enables the amino terminus to function more efficiently.

Herpes simplex virus 1 ICP4 forms complexes with TFIID and mediator in virus-infected cells

The infected cell polypeptide 4 (ICP4) of herpes simplex virus 1 (HSV-1) is a regulator of viral transcription that is required for productive infection. Since viral genes are transcribed by cellular RNA polymerase II (RNA pol II), ICP4 must interact with components of the pol II machinery to regulate viral gene expression. It has been shown previously that ICP4 interacts with TATA box-binding protein (TBP), TFIIB, and the TBP-associated factor 1 (TAF1) in vitro. In this study, ICP4-containing complexes were isolated from infected cells by tandem affinity purification (TAP). Forty-six proteins that copurified with ICP4 were identified by mass spectrometry. Additional copurifying proteins were identified by Western blot analysis. These included 11 components of TFIID and 4 components of the Mediator complex. The significance of the ICP4-Mediator interaction was further investigated using immunofluorescence and chromatin immunoprecipitation. Mediator was found to colocalize with ICP4 starting at early and continuing into late times of infection. In addition, Mediator was recruited to viral promoters in an ICP4-dependent manner. Taken together, the data suggest that ICP4 interacts with components of TFIID and Mediator in the context of viral infection, and this may explain the broad transactivation properties of ICP4.

DNA-dependent oligomerization of herpes simplex virus type 1 regulatory protein ICP4

The human herpes simplex virus type 1 regulatory protein ICP4 binds DNA as a dimer and forms a single protein-DNA complex (A complex) with short DNA probes. ICP4 oligomerized in a DNA-dependent manner, forming two or more protein-DNA complexes with longer DNA fragments containing a single DNA binding site. When resolved electrophoretically, one or more low-mobility DNA-protein complexes follow the fast-moving A complex. The major protein-DNA complex (B complex) formed by ICP4 with long DNA probes migrates just behind the A complex in the electric field, implying the oligomerization of ICP4 on the DNA. Binding experiments with circularly permutated DNA probes containing one ICP4 binding site revealed that about 70 bp of nonspecific DNA downstream of the cognate ICP4 binding site was required for efficient B complex formation. In addition, the C-terminal domain of ICP4 was found to be required for DNA-dependent oligomerization and B complex formation. Gel mobility shift analysis of protein-DNA complexes, combined with supershift analysis using different monoclonal antibodies, indicated that the B complex contained two ICP4 dimers. DNase I footprinting of ICP4-DNA complexes showed that one ICP4 dimer contacts the specific binding site and another ICP4 dimer contacts nonspecific DNA in the B complex. DNA-dependent oligomerization increased the affinity of ICP4 for relatively weak binding sites on large DNA molecules. The results of this study suggest how ICP4 may use multiple weak binding sites to aid in transcription activation.

Temperature-dependent conformational changes in herpes simplex virus ICP4 that affect transcription activation

The C-terminal 500 amino acids of herpes simplex virus type 1 ICP4 are required for full activator function and viral growth and are known to participate in interactions consistent with the role of ICP4 as an activator of transcription. Oligonucleotide mutagenesis was used to target stretches of amino acids that are conserved with the ICP4 analogs of other alphaherpesviruses and were also predicted to be exposed on the surface of the molecule. Seven mutants were isolated that possessed one to three amino acid changes to the residue alanine in four regions between residues 1000 and 1200. The mutants generated were analyzed first in transfection assays and subsequently after introduction into the viral genome. A number of phenotypes representing different degrees of functional impairment were observed. In transient assays conducted at 37 degrees C, mutant M2 was indistinguishable from wild-type ICP4. Mutants M6 and M7 were marginally impaired. M3, M4, and M5 were more significantly impaired but still able to activate transcription, and M1 was completely impaired. In the context of the viral genome, M1, M3, and M7 were found to be temperature sensitive for growth. All three overproduced immediate-early (IE) proteins at the nonpermissive temperature (NPT). M3 and M7 produced early but not late proteins, and M1 produced neither early nor late proteins, at the NPT. The ICP4 proteins synthesized by all of the mutants tested were able to bind to specific ICP4 binding sites in electrophoretic mobility shift experiments. However, the DNA-protein complexes formed with the ICP4 from M1, M3, or M7 produced at the NPT possessed altered mobility. These complexes were not supershifted by a monoclonal antibody that recognizes an epitope in the C terminus; however, they were supershifted by a monoclonal antibody that recognizes the N terminus. The results suggest that the mutant forms of ICP4, while able to bind to DNA, are conformationally altered at the NPT, thus impairing the ability of the protein to activate transcription to different extents. The complete lack of ICP4 function characteristic of the M1 protein, and the inability of all the mutants to attenuate IE gene expression, suggest that the mutations additionally affect functions of the N terminus to different extents.

Visualization of parental HSV-1 genomes and replication compartments in association with ND10 in live infected cells

The relative location of active and repressed genes within the nucleus is becoming recognized as a significant factor in the control of gene expression. We have developed systems to visualize parental and replicated herpes simplex virus type 1 (HSV-1) amplicon genomes in association with PML nuclear bodies (ND10) in live cells. Plasmids containing viral replication and packaging signals, a gene expressing enhanced yellow fluorescent protein linked to the tetracycline repressor DNA binding domain and 14 copies of the tetracycline operator sequence were used to produce amplicon genomes packaged into normal viral particles. The frequency of the juxtaposition of viral genomes and ND10 was substantially increased by inclusion of an active HSV-1 Early gene transcription unit, indicating that the association is neither random nor passive. Furthermore, the ND10-associated genomes preferentially progressed to form viral replication compartments. Thus, active viral transcription contributes to the efficiency of viral genome association with ND10, and this in turn increases the probability that the genome will engage in active DNA replication. These studies in live cells provide a novel insight into virus-ND10 interactions and provide compelling visualization of their functional relevance.

Identification of a motif in the C terminus of herpes simplex virus regulatory protein ICP4 that contributes to activation of transcription

Expression of most viral genes during productive infection by herpes simplex virus is regulated by the viral protein ICP4 (also called IE175 or Vmw175). The N-terminal portion of ICP4 contains well-defined transactivation, DNA binding, and dimerization domains that contribute to promoter regulation. The C-terminal half of ICP4 contributes to the activity of ICP4, but the functional motifs have not been well mapped. To localize functional motifs in the C-terminal half of ICP4, we have compared the relative specific activities of ICP4 variants in transient-transfection assays. Deletion of the C-terminal 56 residues reduces the specific activity more than 10-fold. Mutational analysis identified three consecutive residues (1252 to 1254) that are conserved in ICP4 orthologs and are essential for full activity, especially in the context of ICP4 variants with a deletion in the N-terminal transactivation domain. Recombinant viruses that encode variants of ICP4 with mutations in the N-terminal transactivation domain and/or the extreme C terminus were constructed. The phenotypes of these recombinant viruses support the hypothesis that efficient promoter activation by ICP4 requires motifs at both the N and C termini. The data suggest that the C terminus of ICP4 functions not as an independent transactivation domain but as an enhancer of the ICP4 N-terminal transactivation domain. The data provide further support for the hypothesis that some ICP4 motifs required for promoter activation are not required for promoter repression and suggest that ICP4 utilizes different cellular factors for activation or repression of viral promoters.

The equine herpesvirus 1 immediate-early protein interacts with EAP, a nucleolar-ribosomal protein

The equine herpesvirus 1 (EHV-1) immediate-early (IE) phosphoprotein is essential for the activation of transcription from viral early and late promoters and regulates transcription from its own promoter. The IE protein of 1487 amino acids contains a serine-rich tract (SRT) between residues 181 and 220. Deletion of the SRT decreased transactivation activity of the IE protein. Previous results from investigation of the ICP4 protein, the IE homolog of herpes simplex virus 1 (HSV-1), revealed that a domain containing a serine-rich tract interacts with EAP (Epstein-Barr virus-encoded small nuclear RNA-associated protein), a 15-kDa nucleolar-ribosomal protein (R. Leopardi, and B. Roizman, Proc. Natl. Acad. Sci. USA 93, 4572-4576, 1996). DNA binding assays revealed that (i) glutathione S-transferase (GST)-EAP disrupted the binding of HSV-1 ICP4 to its cognate DNA in a dose-dependent manner, (ii) GST-EAP interacted with the EHV-1 IE protein, but did not disrupt its binding to its cognate site in viral DNA. GST-pulldown assays indicated that the SRT of the IE protein is required for physical interaction with EAP. The IE protein and EAP colocalized in the cytoplasm of the infected equine ETCC cells at late times of the infection cycle. This latter finding may be important in EHV-1 gene regulation since late viral gene expression is greatly influenced by the EICP0 trans-activator protein whose function is antagonized by the IE protein.

Herpes simplex virus type 1 ICP4 promotes transcription preinitiation complex formation by enhancing the binding of TFIID to DNA

Infected-cell polypeptide 4 (ICP4) of herpes simplex virus type 1 (HSV-1) activates the expression of many HSV genes during infection. It functions along with the cellular general transcription factors to increase the transcription rates of genes. In this study, an HSV late promoter consisting of only a TATA box and an INR element was immobilized on a magnetic resin and incubated with nuclear extracts or purified TFIID in the presence and absence of ICP4. Analysis of the complexes formed on these promoters revealed that ICP4 increased the formation of transcription preinitiation complexes (PICs) in a TATA box-dependent manner, as determined by the presence of ICP4, TFIID, TFIIB, and polymerase II on the promoter. With both nuclear extract and purified TFIID, it was determined that ICP4 helped TFIID bind to the promoter and the TATA box. These observations differed from those for the activator Gal4-VP16. As previously observed by others, Gal4-VP16 also increased the formation of PICs without helping TFIID bind to the promoter, suggesting that ICP4 and VP16 differ in their mechanism of activation and that ICP4 functions to facilitate PIC formation at an earlier step in the formation of PICs. We also observed that the DNA binding activity of ICP4 was not sufficient to help TFIID bind to the promoter and that the region of ICP4 that was responsible for this activity is located between residues 30 and 274. Taken together these results demonstrate that a specific region of ICP4 helps TFIID bind to the TATA box and that this in turn facilitates the formation of transcription PICs.

Herpes simplex virus type 1 infection stimulates p38/c-Jun N-terminal mitogen-activated protein kinase pathways and activates transcription factor AP-1

Cells respond to environmental stress and proinflammatory cytokines by stimulating the Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and the p38 mitogen-activated protein kinase cascades. Infection of eukaryotic cells with herpes simplex virus type 1 (HSV-1) resulted in stimulation of both JNK/SAPK and p38 mitogen-activated protein kinase after 3 h of infection, and activation reached a maximum of 4-fold by 9 h post-infection. By using a series of mutant viruses, we showed that the virion transactivator protein VP16 stimulates p38/JNK, whereas no immediate-early, early, or late viral expressed gene is involved. We identified the stress-activated protein kinase kinase 1 as an upstream activator of p38/JNK, and we demonstrated that activation of AP-1 binding proceeded p38/JNK stimulation. During infection, the activated AP-1 consisted mainly of JunB and JunD with a simultaneous decrease in the cellular levels of Jun protein. We suggest that activation of the stress pathways by HSV-1 infection either represents a cascade triggered by the virus to facilitate the lytic cycle or a defense mechanism of the host cell against virus invasion.

The polyserine tract of herpes simplex virus ICP4 is required for normal viral gene expression and growth in murine trigeminal ganglia

ICP4 of herpes simplex virus (HSV) is essential for productive infection due to its central role in the regulation of HSV transcription. This study identified a region of ICP4 that is not required for viral growth in culture or at the periphery of experimentally inoculated mice but is critical for productive growth in the trigeminal ganglia. This region of ICP4 encompasses amino acids 184 to 198 and contains 13 nearly contiguous serine residues that are highly conserved among the alphaherpesviruses. A mutant in which this region is deleted (DeltaSER) was able to grow on the corneas of mice and be transported back to the trigeminal ganglia. DeltaSER did not grow in the trigeminal ganglia but did express low levels of several immediate-early (ICP4 and ICP27) and early (thymidine kinase [tk] and UL42) genes. It expressed very low levels of the late gC gene and did not appear to replicate DNA. This pattern of gene expression was similar to that observed for a tk mutant, dlsptk. Both DeltaSER and dlsptk expressed higher levels of the latency-associated transcript (LAT) per genome earlier in infected ganglia than did the wild-type virus, KOS. However, infected ganglia from all three viruses accumulated the same level of LAT per genome at 30 days postinfection (during latency). The data suggest that the polyserine tract of ICP4 provides an activity that is required for lytic infection in ganglia to progress to viral DNA synthesis and full lytic gene expression. In the absence of this activity, higher levels of LAT per genome accumulate earlier in infection than with wild-type virus.

The high mobility group protein 1 is a coactivator of herpes simplex virus ICP4 in vitro

ICP4 is an activator of herpes simplex virus early and late gene transcription during infection and in vitro can efficiently activate the transcription of a core promoter template containing only a TATA box and an initiator element. In this study, we noted that the extent of activation by ICP4 in vitro was highly dependent on the purity of TFIID when recombinant TFIIB, TFIIE, and TFIIF were used as sources of these factors. ICP4 efficiently activated transcription with a crude TFIID fraction. However, when immunoaffinity-purified TFIID was used in place of the less pure TFIID, ICP4 activated transcription to a significantly lesser extent. This finding indicated that the crude TFIID fraction may contain additional factors that serve as coactivators of ICP4. To test this hypothesis, the crude TFIID preparation was further fractionated by gel filtration chromatography. The TFIID that eluted from the column lacked the hypothesized coactivator activity. A fraction well separated from TFIID contained an activity that when added with the TFIID fraction resulted in higher levels of transcription in the presence ICP4. Further purification of the coactivator-containing fraction resulted in the isolation of a single 30-kDa polypeptide (p30). p30 was also shown to serve as a coactivator of ICP4 with immunoaffinity-purified TFIID; however, p30 had no effect on basal transcription. Amino acid sequence analysis revealed that p30 was the high mobility group protein 1, which has been shown to facilitate the formation of higher-order DNA-protein complexes.

Herpes simplex virus immediate-early proteins ICP0 and ICP4 activate the endogenous human alpha-globin gene in nonerythroid cells

Globin genes are normally expressed only in erythroid cell lineages. However, we found that the endogenous alpha-globin gene is activated following infection of human fibroblasts and HeLa cells with herpes simplex virus (HSV), leading to accumulation of correctly initiated transcripts driven by the alpha-globin promoter. The alpha1- and alpha2-globin genes were both induced, but expression of beta- or zeta-globin genes could not be detected. Experiments using HSV mutants showed that null mutations in the genes encoding the viral immediate-early proteins ICP4 and ICP22 reduced induction approximately 10-fold, while loss of ICP0 function had a smaller inhibitory effect. Transient transfection experiments showed that ICP0 and ICP4 are each sufficient to trigger detectable expression of the endogenous gene, while ICP22 had no detectable effect in this assay. ICP4 also strongly enhanced expression of transfected copies of the alpha2-globin gene. In contrast, the adenovirus E1a protein did not activate the endogenous gene and inhibited expression of the plasmid-borne alpha2-globin gene. Previous studies have led to the hypothesis that chromosomal alpha-globin genes are subject to chromatin-dependent repression mechanism that prevents expression in nonerythroid cells. Our data suggest that HSV ICP0 and ICP4 either break or bypass this cellular gene silencing mechanism.

Identification of a promoter-specific transactivation domain in the herpes simplex virus regulatory protein ICP4

ICP4 is expressed during the immediate-early phase of infection by herpes simplex virus (HSV) and activates transcription of viral genes during subsequent phases of productive infection. Several members of the alpha-herpesvirus family encode regulatory proteins that have extensive homology with ICP4 and exhibit a transactivation domain (TAD) at the N terminus. The portions of ICP4 required for nuclear localization, DNA binding, and dimerization have been defined, but a domain that is specifically required for transactivation has not been identified. We have defined a promoter-specific ICP4 TAD by analysis of the activity of GAL4-ICP4 fusion proteins cotransfected into HeLa cells with a luciferase reporter gene linked to a promoter with five GAL4 binding sites. The transactivation activity of GAL4-ICP4 hybrids is located entirely within the first 139 residues of ICP4 and is significantly less potent than the activity of GAL4-TAD hybrids derived from ICP4 homologs. ICP4 residues 97 to 109 are a critical component of this N-terminal TAD. Transient transfection assays performed with nonfusion forms of ICP4 and luciferase genes linked to the HSV glycoprotein D (gD) or thymidine kinase (tk) promoter revealed that ICP4 residues 97 to 109 are required for induction of the gD promoter but are not required for induction of the tk promoter. Comparative experiments with ICP4 homologs revealed that the pseudorabies virus TAD is a potent activator of the gD promoter and a weak activator of the tk promoter. Complementation assays revealed that loss of ICP4 residues 97 to 109 reduced the yield of virus from infected cells nearly 500-fold compared to wild-type ICP4. We conclude that ICP4 residues 97 to 109 are a core component of a promoter-specific transactivation domain that is required for efficient replication of herpes simplex virus.

Interaction of the viral activator protein ICP4 with TFIID through TAF250

ICP4 of herpes simplex virus is responsible for the activation of viral transcription during infection. It also efficiently activates and represses transcription in vitro depending on the promoter context. The contacts made between ICP4 and the cellular proteins that result in activated transcription have not been identified. The inability of ICP4 to activate transcription with TATA-binding protein in place of TFIID and the requirement for an initiator element for efficient ICP-4-activated transcription suggest that coactivators, such as TBP-associated factors, are involved (B. Gu and N. DeLuca, J. Virol. 68:7953-7965, 1994). In this study we showed that ICP4 activates transcription in vitro using an immunopurified TFIID, indicating that TBP-associated factors may be a sufficient subset of coactivators for ICP4-activated transcription. Similar to results seen in vivo, the presence of the ICP4 C-terminal region (amino acids 774 to 1298) was important for activation in vitro. With epitope-tagged ICP4 molecules in immunoaffinity experiments, it was shown that the C-terminal region was also required for ICP4 to interact with TFIID present in a crude transcription factor fraction. In the same assay, ICP4 was unable to interact with the basal transcription factors, TFIIB, TFIIE, TFIIF, and TFIIH and RNA polymerase II. ICP4 could also interact with TBP, independent of the C-terminal region. However, reflective of the interaction between ICP4 and TFIID, the ICP4 C-terminal region was required for an interaction with FAF250-TBP complexes and with TAF250 alone. Therefore, the interfaces or conformation of TBP mediating the interaction between ICP4 and TBP in solution is probably masked when TBP is bound to TAF250. With a series of mutant ICP4 molecules purified from herpes simplex virus-infected cells, it was shown that ICP4 molecules that can bind DNA and interact with TAF250 could activate transcription. Taken together, these results demonstrate that ICP4 interaction with TFIID involves the TAF250 molecule and the C-terminal region of ICP4 and that this interaction is part of the mechanism by which ICP4 activates transcription.

Repression of the alpha0 gene by ICP4 during a productive herpes simplex virus infection

During a productive infection by herpes simplex virus type 1 (HSV-1), ICP4, the major regulatory protein encoded by the alpha4 gene, binds to its transcription initiation site and represses the accumulation of alpha4 RNA. Evidence suggests that the degree of repression by ICP4 is a function of the absolute distance of an ICP4 binding site 3' from a TATA box. However, repression of HSV-1 gene expression by ICP4 through binding sites located 5' of TATA boxes, as in the case of the alpha0 gene, has not been adequately addressed. To this end, recombinant alpha0 promoters with various arrays of ICP4 binding sites flanking the alpha0 TATA box were constructed and recombined into the HSV-1 genome. Our results demonstrate the following. (i) Destruction of the endogenous alphaO ICP4 binding site, located 5' of the TATA box, results in derepression of alpha0 protein and RNA accumulation in infected Vero cells. (ii) The degree of alpha0 derepression is equivalent to that reported for the alpha4 gene following destruction of the ICP4 binding site at the alpha4 mRNA cap site in HSV-1. (iii) Introduction of an ICP4 binding site at the alpha0 mRNA cap site represses the accumulation of alpha0 RNA greater than threefold relative to the wild type. (iv) Changes in the abundance of alpha0 protein and RNA in infected cells do not affect replication or growth of HSV-1 in tissue culture. Our findings are consistent with the conclusion that alpha0 transcription is repressed by ICP4. These results demonstrate that repression by ICP4 can occur through binding sites located 5' of virus gene TATA boxes in HSV-1. Thus, models addressing repression of HSV-1 gene expression by ICP4 should incorporate the role of binding sites located 5', as well as 3', of virus gene TATA boxes.

Analysis of phosphorylation sites of herpes simplex virus type 1 ICP4

The herpes simplex virus ICP4 protein is required for induction of early and late viral gene transcription as well as for repression of expression of its own gene and several other viral genes. Several electrophoretic forms of ICP4 have been observed, and phosphorylation is thought to contribute to this heterogeneity and possibly to the multiple functions of ICP4. To define the complexity of the site(s) of phosphorylation of ICP4 and to initiate mapping of this site(s), we have performed two-dimensional phosphopeptide mapping of wild-type and mutant forms of ICP4 labeled in infected cells or in vitro. Wild-type ICP4 labeled in infected cells shows a complex pattern of phosphopeptides, and smaller mutant forms of ICP4 show progressively fewer phosphopeptides, arguing that multiple sites on ICP4 are phosphorylated. The serine-rich region of ICP4, residues 175 to 198, was shown to be a site for phosphorylation. Furthermore, the serine-rich region itself or the phosphorylation of this region increases phosphorylation of all phosphopeptides. A mutant ICP4 molecule lacking the serine-rich region showed low levels of phosphorylation by protein kinase A or protein kinase C in vitro. These results suggest that there may be a sequential phosphorylation of ICP4, with phosphorylation of the serine-rich region stimulating phosphorylation of the rest of the molecule. In addition, purified ICP4 showed an associated kinase activity or an autophosphorylation activity with properties different from those of protein kinase A or protein kinase C.

Role of protein kinase A and the serine-rich region of herpes simplex virus type 1 ICP4 in viral replication

Efficient expression of herpes simplex virus genes requires the synthesis of functional ICP4, a nuclear phosphoprotein that contains a prominent serine-rich region between amino acids 142 and 210. Residues in this region not only are potential sites for phosphorylation but also are involved in the functions of ICP4. By comparing the growth of a virus in which this region is deleted (d8-10) with wild-type virus (KOS) in PC12 cells or PC12 cells that are deficient in cyclic AMP-dependent protein kinase (PKA), two observations were made: (i) the growth of wild-type virus was impaired by 1 to 2 orders of magnitude in the PKA-deficient cells, indicating the involvement of PKA in the growth cycle of herpes simplex virus type 1, and (ii) while the growth of d8-10 was impaired by almost 2 orders of magnitude in wild-type cells, it was not further impaired (as was that of wild-type virus) in PKA-deficient cells, implicating the region deleted in d8-10 as a possible target for cellular PKA. In trigeminal'ganglia of mice, the d8-10 mutant virus grew poorly; however, it established latency in nearly 90% of ganglia tested. Studies of the phosphorylation of wild-type and d8-10 ICP4 proteins revealed that the serine-rich region is a major determinant for phosphorylation of ICP4 in vivo and that the phosphorylation state could change as a function of the PKA activity. Consistent with this observation, the serine-rich region of ICP4 was shown to be a target for PKA in vitro. While intact ICP4 was readily phosphorylated by ICP4 in vitro, the d8-10 mutant ICP4 was not. Moreover, a synthethic peptide representing a sequence in the serine tract that is predicted to be a substrate for PKA was phosphorylated by PKA in vitro, having a Km within the physiological range. These data suggest that PKA plays a role in viral growth through phosphorylation of one or more sites on the ICP4 molecule.

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)

Mapping of a functional region conferring nuclear localization of pseudorabies virus immediate-early protein

The immediate-early protein (IE180) of pseudorabies virus (PrV) is localized predominantly in the nuclei of infected cells. To define the nuclear localization signals within IE180, we prepared truncated mutants of IE180 and analyzed their localization in the transfected cells by indirect immunofluorescence. Analysis of mutants truncated from the carboxy-terminal end of the 1460-amino acid polypeptide showed that two regions including a short sequence of basic amino acid residues were associated with the nuclear localization of IE180. To assess whether these regions substantially function as signals for nuclear localization of the IE180 molecule, we then constructed two deletion mutants lacking each region. A mutant lacking amino acids 333 to 575 was detected in the nuclei of the transfected cells, whereas the other mutant lacking amino acids 900 to 950 was detected mainly in the cytoplasm. These results suggest that the region of amino acids 900 to 950 is responsible for nuclear localization of IE180.

Functional interactions between herpes simplex virus immediate-early proteins during infection: gene expression as a consequence of ICP27 and different domains of ICP4

Two of the five immediate-early gene products, ICP4 and ICP27, expressed by herpes simplex virus type 1 have profound effects on viral gene expression and are absolutely essential for virus replication. Functional interactions between ICP4 and ICP27 may contribute to establishing the program of viral gene expression that ensues during lytic infection. To evaluate this possibility, viral mutants simultaneously deleted for ICP27 and defined functional domains of ICP4 were constructed. These mutant viruses allowed a comparison of gene expression as a function of different domains of ICP4 in the presence and absence of ICP27. Gene expression in the absence of both ICP4 and ICP27 was also examined. The results of this study demonstrate a clear involvement for ICP27 in the induction of early genes, in addition to its known role in enhancing late gene expression during viral infection. In the absence of both ICP4 and ICP27, viral early gene expression, as measured by the accumulation of thymidine kinase and ICP6 messages was dramatically reduced relative to the amounts of these messages seen in the absence of only ICP4. Therefore, elevated levels of early gene expression as a consequence of ICP27 occurred in the absence of any ICP4 activity. Evidence is also presented regarding the modulation of the ICP4 repression function by ICP27. When synthesized in the absence of ICP27, a mutant ICP4 protein was impaired in its ability to repress transcription from the L/ST promoter in the context of viral infection and in vitro. This defect correlated with the loss of the ability of this mutant protein to bind to its recognition sequence when produced in infected cells in the absence of ICP27. These observations indicate that ICP27 can regulate the activity of at least one domain of the ICP4 protein as well as contribute to elevated early gene expression independently of ICP4.

Repression of activator-mediated transcription by herpes simplex virus ICP4 via a mechanism involving interactions with the basal transcription factors TATA-binding protein and TFIIB

Infected-cell polypeptide 4 (ICP4) of herpes simplex virus is both a transcriptional activator and a repressor. It has been previously demonstrated that both SP1-activated transcription and USF-activated transcription are repressed by ICP4 without affecting basal transcription (B. Gu, R. Rivera-Gonzalez, C. A. Smith, and N. A. DeLuca, Proc. Natl. Acad. Sci. USA 90:9528-9532, 1993; R. Rivera-Gonzalez, A. N. Imbalzano, B. Gu, and N.A. DeLuca, Virology 202:550-564, 1994). In this study, it was found that ICP4 repressed the activation function of two other activators, VP16 and ICP4 itself, in vitro. ICP4 inhibited transcription by interfering with the formation of transcription initiation complexes without affecting transcription elongation. Repression of activator function required that an ICP4 DNA binding site was present in one orientation within approximately 45 bp 3' to the TATA box. DNA binding by ICP4 was necessary but not sufficient for repression. ICP4 has been shown to form tripartite complexes cooperatively with the TATA box-binding protein and TFIIB on DNA containing an ICP4 binding site and a TATA box (C. A. Smith, P. Bates, R. Rivera-Gonzalez, B. Gu, and N. DeLuca, J. Virol. 67:4676-4687, 1993). A region of ICP4 that enables the molecule to form tripartite complexes was also required in addition to the DNA binding domain for efficient repression. Moreover, repression was observed only when the ICP4 binding site was in a position that resulted in the formation of tripartite complexes. Together, the data suggest that ICP4 represses transcription by binding to DNA in a precise way so that it may interact with the basal transcription complex and inhibit some general step involved in the function of activators. The steps or interactions involved in transcriptional activation that are inhibited by ICP4 are discussed.

Nuclear localization and transcriptional activation activities of truncated versions of the immediate-early gene product of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) immediate-early (IE) gene product encodes a nuclear regulatory protein capable of negatively autoregulating its own promoter, transactivating representative EHV-1 early promoters, and acting in a concerted fashion with accessory EHV-1 regulatory factors to transactivate EHV-1 late promoters. To identify IE amino acid sequences involved in nuclear localization and to examine the contribution of C-terminal portions of the IE polypeptide to transactivation, vectors that express various carboxyterminally truncated IE polypeptides were constructed. It is demonstrated that amino acids 963 through 970 of the 1,487-amino-acid IE protein are required for efficient localization of the truncated IE polypeptides to the nuclei of transfected cells. In addition, it is demonstrated that the first 970 amino acids of the IE gene product are sufficient to transactivate the EHV-1 thymidine kinase promoter to significant levels (i.e., approximately 40% of the level of wild-type activation).

Repression of the herpes simplex virus 1 alpha 4 gene by its gene product (ICP4) within the context of the viral genome is conditioned by the distance and stereoaxial alignment of the ICP4 DNA binding site relative to the TATA box

Infected cell protein no. 4 (ICP4), the major regulatory protein encoded by the alpha 4 gene of herpes simplex virus 1, binds to a site (alpha 4-2) at the transcription initiation site of the alpha 4 gene. An earlier report described the construction of recombinant viruses that contained chimeric genes (alpha 4-tk) that consisted of the 5' untranscribed and transcribed noncoding domains of the alpha 4 gene fused to the coding sequences of the thymidine kinase gene and showed that disruption of the alpha 4-2 binding site by mutagenesis derepressed transcription of this gene (N. Michael and B. Roizman, Proc. Natl. Acad. Sci. USA 90:2286-2290, 1993). This experimental design was used to determine the effect of displacement of the alpha 4-2 binding site on the repression of alpha 4 gene transcription by ICP4. We report the following findings. (i) In the absence of the alpha 4-2 binding site, at 4 h after infection, alpha 4-tk RNA levels increased 10-fold relative to the corresponding RNA levels of a gene that contained the alpha 4-2 site at its natural location. Displacement of the alpha 4-2 binding site by approximately one, two, and three turns of the DNA helix, i.e., by 10, 21, and 30 nucleotides downstream of the original site, increased the concentration of alpha 4-tk RNA 2.4-, 3.5-, and 5.8-fold, respectively. (ii) Displacement of 16 nucleotides, i.e., approximately 1.5 helical turns, increased the accumulation of alpha 4-tk by 5.3-fold, i.e., more than predicted by displacement alone. (iii) At 8 h after infection in the absence of the binding site, the accumulation of alpha 4-tk RNA increased 13.6-fold. However, in cells infected with recombinants that carried displaced alpha 4-2 binding sites, RNA accumulation decreased relative to the levels seen at 4 h after infection. The insertion of DNA sequences in order to displace the alpha 4-2 binding site had no effect on accumulation of RNA in the presence of cycloheximide, i.e., in the absence of ICP4, or on maximum accumulation of alpha 4-tk RNA in the absence of the alpha 4-2 binding site.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutational analysis of varicella-zoster virus major immediate-early protein IE62

The varicella-zoster virus (VZV) open reading frame 62 encodes an immediate-early protein (IE62) that transactivates expression of various VZV promoters and autoregulates its own expression in transient expression assays. In Vero cells, IE62 was shown to transactivate the expression of all putative immediate-early (IE) and early (E) genes of VZV with an up-regulating effect at low intracellular concentrations. To define the functional domains involved in the regulatory properties of IE62, a large number of in-frame insertions and deletions were introduced into a plasmid-borne copy of the gene encoding IE62. Studies of the regulatory activities of the resultant mutant polypeptides in transient expression assays allowed to delineate protein regions important for repression of its own promoter and for transactivation of a VZV putative immediate-early gene (ORF61) promoter and an early gene (ORF29) promoter. This mutational analysis resulted in the identification of a new functional domain situated at the border between regions 4 and 5 which plays a crucial role in the IE62 regulatory functions. This domain turned out to be very well conserved amongst homologous alphaherpesvirus regulatory proteins and appeared to be rich in bulky hydrophobic and proline residues, similar to the proline-rich region of the CAAT box binding protein CTF-1. By immunofluorescence, a nuclear localization signal has been mapped in region 3.

Requirements for activation of the herpes simplex virus glycoprotein C promoter in vitro by the viral regulatory protein ICP4

During infection with herpes simplex virus, infected-cell polypeptide 4 (ICP4) activates transcription of most herpes simplex virus genes. In the present study, the mechanism of activation of transcription by ICP4 was investigated by using a reconstituted in vitro system with fractionated and purified general transcription factors, coupled with DNA-binding assays. The templates used in the reactions included regions of the gC and thymidine kinase (tk) promoters in plasmids, and on isolated fragments, allowing for the evaluation of the potential function of naturally occurring and inserted ICP4-binding sites and elements of the core promoter. ICP4 efficiently activated transcription of the gC promoter by facilitating the formation of transcription initiation complexes. ICP4 could not substitute for any of the basal transcription factors. Moreover, TATA-binding protein (TBP) could not substitute for TFIID in activation, suggesting a requirement for TBP-associated factors. Interactions between ICP4 and DNA 3' to the start site was necessary for activation of the gC promoter. The requirement for DNA-protein contacts could be met either by the presence of an ICP4-binding site in the gC leader, by the presence of a site more than 150 nucleotides further downstream, by an inserted site that normally acts to repress transcription, or by the addition of sufficient non-site-containing DNA. The gC TATA box and start site, or initiator element (inr), were individually sufficient for activation by ICP4 and together contributed to optimal activation. In contrast to gC, the tk promoter was poorly activated in the reconstituted system. However, the tk TATA box was efficiently activated when the tk start site region was replaced with the gC inr, suggesting that activation was mediated through the inr and inr-binding proteins. In addition, mutation of the inr core resulted in a gC promoter that was very poorly activated by ICP4. The results of this and previous studies demonstrate that ICP4 activates transcription in a complex manner involving contacts with DNA 3' to the start site, TBP, TFIIB, TBP-associated factors, and possibly proteins functioning at the start site of transcription.

Physical interaction between the herpes simplex virus type 1 immediate-early regulatory proteins ICP0 and ICP4

The herpes simplex virus type 1 immediate-early protein ICP0 enhances expression of a spectrum of viral genes alone and synergistically with ICP4. To test whether ICP0 and ICP4 interact physically, we performed far-Western blotting analysis of proteins from mock-, wild-type-, and ICP4 mutant virus-infected cells with in vitro-synthesized [35S]Met-labeled ICP0 and ICP4 as probes. The ICP4 and ICP0 polypeptides synthesized in vitro exhibited molecular weights similar to those of their counterparts in herpes simplex virus type 1-infected cells, and the in vitro-synthesized ICP4 was able to bind to a probe containing the ICP4 consensus binding site. Far-Western blotting experiments demonstrated that ICP0 interacts directly and specifically with ICP4 and with itself. To further define the interaction between ICP0 and ICP4, we generated a set of glutathione S-transferase (GST)-ICP0 fusion proteins that contain GST and either ICP0 N-terminal amino acids 1 to 244 or 1 to 394 or C-terminal amino acids 395 to 616 or 395 to 775. Using GST-ICP0 fusion protein affinity chromatography and in vitro-synthesized [35S]Met-labeled ICP0 and ICP4, ICP4 was shown to interact preferentially with the fusion protein containing ICP0 C-terminal amino acids 395 to 775, whereas ICP0 interacted efficiently with both the N-terminal GST-ICP0 fusion proteins and the C-terminal GST-ICP0 fusion proteins containing amino acids 395 to 775. Fusion protein affinity chromatography also demonstrated that the C-terminal 235 amino acid residues of ICP4 are important for efficient interaction with ICP0. Collectively, these results reveal a direct and specific physical interaction between ICP0 and ICP4.

Improved cell survival by the reduction of immediate-early gene expression in replication-defective mutants of herpes simplex virus type 1 but not by mutation of the virion host shutoff function

Derivatives of herpes simplex virus type 1 (HSV-1) have elicited considerable interest as gene transfer vectors because of their ability to infect a wide range of cell types efficiently, including fully differentiated neurons. However, it has been found that infection of many types of cell with vectors derived from replication-defective mutants of HSV-1 is associated with cytopathic effects (CPE). We have previously shown that viral gene expression played an important role in the induction of CPE caused by an HSV-1 mutant deleted for the essential immediate-early gene 3 (IE 3) (P.A. Johnson, A. Miyanohara, F. Levine, T. Cahill, and T. Friedmann, J. Virol. 66:2952-2965, 1992). We have investigated which viral genes might be responsible for CPE by comparing the ability of each of the individual genes expressed by an IE 3 deletion mutant during a nonproductive infection to inhibit biochemical transformation after cotransfection of BHK or CV-1 cells with a selectable marker gene. Transfection of IE genes 1,2, and 4 individually all caused a marked inhibition of colony formation, while transfection of IE 5 and the large subunit of ribonucleotide reductase had little effect. These results suggested that it would be necessary to mutate or reduce the expression of nearly all HSV-1 IE genes to reduce virus-induced CPE. Therefore, we have used VP16 mutants, which are unable to transinduce IE gene expression (C. I. Ace, T. A. McKee, J. M. Ryan, J. M. Cameron, and C. M. Preston, J. Virol. 63:2260-2269, 1989), to derive two replication-defective strains: 14H delta 3, which is deleted for both copies of IE 3, and in 1850 delta 42, which has a deletion in the essential early gene UL42. The IE 3-VP16 double mutant, 14H delta 3, is significantly less toxic than a single IE 3 deletion mutant over a range of multiplicities of infection, as measured in a cell-killing assay, and has an enhanced ability to persist in infected cells in a biologically retrievable form. In contrast, the UL42-VP16 double mutant, in 1850 delta 42, showed reduced toxicity only at low multiplicities of infection. To test the role of the virion host shutoff function as an additional candidate to influence virus-induced CPE, we have introduced a large insertion mutation into the virion host shutoff gene of an IE 3 deletion mutant and the double mutant 14H delta 3.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of a 34-amino-acid segment implicated in dimerization of the herpes simplex virus type 1 ICP4 polypeptide by a dimerization trap

The herpes simplex virus type 1 immediate-early protein ICP4 plays an essential role in the regulation of the expression of all viral genes. It is the major trans activator of early and late genes and also has a negative regulatory effect on immediate-early gene transcription. ICP4 is a sequence-specific DNA-binding protein and has always been purified in a dimeric form. The part of the protein that consists of the entire highly conserved region 2 and of the distal portion of region 1 retains the ability to specifically associate with DNA and to form homodimers in solution. In an attempt to map the dimerization domain of ICP4, we used a dimerization trap assay, in which we screened deletion fragments of this 217-amino-acid stretch for sequences that could confer dimerization properties on a heterologous cellular transcription factor (LFB1), which binds to its cognate DNA sequence only as a dimer. The analysis of these chimeric proteins expressed in vitro ultimately identified a stretch of 34 amino acids (343 to 376) that could still confer DNA-binding activity on the LFB1 reporter protein and thus apparently contained the ICP4 dimerization motif. Consistent with this result, a truncated ICP4 protein containing amino acids 343 to 490, in spite of the complete loss of DNA-binding activity, appeared to retain the capacity to form a heterodimer with a longer ICP4 peptide after coexpression in an in vitro translation system.

Identification and zinc dependence of the bovine herpesvirus 1 transactivator protein BICP0

Bovine herpesvirus 1 (BHV-1) specifies and unspliced early 2.6-kb RNA (ER2.6) which is 3' coterminal with exon 2 of the 2.9-kb immediate-early (IE) RNA. The two transcripts have a common open reading frame (676 codons). The predicted protein, designated BHV-1 infected cell protein 0 (BICP0), contains a zinc finger domain with homology to ICP0 of herpes simplex virus type 1 and protein 61 of varicella-zoster virus, and depending on the promoter, it acts as a strong activator or as a repressor in transient expression assays. In situ immunoadsorbent assays using antisera against synthetic oligopeptides demonstrated that BICP0 accumulates in nuclei of BHV-1-infected cells, as expected for an IE gene product involved in gene regulation. Western blots (immunoblots) revealed a BHV-1-specific 97-kDa protein which was detectable during the IE phase and also at later periods of infection, indicating that the kinetics of BICP0 synthesis is consistent with the switch from IER2.9 to ER2.6. To confirm that ER2.6 encoded the 97-kDa BICP0 protein, a DNA fragment containing BICP0-coding sequences was inserted into the Autographa californica baculovirus genome. A recombinant protein, identified by its reactivity with antipeptide sera, exhibited the same electrophoretic mobility as BICP0 specified by BHV-1. We microinjected Xenopus oocytes with a BICP0 effector plasmid and a promoter-chloramphenicol acetyltransferase plasmid. BICP0-induced stimulation of this promoter was strongly reduced when intracellular zinc was chelated by thionein, indicating that the effect of BICP0 is zinc dependent.

Identification of functional regions of herpes simplex virus glycoprotein gD by using linker-insertion mutagenesis

Glycoprotein gD is a component of the herpes simplex virus (HSV) envelope essential for virus entry into susceptible cells. Previous studies using deletion and point mutations identified a functional domain of HSV-1 gD (gD-1) from residues 231 to 244. However, many of the deletion mutations had global effects on gD-1 structure, thus precluding assessment of the functional role of large portions of the protein. In this study, we constructed a large panel of linker-insertion mutants in the genes for gD-1 and HSV-2 gD (gD-2). The object was to create mutations which would have only localized effects on protein structure but might have profound effects on gD function. The mutant proteins were expressed in transiently transfected L cells. Monoclonal antibodies (MAbs) were used as probes of gD structure. We also examined protein aggregation and appearance of the mutant glycoproteins on the transfected cell surface. A complementation assay measured the ability of the mutant proteins to rescue the infectivity of the gD-null virus, FgD beta, in trans. Most of the mutants were recognized by one or more MAbs to discontinuous epitopes, were transported to the transfected cell surface, and rescued FgD beta virus infectivity. However, some mutants which retained structure were unable to complement FgD beta. These mutants were clustered in four regions of gD. Region III (amino acids 222 to 246) overlaps the region previously defined by gD-1 deletion mutants. The others, from 27 through 43 (region I), from 125 through 161 (region II), and from 277 to 310 (region IV), are newly described. Region IV, immediately upstream of the transmembrane anchor sequence, was previously postulated to be part of a putative stalk structure. However, residues 277 to 300 are directly involved in gD function. The linker-insertion mutants were useful for mapping MAb AP7, a previously ungrouped neutralizing MAb, and provided further information concerning other discontinuous epitopes. The mapping data suggest that regions I through IV are physically near each other in the folded structure of gD and may form a single functional domain.

The DNA binding domains of the varicella-zoster virus gene 62 and herpes simplex virus type 1 ICP4 transactivator proteins heterodimerize and bind to DNA

The product of varicella-zoster virus gene 62 (VZV 140k) is the functional counterpart of the major transcriptional regulatory protein of herpes simplex virus type 1 (HSV-1), ICP4. We have found that the purified bacterially expressed DNA binding domain of VZV 140k (residues 417-647) is a stable dimer in solution. As demonstrated by the appearance of a novel protein--DNA complex of intermediate mobility in gel retardation assays, following in vitro co-translation of a pair of differently sized VZV 140k DNA binding domain peptides, the 140k DNA binding domain peptide binds to DNA as a dimer. In addition, the DNA binding domain peptide of HSV-1 ICP4 readily heterodimerizes with the VZV 140k peptide on co-translation, indicating that HSV-1 ICP4 and VZV 140k possess very similar dimerization interfaces. It appears that only one fully wild type subunit of the dimer is sufficient to mediate sequence specific DNA recognition in certain circumstances. Co-immunoprecipitation analysis of mutant DNA binding domain peptides, co-translated with an epitope-tagged ICP4 DNA binding domain, shows that the sequence requirements for dimerization are lower than those necessary for DNA binding.

Mutation of a single lysine residue severely impairs the DNA recognition and regulatory functions of the VZV gene 62 transactivator protein

The product of varicella-zoster virus gene 62 (VZV 140k) is a potent transactivator protein. We have identified a region within the DNA binding domain of VZV 140k that shows a striking similarity to the DNA recognition helix of the homeodomain, with an especially highly conserved quartet of residues, WLQN. The 140k protein has functional counterparts within the other alphaherpesviruses, which include the major transcriptional regulatory protein of HSV-1, (ICP4), and the WLQN region is highly conserved among the members of this family of viral transactivators. Substitution of VZV 140k residue lysine 548, just adjacent to the WLQN region, drastically reduces the DNA binding activity of the 140k DNA binding domain and the intact 140k mutant protein fails to activate gene expression. Substitutions of two other VZV 140k residues in this conserved WLQN region result in alterations to the DNA binding interaction and reduced transactivation activities. All three mutations act at the level of DNA recognition, as they have no apparent effect on the dimerization state, solubility or efficiency of expression of the mutant peptides.

Amino acid substitution mutations in the herpes simplex virus ICP27 protein define an essential gene regulation function

ICP27 is an essential herpes simplex virus type 1 (HSV-1) alpha protein that is required for the transition from the beta to the gamma phase of infection. To identify functional regions of ICP27, we constructed 16 plasmids that contain nucleotide substitution mutations in the ICP27 gene. The mutations created XhoI restriction sites, altered one or two codons, and were spaced at semiregular intervals throughout the coding region. Three mutations completely inactivated an essential function of ICP27, as demonstrated by the inability of the transfected plasmids to complement the growth of an HSV-1 ICP27 deletion mutant. These mutations, M11, M15, and M16, mapped in the carboxyl-terminal one-third of ICP27 at residues 340 and 341, 465 and 466, and 488, respectively. In cotransfection assays, all three defective-plasmid mutants retained the transrepression function of ICP27 but were defective at transactivation. To define the lytic functions that are mediated by the transactivation activity of ICP27, we engineered HSV-1 recombinants containing the M11, M15, or M16 mutation. All three viral mutants failed to grow in Vero cells and possessed similar phenotypes. The viral mutants replicated their DNA similarly to the wild-type virus but showed several defects in viral gene expression. These were a failure to down-regulate alpha and beta genes at late times after infection and an inability to induce certain gamma-2 genes. Our results demonstrate that the transactivation function of ICP27 (as it is defined in cotransfection assays) mediates an essential gene regulation function during the HSV-1 infection. This activity is not required for ICP27-dependent enhancement of viral DNA replication. Our work supports and extends previous studies which suggest that ICP27 carries out two distinct regulatory activities during the HSV-1 infection.

Binding sites for the herpes simplex virus immediate-early protein ICP4 impose an increased dependence on viral DNA replication on simple model promoters located in the viral genome

We examined the ability of binding sites for the herpes simplex virus immediate-early protein ICP4 to alter the regulation of closely linked promoters by placing strong ICP4 binding sites upstream or downstream of simple TATA promoters in the intact viral genome. We found that binding sites strongly reduced the levels of expression at early times postinfection and that this effect was partially overcome after the onset of viral DNA replication. These data confirm that DNA-bound ICP4 can inhibit the activity of a closely linked promoter and raise the possibility that ICP4 binding sites contribute to temporal regulation during infection.

ICP4, the major transcriptional regulatory protein of herpes simplex virus type 1, forms a tripartite complex with TATA-binding protein and TFIIB

The ICP4 protein of herpes simplex virus can either increase or decrease the rate of transcription mediated by RNA polymerase II, depending on the target promoter. The interplay of DNA-protein and protein-protein contacts determining ICP4 function has yet to be characterized, and consequently the molecular mechanism by which the protein acts remains unclear. ICP4 can transactivate minimal promoters containing only TATA homologies, and therefore it is reasonable to hypothesize that ICP4 works by influencing the TATA-dependent assembly of general transcription factors via specific protein-protein interactions. This study directly addresses this hypothesis by determining whether ICP4 affects the assembly of general transcription factors on templates bearing a TATA box and an ICP4-binding site. Using gel retardation and footprinting assays, we found that ICP4 forms a tripartite complex with TFIIB and either the TATA-binding protein (TBP) or TFIID. The formation of this complex was not the result of simple tripartite occupancy of the DNA but the consequence of protein-protein interactions. In the presence of all three proteins, the affinity of ICP4 and TBP for their respective binding sites was substantially increased. Using mutant derivatives of ICP4 and defective versions of promoters, we also demonstrated that the ability of ICP4 to regulate gene expression correlated with its ability to form a tripartite complex with TFIIB and TBP in vitro.

The transcriptional activation domain of varicella-zoster virus open reading frame 62 protein is not conserved with its herpes simplex virus homolog

Varicella-zoster virus (VZV) open reading frame 62 (ORF62) encodes an immediate-early protein that transactivates expression of VZV, herpes simplex virus (HSV), and cellular genes in transient expression assays. VZV ORF62 is homologous to HSV ICP4 and pseudorabies virus immediate-early (IE180) proteins. All three viral proteins have conserved DNA binding domains that recognize similar sites in their corresponding promoters. Here, we show that the transcriptional activation domain of ORF62 is located near the amino terminus of the protein and is not conserved with the activation domain of ICP4. A 161-amino-acid activation domain of ORF62 activates transcription to a level comparable to that of the potent HSV VP16 activation domain; much of the activity is contained in the first 90 amino acids of ORF62. Deletion of the activation domain from full-length ORF62 markedly reduced transactivating activity. These experiments indicate that while VZV ORF62 and HSV ICP4 have conserved amino acid sequences, including their DNA binding domains, the transcriptional activation domains are poorly conserved.

Repression of the herpes simplex virus 1 alpha 4 gene by its gene product occurs within the context of the viral genome and is associated with all three identified cognate sites

The infected cell protein 4 (ICP-4), the major regulatory protein encoded by the a4 gene of the herpes simplex virus 1, binds two sites (alpha 4-1 proximal, alpha 4-1 distal) at the 5'-untranscribed domain and at the transcription initiation site (alpha 4-2) of the alpha 4 gene. Chimeric genes consisting of the 5'-untranscribed and transcribed noncoding domains of the alpha 4 gene fused to the coding sequences of the thymidine kinase gene were mutagenized to abolish binding of ICP-4 by substitution of bases, including the guanines whose methylation interferes with binding of the protein, and recombined into the viral genome. The cytoplasmic RNAs extracted from infected cells treated with cycloheximide, from untreated infected cells maintained for 4 or 8 hr, and from cells infected first with a virus deleted in the alpha 22 gene and 3 hr later with the test viruses were tested in RNase protection assay for amounts of the chimeric gene RNA relative to amounts of alpha 22 gene RNA. We report the following: (i) Mutation of the alpha 4-2 binding site resulted in a 5-to 6-fold higher accumulation of chimeric gene RNA at 4 hr and as much as 15-fold higher accumulation by 8 hr after infection. (ii) Mutations of alpha 4-1 sites by themselves had no effect on RNA accumulation. However, mutagenesis of all three sites significantly increased mRNA amounts above the levels seen in cells infected with alpha 4-2 site mutants. (iii) The mutations have no effect on accumulation of alpha 4 mRNA in the absence of ICP-4 synthesis and, therefore, the mutations had no effect on RNA stability or transcription rate. (iv) Accumulation of alpha 4 mRNA relative to that of alpha 22 mRNA is highest in the presence of cycloheximide and decreases with time after infection. We conclude that ICP-4 autoregulates the transcription of its own gene in infected cells and that binding of ICP-4 to three sites in its promoter is additive in its effects on this process.

The DNA binding domain of the varicella-zoster virus gene 62 protein interacts with multiple sequences which are similar to the binding site of the related protein of herpes simplex virus type 1

Varicella-zoster virus gene 62 encodes a protein with predicted Mr of 140,000D (VZV 140k) that shares extensive predicted amino acid sequence homology with the major immediate early (IE) transcriptional regulator protein of herpes simplex virus type 1 (HSV-1) Vmw175. The integrity of highly conserved region 2 is essential for the DNA binding and transcriptional regulatory functions of Vmw175. Similarly, an insertion mutation in region 2 (codons 468-641) of 140k eliminates the transcriptional repression and activation functions of this protein. We have expressed a fragment of 140k which encompasses region 2 as a non-fusion polypeptide in bacteria. This 140k DNA binding domain peptide (codons 417-646) binds to numerous DNA sequences throughout the VZV gene 62 promoter region. It induces multiple regions of protection from DNase I digestion, flanked by sites of DNase I hypersensitivity. Several of the sites recognized can be considered to be divergent forms of the consensus sequence which is recognized by Vmw175. However, by use of a panel of mutagenized probe fragments, we found that the 140k DNA binding domain was less sequence-specific than Vmw175 in its interactions with DNA. Consistent with this, the homologous Vmw175 DNA binding domain, and also intact Vmw175, recognize the gene 62 binding sites much less efficiently than the 140k DNA binding domain. Also in contrast to the situation with Vmw175, the 140k DNA binding domain failed to induce DNA bending when occupying the binding sites in its own promoter. Deletion analysis has mapped the minimal DNA binding domain of the VZV 140k protein, as measured in gel retardation analysis, to lie within residues 472 to 633. The differences in binding characteristics of the DNA binding domains of the homologous VZV 140k and HSV-1 Vmw175 IE proteins may account for the subtle differences in their regulatory activities in transfection assays and during virus growth in tissue culture.

Differences in the poly(ADP-ribosyl)ation patterns of ICP4, the herpes simplex virus major regulatory protein, in infected cells and in isolated nuclei

Infected-cell protein 4 (ICP4), the major regulatory protein in herpes simplex viruses 1 and 2, was previously reported to accept 32P from [32P]NAD in isolated nuclei. This modification was attributed to poly(ADP-ribosyl)ation (C. M. Preston and E. L. Notarianni, Virology 131:492-501, 1983). We determined that an antibody specific for poly(ADP-ribose) reacts with ICP4 extracted from infected cells, electrophoretically separated in denaturing gels, and electrically transferred to nitrocellulose. Our results indicate that all forms of ICP4 observed in one-dimensional gel electrophoresis are poly(ADP-ribosyl)ated. Poly(ADP-ribose) on ICP4 extracted from infected cells was resistant to cleavage by purified poly(ADP-ribose) glycohydrolase unless ICP4 was in a denatured state. Poly(ADP-ribose) added to ICP4 in isolated nuclei was sensitive to this enzyme. This result indicates that the two processes are distinct and may involve different sites on the ICP4 molecule.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

Complete nucleotide sequence of the Marek's disease virus ICP4 gene

The Marek's disease virus (MDV) gene encoding a homologue to the ICP4 protein of herpes simplex virus has been mapped to BamHl fragment A based on the physical map of the MDV genome (Fukuchi et al., 1984). The gene lies completely within the inverted repeat flanking the unique short region of the genome. The complete nucleotide sequence of the MDV ICP4 gene has been determined. The coding region is 4245 nucleotides long and has an overall G+C content of 52%. The MDV ICP4 protein is predicted to have a structure similar to that of ICP4-like proteins of other herpesviruses in that it has five distinct regions, the second and fourth of which are highly conserved. In addition, the protein contains the characteristic run of serine residues located toward its amino terminus. The MDV ICP4 gene is expressed in MDV-infected chicken embryo fibroblasts.

Characterization of the regulatory functions of the equine herpesvirus 1 immediate-early gene product

Use of the translation-inhibiting drug cycloheximide has indicated that the equine herpesvirus 1 (EHV-1) immediate-early (IE) gene, the sole EHV-1 IE gene, encodes a major viral regulatory protein since IE mRNA translation is a prerequisite for all further viral gene expression (W.L. Gray, R. P. Baumann, A. T. Robertson, G. B. Caughman, D. J. O'Callaghan, and J. Staczek, Virology 158:79-87, 1987). An EHV-1 IE gene expression vector (pSVIE) in combination with chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs was used in transient transfection assays to characterize the regulatory functions of the IE gene product. These experiments demonstrated that (i) the EHV-1 IE gene product is a bifunctional protein capable of both positive and negative modulation of gene expression; (ii) the IE gene product possesses an autoregulatory function which represses the IE promoter; (iii) IE autoregulation is dependent on IE promoter sequences mapping within positions -288 to +73 relative to the transcription initiation site (+1) of the IE gene; (iv) the IE gene product can independently activate the EHV-1 tk promoter (an early promoter) by as much as 60-fold; (v) two EHV-1 beta-gamma (leaky late) promoters, those of IR5 (gene 5 in the inverted repeat) and the glycoprotein D gene, demonstrate a requirement for both the IE gene product as well as a gene product encoded within the EHV-1 XbaI G fragment for significant activation; and (vi) the IE gene product is capable of activating heterologous viral promoters.

The ICP4 binding sites in the herpes simplex virus type 1 glycoprotein D (gD) promoter are not essential for efficient gD transcription during virus infection

Activation of the early and late genes of herpes simplex virus type 1 during infection in tissue culture requires functional immediate-early regulatory protein ICP4. ICP4 is a specific DNA-binding protein which recognizes a variety of DNA sequences, many of which contain the consensus ATCGTC. In general, mutations which impair the ability of ICP4 to bind to DNA also eliminate its ability to activate viral early and late promoters both in transfection assays and in the infected cell. However, the role of ICP4 binding sites in the viral genome is unclear; many early and late promoters do not contain consensus binding sites in their vicinity. The glycoprotein D (gD) gene contains two well-characterized ICP4 binding sites upstream of its promoter and a third downstream of the transcription start site. Multimerization of one of these sites has been shown to increase the response of the gD promoter to ICP4 in transfection assays, while their removal reduces stimulation of the gD promoter by ICP4 in vitro. To assess the role of these binding sites during virus infection, we have constructed a recombinant viral genome which has mutations affecting all three. Comparison of the amounts of gD RNA synthesized by the recombinant and wild-type viruses indicated that the mutations had little or no effect on the activity of the gD promoter. Therefore, either the sites have no essential role in gD promoter regulation in the presence of all of the herpes simplex virus type 1 IE polypeptides during a normal infection or they can be functionally substituted by other ICP4 binding sites elsewhere in the genome.

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.