Characterization of herpes simplex virus 1 alpha proteins 0, 4, and 27 with monoclonal antibodies

The infected cell protein 0 of herpes simplex virus 1 dynamically interacts with proteasomes, binds and activates the cdc34 E2 ubiquitin-conjugating enzyme, and possesses in vitro E3 ubiquitin ligase activity

The infected cell protein 0 (ICP0) of herpes simplex virus 1, a promiscuous transactivator shown to enhance the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular proteins and has been linked to the disruption of ND10 and degradation of several proteins. ICP0 contains a RING finger domain characteristic of a class of E3 ubiquitin ligases. We report that: (i) in infected cells, ICP0 interacts dynamically with proteasomes and is bound to proteasomes in the presence of the proteasome inhibitor MG132. Also in infected cells, cdc34, a polyubiquitinated E2 ubiquitin-conjugating enzyme, exhibits increased ICP0-dependent dynamic interaction with proteasomes. (ii) In an in vitro substrate-independent ubiquitination system, the RING finger domain encoded by exon 2 of ICP0 binds cdc34, whereas the carboxyl-terminal domain of ICP0 functions as an E3 ligase independent of the RING finger domain. The results indicate that ICP0 can act as a unimolecular E3 ubiquitin ligase and that it promotes ubiquitin-protein ligation and binds the E2 cdc34. It differs from other unimolecular E3 ligases in that the domain containing the RING finger binds E2, whereas the ligase activity maps to a different domain of the protein. The results also suggest that ICP0 shuttles between nucleus and cytoplasm as a function of its dynamic interactions with proteasomes.

Purification of a bacterially expressed herpes simplex virus type 1 origin binding protein for use in posttranslational processing studies

The origin binding protein (OBP) encoded by the UL9 open reading frame of herpes simplex virus type 1 (HSV-1) plays an essential role in productive infection by promoting the initiation of viral DNA synthesis. In this study, OBP was inducibly expressed in Escherichia coli and purified to homogeneity using a two-step chromatographic separation procedure. The properties of this recombinant OBP (rOBP) were found to be indistinguishable from those of the virus-encoded protein. Since rOBP was synthesized in bacterial cells, it lacked the posttranslational processing which normally occurs in OBP produced in HSV-1-infected mammalian cells and could therefore be exploited in experiments which addressed the effects of protein modification on OBP function. As an initial study, the impact of phosphorylation on enzymatic activity was examined using rOBP which had been treated with a panel of purified cellular kinases. rOBP was found to act as a substrate for nearly all of the kinases tested in (32)P-labeled phosphate transfer assays. However, only phosphorylation by protein kinase A (PKA, or cAMP-dependent protein kinase) was shown to significantly alter the enzymatic properties of rOBP, as it increased by five- to eightfold the ATPase activity associated with this protein. Activation of this critical viral DNA replication enzyme by a cAMP-dependent kinase such as PKA may be of some relevance in the natural course of HSV-1 infections, since reactivation of latent virus is thought to involve both signal transduction events and the induction of viral DNA synthesis. Thus, the expression and purification strategy outlined in this work provides an economical source of unmodified HSV-1 OBP that should prove useful in future in vitro studies.

Herpes simplex virus 1 ICP27 is required for transcription of two viral late (gamma 2) genes in infected cells

The herpes simplex virus infected cell protein 27 (ICP27) is required for the expression of certain early viral proteins and for many late proteins during productive infection. Expression of at least one late (gamma 2) gene, that encoding glycoprotein C, is severely restricted in the absence of functional ICP27. The exact mode of action by which ICP27 induces late gene expression is not known, but the effect is apparent at the mRNA level as demonstrated by Northern blot analysis. To determine whether ICP27 activates late genes via transcriptional or posttranscriptional mechanisms, we initially used nuclear run-on assays to measure transcription of viral genes in Vero cells infected with wild-type (WT) virus or an ICP27 nonsense mutant virus, n504. We observed a 4-fold reduction in the nuclear run-on signal from the coding strand of the gC gene for n504-infected cells compared to that of WT-infected cells. However, interpretation of the results was complicated by the observation of a significant signal from the noncoding strand in these experiments. To obviate the problem of symmetrical transcription, we utilized in vivo RNA pulse-labeling to measure the amount of transcription of viral genes in cells infected with either WT virus or n504 virus. We found a 5- to 10-fold reduction in the transcription of the gC and U(L)47 genes, two late genes, in cells infected with n504 compared to that in cells infected with WT virus. In contrast, transcription of the ICP8 gene, an early gene, was similar in WT and n504 virus-infected cells. We also examined the stability of the gC and U(L)47 gene transcripts in n504-infected cells, and we found it to be comparable to that in WT virus-infected cells, further supporting an effect on transcription. Transcription of the gC and U(L)47 genes by n504 was normal in a cell line that expresses WT ICP27. From these results we conclude that ICP27 is required for transcription of the late gC and U(L)47 genes during productive infection.

Herpes simplex virus IE63 (ICP27) protein interacts with spliceosome-associated protein 145 and inhibits splicing prior to the first catalytic step

The multifunctional herpes simplex virus type 1 (HSV-1) protein IE63 (ICP27) interacts with the essential pre-mRNA splicing factor, spliceosome-associated protein 145 (SAP145), and in infected cells IE63 and SAP145 colocalize. This interaction was reduced or abrogated completely using extracts from cells infected with IE63 viral mutants, with mutations in IE63 KH and Sm homology domains, which do not exhibit host shutoff or inhibit splicing. In the presence of IE63, splicing in vitro was inhibited prior to the first catalytic step and the B/C complex formed during splicing was shifted up in mobility and reduced in intensity. With the use of splicing extracts, IE63 and SAP145 both comigrated with the B/C complex, suggesting that they interact within this complex to inhibit B/C complex formation or conversion. The inhibition of splicing may facilitate the export of viral or cellular transcripts, possibly via other protein partners of IE63. These data provide important new insights into how IE63 influences pre-mRNA processing during HSV-1 infection.

HSV-2 disrupts gap junctional intercellular communication between mammalian cells in vitro

Infection by herpes simplex virus-2 (HSV-2) disrupts both dye and electrical coupling in Vero (African green monkey kidney) cell cultures. Vero cells in vitro were iontophoretically injected with the fluorescent dye Lucifer yellow CH, the spread of which revealed that cells throughout the confluent sheet shared open gap junctions. However, 24 h after infection with the virus (but before cells became rounded), dye always remained only within the target cell. Intracellular electrophysiological measurements of ionic coupling revealed a 0.4 coupling coefficient for adjacent cells in uninfected control cultures. By 3 h following infection significant down-regulation of gap junctions had begun, preceding by many hours any signs of infection visible with the light microscope. Measurements between adjacent cells 3 h post-infection, a period when HSV-2 gene expression is known to be at a maximum, yielded an average coupling coefficient of 0.35. By 6 h post-infection (a period of known viral DNA replication) average coupling coefficient for adjacent cells was 0.25, while by 24 h post-infection the average fill still further to <0.08. A coupling coefficient of <0.08 suggests that infection by HSV-2 completely disabled the gap junctions.

Protective T-cell-based immunity induced in neonatal mice by a single replicative cycle of herpes simplex virus

Newborns are very susceptible to infections because their immune systems are not fully developed and react to antigen exposure preferentially with unresponsiveness. UV-inactivated herpes simplex virus type 1 (HSV-1) represents such an antigen and does not induce an immune response in neonates. In contrast, protective T cells were primed in newborn mice by a single replicative cycle of DISC HSV-1 given once within 24 h of birth. Each of the HSV-1-primed CD4(+) or CD8(+) T cells induced in wild-type or interferon-deficient mice conferred resistance to naive animals exposed to a lethal virus challenge. Inactivated HSV-1, injected at variable doses up to 10(4) times that of DISC HSV-1, was ineffective in inducing any detectable immune responses in neonates. Thus, the capacity of HSV-1 to replicate once, but not the number of virus particles per se, was decisive in inducing protective T-cell-associated immunity in newborn mice.

Phosphorylation of the herpes simplex virus type 1 origin binding protein

The herpes simplex virus type 1 (HSV-1) origin binding protein (OBP), the product of the UL9 gene, is one of seven HSV-encoded proteins required for viral DNA replication. OBP performs multiple functions characteristic of a DNA replication initiator protein, including origin-specific DNA binding and ATPase and helicase activities, as well as the ability to interact with viral and cellular proteins involved in DNA replication. Replication initiator proteins in other systems, including those of other DNA viruses, are known to be regulated by phosphorylation; however, the role of phosphorylation in OBP function has been difficult to assess due to the low level of OBP expression in HSV-infected cells. Using a metabolic labeling and immunoprecipitation approach, we obtained evidence that OBP is phosphorylated during HSV-1 infection. Kinetic analysis of metabolically labeled cells indicated that the levels of OBP expression and phosphorylation increased at approximately 4 h postinfection. Notably, when expressed from a transfected plasmid, a recombinant baculovirus, or a recombinant adenovirus (AdOBP), OBP was phosphorylated minimally, if at all. In contrast, superinfection of AdOBP-infected cells with an OBP-null mutant virus increased the level of OBP phosphorylation approximately threefold, suggesting that HSV-encoded viral or HSV-induced cellular factors enhance the level of OBP phosphorylation. Using HSV mutants inhibited at sequential stages of the viral life cycle, we demonstrated that this increase in OBP phosphorylation is dependent on early protein synthesis and is independent of viral DNA replication. Based on gel mobility shift assays, phosphorylation does not appear to affect the ability of OBP to bind to the HSV origins.

Interaction between herpes simplex virus type 1 IE63 protein and cellular protein p32

The herpes simplex virus type 1 (HSV-1) immediate-early gene IE63 (ICP27), the only HSV-1 regulatory gene with a homologue in every mammalian and avian herpesvirus sequenced so far, is a multifunctional protein which regulates transcriptional and posttranscriptional processes. One of its posttranscriptional effects is the inhibition of splicing of viral and cellular transcripts. We previously identified heterogeneous nuclear ribonucleoprotein (hnRNP) K and casein kinase 2 (CK2) as two protein partners of IE63 (H. Bryant et al., J. Biol. Chem. 274:28991-28998, 1999). Here, using a yeast two-hybrid assay, we identify another partner of IE63, the cellular protein p32. Confirmation of this interaction was provided by coimmunoprecipitation from virus-infected cells and recombinant p32 binding assays. A p32-hnRNP K-CK2 complex, which required IE63 to form, was isolated from HSV-1-infected cells, and coimmunoprecipitating p32 was phosphorylated by CK2. Expression of IE63 altered the cytoplasmic distribution of p32, with some now colocalizing with IE63 in the nuclei of infected and transfected cells. As p32 copurifies with splicing factors and can inhibit splicing, we propose that IE63 together with p32, possibly with other IE63 partner proteins, acts to disrupt or regulate pre-mRNA splicing. As well as contributing to host cell shutoff, this effect could facilitate splicing-independent nuclear export of viral transcripts.

The conserved carboxyl-terminal half of herpes simplex virus type 1 regulatory protein ICP27 is dispensable for viral growth in the presence of compensatory mutations

ICP27 is an essential herpes simplex virus type 1 (HSV-1) immediate-early protein that regulates viral gene expression by poorly characterized mechanisms. Previous data suggest that its carboxyl (C)-terminal portion is absolutely required for productive viral infection. In this study, we isolated M16R, a second-site revertant of a viral ICP27 C-terminal mutant. M16R harbors an intragenic reversion, as demonstrated by the fact that its cloned ICP27 allele can complement the growth of an HSV-1 ICP27 deletion mutant. DNA sequencing demonstrated that the intragenic reversion is a frameshift alteration in a homopolymeric run of C residues at codons 215 to 217. This results in the predicted expression of a truncated, 289-residue molecule bearing 72 novel C-terminal residues derived from the +1 reading frame. Consistent with this, M16R expresses an ICP27-related molecule of the predicted size in the nuclei of infected cells. Transfection-based viral complementation assays confirmed that the truncated, frameshifted protein can partially substitute for ICP27 in the context of viral infection. Surprisingly, its novel C-terminal residues are required for this activity. To see if the frameshift mutation is all that is required for M16R's viability, we re-engineered the M16R ICP27 allele and inserted it into a new viral background, creating the HSV-1 mutant M16exC. An additional mutant, exCd305, was constructed which possesses the frameshift in the context of an ICP27 gene with the C terminus deleted. We found that both M16exC and exCd305 are nonviable in Vero cells, suggesting that one or more extragenic mutations are also required for the viability of M16R. Consistent with this interpretation, we isolated two viable derivatives of exCd305 which grow productively in Vero cells despite being incapable of encoding the C-terminal portion of ICP27. Studies of viral DNA synthesis in mutant-infected cells indicated that the truncated, frameshifted ICP27 protein can enhance viral DNA replication. In summary, our results demonstrate that the C-terminal portion of ICP27, conserved widely in herpesviruses and previously believed to be absolutely essential, is dispensable for HSV-1 lytic replication in the presence of compensatory genomic mutations.

Small dense nuclear bodies are the site of localization of herpes simplex virus 1 U(L)3 and U(L)4 proteins and of ICP22 only when the latter protein is present

The herpes simplex virus 1 U(L)3 and U(L)4 open reading frames are expressed late in infection and are not essential for viral replication in cultured cells in vitro. An earlier report showed that the U(L)4 protein colocalizes with the products of the alpha22/U(S)1.5 genes in small nuclear dense bodies. Here we report that the U(L)3 protein also colocalized in these small nuclear dense bodies and the localization of U(L)3 and U(L)4 proteins in these bodies required the presence of alpha22/U(S)1.5 genes. In cells infected with a mutant lacking intact alpha22/U(S)1.5 genes, U(L)3 was diffused throughout the nucleus even though the overall accumulation of the gamma2 U(L)3 protein was decreased. The results suggest that ICP22 acts both as a regulator of U(L)3 accumulation and as the structural component and anchor of these small dense nuclear bodies.

BAC-VAC, a novel generation of (DNA) vaccines: A bacterial artificial chromosome (BAC) containing a replication-competent, packaging-defective virus genome induces protective immunity against herpes simplex virus 1

This study aimed to exploit bacterial artificial chromosomes (BAC) as large antigen-capacity DNA vaccines (BAC-VAC) against complex pathogens, such as herpes simplex virus 1 (HSV-1). The 152-kbp HSV-1 genome recently has been cloned as an F-plasmid-based BAC in Escherichia coli (fHSV), which can efficiently produce infectious virus progeny upon transfection into mammalian cells. A safe modification of fHSV, fHSVDeltapac, does not give rise to progeny virus because the signals necessary to package DNA into virions have been excluded. However, in mammalian cells fHSVDeltapac DNA can still replicate, express the HSV-1 genes, cause cytotoxic effects, and produce virus-like particles. Because these functions mimic the lytic cycle of the HSV-1 infection, fHSVDeltapac was expected to stimulate the immune system as efficiently as a modified live virus vaccine. To test this hypothesis, mice were immunized with fHSVDeltapac DNA applied intradermally by gold-particle bombardment, and the immune responses were compared with those induced by infection with disabled infectious single cycle HSV-1. Immunization with either fHSVDeltapac or disabled infectious single cycle HSV-1 induced the priming of HSV-1-specific cytotoxic T cells and the production of virus-specific antibodies and conferred protection against intracerebral injection of wild-type HSV-1 at a dose of 200 LD(50). Protection probably was cell-mediated, as transfer of serum from immunized mice did not protect naive animals. We conclude that BAC-VACs per se, or in combination with genetic elements that support replicative amplification of the DNA in the cell nucleus, represent a useful new generation of DNA-based vaccination strategies for many viral and nonviral antigens.

The multifunctional herpes simplex virus IE63 protein interacts with heterogeneous ribonucleoprotein K and with casein kinase 2

Herpes simplex virus type 1 (HSV-1), the prototype alpha-herpesvirus, causes several prominent diseases. The HSV-1 immediate early (IE) protein IE63 (ICP27) is the only regulatory gene with a homologue in every mammalian and avian herpesvirus sequenced so far. IE63 is a multifunctional protein affecting transcriptional and post-transcriptional processes, and it can shuttle from the nucleus to the cytoplasm. To identify interacting cellular proteins, a HeLa cDNA library was screened in the yeast two-hybrid system using IE63 as bait. Several interacting proteins were identified including heterogeneous nuclear ribonucleoprotein K (hnRNP K), a multifunctional protein like IE63, and the beta subunit of casein kinase 2 (CK2), a protein kinase, and interacting regions were mapped. Confirmation of interactions was provided by fusion protein binding assays, co-immunoprecipitation from infected cells, and CK2 activity assays. hnRNP K co-immunoprecipitated from infected cells with anti-IE63 serum was a more rapidly migrating subfraction than hnRNP K immunoprecipitated by anti-hnRNP K serum. Using anti-IE63 serum, both IE63 and hnRNP K were phosphorylated in vitro by CK2, while in immunoprecipitates using anti-hnRNP K serum, IE63 but not hnRNP K was phosphorylated by CK2. These data provide important new insights into how this key viral regulatory protein exerts its functions.

The U(L)3 protein of herpes simplex virus 1 is translated predominantly from the second in-frame methionine codon and is subject to at least two posttranslational modifications

The U(L)3 open reading frame (ORF) has the coding capacity of 235 codons. The proteins reacting with the anti-U(L)3 antibody form in denaturing polyacrylamide gel bands with apparent M(r)s of 34,000, 35,000, 38,000, 40,000, 41,000, and 42,000 and designated 1 to 6, respectively. Studies on their origins revealed the following. (i) The U(L)3 proteins forming all six bands were present in lysates of cells infected with wild-type virus and treated with tunicamycin or monensin or in cells infected with the mutant lacking the gene encoding the U(S)3 protein kinase. (ii) The proteins contained in the slower-migrating bands were absent from cells infected with the mutant lacking the U(L)13 protein kinase. Bands 1 and 3, however were phosphorylated in cells infected with this mutant. (iii) Band 2 protein was absent from cells transfected with a plasmid carrying a substitution of the predicted first methionine codon of the U(L)3 ORF and superinfected with the U(L)3(-) mutant. (iv) Band 1 and 3 proteins were absent from lysates of cells transfected with a plasmid carrying a substitution of the second (M12) methionine codon of the U(L)3 ORF and superinfected with the U(L)3(-) mutant. (v) Cells superinfected with mutants lacking both methionine codons did not accumulate any of the proteins contained in the six bands. (vi) In vitro transcription-translation studies indicated that the translation of band 1 protein was initiated from the second (M12) methionine codon and that band 3 protein represented a U(L)13-independent, posttranslationally processed form of these proteins. The results indicate that the U(L)3 protein of herpes simplex virus 1 is translated predominantly from the second in-frame methionine codon and is subject to at least two posttranslational modifications.

Colocalization of the herpes simplex virus 1 UL4 protein with infected cell protein 22 in small, dense nuclear structures formed prior to onset of DNA synthesis

Herpes simplex virus 1 infected cell protein 22 (ICP22) localizes in small, dense nuclear bodies of primate cells early in infection and in the more diffuse replicative complexes after the onset of DNA synthesis. UL4, a gamma2 protein, localizes in cytoplasm and in the small nuclear structures containing ICP22 but not in replicative complexes. In rabbit skin cells, both ICP22 and UL4 localize in the dense nuclear bodies late in infection. The results suggest that the small nuclear structures perform a function involving both proteins late in infection.

Self-interaction of the herpes simplex virus type 1 regulatory protein ICP27

The herpes simplex virus type 1 (HSV-1) regulatory protein ICP27 is a nuclear phosphoprotein required for viral lytic infection, which acts partly at the posttranscriptional level to affect RNA processing and export. In the present study, we show that ICP27 can interact with itself in vivo. Immunofluorescent staining of cells expressing both an ICP27 mutant with a deletion of the major nuclear localization signal and wild-type ICP27 showed that the mutant protein was efficiently imported into the nucleus in the majority of the cotransfected cells, suggesting heterodimer formation between the wild-type and mutant proteins. Coimmunoprecipitation experiments using epitope-tagged wild-type ICP27 and a series of ICP27 mutants with deletions and insertions in important functional regions of the protein revealed that the C-terminal cysteine-histidine-rich zinc-finger-like region of ICP27 was required for the self-association. Furthermore the self-association was also shown in yeast using two-hybrid assays, and again, an intact C-terminal zinc-finger-like region was required for the interaction. This study provides biochemical evidence that ICP27 may function as a multimer in infected cells.

A novel cellular protein, p60, interacting with both herpes simplex virus 1 regulatory proteins ICP22 and ICP0 is modified in a cell-type-specific manner and Is recruited to the nucleus after infection

Herpes simplex virus 1 encodes two multifunctional regulatory proteins, infected-cell proteins 22 and 0 (ICP22 and ICP0). ICP0 is a promiscuous transactivator, whereas ICP22 is required in vivo and for efficient replication and expression of a subset of late (gamma2) genes in rodent or rabbit cell lines and in primary human cell strains (restrictive cells) but not in HEp-2 or Vero (permissive) cells. We report the identification in the yeast two-hybrid system of a cellular protein designated p60 that interacts with ICP22. This protein (apparent Mr of 60,000) has not been previously described and has no known motifs. Analyses of p60 revealed the following. (i) p60 bound fast-migrating, underprocessed wild-type ICP22 and ICP22 lacking the carboxyl-terminal 24 amino acids but not ICP22 lacking the carboxyl-terminal 40 amino acids, whereas the previously identified cellular protein p78 (R. Bruni and B. Roizman, J. Virol. 72:8525-8531, 1998) bound all forms of ICP22. The interaction of p60 with only one isoform of ICP22 supports that hypothesis that each isoform of herpes simplex virus proteins performs a specific function that may be different from that of other isoforms. (ii) p60 also bound ICP0; the binding of ICP0 was independent of that of ICP22. (iii) p60 localized in uninfected rabbit skin cells in both nuclei and cytoplasm. In rabbit skin cells infected with wild-type virus, p60 was posttranslationally processed to a higher apparent Mr but was not redistributed. Posttranslational processing required the presence of the genes encoding ICP22 and UL13 protein kinase. (iv) In uninfected HEp-2 cells, p60 localized primarily in nuclei. Soon after infection with wild-type virus, the p60 localized in discrete small nuclear structures with ICP0. Late in infection, both ICP0 and p60 tended to disperse but p60 did not change in apparent Mr. The localization of p60 was independent of ICP22, but p60 tended to be more localized in small nuclear structures and less dispersed in cells infected with mutants lacking the genes encoding the UL13 or US3 protein kinases. The results suggest that posttranslational modification of p60 is mediated either by ICP0 (permissive cells) or by ICP22 and UL13 protein kinase (restrictive rabbit skin cells) and that the restrictive phenotype of rabbit skin cells may be related to the failure to process p60 by mutants lacking the genes encoding UL13 or ICP22.

Analysis of the phosphorylation sites of herpes simplex virus type 1 regulatory protein ICP27

The herpes simplex virus type 1 (HSV-1) regulatory protein ICP27 is a 63-kDa phosphoprotein required for viral replication. ICP27 has been shown to contain both stable phosphate groups and phosphate groups that cycle on and off during infection (K. W. Wilcox, A. Kohn, E. Sklyanskaya, and B. Roizman, J. Virol. 33:167-182, 1980). Despite extensive genetic analysis of the ICP27 gene, there is no information available about the sites of the ICP27 molecule that are phosphorylated during viral infection. In this study, we mapped several of the phosphorylation sites of ICP27 following in vivo radiolabeling. Phosphoamino acid analysis showed that serine is the only amino acid that is phosphorylated during infection. Two-dimensional phosphopeptide mapping showed a complex tryptic phosphopeptide pattern with at least four major peptides and several minor peptides. In addition, ICP27 purified from transfected cells yielded a similar phosphopeptide pattern, suggesting that cellular kinases phosphorylate ICP27 during viral infection. In vitro labeling showed that protein kinase A (PKA), PKC, and casein kinase II (CKII) were able to differentially phosphorylate ICP27, resulting in distinct phosphopeptide patterns. The major phosphorylation sites of ICP27 appeared to cluster in the N-terminal portion of the protein, such that a frameshift mutant that encodes amino acids 1 to 163 yielded a phosphopeptide pattern very similar to that seen with the wild-type protein. Further, using small deletion and point mutations in kinase consensus sites, we have elucidated individual serine residues that are phosphorylated in vivo. Specifically, the serine at residue 114 was highly phosphorylated by PKA and the serine residues at positions 16 and 18 serve as targets for CKII phosphorylation in vivo. These kinase consensus site mutants were still capable of complementing the growth of an ICP27-null mutant virus. Interestingly, phosphorylation of the serine at residue 114, which lies within the major nuclear localization signal, appeared to modulate the efficiency of nuclear import of ICP27.

Comparison of the intranuclear distributions of herpes simplex virus proteins involved in various viral functions

Herpesviral transcription, DNA synthesis, and capsid assembly occur within the infected cell nucleus. To further define the spatial relationship among these processes, we have examined the intranuclear distributions of viral DNA replication, gene regulatory, and capsid proteins using dual label immunofluorescence and confocal microscopy. We observed that several of the viral DNA replication proteins localize preferentially to punctate structures within replication compartments while the major transcriptional activator, ICP4, and the ICP27 regulatory protein show a more diffuse distribution within replication compartments. The viral proteins that show a punctate distribution in replication compartments redistribute from these compartments to prereplicative sites when viral DNA replication is inhibited, whereas viral proteins that show a diffuse distribution remain within replication compartments when viral DNA replication is inhibited. Thus the sites of viral DNA replication and late transcription appear to be distinct but codistribute within the boundaries of replication compartments. The major capsid protein, ICP5, also localizes initially to a diffuse distribution within replication compartments, but during the time of maximal progeny virus assembly, ICP5 becomes localized to punctate structures within replication compartments that are often near the punctate structures occupied by viral DNA replication proteins. Hence the processes of viral DNA replication, late transcription, and capsid assembly show a general overlapping distribution within replication compartments but appear to be located at distinct sites within these regions of the infected cell nucleus.

Herpes simplex virus 1 regulatory protein ICP22 interacts with a new cell cycle-regulated factor and accumulates in a cell cycle-dependent fashion in infected cells

The herpes simplex virus 1 infected cell protein 22 (ICP22), the product of the alpha22 gene, is a nucleotidylylated and phosphorylated nuclear protein with properties of a transcriptional factor required for the expression of a subset of viral genes. Here, we report the following. (i) ICP22 interacts with a previously unknown cellular factor designated p78 in the yeast two-hybrid system. The p78 cDNA encodes a polypeptide with a distribution of leucines reminiscent of a leucine zipper. (ii) In uninfected and infected cells, antibody to p78 reacts with two major bands with an apparent Mr of 78,000 and two minor bands with apparent Mrs of 62, 000 and 55,000. (ii) p78 also interacts with ICP22 in vitro. (iii) In uninfected cells, p78 was dispersed largely in the nucleoplasm in HeLa cells and in the nucleoplasm and cytoplasm in HEp-2 cells. After infection, p78 formed large dense bodies which did not colocalize with the viral regulatory protein ICP0. (iv) Accumulation of p78 was cell cycle dependent, being highest very early in S phase. (v) The accumulation of ICP22 in synchronized cells was highest in early S phase, in contrast to the accumulation of another protein, ICP27, which was relatively independent of the cell cycle. (vi) In the course of the cell cycle, ICP22 was transiently modified in an aberrant fashion, and this modification coincided with expression of p78. The results suggest that ICP22 interacts with and may be stabilized by cell cycle-dependent proteins.

Interactions between a herpes simplex virus regulatory protein and cellular mRNA processing pathways

The herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP27 performs essential functions during viral lytic infection. Studies with viral mutants have demonstrated that ICP27 affects the shutoff of host protein synthesis, HSV-1 DNA replication, and the expression of viral early and late genes. Mounting evidence has been presented to demonstrate that ICP27 functions predominantly at the posttranscriptional level by affecting mRNA processing. That is, ICP27 alters poly(A) site usage, impairs host cell splicing, and facilitates the export of viral intronless mRNAs. These diverse effects occur by the interaction of ICP27 with viral and host proteins and by binding RNA. To define the precise mechanisms by which ICP27 affects RNA processing pathways, it is necessary to identify all of the molecular interactions of ICP27 in vivo and to determine the functional significance of these interactions. In vivo approaches will be emphasized here. Protein-protein interactions have been analyzed by coimmunoprecipitation studies, followed by immunoblotting to confirm the identity of coprecipitating proteins. Indirect immunofluorescence staining has been performed on cells treated with RNA polymerase II inhibitors to determine the intracellular distribution of ICP27 related to its RNA export function. Finally, in vivo UV irradiation has been used to covalently cross-link ICP27 to mRNAs in direct contact. This was followed with procedures to isolate and analyze the protein-RNA complexes. These studies have revealed several splicing complex proteins with which ICP27 interacts and have identified a number of intronless RNA transcripts to which ICP27 binds in the nucleus and cytoplasm in its role in RNA transport.

A virus with a mutation in the ICP4-binding site in the L/ST promoter of herpes simplex virus type 1, but not a virus with a mutation in open reading frame P, exhibits cell-type-specific expression of gamma(1)34.5 transcripts and latency-associated transcripts

The herpes simplex virus type 1 L/S junction-spanning transcripts (L/STs) are a family of multisized transcripts expressed at high levels in cells infected with mutant viruses that (i) do not express ICP4, (ii) specify forms of ICP4 unable to bind to the consensus ICP4 binding site, or (iii) contain mutations in the ICP4 binding site located at the transcriptional start site of the L/STs. By extension, the failure to detect the L/STs in wild-type virus-infected cells is due to the repressive effect of ICP4 bound to its cognate binding site upstream of the L/ST transcription initiation site. ORF-P, the first and largest open reading frame (ORF) encoded by the L/STs, overlaps >90% of the ORF encoding ORF-34.5, a putative neurovirulence factor, which is transcribed from the opposite DNA strand. Viruses with mutations in the overlapping region of ORF-P and ICP34.5 exhibit premature shutoff of infected-cell protein synthesis and are highly attenuated following intracranial inoculation of juvenile mice. To determine whether the premature protein shutoff and neuroattenuated phenotypes of ORF-P ORF-34.5 double mutants are a consequence of alterations in ORF-P, ORF-34.5, or both, viruses containing mutations only in ORF-P or only in the ICP4 binding site in the L/ST promoter were isolated and characterized. Mutant virus L/ST-n38 contains a single-base-pair transition mutation in ORF-P codon 38, resulting in translational termination of the ORF-P protein (OPP). This mutation does not alter the amino acid sequence of ICP34.5. Expression of a truncated form of OPP by mutant virus L/ST-n38 did not result in premature shutoff of infected-cell protein synthesis and produced no other observable phenotype relative to wild-type virus in in vitro tests. Moreover, the 50% lethal dose (LD50) of L/ST-n38 was comparable to that of wild-type virus following intracranial inoculation of 3-week-old mice, as were the latency and reactivation phenotypes of the virus. These properties of L/ST-n38 indicate that the attenuated phenotype of ORF-P ORF-34.5 double mutants is a consequence of mutations that affect the function of ICP34.5 and not the function of OPP. Mutant virus LST-4BS contains four single-base-pair substitutions in the ICP4 binding site in the L/ST promoter that abrogate the binding of ICP4 to this site, leading to high-level expression of the L/STs and OPP. LST-4BS induced premature shutoff of viral and cellular protein synthesis and was slightly growth restricted in cells of neural lineage (SK-N-SH human neuroblastoma cells) but was wild type for these two parameters in cells of nonneural lineage (immortalized primate Vero cells). Of particular interest was the observation that L/ST-4BS exhibited cell-type-specific expression of both the gamma(1)34.5 transcripts and the latency-associated transcripts (LATs). Thus, expression of these transcripts was barely detectable in cells of neural lineage (NB41A3 mouse neuroblastoma cells) but was wild type in Vero cells. In vivo, L/ST-4BS was reactivated from mouse trigeminal ganglia with reduced efficiency and delayed kinetics relative to wild-type virus. L/ST-4BS was completely attenuated for neurovirulence (LD50 > 10(6) PFU) relative to wild-type virus (LD50 < 900 PFU), although the four single-base-pair substitutions lie outside the coding region for the neurovirulence factor, ICP34.5. Collectively, the complex in vitro and in vivo phenotypes of L/ST-4BS can be attributed to (i) disruptions of the ICP4 binding site in the L/ST promoter and subsequent overexpression of the L/STs and OPP; (ii) alterations in ORF-O, which is also mutated in L/ST-4BS; or (iii) alterations in other cryptic genes or cis-acting elements.

UL13 protein kinase of herpes simplex virus 1 complexes with glycoprotein E and mediates the phosphorylation of the viral Fc receptor: glycoproteins E and I

Herpes simplex virus 1 encodes a Fc receptor consisting of glycoproteins E (gE) and I (gI) and two protein kinases specified by UL13 and US3, respectively. We report the following: (i) Antibody to UL13 formed immune complexes containing gE and gI in addition to UL13 protein. Immune complexes formed by monoclonal antibody to gE, but not those formed by monoclonal antibody to gI, also contained the UL13 protein. This association may reflect direct interaction between gE and UL13 inasmuch as IgG in preimmune rabbit serum and an antiserum made against another viral protein which does not react with the UL13 protein directly also bound gE and UL13. (ii) In cells infected with the wild-type virus, gE formed two sharp bands and a diffuse, slower migrating band. The slower sharp band was undetectable, and the diffuse slower migrating forms of gE were diminished in lysates of cells infected with a mutant virus lacking the UL13 gene (DeltaUL13). (iii) Both gE and gI were labeled with 32Pi in cells infected with wild-type or the DeltaUL13 virus, but the labeling was significantly stronger in cells infected with the wild-type virus than in those infected with the DeltaUL13 virus. (iv) In an in vitro protein kinase assay, UL13 immunoprecipitated from cells infected with wild-type virus labeled gE in the presence of [gamma-32P]ATP. This activity was absent in precipitates from cells infected with DeltaUL13 virus. The labeled gE comigrated with the slower, sharp band of gE. (v) gI present in the UL13 immune complex was also phosphorylated in the in vitro kinase assay. (vi) The cytoplasmic domain of gE contains recognition sequences for phosphorylation by casein kinase II (CKII). Exogenous CKII phosphorylated gE in immune complexes from lysates of cells infected with the DeltaUL13 mutant or in immune complexes from lysates of cells infected with wild-type virus that had been heated to inactivate all endogenous kinase activity including that of UL13. In both instances, CKII phosphorylated gE in both the slow and fast migrating sharp bands. We conclude that UL13 physically associates with gE and mediates the phosphorylation of gE and gI. UL13 may also be a determinant in posttranslational processing of gE.

Mutational analysis of the herpes simplex virus type 1 ICP0 C3HC4 zinc ring finger reveals a requirement for ICP0 in the expression of the essential alpha27 gene

The herpes simplex virus type 1 (HSV-1) immediate-early (IE) protein ICP0 has been implicated in the regulation of viral gene expression and the reactivation of latent HSV-1. Evidence demonstrates that ICP0 is an activator of viral gene expression yet does not distinguish between a direct or indirect role in this process. To further our understanding of the function of ICP0 in the context of the virus life cycle, site-directed mutagenesis of the consensus C3HC4 zinc finger domain was performed, and the effects of these mutations on the growth and replication of HSV-1 were assessed. We demonstrate that alteration of any of the consensus C3HC4 cysteine or histidine residues within this domain abolishes ICP0-mediated transactivation, alters the intranuclear localization of ICP0, and significantly increases its stability. These mutations result in severe defects in the growth and DNA replication of recombinant herpesviruses and in their ability to initiate lytic infections at low multiplicities of infection. These viruses, at low multiplicities of infection, synthesize wild-type levels of the IE proteins ICP0 and ICP4 at early times postinfection yet exhibit significant decreases in the synthesis of the essential IE protein ICP27. These findings reveal a role for ICP0 in the expression of ICP27 and suggest that the multiplicity-dependent growth of alpha0 mutant viruses results partially from reduced levels of ICP27.

Transcription of the derepressed open reading frame P of herpes simplex virus 1 precludes the expression of the antisense gamma(1)34.5 gene and may account for the attenuation of the mutant virus

Open reading frame P (ORF P), located at the 3' terminus of the 8.5-kb DNA sequence transcribed during latency and almost completely antisense to the gamma(1)34.5 gene, is naturally repressed by infected cell protein 4 (ICP4), the major herpes simplex virus 1 regulatory protein. Earlier studies on cells infected with a mutant in which the expression of ORF P is derepressed have shown that (i) the accumulation of the alpha infected cell proteins 0 (ICP0) and 22 (ICP22), the products of spliced mRNAs, is reduced congruent with the binding of ORF P protein to p32, a component of the ASF/SF2 splicing factors, (ii) ORF P protein colocalizes with spliceosomes, (iii) both gamma(1)34.5 mRNA and protein are virtually undetectable, and (iv) the virus is attenuated on intracerebral inoculation in mice. We report the construction and characterization of two recombinant viruses: R7546, in which ORF P transcription was derepressed and the initiator methionine codon was replaced; and R7547, in which both mutations were repaired to the wild-type genotype. The mutations in R7546 do not alter the amino acid sequence of the gamma(1)34.5 gene. We report that (i) the reduction in the accumulation of gamma(1)34.5 mRNA and protein in cells infected with mutant viruses expressing derepressed ORF P genes reflects the effects of antisense transcription of ORF P rather than a function of ORF P protein, (ii) the attenuated phenotype of the viruses carrying derepressed ORF P genes is due largely to the absence of the gamma(1)34.5 protein, and (iii) the reduction in accumulation of ICP0 and ICP22 requires the expression of ORF P protein.

The product of ORF O located within the domain of herpes simplex virus 1 genome transcribed during latent infection binds to and inhibits in vitro binding of infected cell protein 4 to its cognate DNA site

The partially overlapping ORF P and ORF O are located within the domains of the herpes simplex virus 1 genome transcribed during latency. Earlier studies have shown that ORF P is repressed by infected cell protein 4 (ICP4), the major viral regulatory protein, binding to its cognate site at the transcription initiation site of ORF P. The ORF P protein binds to p32, a component of the ASF/SF2 alternate splicing factors; in cells infected with a recombinant virus in which ORF P was derepressed there was a significant decrease in the expression of products of key regulatory genes containing introns. We report that (i) the expression of ORF O is repressed during productive infection by the same mechanism as that determining the expression of ORF P; (ii) in cells infected at the nonpermissive temperature for ICP4, ORF O protein is made in significantly lower amounts than the ORF P protein; (iii) the results of insertion of a sequence encoding 20 amino acids between the putative initiator methionine codons of ORF O and ORF P suggest that ORF O initiates at the methionine codon of ORF P and that the synthesis of ORF O results from frameshift or editing of its RNA; and (iv) glutathione S-transferase-ORF O fusion protein bound specifically ICP4 and precluded its binding to its cognate site on DNA in vitro. These and earlier results indicate that ORF P and ORF O together have the capacity to reduce the synthesis or block the expression of regulatory proteins essential for viral replication in productive infection.

Varicella-zoster virus open reading frame 4 encodes an immediate-early protein with posttranscriptional regulatory properties

Varicella-zoster virus (VZV) encodes four putative immediate-early proteins based on sequence homology with herpes simplex virus type 1: the products of ORF4, -61, -62, and -63. Until now, only two VZV proteins have been described as being truly expressed with immediate-early kinetics (IE62 and IE63). The ORF4-encoded protein can stimulate gene expression either alone or in synergy with the major regulatory protein IE62. Making use of a sequential combination of transcription and protein synthesis inhibitors (actinomycin D and cycloheximide, respectively), we demonstrated the immediate-early nature of the ORF4 gene product, which can thus be named IE4. The fact that IE4 is expressed with kinetics similar to that of IE62 further underlines the possible cooperation between these two VZV proteins in gene expression. Analysis of the IE4-mediated autologous or heterologous viral gene expression at the mRNA levels clearly indicated that IE4 may have several mechanisms of action. Activation of the two VZV genes tested could occur partly by a posttranscriptional mechanism, since increases in chloramphenicol acetyltransferase (CAT) mRNA levels do not account for the stimulation of CAT activity. On the other hand, stimulation of the human immunodeficiency virus type 1 long terminal repeat- or the cytomegalovirus promoter-associated CAT activity is correlated with an increase in the corresponding CAT mRNA.

The UL13 protein kinase and the infected cell type are determinants of posttranslational modification of ICP0

The herpes simplex virus infected-cell protein 0 (ICP0) acts as a promiscuous transactivator of genes introduced into eukaryotic cells by transfection or infection. The protein is highly posttranslationally modified by phosphorylation and nucleotidylylation. We have examined the electrophoretic mobility and phosphorylation of ICP0 in Vero and rabbit skin cells infected with wild-type virus or viruses from which the UL13 gene (DeltaUL13) encoding a protein kinase or the alpha22/US1.5 genes (Deltaalpha22/DeltaUS1.5) encoding putative transcriptional factors has been deleted. We report the following: (i) The accumulation of ICP0 and the electrophoretic mobility of ICP0 were dependent on the nature of the infected cell type and the presence of UL13. ICP0 encoded by wild-type virus accumulated to maximum levels earlier in infected Vero cells and its electrophoretic mobility was slower than that made in rabbit skin cells. In both Vero and rabbit skin cells infected with the DeltaUL13 virus, the prevailing ICP0 form migrated faster than that accumulating in the corresponding cells infected with wild-type virus. (ii) The alteration in electrophoretic mobility of ICP0 made in cells infected with DeltaUL13 virus was due to the absence of the UL13 protein and not to failure of posttranslational modification of Deltaalpha22/DeltaUS1.5 proteins inasmuch as the mobility of ICP0 in cells infected with Deltaalpha22/DeltaUS1.5 virus could not be differentiated from that of wild-type infected cells. (iii) ICP0 is extensively phosphorylated in infected cells even in the absence of UL13 protein. ICP0 is, however, a substrate for the UL13 kinase inasmuch as ICP0 was phosphorylated in mixtures of immune complexes of ICP0 and UL13. Complexes containing ICP0 only or infected cell lysate proteins reacting with preimmune serum from the rabbit immunized with UL13 protein failed to phosphorylate ICP0. (iv) In the absence of UL13, ICP22 is overproduced-an imbalance attributed to UL13. Thus, ICP22 regulates both the utilization of splice acceptor sites and the longevity of ICP0 mRNA (K. L. Carter and B. Roizman, 1996, Proc. Natl. Acad. Sci. USA 93, 12535-12540); UL13 is involved in the posttranslational modification of ICP0 and is required for both posttranslational processing and control of abundance of ICP22.

The range and distribution of murine central nervous system cells infected with the gamma(1)34.5- mutant of herpes simplex virus 1

Wild-type herpes simplex virus 1 (HSV-1) multiplies, spreads, and rapidly destroys cells of the murine central nervous system (CNS). In contrast, mutants lacking both copies of the gamma(1)34.5- gene have been shown to be virtually lacking in virulence even after direct inoculation of high-titered virus into the CNS of susceptible mice (J. Chou, E. R. Kern, R. J. Whitley, and B. Roizman, Science 250:1262-1266, 1990). To investigate the host range and distribution of infected cells in the CNS of mice, 4- to 5-week-old mice were inoculated stereotaxically into the caudate/putamen with 3 x 10(5) PFU of the gamma(1)34.5- virus R3616. Four-micrometer-thick sections of mouse brains removed on day 3, 5, or 7 after infection were reacted with a polyclonal antibody directed primarily to structural proteins of the virus and with antibodies specific for neurons, astrocytes, or oligodendrocytes. This report shows the following: (i) most of the tissue damage caused by R3616 was at the site of injection, (ii) the virus spread by retrograde transport from the site of infection to neuronal cell nuclei at distant sites and to ependymal cells by cerebrospinal fluid, (iii) the virus infected neurons, astrocytes, oligodendrocytes, and ependymal cells and hence did not discriminate among CNS cells, (iv) viral replication in some neurons could be deduced from the observation of infected astrocytes and oligodendrocytes at distant sites, and (v) infected cells were being efficiently cleared from the nervous system by day 7 after infection. We conclude that the gamma(1)34.5- attenuation phenotype is reflected in a gross reduction in the ability of the virus to replicate and spread from cell to cell and is not due to a restricted host range. The block in viral replication appears to be a late event in viral replication.

Physical and functional interactions between herpes simplex virus immediate-early proteins ICP4 and ICP27

The ordered expression of herpes simplex virus type 1 (HSV-1) genes, during the course of a productive infection, requires the action of the virus immediate-early regulatory proteins. Using a protein interaction assay, we demonstrate specific in vitro protein-protein interactions between ICP4 and ICP27, two immediate-early proteins of HSV-1 that are essential for virus replication. We map multiple points of contact between these proteins. Furthermore, by coimmunoprecipitation experiments, we demonstrate the following. (i) ICP4-ICP27 complexes are present in extracts from HSV-1 infected cells. (ii) ICP27 binds preferentially to less modified forms of ICP4, a protein that is extensively modified posttranslationally. We also demonstrate, by performing electrophoretic mobility shift assays and supershifts with monoclonal antibodies to ICP4 or ICP27, that both proteins are present in a DNA-protein complex with a noncanonical ICP4 binding site present in the HSV thymidine kinase (TK) gene. ICP4, in extracts from cells infected with ICP27-deficient viruses, is impaired in its ability to form complexes with the TK site but not with the canonical site from the alpha4 gene. However, ICP4 is able to form complexes with the TK probe, in the absence of ICP27, when overproduced in mammalian cells or expressed in bacteria. These data suggest that the inability of ICP4 from infected cell extracts to bind the TK probe in the absence of ICP27 does not reflect a requirement for the physical presence of ICP27 in the complex. Rather, they imply that ICP27 is likely to modulate the DNA binding activity of ICP4 by affecting its posttranslational modification status. Therefore, we propose that ICP27, in addition to its established role as a posttranscriptional regulator of virus gene expression, may also modulate transcription either through direct or indirect interactions with HSV regulatory regions, or through its ability to modulate the DNA binding activity of ICP4.

The RGG box motif of the herpes simplex virus ICP27 protein mediates an RNA-binding activity and determines in vivo methylation

ICP27 is an essential herpes simplex virus type 1 nuclear regulatory protein that is required for efficient viral gene expression. Although the mechanism by which ICP27 regulates genes is unknown, a variety of evidence suggests that it functions posttranscriptionally, and recent studies indicate that it is an RNA-binding protein. Previously, we noted that a short arginine- and glycine-rich sequence in ICP27 (residues 138 to 152) is similar to an RGG box motif, a putative RNA-binding determinant found in a number of cellular proteins (W. Mears, V. Lam, and S. Rice, J. Virol. 69:935-947, 1995). In the present study, we have further investigated ICP27's association with RNA and examined the role of the RGG box in RNA binding. We find that ICP27 binds efficiently to RNA homopolymers composed of poly(G) and weakly to poly(U) RNA homopolymers. Poly(G) binding activity maps to the N-terminal 189 residues of ICP27 and requires the RGG box sequence. Using a northwestern blotting assay, we demonstrate that the RGG box alone (residues 140 to 152) can mediate RNA binding when attached to a heterologous protein. As many cellular RGG box proteins are methylated on arginine residues, we also investigated the in vivo methylation status of ICP27. Our results demonstrate that ICP27 is methylated in herpes simplex virus-infected cells. Methylation is dependent on the presence of the RGG box, suggesting that one or more arginine residues in the RGG box sequence are modified. These data demonstrate that ICP27 displays the characteristics of an RGG box-type RNA-binding protein.

The UL3 open reading frame of herpes simplex virus type 1 codes for a phosphoprotein

Based on sequence analysis, the protein encoded by the UL3 open reading frame (ORF) of herpes simplex virus type 1 (HSV-1) was predicted to contain an N glycosylation site and to be a glycoprotein. To determine if this prediction was correct, we cloned and expressed the DNA encoding the complete sequence of the UL3 ORF in a baculovirus expression system. Western blotting was done using polyclonal antibody raised against synthetic UL3 peptides. Two major baculovirus-UL3 expressed protein bands with apparent molecular weights of 30 kDa and 31 kDa, and two minor protein bands with apparent molecular weights of 29 kDa and 33 kDa were detected. None of the expressed UL3 protein species were susceptible to tunicamycin treatment, suggesting that they were not N-linked glycosylated. Cell fractionation studies indicated that the UL3 protein was localized in the cytoplasmic and nuclear portion of the cells, rather than the cell membrane, again suggesting a lack of glycosylation. In contrast, the baculovirus expressed UL3 protein was phosphorylated as judged by 32Pi-labeling. Immunoprecipitation followed by SDS-PAGE demonstrated a single 32Pi-labeled UL3 related band with an apparent molecular weight of 33 kDa, indicating that the UL3 protein was a phosphoprotein. Antibodies produced in mice vaccinated with baculovirus-UL3 protein reacted with two UL3 related HSV-1 bands on Western blots. These protein bands had apparent molecular weights of 27 and 33 kDa and presumably represent the unphosphorylated and phosphorylated forms of UL3.

Open reading frame P--a herpes simplex virus gene repressed during productive infection encodes a protein that binds a splicing factor and reduces synthesis of viral proteins made from spliced mRNA

The open reading frame P (ORF P) is located in the domain and on the DNA strand of the herpes simplex virus 1 transcribed during latent infection. ORF P is not expressed in productively infected cells as a consequence of repression by the binding of the major viral regulatory protein to its high-affinity binding site. In cells infected with a mutant virus carrying a derepressed gene, ORF P protein is extensively posttranslationally processed. We report that ORF P interacts with a component of the splicing factor SF2/ASF, pulls down a component of the SM antigens, and colocalizes with splicing factors in nuclei of infected cells. The hypothesis that ORF P protein may act to regulate viral gene expression, particularly in situations such as latently infected sensory neurons in which the major regulatory protein is not expressed, is supported by the evidence that in cells infected with a mutant in which the ORF P gene was derepressed, the products of the regulatory genes alpha 0 and alpha 22 are reduced in amounts early in infection but recover late in infection. The proteins encoded by these genes are made from spliced mRNAs, and the extent of recovery of these proteins late in infection correlates with the extent of accumulation of post-translationally processed forms of ORF P protein.

Overexpression of the herpes simplex virus type 1 immediate-early regulatory protein, ICP27, is responsible for the aberrant localization of ICP0 and mutant forms of ICP4 in ICP4 mutant virus-infected cells

ICP0 and ICP4 are immediate-early regulatory proteins of herpes simplex virus type 1. Previous studies by Knipe and Smith demonstrated that these two proteins are characteristically observed in the nuclei of wild-type virus-infected cells but predominantly in the cytoplasms of cells infected with several ICP4 temperature-sensitive (ts) mutant viruses at the nonpermissive temperature (NPT) (D. M. Knipe and J. L. Smith, Mol. Cell. Biol. 6:2371-2381, 1986). Consistent with this observation, it has been shown previously that ICP0 is present predominantly in the cytoplasms of cells infected with an ICP4 null mutant virus (n12) at high multiplicities of infection and that the level of ICP27, a third viral regulatory protein, plays an important role in determining the intracellular localization of ICP0 (Z. Zhu, W. Cai, and P. A. Schaffer, J. Virol. 68:3027-3040, 1994). To address whether the cytoplasmic localization of ICP0 is a common feature of cells infected with all ICP4 mutant viruses or whether mutant ICP4 polypeptides, together with ICP27, determine the intracellular localization of ICP0, we used double-staining immunofluorescence tests to examine the intracellular staining patterns of ICP0 and ICP4 in cells infected with an extensive series of ICP4 mutant viruses. In these tests, compared with the localization pattern of ICP0 in wild-type virus-infected cells, more ICP0 was detected in the cytoplasms of cells infected with all ICP4 mutants tested at high multiplicities of infection. Each of the mutant forms of ICP4 exhibiting predominantly cytoplasmic staining contains both the nuclear localization signal and the previously mapped ICP27-responsive region (Z. Zhu and P. A. Schaffer, J. Virol. 69:49-59, 1995). No correlation between the intracellular staining patterns of ICP0 and mutant forms of ICP4 was demonstrated, suggesting that mutant ICP4 polypeptides per se are not responsible for retention of ICP0 in the cytoplasm. This observation was confirmed in studies of cells cotransfected with plasmids expressing ICP0 and mutant forms of ICP4, in which the staining pattern of ICP0 was not changed in the presence of mutant ICP4 proteins. Studies of cells infected at low multiplicities with a variety of ICP4 ts mutant viruses at the NPT showed that both ICP0 and ts forms of ICP4 were localized predominantly within the nucleus. These observations are a further indication that the aberrant localization of the ts forms of ICP4 at the NPT is not a direct result of specific mutations in the ICP4 gene. In the final series of tests, the localization of ICP0 in cells infected with a double-mutant virus unable to express either ICP4 or ICP27 was examined. In these tests, ICP0 was detected exclusively in the nuclei of Vero cells but in both the nuclei and the cytoplasms of ICP27-expressing cells infected with the double mutant. These results demonstrate that ICP27, rather than the absence of functional ICP4, is responsible for the cytoplasmic localization of ICP0 in ICP4 mutant virus-infected cells. Taken together, these findings demonstrate that the aberrant localization of ICP0 and certain mutant forms of ICP4 in cells infected with ICP4 mutant viruses is mediated by high levels of ICP27 resulting from the inability of mutant forms of ICP4 to repress the expression of ICP27.

Herpes simplex ICP27 mutant viruses exhibit reduced expression of specific DNA replication genes

Herpes simplex virus type 1 mutants with certain lesions in the ICP27 gene show a 5- to 10-fold reduction in viral DNA synthesis. To determine how ICP27 promotes amplification of viral DNA, we examined the synthesis, accumulation, and stability of the essential viral replication proteins and steady-state levels of the replication gene transcripts throughout the course of ICP27 mutant virus infections. These studies reveal that in the absence of ICP27, expression of the UL5, UL8, UL52, UL9, UL42, and UL30 genes is significantly reduced at the level of mRNA accumulation. In contrast to that of these beta genes, ICP8 expression is unaltered in mutant virus-infected cells, indicating that ICP27 selectively stimulates only a subset of herpes simplex virus beta genes. Analysis of multiple ICP27 mutant viruses indicates a quantitative correlation between the ability of these mutants to replicate viral DNA and the level of replication proteins produced by each mutant. Therefore, we conclude that the primary defect responsible for restricted viral DNA synthesis in cells infected with ICP27 mutants is insufficient expression of most of the essential replication genes. Of further interest, this analysis also provides new information about the structure of the UL52 gene transcripts.

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.

The promoter and transcriptional unit of a novel herpes simplex virus 1 alpha gene are contained in, and encode a protein in frame with, the open reading frame of the alpha 22 gene

The herpes simplex virus type 1 genome encodes a set of genes (alpha genes) expressed in the absence of de novo viral protein synthesis. Earlier studies have shown that the product of the alpha 22 gene, a member of this set, is nucleotidylylated by casein kinase II and phosphorylated by viral protein kinases encoded by UL13 and US3. Mutants lacking the carboxyl-terminal domain starting with amino acid 200 exhibit reduced capacity to replicate in primary human cell strains or in cells of rodent derivation and also exhibit reduced expression of a subset of gamma or late genes. We report that the domain of the alpha 22 gene is transcribed by two 3'-coterminal mRNAs. The longer transcript reported encodes the 420-amino-acid alpha 22 protein, whereas the shorter transcript reported here encodes a protein containing the carboxyl-terminal 273 amino acids of the alpha 22 protein. The shorter gene is designated US1.5. The US1.5 mRNA is synthesized in cells infected and maintained in the presence of cycloheximide and under other conditions which restrict viral gene expression to alpha genes. In-frame insertion of linkers encoding 18, 21, or 22 amino acids after codon 200 or 240 of the alpha 22 protein did not affect the known functions or phenotype associated with the wild-type alpha 22 gene or its product. Earlier studies have placed the nucleotidylylated sequences in the amino-terminal portion of the protein. The results of these studies indicate that the US1.5 gene encodes the functions associated with replication in human primary or rodent cells and optimal expression of alpha 0 and gamma genes. This finding brings the number of genes known to map in the unique short region of the herpes simplex virus type 1 DNA to 14 and the total number of different genes to 78.

The C-terminal repressor region of herpes simplex virus type 1 ICP27 is required for the redistribution of small nuclear ribonucleoprotein particles and splicing factor SC35; however, these alterations are not sufficient to inhibit host cell splicing

Herpes simplex virus type 1 infection results in a reorganization of antigens associated with the small nuclear ribonucleoprotein particles (snRNPs), resulting in the formation of prominent clusters near the nuclear periphery. In this study, we show that the immediate-early protein ICP27, which is involved in the impairment of host cell splicing and in the changes in the distribution of snRNPs, is also required for reassorting the SR domain splicing factor SC35. Other viral processes, such as adsorption and penetration, shutoff of host protein synthesis, early and late gene expression, and DNA replication, do not appear to play a role in changing the staining pattern of splicing antigens. Furthermore, the C-terminal repressor region of ICP27, which is required for the inhibitory effects on splicing, also is involved in redistributing the snRNPs and SC35. During infection or transfection with five different repressor mutants, the speckled staining pattern characteristic of uninfected cells was seen and the level of a spliced target mRNA was not reduced. Infections in the presence of activator mutants showed a redistributed snRNP pattern and a decreased accumulation of spliced target mRNA. Moreover, two arginine-rich regions in the N-terminal half of ICP27 were not required for the redistribution of snRNPs or SC35. Substitution of these regions with a lysine-rich sequence from simian virus 40 large-T antigen resulted in a redistribution of splicing antigens. Unexpectedly, a repressor mutant with a ts phenotype showed a redistributed staining pattern like that seen with wild-type infected cells. During infections with this ts mutant, splicing was not inhibited, as shown in this and previous studies, confirming its repressor phenotype. Furthermore, both the mutant and the wild-type protein colocalized with snRNPs. Therefore, the redistribution of snRNPs and SC35 correlates with ICP27-mediated impairment of host cell splicing, but these alterations are not sufficient to fully inhibit splicing. This indicates that active splicing complexes are still present even after dramatic changes in the organization of the snRNPs.

Initial characterization of the membrane-associated form of ICP4 of herpes simplex virus type 1

The immediate-early gene product, ICP4, of herpes simplex virus type 1 (HSV-1) is one of the major transcriptional regulatory proteins in the virus replicative process and is localized primarily within the nucleus soon after its synthesis. Earlier studies have shown that detectable amounts of ICP4 are also associated with the plasma membranes of infected cells (F. Yao and R. J. Courtney, J. Virol. 65:1516-1524, 1991). To extend our understanding of the properties of the membrane-associated ICP4, we have used various electrophoretic techniques, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, and isoelectric focusing, to compare the membrane- and nuclear-associated forms of ICP4. The data from all of these methods revealed that a single unique form of ICP4 associates with plasma membranes of HSV-1-infected cells. While multiple forms of ICP4 were detected in infected cell nuclei, the membrane-associated form of ICP4 appeared to have a lower apparent molecular weight and a more acidic pI than the various forms of ICP4 found in infected cell nuclei. These results suggest that a novel form of ICP4 may associate with plasma membranes of HSV-1-infected cells. A recombinant adenovirus, AdICP4 (encoding an ICP4 protein), was used to determine the role that other herpesvirus proteins may play in the membrane association of ICP4. The results suggest that the expression of other HSV-1 proteins is not required for the membrane association of ICP4.

Arginine-rich regions succeeding the nuclear localization region of the herpes simplex virus type 1 regulatory protein ICP27 are required for efficient nuclear localization and late gene expression

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is an essential regulatory protein that localizes to the nuclei of infected cells. The strong nuclear localization signal (NLS) of ICP27 was identified recently and shown to reside in the amino-terminal portion of the polypeptide from residues 110 to 137 (W.E. Mears, V. Lam, and S.A. Rice, J. Virol. 69:935-947, 1995). There are also two arginine-rich regions directly succeeding the NLS. The first of these arginine-rich sequences (residues 141 to 151), together with the NLS, has been shown by Mears et al. to form the nucleolar localization signal. Arginine-rich motifs are common in domains involved in nuclear localization and RNA binding. To analyze the role of the arginine-rich regions in ICP27, we constructed stably transformed cell lines containing ICP27 mutants with deletions of all or parts of the NLS and arginine-rich regions. We also constructed mutants in which these regions were replaced with heterologous NLSs or RNA-binding domains. Characterization of these mutants indicated that the arginine-rich regions were required but not sufficient for wild-type localization of ICP27. More importantly, the NLS and arginine-rich regions were also essential to the function of ICP27. Mutants lacking these sequences were defective in late gene expression during infection even when ICP27 was properly localized to the nucleus by substitution of the NLS from simian virus 40 large T antigen. Further, the defect in late gene expression could not be overcome by replacement with the highly basic RNA-binding domain of human immunodeficiency virus type 1 Tat. The deficiency in late gene expression was independent of ICP27's role in stimulating viral DNA replication. In addition, localization of the HSV-1 proteins ICP4, ICP0, and ICP8 was unaffected by ICP27 mutants in this region. These results suggest that the arginine-rich regions are required for efficient nuclear localization and for the regulatory activity of ICP27 involved in viral late gene expression.

The regulation of synthesis and properties of the protein product of open reading frame P of the herpes simplex virus 1 genome

Open reading frame P (ORF P) maps in the inverted repeat sequence ab and b'a' flanking the long unique (UL) sequence of the herpes simplex virus 1 genome, within the sequence reported to be transcribed during latent infection of sensory neurons. Both the protein and the RNA were previously reported to be expressed only in cells infected with a deletion mutant or with a mutant carrying a ts lesion in the alpha 4 gene encoding the infected cell protein no. 4 (ICP4), a major regulatory protein of the virus. In this report we show that (i) disruption of the ICP4 DNA binding site by replacement mutagenesis resulted in the overexpression of ORF P protein even at permissive temperatures, leading to productive infection; (ii) the expression of ORF P does not require prior viral protein synthesis; (iii) late in infection the ORF protein P is processed into multiple forms characterized by a slower electrophoretic mobility in denaturing gels; (iv) ORF P protein accumulates in nuclei of infected cells; and (v) in some nuclei of infected cells, ORF P protein is organized in the form of rods traversing the nucleus from the basolateral to the apical side. We conclude that ORF P has many of the properties predictive of a viral gene group, which we designate pre-alpha. Specifically, these could be induced by the alpha transinducing factor (also known as VP16) carried in the virion; they would be firmly shut off by the onset of expression of alpha genes required for productive infection; and in the absence of repressive effects of ICP4, their expression could be dependent on the number of viral DNA copies available for transcription. Finally, the productively infected cell would evolve a way of disposing excess pre-alpha proteins by posttranslational processing.

Regulatory function of the equine herpesvirus 1 ICP27 gene product

The UL3 protein of equine herpesvirus 1 (EHV-1) KyA strain is a homolog of the ICP27 alpha regulatory protein of herpes simplex virus type 1 (HSV-1) and the ORF 4 protein of varicella-zoster virus. To characterize the regulatory function of the UL3 gene product, a UL3 gene expression vector (pSVUL3) and a vector expressing a truncated version of the UL3 gene (pSVUL3P) were generated. These effector plasmids, in combination with an EHV-1 immediate-early (IE) gene expression vector (pSVIE) and chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs, were used in transient transfection assays. These assays demonstrated that the EHV-1 UL3 gene product is a regulatory protein that can independently trans activate the EHV-1 IE promoter; however, this effect can be inhibited by the repressive function of the IE gene product on the IE promoter (R. H. Smith, G. B. Caughman, and D. J. O'Callaghan, J. Virol. 66:936-945, 1992). In the presence of the IE gene product, the UL3 gene product significantly augments gene expression directed by the promoters of three EHV-1 early genes (thymidine kinase; IR4, which is the homolog of HSV-1 ICP22; and UL3 [ICP27]) and the promoter of the EHV-1 late gene IR5, which is the homolog of HSV-1 US10. Sequences located at nucleotides -123 to +20 of the UL3 promoter harbor a TATA box, SP1 binding site, CAAT box, and octamer binding site and, when linked to the CAT reporter gene, are trans activated to maximal levels by the pSVIE construct in transient expression assays. Results from CAT assays also suggest that the first 11 amino acids of the UL3 protein are not essential for the regulatory function of the UL3 gene product.

ICP27 immediate early gene, glycoprotein K (gK) and DNA helicase homologues of infectious laryngotracheitis virus (gallid herpesvirus 1) SA-2 strain

A 4.8 kilobase segment located at the left-terminal in the unique long (UL) region of infectious laryngotracheitis virus (ILTV) SA-2 strain contained three open reading frames (ORFs). The first of 421 amino acids (aa) was located at map units 0.065 to 0.07, and its predicted 48 kiloDaltons (kDa) protein product has significant homology to the immediate early regulatory protein ICP27 (UL54) of herpes simplex virus type-1 (HSV-1), to varicella-zoster virus (VZV) ORF4 and to equine herpesvirus 1 (EHV-1) ORF5. The zinc finger conserved in the C-terminal of the proteins from HSV-1, VZV and EHV-1, is poorly conserved in ILTV homologue. The second ORF of 336 aa, located at map units 0.075 to 0.08, has a predicted molecular weight (MW) of 38 kDa with significant homology to glycoprotein K (gK) of HSV-1 (UL53), ORF5 of VZV and ORF6 of EHV-1. ILTV gK has features characteristic of a membrane-bound glycoprotein. The 3' region of a third ORF was located at map units 0.08 to 0.095. Translation of the sequence revealed significant homology to the 3'-region of the DNA helicase-primase complex protein (UL52) of HSV-1, ORF6 of VZV and ORF 7 of EHV-1. Northern blot analyses were used to characterize the ILTV ICP27, gK and DNA helicase mRNAs. The data revealed that ILTV ICP27 is an immediate early gene that encodes a 1.6 kb mRNA, ILTV gK encodes a late transcript of 1.8 kb, while ILTV DNA helicase encodes a late transcript of 3.7 kb.

Identification of nuclear and nucleolar localization signals in the herpes simplex virus regulatory protein ICP27

Previous work has shown that the herpes simplex virus type 1 (HSV-1) regulatory protein ICP27 localizes to the cell nucleus and that certain mutant ICP27 polypeptides localize preferentially in nucleoli. To map the signals in ICP27 which mediate its nuclear localization, we identified the portions of ICP27 which can direct a cytoplasmic protein, pyruvate kinase (PK), to nuclei. Our results demonstrate that ICP27 contains multiple nuclear localization signals (NLSs) that function with differing efficiencies. First, ICP27 possesses a strong NLS, mapping to residues 110 to 137, which bears similarity to the bipartite NLSs found in Xenopus laevis nucleoplasmin and other proteins. Second, ICP27 possesses one or more weak NLSs which map to a carboxyl-terminal portion of the protein between residues 140 and 512. Our PK-targeting experiments also demonstrate that ICP27 contains a relatively short sequence, mapping to residues 110 to 152, that can function as a nucleolar localization signal (NuLS). This signal includes ICP27's strong NLS as well as 15 contiguous residues which consist entirely of arginine and glycine. This latter sequence is very similar to an RGG box, a putative RNA-binding motif found in a number of cellular proteins which are involved in nuclear RNA processing. To confirm the results of the PK-targeting experiments, we mutated the ICP27 gene by deleting sequences encoding either the strong NLS or the RGG box. Deletion of the strong NLS (residues 109 to 138) resulted in an ICP27 molecule that was only partially defective for nuclear localization, while deletion of the RGG box (residues 139 to 153) resulted in a molecule that was nuclear localized but excluded from nucleoli. Recombinant HSV-1s bearing either of these deletions were unable to replicate efficiently in Vero cells, suggesting that ICP27's strong NLS and RGG box carry out important in vivo functions.

Evaluation of colocalization interactions between the IE110, IE175, and IE63 transactivator proteins of herpes simplex virus within subcellular punctate structures

A number of previous studies have implied that three herpes simplex virus-encoded nuclear transactivator proteins, IE175 (ICP4), IE110 (ICP0), and IE63 (ICP27), may cooperate in transcriptional and posttranscriptional stimulation of viral gene expression. Using double-label immunofluorescence assays (IFA) in transient expression assays, we have examined the intracellular localization of these three proteins in DNA-transfected cells. The IE110 protein on its own forms spherical punctate domains within the nucleus, whereas the IE175 and IE63 proteins alone give uniform and speckled diffuse patterns, respectively. In infected cells, the IE110 punctate granules have been shown to correspond to novel preexisting subnuclear structures referred to as ND10 domains or PODs that contain a variety of cellular proteins, including SP100 and the PML proto-oncogene product. Cotransfection experiments with wild-type nuclear forms of both IE175 and IE110 provided direct evidence for partial redistribution of IE175 into the same punctate granules that contained IE110. Surprisingly, nuclear forms of IE110 were found to move a cytoplasmic form of IE175 into nuclear punctate structures, and a cytoplasmic form of IE110 was able to retain nuclear forms of IE175 in cytoplasmic punctate structures. Therefore, the punctate characteristic of IE110 appeared to both dominate the interactions and override the normal nuclear localization signals. The domains responsible for the interaction mapped to between codons 518 and 768 in 1E110 and to between codons 835 and 1029 in IE175. Importantly, a truncated nuclear form of the 1,298-amino-acid IE175 protein, which lacked the C-terminal domain beyond codon 834, was found to be excluded from the IE110 punctate granules. Cotransfection of nuclear or cytoplasmic IE110 with a truncated nuclear form of IE63 also led to partial redistribution of IE63 into either nuclear or cytoplasmic punctate granules containing IE110. Both the IE63-IE110 and IE175-IE110 colocalization interactions were demonstrated in Vero cells but not in 293 cells. Consequently, they differ from IE110 self-interactions, which correlate with in vitro dimerization and occur efficiently in both cell types. These interactions may help to explain the altered promoter target specificity and synergism observed when IE175 is cotransfected with IE110 in transactivation studies.

Intracellular localization of the herpes simplex virus type 1 major transcriptional regulatory protein, ICP4, is affected by ICP27

Infected-cell protein 4 (ICP4) is the major transcriptional activator of herpes simplex virus (HSV) gene expression during productive infection. ICP0 has broad transactivating activity for all classes of HSV genes as well as cellular genes and genes of heterologous viruses. Together, the transactivating activities of ICP4 and ICP0 are synergistic. ICP27, which alone does not exhibit major transregulatory activity, is able to differentially activate and repress viral gene expression induced by ICP4 and ICP0. Thus, ICP27 plays a modulatory role in viral gene expression. In order to explore the functional relationships among ICP4, ICP0, and ICP27 in the regulation of viral gene expression, we have used indirect immunofluorescence to examine the intracellular localization of ICP4 in cells infected with wild-type virus or with mutant viruses that did not express functional forms of ICP0 or ICP27. Although ICP4 localized to both the nuclei and cytoplasm of cells infected with either the wild-type virus or an ICP0 null mutant virus, this protein was present exclusively in the nuclei of cells infected with an ICP27 null mutant virus, suggesting that ICP27 is able to inhibit the nuclear localization of ICP4 during virus infection. Transient expression assays with pairs of plasmids that express wild-type forms of ICP4 and ICP0 or of ICP4 and ICP27 demonstrated that ICP27 has a significant inhibitory effect on the nuclear localization of ICP4, confirming the observations made with the mutant-virus-infected cells. By using a plasmid expressing wild-type ICP4 and a series of ICP27 mutant plasmids in transient expression assays, the C-terminal half of ICP27 was shown to be required for its inhibitory effect on the nuclear localization of ICP4. In similar studies using a series of ICP4 mutant plasmids, the region of ICP4 responsive to wild-type ICP27 was mapped to the C-terminal portion of the molecule between amino acid residues 820 and 1029. The level of expression of ICP27 was shown to have a significant effect on the intracellular localization of ICP4 in transient assays. These findings are consistent with previous studies in which ICP27 was shown to have an inhibitory effect on the nuclear localization of ICP0 (Z. Zhu, W. Cai, and P. A. Schaffer J. Virol. 68:3027-3040, 1994). Thus, ICP27 has a significant inhibitory effect on the ability of the two major HSV type 1 (HSV-1) regulatory proteins to localize to the nucleus. Collectively, these findings indicate that cooperative regulation of HSV-1 gene expression may well involve intracellular compartmental constraints.

Identification of proteins specified by porcine epidemic diarrhoea virus

Up to now, little was known about the proteins of porcine epidemic diarrhoea virus (PEDV). Using (i) metabolic labelling, (ii) antisera directed against synthetic peptides and monoclonal antibodies, and (iii) eukaryotic and prokaryotic expression systems, we have started to identify and characterize these proteins. The nucleocapsid protein (N) of PEDV was identified as a phosphoprotein with a relative mobility (M(r)) of 57 k. No additional phosphoproteins were detected. At least three glycoproteins were virion associated. The 180/200 k band most probably represented the surface glycoprotein (S), whereas the 27 k protein was the membrane protein (M). The 21 k species could not yet be identified. A rabbit anti-M protein antiserum was generated by using synthetic peptides corresponding to the C-terminus of M. The specificity of this serum was reconfirmed by transient expression of the M-gene in Vero cells followed by immunofluorescence. In order to generate antisera against the putative gene products of ORF3 and of the internal N reading frames (I-1, I-2, I-3), additional anti-peptide sera were raised. The putative ORF3 product which had been tagged by a 6 Histidine tail, was expressed in E. coli and purified by nickel chelate affinity chromatography before 2-dimensional polyacrylamide gel electrophoresis and immunostaining with a rabbit anti-peptide serum directed against the N-terminus of the ORF3 product. Transient expression of the I-1 and I-3 reading frames was used to confirm the specificity of the corresponding anti-peptide sera. The immunological tools presented in this paper are now being used to identify the putative corresponding gene products specified by PEDV.

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.

Transformed cells producing the glycoprotein D of HSV-1 are resistant to infection with clinical strains of HSV

The generality of the resistance exhibited by gD producing cells to HSV-1 infection was tested. We tested three different cell lines producing various amounts of gD for resistance against three HSV-1 strains. The strains used were the prototype laboratory F strain and two recently isolated low passage local clinical strains, VG and VD. The results indicate that: (i) the resistance of the cell lines is directly related to the amount of gD they produce, (ii) the cell lines showed greater resistance against the two local clinical HSV-1 strains than against the laboratory strain, and (iii) the resistance is not mediated at the level of virus adsorption to the cell membranes.