Identification and transcriptional mapping of genes encoded at the IR/Us junction of equine herpesvirus type 1

Development and characterization of ORF68 negative equine herpes virus type-1, Ab4p strain

Equine herpesvirus-1 (EHV-1) is an important pathogen, which infects horses worldwide with high morbidity but low mortality rates. The respiratory disorders and abortions are the most common indicators. Ab4p (an abortigenic and paralytic virus) is one of the most important and virulent strains. The development and functional characterization of the open reading frame-68 (ORF68) negative EHV-1 Ab4p mutants and an assessment of their roles in the infection at the cellular level were the main targets of the current study. Escherichia coli DH10β containing the Ab4p bacterial artificial chromosome (pAb4pBAC) and Red/ET expression vector were used to develop different ORF68 mutants. Multi-step growth kinetic experiments were conducted in order to evaluate the growth properties of the constructed mutant viruses. Growth of the Ab4pΔORF68 showed the lowest titer, compared to the Ab4pΔORF68R, Ab4pΔORF68R non-sense, and the parent Ab4p viruses without any significant difference (P > 0.05). The growth of the mutant viruses was almost similar across the cell types, but viruses growth was more efficient in FHK cells as judged by the number of the obtained virus particles. The plaque size of Ab4pΔORF68 was significantly (40%) smaller than those of Ab4p (P < 0.01), Ab4pΔORF68R, and Ab4pΔORF68R non-sense viruses which confirmed the importance of ORF68 protein in the cell-to-cell transmission of EHV-1. Subcellular localization of the green fluorescent protein (GFP) ORF68 gene fusion product showed late expression with intranuclear localization of the transfected cells while immunofluorescent antibody technique (IFAT) localized it at the nucleus and nuclear membranes of the infected cells. Hence, it could be concluded that ORF68 protein may not be essential for EHV-1 Ab4p growth but plays a crucial role in virus penetration and transmission at the cellular level. Therefore, the generated EHV-1 ORF68 negative mutant could be a prospective candidate for the development of a vaccine marker.

Prevalence of equine herpesvirus type 1 strains of neuropathogenic genotype in a major breeding area of Japan

A single non-synonymous nucleotide substitution of guanine (G) for adenine (A) at position 2254 in the viral DNA polymerase gene (encoded by open reading frame [ORF] 30) of equine herpesvirus type 1 (EHV-1) has been significantly associated with neuropathogenic potential in strains of this virus. To estimate the prevalence of EHV-1 strains with the neuropathogenic genotype (ORF30 G(2254)) in the Hidaka district--a major horse breeding area in Japan--we analyzed the ORF30 genomic region in cases of EHV-1 infection in this area during the years 2001-2010. Of the 113 cases analyzed, 3 (2.7%) were induced by ORF30 G(2254) strains. This prevalence is lower than those observed in the U.S.A. (10.8-19.4%), Argentina (7.4%), France (24%), and Germany (10.6%).

Properties of an equine herpesvirus 1 mutant devoid of the internal inverted repeat sequence of the genomic short region

The 150 kbp genome of equine herpesvirus-1 (EHV-1) is composed of a unique long (UL) region and a unique short (Us) segment, which is flanked by identical internal and terminal repeat (IR and TR) sequences of 12.7 kbp. We constructed an EHV-1 lacking the entire IR (vL11ΔIR) and showed that the IR is dispensable for EHV-1 replication but that the vL11ΔIR exhibits a smaller plaque size and delayed growth kinetics. Western blot analyses of cells infected with vL11ΔIR showed that the synthesis of viral proteins encoded by the immediate-early, early, and late genes was reduced at immediate-early and early times, but by late stages of replication reached wild type levels. Intranasal infection of CBA mice revealed that the vL11ΔIR was significantly attenuated as mice infected with the vL11ΔIR showed a reduced lung viral titer and greater ability to survive infection compared to mice infected with parental or revertant virus.

Molecular analysis of US10, S3, and US2 in duck enteritis virus

A 4554-bp fragment was amplified from the DEV C-KCE vaccine strain by single oligonucleotide nested polymerase chain reaction with partially known sequences for the DEV US1 and US10 genes. Three open reading frames containing the genes encoding US10, S3, and US2 were predicted using the Editseq program (DNAStar). The S3 and US2 genes have the same transcription orientation but are oriented head-to-head with respect to US10. The promoters and polyadenylation signals were predicted. Two poly A sequences were predicted in S3, but none were predicted in US2. These results provide partial sequence of US region for the physical map of the DEV genome. Phylogenetic analysis suggests that the DEV C-KCE strain is more closely related to Mardivirus in the alphaherpesvirus subfamily of the Herpesviridae.

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

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

The pseudorabies virus Us2 protein, a virion tegument component, is prenylated in infected cells

The Us2 gene is conserved among alphaherpesviruses, but its function is not known. We demonstrate here that the pseudorabies virus (PRV) Us2 protein is synthesized early after infection and localizes to cytoplasmic vesicles and to the plasma membrane, despite the lack of a recognizable signal sequence or membrane-spanning domain. Us2 protein is also packaged as part of the tegument of mature virions. The Us2 carboxy-terminal four amino acids comprise a CAAX motif, a well-characterized signal for protein prenylation. Treatment of infected cells with lovastatin, a drug that disrupts protein prenylation, changed the relative electrophoretic mobility of Us2 in sodium dodecyl sulfate-polyacrylamide gels. In addition, lovastatin treatment caused a dramatic relocalization of Us2 to cytoplasmic punctate structures associated with microtubules, which appeared to concentrate over the microtubule organizing center. When the CAAX motif was changed to GAAX and the mutant protein was synthesized from an expression plasmid, it concentrated in punctate cytoplasmic structures reminiscent of Us2 localization in infected cells treated with lovastatin. We suggest that prenylation of PRV Us2 protein is required for proper membrane association. Curiously, the Us2 protein isolated from purified virions does not appear to be prenylated. This is the first report to describe the prenylation of an alphaherpesvirus protein.

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.

The equine herpesvirus 1 Us2 homolog encodes a nonessential membrane-associated virion component

Experiments were conducted to analyze the equine herpesvirus 1 (EHV-1) gene 68 product which is encoded by the EHV-1 Us2 homolog. An antiserum directed against the amino-terminal 206 amino acids of the EHV-1 Us2 protein specifically detected a protein with an Mr of 34,000 in cells infected with EHV-1 strain RacL11. EHV-1 strain Ab4 encodes a 44,000-Mr Us2 protein, whereas vaccine strain RacH, a high-passage derivative of RacL11, encodes a 31,000-Mr Us2 polypeptide. Irrespective of its size, the Us2 protein was incorporated into virions. The EHV-1 Us2 protein localized to membrane and nuclear fractions of RacL11-infected cells and to the envelope fraction of purified virions. To monitor intracellular trafficking of the protein, the green fluorescent protein (GFP) was fused to the carboxy terminus of the EHV-1 Us2 protein or to a truncated Us2 protein lacking a stretch of 16 hydrophobic amino acids at the extreme amino terminus. Both fusion proteins were detected at the plasma membrane and accumulated in the vicinity of nuclei of transfected cells. However, trafficking of either GFP fusion protein through the secretory pathway could not be demonstrated, and the EHV-1 Us2 protein lacked detectable N- and O-linked carbohydrates. Consistent with the presence of the Us2 protein in the viral envelope and plasma membrane of infected cells, a Us2-negative RacL11 mutant (L11DeltaUs2) exhibited delayed penetration kinetics and produced smaller plaques compared with either wild-type RacL11 or a Us2-repaired virus. After infection of BALB/c mice with L11DeltaUs2, reduced pathogenicity compared with the parental RacL11 virus and the repaired virus was observed. It is concluded that the EHV-1 Us2 protein modulates virus entry and cell-to-cell spread and appears to support sustained EHV-1 replication in vivo.

The equine herpesvirus 1 IR6 protein that colocalizes with nuclear lamins is involved in nucleocapsid egress and migrates from cell to cell independently of virus infection

The equine herpesvirus 1 (EHV-1) IR6 protein forms typical rod-like structures in infected cells, influences virus growth at elevated temperatures, and determines the virulence of EHV-1 Rac strains (Osterrieder et al., Virology 226:243-251, 1996). Experiments to further elucidate the functions and properties of the IR6 protein were conducted. It was shown that the IR6 protein of wild-type RacL11 virus colocalizes with nuclear lamins very late in infection as demonstrated by confocal laser scan microscopy and coimmunoprecipitation experiments. In contrast, the mutated IR6 protein encoded by the RacM24 strain did not colocalize with the lamin proteins at any time postinfection (p.i.). Electron microscopical examinations of ultrathin sections were performed on cells infected at 37 and 40 degreesC, the latter being a temperature at which the IR6-negative RacH virus and the RacM24 virus are greatly impaired in virus replication. These analyses revealed that nucleocapsid formation is efficient at 40 degreesC irrespective of the virus strain. However, whereas cytoplasmic virus particles were readily observed at 16 h p.i. in cells infected with the wild-type EHV-1 RacL11 or an IR6-recombinant RacH virus (HIR6-1) at 40 degreesC, virtually no capsid translocation to the cytoplasm was obvious in RacH- or RacM24-infected cells at the elevated temperature, demonstrating that the IR6 protein is involved in nucleocapsid egress. Transient transfection assays using RacL11 or RacM24 IR6 plasmid DNA and COS7 or Rk13 cells, infection studies using a gB-negative RacL11 mutant (L11DeltagB) which is deficient in direct cell-to-cell spread, and studies using lysates of IR6-transfected cells demonstrated that the wild-type IR6 protein is transported from cell to cell in the absence of virus infection and can enter cells by a yet unknown mechanism.

Genomic organization of the canine herpesvirus US region

Canine herpesvirus (CHV) is an alpha-herpesvirus of limited pathogenicity in healthy adult dogs and infectivity of the virus appears to be largely limited to cells of canine origin. CHV's low virulence and species specificity make it an attractive candidate for a recombinant vaccine vector to protect dogs against a variety of pathogens. As part of the analysis of the CHV genome, the authors determined the complete nucleotide sequence of the CHV US region as well as portions of the flanking inverted repeats. Seven full open reading frames (ORFs) encoding proteins larger than 100 amino acids were identified within, or partially within the CHV US: cUS2, cUS3, cUS4, cUS6, cUS7, cUS8 and cUS9; which are homologs of the herpes simplex virus type-1 US2; protein kinase; gG, gD, gI, gE; and US9 genes, respectively. An eighth ORF was identified in the inverted repeat region, cIR6, a homolog of the equine herpesvirus type-1 IR6 gene. The authors identified and mapped most of the major transcripts for the predicted CHV US ORFs by Northern analysis.

An equine herpesvirus-1 gene 71 deletant is attenuated and elicits a protective immune response in mice

The pathogenesis of pulmonary infection and the immune response following intranasal inoculation of mice with two equine herpesvirus type 1 (EHV-1) deletion mutants have been assessed. The mutants, ED71 and ED75, have deletions in genes 71 (EUS4) and 75 (10K), respectively. Deletions were replaced by the Escherichia coli lacZ gene driven by the simian virus 40 (SV40) early promoter. It has previously been shown that the protein products of genes 71 and 75 are dispensable in vitro but that removal of gene 71 results in a defect in virus maturation and capsid envelopment which impairs the ability of mutant virus to spread via release and readsorption. This study demonstrated that the 192-kDa gene 71 product is required for full expression of virulence in mice, whereas the putative 10-kDa product of gene 75 has minimal effect. Both mutants exhibited the same tissue and cytotropism as wild-type EHV-1 and induced both humoral and cell-mediated immune responses indistinguishable from those induced by the parental strain. Irrespective of the reduced pathogenicity of the gene 71 mutant, infected mice were protected against a challenge with wild-type EHV-1. These findings highlight the potential of ED71 as a vaccine candidate.

The ICP22 protein of equine herpesvirus 1 cooperates with the IE protein to regulate viral gene expression

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 EHV-1 EICP22 protein, a homolog of ICP22 of herpes simplex virus, increased the in vitro DNA binding activity of the IE protein for sequences in the IE, early, and late promoters. The EICP22 protein affected the rate as well as the extent of the IE protein binding to promoter DNA sequences. To study the DNA binding activity of the IE protein, Trp493, Gln495, Asn496, and Lys498 of the WLQN region, which is directly involved in DNA binding, were replaced with Ser (IEW493S), Glu (IEQ495E), Ile (IEN496I), and Glu (IEK498E), respectively. Gel shift assays revealed that the glutathione S-transferase (GST)-IEQ495E(407-615) and GST-IEK498E(407-615) proteins failed to bind to the IE promoter, indicating that the Gln and Lys residues are important for the DNA binding activity. In the presence of the GST-EICP22 protein, DNA binding activity of the GST-IEQ495E(407-615) protein was restored, suggesting that the EICP22 protein cooperates with the IE protein to regulate EHV-1 gene expression. Transient-transfection assays also showed that the EICP22 protein allowed the IEQ495E mutant to be functional as a transactivator. These results are unique and may represent an important role for the EICP22 protein in EHV-1 gene regulation.

Equine herpesvirus-1 strain KyA, a candidate vaccine strain, reduces viral titers in mice challenged with a pathogenic strain, RacL

The equine herpesvirus type-1 (EHV-1) strain Kentucky A (KyA) has a long history of repeated passage either in vivo in the Syrian hamster or in vitro in mouse L-M fibroblast tissue culture. This repeated passage in cells other than those of the natural host has caused genomic alterations of the KyA chromosome resulting in deletion of several genes or portions of open reading frames (ORFs). This report presents in vivo data from a mouse model of EHV-1 infection demonstrating the attenuated nature of EHV-1 strain KyA and that intranasal infection with KyA protects animals from subsequent challenge with a pathogenic strain, RacL, by reducing RacL viral titers in the lungs of the challenged animals. Mice infected with KyA exhibit no clinical manifestations of EHV-1 disease and do not experience the wasting that occurs with RacL infection. KyA-infected mice clear virus from the lung by day 5 post-infection (p.i.), whereas RacL infected mice have substantial virus titers (5 x 10(5) pfu/lung) at this time point. Intranasal infection with KyA followed by a challenge with RacL 4 weeks post-KyA infection resulted in a significant (P = 0.0079) reduction in the lung titers of the RacL virus. RacL was identified as the virus present in the lungs of the challenged mice by a PCR assay employing primers to amplify the EUS4 gene which differs in size by 1.2 kilobase pairs (kbp) in the two strains. Importantly, the protection afforded by KyA is long lasting in that challenge with RacL 15 months after KyA infection, results in reduced virus titers and viral clearance by day 5 post-challenge. These results support the further consideration of EHV-1 KyA as a live virus vaccine.

Alterations in the equine herpesvirus type-1 (EHV-1) strain RacH during attenuation

The equine herpesvirus type-1 modified live-vaccine strain RacH (256th passage on porcine embryonic kidney cells) was investigated by restriction-enzyme analysis and compared to representative plaque isolates of the 12th passage (RacL11, RacL22) and 185th passage (RacM24, RacM36). The restriction patterns of all Rac plaque isolates differed compared with reference strain Ab4. The left UL terminus was shortened by 0.1 kbp and a missing BamHI site led to the fusion of the f and t fragments. In some Rac derivatives, losses of restriction sites without deletions were observed: 1. One BamHI site located in the ribosyl reductase gene was missing in RacH, RacM24, RacM36, and RacL22; and 2. An SalI site mapping to the gp14 (gB) gene was absent in RacM24, RacM36 and RacH. An identical deletion of 0.85 kbp in size was found in both copies of the inverted repeat (IR) regions of RacH. The deletion was present only in the terminal IR of the medium-passage derivative RacM36. By contrast, in the genomes of the apathogenic RacM24, as well as the pathogenic plaque isolates RacL11 and RacL22, no deletions in the IRs were detectable. Nucleotide-sequence and Northern-blot analyses revealed that the deletions led to the elimination of one or both copies of the gene 67 (IR6) open-reading frame in RacM36 and RacH and affected the gene 68 (EUS1) in RacH.

Transcriptional analyses of the unique short segment of EHV-1 strain Kentucky A

The unique short (Us) segment of the genome of equine herpesvirus type 1 (EHV-1) strain KyA is comprised of six open reading frames (ORFs) that encode: a) a homolog of the Us2 protein of herpes simplex virus type 1 (HSV-1); b) a serine threonine protein kinase that is a homolog of the HSV-1 Us3 protein; c) a homolog of pseudorabies virus glycoprotein gX and HSV-2 gG; d) a novel glycoprotein, EUS4, not encoded by other herpesviruses sequenced to date; e) a homolog of HSV-1 gD; and f) a homolog of HSV-1 Us9. The KyA strain is a deletion mutant that lacks Us sequences encoding gI, gE, and a potential 10 kD polypeptide, and thus may be useful as a parent virus for the generation of live virus vaccines. To complete the elucidation of the transcriptional program of the Us segment, Northern blot hybridization and S1 nuclease analyses were performed on poly(A)(+)-selected RNA isolated from infected cells maintained under early (phosphonoacetic acid-block) and late conditions. The findings revealed that the gene (EUS2 ORF) encoding the protein kinase is expressed as an early 2.9 kb transcript that overlaps and is 3' coterminal with a 1.6 kb early transcript that encodes the gG/gX homolog (EUS3 ORF). Two transcripts of 1.6 kb and 5.8 kb are 5' coterminal and may both encode the novel glycoprotein gene EUS4. The 1.6 kb transcript terminates at a poly(A) signal site downstream of the EUS4 ORF, and the 5.8 kb transcript terminates within the inverted repeat (IR) segment. Overall, the transcriptional program of the EHV-1 KyA Us segment is complex and exhibits similarities to that of HSV-1 Us segment: a) transcripts arise from both DNA strands; b) some transcripts, including those mapping at the termini of the Us segment, extend into the IR segments and are 3' coterminal with the 1.2 kb IR6 transcript; c) at least one transcript reads through a functional polyadenylation signal; d) some transcripts encoding genes that lie in different reading frames exist as a family of overlapping mRNAs, some in an anti-sense manner. Lastly, of the six Us genes of the EHV-1 KyA strain, only those encoding the EHV-1 protein kinase and the HSV-2 gG/gX homolog are members of the early kinetic class.

The Marek's disease virus (MDV) unique short region: alphaherpesvirus-homologous, fowlpox virus-homologous, and MDV-specific genes

Despite its previous classification as a gammaherpesvirus, primarily due to its lymphotropism, Marek's disease virus (MDV), an oncogenic avian herpesvirus, is phylogenetically more related to the "neurotropic" alphaherpesviruses, characterized by its prototype, herpes simplex virus (HSV) (Buckmaster et al., 1988, J. Gen. Virol. 69, 2033-2042). In this report we present the DNA sequence of an 11,286-bp DNA segment encompassing the entire 11,160-bp-long Us region of the oncogenic avian herpesvirus, Marek's disease virus. Eleven open reading frames (ORFs) likely to code for proteins were identified; of these, 7 represent homologs exclusive to alphaherpesvirus S component genes. These include MDV counterparts of HSV US1 (ICP22), US2, US3 (a serine-threonine protein kinase), US6, US7, and US8 (HSV glycoproteins gD, gI, and gE, respectively), and US10. Three additional ORFs were identified with no apparent relation to any sequences currently present in the SwissProt or GenBank/EMBL databases, while a fourth was found to exhibit significant homology to an uncharacterized fowlpox virus (FPV) ORF. Having precisely identified the IRs-U(s) and U(s)-TRs junctions, we have corrected and clarified their previously reported locations. By characterizing genes encoding three new alphaherpesvirus-related homologs (US1, US8, and US10), completing the sequence for a fourth (US7), and identifying 2 new MDV-specific ORFs (SORF1 and SORF3) and a fowlpox homolog (SORF2), our sequence analysis of the "virulent" GA strain of MDV (vMDV) extends upon that of a 5255-bp segment located in the U(s) region of the "very virulent" RB1B strain of MDV (vvMDV) (Ross et al., 1991, J. Gen. Virol. 72, 939-947; 949-954). These two sequences were found to exhibit 99% identity at both nucleotide and predicted amino acid levels. Combined with the fact that MDV U(s) sequences failed to show statistically significant CpG deficiencies, our analysis is consistent with MDV bearing a closer phylogenetic relation to alphaherpesviruses than to gammaherpesviruses. Because alphaherpesvirus-specific U(s) region genes are primarily nonessential for virus replication, they are thought to be important biological property determinants. Thus, our sequence provides a foundation for further MDV studies aimed at resolving the apparent discrepancy between MDV's genetic and biologic properties.

Identification and initial characterization of the IR6 protein of equine herpesvirus 1

The IR6 gene of equine herpesvirus 1 (EHV-1) is a novel gene that maps within each inverted repeat (IR), encodes a potential protein of 272 amino acids, and is expressed as a 1.2-kb RNA whose synthesis begins at very early times (1.5 h) after infection and continues throughout the infection cycle (C. A. Breeden, R. R. Yalamanchili, C.F. Colle, and D.J. O'Callaghan, Virology 191:649-660,1992). To identify the IR6 protein and ascertain its properties, we generated an IR6-specific polyclonal antiserum to a TrpE/IR6 fusion protein containing 129 amino acids (residues 134 to 262) of the IR6 protein. This antiserum immunoprecipitated a 33-kDa protein generated by in vitro translation of mRNA transcribed from a pGEM construct (IR6/pGEM-3Z) that contains the entire IR6 open reading frame. The anti-IR6 antibody also recognized an infected-cell protein of approximately 33 kDa that was expressed as early as 1 to 2 h postinfection and was synthesized throughout the infection cycle. A variety of biochemical analyses including radiolabeling the IR6 protein with oligosaccharide precursors, translation of IR6 mRNA in the presence of canine pancreatic microsomes, radiolabeling the IR6 protein in the presence of tunicamycin, and pulse-chase labeling experiments indicated that the two potential sites for N-linked glycosylation were not used and that the IR6 protein does not enter the secretory pathway. To address the possibility that the unique IR6 gene encodes a novel regulatory protein, we transiently transfected an IR6 expression construct into L-M fibroblasts alone or with an immediate-early gene expression construct along with a representative EHV-1 immediate-early, early, or late promoter-chloramphenicol acetyltransferase reporter construct. The results indicated that the IR6 protein does not affect the expression of these representative promoter constructs. Interestingly, the IR6 protein was shown to be phosphorylated and to associate with purified EHV-1 virions and nucleocapsids. Lastly, immunofluorescence and laser-scanning confocal microscopic analyses revealed that the IR6 protein is distributed throughout the cytoplasm at early times postinfection and that by 4 to 6 h it appears as "dash-shaped" structures that localize to the perinuclear region. At late times after infection (8 to 12 h), these structures assemble around the nucleus, and three-dimensional image analyses reveal that the IR6 protein forms a crown-like structure that surrounds the nucleus as a perinuclear network.

Identification and characterization of the ICP22 protein of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of herpes simplex virus type 1 ICP22 is differently expressed from the fourth open reading frame of the inverted repeat (IR4) as a 1.4-kb early mRNA and a 1.7-kb late mRNA which are 3' coterminal (V. R. Holden, R. R. Yalamanchili, R. N. Harty, and D. J. O'Callaghan, J. Virol. 66:664-673, 1992). To extend the characterization of IR4 at the protein level, the synthesis and intracellular localization of the IR4 protein were investigated. Antiserum raised against either a synthetic peptide corresponding to amino acids 270 to 286 or against a TrpE-IR4 fusion protein (IR4 residues 13 to 150) was used to identify the IR4 protein. Western immunoblot analysis revealed that IR4 is expressed abundantly from an open reading frame composed of 293 codons as a family of proteins that migrate between 42 to 47 kDa. The intracellular localization of IR4 was examined by cell fractionation, indirect immunofluorescence, and laser-scanning confocal microscopy. These studies revealed that IR4 is localized predominantly in the nucleus and is dispersed uniformly throughout the nucleus. Interestingly, when IR4 is expressed transiently in COS-1 or LTK- cells, a punctate staining pattern within the nucleus is observed by indirect immunofluorescence. Cells transfected with an IR4 mutant construct that encodes a C-terminal truncated (19 amino acids) IR4 protein exhibited greatly reduced intranuclear accumulation of the IR4 protein, indicating that this domain possesses an important intranuclear localization signal. Western blot analysis of EHV-1 virion proteins revealed that IR4 proteins are structural components of the virions. Surprisingly, the 42-kDa species, which is the least abundant and the least modified form of the IR4 protein family in infected cell extracts, was the most abundant IR4 protein present in purified virions.