Herpes simplex virus type 1 protein IE63 affects the nuclear export of virus intron-containing transcripts

Molecular Mechanism of SR Protein Kinase 1 Inhibition by the Herpes Virus Protein ICP27

Serine-arginine (SR) protein kinase 1 (SRPK1) catalyzes the phosphorylation of SR proteins, which are a conserved family of splicing factors that contain a domain rich in arginine and serine repeats. SR proteins play important roles in constitutive pre-mRNA splicing and are also important regulators of alternative splicing. During herpes simplex virus infection, SRPK1 is inactivated and its cellular distribution is markedly altered by interaction with the viral protein ICP27, resulting in hypophosphorylation of SR proteins. Mutational analysis previously showed that the RGG box motif of ICP27 is required for interaction with SRPK1; however, the mechanism for the inhibition and the exact role of the RGG box was unknown. Here, we used solution nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC) to demonstrate that the isolated peptide comprising the RGG box of ICP27 binds to SRPK1 with high affinity, competing with a native substrate, the SR repeat region of SR protein SRSF1. We determined the crystal structure of the complex between SRPK1 and an RGG box peptide, which revealed that the viral peptide binds to the substrate docking groove, mimicking the interactions of SR repeats. Site-directed mutagenesis within the RGG box further confirmed the importance of selected arginine residues for interaction, relocalization, and inhibition of SRPK1 in vivo Together these data reveal the molecular mechanism of the competitive inhibition of cellular SRPK1 by viral ICP27, which modulates SRPK1 activity.IMPORTANCE Serine arginine (SR) proteins are a family of mRNA regulatory proteins that can modulate spliceosome association with different splice sites and therefore regulate alternative splicing. Phosphorylation within SR proteins is necessary for splice-site recognition, and this is catalyzed by SR protein kinase 1 (SRPK1). The herpes simplex virus (HSV-1) protein ICP27 has been shown previously to interact with and downregulate SRPK1 activity in vivo; however, the molecular mechanism for this interaction and inhibition was unknown. Here, we demonstrate that the isolated peptide fragment of ICP27 containing RGG box binds to SRPK1 with high affinity, and competes with a native cellular substrate. Elucidation of the SRPK1-RGG box crystal structure further showed that a short palindromic RGRRRGR sequence binds in the substrate docking groove of SRPK1, mimicking the binding of SR repeats of substrates. These data reveal how the viral protein ICP27 inactivates SRPK1, promoting hypophosphorylation of proteins regulating splicing.

Nuclear-cytoplasmic compartmentalization of the herpes simplex virus 1 infected cell transcriptome is co-ordinated by the viral endoribonuclease vhs and cofactors to facilitate the translation of late proteins

HSV1 encodes an endoribonuclease termed virion host shutoff (vhs) that is produced late in infection and packaged into virions. Paradoxically, vhs is active against not only host but also virus transcripts, and is involved in host shutoff and the temporal expression of the virus transcriptome. Two other virus proteins-VP22 and VP16 -are proposed to regulate vhs to prevent uncontrolled and lethal mRNA degradation but their mechanism of action is unknown. We have performed dual transcriptomic analysis and single-cell mRNA FISH of human fibroblasts, a cell type where in the absence of VP22, HSV1 infection results in extreme translational shutoff. In Wt infection, host mRNAs exhibited a wide range of susceptibility to vhs ranging from resistance to 1000-fold reduction, a variation that was independent of their relative abundance or transcription rate. However, vhs endoribonuclease activity was not found to be overactive against any of the cell transcriptome in Δ22-infected cells but rather was delayed, while its activity against the virus transcriptome and in particular late mRNA was minimally enhanced. Intriguingly, immediate-early and early transcripts exhibited vhs-dependent nuclear retention later in Wt infection but late transcripts were cytoplasmic. However, in the absence of VP22, not only early but also late transcripts were retained in the nucleus by a vhs-dependent mechanism, a characteristic that extended to cellular transcripts that were not efficiently degraded by vhs. Moreover, the ability of VP22 to bind VP16 enhanced but was not fundamental to the rescue of vhs-induced nuclear retention of late transcripts. Hence, translational shutoff in HSV1 infection is primarily a result of vhs-induced nuclear retention and not degradation of infected cell mRNA. We have therefore revealed a new mechanism whereby vhs and its co-factors including VP22 elicit a temporal and spatial regulation of the infected cell transcriptome, thus co-ordinating efficient late protein production.

Intron retention in viruses and cellular genes: Detention, border controls and passports

Intron retention (IR), where one or more introns remain in the RNA after splicing, was long thought to be rare in mammalian cells, albeit common in plants and some viruses. Largely due to the development of better methods for RNA analysis, it has now been recognized that IR is much more common than previously thought and that this mechanism is likely to play an important role in mammalian gene regulation. To date, most publications and reviews about IR have described the resulting mRNAs as "dead end" products, with no direct consequence for the proteome. However, there are also many reports of mRNAs with retained introns giving rise to alternative protein isoforms. Although this was originally revealed in viral systems, there are now numerous examples of bona fide cellular proteins that are translated from mRNAs with retained introns. These new isoforms have sometimes been shown to have important regulatory functions. In this review, we highlight recent developments in this area and the research on viruses that led the way to the realization of the many ways in which mRNAs with retained introns can be regulated. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing RNA Export and Localization > Nuclear Export/Import RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Regulation of viral gene expression by duck enteritis virus UL54

Duck enteritis virus (DEV) UL54 is a homologue of human herpes simplex virus-1 (HSV-1) ICP27, which plays essential regulatory roles during infection. Our previous studies indicated that DEV UL54 is an immediate-early protein that can shuttle between the nucleus and the cytoplasm. In the present study, we found that UL54-deleted DEV (DEV-ΔUL54) exhibits growth kinetics, a plaque size and a viral DNA copy number that are significantly different from those of its parent wild-type virus (DEV-LoxP) and the revertant (DEV-ΔUL54 (Revertant)). Relative viral mRNA levels, reflecting gene expression, the transcription phase and the translation stage, are also significantly different between DEV-ΔUL54-infected cells and DEV-LoxP/DEV-ΔUL54 (Revertant)-infected cells. However, the localization pattern of UL30 mRNA is obviously changed in DEV-ΔUL54-infected cells. These findings suggest that DEV UL54 is important for virus growth and may regulate viral gene expression during transcription, mRNA export and translation.

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

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

Genetic evidence for a connection between Rous sarcoma virus gag nuclear trafficking and genomic RNA packaging

The packaging of retroviral genomic RNA (gRNA) requires cis-acting elements within the RNA and trans-acting elements within the Gag polyprotein. The packaging signal psi, at the 5' end of the viral gRNA, binds to Gag through interactions with basic residues and Cys-His box RNA-binding motifs in the nucleocapsid. Although specific interactions between Gag and gRNA have been demonstrated previously, where and when they occur is not well understood. We discovered that the Rous sarcoma virus (RSV) Gag protein transiently localizes to the nucleus, although the roles of Gag nuclear trafficking in virus replication have not been fully elucidated. A mutant of RSV (Myr1E) with enhanced plasma membrane targeting of Gag fails to undergo nuclear trafficking and also incorporates reduced levels of gRNA into virus particles compared to those in wild-type particles. Based on these results, we hypothesized that Gag nuclear entry might facilitate gRNA packaging. To test this idea by using a gain-of-function genetic approach, a bipartite nuclear localization signal (NLS) derived from the nucleoplasmin protein was inserted into the Myr1E Gag sequence (generating mutant Myr1E.NLS) in an attempt to restore nuclear trafficking. Here, we report that the inserted NLS enhanced the nuclear localization of Myr1E.NLS Gag compared to that of Myr1E Gag. Also, the NLS sequence restored gRNA packaging to nearly wild-type levels in viruses containing Myr1E.NLS Gag, providing genetic evidence linking nuclear trafficking of the retroviral Gag protein with gRNA incorporation.

Kaposi's sarcoma-associated herpesvirus ORF57 in viral RNA processing

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 (MTA, mRNA transcript accumulation) is a multifunctional regulator of the expression of viral lytic genes. KSHV ORF57 is expressed during viral lytic infection and is essential for virus production. Like its homologues in the herpesvirus family, ORF57 promotes the accumulation (stabilization) and export of viral intronless RNA transcripts by a mechanism which remains to be defined. The ORF57-Aly/REF interaction plays only a small role in viral RNA export. Although other members of the family generally inhibit the splicing of cellular RNAs, KSHV ORF57 and EBV EB2, in sharp contrast, stimulate viral RNA splicing for the expression of viral intron-containing genes. The functions of KSHV ORF57 are independent of transcription and of other viral proteins; instead, these functions always rely on cellular components and occur in various protein-RNA complexes. ORF57 may synergize with KSHV ORF50 to transactivate a subset of viral promoters by an unknown mechanism. Thus, some functions of ORF57 have been conserved while others have diverged from its homologues as ORF57 adapted over evolution to KSHV biology and pathogenesis.

Alcelaphine herpesvirus-1 open reading frame 57 encodes an immediate-early protein with regulatory function

Alcelaphine herpesvirus-1 (AlHV-1) is the causative agent of Malignant Catarrhal fever, a lymphoproliferative and degenerative disease of large ruminants and ungulate species. The Alcelaphine Herpesvirus-1 gene product encoded by open reading frame 57 (ORF 57) is the positional homologue of the ORF 57 of Herpes Virus Saimiri (HVS), Kaposi's Sarcoma associated herpesvirus (KSHV) and Murine Gammaherpesvirus 68 (MHV 68), the Epstein-Barr virus BMLF1 gene, the herpes simplex virus (HSV-1) ICP 27 and the IE 4 gene of Varicella Zoster virus (VZV). In these viruses the ORF 57 gene product is expressed very early and encodes a regulatory protein, which is essential for viral replication acting both at the transcriptional and post-transcriptional levels. The function of ORF 57 gene product in the life cycle of AlHV-1 however remains unknown. Here we examined the expression of this gene and the function of its product. We have demonstrated that it is expressed very early in infection and have shown that the ORF57 gene product activates the promoter of another classical transactivator gene ORF50. It activates ORF50 promoter driving expression of an intron-less reporter gene to 50 fold and does not have any effect on an intron-containing reporter gene driven by the ORF 50 promoter. The 50 fold increase in the luciferase activity was not correlated with a similar fold increase in the luciferase RNA levels indicating that ORF 57 protein acts at a post-transcriptional level to regulate gene expression.

RNA binding by the herpes simplex virus type 1 nucleocytoplasmic shuttling protein UL47 is mediated by an N-terminal arginine-rich domain that also functions as its nuclear localization signal

The function of the alphaherpesvirus UL47 tegument protein has not yet been defined. Nonetheless, previous studies with transfected cells have shown that both the herpes simplex virus type 1 homologue (hUL47, or VP13/14) and the bovine herpesvirus type 1 (BHV-1) homologue (bUL47, or VP8) have the capacity to shuttle between the nucleus and the cytoplasm. Furthermore, hUL47 packaged into the virion has also been shown to bind several individual virus-specific RNA transcripts. Here, we extend these observations and show that hUL47 binds a wide range of RNA species in vitro. It has a high affinity for polyadenylated transcripts but has no apparent selectivity for virus-encoded RNA over cellular RNA. We also show that the virion population of bUL47 binds RNA in vitro. However, while purified recombinant hUL47 retains its RNA binding activity, recombinant bUL47 does not, suggesting that the BHV-1 homologue may require virus-induced modification for its activity. We identify the minimal RNA binding domain in hUL47 as a 26-residue N-terminal peptide containing an arginine-rich motif that is essential but not sufficient for optimal RNA binding, and we demonstrate that this RNA binding domain incorporates the hUL47 minimal nuclear localization signal. In addition, we show that soon after hUL47 is expressed during infection, it colocalizes in the infected cell nucleus with ICP4, the major virus transcriptional activator. Using RNA immunoprecipitations, we demonstrate that hUL47 is also bound in vivo to at least one viral transcript, the ICP0 mRNA. Taken together, these results suggest that hUL47 may play a role in RNA biogenesis in the infected cell.

Herpes simplex virus type 1 ICP27-dependent activation of NF-kappaB

The ability of herpes simplex virus type 1 (HSV-1) to activate NF-kappaB has been well documented. Beginning at 3 to 5 h postinfection, HSV-1 induces a robust and persistent nuclear translocation of an NF-kappaB-dependent (p50/p65 heterodimer) DNA binding activity, as measured by electrophoretic mobility shift assay. Activation requires virus binding and entry, as well as de novo infected-cell protein synthesis, and is accompanied by loss of both IkappaBalpha and IkappaBbeta. In this study, we identified loss of IkappaBalpha as a marker of NF-kappaB activation, and infection with mutants with individual immediate-early (IE) regulatory proteins deleted indicated that ICP27 was necessary for IkappaBalpha loss. Analysis of both N-terminal and C-terminal mutants of ICP27 identified the region from amino acids 21 to 63 as being necessary for IkappaBalpha loss. Additional experiments with mutant viruses with combinations of IE genes deleted revealed that the ICP27-dependent mechanism of NF-kappaB activation may be augmented by functional ICP4. We also analyzed two additional markers for NF-kappaB activation, phosphorylation of the p65 subunit on Ser276 and Ser536. Phosphorylation of both serines was induced upon HSV infection and required functional ICP4 and ICP27. Pharmacological inhibitor studies revealed that both IkappaBalpha and Ser276 phosphorylation were dependent on Jun N-terminal protein kinase activity, while Ser536 phosphorylation was not affected during inhibitor treatment. These results demonstrate that there are several layers of regulation of NF-kappaB activation during HSV infection, highlighting the important role that NF-kappaB may play in infection.

Characterization of a spliced exon product of herpes simplex type-1 latency-associated transcript in productively infected cells

The latency-associated transcripts (LATs) of herpes simplex virus type-1 (HSV-1) are the only viral RNAs accumulating during latent infections in the sensory ganglia of the peripheral nervous system. The major form of LAT that accumulates in latently infected neurons is a 2 kb intron, spliced from a much less abundant 8.3 primary transcript. The spliced exon mRNA has been hard to detect. However, in this study, we have examined the spliced exon RNA in productively infected cells using ribonuclease protection (RPA), and quantitative RT-PCR (q-PCR) assays. We were able to detect the LAT exon RNA in productively infected SY5Y cells (a human neuronal cell line). The level of the LAT exon RNA was found to be approximately 5% that of the 2 kb intron RNA and thus is likely to be relatively unstable. Quantitative RT-PCR (q-PCR) assays were used to examine the LAT exon RNA and its properties. They confirmed that the LAT exon mRNA is present at a very low level in productively infected cells, compared to the levels of other viral transcripts. Furthermore, experiments showed that the LAT exon mRNA is expressed as a true late gene, and appears to be polyadenylated. In SY5Y cells, in contrast to most late viral transcripts, the LAT exon RNA was found to be mainly nuclear localized during the late stage of a productive infection. Interestingly, more LAT exon RNA was found in the cytoplasm in differentiated compared to undifferentiated SY5Y cells, suggesting the nucleocytoplasmic distribution of the LAT exon RNA and its related function may be influenced by the differentiation state of cells.

Herpes simplex virus ICP27 activation of stress kinases JNK and p38

We previously reported that herpes simplex virus type 1 (HSV-1) can activate the stress-activated protein kinases (SAPKs) p38 and JNK. In the present study, we undertook a comprehensive and comparative analysis of the requirements for viral protein synthesis in the activation of JNK and p38. Infection with the UL36 mutant tsB7 or with UV-irradiated virus indicated that both JNK and p38 activation required viral gene expression. Cycloheximide reversal or phosphonoacetic acid treatment of wild-type virus-infected cells as well as infection with the ICP4 mutant vi13 indicated that only the immediate-early class of viral proteins were required for SAPK activation. Infection with ICP4, ICP27, or ICP0 mutant viruses indicated that only ICP27 was necessary. Additionally, we determined that in the context of virus infection ICP27 was sufficient for SAPK activation and activation of the p38 targets Mnk1 and MK2 by infecting with mutants deleted for various combinations of immediate-early proteins. Specifically, the d100 (0-/4-) and d103 (4-/22-/47-) mutants activated p38 and JNK, while the d106 (4-/22-/27-/47-) and d107 (4-/27-) mutants did not. Finally, infections with a series of ICP27 mutants demonstrated that the functional domain of ICP27 required for activation was located in the region encompassing amino acids 20 to 65 near the N terminus of the protein and that the C-terminal transactivation activity of ICP27 was not necessary.

The herpes simplex virus ICP27 protein: a multifunctional post-transcriptional regulator of gene expression

The herpes simplex virus 1 ICP27 is an essential, highly conserved protein involved in various steps of herpes simplex virus 1 gene regulation as well as in the shut-off of host gene expression during infection. It functions primarily at the post-transcriptional level in inhibiting precursor mRNA splicing and in promoting nuclear export of viral transcripts. These activities are discussed.

Control of VP16 translation by the herpes simplex virus type 1 immediate-early protein ICP27

Herpes simplex virus (HSV) ICP27 is an essential and multifunctional regulator of gene expression that modulates the synthesis and maturation of viral and cellular mRNAs. Processes that are affected by ICP27 include transcription, pre-mRNA splicing, polyadenylation, and nuclear RNA export. We have examined how ICP27 influences the expression of the essential HSV tegument protein and transactivator of immediate-early gene expression VP16. We monitored the effects of ICP27 on the levels, nuclear export, and polyribosomal association of VP16 mRNA and on the amount and stability of VP16 protein. Deletion of ICP27 reduced the levels of VP16 mRNA without altering its nuclear export or the stability of the encoded protein. However, the translational yield of the VP16 mRNA produced in the absence of ICP27 was reduced 9- to 80-fold relative to that for wild-type infection, suggesting a defect in translation. In the absence of ICP27, the majority of cytoplasmic VP16 mRNA was not associated with actively translating polyribosomes but instead cosedimented with 40S ribosomal subunits, indicating that the translational defect is likely at the level of initiation. These effects were mRNA specific, as polyribosomal analysis of two cellular transcripts (glyceraldehyde-3-phosphate dehydrogenase and beta-actin) and two early HSV transcripts (thymidine kinase and ICP8) indicated that ICP27 is not required for efficient translation of these mRNAs. Thus, we have uncovered a novel mRNA-specific translational regulatory function of ICP27.

Evidence for a direct interaction between HSV-1 ICP27 and ICP8 proteins

Herpes simplex virus 1 (HSV-1) ICP27 and ICP8 proteins have both been implicated in the transcription of late genes and regulation of viral gene expression. We showed previously that ICP27 and ICP8 associate with the RNAP II holoenzyme (Zhou and Knipe, J. Virol. 76, 5893-5904). Here, we demonstrate that ICP27 and ICP8 coprecipitate from lysates of HSV-1-infected HEp2 cells and from lysates of insect cells expressing ICP27 and ICP8, the latter being in the absence of other HSV-1 proteins. By expressing and purifying hexahistidine-tagged ICP8 (His-ICP8) and maltose binding protein (MBP)-tagged ICP27 (MBP-27) proteins and performing in vitro immunoprecipitation and pull-down assays, we also demonstrate that ICP27 and ICP8 coprecipitate in the absence of other viral or cellular proteins. Taken together, these data provide evidence that ICP27 and ICP8 interact directly in vitro and in infected cells. We hypothesize that the ICP27-ICP8 interaction plays a role in the stimulation of late gene transcription.

A posttranscriptional regulator of Kaposi's sarcoma-associated herpesvirus interacts with RNA-binding protein PCBP1 and controls gene expression through the IRES

Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8, HHV-8) belongs to the gamma-herpesvirus subfamily. The KSHV ORF57 gene is thought to be a homolog of posttranscriptional regulators that are conserved in the herpesvirus family and are essential for replication. We generated specific monoclonal antibodies (mAbs) against the ORF57 protein that detected the 51-kDa protein expressed in the nucleus of KSHV-infected cells. We also found that the ORF57 protein interacted with poly(rC)-binding protein 1 (PCBP1), a cellular RNA-binding, posttranscriptional regulator. ORF57's interaction with PCBP1 enhanced the activity of not only poliovirus internal ribosome-entry site (IRES)-dependent translation but also X-linked inhibitor of apoptosis (XIAP) and KSHV vFLIP IRES. Actually, when ORF57 expression was induced by the expression of replication and transcription activator (RTA) in KSHV-infected cells, the expression of XIAP was enhanced. These results suggest that ORF57 binds to PCBP1 as a functional partner for posttranscriptional regulation and is involved in the regulation of the expression of both cellular and viral genes through IRESs.

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.

Herpes simplex virus 1 infection is required to produce ICP27 immunoreactive triplet forms when ribosomal aminoacyl-tRNA translocation is blocked by cycloheximide

Infected cell protein (ICP) 27 is an essential herpes simplex virus type 1 (HSV-1) phosphoprotein required for optimal viral DNA and early or late gene synthesis. Three slow-migrating immunoreactive species were detected using multiple anti-ICP27 antibodies following HSV-1 infection of HEp-2 and Vero cells in the presence of cycloheximide (CHX). Generation of the protein triplet moieties required transcription of the alpha27 gene. These forms were observed following infection with a series of recombinant viruses that produce truncated ICP27 polypeptides, suggesting that alternative splicing is not involved in the process. These ICP27 species were not observed following translation inhibition by puromycin (PUR). Synthesis of the triplet occurred by 6 hpi and CHX addition as late as 3 hpi still enabled their production. That the ICP27 species were detected in uninfected ICP27-expressing cells without CHX, but not in its presence, suggests a mechanism in which virus infection is required to produce the forms when ribosomal aminoacyl-transfer RNA (tRNA) translocation is blocked.

Identification of herpes simplex virus RNAs that interact specifically with regulatory protein ICP27 in vivo

Herpes simplex virus type 1 (HSV-1) protein ICP27 has an essential regulatory role during viral replication, in part by post-transcriptional control of gene expression, and has a counterpart in all herpes viruses sequenced so far. Although much is known about the functions of this signature herpesvirus protein, little is known about its RNA binding capabilities; ICP27 interacts with specificity for a subset of intronless HSV-1 RNAs and poly(G), through its RGG box. We performed an in vivo yeast three-hybrid screen of an HSV-1 genomic library, searching for ICP27 interacting RNAs. Comparable with a yeast genomic screen, 24 of 55 single inserts mapped to antisense strands of HSV-1 transcribed regions or non-transcribed regions. The 31 HSV-1 sense RNAs identified were 35 to 225 nucleotides in length and interacted with preferred specificity for ICP27 as compared with an unrelated RNA-binding protein. They map to 10 monocistronic and 10 polycistronic transcripts of all kinetic classes and represent 28 open reading frames encoding predominantly essential viral proteins with roles in viral DNA replication and virion maturation. Several studies show regulatory effects by ICP27 on the majority of these transcripts, consistent with its regulation of the early-late switch in the HSV-1 life cycle. Deletion of the ICP27 RGG box and the ICP27 M15 mutation, both lethal in virus, abolished or severely reduced the ICP27-RNA interactions, indicating their biological relevance. The study facilitates continued study of gene regulation by ICP27 by further defining its interactions with viral RNAs.

The Epstein-Barr virus SM protein is functionally similar to ICP27 from herpes simplex virus in viral infections

The herpes simplex virus type 1 (HSV-1) ICP27 protein is an essential RNA-binding protein that shuttles between the nucleus and cytoplasm to increase the cytoplasmic accumulation of viral late mRNAs. ICP27 homologs have been identified in each of the herpesvirus subfamilies, and accumulating evidence indicates that homologs from the gammaherpesvirus subfamily function similarly to ICP27. In particular, the Epstein-Barr virus (EBV) SM protein posttranscriptionally regulates gene expression, binds RNA in vitro and in vivo, and shuttles between the nucleus and cytoplasm. To determine if these two proteins function through a common mechanism, the ability of EBV SM to complement the growth defect of an HSV-1 ICP27-null virus was examined in a transient-expression assay. ICP27 stimulated the growth of the null mutant more efficiently than did SM, but the ability of SM to compensate for the ICP27 defects suggests conservation of common functions. To assay for complementation in the context of a viral infection, the growth properties of an HSV recombinant expressing SM in an ICP27-null background were analyzed. SM stimulated growth of the recombinant, although this growth was reduced by comparison to that of an ICP27-expressing virus. By contrast, an HSV recombinant expressing an SM mutant allele defective for transactivation activity and nucleocytoplasmic shuttling did not grow at all. These results suggest that SM and ICP27 may regulate gene expression through a common pathway that is evolutionarily conserved in herpesviruses.

Association of herpes simplex virus type 1 ICP8 and ICP27 proteins with cellular RNA polymerase II holoenzyme

Herpes simplex virus 1 (HSV-1) infection causes the shutoff of host gene transcription and the induction of a transcriptional program of viral gene expression. Cellular RNA polymerase II is responsible for transcription of all the viral genes, but several viral proteins stimulate viral gene transcription. ICP4 is required for all delayed-early and late gene transcription, ICP0 stimulates transcription of viral genes, and ICP27 stimulates expression of some early genes and transcription of at least some late viral genes. The early DNA-binding protein, ICP8, also stimulates late gene transcription. We therefore investigated which HSV proteins interact with RNA polymerase II. Using immunoprecipitation and Western blotting methods, we observed the coprecipitation of ICP27 and ICP8 with RNA polymerase II holoenzyme. The association of ICP27 with RNA polymerase II was detectable as early as 3 h postinfection, while ICP8 association became evident by 5 h postinfection, and the association of both was independent of viral DNA synthesis. Infections with ICP27 gene mutant viruses revealed that ICP27 is required for the association of ICP8 with RNA polymerase II, while studies with ICP8 gene deletion mutants showed no apparent role for ICP8 in the association of ICP27 with RNA polymerase II. The association of ICP27 and ICP8 with RNA polymerase II holoenzyme appeared to be independent of nucleic acids. We hypothesize that the interaction of ICP27 with RNA polymerase II holoenzyme reflects its role in stimulating early and late gene expression and/or its role in inhibiting host transcription and that the interaction of ICP8 with RNA polymerase II holoenzyme reflects its role in stimulating late gene transcription.

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.

A single amino acid substitution in the ICP27 protein of herpes simplex virus type 1 is responsible for its resistance to leptomycin B

Leptomycin B (LMB) is a specific inhibitor of Crm1-dependent nuclear export of proteins. The replication of herpes simplex virus (HSV) is normally highly sensitive to LMB; a resistant HSV variant, however, was isolated by serial passages of the virus. Analysis of marker transfer and viral DNA sequences revealed that a single amino acid substitution within the ICP27 gene is responsible for conferring this resistance.

Accumulation of herpes simplex virus type 1 early and leaky-late proteins correlates with apoptosis prevention in infected human HEp-2 cells

We previously reported that a recombinant ICP27-null virus stimulated, but did not prevent, apoptosis in human HEp-2 cells during infection (M. Aubert and J. A. Blaho, J. Virol. 73:2803-2813, 1999). In the present study, we used a panel of 15 recombinant ICP27 mutant viruses to determine which features of herpes simplex virus type 1 (HSV-1) replication are required for the apoptosis-inhibitory activity. Each virus was defined experimentally as either apoptotic, partially apoptotic, or nonapoptotic based on infected HEp-2 cell morphologies, percentages of infected cells with condensed chromatin, and patterns of specific cellular death factor processing. Viruses d27-1, d1-5, d1-2, M11, M15, M16, n504R, n406R, n263R, and n59R are apoptotic or partially apoptotic in HEp-2 cells and severely defective for growth in Vero cells. Viruses d2-3, d3-4, d4-5, d5-6, and d6-7 are nonapoptotic, demonstrating that ICP27 contains a large amino-terminal region, including its RGG box RNA binding domain, which is not essential for apoptosis prevention. Accumulations of viral TK, VP16, and gD but not gC, ICP22, or ICP4 proteins correlated with prevention of apoptosis during the replication of these viruses. Of the nonapoptotic viruses, d4-5 did not produce gC, indicating that accumulation of true late gene products is not necessary for the prevention process. Analyses of viral DNA synthesis in HEp-2 cells indicated that apoptosis prevention by HSV-1 requires that the infection proceeds to the stage in which viral DNA replication takes place. Infections performed in the presence of the drug phosphonoacetic acid confirmed that the process of viral DNA synthesis and the accumulation of true late (gamma(2)) proteins are not required for apoptosis prevention. Based on our results, we conclude that the accumulation of HSV-1 early (beta) and leaky-late (gamma(1)) proteins correlates with the prevention of apoptosis in infected HEp-2 cells.

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.

Identification of nuclear export signal in UL37 protein of herpes simplex virus type 2

The UL37 gene of herpes simplex virus (HSV) encodes a 120-kDa phosphoprotein associated with the virion. In this study, we have generated a rabbit polyclonal antiserum against HSV-2 UL37 protein, and examined its intracellular localization by immunofluorescence study. In infected cells, specific fluorescence was detectable in the perinuclear region. In transfected cells, UL37 protein was observed mainly in the cytoplasm. Transfection assays of deletion mutants of UL37 protein suggested that the leucine rich region (LRR) containing amino acids 263-273 may be important for cytoplasmic localization. Deletion of the LRR or substitution of the leucine residues resulted in nuclear remaining of UL37 protein. Moreover, the LRR could export green fluorescent protein (GFP) to the cytoplasm as a fusion protein and this export was blocked by leptomycin B treatment, indicating that the LRR acted as a nuclear export signal. These results suggest that UL37 protein fulfills a role as a shuttle between the nucleus and the cytoplasm through the LRR.

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.

Processing of alpha-globin and ICP0 mRNA in cells infected with herpes simplex virus type 1 ICP27 mutants

Herpes simplex virus (HSV) ICP27 is an essential and multifunctional regulator of viral gene expression that modulates RNA splicing, polyadenylation, and nuclear export. We have previously reported that ICP27 causes the cytoplasmic accumulation of unspliced alpha-globin pre-mRNA. Here we examined the effects of a series of ICP27 mutations that alter important functional regions of the protein on the processing and nuclear transport of alpha-globin and HSV ICP0 RNA. The results demonstrate that ICP27 mutants that are impaired for growth in noncomplementing cells, including mutants in the N- and C-terminal regions, are defective in the accumulation of alpha-globin pre-mRNA. Unexpectedly, several mutants that are competent to repress the expression of reporter genes in transient transfection assays failed to accumulate unspliced RNA, implying that different mechanisms are responsible for transrepression and pre-mRNA accumulation. Several mutants caused a marked increase in the length and heterogeneity of the alpha-globin mRNA poly(A) tail, suggesting that ICP27 may directly or indirectly affect the regulation of poly(A) polymerase. ICP27 was also required for the accumulation of multiple ICP0 intron-bearing transcripts, but this effect displayed a mutational sensitivity profile different from that of accumulation of unspliced alpha-globin RNA. Moreover, unlike spliced and unspliced alpha-globin RNAs, which were efficiently exported to the cytoplasm, spliced and intron-containing ICP0 transcripts were predominantly nuclear in localization, and ICP27 was not required for nuclear retention of the spliced message. We propose that these transcript- and ICP27 allele-specific differences may be explained by the presence of a strong cis-acting ICP27 response element in the alpha-globin transcript.

Identification of an export control sequence and a requirement for the KH domains in ICP27 from herpes simplex virus type 1

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is an RNA-binding protein that performs multiple functions required for the expression of HSV-1 genes during a productive infection. One essential function involves shuttling between the nucleus and the cytoplasm. Some of the domains identified in ICP27 include a leucine-rich nuclear export sequence (NES), a nuclear localization signal, three KH-like RNA-binding domains, and an RGG-box type RNA-binding motif. To study the contribution of two of the essential domains in ICP27 to HSV gene expression, we generated recombinant herpesviruses carrying deleterious mutations in the NES and KH domains of ICP27. To accomplish this, we fused the green fluorescent protein (GFP) to ICP27 and utilized fluorescence as a marker to isolate recombinant herpesviruses. Fusion of GFP to wild-type ICP27 did not disturb its localization or function or significantly reduce virus yield. Analysis of HSV gene expression in cells infected with a recombinant virus carrying a point mutation in the first KH-like RNA-binding domain revealed that nuclear export of ICP27 was not blocked, and the expression of only a subset of ICP27-dependent late genes was affected. These findings suggest that individual KH-like RNA-binding motifs in ICP27 may be involved in binding distinct RNAs. Analysis of recombinant viruses carrying a lethal mutation in the NES of ICP27 was not accomplished because this mutation results in a strong dominant-negative phenotype. Finally, we demonstrate that shuttling by ICP27 is regulated by an export control sequence adjacent to its NES that functions like the inhibitory sequence element found adjacent to the NES of NS1 from influenza virus.

Epstein-Barr virus EB2 protein exports unspliced RNA via a Crm-1-independent pathway

Human herpesviruses encode posttranscriptional activators that are believed to up-regulate viral replication by facilitating early and late gene expression. We have reported previously that the Epstein-Barr virus protein EB2 (also called M or SM) promotes nuclear export of RNAs that are poor substrates for spliceosome assembly, an effect that closely resembles the human immunodeficiency virus type 1 Rev-dependent nuclear export of unspliced viral RNA. Here we present experimental data showing that EB2 efficiently promotes the nuclear export of unspliced RNA expressed from a Rev reporter construct. Site-directed mutagenesis as well as domain swapping experiments indicate that a leucine-rich region found in the EB2 protein, which matches the consensus sequence for the leucine-rich nuclear export signal, is not a nuclear export signal per se. Accordingly, leptomycin B (LMB), a specific Crm-1 inhibitor, impairs Rev- but not EB2-dependent nuclear export of unspliced RNA. Moreover, EB2 nucleocytoplasmic shuttling visualized by a heterokaryon assay is, unlike Rev shuttling, not affected by LMB. We also show that overexpression of an N-terminal deletion mutant of Nup214/can, a major nucleoporin of the nuclear pore complex involved in several aspects of nuclear transport, blocks both Rev- and EB2-dependent nuclear export of RNA. These results strongly suggest that EB2 nuclear export of unspliced RNA is mediated by a Crm-1-independent pathway.

The herpes simplex virus 1 Us11 protein cooperates with suboptimal amounts of human immunodeficiency virus type 1 (HIV-1) Rev protein to rescue HIV-1 production

The human immunodeficiency virus type 1 (HIV-1) RNA-binding Rev protein governs the expression of structural and enzymatic viral proteins at a posttranscriptional level. Binding of Rev to the stem-loop IIB (SLIIB) sequence of the Rev-response element (RRE) within unspliced and singly spliced viral mRNAs and to the nuclear export signal-binding receptor, hCRM1 (or exportin 1), is required for the export of these transcripts to the cytoplasm. We have previously shown that herpes simplex virus type 1 (HSV-1) RNA-binding Us11 protein is able to bind the RRE and substitute for Rev in inducing the expression of HIV-1 envelope glycoproteins. We show here that Us11 cannot substitute for Rev in rescuing a rev-deleted HIV-1 provirus. However, HIV-1 production is observed when Us11 is expressed with suboptimal amounts of Rev. An in vivo RNA-protein binding assay indicates that Us11 is unable to directly interact with the SLIIB RNA but can bind Rev assembled on that stem-loop structure. This association of US11 with Rev, which was confirmed by in vivo coimmunoprecipitation and GST-pulldown assays, therefore underlies a biological Us11-Rev cooperation. Furthermore this cooperation was shown to remain susceptible to the effect of leptomycin B, which blocks the binding of hCRM1 to the nuclear export signal of Rev. These observations performed with intron-containing constructs provide evidence that HSV-1 Us11 protein is not directly involved in the cytoplasmic accumulation of viral mRNAs but may be rather acting as an auxiliary protein, thus allowing this retroviral protein to fulfill the nuclear export of these transcripts and to rescue HIV-1 production.

Kaposi's sarcoma-associated herpesvirus open reading frame 57 encodes a posttranscriptional regulator with multiple distinct activities

Open reading frame (ORF) 57 of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a homolog of known posttranscriptional regulators that are essential for replication in other herpesviruses. Here, we examined the expression of this gene and the function(s) of its product. KSHV ORF 57 is expressed very early in infection from a 1.6-kb spliced RNA bearing several in-frame initiation codons. Its product is a nuclear protein that, in transient assays, has little effect on the expression of luciferase reporter genes driven by a variety of KSHV and heterologous promoters. However, ORF 57 protein enhances the accumulation of several viral transcripts, in a manner suggesting posttranscriptional regulation. These transcripts include not only known cytoplasmic mRNAs (e.g., ORF 59) but also a nuclear RNA (nut-1) that lacks coding potential. Finally, ORF 57 protein can also modulate the effects of the ORF 50 gene product, a classical transactivator known to be required for lytic induction. The expression from some (e.g., nut-1) but not all (e.g., tk) ORF 50-responsive promoters can be synergistically enhanced by coexpression of ORF 50 and ORF 57. This effect is not due to upregulation of ORF 50 expression but rather to a posttranslational enhancement of the transcriptional activity of ORF 50. These data indicate that ORF 57 is a powerful pleiotropic effector that can act on several posttranscriptional levels to modulate the expression of viral genes in infected cells.

Herpes simplex virus ICP27 induces cytoplasmic accumulation of unspliced polyadenylated alpha-globin pre-mRNA in infected HeLa cells

Transcripts of most intron-bearing cellular genes must be processed by the splicing machinery in order to efficiently accumulate and gain access to the cytoplasm. However, we found that herpes simplex virus induces cytoplasmic accumulation of both spliced and unspliced polyadenylated alpha-globin RNAs in infected HeLa cells. Accumulation of the unspliced RNA required the immediate-early protein ICP27, and ICP27 was sufficient (in combination with ICP4) to produce this effect in a transient-transfection assay. However, expression of ICP27 did not markedly alter the levels of fully spliced alpha-globin transcripts in infected cells. These data demonstrate that the previously documented effects of ICP27 on the cellular splicing apparatus do not greatly inhibit splicing of alpha-globin RNA and argue that ICP27 induces a splicing-independent pathway for alpha-globin RNA accumulation and nuclear export.

Herpesvirus mRNAs are sorted for export via Crm1-dependent and -independent pathways

Cellular pre-mRNA splicing is inhibited by ICP27, a herpes simplex virus regulatory protein, resulting in the shutoff of host protein synthesis. Here we reveal that ICP27 also mediates the export of some virus RNAs via a Crm1-dependent pathway and present evidence that independent domains are required for these functions. Sorting of some viral mRNAs for nuclear export requires Crm1, while other virus mRNAs are exported via another pathway.

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

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

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.

Nuclear export factor CRM1 interacts with nonstructural proteins NS2 from parvovirus minute virus of mice

The nonstructural NS2 proteins of autonomous parvoviruses are known to act in a host cell-dependent manner and to play a role in viral DNA replication, efficient translation of viral mRNA, and/or encapsidation. Their exact function during the parvovirus life cycle remains, however, still obscure. We report here the characterization of the interaction with the NS2 proteins from the parvovirus minute virus of mice (MVM) and rat as well as mouse homologues of the human CRM1 protein, a member of the importin-beta family recently identified as an essential nuclear export factor. Using the two-hybrid system, we could detect the interaction between the carboxy-terminal region of rat CRM1 and each of the three isoforms of NS2 (P [or major], Y [or minor], and L [or rare]). NS2 proteins were further shown to interact with the full-length CRM1 by coimmunoprecipitation experiments using extracts from both mouse and rat cell lines. Our data show that CRM1 preferentially binds to the nonphosphorylated isoforms of NS2. Moreover, we observed that the treatment of MVM-infected cells with leptomycin B, a drug that specifically inhibits the CRM1-dependent nuclear export pathway, leads to a drastic accumulation of NS2 proteins in the nucleus. Both NS2 interaction with CRM1 and nuclear accumulation upon leptomycin B treatment strongly suggest that these nonstructural viral proteins are actively exported out of the nuclei of infected cells via a CRM1-mediated nuclear export pathway.

Antiviral activity of enterocin CRL35 against herpesviruses

Enterocin CRL35 is an antibacterial polypeptide of 3.5 x 10(3) Da produced by Enterococcus faecium CRL35. A series of experiments are described that show the enterocin also had antiviral activity against thymidine-kinase positive (tk+) and deficient (tk-) strains of herpes simplex (HSV) type 1 and 2 in Vero and BHK-21 cells. This activity was observed at 100 microg/ml, 15-fold lower than the cytotoxic concentration. In both cell lines there was a 2 log inhibition of infectivity. The compound inhibited viral multiplication in a dose-dependent manner and had no virucidal effect. Enterocin CRL35 also inhibited the virion-associated host shutoff in infected Vero cells showing that intracellular viral multiplication was affected.

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.

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.

Mta has properties of an RNA export protein and increases cytoplasmic accumulation of Epstein-Barr virus replication gene mRNA

The Epstein-Barr virus (EBV) Zta and Mta regulatory proteins were previously found to be required for efficient replication of oriLyt in cotransfection-replication assays, but the contribution of Mta to the replication process was unknown. We now demonstrate that Mta regulates replication gene expression. Using the polymerase processivity factor BMRF1 as an example, we found that in transfected cells, total BMRF1 mRNA levels were unaffected by Mta but that the amounts of cytoplasmic BMRF1 RNA and protein were greatly increased in the presence of Mta. Mta also increased cytoplasmic accumulation of the BALF2, BALF5, BSLF1, and BBLF4 replication gene mRNAs but did not affect cytoplasmic levels of BBLF2/3 mRNA. Thus, five of the six core replication genes require Mta for efficient accumulation of cytoplasmic RNA. The contribution of Mta to posttranscriptional RNA processing was examined. Examination of Mta localization in transfected cells by indirect immunofluorescence revealed that Mta colocalized with the splicing factor SC35. We also found that Mta has RNA binding activity. Glutathione S-transferase-Mta bound to BMRF1 and BMLF1 transcripts but not to a control cellular gene RNA. Mta contains a consensus leucine-rich nuclear export signal. Such signal sequences are characteristic of proteins that undergo nuclear export. Examination of Mta localization in a heterokaryon assay provided evidence that Mta shuttles between the nucleus and the cytoplasm. Our experiments indicate that Mta functions in RNA processing and transport and mediates cytoplasmic accumulation of a number of EBV early mRNAs.

The Epstein-Barr virus (EBV) SM protein enhances pre-mRNA processing of the EBV DNA polymerase transcript

The Epstein-Barr virus (EBV) DNA polymerase (pol) mRNA, which contains a noncanonical polyadenylation signal, UAUAAA, is cleaved and polyadenylated inefficiently (S. C. S. Key and J. S. Pagano, Virology 234:147-159, 1997). We postulated that the EBV early proteins SM and M, which appear to act posttranscriptionally and are homologs of herpes simplex virus (HSV) ICP27, might compensate for the inefficient processing of pol pre-mRNA. Here we show that the SM and M proteins interact with each other in vitro. In addition, glutathione S-transferase-SM/M fusion proteins precipitate the heterogeneous ribonucleoprotein (hnRNP) C1 splicing protein. Further, the SM protein is coimmunoprecipitated from SM-expressing cell extracts with an antibody to the hnRNP A1/A2 proteins, which are splicing and nuclear shuttling proteins. Finally, the amount of processed EBV DNA polymerase mRNA was increased three- to fourfold in a HeLa cell line expressing SM; this increase was not due to enhanced transcription. Thus, inefficient processing of EBV pol RNA by cellular cleavage and polyadenylation factors appears to be compensated for and may be regulated by the early EBV protein, SM, perhaps via RNA 3'-end formation.

The role of herpes simplex virus ICP27 in the regulation of UL24 gene expression by differential polyadenylation

Herpes simplex virus specifies two sets of transcripts from the UL24 gene, short transcripts (e.g., 1.4 kb), processed at the UL24 poly(A) site, and long transcripts (e.g., 5.6 kb), processed at the UL26 poly(A) site. The 1.4- and 5.6-kb transcripts initiate from the same promoter but are expressed with early and late kinetics, respectively. Measurements of transcript levels following actinomycin D treatment of infected cells revealed that the 1.4- and 5.6-kb UL24 transcripts have similar stabilities, consistent with UL24 transcript kinetics being regulated by differential polyadenylation rather than by differential stabilities. Although the UL24 poly(A) site, which gives rise to short transcripts, is encountered first during processing, long transcripts processed at the UL26 site are equally or more abundant; thus, operationally, the UL24 site is weak. Using a series of viral ICP27 mutants, we investigated whether ICP27, which has been suggested to stimulate the usage of weak poly(A) sites, stimulates 1.4-kb transcript accumulation. We found that accumulation of 1.4-kb transcripts did not require ICP27 during viral infection. Rather, ICP27 was required for full expression of 5.6-kb transcripts, and the decrease in 5. 6-kb transcripts relative to 1.4-kb transcripts was not due solely to reduced DNA synthesis. Our results indicate that temporal expression of UL24 transcripts can be regulated by differential polyadenylation and that although ICP27 is not required for processing at the operationally weak UL24 poly(A) site, it does modulate 5.6-kb transcript levels at a step subsequent to transcriptional initiation.

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.

ICP27 mediates HSV RNA export by shuttling through a leucine-rich nuclear export signal and binding viral intronless RNAs through an RGG motif

Infection of metazoan cells with some viruses alters the balance of cellular mRNA export to favor viral RNA export and to retain cellular transcripts in the nucleus. Here, evidence is presented to show that the herpes simplex virus 1 (HSV-1) essential regulatory protein ICP27, which inhibits host cell-splicing, resulting in the accumulation of unspliced transcripts in the nucleus, mediates RNA export of viral intronless mRNAs. ICP27 was shown to shuttle between the nucleus and cytoplasm through a leucine-rich nuclear export signal, which alone was able to direct the export of the heterologous green fluorescent protein. In vivo UV irradiation studies demonstrated that ICP27 could be crosslinked to poly(A)+ RNA in the nucleus and the cytoplasm, supporting a role in export. Furthermore, the amount of hnRNP A1, which has been implicated in the export of cellular spliced mRNAs, that was bound to poly(A)+ RNA in HSV-1-infected cells was reduced compared with uninfected cells. In addition, it was demonstrated that ICP27 bound seven intronless HSV-1 transcripts in both the nucleus and the cytoplasm, and export of these transcripts was diminished substantially during infection with an ICP27 null mutant virus. In contrast, ICP27 did not bind to two HSV-1 mRNAs that undergo splicing. Finally, binding of ICP27 to RNA in vivo required an arginine-glycine region that resembles an RGG box. These results indicate that ICP27 is an important viral export factor that promotes the transport of HSV-1 intronless RNAs.

The herpes simplex virus immediate-early protein ICP27 shuttles between nucleus and cytoplasm

ICP27 is an essential herpes simplex virus type 1 (HSV-1) nuclear protein which regulates viral early and late genes during infection. The exact mechanism by which ICP27 modulates viral gene expression is unknown, but considerable evidence suggests that it functions posttranscriptionally. In this study, we have asked whether ICP27, like some other viral and cellular posttranscriptional regulatory proteins, shuttles between the nuclear and cytoplasmic compartments of the cell. Using an interspecies heterokaryon assay, we demonstrate that ICP27, but not the HSV-1 nuclear proteins ICP4 or ICP8, is an efficient shuttling protein. ICP27's shuttling ability does not depend on viral infection or other HSV-1 proteins, as it shuttles even when transiently expressed in uninfected cells. To understand the importance of shuttling for ICP27's regulatory functions, we examined several mutant forms of ICP27 to see whether they exhibited altered shuttling. We identified three ICP27 mutations which partially disrupt shuttling, as well as one mutation, M15, which completely abrogates this activity. The M15 mutation alters residues 465 and 466 near the carboxyl terminus of ICP27 and was previously shown to inactivate ICP27's ability to induce certain viral late mRNAs. These results suggest that ICP27's nuclear shuttling activity is involved in its viral late gene activation function.

Shuttling of the herpes simplex virus type 1 regulatory protein ICP27 between the nucleus and cytoplasm mediates the expression of late proteins

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is required posttranscriptionally for the expression of HSV-1 late genes during a productive infection. ICP27 also inhibits host cell pre-mRNA splicing, effectively shutting off host cell protein synthesis. Here we describe intragenic suppressors of LG4, a virus with a conditional lethal mutation in the gene encoding ICP27. At the restrictive temperature, tsICP27 from LG4 fails to inhibit host cell pre-mRNA splicing and to activate the expression of HSV-1 late-gene products. Although the suppressors of LG4 restore virus growth, they still fail to inhibit host cell pre-mRNA splicing. Thus, the role of ICP27 in the synthesis of late proteins is independent of host shutoff. In HSV-1-infected cells, ICP27 shuttles between the nucleus and the cytoplasm. Shuttling of ICP27 occurs only at late times during infection. In transfected cells, ICP27 shuttling was dependent on coexpression of RNA from a late HSV-1 gene. While shuttling does not occur in cells infected with LG4 at 39.5 degrees C, the suppressors of LG4 restore shuttling. Temperature shift experiments correlate the defect in shuttling with the temperature-sensitive phenotype of LG4. These data provide a correlation between shuttling of ICP27 and the expression of HSV-1 late-gene products. We propose that ICP27 regulates late-gene protein synthesis by facilitating the export of late RNAs.