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.

Herpes simplex virus type 1 (HSV-1) protein ICP27 facilitates the export of viral intronless mRNAs. ICP27 shuttles between the nucleus and cytoplasm, which has been shown to require a leucine-rich nuclear export sequence (NES). ICP27 export was reported to be sensitive to the CRM1 inhibitor leptomycin B (LMB) in HSV- In vitro export assays showed that ICP27 export was not sensitive to LMB but was blocked by a dominant-negative TAP deletion mutant lacking the nucleoporin interaction domain. These data suggest that ICP27 uses the TAP pathway to export viral RNAs. Interestingly, the leucine-rich N-terminal sequence was required for efficient export, even though ICP27 export was LMB insensitive. Thus, this region is required for efficient ICP27 export but does not function as a CRM1-dependent NES.Eukaryotic pre-mRNAs are synthesized in the nucleus by RNA polymerase II, after which they are processed by capping at the 5Ј end, cleavage and polyadenylation to form the 3Ј end, and splicing to remove intervening sequences. After processing, mRNAs must be exported through the nuclear pore complex (NPC) to the cytoplasm for translation. Export of mRNAs requires processing, packaging by RNA-binding proteins into ribonucleoprotein (RNP) complexes, recognition by export factors and translocation through the NPC. Three classes of factors appear to be required for mRNA export: adapter proteins that bind directly to the mRNA, receptor proteins that recognize and bind to adapter proteins, and NPC components termed nucleoporins that mediate export across the nuclear membrane (21, 58).Two pathways have been uncovered that appear to be responsible for the export of mRNA. The first export receptor to be identified was CRM1, which recognizes nuclear export sequences (NES) that consist of short hydrophobic stretches that are leucine-rich. This sequence was first revealed in the HIV-1 export protein Rev and in the cellular protein PKI (7,8). Although Rev interacts with CRM1 to export human immunodeficiency virus (HIV) env mRNA, substantial evidence has shown that CRM1 is not a major contributor to mRNA export in metazoans or yeast (6, 35). The human protein TAP and its yeast ortholog Mex67p are the best candidates for mRNA export receptors because they have been shown to shuttle between the nucleus and cytoplasm, cross-link to poly(A) ϩ RNA, localize at the nuclear pores, and interact directly with nucleoporins (1,2,16,17). TAP was first identified as the cellular factor that interacts with the constitutive transport element (CTE) present in RNAs from type D retroviruses and TAP promotes the export of CTE-containing transcripts (2, 11, 16). In yeast, Mex67p is required for poly(A) ϩ RNA export and cellular viability (45, 47). Further, overexpression of TAP in Xenopus oocytes or mammalian cells stimulates mRNA export (3, 14) and inactivation of TAP in Caenorhabditis elegans by RNA interference blocked nuclear export of poly(A) ϩ RNA (56), indicating a direct role in mRNA export.In metazoans, pre-mRNA splicing is required for rapid and ef...

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Infection with some viruses can alter cellular mRNA processing to favor viral gene expression. We present evidence that herpes simplex virus 1 (HSV-1) protein ICP27, which contributes to host shut-off by inhibiting pre-mRNA splicing, interacts with essential splicing factors termed SR proteins and affects their phosphorylation. During HSV-1 infection, phosphorylation of several SR proteins was reduced and this correlated with a subnuclear redistribution. Exogenous SR proteins restored splicing in ICP27-inhibited nuclear extracts and SR proteins isolated from HSV-1-infected cells activated splicing in uninfected S100 extracts, indicating that inhibition occurs by a reversible mechanism. Spliceosome assembly was blocked at the pre-spliceosomal complex A stage. Furthermore, we show that ICP27 interacts with SRPK1 and relocalizes it to the nucleus; moreover, SRPK1 activity was altered in the presence of ICP27 in vitro. We propose that ICP27 modi®es SRPK1 activity resulting in hypophosphorylation of SR proteins impairing their ability to function in spliceosome assembly.

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Herpes simplex virus 1 (HSV-1) ICP27 has been shown to interact with RNA polymerase II (RNAP II) holoenzyme. Here, we show that ICP27 interacts with the C-terminal domain (CTD) of RNAP II and that ICP27 mutants that cannot interact fail to relocalize RNAP II to viral transcription sites, suggesting a role for ICP27 in RNAP II recruitment. Using monoclonal antibodies specific for different phosphorylated forms of the RNAP II CTD, we found that the serine-2 phosphorylated form, which is found predominantly in elongating complexes, was not recruited to viral transcription sites. Further, there was an overall reduction in phosphoserine-2 staining. Western blot analysis revealed that there was a pronounced decrease in the phosphoserine-2 form and in overall RNAP II levels in lysates from cells infected with wild-type HSV-1. There was no appreciable difference in cdk9 levels, suggesting that protein degradation rather than dephosphorylation was occurring. Treatment of infected cells with proteasome inhibitors MG-132 and lactacystin prevented the decrease in the phosphoserine-2 form and in overall RNAP II levels; however, there was a concomitant decrease in the levels of several HSV-1 late proteins and in virus yield. Proteasomal degradation has been shown to resolve stalled RNAP II complexes at sites of DNA damage to allow 3 processing of transcripts. Thus, we propose that at later times of infection when robust transcription and DNA replication are occurring, elongating complexes may collide and proteasomal degradation may be required for resolution.ICP27 is a multifunctional regulatory protein that is required for herpes simplex virus 1 (HSV-1) productive infection. This 63-kDa phosphoprotein is expressed with immediate-early kinetics, and it is required for appropriate expression of viral early and late gene products; in addition, it contributes to the shutoff of host protein synthesis (for review, see reference 54). ICP27 has been demonstrated to function posttranscriptionally at the level of RNA processing and export (4,35,(49)(50)(51), and recently evidence for a role in translation initiation has been presented (15, 16). Further, ICP27 also appears to contribute to the transcriptional regulation of HSV-1 early and late genes (23, 61). Thus, ICP27 appears to function in all stages of viral gene expression from transcription through translation. The mechanisms of some of its effects on gene expression have been elucidated. At early times after infection, ICP27 interacts with several splicing factors (4, 51), including members of a family of essential splicing factors termed SR proteins, and affects their phosphorylation. This results in the blockage of prespliceosomal assembly, which in turn contributes to the shutoff of host protein synthesis because cellular pre-mRNAs cannot be properly processed (51). At later times, beginning at about 6 h after infection, ICP27 begins to shuttle between the nucleus and cytoplasm (6,38,42,49,56). ICP27 binds to viral mRNAs (37,49,55) and facilitates their export to the cytopla...

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is essential for viral replication (56). ICP27 functions principally at the posttranscriptional level, affecting RNA processing and export (37,58,61). Early in infection, ICP27 associates with spliceosomal proteins (45,59,60) and mediates an inhibition of host cell splicing (3,19,34,62). This process contributes to the shutoff of host protein synthesis because cellular pre-mRNAs are incompletely spliced and thus are retained in the nucleus in stalled spliceosomal complexes. ICP27 inhibits host cell splicing by recruiting a primarily cytoplasmic kinase, termed SR protein kinase 1, to the nucleus, where its interaction with ICP27 alters its ability to phosphorylate essential splicing factors, termed SR proteins (62). This process results in stalled splicing complex formation (3,34,62). In metazoans, the nuclear export of mRNAs has been linked to pre-mRNA splicing (36,47,55). The basis of this connection was revealed by the discovery of a protein complex that is deposited on pre-mRNAs undergoing splicing at a specific position upstream of exon junctions (30)(31)(32)49). This exon junction complex (EJC) consists of at least six proteins, which have been shown to function in splicing, RNA export, cytoplasmic localization, mRNA surveillance, and translational efficiency (14,28,30,73). Here we show that these structures to which Aly/REF was redistributed colocalized with ICP4 and thus are sites of HSV-1 transcription. Further, ICP27 mutants that are unable to interact with Aly/REF were unable to recruit Aly/REF to centers of ICP4 staining; instead, Aly/REF remained associated with splicing factor SC35. However, a failure to interact with Aly/REF did not impair the export of ICP27 to the cytoplasm at late times after infection. Further, although it has been suggested that efficient shuttling of ICP27 requires RNA binding (67,68), an ICP27 mutant that lacks the essential RGG box RNA binding domain and thus cannot bind RNA (40, 58) was efficiently exported to the cytoplasm, whereas an ICP27 mutant that has a mutation in a predicted KH domain and that is able to bind RNA was largely retained in the nucleus.To further explore the export requirements for ICP27, we investigated its interaction with TAP/NXF1, the cellular mRNA export receptor. ICP27 was shown to interact

While the majority of metazoan genes and those of the DNA viruses which infect them contain introns which require RNA splicing, herpes simplex virus type 1 (HSV-1) encodes relatively few spliced products. We previously showed that the HSV-1 immediate-early protein ICP27 decreased the levels of spliced target mRNAs in transfections and spliced cellular mRNAs during infection, suggesting that ICP27 may function in impairing host cell splicing. Here, we show that during infections with the wild type, but not in infections with an ICP27 viral mutant termed 27-LacZ, precursor RNA accumulated for a virus transcript which contained introns. Pre-mRNA accumulation in the nucleus was greater than that in the cytoplasm, indicating that splicing rather than transport was affected. Furthermore, splicing of a jI-globin pre-mRNA substrate was inhibited in nuclear extracts from wild-type-infected cells but not in extracts from cells infected with 27-LacZ. The inhibitory activity in extracts from wild-type-infected cells was able to reduce the splicing efficiency of competent extracts in biochemical complementation assays. ICP27 appeared to be responsible for this decrease, because the splicing activity of an extract from cells infected with an ICP27 ts mutant was significantly reduced after incubation of the extract at the permissive temperature to allow renaturation of the conformationally defective ICP27 protein. These results strongly suggest that HSV-1 infection inhibits host cell splicing through the action of ICP27.