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

Serine-arginine protein kinases and their targets in viral infection and their inhibition

Accumulating evidence has consolidated the interaction between viral infection and host alternative splicing. Serine-arginine (SR) proteins are a class of highly conserved splicing factors critical for the spliceosome maturation, alternative splicing and RNA metabolism. Serine-arginine protein kinases (SRPKs) are important kinases that specifically phosphorylate SR proteins to regulate their distribution and activities in the central pre-mRNA splicing and other cellular processes. In addition to the predominant SR proteins, other cytoplasmic proteins containing a serine-arginine repeat domain, including viral proteins, have been identified as substrates of SRPKs. Viral infection triggers a myriad of cellular events in the host and it is therefore not surprising that viruses explore SRPKs-mediated phosphorylation as an important regulatory node in virus-host interactions. In this review, we briefly summarize the regulation and biological function of SRPKs, highlighting their involvement in the infection process of several viruses, such as viral replication, transcription and capsid assembly. In addition, we review the structure-function relationships of currently available inhibitors of SRPKs and discuss their putative use as antivirals against well-characterized viruses or newly emerging viruses. We also highlight the viral proteins and cellular substrates targeted by SRPKs as potential antiviral therapeutic candidates.

Stress Induced Transcription Factors Transactivate the Herpes Simplex Virus 1 Infected Cell Protein 27 (ICP27) Transcriptional Enhancer

Following acute infection, herpes simplex virus 1 (HSV-1) establishes lifelong latency in neurons, including sensory neurons within trigeminal ganglia. During latency, lytic cycle viral gene expression is silenced. However, stressful stimuli can trigger reactivation from latency. The viral tegument protein, VP-16, transactivates all immediate early (IE) promoters during productive infection. Conversely, cellular factors are expected to trigger viral gene expression during early stages of reactivation from latency and in non-neuronal cells that do not support high levels of productive infection. The glucocorticoid receptor (GR), synthetic corticosteroid dexamethasone, and certain stress-induced transcription factors cooperatively transactivate infected cell protein 0 (ICP0) and ICP4 promoters. Since ICP27 protein expression is required for productive infection, we hypothesized that the ICP27 promoter is transactivated by stress-induced transcription factors. New studies have demonstrated that ICP27 enhancer sequences were transactivated by GR and Krüppel-like factor 15 (KLF15). Mutation of a consensus Sp1 binding site within ICP27 enhancer sequences impaired transactivation by GR and KLF15. Chromatin immunoprecipitation studies have demonstrated that GR and KLF15 occupy ICP27 promoter sequences during productive infection. Cells transfected with an ICP27 enhancer fragment revealed the GR and KLF15 occupancy of ICP27 enhancer sequences required the intact Sp1 binding site. Notably, GR and KLF15 form a feed-forward transcription loop in response to stress, suggesting these cellular factors promote viral replication following stressful stimuli.

Strength in Diversity: Nuclear Export of Viral RNAs

The nuclear export of cellular mRNAs is a complex process that requires the orchestrated participation of many proteins that are recruited during the early steps of mRNA synthesis and processing. This strategy allows the cell to guarantee the conformity of the messengers accessing the cytoplasm and the translation machinery. Most transcripts are exported by the exportin dimer Nuclear RNA export factor 1 (NXF1)-NTF2-related export protein 1 (NXT1) and the transcription-export complex 1 (TREX1). Some mRNAs that do not possess all the common messenger characteristics use either variants of the NXF1-NXT1 pathway or CRM1, a different exportin. Viruses whose mRNAs are synthesized in the nucleus (retroviruses, the vast majority of DNA viruses, and influenza viruses) exploit both these cellular export pathways. Viral mRNAs hijack the cellular export machinery via complex secondary structures recognized by cellular export factors and/or viral adapter proteins. This way, the viral transcripts succeed in escaping the host surveillance system and are efficiently exported for translation, allowing the infectious cycle to proceed. This review gives an overview of the cellular mRNA nuclear export mechanisms and presents detailed insights into the most important strategies that viruses use to export the different forms of their RNAs from the nucleus to the cytoplasm.

Host shutoff activity of VHS and SOX-like proteins: role in viral survival and immune evasion

BACKGROUND

Host shutoff refers to the widespread downregulation of host gene expression and has emerged as a key process that facilitates the reallocation of cellular resources for viral replication and evasion of host antiviral immune responses.

MAIN BODY

The Herpesviridae family uses a number of proteins that are responsible for host shutoff by directly targeting messenger RNA (mRNA), including virion host shutoff (VHS) protein and the immediate-early regulatory protein ICP27 of herpes simplex virus types 1 (HSV-1) and the SOX (shutoff and exonuclease) protein and its homologs in Gammaherpesvirinae subfamilies, although these proteins are not homologous. In this review, we highlight evidence that host shutoff is promoted by the VHS, ICP27 and SOX-like proteins and that they also contribute to immune evasion.

CONCLUSIONS

Further studies regarding the host shutoff proteins will not only contribute to provide new insights into the viral replication, expression and host immune evasion process, but also provide new molecular targets for the development of antiviral drugs and therapies.

Oncogenic Marek's disease herpesvirus encodes an isoform of the conserved regulatory immediate early protein ICP27 generated by alternative promoter usage

Herpesvirus gene expression is temporally regulated, with immediate early (IE), early (E) and late (L) genes. ICP27, which is involved in post-transcriptional regulation, is the only IE gene product conserved in all herpesviruses. We show here that the ICP27 transcript of the oncogenic Marek's disease virus shares the same polyadenylation signal as the bicistronic glycoprotein K-ICP27 transcript and is regulated by alternative promoter usage, with transcription from its own promoter (pICP27) or that of gK (pgK). The pgK can generate a spliced ICP27 transcript yielding an N-terminal-deleted ICP27 isoform (ICP27ΔN) that, like ICP27, co-localizes with the SR protein in infected cells, but with a diffuse nuclear distribution. The pICP27 includes functional responsive elements (REs) for SP1, AP1 and CREB, is essentially active during the lytic phase and leads to exclusive expression of the native form of ICP27. The alternative promoter, pgK, including active REs for GATA, P53 and CREB, preferentially generates the gK transcript during the lytic phase and the spliced ICP27 transcript (ICP27ΔN) during the latent phase. An analysis of the DNA methylation marks of each promoter showed that pgK was systematically demethylated, whereas pICP27 was methylated during latency and demethylated during the lytic stage. Thus, MDV ICP27 gene expression is dependent on alternative promoters, the usage of which is regulated by DNA methylation, which differs between viral stages.

Nuclear localization signals of varicella zoster virus ORF4

The varicella zoster virus (VZV) ORF4 protein, one of immediate-early genes protein, is associated with the tegument in purified virions. ORF4 protein functions at both transcriptional and post-transcriptional levels, present during different phase of whole VZV life cycle. ORF4 protein acts as a nucleocytoplasm shuttle protein, the precise nuclear location signals (NLS) and molecular mechanisms of nucleocytoplasm transport are not elucidated. At this study, we constructed a series of mutants, used fluorescence microscopy and Co-IP analysis to identify an unconventional bipartite NLS ((130)RKHRDRSLSNRRRRP(144)) in VZV ORF4. This study also demonstrates that nuclear import of VZV ORF4 occurs via a Ran-dependent pathway with importin-α5 and importin-β1. Additionally, NLS function of ORF4 is independent from VZV ORF62 protein. ORF62 protein cannot influence the intracellular distribution of ORF4 protein without NLS. So interaction between ORF4 and ORF62 protein is speculated to occur in nucleus. Thus, NLS is indispensable for the post-transcriptional function of ORF4.

Chromatin immunoprecipitation and microarray analysis suggest functional cooperation between Kaposi's Sarcoma-associated herpesvirus ORF57 and K-bZIP

The Kaposi's sarcoma-associated herpesvirus (KSHV) open reading frame 57 (ORF57)-encoded protein (Mta) is a multifunctional regulator of viral gene expression. ORF57 is essential for viral replication, so elucidation of its molecular mechanisms is important for understanding KSHV infection. ORF57 has been implicated in nearly every aspect of viral gene expression, including transcription, RNA stability, splicing, export, and translation. Here we demonstrate that ORF57 interacts with the KSHV K-bZIP protein in vitro and in cell extracts from lytically reactivated infected cells. To further test the biological relevance of the interaction, we performed a chromatin immunoprecipitation and microarray (ChIP-chip) analysis using anti-ORF57 antibodies and a KSHV tiling array. The results revealed four specific areas of enrichment, including the ORF4 and K8 (K-bZIP) promoters, as well as oriLyt, all of which interact with K-bZIP. In addition, ORF57 associated with DNA corresponding to the PAN RNA transcribed region, a known posttranscriptional target of ORF57. All of the peaks were RNase insensitive, demonstrating that ORF57 association with the viral genome is unlikely to be mediated exclusively by an RNA tether. Our data demonstrate that ORF57 associates with the viral genome by using at least two modes of recruitment, and they suggest that ORF57 and K-bZIP coregulate viral gene expression during lytic infection.

Delineation of a core RNA element required for Kaposi's sarcoma-associated herpesvirus ORF57 binding and activity

The Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 protein is an essential multifunctional regulator of gene expression. ORF57 interaction with RNA is necessary for ORF57-mediated posttranscriptional functions, but little is known about the RNA elements that drive ORF57-RNA specificity. Here, we investigate the cis-acting factors on the KSHV PAN RNA that dictate ORF57 binding and activity. We show that ORF57 binds directly to the 5' end of PAN RNA in KSHV-infected cells. Furthermore, we employ in vitro and cell-based assays to define a 30-nucleotide (nt) core ORF57-responsive element (ORE) that is necessary and sufficient for ORF57 binding and activity. Mutational analysis of the core ORE further suggests that a 9-nt sequence is a specific binding site for ORF57. These studies provide insight into ORF57 specificity determinants and lay a foundation for future analyses of cellular and viral ORF57 targets.

Identification of nuclear and nucleolar localization signals of pseudorabies virus (PRV) early protein UL54 reveals that its nuclear targeting is required for efficient production of PRV

The pseudorabies virus (PRV) early protein UL54 is a homologue of herpes simplex virus 1 (HSV-1) immediate-early protein ICP27, which is a multifunctional protein that is essential for HSV-1 infection. In this study, the subcellular localization and nuclear import signals of PRV UL54 were characterized. UL54 was shown to predominantly localize to the nucleolus in transfected cells. By constructing a series of mutants, a functional nuclear localization signal (NLS) and a genuine nucleolar localization signal (NoLS) of UL54 were for the first time identified and mapped to amino acids (61)RQRRR(65) and (45)RRRRGGRGGRAAR(57), respectively. Additionally, three recombinant viruses with mutations of the NLS and/or the NoLS in UL54 were constructed based on PRV bacterial artificial chromosome (BAC) pBecker2 to test the effect of UL54 nuclear targeting on viral replication. In comparison with the wild-type virus, a recombinant virus harboring an NLS or NoLS mutation of UL54 reduced viral production to different extents. However, mutations of both the NLS and NoLS targeted UL54 to the cytoplasm in recombinant virus-infected cells and significantly impaired viral replication, comparable to the UL54-null virus. In addition, a virus lacking the NLS or the NoLS displayed modest defects in viral gene expression and DNA synthesis. However, deletion of both the NLS and the NoLS resulted in severe defects in viral gene expression and DNA synthesis, as well as production of infectious progeny. Thus, we have identified a classical NLS and a genuine NoLS in UL54 and demonstrate that the nuclear targeting of UL54 is required for efficient production of PRV.

ICP27 protein of Marek's disease virus interacts with SR proteins and inhibits the splicing of cellular telomerase chTERT and viral vIL8 transcripts

All herpesviruses have a post-transcriptional regulatory protein that prevents precursor mRNA splicing and leads to the shutting off of host protein synthesis. The ICP27 protein of herpes simplex virus 1 (HSV-1) is the prototype of these proteins. Marek's disease virus (MDV-1), an alphaherpesvirus that induces lymphoma in birds, also has an ICP27 protein that is produced in lytic MDV-1-infected cells. We characterized this protein. We demonstrated ICP27 production in latently infected MSB-1 cells, but only on MDV-1 reactivation. ICP27 was found predominantly in specific structures within the nucleus. The ICP27 of MDV-1 colocalized and interacted with SR proteins. We demonstrated inhibitory effects of MDV-1 ICP27 on the splicing of both the viral vIL8 and cellular chTERT (telomerase reverse transcriptase) genes. Thus, the ICP27 of MDV-1 plays a similar role to the ICP27 of HSV-1 and may be involved in MDV-1 replication and the development of Marek's disease.