Mapping of functional regions conferring nuclear localization and RNA-binding activity of pseudorabies virus early protein UL54

Establishment of an In Vitro Model of Pseudorabies Virus Latency and Reactivation and Identification of Key Viral Latency-Associated Genes

Alphaherpesviruses infect humans and most animals. They can cause severe morbidity and mortality. The pseudorabies virus (PRV) is a neurotropic alphaherpesvirus that can infect most mammals. The PRV persists in the host by establishing a latent infection, and stressful stimuli can induce the latent viruses to reactivate and cause recurrent diseases. The current strategies of antiviral drug therapy and vaccine immunization are ineffective in eliminating these viruses from the infected host. Moreover, overspecialized and complex models are also a major obstacle to the elucidation of the mechanisms involved in the latency and reactivation of the PRV. Here, we present a streamlined model of the latent infection and reactivation of the PRV. A latent infection established in N2a cells infected with the PRV at a low multiplicity of infection (MOI) and maintained at 42 °C. The latent PRV was reactivated when the infected cells were transferred to 37 °C for 12 to 72 h. When the above process was repeated with a UL54-deleted PRV mutant, it was observed that the UL54 deletion did not affect viral latency. However, viral reactivation was limited and delayed. This study establishes a powerful and streamlined model to simulate PRV latency and reveals the potential role of temperature in PRV reactivation and disease. Meanwhile, the key role of the early gene UL54 in the latency and reactivation of PRV was initially elucidated.

Evaluation of the impact of ul54 gene-deletion on the global transcription and DNA replication of pseudorabies virus

Pseudorabies virus (PRV) is an animal alphaherpesvirus with a wide host range. PRV has 67 protein-coding genes and several non-coding RNA molecules, which can be classified into three temporal groups, immediate early, early and late classes. The ul54 gene of PRV and its homolog icp27 of herpes simplex virus have a multitude of functions, including the regulation of viral DNA synthesis and the control of the gene expression. Therefore, abrogation of PRV ul54 function was expected to exert a significant effect on the global transcriptome and on DNA replication. Real-time PCR and real-time RT-PCR platforms were used to investigate these presumed effects. Our analyses revealed a drastic impact of the ul54 mutation on the genome-wide expression of PRV genes, especially on the transcription of the true late genes. A more than two hour delay was observed in the onset of DNA replication, and the amount of synthesized DNA molecules was significantly decreased in comparison to the wild-type virus. Furthermore, in this work, we were able to successfully demonstrate the utility of long-read SMRT sequencing for genotyping of mutant viruses.

Duck enteritis virus UL54 is an IE protein primarily located in the nucleus

Background

The UL54 protein of Duck Enteritis Virus (DEV) is a homolog of herpes simplex virus-1 (HSV-1) immediate-early infectious cell protein 27 (ICP27), a multifunctional protein essential for viral infection. Nonetheless, there is little information on the UL54 protein of DEV.

Methods

The UL54 gene was cloned into the pPAL7 vector, and the recombinant protein, expressed in the E. coli Rosetta, was used to produce a specific antibody. Using this antibody, Western blotting and indirect immunofluorescence analysis (IFA) were used to analyze the expression level and intracellular localization, respectively, of UL54 in DEV-infected cells at different times. Real-time quantitative reverse transcription PCR (RT-PCR) and the pharmacological inhibition test were utilized to ascertain the kinetic class of the UL54 gene.

Results

UL54 was expressed as a fusion protein of approximately 66.0 kDa using the prokaryotic expression system, and this protein was used to generate the specific anti-UL54 antibody. The UL54 protein was initially diffusely distributed throughout the cytoplasmic region; then, after 2 h, it gradually distributed into the nucleus, peaking at 24 h, and complete localization to the nucleus was observed thereafter. The UL54 transcript was detected as early as 0.5 h, and peak expression was observed at 24 h. The UL54 gene was insensitive to the DNA polymerase inhibitor Ganciclovir (GCV) and the protein synthesis inhibitor Cycloheximide (CHX), both of which confirmed that UL54 was an immediate early gene.

Conclusions

The DEV UL54 gene was expressed in a prokaryotic expression system and characterized for expression level, intracellular localization and gene kinetic class. We propose that these results will provide the foundation for further functional analyses of this gene.

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.

Characterization of molecular determinants for nucleocytoplasmic shuttling of PRV UL54

The pseudorabies virus (PRV) early protein UL54 is a homologue of the herpes simplex virus 1 (HSV-1) immediate-early protein ICP27, which is a multifunctional protein and essential for HSV-1 infection. To determine if UL54 might shuttle between the nucleus and cytoplasm, as has been shown for its homologues in human herpesviruses, the molecular determinants for its nucleocytoplasmic shuttling were investigated. Heterokaryon assays demonstrated that UL54 was a nucleocytoplasmic shuttling protein and this property could not be blocked by leptomycin B, an inhibitor of chromosome region maintenance 1 (CRM1). However, TAP/NXF1 promoted the nuclear export of UL54 and interacted with UL54, suggesting that UL54 shuttles between the nucleus and the cytoplasm via a TAP/NXF1, but not CRM1, dependent nuclear export pathway. Furthermore, UL54 was demonstrated to target to the nucleus through a classic Ran-, importin β1- and α5-dependent nuclear import mechanism.

UL54-null pseudorabies virus is attenuated in mice but productively infects cells in culture

The pseudorabies virus (PRV) UL54 homologs are important multifunctional proteins with roles in shutoff of host protein synthesis, transactivation of virus and cellular genes, and regulation of splicing and translation. Here we describe the first genetic characterization of UL54. We constructed UL54 null mutations in a PRV bacterial artificial chromosome using sugar suicide and lambdaRed allele exchange systems. Surprisingly, UL54 is dispensable for growth in tissue culture but exhibits a small-plaque phenotype that can be complemented in trans by both the herpes simplex virus type 1 ICP27 and varicella-zoster virus open reading frame 4 proteins. Deletion of UL54 in the virus vJSdelta54 had no effect on the ability of the virus to shut off host cell protein synthesis but did affect virus gene expression. The glycoprotein gC accumulated to lower levels in cells infected with vJSdelta54 compared to those infected with wild-type virus, while gK levels were undetectable. Other late gene products, gB, gE, and Us9, accumulated to higher levels than those seen in cells infected with wild-type virus in a multiplicity-dependent manner. DNA replication is also reduced in cells infected with vJSdelta54. UL54 appears to regulate UL53 and UL52 at the transcriptional level as their respective RNAs are decreased in cells infected with vJSdelta54. Interestingly, vJSdelta54 is highly attenuated in a mouse model of PRV infection. Animals infected with vJSdelta54 survive twice as long as animals infected with wild-type virus, and this results in delayed accumulation of virus-specific antigens in skin, dorsal root ganglia, and spinal cord tissues.