Nuclear export of VP19C is not essential for replication of herpes simplex virus type 1
- You Li†1, 3,
- Dongwei Mao†2,
- Guoda Ma1,
- Lili Cui1,
- Hua Tao5,
- Haihong Zhou5,
- Wandong Liang4,
- Bin Zhao1, 5Email author and
- Keshen Li1Email author
© Li et al.; licensee BioMed Central Ltd. 2014
Received: 19 July 2014
Accepted: 7 September 2014
Published: 19 September 2014
Herpes simplex virus (HSV) type 1 has a 152 kb double-stranded DNA genome that may encode more than 80 gene products, many of which remain uncharacterized. The HSV-1 triplex is a complex of three protein subunits, VP19C and a dimer of VP23 that is essential for capsid assembly. Previous studies have demonstrated that HSV-1 VP19C contains an atypical nuclear localization signal and a functional nuclear export signal (NES), which are both important for the nucleocytoplasmic shuttling of VP19C. However, whether the VP19C NES is required for efficient HSV-1 production is unknown.
In the present study, a VP19C NES-mutated recombinant virus was generated by using bacterial artificial chromosome recombineering technology to investigate the role of VP19C nuclear export in HSV-1 replication. Our results demonstrate that the growth curves, plaque areas, subcellular localization and viral gene expression are indistinguishable between the VP19C NES-mutated virus and the wild-type virus.
Our findings reported herein indicate abrogation of the nuclear export of VP19C did not affect HSV-1 replication and viral gene expression.
KeywordsVP19C Nuclear export signal (NES) Bacterial artificial chromosome (BAC) Replication
Construction of a VP19C NES-mutated BAC with recombineering technology
Primers used for generation of recombinant VP19C-NESm/17-37BAC
Abrogation of VP19C nuclear export did not affect HSV-1 replication
To investigate the effect of the NES mutation of VP19C on the life cycle of HSV-1, 1–2 μg of recombinant pVP19C NESm/17-37BAC DNA was electroporated into Vero cells to reconstitute the VP19C NESm/17-37BAC virus (Figure 2B). Sequence analysis confirmed that the NES mutation in VP19C was not repaired during viral replication (Figure 2C), which indicated that the NES is not required for virus growth in vitro.
To compare the in vitro growth properties of VP19C NESm/17-37BAC virus with those of the parental virus and wild-type HSV-1, single step growth kinetics were determined as described previously . As shown in Figure 2D, the VP19C NESm/17-37BAC virus showed growth kinetics almost identical to that of the parental 17-37BAC virus and wild-type HSV-1 on Vero cells (Figure 2D). Plaque diameters were measured following infection of Vero cells with VP19C NESm/17-37BAC, 17-37BAC or wild-type HSV-1 viruses in 6-well plates. There were no significant differences observed between the plaque diameters of the recombinant virus, the parental virus and the wild-type virus (Figure 2E).
Abrogation of VP19C nuclear export did not affect subcellular localization of VP19C and viral gene expression
Immunofluorescence staining was applied to visualize the subcellular localization of VP19C in wild-type HSV-1 and VP19C NESm/17-37BAC virus-infected cells. As shown in Figure 2F, VP19C displayed a nuclear localization in VP19C NESm/17-37BAC virus-infected cells as it did in wild-type HSV-1-infected cells, indicating that the NES mutation of VP19C did not affect its nuclear localization in infection background.
Western blot was also performed to detect the expression of virion proteins VP19C, VP23 and VP22. As it is shown in Figure 2G and H, the expression of VP19C, VP23 and VP22 was unchanged. These results collectively indicated that abrogation of VP19C nuclear export does not affect subcellular localization of VP19C and viral gene expression.
The HSV-1 capsid is the most morphologically regular component of the virus structure. VP19C is an integral capsid protein that interacts with VP23 to form the triplex, which is the unit that forms the capsid. Capsid assembly of HSV-1 is known to take place in the nuclei of infected cells. It is reported that correct transport of the protein components to the site of capsid assembly is an important function of VP19C . Because of its nuclear localization signal (NLS) and NES, VP19C is able to shuttle between the nucleus and the cytoplasm as part of a chromosome region maintenance 1 (CRM1) dependent pathway as well as through the atypical Ran- and importin β-dependent, but not importin α5, mechanisms . These lines of evidence suggest that VP19C and VP23 may interact in the cytoplasm, and then, VP23 is translocated into the nucleus because of the NLS on VP19C to participate in capsid assembly.
The hydrophobic residues in the NES are important for nuclear export of VP19C, and VP19C is exported from the nucleus through the NES via a CRM1-dependent pathway. However, CRM1-mediated transport is highly subjected to various regulation processes, including the masking of NES, phosphorylation and heterodimerization of the protein and formation of disulfide bonds by an oxidative process . Phosphorylation of residues in the vicinity of the NES and NLS may regulate the intracellular distribution of a protein . For example, several viral proteins, such as the Rabies virus P-protein and chicken anemia virus VP3, regulate their NLS/NES activity via phosphorylation . Additionally, the availability of specific cofactors may also influence this regulation. The mechanism by which subcellular localization is regulated and whether it is regulated over the course of HSV-1 infection will be interesting to determine.
BAC recombineering allows for the rapid and efficient modification of viral genomes in E. coli, thus providing an invaluable tool for the genetic manipulation of herpesvirus genomes and the production of altered forms of viruses to decipher the specific function of a gene in the virus life cycle and pathogenesis . In the present study, we constructed a VP19C NES-mutated recombinant virus using BAC recombineering technology. The recombinant virus with the NES mutation in VP19C did not impair virus replication, and viral gene expression was also unaffected. Our previous study identified a non-classical NLS of VP19C, and the nuclear import of VP19C is required for efficient HSV-1 production . These lines of evidence together suggest that the role of nuclear import and export may function differently in HSV-1 lytic replication. HSV-1 ICP27 was shown to shuttle between the nucleus and cytoplasm through a leucine-rich NES in a CRM1-dependent pathway . However, ICP27 shuttling is not involved in viral DNA replication, but affects the expression of HSV-1 late genes . Li et al. also identified a functional NLS and NES in ORF45 of Kaposi’s sarcoma-associated herpesvirus 8, and demonstrated that the NES is important for ORF45 nucleocytoplasmic shuttling. However, only the NLS-mutated virus had impaired virus production, and the NES-mutated virus did not affect viral replication . Additionally, because VP19C could interact with other cellular proteins (such as nuclear pore complexs) or viral proteins (such as UL25) , it cannot be excluded that VP19C may export from the nucleus through other pathways in the context of infection. Although the NES seems dispensable for HSV-1 lytic replication in the present study, it might be important for certain ancillary functions that could facilitate its interaction with the host.
In conclusion, our study provides evidence that the nuclear export of VP19C is not essential for HSV-1 replication and viral gene expression. The potential role of nucleocytoplasmic shuttling of VP19C played in the HSV-1 viral life-cycle still needs to be further elucidated.
Herpes simplex virus type 1
Nuclear export signal
Nuclear localization signal
Bacterial artificial chromosome
Chromosome region maintenance 1
- E. coli:
Restricted fragment length polymorphisms
This work was supported by funding from the National Nature Science Foundation of China (grant numbers 31171219, 81271213, 81070878, 81271214, 81300929 and 81261120404). We thank Dr. Frazer J. Rixon, Dr. C. Sweet, Dr. David Leib, Dr. Gregory Smith and Dr. Nikolaus Osterrieder for the generous gift of the antibodies against VP19C and VP23, the antibody against VP22, 17-37BAC, E. coli GS1783 and pEPkan-S, respectively.
- Rixon FJ, Addison C, McGregor A, Macnab SJ, Nicholson P, Preston VG, Tatman JD: Multiple interactions control the intracellular localization of the herpes simplex virus type 1 capsid proteins. J Gen Virol. 1996, 77 (Pt 9): 2251-2260.View ArticlePubMedGoogle Scholar
- Zhao L, Zheng C: The first identified nucleocytoplasmic shuttling herpesviral capsid protein: herpes simplex virus type 1 VP19C. PLoS One. 2012, 7: e41825. 10.1371/journal.pone.0041825PubMed CentralView ArticlePubMedGoogle Scholar
- Gierasch WW, Zimmerman DL, Ward SL, Vanheyningen TK, Romine JD, Leib DA: Construction and characterization of bacterial artificial chromosomes containing HSV-1 strains 17 and KOS. J Virol Methods. 2006, 135: 197-206. 10.1016/j.jviromet.2006.03.014View ArticlePubMedGoogle Scholar
- Li Y, Zhao L, Wang S, Xing J, Zheng C: Identification of a novel NLS of herpes simplex virus type 1 (HSV-1) VP19C and its nuclear localization is required for efficient production of HSV-1. J Gen Virol. 2012, 93: 1869-1875. 10.1099/vir.0.042697-0View ArticlePubMedGoogle Scholar
- Adamson WE, McNab D, Preston VG, Rixon FJ: Mutational analysis of the herpes simplex virus triplex protein VP19C. J Virol. 2006, 80: 1537-1548. 10.1128/JVI.80.3.1537-1548.2006PubMed CentralView ArticlePubMedGoogle Scholar
- Kutay U, Guttinger S: Leucine-rich nuclear-export signals: born to be weak. Trends Cell Biol. 2005, 15: 121-124. 10.1016/j.tcb.2005.01.005View ArticlePubMedGoogle Scholar
- Hall RN, Meers J, Fowler E, Mahony T: Back to BAC: The Use of Infectious Clone Technologies for Viral Mutagenesis. Viruses. 2012, 4: 211-235. 10.3390/v4020211PubMed CentralView ArticlePubMedGoogle Scholar
- Lengyel J, Strain AK, Perkins KD, Rice SA: ICP27-dependent resistance of herpes simplex virus type 1 to leptomycin B is associated with enhanced nuclear localization of ICP4 and ICP0. Virology. 2006, 352: 368-379. 10.1016/j.virol.2006.04.044View ArticlePubMedGoogle Scholar
- Rice SA, Lam V: Amino acid substitution mutations in the herpes simplex virus ICP27 protein define an essential gene regulation function. J Virol. 1994, 68: 823-833.PubMed CentralPubMedGoogle Scholar
- Li X, Zhu F: Identification of the nuclear export and adjacent nuclear localization signals for ORF45 of Kaposi's sarcoma-associated herpesvirus. J Virol. 2009, 83: 2531-2539. 10.1128/JVI.02209-08PubMed CentralView ArticlePubMedGoogle Scholar
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