Characterization of a baculovirus nuclear localization signal domain in the late expression factor 3 protein

A CRM1-dependent nuclear export signal in Autographa californica multiple nucleopolyhedrovirus Ac93 is important for the formation of intranuclear microvesicles

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) Ac93 is highly conserved in all sequenced baculovirus genomes, and it plays important roles in both the nuclear egress of nucleocapsids and the formation of intranuclear microvesicles. In this study, we characterized a cellular CRM1-dependent nuclear export signal (NES) of AcMNPV Ac93. Bioinformatic analysis revealed that AcMNPV Ac93 may contain an NES at amino acids 115-125. Green fluorescent protein (GFP) fused to the NES (GFP:NES) of AcMNPV Ac93 is localized to the cytoplasm of transfected cells. Multiple point mutation analysis demonstrated that NES is important for the nuclear export of GFP:NES. Bimolecular fluorescence complementation experiments and co-immunoprecipitation assays confirmed that Ac93 interacts with Spodoptera frugiperda CRM1 (SfCRM1). However, AcMNPV Ac34 inhibits cellular CRM1-dependent nuclear export of GFP:NES. To determine whether the NES in AcMNPV Ac93 is important for the formation of intranuclear microvesicles, an ac93-null AcMNPV bacmid was constructed; the wild-type and NES-mutated Ac93 were reinserted into the ac93-null AcMNPV bacmid. Immunofluorescence analysis showed that Ac93 and SfCRM1 were predominantly colocalized at intranuclear microvesicles in infected cells, while the construct containing point mutations at residues 123 and 125 of Ac93 resulted in a defect in budded virus production and the abolishment of intranuclear microvesicles. Together, these data demonstrate that Ac93 contains a functional NES, which is required for the production of progeny viruses and the formation of intranuclear microvesicles.IMPORTANCEAutographa californica multiple nucleopolyhedrovirus (AcMNPV) Ac93 is important for the formation of intranuclear microvesicles. However, how the baculovirus manipulates Ac93 for the formation of intranuclear microvesicles is unclear. In this study, we identified a nuclear export signal (NES) at amino acids 115-125 of AcMNPV Ac93. Our results showed that the NES is required for the interaction between Ac93 and Spodoptera frugiperda CRM1 (SfCRM1). However, AcMNPV Ac34 inhibits the nuclear export of green fluorescent protein fused to the NES. Our analysis revealed that Ac93 and SfCRM1 were predominantly colocalized at intranuclear microvesicles in AcMNPV-infected cells. Together, our results indicate that Ac93 participates in the formation of intranuclear microvesicles via the Ac93 NES-mediated CRM1 pathway.

Role of the Bombyx mori nucleopolyhedrovirus LEF3 acetylation on viral replication

Late expression factor 3 (LEF3) is a single-stranded DNA binding protein of Bombyx mori nucleopolyhedrovirus (BmNPV) with multiple functions. It is an essential factor for viral DNA replication and plays an important regulatory role during BmNPV infection. Our recent quantitative analysis of protein acetylome revealed for the first time that LEF3 can be acetylated at four lysine residues during the viral infection, but the underlying mechanism is unknown. Among the modification sites, two of them (K18 and K27) are located in the conserved nuclear localization sequence region. The acetylation level for K18 especially was up-regulated approximately 7.4 times after 36 h of post-infection. To understand the regulatory function of this modification, site-direct mutagenesis for acetylated mimic (K18Q) or deacetylated mimic (K18R) mutants was performed on LEF3. The fluorescence analysis results showed that the replication capacity of the virus was significantly reduced after K18 acetylation. Meanwhile, co-localization analysis revealed that acetylation at K18 caused LEF3 to lose its nuclear targeting ability and affected the interaction between LEF3 and P143, retaining P143 in the cytoplasm. And further Yeast two-hybrid analysis results also confirmed that the acetylation at K18 did affect the interaction between LEF3 and P143. In conclusion, the acetylation of LEF3 at K18 might act as one of the antiviral strategies for silkworm host by affecting nuclear localization of LEF3, interaction with P143, and then blocking viral replication.

Autographa Californica Multiple Nucleopolyhedrovirus orf13 Is Required for Efficient Nuclear Egress of Nucleocapsids

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf13 (ac13) is a conserved gene in all sequenced alphabaculoviruses. However, its function in the viral life cycle remains unknown. In this study, we found that ac13 was a late gene and that the encoded protein, bearing a putative nuclear localization signal motif, colocalized with the nuclear lamina. Deletion of ac13 did not affect viral genome replication, nucleocapsid assembly or occlusion body (OB) formation, but reduced virion budding from infected cells by approximately 400-fold compared with the wild-type virus. Deletion of ac13 substantially impaired the egress of nucleocapsids from the nucleus to the cytoplasm, while the OB morphogenesis was unaffected. Taken together, our results indicated that ac13 was required for efficient nuclear egress of nucleocapsids during virion budding, but was dispensable for OB formation.

The Motif of 76KRKCSK in Bm65 Is an Efficient Nuclear Localization Signal Involved in Production of Infectious Virions

orf65 (Bm65) of Bombyx mori nucleopolyhedrovirus (BmNPV) codes for a putative 104-amino-acid protein containing three cysteine residues with a putative molecular mass of 12.2 kDa. Previous studies have showed that Bm65 accumulates mainly in nucleus and involved in the repair of UV-damaged DNA. However, the mechanism of nuclear import of Bm65 remains unclear. In this study, a SDS-stable Bm65 tetramer was found in BmNPV-infected BmN cells, and alanine substitutions for the three cysteine residues did not affect the formation of Bm65 tetramer. Additionally, a basic amino acid cluster of the Bm65 protein was identified as an efficient nuclear localization signal (NLS). Firstly, transient expression of GFP-fused truncated Bm65 variants revealed that the ⁷⁶KRKCSK motif functions as the NLS. This was also confirmed by alanine substitution in the ⁷⁶KRKCSK motif, which caused attenuated nuclear localization of Bm65. Next, the ⁷⁶KRKCSK motif-mutated bacmid was generated and the ⁷⁶KRKCSK motif was also found to be important for nuclear localization of Bm65 in BmNPV-infected conditions. Lastly, analyses of flag-tagged Bm65 expressing bacmids revealed that the mutations in ⁷⁶KRKCSK motif did not affect the synthesis of Bm65 tetramer, but severely impaired production levels of infectious virions. In conclusion, Bm65 exists in mainly a tetrameric form in virus-infected cells, which may be involved with production levels of infectious virions.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

Identification of a novel bipartite nuclear localization signal in the DNA polymerase of the betabaculovirus Pieris rapae granulovirus

DNA polymerase (DNApol) is highly conserved in all baculoviruses and plays an essential role in viral DNA replication. Previous results showed that the DNApol of the betabaculovirus Pieris rapae granulovirus (PiraGV) can localize in the nucleus. However, it is not clear how the DNApol is transported into the nucleus. Bioinformatic and GFP localization analysis showed that PiraGV DNApol contains a nuclear localization signal (NLS) at aa 4-25 (LFKRKLDEPPTDHTLVKAIKLS) of the N-terminus that does not match either the classical monopartite or the bipartite NLS consensus sequence. Multiple-point-substitution analysis confirmed that the NLS is required for transport of PiraGV DNApol into the nucleus. We also substituted the NLS of the PiraGV DNApol for that of the alphabaculovirus Spodoptera litura nuclear polyhedrosis virus (SpltNPV) DNApol. A viral growth curve and quantitative real-time PCR revealed that the substitution impaired viral DNA replication and resulted in a reduction in virus production. Together, our results show that PiraGV contains a novel NLS and that the NLS cannot efficiently replace that of SpltNPV DNApol for viral DNA synthesis and virus production.

The 91-205 amino acid region of AcMNPV ORF34 (Ac34), which comprises a potential C3H zinc finger, is required for its nuclear localization and optimal virus multiplication

During baculovirus infection, most viral proteins must be imported to the nucleus to support virus multiplication. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf34 (ac34) is an alphabaculovirus unique gene that is required for optimal virus production. Ac34 distributes in both the cytoplasm and the nuclei of virus-infected Sf9 cells, but contains no conventional nuclear localization signal (NLS). In this study, we investigated the nuclear targeting domains in Ac34. Transient expression assays showed that Ac34 localized in both the cytoplasm and the nuclei of Sf9 cells, indicating that no viral protein is required for Ac34 nuclear localization. Subcellular localization analysis of Ac34 truncations and internal deletions fused with green fluorescent protein in plasmid-transfected Sf9 cells identified that the 91-205 amino acid (aa) region is required for Ac34 nuclear localization. Mutations in a potential C3H zinc finger (aa 116-131) in Ac34 resulted in exclusive cytoplasmic distribution of GFP:Ac34, suggesting that the zinc finger is required for Ac34 nuclear localization. To assess the functional importance of Ac34 in the nucleus during virus replication, recombinant AcMNPV bacmids containing a series of Ac34 truncations, internal deletions, or site mutations fused with HA tags were constructed. Subcellular localization analysis showed that Ac34 with internal deletions in aa 91-205 or site mutations in the potential zinc finger was predominantly distributed in the cytoplasm. Viral plaque assays and virus growth curves indicated that disruption of Ac34 nuclear localization significantly impaired virus replication. Taken together, our findings demonstrated that the nuclear localization of Ac34 requires the 91-205 aa region and its nuclear localization is essential for optimal virus replication.

Rescue of dnapol-null Autographa californica multiple nucleopolyhedrovirus with DNA polymerase (DNApol) of Spodoptera litura nucleopolyhedrovirus (SpltNPV) and identification of a nuclear localization signal in SpltNPV DNApol

DNA polymerase (DNApol) is highly conserved in all baculoviruses and plays an essential role in viral DNA replication. It determines the fidelity of baculovirus DNA replication by inserting the correct nucleotides into the primer terminus and proofreading any mispaired nucleotides. DNApols of groups I and II of the genus Alphabaculovirus in the family Baculoviridae share many common structural features. However, it is not clear whether a group I Autographa californica multiple nucleopolyhedrovirus (AcMNPV) DNApol can be substituted by a group II NPV DNApol. Here we report the successful generation of AcMNPV dnapol-null virus being rescued by a group II Spodoptera litura NPV (SpltNPV) dnapol (Bac-AcΔPol : Slpol). Viral growth curves and quantitative real-time PCR showed that the dnapol replacement reduced the level of viral production and DNA replication of Bac-AcΔPol : SlPol compared with WTrep, a native dnapol insertion in an AcMNPV dnapol-null virus. Light microscopy showed that production of occlusion bodies for Bac-AcΔPol : Slpol was reduced. We also identified a nuclear localization signal (NLS) for the SpltNPV DNApol C terminus at residues 827-838 by mutational analysis and confocal microscopy. Multiple point substitution of SpltNPV DNApol NLS abrogated virus production and viral DNA replication. Overall, these data suggested that the NLS plays an important role in SpltNPV DNApol nuclear localization and that SpltNPV DNApol cannot efficiently substitute the AcMNPV DNApol in AcMNPV.

Iflavirus increases its infectivity and physical stability in association with baculovirus

Virus transmission and the prevalence of infection depend on multiple factors, including the interaction with other viral pathogens infecting the same host. In this study, active replication of an iflavirus, Spodoptera exigua iflavirus 1 (order Picornavirales) was observed in the offspring of insects that survived following inoculation with a pathogenic baculovirus, Spodoptera exigua multiple nucleopolyhedrovirus. Tracking the origin of the iflavirus suggested the association of this virus with the occlusion bodies of the baculovirus. Here we investigated the effect of this association on the stability and infectivity of both viruses. A reduction in baculovirus pathogenicity, without affecting its infectivity and productivity, was observed when associated with the iflavirus. In contrast, viral association increased the infectivity of the iflavirus and its resistance to ultraviolet radiation and high temperature, two of the main factors affecting virus stability in the field. In addition, electron microscopy analysis revealed the presence of particles resembling iflavirus virions inside the occlusion bodies of the baculovirus, suggesting the possible co-occlusion of both viruses. Results reported here are indicative of facultative phoresis of a virus and suggest that virus–virus interactions may be more common than currently recognized, and may be influential in the ecology of baculovirus and host populations and in consequence in the use of baculoviruses as biological insecticides.

Nuclear Translocation Sequence and Region in Autographa californica Multiple Nucleopolyhedrovirus ME53 That Are Important for Optimal Baculovirus Production

Autographa californicamultiple nucleopolyhedrovirus (AcMNPV) is in the familyBaculoviridae, genusAlphabaculovirus AcMNPVme53is a highly conserved immediate early gene in all lepidopteran baculoviruses that have been sequenced and is transcribed up to late times postinfection. Althoughme53is not essential for viral DNA synthesis, infectious budded virus (BV) production is greatly attenuated when it is deleted. ME53 associates with the nucleocapsid on both budded virus and occlusion-derived virus, but not with the virus envelope. ME53 colocalizes in plasma membrane foci with the envelope glycoprotein GP64 in a GP64-dependent manner. ME53 localizes in the cytoplasm early postinfection, and despite the lack of a reported nuclear localization signal (NLS), ME53 translocates to the nucleus at late times postinfection. To map determinants of ME53 that facilitate its nuclear translocation, recombinant AcMNPV bacmids containing a series of ME53 truncations, internal deletions, and peptides fused with hemagglutinin (HA) or green fluorescent protein (GFP) tags were constructed. Intracellular-localization studies identified residues within amino acids 109 to 137 at the N terminus of ME53 that acted as the nuclear translocation sequence (NTS), facilitating its nuclear transport at late times postinfection. The first 100 N-terminal amino acids and the last 50 C-terminal amino acids of ME53 are dispensable for high levels of budded virus production. The region within amino acids 101 to 398, which also contains the NTS, is critical for optimal levels of budded virus production.

IMPORTANCE

Baculovirusme53is a conserved immediate early gene found in all sequenced lepidopteran alpha- and betabaculoviruses. We first identified residues within amino acids 109 to 137 at the N terminus that act as the ME53 nuclear translocation sequence (NTS) to facilitate its nuclear translocation and defined an internal region within amino acids 101 to 398, which includes the NTS, as being necessary for optimal budded virus production. Altogether, these results indicate a previously unidentified nuclear role that ME53 plays in virus replication.

Autographa californica multiple nucleopolyhedrovirus DNA polymerase C terminus is required for nuclear localization and viral DNA replication

The DNA polymerase (DNApol) of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is essential for viral DNA replication. The DNApol exonuclease and polymerase domains are highly conserved and are considered functional in DNA replication. However, the role of the DNApol C terminus has not yet been characterized. To identify whether only the exonuclease and polymerase domains are sufficient for viral DNA replication, several DNApol C-terminal truncations were cloned into a dnapol-null AcMNPV bacmid with a green fluorescent protein (GFP) reporter. Surprisingly, most of the truncation constructs, despite containing both exonuclease and polymerase domains, could not rescue viral DNA replication and viral production in bacmid-transfected Sf21 cells. Moreover, GFP fusions of these same truncations failed to localize to the nucleus. Truncation of the C-terminal amino acids 950 to 984 showed nuclear localization but allowed for only limited and delayed viral spread. The C terminus contains a typical bipartite nuclear localization signal (NLS) motif at residues 804 to 827 and a monopartite NLS motif at residues 939 to 948. Each NLS, as a GFP fusion peptide, localized to the nucleus, but both NLSs were required for nuclear localization of DNApol. Alanine substitutions in a highly conserved baculovirus DNApol sequence at AcMNPV DNApol amino acids 972 to 981 demonstrated its importance for virus production and DNA replication. Collectively, the data indicated that the C terminus of AcMNPV DNApol contains two NLSs and a conserved motif, all of which are required for nuclear localization of DNApol, viral DNA synthesis, and virus production.

IMPORTANCE

The baculovirus DNA polymerase (DNApol) is a highly specific polymerase that allows viral DNA synthesis and hence virus replication in infected insect cells. We demonstrated that the exonuclease and polymerase domains of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) alone are insufficient for viral DNA synthesis and virus replication. Rather, we identified three features, including two nuclear localization signals and a highly conserved 10-amino-acid sequence in the AcMNPV DNApol C terminus, all three of which are important for both nuclear localization of DNApol and for DNApol activity, as measured by viral DNA synthesis and virus replication.

Characterization of protein-protein interaction domains within the baculovirus Autographa californica multiple nucleopolyhedrovirus late expression factor LEF-3

Autographa californica nucleopolyhedrovirus late expression factor 3 (LEF-3) is required for late viral gene expression probably through its numerous functions related to DNA replication, including nuclear localization of the virus helicase P143 and binding to ssDNA. LEF-3 appears to interact with itself as a homo-oligomer, although the details of this oligomeric structure are not yet known. To examine LEF-3-LEF-3 interactions, a bimolecular fluorescent protein complementation assay was used. Pairs of recombinant plasmids expressing full-length LEF-3 fused to one of two complementary fragments (V1 or V2) of a variant of yellow fluorescent protein named 'Venus' were constructed. Plasmids expressing fusions with complementary fragments of Venus were co-transfected into Sf21 cells and analysed by fluorescence microscopy. Co-transfected plasmids expressing full-length V1-LEF-3 and V2-LEF-3 showed positive fluorescence, confirming the formation of homo-oligomers. A series of truncated V1/V2-LEF-3 fusions was constructed and used to investigate interactions with one another as well as with full-length LEF-3.

Baculovirus nuclear import: open, nuclear pore complex (NPC) sesame

Baculoviruses are one of the largest viruses that replicate in the nucleus of their host cells. During infection, the rod-shape, 250-nm long nucleocapsid delivers its genome into the nucleus. Electron microscopy evidence suggests that baculoviruses, specifically the Alphabaculoviruses (nucleopolyhedroviruses) and the Betabaculoviruses (granuloviruses), have evolved two very distinct modes for doing this. Here we review historical and current experimental results of baculovirus nuclear import studies, with an emphasis on electron microscopy studies employing the prototypical baculovirus Autographa californica multiple nucleopolyhedrovirus infecting cultured cells. We also discuss the implications of recent studies towards theories of nuclear transport mechanisms.

Identification of a domain of the baculovirus Autographa californica multiple nucleopolyhedrovirus single-strand DNA-binding protein LEF-3 essential for viral DNA replication

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) lef-3 is one of nine genes required for viral DNA replication in transient assays. LEF-3 is predicted to contain several domains related to its functions, including nuclear localization, single-strand DNA binding, oligomerization, interaction with P143 helicase, and interaction with a viral alkaline nuclease. To investigate the essential nature of LEF-3 and the roles it may play during baculovirus DNA replication, a lef-3 null bacmid (bKO-lef3) was constructed in Escherichia coli and characterized in Sf21 cells. The results showed that AcMNPV lef-3 is essential for DNA replication, budded virus production, and late gene expression in vivo. Cells transfected with the lef-3 knockout bacmid produced low levels of early proteins (P143, DNA polymerase, and early GP64) and no late proteins (P47, VP39, or late GP64). To investigate the functional role of domains within the LEF-3 open reading frame in the presence of the whole viral genome, plasmids expressing various LEF-3 truncations were transfected into Sf21 cells together with bKO-lef3 DNA. The results showed that expression of AcMNPV LEF-3 amino acids 1 to 125 was sufficient to stimulate viral DNA replication and to support late gene expression. Expression of Choristoneura fumiferana MNPV lef-3 did not rescue any LEF-3 functions. The construction of a LEF-3 amino acid 1 to 125 rescue bacmid revealed that this region of LEF-3, when expressed in the presence of the rest of the viral genome, stimulated viral DNA replication and late and very late protein expression, as well as budded virus production.

Autographa californica multiple nucleopolyhedrovirus LEF-2 is a capsid protein required for amplification but not initiation of viral DNA replication

The late expression factor 2 gene (lef-2) of baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been identified as one of the factors essential for origin-dependent DNA replication in transient expression assays and has been shown to be involved in late/very late gene expression. To study the function of lef-2 in the life cycle of AcMNPV, lef-2 knockout and repair bacmids were generated by homologous recombination in Escherichia coli. Growth curve analysis showed that lef-2 was essential for virus production. Interestingly, a DNA replication assay indicated that lef-2 is not required for the initiation of viral DNA replication and that, rather, it is required for the amplification of DNA replication. lef-2 is also required for the expression of late and very late genes, as the expression of these genes was abolished by lef-2 deletion. Temporal and spatial distributions of LEF-2 protein in infected cells were also analyzed, and the data showed that LEF-2 protein was localized to the virogenic stroma in the nuclei of the infected cells. Analysis of purified virus particles revealed that LEF-2 is a viral protein component of both budded and occlusion-derived virions, predominantly in the nucleocapsids of the virus particles. This observation suggests that LEF-2 may be required immediately after virus entry into host cells for efficient viral DNA replication.