Overexpression of Bm65 correlates with reduced susceptibility to inactivation by UV light

Bombyx mori RPL13 participates in UV-induced DNA damage repair of B. mori nucleopolyhedrovirus through interaction with Bm65

Ribosomal protein L13 (RPL13) is highly conserved in evolution. At present, the properties and functions of RPL13 have not been characterised in insects. In this study, Bombyx mori RPL13 (BmRPL13) was first found to be specifically recruited to the sites of ultraviolet (UV)-induced DNA damage and contributed to UV damage repair. Escherichia coli expressing BmRPL13 showed better resistance to UV radiation. After knocking down the expression of BmRPL13 in BmN cells, the repair speed of UV-damaged DNA slowed down. The further results showed that BmRPL13 interacted with B. mori nucleopolyhedrovirus (BmNPV) ORF65 (Bm65) protein to locate at the UV-induced DNA damage sites of BmNPV and helped repair UV-damaged viral DNA.

BmNPV Orf 65 (Bm65) Is Identified as an Endonuclease Directly Facilitating UV-Induced DNA Damage Repair

Baculoviruses have been used as biopesticides for the control of Lepidoptera larvae. However, solar UV radiation reduces the activity of baculovirus. In this study, an UV endonuclease, Bm65, was found encoded in the genome of Bombyx mori nuclear polyhedrosis virus (BmNPV). Bm65 (the ortholog of AcMNPV orf79) was guided by a key nuclear localization signal to enter the nucleus and accumulated at UV-induced DNA damage sites. Subsequent results further showed that Bm65-mediated DNA damage repair was not the only UV damage repair pathway of BmNPV. BmNPV also used host DNA repair proteins to repair UV-induced DNA damage. In summary, these results revealed that Bm65 was very important in UV-induced DNA damage repair of BmNPV, and BmNPV repaired UV-damaged DNA through a variety of ways. IMPORTANCE Baculovirus biopesticides are environmentally friendly insecticides and specifically infect invertebrates. UV radiation from the sunlight greatly reduces the activity of baculovirus biopesticides. However, the molecular mechanisms of most baculoviruses to repair UV-induced DNA damage remain unclear. Nucleotide excision repair (NER) is a major DNA repair pathway that removes UV-induced DNA lesions. At present, there are few reports about the nucleotide excision repair pathway in viruses. Here, we showed for the first time that the baculovirus Bm65 endonuclease actually cleaved UV-damaged DNA. Meanwhile, we found that BmNPV used both viral-encoded enzymes and host DNA damage repair proteins to reverse UV-induced DNA damage. These results will provide a reference for the research of UV damage repair of other viruses.

Bombyx mori Rad23 (BmRad23) contributes to the repair of UV-damaged BmNPV

Baculoviruses have been developed as long-term and environmentally friendly biopesticides. However, solar ultraviolet radiation can reduce the activity of baculovirus. Radiation sensitive 23 (Rad23) can recognize DNA damage and is involved with nucleotide excision repair (NER). In the current study, BmRad23 was accumulated mainly within the nucleus. Host cell reactivation (HCR) assays have shown that BmRad23 significantly facilitated the expression of UV-damaged mCherry reporter gene. Reverse transcription quantitative PCR (RT-qPCR) result showed that the mRNA expression level of BmRad23 was increased in (Bombyx mori nuclear polyhedrovirus, BmNPV) BmNPV-infected BmN cells. However, the expression of BmRad23 was increased significantly when BmNPV budded viruses (BVs) or BmN cells were irradiated with UV light. Overexpression of BmRad23 promoted the mRNA levels of two UV-induced DNA damage repair genes which were from Bombyx mori and BmNPV, respectively. Meanwhile, the overexpression of BmRad23 in BmN cells was conducive to the proliferation of BmNPV and UV-damaged BmNPV. The recombinant BmNPV BVs expressing BmRad23 showed stronger resistance to UV radiation than the control virus. In conclusion, the results revealed that BmRad23 contributed to the proliferation of BmNPV and the repair of UV-damaged BmNPV, which would provide a reference for the development of efficient baculovirus pesticides against UV radiation.

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.

BmNPV infection correlates with the enhancement of the resistance of Bombyx mori cells to UV radiation

At present, the effect of ultraviolet (UV) radiation on the interaction between Bombyx mori nucleopolyhedrovirus (BmNPV) and host remains unclear. In the current study, UV treatment significantly reduced the activity of BmNPV budded viruses (BVs), and UV-damaged BmN cells were not conducive to BmNPV proliferation. BmNPV infection significantly reduced the viability of host cells, but increased the viability of high-dose UV-treated host cells. Furthermore, the quantitative reverse-transcription PCR (qPCR) results suggested that BmNPV and Bombyx mori might mutually use the same DNA repair proteins for repairing UV-induced damage and BmNPV infection promote the ability of host cells to repair UV-induced damage.

The complete genome sequence of a second alphabaculovirus from the true armyworm, Mythimna unipuncta: implications for baculovirus phylogeny and host specificity

The Mythimna unipuncta nucleopolyhedrovirus isolate KY310 (MyunNPV-KY310) is an alphabaculovirus isolated from a true armyworm (Mythimna unipuncta) population in Kentucky, USA. Occlusion bodies of this virus were examined by electron microscopy and the genome sequence was determined by 454 pyrosequencing. MyunNPV-KY310 occlusion bodies consisted of irregular polyhedra measuring 0.8-1.8 µm in diameter and containing multiple virions, with one to six nucleocapsids per virion. The genome sequence was determined to be 156,647 bp with a nucleotide distribution of 43.9% G+C. 152 ORFs and six homologous repeat (hr) regions were annotated for the sequence, including the 38 core genes of family Baculoviridae and an additional group of 26 conserved alphabaculovirus genes. BLAST queries and phylogenetic inference confirmed that MyunNPV-KY310 is most closely related to the alphabaculovirus Leucania separata nucleopolyhedrovirus isolate AH1, which infects Mythimna separata. In contrast, MyunNPV-KY310 did not exhibit a close relationship with Mythimna unipuncta nucleopolyhedrovirus isolate #7, an alphabaculovirus from the same host species. MyunNPV-KY310 lacks the gp64 envelope glycoprotein, which is a characteristic of group II alphabaculoviruses. However, this virus and five other alphabaculoviruses lacking gp64 are placed outside the group I and group II clades in core gene phylogenies, further demonstrating that viruses of genus Alphabaculovirus do not occur in two monophyletic clades. Potential instances of MyunNPV-KY310 ORFs arising by horizontal transfer were detected. Although there are now genome sequences of four different baculoviruses from M. unipuncta, comparison of their genome sequences provides little insight into the genetic basis for their host specificity.

Evidence for the role of BmNPV Bm65 protein in the repair of ultraviolet-induced DNA damage

It is unclear how, or to what extent, baculovirus DNA that has been damaged by ultraviolet (UV) light is repaired during infection and replication. In our previous study, expression of Bombyx mori nucleopolyhedrovirus (BmNPV) ORF Bm65, a homolog of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac79, correlated with decreased inactivation of virus by UV irradiation. In the current study, we accumulated more evidence pointing to a role for Bm65 in repair of UV-induced DNA damage. The localization of Bm65 was studied using enhanced green fluorescent protein (EGFP) fusion constructs expressed in BmN cells transfected with a Bm65 expression plasmid. The results indicate that Bm65-EGFP accumulates in the nucleus. A host cell reactivation assay showed that Bm65 significantly increased the expression of UV-damaged mCherry reporter gene. An assay measuring cyclobutane pyrimidine dimers (CPDs) in UV-irradiated BmN cells found that CPD quantity was decreased in cells transfected with a Bm65 expression plasmid. We also showed that after UVC treatment, the viability of Bm65-transfected cells was higher than that of egfp-transfected cells. These results suggest that Bm65 may be involved in the repair of baculovirus DNA that has been damaged by UV light.

Regulation of BmBDV NS1 by phosphorylation: Impact of mutagenesis at consensus phosphorylation sites on ATPase activity and cytopathic effects

Bombyx mori bidensovirus (BmBDV) is a single-stranded DNA virus belonging to the Bidensovirus genus, Bidnaviridae family. Previous studies showed that parvovirus nonstructural protein 1 (NS1) contains endonuclease, helicase, and ATPase activities and that these activities are regulated by serine/threonine phosphorylation. We have reported that residue Thr-184 site of BmBDV NS1 is phosphorylated, and that residues of Thr-181 and Thr-191 are potentially phosphorylated. However, whether phosphorylation affects BmBDV NS1 activities remains unclear. In this study, the substitution of threonine with Glycine at positions 181, 184 and 191 of BmBDV NS1 reduced its ATPase activity. After wild-type NS1 was treated with calf intestinal alkaline phosphatase (CIP), ATPase activity decreased significantly. Moreover, silkworms that were injected with recombinant viruses carrying these NS1 mutations exhibited significant increases in the median lethal time to death compared with silkworms that were injected with a virus that expressed wild-type NS1. In conclusion, these results showed that the ATPase activity and virulence of BmBDV NS1 are regulated via phosphorylation.