Bombyx mori is one of the key lepidopteran model species, and is economically important for silk production and proteinaceous drug expression. Baculovirus and insect host are important natural biological models for studying host-pathogen interactions. The impact of Bombyx mori nucleopolyhedrovirus (
Hematological and neurological expressed 1-like (HN1L) protein is an evolutionarily conserved protein that plays an important role in embryonic development. It has been reported that HN1L is involved in the process of cell growth and cancer formation and that cell cycle arrest occurs during suppression of HN1L expression. Previous studies have demonstrated that the expression levels of the Bombyx mori HN1L protein were significantly downregulated in Bombyx mori Nucleopolyhedrovirus (BmNPV) infected silkworm cells. Transient transfections were performed with plasmids for pIEX-1-HN1L expression in Bombyx mori ovarian cells (BmN) in order to explore the effect of the HN1L protein on the growth of silkworm cells and its regulatory role in the process of viral infection. Cellular localization analysis revealed that HN1L was localized in the cytoplasm and that its upregulation could significantly enhance cellular activity. Furthermore, HN1L could promote G1/S phase conversion, thereby contributing to cell proliferation. Upon infection of BmN cells with BmNPV, the induction of apoptosis increased, although HN1L overexpression could inhibit DNA fragmentation, suggesting that the HN1L protein could inhibit cell apoptosis induced by viral invasion. In addition, Western blotting indicated that the HN1L protein inhibited the activation of caspase-9 zymogen and the expression of Bax protein, although it promoted Bcl-2 expression. Flow cytometry analysis further confirmed that overexpression of HN1L significantly inhibited apoptosis induced by BmNPV infection. Consequently, we demonstrated that BmN HN1L is a protein with multiple functions, which enhanced cell activity, regulated the cell cycle and induced an anti-apoptotic response by BmNPV infection.
Background Bombyx mori has become an important model organism for many fundamental studies. Bombyx mori nucleopolyhedrovirus (BmNPV) is a significant pathogen to Bombyx mori, yet also an efficient vector for recombinant protein production. A previous study indicated that acetylation plays many vital roles in several cellular processes of Bombyx mori while global phosphorylation pattern upon BmNPV infection remains elusive.MethodEmploying tandem mass tag (TMT) labeling and phosphorylation affinity enrichment followed by high-resolution LC-MS/MS analysis and intensive bioinformatics analysis, the quantitative phosphoproteome in Bombyx mori cells infected by BmNPV at 24 hpi with an MOI of 10 was extensively examined.ResultsTotally, 6480 phosphorylation sites in 2112 protein groups were identified, among which 4764 sites in 1717 proteins were quantified. Among the quantified proteins, 81 up-regulated and 25 down-regulated sites were identified with significant criteria (the quantitative ratio above 1.3 was considered as up-regulation and below 0.77 was considered as down-regulation) and with significant p-value (p < 0.05). Some proteins of BmNPV were also hyperphosphorylated during infection, such as P6.9, 39 K, LEF-6, Ac58-like protein, Ac82-like protein and BRO-D.ConclusionThe phosphorylated proteins were primary involved in several specific functions, out of which, we focused on the binding activity, protein synthesis, viral replication and apoptosis through kinase activity.
39K protein was proven to be the most notable hyperphosphorylated protein of Bombyx mori nucleopolyhedrovirus (BmNPV), since its amino acid 136 has the highest phosphorylation ratio (16.683) among all of BmNPV phosphorylated proteins (Shobahah et al., 2017). Therefore, we inserted positive (mimicking phosphorylation) and negative (mimicking dephosphorylation) mutants of the highest phosphorylated site of 39K and the wild type 39k into the 39k-ko-Bacmid using the Bac-to-Bac system. These three kinds of recombinant Bacmids along with wild type and knocked-out Bacmid were then transfected into the Bombyx mori cells (BmN) and further investigated by qPCR analysis. The results of the qPCR showed that 39K phosphorylation had no significant effect on viral genome replication, unlike the positive mutation that reduced the viral gene transcription drastically compared to the wild type. Thus, phosphorylation of BmNPV 39K protein does not appear to be the essential mechanism for viral genome replication, even though it has an important role in the viral gene transcription.
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