Determination of the proteins encoded by BmBDV VD1-ORF4 and their interacting proteins in BmBDV-infected midguts.

Bombyx mori bidensovirus (BmBDV) VD1-ORF4 consists of 3,318 nucleotides, which codes for a predicted protein with molecular weight of about 127 kDa. However, the authentic proteins encoded by VD1-ORF4 in silkworm midguts infected with BmBDV and their interacting proteins are still unclear. In this study, Western blot analysis revealed that a 127-kDa protein was confirmed to be translated from the VD1-ORF4 transcript using polyclonal antibodies and monoclonal antibodies against VD1-ORF4 deduced amino acid. Moreover, four smaller proteins with molecular weight of about 70, 60, 53, and 42 kDa were also examined in the infected midguts. Transient expression assay indicated that the expression amount of VD1-ORF4 fused with egfp was at least 30-fold lower than that of egfp gene, and immunofluorescence staining result indicated that these proteins encoded by VD1-ORF4 were located in both the cytoplasm and nucleus. Co-immunoprecipitation result showed that Aminopeptidase and Heat shock protein 90 can be captured by these proteins encoded by VD1-ORF4. In conclusion, multiple proteins were produced from the transcripts of VD1-ORF4 gene by an uncertain expression strategy, which may play important roles in viral replication and assembly.