Bombyx mori nucleopolyhedrovirus ORF51 encodes a budded virus envelope associated protein

Bombyx mori nucleopolyhedrovirus (BmNPV) ORF 51 (Bm51) is a gene present in many lepidopteran NPVs, but its function is unknown. In this study, Bm51 was characterized. Transcripts of Bm51 were detected from 4.5 through 72 hour post infection (h p.i.) by RT-PCR. The corresponding protein was detected from 6 to 72 h p.i. in BmNPV-infected BmN cells by western blot analysis using a polyclonal antibody against Bm51. Western blot assay of occlusion-derived virus and budded virus (BV) preparations revealed that Bm51 encodes a 23-kDa structural protein that is associated with BV and is located in the envelope fraction of budded virions. The protein was temporarily called BV-E23. In addition immunofluorescence microscopy demonstrated that the protein was present within the cytoplasm and nuclei in virus-infected cells. In conclusion, the available data suggest that Bm51 is a functional ORF of BmNPV and encodes a protein expressed in the early stage of the infection cycle that is associated with the BV envelope.