Proliferation of Bombyx mori nucleopolyhedrovirus strain H4 in BmN cells is enhanced by exchange of the F gene sequence with type strain T3.

The Bombyx mori nucleopolyhedrovirus (BmNPV)-based baculoviral expression vector system is among the most efficient expression vector systems for eukaryotic proteins especially when used in combination with silkworms as a host. We newly isolated a novel BmNPV strain (BmNPV H4) in Hokkaido, Japan that outperforms the type strain T3 in terms of both proliferation and expression of polyhedrin protein in silkworm larvae; however, it proliferates poorly in the BmN cell line. We inferred the gene responsible for the differences in proliferation between viral strains by quantifying amino acid similarity distances in protein functional domains and identifying highly divergent alleles between the H4 and T3 strains. Among proteins that differ markedly in functional domain sequence between H4 and T3, we identified the F gene, which encodes the F protein, as a putative cause of proliferative differences between the two strains. Using recombinant viruses with the F protein-coding sequence exchanged between H4 and T3, we determined that the T3 F protein increases H4 proliferation in BmN while the H4 F protein does not improve T3 proliferation in silkworm larvae. Our results suggest that the BmNPV F protein can strongly affect viral proliferation in a genetic background-specific manner and may be an important target for manipulating the proliferation characteristics of BmNPV-based expression vectors.