Antiviral substance in the silkworm gut juice against a nuclear polyhedrosis virus of the silkworm, Bombyx mori

A protein showing strong antiviral activity against Bombyx mori nucleopolyhedrovirus (BmNPV) was purified from the digestive juice of B. mori larvae. The molecular mass of this protein was 24 271 Da. Partial N-terminal amino acid sequence of the protein was determined and cDNA was cloned based on the amino acid sequence. A homology search of the deduced amino acid sequence of the cDNA showed 94% identity with B. mori serine protease so the protein was designated B. mori serine protease-2 (BmSP-2). Analysis of BmSP-2 gene expression showed that this gene is expressed in the midgut but not in other tissues. In addition, BmSP-2 gene was shown to not be expressed in the molting and wandering stages, indicating that the gene is hormonally regulated. Our results suggest that BmSP-2, an insect digestive enzyme, can be a potential antiviral factor against BmNPV at the initial site of viral infection.