Purification and partial characterization of an induced antibacterial protein in the silkworm, Bombyx mori

Injection of live Escherichia coli into larvae of the silkworm, Bombyx mori, induces antibacterial activity in the hemolymph. The major induced antibacterial activity was purified in two steps by CM-Sephadex C-50 and Sephadex G-100 column chromatography. After trypsin treatment, the purified antibacterial protein lost its activity and the antibacterial activity was found to be partially heat labile. The purified protein was a single polypeptide chain of molecular weight 16 kDa. The 20 N-terminal amino acid sequence of the protein was determined and this sequence showed homology with the N-terminal amino acid sequence of lysozymes reported in other species. The purified protein was found to have comparable antibacterial activity against both E. coli and Micrococcus luteus. The purification of antibacterial protein and the antibacterial properties of the purified protein are discussed.