Identification of Viral Structural Polypeptides in the Midgut and Feces of the Silkworm, Bombyx mori, Infected with Bombyx Densovirus Type 2

Abstract Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) analysis showed that Bombyx mori densovirus type 2 (BmDNV-2) contained six structural polypeptides with apparent molecular weights ( M r s) of 120,000 (120K), 118, 53, 51, 49, and 46.5K. Among these structural polypeptides, four polypeptides were detected by immnnoblot analysis with anti-BmDNV-2 antibody raised against purified BmDNV-2 virions. Using such antibody for immunoblot analysis, these four viral structural polypeptides were identified in the midgut epithelium, midgut contents, and feces from BmDNV-2-infected silkworm larvae. The relative proportion of viral structural polypeptides in the midgut epithelium was different from that in the midgut contents or feces, while midgut contents and feces were similar in their viral structural polypeptide camposition. Conversion of the 53K polypeptide in the midgut epithelium to 51, 49, and 46.5K polypeptides was achieved in vitro at acid and alkaline pHs. Since protease inhibitors specific for cysteine and serine proteases reduced this conversion, proteolytic cleavage appeared to be responsible for this conversion. Peptide mapping of structural polypeptides suggested that 53, 51, 49, and 46.5K polypeptides shared a common primary amino acid sequence. Post-translational modification may be involved in the generation of structural polypeptides and the maturation of the BmDNV-2 virion.