Insect Immunity: Isolation and Structure of Cecropins B and D from Pupae of the Chinese Oak Silk Moth, Antheraea pernyi

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Abstract
The immune system in the Chinese oak silk moth, A. pernyi, was compared with that of the Cecropia moth. Antibacterial activity against Escherichia coli was induced in diapausing pupae by injection of viable E. coli or Enterobacter cloacae. The activity reached a maximum on day 7-8 after which the response gradually declined. The pupae produced a set of immune proteins with P4 and P5 as major labeled components similar to that earlier found in Cecropia. The major antibacterial factor in A. pernyi was cecropin D. A procedure is described for the isolation of cecropin B and D, which is in principle similar to the one used for the isolation of the corresponding cecropins from Cecropia pupae. Amino acid sequence analyses of the A. pernyi cecropins show the D form to contain 36 amino acid residues and that both cecropins have blocked C-termini. The general structure of cecropins having a charged N-terminal region (residues 1-21) followed by a long hydrophobic stretch (residues 22-32) is well conserved. Cecropin B and D from A. pernyi differ from corresponding proteins in Cecropia by 4 and 3 conservative amino acid replacements, respectively. The homology between the cecropins from the 2 insects suggests that they originate from a single ancestral gene. The antibacterial activity was tested against 9 different bacterial species. Evolutionary aspects of the cecropins are discussed.
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