Insect immunity: isolation and structure of cecropin D and four minor antibacterial components from Cecropia pupae.

We have investigated low molecular weight antibacterial proteins from the Cecropia moth. Hyalophora cecropia. In addition to the previously described cecropins A and B, five new antibacterial proteins were discovered, the cecropins C, D, E and F, and the factor G. A scheme for the purification of these factors is presented. Cecropin D is a major cecropin, its amino acid sequence, WNPFKELEKVGQRVRDAVISAGPAVATVAQATALAK, shows homology to cecropin A and B. Like these cecropins, cecropin D has a block C-terminal. The previously tentative C-terminal sequence of cecropin A is also confirmed. It is concluded that the three major cecropins, A, B and D, are products of three different genes that are derived from a common ancestor. The cecropins C, E and F were present in very low amounts, and thus their primary structures could not be fully elucidated. Cecropin C has an amino acid sequence that up to residue 37 is identical to the sequence of A, though it lacks the C-terminal blocking group. It may be a precursor or degradation product of cecropin A. The minor cecropin E shows a similar relation to cecropin D. Cecropin F has a single amino acid replacement (17 Asp leads to Asn) compared to cecropin D, and is probably a product of an allele that is present at a low frequency in the population. The primary structure of the factor G could not be determined, however its amino acid composition is different from that of the cecropins. All the major cecropins were found to be efficient against several gram-positive and gram-negative bacterial strains. No significant difference was found between them in their activity against Escherichia coli, though against some less susceptible bacteria the most basic cecropins were more effective, the activity falling in the series B greater than A much greater than D.