Function of antimicrobial proteins in insects

We have isolated and characterized various antimicrobial proteins from the haemolymph of Sarcophaga peregrina (flesh fly) larvae. Of these the sarcotoxin I family is a group of proteins mainly active against Gram-negative bacteria whereas sapecin is active mainly against Gram-positive bacteria. In addition to its function in defence, sapecin also plays a role in insect development. Recently, we identified a hendecapeptide of the sapecin homologue sapecin B that has the same antibacterial activity as the original sapecin B. Both sarcotoxin I and sapecin are inducible proteins synthesized de novo by the fat body and/or haemocytes and secreted into the haemolymph when the insect is in the acute phase response to bacterial infection. Antifungal protein (AFP) is constitutively present in the haemolymph and is active against certain fungi but not bacteria. These various antimicrobial proteins interact with microbial membranes. Sarcotoxin I interferes with membrane functions such as ATP synthesis and amino acid transport. The fungicidal activity of AFP is enhanced synergistically by sarcotoxin I, although sarcotoxin I alone has no appreciable antifungal activity. It is clear that the flesh fly has the ability to mount a potent defence response against microbial parasites by mobilizing several antimicrobial proteins.