Primary structure of sarcotoxin I, an antibacterial protein induced in the hemolymph of Sarcophaga peregrina (flesh fly) larvae.

The primary structure of sarcotoxin I, a potent bactericidal protein induced in the hemolymph of larvae of Sarcophaga peregrina (flesh fly), was investigated. Sarcotoxin I was a mixture of three proteins (sarcotoxins IA, IB, and IC) with almost identical primary structures. These proteins were found to consist of 39 amino acid residues and to differ in only 2-3 amino acid residues. The amino-terminal half of the molecules was rich in charged amino acids and was hydrophilic, whereas the carboxyl-terminal half was hydrophobic. It is suggested that the carboxyl-terminal half of sarcotoxin I penetrates into the bacterial membrane and that its amino-terminal half rich in basic amino acid residues interacts with acidic phospholipids in the bacterial membrane, resulting in perturbation of the membrane and loss of viability of the bacteria.