Drosocin
Drosocin is a 19-residue long antimicrobial peptide (AMP) of flies first isolated in the fruit fly "Drosophila melanogaster", and later shown to be conserved throughout the genus "Drosophila". Drosocin is regulated by the NF-κB Imd signalling pathway in the fly. Structure and function. Drosocin is primarily active against Gram-negative bacteria. The peptide is proline-rich with proline-arginine repeats, as well a critical threonine residue. This threonine is "O"-glycosylated, which is required for antimicrobial activity. This "O"-glycosylation can be performed either by mono- or disaccharides, which have different activity spectra. Like the antimicrobial peptides pyrrhocoricin and  abaecin, drosocin binds to bacterial DnaK, inhibiting cell machinery and replication. The action of these drosocin-like peptides is potentiated by the presence of pore-forming peptides, which facilitates the entry of drosocin-like peptides into the bacterial cell. Proline-rich peptides such as drosocin can also bind to microbe ribosomes, preventing protein translation. In the absence of pore-forming peptides, the related AMP pyrrhocoricin is taken into the bacteria by the action of uptake permeases. In "Drosophila melanogaster" the "Drosocin" gene is specifically important for the fly defense against infection by "Enterobacter cloacae" bacteria, supporting previous "in vitro" work  showing Drosocin is active against "E. cloacae". The "Drosocin" gene of "Drosophila neotestacea" uniquely encodes tandem repeats of "Drosocin "mature peptides between cleavage sites. As a result, a single protein gets chopped up into multiple Drosocin peptides. This tandem repeat structure is also found in the honeybee AMP apidaecin, and is hypothesized as an evolutionary mechanism to increase the speed of the immune response and AMP production. Molecular structure. The bolded threonine residue acts as a site for O-glycosylation, also found in the AMPs abaecin and pyrrhocoricin. The underlined PRP motifs are key to the binding of such peptides  to the DnaK proteins of bacteria. "D. melanogaster drosocin: GKPRPYSPRPTSHPRPIRV"