Inhibition of melanization by a Nasonia defensin-like peptide: Implications for host immune suppression

Nasonia vitripennis (Hymenoptera: Pteromalidae) is a small parasitic wasp which stings and lays eggs in pupae of various fly species, such as blow flies and flesh flies. Recently, Werren et al. reported several protease inhibitor-like peptides in N. vitripennis which includes cysteine-rich venom peptides with Kunitz type motif, the trypsin inhibitor-like type motif typically contains 10 cysteine residues that form five disulfide bridges and also some Kazal-type inhibitors. But functional data of these peptides is lacking. Here, we describe a new N. vitripennis defensin-like peptide (DLP) (nasonin-3) that has a different structural architecture from those mentioned above. Nasonin-3 adopts a typical defensin-like structure with a conserved cysteine-stabilized α-helical and β-sheet (CSαβ) motif. This recombinant peptide is devoid of antimicrobial activity, but it inhibits the melanization of its host's haemocytes through decrease of the PO activity. Sequence and structure comparisons allow us identifying a putatively functional motif related to the protease inhibitory activity.