Evidence for the existence of insect defensin-like peptide in scorpion venom.

Insect defensin refers to a group of antibacterial peptides derived from a variety of insect species as well as from scorpion and possessing a three-dimensional structure highly similar to that of scorpion toxins. A full-length cDNA encoding an insect defensin-like peptide was isolated from the venom gland cDNA library of the Chinese scorpion Buthus martensii Karsch. The precursor, the overall organization of which is similar to that of insect defensins, consists of 61 amino acid residues with a putative signal peptide of 15 residues, a propeptide of 7 residues, and a mature peptide of 39 residues (named BmTXKS2). The positions of six cysteines and a conserved glycine in mature BmTXKS2 are the same as those in LqDef, the first defensin found in scorpions, which suggests these peptides should present a similar cysteine-stabilized alphabetamotif. Phylogenetic analysis further shows that the structure of BmTXKS2 is closer to that of ancient defensins (e.g., LqDef and AaDef, two insect defensins present in the scorpion hemolymph) than to scorpion toxins.