Structure-function studies of omega-atracotoxin, a potent antagonist of insect voltage-gated calcium channels

The omega-atracotoxins are a family of 36 to 37-residue peptide neurotoxins that block insect but not mammalian voltage-gated calcium channels. The hi gh phylogenetic specificity of these toxins recommends them as lend compoun ds for targeting insects that have developed resistance to chemical pestici des. We have begun to examine structure-function relationships in the omega -atracotoxins in order to explore the molecular basis of their activity and phylogenetic specificity. By probing the venom of the Blue Mountains funne l-web spidor. Hadronyche versuta, for insecticidal toxins with masses close to that of omega-atracotoxin-Hv1a (omega-ACTX-Hv1a), we have isolated and sequenced five additional w-atracotoxins. Five of the six omega-atracotoxin s isolated from the venom of H. versuta (omega-ACTX-Hv1a to -Hv1e) differ f rom one another by only 1-3 residues and have similar insecticidal potencie s. In contrast, omega-ACTX-Hv1f differs from the other toxins by up to 10 r esidues and it has markedly reduced insecticidal potency, thus providing in formation on key functional residues. The new atracotoxin sequences have re vealed that the three N-terminal residues are highly conserved. Despite the fact that these residues ore structurally disordered in solution we show h ere, by a series of N-terminal truncations, that they contribute significan tly to insecticidal potency. However, loss of activity does not correlate w ith deletion of highly conserved residues, which leads us to propose that t he disposition of the N-terminal charge, rather than the chemical propertie s of the N-terminal residues themselves, may be critical for the activity o f omega-atracotoxin on insect calcium channels.