The ω-atracotoxins:: Selective blockers of insect M-LVA and HVA calcium channels

The omega-atracotoxins (omega-ACTX) are a family of arthropod-selective peptide neurotoxins from Australian funnel-web spider venoms (Hexathelidae: Atracinae) that are candidates for development as biopesticides. We isolated a 37-residue insect-selective neurotoxin, WACTX-Ar1a, from the venom of the Sydney funnel-web spider Atrax robustus, with high homology to several previously characterized members of the omega-ACTX-1 family. The peptide induced potent excitatory symptoms, followed by flaccid paralysis leading to death, in acute toxicity tests in house crickets. Using isolated smooth and skeletal nerve-muscle preparations, the toxin was shown to lack overt vertebrate toxicity at concentrations up to 1 LM. To further characterize the target of the omega-ACTXs, voltage-clamp analysis using the whole-cell patch-clamp technique was undertaken using cockroach dorsal Impaired median neurons. It is shown here for the first time that omega-ACTX-Ar1a, and its homolog w-ACTX-Hv1a from Hadronyche versuta, reversibly block both mid-low- (M-LVA) and high-voltage-activated (HVA) insect calcium channel (Ca-v) currents. This block occurred in the absence of alterations in the voltage-dependence of Ca-v channel activation, and was voltage-independent, suggesting that omega-ACTX-1 family toxins are pore blockers rather than gating modifiers. At a concentration of 1 mu M omega-ACTX-Ar1a failed to significantly affect global Kv channel currents. However, 1 mu M (omega-ACTX-Ar1a caused a modest 18% block of insect Na, channel currents, similar to the minor block of Na, channels reported for other insect Ca, channel blockers such as omega-agatoxin IVA. These findings validate both M-LVA and HVA Cav channels as potential targets for insecticides. (C) 2007 Published by Elsevier Inc.