The Janus-faced atracotoxins are specific blockers of invertebrate KCa channels

The Janus- faced atracotoxins are a unique family of excitatory peptide toxins that contain a rare vicinal disulfide bridge. Although lethal to a wide range of invertebrates, their molecular target has remained enigmatic for almost a decade. We demonstrate here that these toxins are selective, high- affinity blockers of invertebrate Ca2+ -activated K+ (K-Ca) channels. Janus-faced atracotoxin (J-ACTX)-Hv1c, the prototypic member of this toxin family, selectively blocked KCa channels in cockroach unpaired dorsal median neurons with an IC50 of 2 nM, but it did not significantly affect a wide range of other voltage-activated K+, Ca2+ or Na+ channel subtypes. J-ACTX- Hv1c blocked heterologously expressed cockroach large-conductance Ca2+ -activated K+ (pSlo) channels without a significant shift in the voltage dependence of activation. However, the block was voltage-dependent, indicating that the toxin probably acts as a pore blocker rather than a gating modifier. The molecular basis of the insect selectivity of J-ACTX-Hv1c was established by its failure to significantly inhibit mouse mSlo currents ( IC50 similar to 10 mu m) and its lack of activity on rat dorsal root ganglion neuron K-Ca channel currents. This study establishes the Janus- faced atracotoxins as valuable tools for the study of invertebrate K-Ca channels and suggests that K-Ca channels might be potential insecticide targets.