The Janus-faced atracotoxins are specific blockers of invertebrate K(Ca) channels

Abstract

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 calcium activated K+ (KCa) channels. 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 potassium (KV), calcium (CaV), or sodium (NaV) channel subtypes. J ACTX-Hv1c blocked heterologously expressed cockroach BKCa (pSlo) channels without a significant shift in the voltage-dependence of activation. However, the block was voltage-dependent, indicating that the toxin likely 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 ~10 μM) and its lack of activity on rat dorsal root ganglion neuron IK(Ca). This study establishes the Janus-faced atracotoxins as valuable tools for the study of invertebrate KCa channels and suggests that KCa channels might be a potential insecticide target.