Small molecule modulation of the Drosophila Slo channel elucidated by cryo-EM

Slowpoke (Slo) potassium channels display extraordinarily high conductance, are synergistically activated by a positive transmembrane potential and high intracellular Ca(2+) concentrations and are important targets for insecticides and antiparasitic drugs. However, it is unknown how these compounds modulate ion translocation and whether there are insect-specific binding pockets. Here, we report structures of Drosophila Slo in the Ca(2+)-bound and Ca(2+)-free form and in complex with the fungal neurotoxin verruculogen and the anthelmintic drug emodepside. Whereas the architecture and gating mechanism of Slo channels are conserved, potential insect-specific binding pockets exist. Verruculogen inhibits K(+) transport by blocking the Ca(2+)-induced activation signal and precludes K(+) from entering the selectivity filter. Emodepside decreases the conductance by suboptimal K(+) coordination and uncouples ion gating from Ca(2+) and voltage sensing. Our results expand the mechanistic understanding of Slo regulation and lay the foundation for the rational design of regulators of Slo and other voltage-gated ion channels.