Extracellular K ⁺ modulates the pore conformations of Cys-loop receptor anion channels

K ⁺ is an essential cation for life, but no eukaryotic membrane protein with a modulatory site for extracellular K ⁺ has been discovered. Here, we report that a Cys-loop receptor, CG12344/DmAlka, expressed in the Drosophila nervous system, is selectively modulated by physiological concentration of extracellular K ⁺ . Structural prediction, electrophysiology and phylogenetic analysis of DmAlka revealed the extracellular K ⁺ binding site that mimics the hydrated chemical environment for K ⁺ , as observed in K ⁺ channel pore. Furthermore, we found that K ⁺ binding induces a previously unrecognized “mode-switching,” altering properties ranging from ligand sensitivity to ion selectivity. Notably, a human glycine receptor variant also exhibited similar mechanisms. Our study reveals a novel regulatory mechanism of Cys-loop receptors that directly links the extracellular K ⁺ signaling to Cl ^- conductance in animals.