Structural basis for excitatory neuropeptide signaling

Rapid chemo-electric signaling between neurons is mediated by ligand-gated ion channels, cell-surface proteins with an extracellular ligand-binding domain and a membrane-spanning ion channel domain ¹ . The degenerin/epithelial sodium channel (DEG/ENaC) superfamily, which occurs throughout the animal kingdom, is unique in its diversity of gating stimuli, with some DEG/ENaCs gated by conventional ligands such as neuropeptides, and others gated by e.g. pH, mechanical force, or enzymatic activity ^2-5 . The mechanism by which ligands bind to and activate DEG/ENaCs is poorly understood. We have therefore dissected the structural basis for neuropeptide binding and gating in a neuropeptide-gated DEG/ENaC, FMRFamide-gated sodium channel 1 (FaNaC1) from the annelid worm Malacoceros fuliginosus ⁶ , using cryo-electron microscopy. High-resolution structures of FaNaC1 in the ligand-free resting state and in several ligand-bound states reveal the ligand-binding site and capture the ligand-induced conformational changes that mediate channel gating. Complementary mutagenesis experiments confirm the functional roles of particular amino acid residues implicated by the structures. Our results illuminate ligand-induced channel gating in DEG/ENaCs and offer a structural template for the experimental dissection of channel pharmacology and ion conduction in a characteristically metazoan ion channel superfamily.