Functionally important binding site for a volatile anesthetic in a voltage-gated sodium channel identified by X-ray crystallography
Voltage-gated sodium channels (VGSCs), key mediators of excitability and synaptic transmission, are established and functionally relevant targets for volatile anaesthetic (VA) action. Using the structurally homologous prokaryotic VGSCs NavMs and NaChBac as models, we present a structure-function analysis of VGSC-VA interactions. We report that multiple VAs compete for binding sites on NavMs, and that these direct interactions mediate functional effects of sevoflurane on NavMs that mirror those attributed to VA effects in eukaryotic VGSCs, including human isoforms. Using X-ray crystallography, we determined the first atomic-resolution structure of a VA bound to a VGSC, showing sevoflurane displacing lipids to bind in an intramembranous hydrophobic pocket of NavMs. A conserved tyrosine residue within this binding site is critical for channel gating, and its substitution with alanine abolishes sevoflurane binding and selectively eliminates the characteristic anaesthetic-induced hyperpolarising shift of steady-state inactivation that reduces neuronal excitability at physiological membrane potentials. Finally, we provide evidence supporting VA action at the conserved sites in human VGSC isoforms. These findings define the first VA binding site in a VGSC. A membrane-mediated access pathway to the binding site leads to negative modulation of channel function that reduces neuronal activity and excitatory synaptic transmission in general anaesthesia.