Structural basis of inhibition of human Na _V 1.8 by the tarantula venom peptide Protoxin-I

Voltage-gated sodium channels (Na _V s) selectively permit diffusion of sodium ions across the cell membrane and, in excitable cells, are responsible for propagating action potentials. One of the nine human Na _V isoforms, Na _V 1.8, is a promising target for analgesics, and selective inhibitors are of interest as therapeutics. One such inhibitor, the gating-modifier peptide Protoxin-I derived from tarantula venom, blocks channel opening by shifting the activation voltage threshold to more depolarised potentials, but the structural basis for this inhibition has not previously been determined. Using monolayer graphene grids, we report the cryogenic electron microscopy structures of full-length human apo-Na _V 1.8 and the Protoxin-I-bound complex at 3.1 Å and 2.8 Å resolution, respectively. The apo structure shows an unexpected movement of the Domain I S4-S5 helix, and VSD _I was unresolvable. We find that Protoxin-I binds to and displaces the VSD _II S3-S4 linker, hindering translocation of the S4 _II helix during activation.