Structural basis of inhibition of human Na_V1.8 by the tarantula venom peptide Protoxin-I

Voltage-gated sodium channels (Na_Vs) 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_V1.8, 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_V1.8 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.