Fig. 2: Open state conformation and NTD-S6I segment interaction.
a Intracellular view of the activation gate formed by the S6 helices from D1 to DIV. The average diameters at the top (black dashed lines) and bottom (orange dashed lines) layers of the gate are ~10 Å. Side chains of AG lining residues are shown as sticks. b Comparison of pore radius profiles along the ion conduction pathway for hNav1.5 (orange), Nav1.5-E1784K (black), and rNav1.5c-BTX-B. (PDB ID: 7DTC, green). c Time-averaged water number densities along the pore axis under transmembrane voltages (TMV) of 0 mV (black), 745 mV (red), and 969 mV (blue). At 0 mV, the activation gate region is dehydrated; hydration increases at 745 mV and becomes continuous at 969 mV. The cyan dotted line indicates bulk water density, and shaded regions represent standard error. d Representative frames from an MD simulation showing Na+ (magenta sphere) permeating through the SF, CC, and AG. e Structural alignment of hNav1.5 with rNav1.5c-BTX-B (PDB: 8T6L, color: pale cyan) shows the relative position of NTD and S4-S5I, based on superposition of S1–S3 segments of VSDI. In hNav1.5, the hydrogen bond between S61 (NTD) and S234 (S4–S5 linker) observed in rNav1.5c-BTX-B (S62 and S235) is disrupted, and the NTD is displaced downward. f Cryo-EM densities of residues involved in the NTD-S6I interaction. g Intracellular view of the interface between the NTD and the S6I segment. R433 and K430 of S6I are positioned near D57 of the NTD. Inset i, potential salt bridges between D57 and both R433 and K430. Inset ii, structural alignment with rNav1.5c-BTX-B reveals a 12.5° tilt and a downward shift of S6I toward the NTD. h Time course of inactivation (τ) from sodium current recordings shows that K430D and D57K have a slower inactivation decay. Four independent experiments (N) were performed to obtain the number of cells (n) in each group. The data are presented as mean ± SEM. One-way ANOVA and Post-Hoc Tukey test were used for statistical analysis, *p < 0.05, WT vs K430D * p = 0.008 and WT vs D57K * p = 0.01.
