Fig. 6: Mechanism for the agonistic effects of BTX-B.
a The gating hinge and the π-helices in the structure of BTX-B bound rNaV1.5c. Top view of the pore with only S5 and S6 segments shown for clarity. The gating hinge residues G401, G929, G1459, and S1761 on S6 from DI to DIV (GGGS motif) and their adjacent pore-facing residues S402, N930, S1460, and F1762 (SNSF motif) that have been shown to interact with Site II neurotoxins are shown as sticks colored according to their domains26,27,50,52. This section of S6 in DI and DIII adopt the π-helix configuration as indicated by the π-helix bulging while the section in DII and DIV are α-helix. BTX-B sites IIB1 and IIB2 are shown as sticks overlaid with transparent van der Waals sphere in cyan and bright green, respectively. Red arrows indicate the positions of I1758 and I1759. b The dual receptor sites for BTX-B are compatible with rNaV1.5c in the open state. Superposition between the BTX-B bound rNaV1.5c (DI—dark blue, DII—dark green, DIII—dark yellow, DIV—dark red, DIII-DIV linker—orange) and the rNaV1.5c QQQ in the Open state (DI—light blue, DII—light green, DIII—light yellow, DIV—pink, DIII-DIV linker—light orange) (PDB: 7FBS)54. Major conformational changes are observed in the C-terminal end of DIV-S6 (dark red vs. pink) starting at the conserved gating residue N1767 and in the DIII-DIV linker (dark orange vs. light orange) responsible for fast inactivation. The N1767 side chains from both structures are shown as sticks to indicate where the rotation of DIV-S6 starts. Site IIB1 and IIB2 are shown as bright green and cyan sticks, respectively. c Close-up view focusing on Site IIB1 (bright green sticks). The rotation of the DIV-S6 from the BTX-B bound (red) to the open-state structure (pink) replaces the M1768 side chain with Y1769 that can hypothetically form a hydrogen bond with IIB1 (yellow dash) to further stabilize the open state. However, Y1769 does not contribute significantly to the binding of BTX-B based on mutagenesis studies.
