Binding of the anticonvulsant drug lamotrigine and the neurotoxin batrachotoxin to voltage-gated sodium channels induces conformational changes associated with block and steady-state activation

Voltage-gated sodium channels are dynamic membrane proteins characterized by rapid conformational changes that switch the molecule between closed resting, activated, and inactivated states. Sodium channels are specifically blocked by the anticonvulsant drug lamotrigine, which preferentially binds to the channel pore in the inactivated open state. Batrachotoxin is a lipid-soluble alkaloid that causes steady-state activation and binds in the inner pore of the sodium channel with overlapping but distinct molecular determinants from those of lamotrigine. Using circular dichroism spectroscopy on purified voltage-gated sodium channels from Electrophorus electricus, the secondary structures associated with the mixture of states present at equilibrium in the absence of these ligands were compared with specific stabilized states in their presence. As the channel shifts to open states, there appears to be a significant change in secondary structure to a more alpha-helical conformation. The observed changes are consistent with increased order involving the S6 segments that form the pore, the domain III-IV linker, and the P-loops that form the outer pore and selectivity filter. A molecular model has been constructed for the sodium channel based on its homology with the pore-forming regions of bacterial potassium channels, and automated docking of the crystal structure of lamotrigine with this model produces a structure in which the close contacts of the drug are with the residues previously identified by mutational studies as forming the binding site for this drug.