Molecular determinants of coupling between the domain III voltage sensor and pore of a sodium channel

In a voltage-dependent sodium channel, the activation of voltage sensors upon depolarization leads to the opening of the pore gates. To elucidate the principles underlying this conformational coupling, we investigated a putative gating interface in domain III of the sodium channel using voltage-clamp fluorimetry and tryptophan-scanning mutagenesis. Most mutations have similar energetic effects on voltage-sensor activation and pore opening. However, several mutations stabilized the activated voltage sensor while concurrently destabilizing the open pore. When mapped onto a homology model of the sodium channel, most localized to hinge regions of the gating interface. Our analysis shows that these residues are involved in energetic coupling of the voltage sensor to the pore when both are in resting and when both are in activated conformations, supporting the notion that electromechanical coupling in a voltage-dependent ion channel involves the movement of rigid segments connected by elastic hinges.