Gating pore currents and the resting state of Na_v1.4 voltage sensor domains

摘要

Mammalian voltage-gated sodium channels are composed of four homologous voltage sensor domains (VSDs; Dl, Dll, Dill, and DIV) in which their S4 segments contain a variable number of positively charged residues. We used single histidine (H) substitutions of these charged residues in the Na_v1.4 channel to probe the positions of the S4 segments at hyperpolarized potentials. The substitutions led to the formation of gating pores that were detected as proton leak currents through the VSDs. The leak currents indicated that the mutated residues are accessible from both sides of the membrane. Leak currents of different magnitudes appeared in the DI/R1H, DII/R1H, and DIII/R2H mutants, suggesting that the resting state position of S4 varies depending on the domain. Here, DI/R1H indicates the first arginine R1, in domain Dl, has been mutated to histidine. The single R1H, R2H, and R3H mutations in DIV did not produce appreciable proton currents, indicating that the VSDs had different topologies. A structural model of the resting states of the four VSDs of Na_v1.4 relaxed in their membrane/solution environment using molecular dynamics simulations is proposed based on the recent Na_vAb sodium channel X-ray structure. The model shows that the hydrophobic septa that isolate the intracellular and the extracellular media within the Dl, Dll, and Dill VSDs are ～2 A long, similar to those of K_v channels. However, the septum of DIV is longer, which prevents water molecules from hydrating the center of the VSD, thus breaking the proton conduction pathway. This structural model rationalizes the activation sequence of the different VSDs of the Na_v1.4 channel.