Molecular architecture of a sodium channel S6 helix: radial tuning of the voltage-gated sodium channel 1.7 activation gate.

BACKGROUND: In-frame deletion mutation (Del-L955) in NaV1.7 sodium channel from a kindred with erythromelalgia hyperpolarizes activation. RESULTS: Del-L955 twists the S6 helix, displacing the Phe960 activation gate. Replacement of Phe960 at the correct helical position depolarizes activation. CONCLUSION: Radial tuning of the activation gate is critical to the activation of NaV1.7 channel. SIGNIFICANCE: Structural modeling guided electrophysiology reveals the functional importance of radial tuning of the S6 segment. Voltage-gated sodium (NaV) channels are membrane proteins that consist of 24 transmembrane segments organized into four homologous domains and are essential for action potential generation and propagation. Although the S6 helices of NaV channels line the ion-conducting pore and participate in channel activation, their functional architecture is incompletely understood. Our recent studies show that a naturally occurring in-frame deletion mutation (Del-L955) of NaV1.7 channel, identified in individuals with a severe inherited pain syndrome (inherited erythromelalgia) causes a substantial hyperpolarizing shift of channel activation. Here we took advantage of this deletion mutation to understand the role of the S6 helix in the channel activation. Based on the recently published structure of a bacterial NaV channel (NaVAb), we modeled the WT and Del-L955 channel. Our structural model showed that Del-L955 twists the DII/S6 helix, shifting location and radial orientation of the activation gate residue (Phe(960)). Hypothesizing that these structural changes produce the shift of channel activation of Del-L955 channels, we restored a phenylalanine in wild-type orientation by mutating Ser(961) (Del-L955/S961F), correcting activation by ∼10 mV. Correction of the displaced Phe(960) (F960S) together with introduction of the rescuing activation gate residue (S961F) produced an additional ∼6-mV restoration of activation of the mutant channel. A simple point mutation in the absence of a twist (L955A) did not produce a radial shift and did not hyperpolarize activation. Our results demonstrate the functional importance of radial tuning of the sodium channel S6 helix for the channel activation.