Molecular architecture of a sodium channel S6 helix: radial tuning of the Nav1.7 activation gate

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-Leu955) of NaV1.7 channel, identified in individuals with a severe inherited pain syndrome (inherited erythromelalgia, IEM) 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-Leu955 channel. Our structural model showed that Del-Leu955 twists the DII/S6 helix, shifting location and radial orientation of the activation gate residue (Phe960). Hypothesizing that these structural changes produce the activation shift of DelLeu955 channels, we restored a phenylalanine in wild-type orientation by mutating Ser961 (DelLeu955/Ser961Phe), correcting activation by ~10 mV. Correction of the displaced Phe960 (Phe960Ser), together with introduction of the rescuing activation gate residue (Ser961Phe) produced an additional ~6 mV restoration of activation of the mutant channel. A simple point mutation in the absence of a twist (Leu955Ala) 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 gate.