Mechanism of Slow Inactivation in Prokaryotic Voltage-Gated Sodium Channel

Abstract

Voltage-gated sodium channels (Nav) play a crucial role in electrically excitable cells by mediating the rapid influx of Na+ and thereby initiating the action potential in response to a depolarizing stimulus. In excitable cells, Nav activates and then undergoes fast and slow inactivation. Fast inactivation occurs within the time frame of a single action potential, whereas slow inactivation is produced by prolonged depolarization, mediating the availability of sodium channels over extended period of time. Mutations that alter slow inactivation are associated with several diseases such as hyperkalemic periodic analysis and long-QT syndrome. However, the molecular mechanism of slow inactivation and modulation by drug binding are still not completely known. In order to establish a structural framework of slow inactivation in molecular level, we used the prokaryotic homologue NavSP1 from Silicibacter pomeroyi as a model system. NavSP1 shares very similar overall architecture, activation, inactivation properties, and similar drug binding sites with the eukaryotic homologues and thereby proves to be an excellent model to be studied in native environment. We investigated how channel opening or the activation of voltage sensor initiates slow inactivation by evaluating the changes in conformational dynamics of the channel pore in the presence and absence of the voltage sensor. Our approach combined structural dynamics by EPR spectroscopy (CW and DEER methods) and functional studies by electrophysiological methods. Evaluation of the changes in the conformational dynamics and solvent accessibility in the turret and selectivity filter region show structural rearrangement of the outer pore region during slow inactivation.