Open and Closed States of the NaVAb Activation Gate

Abstract

The bacterial voltage-gated sodium channel NavAb is a structural model for eukaryotic voltage-gated sodium channels, and it has been crystallized in both the pre-open and inactivated states (Payanadeh et al 2011, 2012). These structures revealed the general architecture of bacterial voltage-gated sodium channels and provided insight into mechanisms of voltage-sensing, electromechanical coupling, and the selective conductance of sodium ions. Comparison with structures of other bacterial sodium channels (Zhang et al 2012, Bagneris et al 2013) has also informed hypotheses of activation gating, an opening transition now thought to involve twisting movements of the S6 helix that ultimately result in an iris-like dilation of the intracellular ends of the four S6 segments. We sought to capture closed and open states in the same channel protein through x-ray crystallography and electrophysiological studies of mutations in the S6 of NavAb. Our studies show that paired mutations of hydrophobic residues in the S6 helix can uncouple voltage-sensor movement from pore opening and thereby lock the channel in a permanently closed state in which the intracellular ends of the four S6 helices interact tightly with each other. On the other hand, truncation of the intracellular extensions of the S6 segments results in an open conformation of the activation gate. These analyses allow for comparison of the closed, open, and inactivated states of NavAB's activation gate, a first for a voltage-gated sodium channel. Our results help confirm and sharpen the previously hypotheses for the pore-opening transition of sodium channels and reveal key conformational changes that connect the closed, open, and inactivated states of NavAb.