Structure of a Prokaryotic Sodium Channel Pore Reveals Essential Gating Elements and an Outer Ion Binding Site Common to Eukaryotic Channels

Voltage-gated sodium channels (Nays) are central elements of cellular excitation. Notwithstanding advances from recent bacterial Na-v (BacNa(v)) structures, key questions about gating and ion selectivity remain. Here, we present a closed conformation of Na(v)Ae1p, a pore-only BacNav derived from Na(v)Ae1, a BacNa(v) from the arsenite oxidizer Alkalilimnicola ehrlichei found in Mono Lake, California, that provides insight into both fundamental properties. The structure reveals a pore domain in which the pore-lining S6 helix connects to a helical cytoplasmic tail. Electrophysiological studies of full-length BacNa(v)s show that two elements defined by the NavAe1p structure, an S6 activation gate position and the cytoplasmic tail neck, are central to BacNav gating. The structure also reveals the selectivity filter ion entry site, termed the outer ion site. Comparison with mammalian voltage-gated calcium channel (Ca-v) selectivity filters, together with functional studies, shows that this site forms a previously unknown determinant of Cav high-affinity calcium binding. Our findings underscore commonalities between BacNavs and eukaryotic voltage-gated channels and provide a framework for understanding gating and ion permeation in this superfamily. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.