Structural Investigation of a Bacterial Voltage-Gated Sodium Channel NaVRh

Abstract

Voltage-gated sodium (Nav) channels are essential for the rapid depolarization of nerve and muscle, and are important drug targets. Elucidation of the structures and functional mechanisms of Nav channels will shed light on fundamental ion channel mechanisms and facilitate potential clinical applications. A family of bacterial Nav channels, exemplified by NaChBac (Na+-selective Channel of Bacteria), provides a good model system for structure-function analysis. We determined the crystal structure of NavRh, a NaChBac orthologue from marine bacteria, Rickettsiales sp. HIMB114 (denoted Rh), at 3.05 A resolution. The channel comprises an asymmetric tetramer. The carbonyl oxygen atoms of Thr178 and Leu179 constitute an inner site within the selectivity filter (178TLSSWE183) where a hydrated Ca2+ can bind and resides in the crystal structure. The outer mouth of the Na+ selectivity filter, defined by Ser181 and Glu183, is closed, as is the activation gate at the intracellular side of the pore. The voltage sensors adopt a depolarized conformation with all the gating charges exposing to the extracellular side. We hypothesize that NavRh is captured in an inactivated conformation. Comparison of NavRh with NavAb reveals that the VSD segments undergo discordant conformational shifts concurrent with domain rotation that may underlie the electromechanical coupling mechanism of voltage-gated channels.