Three in a row-how sodium ions cross the channel

Abstract

Sodium channels are central to a host of fundamental cellular processes, including sensory perception, pain, and muscle contraction. In order to understand any of these processes in detail, it is necessary to know the atomic structure of the channel proteins both with and without bound sodium ions. In this issue, Naylor et al (2016) present the structure of a bacterial sodium channel tetramer. The three bound, partially hydrated sodium ions line up neatly in a row inside the selectivity filter, providing us with the first detailed insights into ion conduction in sodium channels, and the mechanisms by which sodium and potassium ions are discriminated.

Figure 1. Structure of a bacterial sodium channel tetramer (Na_vMs) with three bound sodium ions

The partially hydrated sodium ions line up in a row inside the selectivity filter (call‐out), providing detailed insights into ion selection and conduction by sodium channels. Negatively charged side chains in the selectivity filter attract sodium ions and allow them to pass, while the larger hydrated potassium ions are excluded. Negatively charged glutamate residues in the filter partially compensate the positive charge of the sodium ions passing through. For clarity, only two of the protomers (red and green) in the sodium channel tetramer are shown.