Crystal structure of a potassium ion transporter, TrkH

Abstract

The TrkH/TrkG/KtrB proteins mediate K+ uptake in bacteria and probably evolved from simple K+ channels by multiple gene duplications or fusions. Here we present the crystal structure of a TrkH from Vibrio parahaemolyticus. TrkH is a homodimer, and each protomer contains an ion permeation pathway. A selectivity filter, similar in architecture to those of K+ channels but significantly shorter, is lined by backbone and side-chain oxygen atoms. Functional studies showed that TrkH is selective for permeation of K+ and Rb+ over smaller ions such as Na+ or Li+. Immediately intracellular to the selectivity filter are an intramembrane loop and an arginine residue, both highly conserved, which constrict the permeation pathway. Substituting the arginine with an alanine significantly increases the rate of K+ flux. These results reveal the molecular basis of K+ selectivity and suggest a novel gating mechanism for this large and important family of membrane transport proteins. All living cells accumulate potassium ions, and a high intracellular potassium concentration is essential for many physiological processes. In bacteria, yeast and plants, potassium ion uptake is achieved by a superfamily of transporters, the SKT proteins. These are thought to have evolved from simple potassium channels, but the mechanism of potassium ion selectivity and transport is unknown. The crystal structure of a bacterial SKT protein, the TrkH potassium transporter from Vibrio parahaemolyticus, has now been determined. Its selectivity filter is similar to that of potassium ion channels, but it is shorter. Biochemical studies suggest that K+ selectivity depends on a novel gating mechanism that excludes smaller ions such as Na+ and Li+ in favour of the larger K+ and Rb+. Here, the X-ray crystal structure of TrkH is solved, a protein that mediates potassium uptake in Vibrio parahaemolyticus, a bacterium. The selectivity filter of the ion transporter is very similar to what is seen in (much simpler) potassium ion channels, but it is significantly shorter. Biochemical studies were performed to explore the molecular basis of K+ selectivity, and it is believed that a novel gating mechanism is involved.