Journal Article

Crystal structure of the potassium-importing KdpFABC membrane complex

Huang, Ching-Shin, Pedersen, Bjørn Panyella, Stokes, David L.

2017

Overview

The crystal structure of the bacterial potassium import complex KdpFABC shows how ATP hydrolysis is coupled to potassium transport to maintain cellular homeostasis under low potassium conditions. Potassium pump Potassium homeostasis is fundamental for regulating osmotic pressure, pH and membrane potential. K + import into the cells of many bacteria occurs through several mechanisms, though at low concentrations they employ KdpFABC, a transport protein complex that couples the functionality of a pump-like subunit with a channel-like subunit. Here David Stokes and colleagues report the crystal structure of KdpFABC and show how this complex acts as a potassium pump, coupling ATP hydrolysis to the transport of K + into the cytosol. A tunnel connects the KdpA and KdpB subunits, coupling the ion-binding site to the hydrolysis of ATP. This work offers new insights into the regulation of transporter and channel gating. Cellular potassium import systems play a fundamental role in osmoregulation, pH homeostasis and membrane potential in all domains of life. In bacteria, the kdp operon encodes a four-subunit potassium pump that maintains intracellular homeostasis, cell shape and turgor under conditions in which potassium is limiting 1 . This membrane complex, called KdpFABC, has one channel-like subunit (KdpA) belonging to the superfamily of potassium transporters and another pump-like subunit (KdpB) belonging to the superfamily of P-type ATPases. Although there is considerable structural and functional information about members of both superfamilies, the mechanism by which uphill potassium transport through KdpA is coupled with ATP hydrolysis by KdpB remains poorly understood. Here we report the 2.9 Å X-ray structure of the complete Escherichia coli KdpFABC complex with a potassium ion within the selectivity filter of KdpA and a water molecule at a canonical cation site in the transmembrane domain of KdpB. The structure also reveals two structural elements that appear to mediate the coupling between these two subunits. Specifically, a protein-embedded tunnel runs between these potassium and water sites and a helix controlling the cytoplasmic gate of KdpA is linked to the phosphorylation domain of KdpB. On the basis of these observations, we propose a mechanism that repurposes protein channel architecture for active transport across biomembranes.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/45/269/1151 / 631/45/321/1156 / 631/45/612/1237 / 631/535/1266 / Active transport / Adenosine Triphosphatases - chemistry / Adenosine Triphosphatases - metabolism / BASIC BIOLOGICAL SCIENCES / Binding sites / Cation Transport Proteins - chemistry / Cation Transport Proteins - metabolism / Cell size / Cellular structure / Crystal structure / Crystallography, X-Ray / E coli / Escherichia coli - chemistry / Escherichia coli Proteins - chemistry / Escherichia coli Proteins - metabolism / Homeostasis / Humanities and Social Sciences / Hydrolysis / Ions / letter / Membrane potential / membrane proteins / Membrane Proteins - chemistry / Membrane Proteins - metabolism / Membranes / Models, Molecular / multidisciplinary / Mutation / Observations / Osmoregulation / Phosphorylation / Physiological aspects / Potassium / Potassium - metabolism / Potassium channels / Proteins / Science / Structural members / Structure / Structure-function relationships / Turgor / Water chemistry / x-ray crystallography