Conduction pathway for potassium through the Escherichia coli pump KdpFABC

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Bacteria utilize the KdpFABC complex to maintain intracellular potassium (K+) levels under osmotic stress.
• KdpA acts as a K+ transporter with a selectivity filter, while KdpB is a P-type ATPase driving ion transport.
• Previous models proposed K+ movement through a tunnel connecting KdpA and KdpB.

Purpose of the Study:

• To elucidate the structural and mechanistic details of the KdpFABC complex during K+ transport.
• To investigate the pathway and interactions of potassium ions within the pump.
• To validate the proposed conduction pathway through functional assays.

Main Methods:

• Reconstitution of the KdpFABC complex into lipid nanodiscs.
• Cryo-electron microscopy (cryo-EM) to determine the structure at 2.1 Å resolution.
• ATPase and ion transport assays to assess the impact of mutations.

Main Results:

• A high-resolution structure of the KdpFABC pump in the E1~P·ADP state was obtained.
• Potassium ions were observed within the selectivity filter of KdpA and a binding site in KdpB.
• The study identified water molecules and hydrophobic regions within the ion conduction pathway.
• Mutational analysis confirmed K+ ion passage through the tunnel and a low-affinity release site in KdpB.

Conclusions:

• The KdpFABC complex facilitates K+ transport via a tunnel connecting the KdpA selectivity filter to KdpB.
• A unique partnership between a channel and an ATPase is crucial for maintaining bacterial potassium gradients.
• This work provides critical insights into the mechanism of potassium homeostasis in bacteria.