Structure and mechanism of the K + /H + exchanger KefC.

Intracellular potassium (K + ) homeostasis is fundamental to cell viability. In addition to channels, K + levels are maintained by various ion transporters. One major family is the proton-driven K + efflux transporters, which in gram-negative bacteria is important for detoxification and in plants is critical for efficient photosynthesis and growth. Despite their importance, the structure and molecular basis for K + -selectivity is poorly understood. Here, we report ~3.1 Å resolution cryo-EM structures of the Escherichia coli glutathione (GSH)-gated K + efflux transporter KefC in complex with AMP, AMP/GSH and an ion-binding variant. KefC forms a homodimer similar to the inward-facing conformation of Na + /H + antiporter NapA. By structural assignment of a coordinated K + ion, MD simulations, and SSM-based electrophysiology, we demonstrate how ion-binding in KefC is adapted for binding a dehydrated K + ion. KefC harbors C-terminal regulator of K + conductance (RCK) domains, as present in some bacterial K + -ion channels. The domain-swapped helices in the RCK domains bind AMP and GSH and they inhibit transport by directly interacting with the ion-transporter module. Taken together, we propose that KefC is activated by detachment of the RCK domains and that ion selectivity exploits the biophysical properties likewise adapted by K + -ion-channels.