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.

The strict exchange of protons for sodium ions across cell membranes by Na⁺/H⁺ exchangers is a fundamental mechanism for cell homeostasis. At active pH, Na⁺/H⁺ exchange can be modelled as competition between H⁺ and Na⁺ to an ion-binding site, harbouring either one or two aspartic-acid residues. Nevertheless, extensive analysis on the model Na⁺/H⁺ antiporter NhaA from Escherichia coli, has shown that residues on the cytoplasmic surface, termed the pH sensor, shifts the pH at which NhaA becomes active. It was unclear how to incorporate the pH senor model into an alternating-access mechanism based on the NhaA structure at inactive pH 4. Here, we report the crystal structure of NhaA at active pH 6.5, and to an improved resolution of 2.2 angstrom. We show that at pH 6.5, residues in the pH sensor rearrange to form new salt-bridge interactions involving key histidine residues that widen the inward-facing cavity. What we now refer to as a pH gate, triggers a conformational change that enables water and Na⁺ to access the ion-binding site, as supported by molecular dynamics (MD) simulations. Our work highlights a unique, channel-like switch prior to substrate translocation in a secondary-active transporter. 

Solute carrier (SLC) transporters are membrane transport proteins, which catalyse the movement of nutrients, ions, and drugs across cell membranes. Here, I will present our contribution to understanding the mechanism of the sodium/proton exchangers (NHE), belonging to the SLC9 family of membrane transporters. NHEs exchange sodium ions for protons across biological membranes, which is a critical reaction for the fine-tuning of cytoplasmic and organelle pH, sodium levels and volume homeostasis. Dysfunction of NHE members has been linked to a number of diseases and disorders, such as hypertension, heart failure, autism spectrum disorder, epilepsy and the susceptibility of long COVID. Protein structures are important for developing mechanistic models, but due to technical challenges only bacterial homologue structures of NHE proteins were previously available.

Accumulating many years of effort, we were able to determine the first structure of a mammalian Na⁺/H⁺ exchanger, the endosomal isoform NHE9 by single-particle cryo-EM. The structure of NHE9 demonstrated that NHE proteins are architecturally most similar to bacterial homologues with 13-TM segments and likely operated by a similar elevator mechanism (I). Interestingly, native MS and thermal-shift assays indicted that the NHE9 homodimer is stabilized by the binding of a rare lipid only found in late endosomes, which implies the cell may use this lipid as means to switch-on NHE9 activity once it reaches its correct functional localization. We further provided evidence that the large cytoplasmic tail in NHE proteins likely acts in an auto-inhibitory manner. It is only removed upon the binding of extrinsic proteins (II). Indeed, the first structure of a potassium specific K⁺/H⁺ exchanger KefC reveals how its cytoplasmic tail restricts movement of the ion-transporting domain to directly inhibit transport. The structure of KefC is also the first ion-bound state seen for this family and, unlike to the modeled Na⁺/H⁺ exchanger sites with a hydrated Na⁺ ion, coordinates K⁺ as a dehydrated ion (IV). Lastly, we determining the structure of a bacterial Na⁺/H⁺ exchanger NhaA to high-resolution at an active pH of 6.5. With this structure we demonstrated how a cytoplasmic “pH gate” controlled by the pH activated NhaA (III).

Na⁺/H⁺ exchangers (NHEs) are ancient membrane-bound nanoma- chines that work to regulate intracellular pH, sodium levels and cell volume. NHE activities contribute to the control of the cell cycle, cell proliferation, cell migration and vesicle trafficking. NHE dysfunction has been linked to many diseases, and they are targets of pharma- ceutical drugs. Despite their fundamental importance to cell home- ostasis and human physiology, structural information for the mammalian NHEs was lacking. Here, we report the cryogenic elec- tron microscopy structure of NHE isoform 9 (SLC9A9) from Equus caballus at 3.2 Å resolution, an endosomal isoform highly expressed in the brain and associated with autism spectrum (ASD) and atten- tion deficit hyperactivity (ADHD) disorders. Despite low sequence identity, the NHE9 architecture and ion-binding site are remarkably most similar to distantly related bacterial Na⁺/H⁺ antiporters with 13 transmembrane segments. Collectively, we reveal the conserved architecture of the NHE ion-binding site, their elevator-like structural transitions, the functional implications of autism disease mutations and the role of phosphoinositide lipids to promote homodimerization that, together, have important physiological ramifications.

Here, we present a simple overexpression condition for high-throughput screening of membrane proteins in Escherichia coli. For the vast majority of bacterial membrane protein targets tested the MEMbrane protein Single shoT Amplification Recipe-MemStarleads to high production yields of target protein. The use of MemStar has facilitated structural studies of several transport proteins.