The new study, published in PLOS Biology, addresses a long-standing puzzle in the biophysics of fluoride transport. FEX channels belong to a broader family of anion-selective channels known as Fluc proteins, which are remarkable for their extreme selectivity: they permit fluoride ions to flow through at high rates while excluding nearly all other anions, including the chemically similar chloride. Earlier work had established that FEX activity depends on sodium, meaning the channels do not simply export fluoride on their own but require the presence of sodium ions to function. What remained unclear was how sodium achieves this activation, where the ions bind, and whether other monovalent cations, the positively charged ions such as lithium, potassium, and rubidium that abound in biological environments, might influence the process. Because the cellular milieu is a crowded soup of competing ions, the answer could matter for real organisms, not just purified proteins in a test tube.

To dissect the mechanism, the researchers turned to a reconstituted system, a minimalist experimental platform in which purified FEX proteins are embedded into artificial lipid membranes. This approach strips away the confounding complexity of a living cell and allows the researchers to control precisely which ions are present on either side of the membrane. By measuring fluoride flux under different ionic conditions, they could isolate the contribution of each cation to channel activity. The results revealed that sodium and lithium, despite their chemical similarity as small alkali metal ions, have sharply divergent effects. Sodium supports robust channel activity, acting as the activating cofactor that the fluoride exporter needs to transport its anionic cargo. Lithium, by contrast, does not activate the channel at all. Worse, it actively interferes with sodium-dependent activation.

The functional assays showed that lithium behaves as a competitive antagonist of channel activation. In other words, lithium ions compete with sodium ions for the same binding site, located at the center of the channel, and when lithium occupies that site, the channel is locked into an inactive state. Competitive antagonism is a familiar concept in pharmacology, where drug molecules block receptor sites to prevent natural ligands from binding, but seeing it play out with two biologically common metal ions competing to regulate an ion channel is unusual. The implication is that the ratio of sodium to lithium in the cellular environment could directly tune how well a fungus or other eukaryote exports fluoride, and therefore how resistant it is to fluoride poisoning.

The team then connected this molecular mechanism to a whole-organism phenotype. In yeast, a standard eukaryotic model organism, the presence of lithium markedly reduced fluoride tolerance. Yeast cells that would normally survive a given dose of fluoride faltered when lithium was also present, because their FEX channels could no longer export the fluoride efficiently. This finding elevates the work from an in vitro biophysical curiosity to a potentially significant environmental and physiological observation. Lithium has not previously been recognized as an environmental antagonist of eukaryotic fluoride export, and the results tie cellular fluoride stress tolerance directly to the abundance of additional ions in the surrounding milieu. Any environment where lithium concentrations rise, whether through natural geochemistry or industrial contamination, could thereby sensitize eukaryotic communities to fluoride that they would otherwise shrug off.

To understand why lithium and sodium have such different effects on the same binding site, the researchers determined a cryo-electron microscopy structure of the FEX channel from the pathogenic fungus Candida albicans in the presence of lithium ions. Cryo-EM, a technique that flash-freezes individual protein molecules in glassy ice and images them with an electron beam, has revolutionized the study of membrane proteins, and it allowed the team to visualize the channel at near-atomic resolution with the antagonist bound. The structure revealed the architecture of the central cation binding site and showed how lithium coordinates there. But a static structure alone could not explain the functional differences, because ion binding sites often look deceptively similar for chemically related ions. The crucial differences, it turned out, lay in the dynamics.

Complementing the cryo-EM structure, the team ran molecular dynamics simulations, computational models that track the motion of every atom in the protein and its surrounding membrane over time. These simulations exposed a fundamental difference in the preferred coordination sphere of the two cations, meaning the geometric arrangement of atoms that each ion prefers to surround itself with. Sodium binding supports a dynamic structure with a broader pore radius, keeping the channel in an open, flexible conformation through which fluoride ions can pass. Lithium coordination, by contrast, favors a more rigid and compact conformation, particularly in the channel’s vestibule, the funnel-shaped entry region that guides ions toward the pore. Lithium’s smaller radius and stronger electrostatic grip on its coordinating atoms essentially stiffen the protein around the binding site.

Those conformational changes propagate through the channel in ways that directly obstruct fluoride transport. The simulations showed that lithium binding perturbs the tilt angle of a pore-lining helix, one of the protein’s structural elements that forms part of the wall of the ion-conducting pathway. At the same time, it alters the rotamer, or side-chain orientation, of a key phenylalanine residue that juts into the pore. Phenylalanine’s bulky aromatic side chain acts as a gatekeeper within the permeation pathway, and when its orientation shifts, the effective opening of the channel narrows. Together, the helix tilt change and the phenylalanine rearrangement constrict the permeation pathway, explaining at the atomic level why lithium-bound channels cannot conduct fluoride even though the ion sits in the very site that normally activates transport.

The study thus delivers a complete mechanistic arc, from organism-level physiology down to individual atoms, and back again. It explains how a eukaryotic fluoride channel is regulated by monovalent cations, identifies the structural determinants of that regulation, and demonstrates that the regulation has real consequences for fluoride resistance in living cells. For biophysicists, the work adds to a growing appreciation that ion channels are not merely passive conduits but are exquisitely sensitive to the ionic composition of their environment, with different cations stabilizing distinct conformational ensembles of the same protein. For microbiologists and ecologists, it suggests that fluoride resistance in nature is not determined by fluoride alone but by the full ionic context in which organisms live, a factor that has been invisible until now.

There are also practical implications worth considering. Candida albicans, the organism whose channel was visualized structurally, is a major human fungal pathogen, and fluoride tolerance mechanisms have been proposed as contributors to fungal fitness in host environments and clinical settings. If lithium or related cations can compromise fluoride export in fungi, the interplay between environmental ion concentrations and microbial survival becomes a variable worth measuring in agricultural, industrial, and medical contexts where fluoride exposure is significant. Conversely, the finding that a simple, environmentally available ion can disable a resistance mechanism hints at new strategies for sensitizing fluoride-resistant organisms. As with any reconstituted and cell-based study, extending these results to more organisms and more complex environments will be the next step, but the core message is already clear: the battle over fluoride toxicity is fought not just with fluoride, but with the sodium and lithium ions that decide whether the cellular escape route stays open.

Subject of Research: Regulation of eukaryotic fluoride exporter channels by monovalent cations and its effect on fluoride resistance

Article Title: Monovalent cations differentially regulate the activity of eukaryotic fluoride channels and impact fluoride resistance

Article References: Kang, C.-Y., Heidari, S., Kolb, S., An, M., Ohi, M. D., Torabifard, H., & Stockbridge, R. B. (2026). Monovalent cations differentially regulate the activity of eukaryotic fluoride channels and impact fluoride resistance. PLOS Biology, 24(9), e3004008. https://doi.org/10.1371/journal.pbio.3004008

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004008

Keywords: fluoride exporter, FEX channels, lithium, sodium dependence, cryo-EM, molecular dynamics, yeast, Candida albicans, fluoride resistance, ion channels, competitive antagonism, PLOS Biology