A bacterial Rhesus transporter retunes a structurally conserved ammonium pore into a reversible nitrogen valve

Life on Earth depends on nitrogen, a fundamental building block for the proteins and DNA that make up every cell. While the air we breathe is full of nitrogen gas, most organisms cannot use it directly; they must harvest it in a simpler, "reduced" form, often as ammonium. To survive, cells need to move this ammonium across their protective outer membranes, but they must do so with extreme care. Ammonium is essential for growth, yet in high concentrations, it becomes toxic, disrupting the delicate chemical balance inside the cell. Nature has solved this problem with a family of molecular machines called transporters. These proteins act as gatekeepers, embedded in the cell membrane, allowing nitrogen to pass through while keeping the cell safe. For decades, scientists believed these machines worked in a single, predictable way: they grabbed ammonium from the outside, stripped it of a hydrogen atom to turn it into a gas, and then pushed the gas through a dry, narrow tunnel into the cell, where it was reassembled. This process was thought to be a one-way street, strictly designed for importing nutrients.

However, a new study challenges this rigid view by looking at a specific bacterial protein called NeRh50, found in a microbe that thrives on ammonia. Researchers discovered that this protein does not just follow the standard one-way import rule. Instead, it uses the same basic structural blueprint as the well-known importers but has been "retuned" to function as a reversible valve. It can pull ammonium in when the cell needs it, but it can also push it out when the cell has too much, acting as a safety release. This finding suggests that evolution does not always need to invent entirely new machines to solve new problems; sometimes, it simply tweaks the settings on an existing one to change its direction and purpose.

The researchers, working with a team from the University of Strathclyde and the University of Oxford, set out to understand how NeRh50 manages this dual behavior. They focused on two specific spots within the protein's central tunnel, or pore, where the ammonium travels. One spot is located at the entrance, and the other is deeper inside, near a pair of histidine molecules that act as a checkpoint. To see what these spots do, the scientists created mutant versions of the protein, essentially swapping out the amino acids at these critical locations for others, and then watched how the protein behaved. They used a technique that allowed them to measure tiny electrical currents generated as ammonium moved through the protein, observing how the flow changed when they altered the chemical environment or the protein's structure.

What they found was a clear division of labor between the two spots. The entrance spot, controlled by a specific amino acid, acts as the gatekeeper for bringing ammonium into the cell. When the researchers disabled this spot, the protein could no longer pull ammonium in effectively, even though the rest of the machine was intact. However, this disabled protein could still push ammonium out. Conversely, the deeper spot, controlled by the histidine pair, was essential for the protein's ability to act as a safety valve. When this spot was altered, the protein could still pull ammonium in, but it lost its ability to push it out or protect the cell from toxic levels of ammonium. This proved that the two functions—importing and exporting—are not locked together; they can be separated and controlled independently within the same molecular structure.

To understand the mechanics of this separation, the team ran detailed computer simulations that tracked the movement of ammonium ions through the protein's tunnel. They discovered that the standard model of ammonium transport, which relies on a dry, dehydrated tunnel that forces the ammonium to lose a hydrogen atom before passing through, only explained part of the story. In the standard import mode, the protein behaves like the classic models, using a water-based pathway to move protons. But in the export or protective mode, the protein behaves differently. The simulations showed that the tunnel in NeRh50 is slightly more open and flexible than in the strict importers. This slight looseness allows the charged ammonium ion to pass through without being forced to lose its hydrogen atom first, provided there is an electrical push from the cell. This "D2O-resistant" pathway, which does not rely on the same water-wire mechanism as the standard importer, is what enables the protein to reverse direction and act as a valve.

The study also looked at how these proteins behave in living yeast cells, which served as a testbed for the bacteria's machinery. When the yeast cells were exposed to a toxic chemical similar to ammonium, the normal NeRh50 protein protected them by pumping the toxin out. But the mutant versions that had lost the ability to export failed to protect the cells, even though they could still import nutrients. This confirmed that the export function is a distinct capability, not just a side effect of the import process. The researchers concluded that the NeRh50 protein is not merely a defective importer or a simple copy of the standard ammonium transporter. It is a sophisticated, dual-purpose machine that has evolved to handle the specific needs of its host organism, which lives in an environment rich in ammonia.

This discovery reshapes how we understand the evolution of these essential proteins. The Amt/Mep/Rh superfamily, which includes NeRh50 and many other transporters across bacteria, fungi, and humans, was previously thought to be a collection of machines with fixed roles. Some were seen as strict importers, while others were thought to be sensors or bidirectional channels. This paper shows that the difference is not in the overall architecture, which remains remarkably conserved, but in the fine-tuning of a few key chemical residues. By changing just a couple of amino acids, evolution can switch a protein from a one-way intake valve to a reversible safety valve, or even uncouple the two functions entirely. It is a demonstration of how nature repurposes ancient, reliable structures to meet new physiological demands, turning a simple tunnel into a dynamic, two-way control system that keeps the cell alive in a changing world.