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

The research explores how proteins evolve to perform new functions without creating entirely novel structures. This principle is particularly evident in membrane transporters, which coordinate substrate selectivity, directionality, flux, and energy cost between the cell and its surroundings. A prominent example is the Amt/Mep/Rh superfamily, which controls the movement of reduced nitrogen across cellular membranes.

Despite sharing a highly conserved ammonium-conducting pore, proteins within this family have diverse physiological roles, such as nitrogen acquisition, sensing, and homeostatic control.

To investigate the transport mechanism of NeRh50, a bacterial Rhesus protein from the ammonia-oxidising bacterium Nitrosomonas europaea, the study employed targeted mutagenesis, electrophysiology, yeast complementation, and molecular dynamics simulations. The findings reveal that NeRh50 does not function merely as an ammonium importer like AmtB.

Instead, it utilizes the conserved Amt/Mep/Rh pore as a branched transport system where ammonium uptake and export-linked transport can be genetically and mechanistically separated. Two critical residues within the pore define this division of labor: one at the external entrance that links ammonium recruitment to productive inward uptake, and another deeper in the pore that facilitates a distinct transport mode for substrate release when intracellular nitrogen levels rise.

The study concludes that conserved landmarks within the Amt/Mep/Rh pore do not dictate a single mechanism. Rather, their local chemistry can be reassigned to generate different transport outputs, enabling NeRh50 to operate as a reversible nitrogen valve. This minimal retuning within an ancient membrane protein scaffold illustrates how small changes can significantly adapt nitrogen handling to meet ecological and physiological demands.