Biology subjects

Bizior, A.

Publications and source records attributed to Bizior, A..

4 recordsLinked to original sources

bioRxiv2026

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

How conserved proteins acquire new physiological functions is a central question in molecular evolution. Rather than inventing new architectures, evolution often repurposes existing scaffolds, preserving core structural features while retuning the molecular logic that connects mechanism to physiology. Membrane transporters offer a powerful test of this principle because substrate selectivity, directionality, flux and energetic cost must be coordinated at the interface between the cell and its environment. The Amt/Mep/Rh superfamily controls one of the most fundamental requirements of cellular life: the movement of reduced nitrogen across biological membranes. Despite sharing a highly conserved ammonium-conducting pore, these proteins support distinct physiological roles, including nitrogen acquisition, sensing and homeostatic control. Here, we combined targeted mutagenesis, electrophysiology, yeast complementation and molecular dynamics simulations to define the transport logic of NeRh50, a bacterial Rhesus protein from the ammonia-oxidising bacterium Nitrosomonas europaea. We show that NeRh50 is not simply an AmtB-like ammonium importer. Instead, it uses the conserved Amt/Mep/Rh pore as a branched transport system in which inward uptake and export-linked transport can be separated genetically and mechanistically. Two conserved pore residues define this division of labour. A residue at the external entrance couples ammonium recruitment to productive inward uptake, whereas a second residue deeper in the pore enables a distinct transport mode required for substrate release when intracellular nitrogen accumulates. Thus, conserved Amt/Mep/Rh pore landmarks do not impose a single mechanism. Their local chemistry can be reassigned to generate different transport outputs, allowing NeRh50 to function as a reversible nitrogen valve. These findings reveal how minimal retuning within an ancient membrane-protein scaffold can rewire transport directionality and adapt nitrogen handling to ecological and physiological demand.

biochemistry↗

bioRxiv2026

Lipid-associated architecture and divergent ammonium transport distinguish a bacterial Rhesus protein

Nitrification is a major process in the global nitrogen cycle, initiated by the oxidation of ammonia to nitrite. The ammonia-oxidising bacterium Nitrosomonas europaea catalyses this first and rate-limiting step and depends on ammonium both as an energy substrate and as a nitrogen source, making ammonium acquisition central to its physiology. Cellular ammonium transport is mediated by the Amt/Mep/Rh superfamily, whose members share a conserved fold despite divergent physiological roles. Remarkably, N. europaea lacks a canonical prokaryotic Amt transporter and instead expresses NeRh50, a Rh-family protein proposed to have been acquired from a eukaryotic lineage. Here, we determine a cryo-EM structure of NeRh50 that reveals tightly associated lipids at the monomer-monomer interfaces, including density consistent with an unusual inverted lipid orientation. Native mass spectrometry and molecular dynamics simulations further support persistent lipid association at these interfaces. NeRh50 also differs functionally from canonical AmtB, selectively mediating electrogenic ammonium transport with ~70-fold lower apparent affinity. Transport is enhanced under alkaline conditions but not by an imposed proton gradient, suggesting membrane-potential dependence. Together, these findings reveal divergence from canonical Amt proteins in both membrane-associated architecture and transport properties, providing insight into the functional and evolutionary diversification of the Amt/Mep/Rh superfamily.

biochemistry↗

bioRxiv2021

A mechanistic switch from selective transporter to an ion channel impairs the filamentation signalling capability of ammonium transceptors in yeast

Ammonium translocation through biological membranes by the ubiquitous Amt-Mep-Rh family of transporters plays a key role in all domains of life. Two highly conserved histidine residues protrude into the lumen of these transporters, forming the familys characteristic Twin-His motif. It has been hypothesized that the motif is essential to confer the selectivity of the transport mechanism. Here, using a combination of in vitro electrophysiology, in vivo yeast functional complementation and in silico molecular dynamics simulations, we demonstrate that variations in the Twin-His motif trigger a mechanistic switch between a specific transporter, depending on ammonium deprotonation, to an unspecific ion channel activity. We therefore propose that there is no selective filter that governs the specificity in Amt-Mep transporters but the inherent mechanism of translocation, dependent on the fragmentation of the substrate, ensures the high specificity of the translocation. We further show that both mechanisms coexist in fungal Mep2 Twin-His variants, disrupting the transceptor function and so inhibiting the filamentation process. These data strongly support a transport mechanism-mediated signalling process in the long-standing debate on the sensory function of Mep2-like transporters.

microbiology↗

bioRxiv2019

A two-lane mechanism for selective biological ammonium transport

The transport of charged molecules across biological membranes faces the dual problem of accommodating charges in a highly hydrophobic environment while maintaining selective substrate translocation. A particular controversy has existed around the mechanism of ammonium exchange by the ubiquitous Amt/Mep/Rh transporter family, an essential process in all kingdoms of life. Here, using a combination of electrophysiology, yeast functional complementation and extended molecular dynamics simulations, we reveal a unique two-lane pathway for electrogenic NH4+ transport in two archetypal members of the family. The pathway underpins a mechanism by which charged H+ and neutral NH3 are carried separately across the membrane after NH4+ deprotonation. This mechanism defines a new principle of achieving transport selectivity against competing ions in a biological transport process.

biochemistry↗