Personal statement

Nitrogen Transport, Sensing and Evolution

My laboratory investigates how microorganisms sense, transport and respond to nitrogen. Nitrogen is essential for life, yet cells must constantly adapt to large changes in its availability and chemical form. We use ammonium transport proteins as a model system to understand the molecular mechanisms that allow cells to detect environmental nutrients and regulate their uptake.

A central question driving our research is how evolution can modify a highly conserved molecular architecture to generate distinct transport mechanisms and biological functions.

Our work initially focused on the bacterial ammonium transporter AmtB and its regulation by the nitrogen-signalling protein GlnK. This established how ammonium transport is integrated into the broader cellular response to nitrogen availability and led us to investigate the transport mechanism itself.

Although Amt/Mep/Rh proteins were traditionally described as passive ammonia channels, our electrophysiological studies demonstrated that transport by AmtB is electrogenic. Together with structural, biochemical and computational approaches, this led us to propose a two-lane mechanism in which NH₄⁺ is recognised and deprotonated within the transporter before NH₃ and H⁺ move through physically distinct pathways across the membrane. These findings challenged the classical NH₃-channel model and established the conserved Twin-His motif at the centre of the transport mechanism.

We are now investigating how this conserved molecular framework has diversified across the Amt/Mep/Rh superfamily. In particular, our work on the bacterial Rhesus protein NeRh50 shows that proteins sharing the same overall structural scaffold can nevertheless display markedly different transport properties. By combining cryo-electron microscopy, electrophysiology, genetics, native mass spectrometry, molecular dynamics simulations and phylogenetic analysis, we are linking changes in protein sequence and membrane-associated architecture to changes in transport mechanism and biological function.

This work is also revealing an important role for the surrounding membrane. Specific lipid interactions can alter transporter architecture and activity, suggesting that the membrane is not simply a passive environment but an integral component of transporter function.

Our broader goal is to understand how ammonium transport and sensing evolved from a common ancestral molecular framework. We are extending our studies across the diversity of the Amt/Mep/Rh family, including canonical transporters, Rhesus proteins and ammonium-sensing proteins fused to intracellular signalling domains.

Ultimately, we aim to move from describing individual transport proteins towards a predictive understanding of function: determining how sequence, structure, membrane environment and evolutionary history combine to define what a transporter does. This should allow us not only to reconstruct how nitrogen sensing and transport evolved, but eventually to use those principles to design new nitrogen-responsive molecular systems.

Selected references

1. Williamson, G. et al. A two-lane mechanism for selective biological ammonium transport. eLife 9, e57183 (2020). doi: 10.7554/eLife.57183.
2. Williamson, G. et al. Coexistence of ammonium transporter and channel mechanisms in Amt-Mep-Rh twin-His variants impairs the filamentation signaling capacity of fungal Mep2 transceptors. mBio 13, e02913-21 (2022). doi: 10.1128/mbio.02913-21.
3. Bizior, A., Williamson, G., Harris, T., Hoskisson, P. A. & Javelle, A. Prokaryotic ammonium transporters: what has three decades of research revealed? Microbiology 169, 001360 (2023). doi: 10.1099/mic.0.001360.
4. Williamson, G., Bizior, A., Harris, T., Pritchard, L., Hoskisson, P. A. & Javelle, A. Biological ammonium transporters from the Amt/Mep/Rh superfamily: mechanism, energetics, and technical limitations. Bioscience Reports 44, BSR20211209 (2024). doi: 10.1042/BSR20211209
5. Williamson, G., Harris, T., Bizior, A., Hoskisson, P. A., Pritchard, L. & Javelle, A. Biological ammonium transporters: evolution and diversification. FEBS Journal 291, 3786–3810 (2024). doi: 10.1111/febs.17059.
6. Maeda, K., Kurata, H., Javelle, A., Westerhoff, H. V. & Boogerd, F. C. Computer experimentation on coli ammonium transport and assimilation reveals mechanisms for energy coupling, balanced futile cycling, and robust growth. Preprint at doi: 10.64898/2026.05.09.723968 (2026).

Key collaborations

Systems biology and kinetic modelling of ammonium transport, Professor Hans V. Westerhoff, Amsterdam, Dr Fred Boogerd, VU Amsterdam, and Professor Toshinari Maeda, Kyushu Institute of Technology. This collaboration combines my expertise in ammonium transport and membrane-protein mechanism with systems biology and kinetic modelling, allowing us to connect transporter biochemistry to cellular physiology. A manuscript from this collaboration is under revision in npj Systems Biology and Applications.

Molecular dynamics of Amt/Mep/Rh transport mechanisms, Professor Ulrich Zachariae, University of Dundee (UK) and Prof Syma Khalid, Universality of Oxford (UK). This collaboration provides atomistic molecular dynamics expertise to model proton transfer, pore hydration, substrate movement and conformational dynamics in ammonium transporters.

Structural biology of bacterial ammonium transporters, Dr Georgia Isom, University of Oxford (UK). This collaboration uses Cryo-EM to obtain structural insight into bacterial Amt/Rh proteins and to link transporter architecture to molecular mechanism.

Native mass spectrometry and lipid-protein interactions, Professor Dame Carol Robinson, University of Oxford (UK). This collaboration investigates how native membrane-protein complexes and lipid interactions influence transporter activity and stability.

Evolution and functional diversification of Amt/Mep/Rh proteins, Dr Leighton Pritchard and Professor Paul A. Hoskisson, University of Strathclyde (UK). This collaboration combines comparative genomics, phylogenetics and microbiology to understand how conserved ammonium transporter architectures diversified across evolution.

Evolution of ammonium transport, LUCA and early nitrogen metabolism, Dr Rika Anderson, Carleton College (USA), Dr Joanne Boden, University of Bristol (UK), and Dr Eva Stüeken, University of  St Andrews (UK). We combine membrane transport, microbial evolution, phylogenomics and early Earth geochemistry. We are developing a synthesis on LUCA relying on environmental ammonium assimilation rather than N₂.