Ammonium’s journey across cell membranes
Ammonia and the proton are transported separately
Bacteria, fungi and plants import ammonium as a major nitrogen source, while in animals this compound is best known for its role in pH homeostasis and the toxicity of its accumulation. Ammonium transport across cell membranes is achieved by a protein family called Mep-Amt-Rh, including human Rhesus antigens.
Ammonium (NH4+) is the substrate recognized by the proteins of this family, but it is the ammonia-neutral form (NH3) that, following a deprotonation step, passes through the hydrophobic transport pore. A study involving the Biology of Membrane Transport Laboratory and published in eLife highlights the particular fate of the proton (H+) released by the dissociation of ammonium. In collaboration with researchers from the University of Strathclyde and the University of Dundee, the ULB team shows that the AmtB ammonium transport protein from the intestinal bacteria E. coli is able to separately ensure the translocation of ammonia and the proton, the ion being carried out by two chains of water molecules. This sophisticated transport mechanism constitutes a parade to distinguish NH4+ molecules from the similar-sized cation, K+.
This study brings a significant advance in the understanding of the molecular mechanism of ammonium transport, a process whose alteration has potentially lethal adverse consequences.
Source and picture : Biology of Membrane Transport Laboratory (ULB)
"eLife Insisght" (en anglais) : Ammonium Transporters: A molecular dual carriageway
Williamson, G., Tamburrino, G., Bizior, A., Boeckstaens, M., Dias Mirandela, G., Bage, M., … Javelle, A. (2020). A two-lane mechanism for selective biological ammonium transport. ELife. https://doi.org/10.7554/eLife.57183