Transport of Ammonium Across Prokaryotic Cell Membranes
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Ammonium transport proteins are present in all domains of life. Recently, four ammonium transport protein (Amt) structures were resolved at high resolution: the AmtB from the eubacteria E. coli, the Amt-1 from the hyperthermophilic archaeon Archaeoglobus fulgidus, the Rh50 from Nitrosomonas europaea and the human Rhesus glycoprotein RhCG. All these proteins share a remarkable degree of structural homology, with eleven transmembrane helices per monomer. A narrow, non-continuous and hydrophobic pore, located at the center of each monomer of the trimeric Amt-1, is thought to be the substrate channel. At the extracellular channel cleft, ammonium selectivity and recruitment occur via a cation-7t interaction with the side chain of W137 that are stabilized by hydrogen bonds to the side chain of S208. The side chains of F96 and F204 block the channel continuity. Next to them the side chains of two highly conserved histidine residues (HI57 and H305) are bridged by an H-bond, with their edges pointing into the cavity. The role of these histidine residues is not clearly understood yet.
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