Ion transport versus gas conduction: function of AMT/Rh-type proteins.
Although lipid membranes exhibit some permeability to the weak base NH3, organisms have developed specialized proteins that increase and regulate the NH3 fluxes across cellular membranes. In humans, the Rh glycoproteins, such as the erythrocyte-specific RhAG and the liver and kidney homologs RhBG and RhCG, are involved in the passage of NH3. Rh glycoproteins have distant relatives, called ammonium transporters (AMTs), in archae and bacteria. The crystal structures of AMTs show that the proteins are homo-trimers and that the center of each monomer forms a pore. AMT/Rh proteins have also been identified in plants. In contrast to the human Rh glycoproteins, these AMTs specifically transport NH4+ or co-transport NH3/H+. Hence, they can transport against NH3 gradients. The molecular determinants for the different transport mechanisms within proteins of the same family are currently unclear. The functional differences between AMT/Rh transporters are likely to be an evolutionary adaptation to different ammonium and nitrogen requirements in bacteria, plants and animals.
Zentrum für Molekularbiologie der Pflanzen (ZMBP), Pflanzenphysiologie, Universität Tübingen, Auf der Morgenstelle 1, D-72076 Tübingen, Germany. uwe.ludewig@zmbp.uni-tuebingen.de
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D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D001798:Blood Proteins / Q000737:chemistry / Q000502:physiology*; D002021:Buffers; D002245:Carbon Dioxide / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry; D027682:Cation Transport Proteins / Q000737:chemistry / Q000502:physiology*; D006023:Glycoproteins / Q000737:chemistry / Q000502:physiology*; D006801:Humans; D007473:Ion Channels / Q000737:chemistry; D017136:Ion Transport; D007668:Kidney / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000502:physiology*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000502:physiology*; D051379:Mice; D008958:Models, Molecular; D010940:Plant Proteins / Q000737:chemistry / Q000502:physiology; D011487:Protein Conformation; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D051381:Rats; D013329:Structure-Activity Relationship
D001798:Blood Proteins; D002021:Buffers; D002352:Carrier Proteins; D027682:Cation Transport Proteins; D006023:Glycoproteins; D007473:Ion Channels; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; C498523:RHAG protein, human; C422995:RHBG protein, human; C413213:RHCG protein, human; C508142:Rh type B glycoprotein, rat; C508143:Rh type C glycoprotein, rat; C486033:RhBG protein, mouse; C401210:Rhag protein, mouse; C498524:Rhag protein, rat; C485880:Rhcg protein, mouse; C088952:ammonium transporters, plant; D002245:Carbon Dioxide; D000641:Ammonia