title: Different transport mechanisms in plant and human AMT/Rh-type ammonium transporters.
abstract: The conserved family of AMT/Rh proteins facilitates ammonium transport across animal, plant, and microbial membranes. A bacterial homologue, AmtB, forms a channel-like structure and appears to function as an NH3 gas channel. To evaluate the function of eukaryotic homologues, the human RhCG glycoprotein and the tomato plant ammonium transporter LeAMT1;2 were expressed and compared in Xenopus oocytes and yeast. RhCG mediated the electroneutral transport of methylammonium (MeA), which saturated with Km = 3.8 mM at pHo 7.5. Uptake was strongly favored by increasing the pHo and was inhibited by ammonium. Ammonium induced rapid cytosolic alkalinization in RhCG-expressing oocytes. Additionally, RhCG expression was associated with an alkali-cation conductance, which was not significantly permeable to NH4+ and was apparently uncoupled from the ammonium transport. In contrast, expression of the homologous LeAMT1;2 induced pHo-independent MeA+ uptake and specific NH4+ and MeA+ currents that were distinct from endogenous currents. The different mechanisms of transport, including the RhCG-associated alkali-cation conductance, were verified by heterologous expression in appropriate yeast strains. Thus, homologous AMT/Rh-type proteins function in a distinct manner; while LeAMT1;2 carries specifically NH4+, or cotransports NH3/H+, RhCG mediates electroneutral NH3 transport.
authors.affiliation: Zentrum für Molekularbiologie der Pflanzen, Pflanzenphysiologie, Universität Tübingen, Tübingen, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Mayer
authors.forename: Gabriel
authors.initials: G
authors.lastname: Schaaf
authors.forename: Isabelle
authors.initials: I
authors.lastname: Mouro
authors.forename: Claude
authors.initials: C
authors.lastname: Lopez
authors.forename: Yves
authors.initials: Y
authors.lastname: Colin
authors.forename: Petra
authors.initials: P
authors.lastname: Neumann
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Cartron
authors.forename: Uwe
authors.initials: U
authors.lastname: Ludewig
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D027682:Cation Transport Proteins / Q000235:genetics / Q000502:physiology*; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D018551:Solanum lycopersicum; D008562:Membrane Glycoproteins / Q000235:genetics / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D008744:Methylamines / Q000378:metabolism; D009865:Oocytes / Q000502:physiology; D018408:Patch-Clamp Techniques; D010940:Plant Proteins / Q000235:genetics / Q000502:physiology*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012441:Saccharomyces cerevisiae; D014981:Xenopus
chemical_list: D027682:Cation Transport Proteins; D008562:Membrane Glycoproteins; D008744:Methylamines; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; C413213:RHCG protein, human; C088952:ammonium transporters, plant; D000641:Ammonia; C027451:methylamine