The Journal of General Physiology
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Published online Jan 30 2006. doi:10.1085/jgp.200509369
The Rockefeller University Press, 0022-1295 $8.00
JGP, Volume 127, Number 2, 133-144
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*Compound via MeSH
*Substance via MeSH
Hazardous Substances DB
*AMMONIA
*METHYLAMINE
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ARTICLE

 Different Transport Mechanisms in Plant and Human AMT/Rh-type Ammonium Transporters

Maria Mayer1, Gabriel Schaaf1, Isabelle Mouro2, Claude Lopez2, Yves Colin2, Petra Neumann1, Jean-Pierre Cartron2, and Uwe Ludewig1

1 Zentrum für Molekularbiologie der Pflanzen, Pflanzenphysiologie, Universität Tübingen, D-72076 Tübingen, Germany
2 INSERM U76, INTS, 75015 Paris, France

Correspondence to Uwe Ludewig: uwe.ludewig{at}zmbp.uni-tuebingen.de

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.

Abbreviations used in this paper: BCECF, 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein; MeA, methylammonium; MES, 2-[N- morpholino]ethanesulfonic acid.


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