Uncoupling of Ionic Currents from Substrate Transport in the Plant Ammonium Transporter AtAMT1;2 [Plant Biology]

March 14th, 2014 by Neuhauser, B., Ludewig, U.

The ammonium flux across prokaryotic, plant and animal membranes is regulated by structurally related ammonium transporters (AMT) and/or related Rhesus (Rh) glycoproteins. Several plant AMT homologs, such as AtAMT1;2 from Arabidopsis, elicit ionic, ammonium-dependent currents when expressed in oocytes. By contrast, functional evidence for the transport of NH3 and the lack of coupled ionic currents has been provided for many Rh proteins. Furthermore, despite of high resolution structures the transported substrate in many bacterial homologs, such as AmtB from E. coli, is still unclear. In a heterologous genetic screen in yeast, AtAMT1;2 mutants with reduced transport activity were identified based on the resistance of yeast to the toxic transport analog methylamine. When expressed in oocytes, the reduced transport capacity was confirmed for either of the mutants Q67K, M72I and W145S. Structural alignments suggest that these mutations were dispersed at subunit contact sites of trimeric AMTs, without direct contact to the pore lumen. Surprisingly, and in contrast to the wild type AtAMT1;2 transporter, ionic currents were not associated with the substrate transport in these mutants. In combination with the evidence from other studies, these data may suggest that the wild type AtAMT1;2 functions as H+/NH3 co-transporter and that the strict substrate coupling with protons is lost by the mutations, is discussed.