Ammonium ion transport by the AMT/Rh homolog TaAMT1;1 is stimulated by acidic pH.
It is unclear how ammonia is transported by proteins from the Amt/Mep/Rh superfamily. We investigated this for the ammonium transporter TaAMT1;1 from wheat expressed in Xenopus oocytes by two-electrode voltage clamp and radio-labeled uptakes. Inward currents were activated by NH (4) (+) or methylammonium ions (MeA(+)). Importantly, currents increased fivefold when the external pH was decreased from 7.4 to 5.5; this type of pH dependence is unique and is a strong indication of NH (4) (+) or MeA(+) transport. This was confirmed by the close correlation between the uptake of radio-labeled MeA(+) and MeA(+)-induced currents. Homology models of members of the Amt/Mep/Rh superfamily exhibited major divergences in their cytoplasmic regions. A point mutation in this region of TaAMT1;1 abolished the pH sensitivity and decreased the apparent affinities for NH (4) (+) and MeA(+). We suggest a model where NH (4) (+) is transported as NH(3) and H(+) via separate pathways but the latter two recombine before leaving the protein.
Søgaard
Rikke
R
Nordic Centre for Water Imbalance Related Disorders, Department of Cellular and Molecular Medicine, Blegdamsvej 3C, University of Copenhagen, Copenhagen, 2200 N, Denmark.
Alsterfjord
Magnus
M
Macaulay
Nanna
N
Zeuthen
Thomas
T