Transport mechanisms in the ammonium transporter family

Ammonium transport is mediated by membrane proteins of the ubiquitous Amt/Rh family Despite the availability of different X-ray structures that provide many insights on the ammonium permeation process. the molecular details of its mechanism remain controversial The X-ray structures have revealed that the pore of the Amt and Rh proteins is characterized by a hydrophobic portion about 12 A long in which electronic density was observed in crystallographic study of AmtB from Escherichia coli. This electronic density was initially only observed when crystals were grown in presence of ammonium salt and was thus attributed to ammonia (NH3) molecules, and lead the authors to suggest that the conduction mechanism in the Amt/Rh proteins involves the single-file diffusion of NH3 molecules However, other X-ray crystallography results and molecular mechanics simulations suggest that the pore of AmtB could also he filled with water molecules The possible presence of water molecules in the pore lumen calls for a reassessment of the growing consensus that Amt/Rh proteins work as plain NH3 channels Indeed, functional experiments on plant ammonium transporters and rhesus proteins suggest a variety of permeation mechanisms including the passive diffusion of NH3 the ant port of NH4+/H+, the transport of NH4+, or the cotransport of NH3/H+ We discuss these mechanisms in light of some recent functional and simulation studies on the Ann B transporter and illustrate how they can be reconciled with the available high resolution X-ray data (C) 2010 Elsevier Masson SAS All rights reserved