Understanding Amt/Mep/Rh: transporting charge through a hydrophobic pore

Background:
Ammonium movement across biological membranes, a fundamental process in all living organisms, is mediated by the ubiquitous Amt/Mep/Rh family of membrane transporters. Ammonium is a vital nitrogen source for bacteria, fungi, and plants and a toxic metabolic waste for animals. The first genes encoding ammonium transporters were identified over 20 years ago in Saccharomyces cerevisiae (Mep) and Arabidopsis thaliana (Amt. In later years, it was shown that the rhesus protein (Rh) is an Amt/Mep ortholog in vertebrates and since then, members of the Amt/Mep/Rh protein family have been identified in almost all sequenced organisms. The physiological importance of this family is underlined by the role of Mep2 in filamentation, a dimorphic transition related to the virulence of pathogenic yeast. Despite 20 years of research, the exact mechanism of Amt/Mep/Rh remains elusive. The crystal structure of AmtB suggested that transport was electroneutral, however functional information demonstrates that transport is electrogenic.
Objectives:
To reconcile the confliciting functional and structural information and propose a mechansitic model for Amt/Mep/Rh mediated ammonium transport using the E. coli ammonium transporter AmtB.
Methods:
Molecular Dyanmic Simulations (MDS) were used to predict the impact of amino acid substitions on AmtB.
Solid-Supported Membrane Electrophysiology (SSME) was used to characterise the transport activity and selectivity of WT and variant AmtB.
Results:
MDS revealed the presence of two water wires within AmtB. SSME demonstrated that these wires are vital for AmtB-mediated transport. The findings suggest that AmtB functions as an NH3/H+ symporter.