The Amt/Mep/Rh family of ammonium transport proteins

The Amt/ Mep/ Rh family of integral membrane proteins comprises ammonium transporters of bacteria, archaea and eukarya, as well as the Rhesus proteins found in animals. They play a central role in the uptake of reduced nitrogen for biosynthetic purposes, in energy metabolism, or in renal excretion. Recent structural information on two prokaryotic Amt proteins has significantly contributed to our understanding of this class, but basic questions concerning the transport mechanism and the nature of the transported substrate, NH3 or NH4+, remain to be answered. Here we review functional and structural studies on Amt proteins and discuss the bioenergetic issues raised by the various mechanistic proposals present in the literature.