Regulation and function of ammonium carriers in bacteria, fungi, and plants

The ammonium transport (Amt) family of proteins comprises a unique and ubiquitous group of integral membrane proteins found in all domains of life. They are present in bacteria, archaea, fungi, plants, and animals, including humans where they are represented by the Rhesus proteins. The Amt proteins have a variety of functions. In bacteria and fungi, they act to scavenge ammonium and to recapture ammonium lost from cells by diffusion across the cell membrane. In fungi, they have also been proposed to act as ammonium sensors to control filamentous growth. In plants, they make a major contribution to nitrogen nutrition and in higher animals; they are involved in ammonium fluxes in kidney and liver. In this paper, we review current knowledge of the biology of Amt proteins in bacteria, fungi, and plants with particular attention to the different functions of the proteins and their modes of regulation.