Bacterial homologs of eukaryotic membrane proteins: the 2-TM-GxN family of Mg2+ transporters

Magnesium is essential for all forms of life. It is the cofactor for many enzymes and plays a key role in many biological processes. Thus, the acquisition of Mg2+ is crucial for cell survival. The best characterized Mg2+ transporters to date belong to the 2- TM- GxN type family of transporters. The name indicates the two C- terminal transmembrane ( TM) domains and a conserved GxN motif present in all members of this family towards the C- terminal end of TM1. In most members of the family, this conserved motif is generally YGMNF. The prototypical member of this family is CorA. Other characterized members of this family include Mrs2p, Alr, Mnr, AtMGT and ZntB. CorA is widely distributed throughout the prokaryotic world. It is the primary Mg2+ uptake system in most bacteria and many Archaea. A homolog, Mrs2p, is a eukaryotic mitochondrial Mg2+ channel. The Mrs2p related AtMGT transporters are found in plants and other eukaryotes. Alr1p and Mnr are Mg2+ transporters found in the plasma membrane of many fungi. ZntB is a bacterial member of the 2-TM- GxN family but mediates efflux of Zn2+ instead of influx of Mg2+. The recent crystal structure of a bacterial CorA shows that the structure of this family is unlike that of any other class of transporter or channel currently known.