MICROBIAL MAGNESIUM TRANSPORT - UNUSUAL TRANSPORTERS SEARCHING FOR IDENTITY

Mg2+ is unique among biological cations because of its charge density and solution chemistry. This is abundantly reflected in its transport systems, studied primarily in Salmonella typhimurium, The constitutive ly expressed CorA transport system is the primary Mg2+ influx pathway for the Bacteria and the Archaea, Its structure of a large N-terminal soluble periplasmic domain with three transmembrane segments at the C- terminus is unique among membrane carriers, and its protein sequence b ears no resemblance to other known proteins. The MgtE transport system can also mediate Mg2+ uptake, but whether this is its primary functio n is not known. MgtE also lacks homology to other known proteins. In c ontrast, the MgtA and MgtB Mg2+ transport systems of enteric bacteria are P-type ATPases by sequence homology, mediating Mg2+ influx with, r ather than against, the Mg2+ electrochemical gradient. They are closel y related to mammalian Ca2+-ATPases. Expression of MgtA and MgtB is un der the control of the PhoPQ two-component regulatory system, importan t in bacterial virulence. In S, typhimurium, MgtB is encoded by a two- gene operon mgtCB; the function of the MgtC protein is unknown, and it lacks close homologues. The ligand for the phoQ membrane sensor kinas e is Mg2+ and, at decreased extracellular Mg2+ concentrations, transcr iption of mgtA and mgtCB are enormously induced. All three genes are a lso induced upon S, typhimurium invasion of epithelial or macrophage c ells. Mutation of these genes has no effect on invasion efficiency, bu t an insertion in mgtC renders S, typhimurium essentially avirulent in the mouse. The physiological roles of the known Mg2+ transport system s are not yet completely defined, Nonetheless, the singular sequence a nd apparent structure of the CorA and MgtE transport proteins, the com plex regulation of MgtA, MgtB and MgtC and their involvement in pathog enesis suggests that further study will be rewarding.