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 constitutively 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 bears no resemblance to other known proteins. The MgtE transport system can also mediate Mg2+ uptake, but whether this is its primary function is not known. MgtE also lacks homology to other known proteins. In contrast, the MgtA and MgtB Mg2+ transport systems of enteric bacteria are P‐type ATPases by sequence homology, mediating Mg2+ influx with, rather than against, the Mg2+ electrochemical gradient. They are closely related to mammalian Ca2+‐ATPases. Expression of MgtA and MgtB is under the control of the PhoPQ two‐component regulatory system, important 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 cl