Magnesium transport in prokaryotes
Possessing the largest hydrated radius, the smallest ionic radius, and the highest charge density among the biologically relevant cations, Mg2+ provides an interesting problem for transport into living cells. Transport systems for Mg2+ have been characterized primarily in Salmonella typhimurium because the well-developed genetics of Gram-negative bacteria make cloning and studying the transporters a viable proposition. The CorA transport system is expressed constitutively and is the major Mg2+ transporter in Eubacteria and Archaea. It has three transmembrane domains, a uniquely large periplasmic domain, and no sequence homology to other proteins. The MgtE Mg2+ transporter also lacks sequence homology to other proteins, and it is unclear if Mg2+ transport is its primary function. The MgtA and MgtB Mg2+ transporters have sequence homology to P-type ATPases. They are more closely related to the mammalian Ca2+–ATPases than to the prokaryotic P-type ATPases. MgtA and MgtB mediate Mg2+ influx with, rather than against, the Mg2+ electrochemical gradient. Unlike corA and mgtE, the mgtA and mgtC/mgtB loci are regulated, being induced by the two-component regulatory system PhoP/PhoQ. PhoQ is a Mg2+ membrane sensor kinase that phosphorylates the transcription factor PhoP under Mg2+–limiting conditions. This factor then induces transcription of mgtA and mgtCB. MgtC, which is encoded by the first gene in the mgtCB operon, has no sequence homology to any known protein and is essential for S.typhimurium virulence in mice and macrophages, but does appear to be a Mg2+ transporter. The physiological roles of these Mg2+ transporters and their mechanisms are not yet completely clear, but initial data indicate that Mg2+ transporters are unique transport systems with unusual mechanisms for mediating Mg2+ movement through the membrane.