Mediates both influx and efflux of magnesium ions (PubMed:3536881, PubMed:2548998, PubMed:2670893, PubMed:9786860, PubMed:10601252, PubMed:10748031). Can also mediate cobalt and nickel uptake, albeit only at extracellular concentrations that are toxic to the cell (PubMed:3536881, PubMed:2670893). Does not transport iron (PubMed:15516579). Alternates between open and closed states. Activated by low cytoplasmic Mg(2+) levels. Inactive when cytoplasmic Mg(2+) levels are high (By similarity). Inhibited by cation hexaammines. Homopentamer. In the absence of Mg(2+), interactions between subunits are weakened, and dimers, trimers and tetramers can be observed in vitro (By similarity). Homotetramer (PubMed:15231793). Constitutively expressed. The central ion permeation pathway is formed by the first transmembrane domain from each of the five subunits. Mg(2+) binding strengthens interactions between subunits and leads to the formation of a symmetrical homopentamer surrounding a closed ion permeation pathway. Co(2+) binding also induces a conformation change. Low Mg(2+) concentrations trigger both a conformation change within each subunit and a loosening of the interactions between subunits. This results in an open ion conduction pathway. In addition, this results in a less symmetrical shape of the whole complex. Is the dominant magnesium transport system under laboratory growth conditions. Belongs to the CorA metal ion transporter (MIT) (TC 1.A.35) family.