Mediates influx of magnesium ions. Alternates between open and closed states. Activated by low cytoplasmic Mg(2+) levels. Inactive when cytoplasmic Mg(2+) levels are high. May also mediate uptake of Co(2+). 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). By secretion stress. 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. 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. Belongs to the CorA metal ion transporter (MIT) (TC 1.A.35) family.