Mediates influx of magnesium ions (PubMed:9573171, PubMed:10748031). Alternates between open and closed states. Activated by low cytoplasmic Mg(2+) levels. Inactive when cytoplasmic Mg(2+) levels are high. Can also mediate Co(2+) uptake (By similarity). Inhibited by cation hexaammines. Homopentamer (PubMed:23091000, PubMed:26051127). In the absence of Mg(2+), interactions between subunits are weakened, and dimers, trimers and tetramers can be observed in vitro (By similarity). Was initially proposed to be a homotetramer, based on the cross-linking studies (PubMed:15231793). This is in contradiction with current 3D-structures that clearly show it is a homopentamer (PubMed:23091000, PubMed:26051127). 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 (PubMed:23091000). Low cytoplasmic 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 (PubMed:26051127). Belongs to the CorA metal ion transporter (MIT) (TC 1.A.35) family.