Mediates influx of magnesium ions. Can also mediate cobalt and manganese uptake (By similarity). Alternates between open and closed states. Activated by low cytoplasmic Mg(2+) levels. Inactive when cytoplasmic Mg(2+) levels are high (By similarity). Homopentamer. In the absence of Mg(2+), interactions between subunits are weakened, and dimers, trimers and tetramers can be observed in vitro (By similarity). 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. Belongs to the CorA metal ion transporter (MIT) (TC 1.A.35) family.