Mediates influx of magnesium ions (PubMed:18276588, PubMed:22722933, PubMed:23781956, PubMed:23112165). Mediates Co(2+) uptake (PubMed:21454699, PubMed:23091000, PubMed:23425532). Has high selectivity for Co(2+) (PubMed:21454699, PubMed:23425532). Alternates between open and closed states. Activated by low cytoplasmic Mg(2+) levels. Inactive when cytoplasmic Mg(2+) levels are high (PubMed:23112165, PubMed:23425532, PubMed:26871634). Inhibited by cobalt hexaammine. Homopentamer (PubMed:22722933, PubMed:23781956, PubMed:16902408, PubMed:16598263, PubMed:16857941, PubMed:23112165, PubMed:23425532, PubMed:26871634). In the absence of Mg(2+), interactions between subunits are weakened, and dimers, trimers and tetramers can be observed in vitro (PubMed:22722933, PubMed:26871634). 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, PubMed:16902408, PubMed:16598263, PubMed:16857941, PubMed:23112165, PubMed:23425532, PubMed:26871634). Co(2+) binding also induces a conformation change (PubMed:21454699). 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 (PubMed:23112165, PubMed:26871634). Belongs to the CorA metal ion transporter (MIT) (TC 1.A.35) family.