Magnesium transport protein, CorA.  The structure at 2.7 Å resolution is known. The CorA monomer has a C-terminal membrane domain containing two transmembrane segments and a large N-terminal cytoplasmic soluble domain. In the membrane, CorA forms a homopentamer shaped like a funnel which binds fully hydrated Mg²⁺ in the periplasm (Maguire 2006). A ring of positive charges are external to the ion-conduction pathway at the cytosolic membrane interface, and highly negatively charged helices in the cytosolic domain  appear to interact with the ring of positive charge to facilitate Mg²⁺ entry. Mg²⁺ ions are present in the cytosolic domain that are well placed to control the interaction of the ring of positive charge and the negatively charged helices, and thus, control Mg²⁺ entry (Maguire 2006).  Gating is achieved by helical rotation upon the binding of a metal ion substrate to the regulatory binding sites. The preference for Co²⁺ over Mg²⁺ has been reported to be determined by the presence of threonine side chains in the channel (Nordin et al. 2013), but more recently, Kowatz and Maguire 2018 showed that Co²⁺ is not a substrate, and that the intersubunit bound Mg²⁺ is not required for function and does not control the open versus closed states.