Ion selectivity mechanism of the MgtE channel for Mg2+ over Ca2+

MgtE is a Mg2+-selective ion channel whose orthologs are widely distributed from prokaryotes to eukaryotes, including humans, and are important participants in the maintenance of cellular Mg2+ homeostasis. The previous high-resolution structure determination of the MgtE transmembrane (TM) domain in complex with Mg2+ ions revealed a recognition mechanism of MgtE for Mg2+ ions. In contrast, the previous Ca2+-bound structure of the MgtE TM domain was determined only at moderate resolution (3.2 angstrom resolution), which was insufficient to visualize the water molecules coordinated to Ca2+ ions. Here, we showed that the metal-binding site of the MgtE TM domain binds to Mg2+ similar to 500-fold more strongly than to Ca2+. We then determined the crystal structure of the MgtE TM domain in complex with Ca2+ ions at a higher resolution (2.5 angstrom resolution), revealing hexahydrated Ca2+. These results provide mechanistic insights into the ion selectivity of MgtE for Mg2+ over Ca2+.