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 Å 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⁺ ∼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 Å resolution), revealing hexahydrated Ca²⁺. These results provide mechanistic insights into the ion selectivity of MgtE for Mg²⁺ over Ca²⁺.