Novel Mg²⁺ binding sites in the cytoplasmic domain of the MgtE Mg²⁺ channels revealed by X-ray crystal structures

MgtE is a Mg2+-selective channel regulated by the intracellular Mg2+ concentration. MgtE family proteins are highly conserved in all domains of life and contribute to cellular Mg2+ homeostasis. In humans, mutations in the SLC41 proteins, the eukaryotic counterparts of the bacterial MgtE, are known to be associated with various diseases. The first MgtE structure from a thermophilic bacterium, Thermus thermophilus, revealed that MgtE forms a homodimer consisting of transmembrane and cytoplasmic domains with a plug helix connecting the two and that the cytoplasmic domain possesses multiple Mg2+ binding sites. Structural and electrophysiological analyses revealed that the dissociation of Mg2+ ions from the cytoplasmic domain induces structural changes in the cytoplasmic domain, leading to channel opening. Thus, previous works showed the importance of MgtE cytoplasmic Mg2+ binding sites. Nevertheless, due to the limited structural information on MgtE from different species, the conservation and diversity of the cytoplasmic Mg2+ binding site in MgtE family proteins remain unclear. Here, we report crystal structures of the Mg2+-bound MgtE cytoplasmic domains from two different bacterial species, Chryseobacterium hispalense and Clostridiales bacterium, and identify multiple Mg2+ binding sites, including ones that were not observed in the previous MgtE structure. These structures reveal the conservation and diversity of the cytoplasmic Mg2+ binding site in the MgtE family proteins.