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

MgtE is a Mg -selective channel regulated by the intracellular Mg concentration. MgtE family proteins are highly conserved in all domains of life and contribute to cellular Mg 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, , 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 Mg binding sites. Structural and electrophysiological analyses revealed that the dissociation of Mg 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 Mg binding sites. Nevertheless, due to the limited structural information on MgtE from different species, the conservation and diversity of the cytoplasmic Mg binding site in MgtE family proteins remain unclear. Here, we report crystal structures of the Mg -bound MgtE cytoplasmic domains from two different bacterial species, and , and identify multiple Mg binding sites, including ones that were not observed in the previous MgtE structure. These structures reveal the conservation and diversity of the cytoplasmic Mg binding site in the MgtE family proteins.