P-type ATPase magnesium transporter MgtA acts as a dimer.

Magnesium (Mg) uptake systems are present in all domains of life, consistent with the vital role of this ion. P-type ATPase Mg importers are required for bacterial growth when Mg is limiting or during pathogenesis. However, insights into their mechanisms of action are missing. Here we solved the cryo-EM structure of the Mg transporter MgtA from Escherichia coli. We obtained high-resolution structures of both homodimeric (2.9 Å) and monomeric (3.6 Å) forms. The dimer structure is formed by multiple contacts between residues in adjacent soluble N and P subdomains. Our structures revealed an ion, assigned as Mg, in the transmembrane segment. Moreover, we detected two cytoplasmic ion-binding sites and determined the structure of the N-terminal tail. Sequence conservation, mutagenesis and ATPase assays indicate dimerization, the ion-binding sites and the N-terminal tail facilitate cation transport or serve regulatory roles.