The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.