Crystal structure of the MgtE Mg2+ transporter

Abstract

The magnesium ion Mg2+ is a vital element involved in numerous physiological processes. Mg2+ has the largest hydrated radius among all cations, whereas its ionic radius is the smallest. It remains obscure how Mg2+ transporters selectively recognize and dehydrate the large, fully hydrated Mg2+ cation for transport. Recently the crystal structures of the CorA Mg2+ transporter were reported. The MgtE family of Mg2+ transporters is ubiquitously distributed in all phylogenetic domains, and human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. However, the MgtE transporters have not been thoroughly characterized. Here we determine the crystal structures of the full-length Thermus thermophilus MgtE at 3.5 A resolution, and of the cytosolic domain in the presence and absence of Mg2+ at 2.3 A and 3.9 A resolutions, respectively. The transporter adopts a homodimeric architecture, consisting of the carboxy-terminal five transmembrane domains and the amino-terminal cytosolic domains, which are composed of the superhelical N domain and tandemly repeated cystathionine-beta-synthase domains. A solvent-accessible pore nearly traverses the transmembrane domains, with one potential Mg2+ bound to the conserved Asp 432 within the pore. The transmembrane (TM)5 helices from both subunits close the pore through interactions with the 'connecting helices', which connect the cystathionine-beta-synthase and transmembrane domains. Four putative Mg2+ ions are bound at the interface between the connecting helices and the other domains, and this may lock the closed conformation of the pore. A structural comparison of the two states of the cytosolic domains showed the Mg2+-dependent movement of the connecting helices, which might reorganize the transmembrane helices to open the pore. These findings suggest a homeostasis mechanism, in which Mg2+ bound between cytosolic domains regulates Mg2+ flux by sensing the intracellular Mg2+ concentration. Whether this presumed regulation controls gating of an ion channel or opening of a secondary active transporter remains to be determined.

Role of CBS and Bateman Domains in Phosphorylation-Dependent Regulation of a CLC Anion Channel

“…Crystal structures of four unrelated proteins have demonstrated that ligand interactions induce relatively large-scale changes in Bateman domain dimer conformations (22,(44)(45)(46). For example, Mg 2þ binding to the Bateman domain dimer of the bacterial channel Mg 2þ MgtE induces an open-to-closed conformational change by shielding the charge-charge repulsion of acidic amino acid residues (22,45,47).…”

“…For example, Mg 2þ binding to the Bateman domain dimer of the bacterial channel Mg 2þ MgtE induces an open-to-closed conformational change by shielding the charge-charge repulsion of acidic amino acid residues (22,45,47). By analogy with MgtE, binding and unbinding Values are means 5 SE (n).…”

“…Bateman domains interact with diverse regulatory ligands, including adenosine nucleosides (18), cytoplasmic ions (22), charged membrane domains (23), DNA and RNA (24,25), and other proteins (26). Interactions with regulatory ligands induce Bateman domain conformational changes that control protein function (19,21).…”

Comparative Analysis of 126 Cyanobacterial Genomes Reveals Evidence of Functional Diversity Among Homologs of the Redox-Regulated CP12 Protein

“…CBS domains have been shown to bind Mg 2+ , singlestranded DNA and RNA, and double-stranded DNA (Kery et al, 1998;McLean et al, 2004;Scott et al, 2004;Hattori et al, 2007Hattori et al, , 2009Sharpe et al, 2008;AguadoLlera et al, 2010;Feng et al, 2010). A potential clue to the function of the CP12 CBS domain is found in the primary structure of all three types of CP12-CBS proteins.…”

Mg 2+ -sensing mechanism of Mg 2+ transporter MgtE probed by molecular dynamics study

“…This regulation is accomplished by converting the physical stimuli or the chemical environment into the structural changes of the transporter protein. Although these regulatory mechanisms have been investigated from a structural point of view for several membrane proteins, including Mg 2ϩ transporters (1)(2)(3)(4), the Ca 2ϩ -activated K ϩ channel MthK (5) AmtB-GlnK (6,7), and an acid-sensing ion channel (8), the molecular mechanism of the regulation, i.e., how these proteins provide a response to the change in the chemical environment, is still poorly understood.…”