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1.A.26 The Mg2+ Transporter-E (MgtE) Family

Several functionally characterized members of the MgtE family have been described. These include MgtE from the Gram-positive bacterium, Bacillus firmus OF4, and MgtE from the Gram-negative bacterium, Providencia stuartii. These proteins are capable of transporting Mg2+ and Co2+ but not Ni2+. The KM values for Mg2+ and Co2+ uptake are in the 50-100μM range. An MgtE homologue from Aeromonas hydrophila expressed in a Salmonella typhimurium strain that is deleted for all Mg2+ transporters complements the defect, both phenotypically and by allowing 57Co2+ uptake (Merino et al., 2001). MstE (1.A.26.1.2), CLC (2.A.49.6.1) and HlyC/CorC (HCC; 9.A.40.1.2) may all share a hydrophilic domain.

Members of the family have been sequenced from a wide variety of bacteria, archaea and eukaryotes. Their phylogeny roughly follows that of the organismal taxonomies. They have sizes that vary considerably from 311 residues for the Methanococcus thermoautotrophicum protein, 463 residues for the Synechocystis homologue, and 513 residues for the human homologue, SLC41A1. The B. firmus transporter and several homologues examined have strongly charged, hydrophilic N-terminal domains (cytoplasmic) followed by a hydrophobic C-terminal domain with 5 putative transmembrane α-helical spanners. A central 100 residues resembles archaeal inositol monophosphate dehydrogenases (Kehres and Maguire, 2002). Kehres and Maguire (2002) suggested that the MgtE proteins are secondary carriers (uniporters?) with inwardly directed polarity. Hattori et al. (2009) have considered MgtE to function by a channel mechanism. They provided evidence that the MgtE cytosolic domain acts as a Mg2+ sensor to regulate gating of the pore in response to the intracellular Mg2+ concentration. This produces a mechanism for the maintenance of homeostasis conditions (Hattori et al., 2009).  Structure, mechanism and regluation have been reviewed (Payandeh et al. 2013; Sahni and Scharenberg 2013).

Mg2+ has the largest hydrated radius among all cations although its ionic radius is the smallest. How do Mg2+ transporters selectively recognize and dehydrate the large, fully hydrated Mg2+ cation for transport? The MgtE family of Mg2+ transporters is distributed in all phylogenetic domains. Human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. However, Hattori et al. (2007) determine the crystal structures of the full-length Thermus thermophilus MgtE at 3.5 Å resolution, and of the cytosolic domain in the presence and absence of Mg2+ at 2.3 Å and 3.9 Å resolutions, respectively. The transporter adopts a homodimeric architecture. The carboxy-terminal domain has five TMSs. The amino-terminal cytosolic part consists of the superhelical N domain and tandemly repeated cystathionine-β-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 helices #5 from both subunits close the pore through interactions with the 'connecting helices', which connect the cystathionine-β-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 homeostatic 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.

The cytosolic domain of MgtE undergoes a Mg2+-dependent structural change, which may gate the ion-conducting pore passing through the transmembrane domain. Ishitani et al. (2008) performed molecular dynamics simulations of the MgtE cytosolic domain which reproduced the structural changes of the cytosolic domain upon binding or releasing Mg2+. The roles of the N and CBS domains in the cytosolic domain and their respective Mg2+ binding sites were proposed.

Maruyama et al. (2012) showed that MgtE exhibits the channel-like electrophysiological property, i.e., Mg2+ transport occurs in accordance to the electrochemical potential of Mg2+. The Mg2+-permeation pathway opens in response to a decrease of the intracellular Mg2+ concentration, while it is completely closed at the intracellular Mg2+ concentration of 10 mM. The crystal structures of the MgtE dimer reveals that the Mg2+-sensing cytoplasmic region consists of the N and CBS domains. The Mg2+-bound state adopts a compact, globular conformation, which is stabilized by the coordination of a number of Mg2+ ions between these domains. On the other hand, in the Mg2+-unbound state, these domains are far apart, and fixed by the crystal packing. Maruyama et al. (2012) reported the backbone resonance assignments of the dimer of the cytoplasmic region of the MgtE from Thermus thermophilus with a molecular weight of 60 KDa, in the Mg2+-unbound state.  t

The crystal structures of bacterial CorA and MgtE Mg2+ channels revealed that both structures are unique, unlike that of any other channel or transporter (Moomaw and Maguire 2008). Although structurally quite different, both CorA and MgtE appear to be gated in a similar manner through multiple Mg2+ binding sites in the cytosolic domain of the channels. These sites essentially serve as Mg2+ 'sensors' of cytosolic Mg2+ concentration.

The transport reaction catalyzed by MgtE proteins is:

Mg2+ (or Co2+) (out) → Mg2+Sahni and Scharenberg 2013).

Mg2+ has the largest hydrated radius among all cations although its ionic radius is the smallest. How do Mg2+ transporters selectively recognize and dehydrate the large, fully hydrated Mg2+ cation for transport? The MgtE family of Mg2+ transporters is distributed in all phylogenetic domains. Human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. However, Hattori et al. (2007) determine the crystal structures of the full-length Thermus thermophilus MgtE at 3.5 Å resolution, and of the cytosolic domain in the presence and absence of Mg2+ at 2.3 Å and 3.9 Å resolutions, respectively. The transporter adopts a homodimeric architecture. The carboxy-terminal domain has five TMSs. The amino-terminal cytosolic part consists of the superhelical N domain and tandemly repeated cystathionine-β-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 helices #5 from both subunits close the pore through interactions with the 'connecting helices', which connect the cystathionine-β-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 homeostatic 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.

The cytosolic domain of MgtE undergoes a Mg2+-dependent structural change, which may gate the ion-conducting pore passing through the transmembrane domain. Ishitani et al. (2008) performed molecular dynamics simulations of the MgtE cytosolic domain which reproduced the structural changes of the cytosolic domain upon binding or releasing Mg2+. The roles of the N and CBS domains in the cytosolic domain and their respective Mg2+ binding sites were proposed.

Maruyama et al. (2012) showed that MgtE exhibits the channel-like electrophysiological property, i.e., Mg2+ transport occurs in accordance to the electrochemical potential of Mg2+. The Mg2+-permeation pathway opens in response to a decrease of the intracellular Mg2+ concentration, while it is completely closed at the intracellular Mg2+ concentration of 10 mM. The crystal structures of the MgtE dimer reveals that the Mg2+-sensing cytoplasmic region consists of the N and CBS domains. The Mg2+-bound state adopts a compact, globular conformation, which is stabilized by the coordination of a number of Mg2+ ions between these domains. On the other hand, in the Mg2+-unbound state, these domains are far apart, and fixed by the crystal packing. Maruyama et al. (2012) reported the backbone resonance assignments of the dimer of the cytoplasmic region of the MgtE from Thermus thermophilus with a molecular weight of 60 KDa, in the Mg2+-unbound state.  t

The crystal structures of bacterial CorA and MgtE Mg2+ channels revealed that both structures are unique, unlike that of any other channel or transporter (Moomaw and Maguire 2008). Although structurally quite different, both CorA and MgtE appear to be gated in a similar manner through multiple Mg2+ binding sites in the cytosolic domain of the channels. These sites essentially serve as Mg2+ 'sensors' of cytosolic Mg2+ concentration.

The transport reaction catalyzed by MgtE proteins is:

Mg2+ (or Co2+) (out) → Mg2+ (or Co2+) (in).

 

References associated with 1.A.26 family:

de Baaij, J.H., M.J. Groot Koerkamp, M. Lavrijsen, F. van Zeeland, H. Meijer, F.C. Holstege, R.J. Bindels, and J.G. Hoenderop. (2013). Elucidation of the distal convoluted tubule transcriptome identifies new candidate genes involved in renal Mg2+ handling. Am. J. Physiol. Renal Physiol 305: F1563-1573. 24089412
Fleig, A., M. Schweigel-Röntgen, and M. Kolisek. (2013). Solute Carrier Family SLC41, what do we really know about it? Wiley Interdiscip Rev Membr Transp Signal 2:. 24340240
Hattori, M., N. Iwase, N. Furuya, Y. Tanaka, T. Tsukazaki, R. Ishitani, M.E. Maguire, K. Ito, A. Maturana, and O. Nureki. (2009). Mg2+-dependent gating of bacterial MgtE channel underlies Mg2+ homeostasis. EMBO. J. 28: 3602-3612. 19798051
Hattori, M., Y. Tanaka, S. Fukai, R. Ishitani, and O. Nureki. (2007). Crystal structure of the MgtE Mg2+ transporter. Nature 448: 1072-1075. 17700703
Hurd, T.W., E.A. Otto, E. Mishima, H.Y. Gee, H. Inoue, M. Inazu, H. Yamada, J. Halbritter, G. Seki, M. Konishi, W. Zhou, T. Yamane, S. Murakami, G. Caridi, G. Ghiggeri, T. Abe, and F. Hildebrandt. (2013). Mutation of the Mg2+ Transporter SLC41A1 Results in a Nephronophthisis-Like Phenotype. J Am Soc Nephrol. [Epub: Ahead of Print] 23661805
Ishitani, R., Y. Sugita, N. Dohmae, N. Furuya, M. Hattori, and O. Nureki. (2008). Mg2+-sensing mechanism of Mg2+ transporter MgtE probed by molecular dynamics study. Proc. Natl. Acad. Sci. USA 105: 15393-15398. 18832160
Kehres, D.G. and M.E. Maguire. (2002). Structure, properties and regulation of magnesium transport proteins. Biometals 15: 261-270. 12206392
Kolisek, M., P. Launay, A. Beck, G. Sponder, N. Serafini, M. Brenkus, E.M. Froschauer, H. Martens, A. Fleig, and M. Schweigel. (2008). SLC41A1 is a novel mammalian Mg2+ carrier. J. Biol. Chem. 283: 16235-16247. 18367447
Mandt, T., Y. Song, A.M. Scharenberg, and J. Sahni. (2011). SLC41A1 Mg2+ transport is regulated via Mg2+-dependent endosomal recycling through its N-terminal cytoplasmic domain. Biochem. J. 439: 129-139. 21696366
Maruyama, T., S. Imai, M. Osawa, M. Hattori, R. Ishitani, O. Nureki, and I. Shimada. (2013). Backbone resonance assignments for the cytoplasmic region of the Mg2+ transporter MgtE in the Mg (2+)-unbound state. Biomol NMR Assign 7: 93-96. 22477092
Merino, S., R. Gavín, M. Altarriba, L. Izquierdo, M.E. Maguire, and J.M. Tomás. (2001). The MgtE Mg2+ transport protein is involved in Aeromonas hydrophila adherence. FEMS Microbiol. Lett. 198: 189-195. 11430413
Moomaw, A.S. and M.E. Maguire. (2008). The unique nature of mg2+ channels. Physiology (Bethesda) 23: 275-285. 18927203
Payandeh, J., R. Pfoh, and E.F. Pai. (2013). The structure and regulation of magnesium selective ion channels. Biochim. Biophys. Acta. 1828: 2778-2792. 23954807
Sahni, J. and A.M. Scharenberg. (2013). The SLC41 family of MgtE-like magnesium transporters. Mol Aspects Med 34: 620-628. 23506895
Sahni, J., Y. Song, and A.M. Scharenberg. (2012). The B. subtilis MgtE magnesium transporter can functionally compensate TRPM7-deficiency in vertebrate B-cells. PLoS One 7: e44452. 22970223
Schmitz, C., F. Deason, and A.L. Perraud. (2007). Molecular components of vertebrate Mg2+-homeostasis regulation. Magnes Res 20: 6-18. 17536484
Smith, R.L., L.J. Thompson, and M.E. Maguire. (1995). Cloning and characterization of MgtE, a putative new class of Mg2+ transporter from Bacillus firmus OF4. J. Bacteriol. 177: 1233-1238. 7868596
Sponder, G., K. Rutschmann, and M. Kolisek. (2013). "Inside-in" or "inside-out"? The membrane topology of SLC41A1. Magnes Res 26: 176-181. 24491491
Wabakken, T., E. Rian, M. Kveine, and H.C. Aasheim. (2003). The human solute carrier SLC41A1 belongs to a novel eukaryotic subfamily with homology to prokaryotic MgtE Mg2+ transporters. Biochem. Biophys. Res. Commun. 306: 718-724. 12810078