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 50-100 μM 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 regulation 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? Human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. Hattori et al. (2007) determined 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.
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. MgtE tightly regulates the intracellular Mg2+ concentration. The dimeric structure is composed of a total of ten transmembrane α-helices, forming a central pore. Intracellular soluble domains constitute the Mg2+ sensor. The ion selectivity for Mg2+ over Ca2+ resides in a central cavity in the transmembrane pore involving a conserved aspartate residue (Asp432) from each monomer. Two different classes of binding sites are present in MgtE: a high affinity site for Mg2+ (Kd 0.3 mM) with low Ca2+ affinity (Kd 80 mM), and a medium affinity site for Mg2+ (Kd 2 mM) and Ca2+ (Kd 6 mM), tentatively assigned to the central cavity and the sensor domain, respectively (Kimura et al. 2018).
The transport reaction catalyzed by MgtE proteins is:
Mg2+ (or Co2+) (out) → Mg2+ (or Co2+) (in).