9.A.19 The Mg²⁺ Transporter-E (MgtE) Family

Two functionally characterized members of the MgtE family have been described: 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 K_M 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).

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). The mode of transport, the nature of the energy source and the potential reversibility of the transport reaction have not been studied. However, the available evidence suggests that the MgtE proteins are secondary carriers (uniporters?) with inwardly directed polarity (Kehres and Maguire, 2002).

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.

The suggested transport reaction catalyzed by MgtE proteins is therefore:

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