text: Two papers describe MgtE, a fourth Mg uptake protein in bacteria unrelated to MgtA/B or CorA. This gene has been sequenced and the protein, 312 amino acids in size, is predicted to contain either four or five TM spanning domains that are closely arranged in the C-terminal part of the protein (see figure). This region of the protein has been identified in the Pfam database as a conserved protein domain (PF01769) and species containing proteins that have this protein domain are roughly equally distributed throughout the Eubacteria and Archaea, although it is quite rare in comparison with the distribution of CorA. However, the diversity of the proteins containing the domain is significantly larger than that of the CorA domain. The Pfam database lists seven distinct groups of MgtE domain containing proteins, of which six contain an archaic or eubacterial member. The expression of MgtE is frequently controlled by a conserved RNA structure, YkoK leader or M-box.
  The figure (right), adapted from Smith et al. (1995) and the PFAM database entry, shows the computer-predicted membrane topology of the MgtE protein in Bacillus firmus OF4. The TM domains are shown in light blue. The CBS domains, named for the protein they were identified in, cystathionine-beta synthase, shown in orange, are identified in the Pfam database as regulatory domains, but the mechanism of action has not yet been described. They are found in several voltage-gated chloride channels. The orientation in the membrane and the positions of the N- and C-termini are indicated. This figure is not drawn to scale. This transporter has recently had its structure solved by x-ray crystallography.
  The MgtE gene was first identified by Smith et al. (1995) during a screen for CorA-like proteins in bacteria and complements the Mg-uptake-deficient S. typhimurium strain MM281 (corA mgtA mgtB), restoring wild type growth on standard media. The kinetics of Mg transport for the protein were not determined, as Mg was unavailable. As a substitute, the uptake of Co was measured and was shown to have a km of 82 μM and a Vmax of 354 pmol min 10 cells. Mg was a competitive inhibitor with a Ki of 50 μM—the Ki of Mg inhibition of Co uptake via CorA is 10 μM. A comparison of the available kinetic data for MgtA and CorA is shown in the table. Clearly, MgtE does not transport Co to the same degree as CorA, and the inhibition of transport by Mg is also less efficient, which suggests that the affinity of MgtE for Mg is lower than that of CorA. The strongest inhibitor of Co uptake was Zn, with a Ki of 20 μM. The transport of Zn by this protein may be as important as that of Mg.
  The table shows a comparison of the transport kinetics of MgtE and CorA, and key kinetic parameter values for them are listed. As shown, the data has been generated at differing incubation temperatures. km and Ki are not significantly altered by the differing incubation temperature. Conversely, Vmax shows a strong positive correlation with temperature, hence the value of Co Vmax for MgtE is not directly comparable with the values for CorA.