text: The first magnesium transporter isolated in any multicellular organism, AtMHX shows no similarity to any previously isolated Mg transport protein. The gene was initially identified in the A. thaliana genomic DNA sequence database, by its similarity to the SLC8 family of Na+/Ca exchanger genes in humans.
  The cDNA sequence of 1990 bp is predicted to produce a 539-amino acid protein. AtMHX is quite closely related to the SLC8 family at the amino acid level and shares a topology with eleven predicted TM domains (Figure A10.5). There is one major difference in the sequence, in that the long non-membranal loop (see Figure A10.5) is 148 amino acids in the AtMHX protein but 500 amino acids in the SLC8 proteins. However, this loop is not well conserved and is not required for transport function in the SLC8 family.
  The AtMHX gene is expressed throughout the plant but most strongly in the vascular tissue. The authors suggest that the physiological role of the protein is to store Mg in these tissues for later release when needed. The protein localisation to the vacuolar membrane supports this suggestion (see also Chapter 1.5).
  The protein transports Mg into the vacuolar space and H out, as demonstrated by electrophysiological techniques. The transport is driven by the ΔpH maintained between the vacuolar space (pH 4.5 – 5.9) and the cytoplasm (pH 7.3 – 7.6) by an H-ATPase. How the transport of Mg by the protein is regulated was not determined. Currents were observed to pass through the protein in both directions, but the Mg out current required a ‘cytoplasmic’ pH of 5.5, a condition not found in plant cells under normal circumstances. In addition to the transport of Mg, Shaul et al. (1999) also showed that the protein could transport Zn and Fe, but did not report on the capacity of the protein to transport other divalent cations (e.g. Co and Ni) or its susceptibility to inhibition by cobalt (III) hexaammine.
  The detailed kinetics of Mg transport have not been determined for AtMHX. However, physiological effects have been demonstrated. When A. thaliana plants were transformed with overexpression constructs of the AtMHX gene driven by the CaMV 35S promoter, the plants over-accumulated the protein and showed a phenotype of necrotic lesions in the leaves, which the authors suggest is caused by a disruption in the normal function of the vacuole, given their observation that the total Mg (or Zn) content of the plants was not altered in the transgenic plants.
   The image has been adapted from Shaul et al. (1999) and Quednau et al. (2004), and combined with an analysis using HMMTOP, this figure shows the computer predicted membrane topology of the AtMHX protein in Arabidopsis thaliana. At this time the topology shown should be considered a tentative hypothesis. The TM domains are shown in light blue, the orientation in the membrane and the positions of the N- and C-termini are indicated, and the figure is not drawn to scale. The α1 and α2 domains, shown in green, are both quite hydrophobic and may both be inserted into the membrane.