page_title: Magnesium transporter
section_title: Protozoan (Paramecium)
hierarchical_section_title: Magnesium transporter / Protozoan (Paramecium)
caption_attribution_description: Author: Revel Drummond
context_page_description: Magnesium transporters are proteins that transport magnesium across the cell membrane. All forms of life require magnesium, yet the molecular mechanisms of Mg²⁺ uptake from the environment and the distribution of this vital element within the organism are only slowly being elucidated.
  The ATPase function of MgtA is highly cardiolipin dependent and has been shown to detect free magnesium in the μM range
  In bacteria, Mg²⁺ is probably mainly supplied by the CorA protein and, where the CorA protein is absent, by the MgtE protein. In yeast the initial uptake is via the Alr1p and Alr2p proteins, but at this stage the only internal Mg²⁺ distributing protein identified is Mrs2p. Within the protozoa only one Mg²⁺ transporter has been identified. In metazoa, Mrs2p and MgtE homologues have been identified, along with two novel Mg²⁺ transport systems TRPM6/TRPM7 and PCLN-1. Finally, in plants, a family of Mrs2p homologues has been identified along with another novel protein, AtMHX.
context_section_description: The transport of Mg²⁺ into Paramecium has been characterised largely by R. R. Preston and his coworkers. Electrophysiological techniques on whole Paramecium were used to identify and characterise Mg²⁺ currents in a series of papers before the gene was cloned by Haynes et al. (2002).
  The open reading frame for the XNTA gene is 1707 bp in size, contains two introns and produces a predicted protein of 550 amino acids. The protein has been predicted to contain 11 TM domains and also contains the α1 and α2 motifs (see figure) of the SLC8 (Na+/Ca²⁺ exchanger) and SLC24 (K+ dependent Na+/Ca²⁺ exchanger) human solute transport proteins. The XntAp is equally similar to the SLC8 and SLC24 protein families by amino acid sequence, but the predicted TM topology is more like that of SLC24, but the similarity is at best weak and the relationship is very distant. The AtMHX protein from plants also shares a distant relationship with the SLC8 proteins.
  The figure shows the predicted TM topology of XntAp. Adapted from Haynes et al. (2002), this figure shows the computer predicted membrane topology of XntAp in Paramecium. The orientation in the membrane was determined using HMMTOP. The TM domains are shown in light blue, the α1 and α2 domains are shown in green. 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 Mg²⁺-dependent currents carried by XntAp are kinetically like that of a channel protein and have an ion selectivity order of Mg²⁺ > Co²⁺, Mn²⁺ > Ca²⁺ — a series again very similar to that of CorA. Unlike the other transport proteins reported so far, XntAp is dependent on intracellular Ca²⁺. The transport is also dependent on ΔΨ, but again Mg²⁺ is not transported to equilibrium, being limited to approximately 0.4 mM free Mg²⁺ in the cytoplasm. The existence of an intracellular compartment with a much higher free concentration of Mg²⁺ (8 mM) was supported by the results.
caption: The TM topology of the XNTA protein
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