The transport of Mg into Paramecium has been characterised largely by R. R. Preston and his coworkers. Electrophysiological techniques on whole Paramecium were used to identify and characterise Mg 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+/Ca exchanger) and SLC24 (K+ dependent Na+/Ca 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 Mg-dependent currents carried by XntAp are kinetically like that of a channel protein and have an ion selectivity order of Mg > Co, Mn > Ca — a series again very similar to that of CorA. Unlike the other transport proteins reported so far, XntAp is dependent on intracellular Ca. The transport is also dependent on ΔΨ, but again Mg is not transported to equilibrium, being limited to approximately 0.4 mM free Mg in the cytoplasm. The existence of an intracellular compartment with a much higher free concentration of Mg (8 mM) was supported by the results.