membrane. Second, the uptake is saturated far below that which the ΔΨ theoretically permits, so the transport of Mg2+ by Mrs2p is likely to be regulated in a similar manner to CorA, possibly by the inactivation of the protein. Third, Mg2+ efflux was observed via Mrs2p upon the artificial depolarisation of the mitochondrial membrane by valinomycin. Finally, the Mg2+ fluxes through Mrs2p are inhibited by cobalt (III) hexaammine. The kinetics of Mg2+ uptake by Mrs2p were determined in the Froschauer et al. (2004) paper on CorA in bacteria. The initial change in free Mg2+ concentration was 150 μM s-1 for wild type and 750 μM s-1 for mitochondria from yeast overexpressing MRS2. No attempt was made to scale the observed transport to the amount of transporter present. Protozoan (Paramecium) 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