There are a large number of proteins yet to be identified that transport Mg. Even in the best studied eukaryote, yeast, Borrelly has reported a Mg/H exchanger without an associated protein, which is probably localised to the Golgi. At least one other major Mg transporter in yeast is still unaccounted for, the one affecting Mg transport in and out of the yeast vacuole. In higher, multicellular organisms, it seems that many Mg transporting proteins await discovery.
The CorA-domain-containing Mg transporters (CorA, Alr-like and Mrs2-like) have a similar but not identical array of affinities for divalent cations. In fact, this observation can be extended to all of the Mg transporters identified so far. This similarity suggests that the basic properties of Mg strongly influence the possible mechanisms of recognition and transport. However, this observation also suggests that using other metal ions as tracers for Mg uptake will not necessarily produce results comparable to the transporter's ability to transport Mg. Ideally, Mg should be measured directly.
Since Mg is practically unobtainable, much of the old data will need to be reinterpreted with new tools for measuring Mg transport, if different transporters are to be compared directly. The pioneering work of Kolisek and Froschauer using mag-fura 2 has shown that free Mg can be reliably measured in vivo in some systems. By returning to the analysis of CorA with this new tool, we have gained an important baseline for the analysis of new Mg transport systems as they are discovered. However, it is important that the amount of transporter present in the membrane is accurately determined if comparisons of transport capability are to be made. This bacterial system might also be able to provide some utility for the analysis of eukaryotic Mg transport proteins, but differences in biological systems of prokaryotes and eukaryotes will have to be considered in any experiment.