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 (XntAp) 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.
Evolution
The evolution of Mg2+ transport appears to have been rather complicated. Proteins apparently based on MgtE are present in bacteria and metazoa, but are missing in fungi and plants, whilst proteins apparently related to CorA are present in all of these groups. The two active transport transporters present in bacteria, MgtA and MgtB, do not appear to have any homologies in higher organisms. There are also Mg2+ transport systems that are found only in the higher organisms.
Types
There are a large number of proteins yet to be identified that transport Mg2+. Even in the best studied eukaryote, yeast, Borrelly has reported a Mg2+/H+ exchanger without an associated protein, which is probably localised to the Golgi. At least one other major Mg2+ transporter in yeast is still unaccounted for, the one affecting Mg2+ transport in and out of the yeast vacuole. In higher, multicellular organisms, it seems that many Mg2+ transporting proteins await discovery.
The CorA-domain-containing Mg2+ 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 Mg2+ transporters identified so far. This similarity suggests that the basic properties of Mg2+ strongly influence the possible mechanisms of recognition and transport. However, this observation also suggests that using other metal ions as tracers for Mg2+ uptake will not necessarily produce results comparable to the transporter's ability to transport Mg2+. Ideally, Mg2+ should be measured directly.
Since 28Mg2+ is practically unobtainable, much of the old data will need to be reinterpreted with new tools for measuring Mg2+ transport, if different transporters are to be compared directly. The pioneering work of Kolisek and Froschauer using mag-fura 2 has shown that free Mg2+ 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 Mg2+ 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 Mg2+ transport proteins, but differences in biological systems of prokaryotes and eukaryotes will have to be considered in any experiment.
Function
Comparing the functions of the characterised Mg2+ transport proteins is currently almost impossible, even though the proteins have been investigated in different biological systems using different methodologies and technologies. Finding a system where all the proteins can be compared directly would be a major advance. If the proteins could be shown to be functional in bacteria (S. typhimurium), then a combination of the techniques of mag-fura 2, quantification of protein in the envelope membrane, and structure of the proteins (X-ray crystal or cryo-TEM) might allow the determination of the basic mechanisms involved in the recognition and transport of the Mg2+ ion. However, perhaps the best advance would be the development of methods allowing the measurement of the protein's function in the patch-clamp system using artificial membranes.
Bacteria
Early research In 1968, Lusk described the limitation of bacterial (Escherichia coli) growth on Mg2+-poor media, suggesting that bacteria required Mg2+ and were likely to actively take this ion from the environment. The following year, the same group and another group, Silver, independently described the uptake and efflux of Mg2+ in metabolically active E. coli cells using 28Mg2+. By the end of 1971, two papers had been published describing the interference of Co2+, Ni2+ and Mn2+ on the transport of Mg2+ in E. coli and in Aerobacter aerogenes and Bacillus megaterium. In the last major development before the cloning of the genes encoding the transporters, it was discovered that there was a second Mg2+ uptake system that showed similar affinity and transport kinetics to the first system, but had a different range of sensitivities to interfering cations. This system was also repressible by high extracellular concentrations of Mg2+ .
CorA
The CorA gene and its corresponding protein are the most exhaustively studied Mg2+ transport system in any organism. Most of the published literature on the CorA gene comes from the laboratory of M. E. Maguire. Recently the group of R. J. Schweyen made a significant impact on the understanding of Mg2+ transport by CorA. The gene was originally named after the cobalt-resistant phenotype in E. coli that was caused by the gene's inactivation.
The gene was genetically identified in E. coli by Park et al., but wasn't cloned until Hmiel et al. isolated the Salmonella enterica serovar Typhimurium (S. typhimurium) homologue. Later it would be shown by Smith and Maguire that the CorA gene was present in 17 gram-negative bacteria. With the large number of complete genome sequences now available for prokaryotes, CorA has been shown to be virtually ubiquitous among the Eubacteria, as well as being widely distributed among the Archaea. The CorA locus in E. coli contains a single open reading frame of 948 nucleotides, producing a protein of 316 amino acids. This protein is well conserved amongst the Eubacteria and Archaea. Between E. coli and S. typhimurium, the proteins are 98% identical, but in more distantly related species, the similarity falls to between 15 and 20%. In the more distantly related genes, the similarity is often restricted to the C-terminal part of the protein, and a short amino acid motif GMN within this region is very highly conserved. The CorA domain, also known as PF01544 in the pFAM conserved protein domain database (http://webarchive.loc.gov/all/20110506030957/http%3A//pfam.sanger.ac.uk/), is additionally present in a wide range of higher organisms, and these transporters will be reviewed below.
The CorA gene is constitutively expressed in S. typhimurium under a wide range of external Mg2+ concentrations. However, recent evidence suggests that the activity of the protein may be regulated by the PhoPQ two-component regulatory system. This sensor responds to low external Mg2+ concentrations during the infection process of S. typhimurium in humans. In low external Mg2+ conditions, the PhoPQ system was reported to suppress the function of CorA and it has been previously shown that the transcription of the alternative Mg2+ transporters MgtA and MgtB is activated in these conditions. Chamnongpol and Groisman suggest that this allows the bacteria to escape metal ion toxicity caused by the transport of other ions, particularly Fe(II), by CorA in the absence of Mg2+. Papp and Maguire offer a conflicting report on the source of the toxicity.
The figure (not to scale) shows the originally published transmembrane (TM) domain topology of the S. typhimurium CorA protein, which was said to have three membrane-spanning regions in the C-terminal part of the protein (shown in blue), as determined by Smith et al.. Evidence for CorA acting as a homotetramer was published by Warren et al. in 2004. In December 2005 the crystal structure of the CorA channel was posted to the RSCB protein structure database. The results showed that the protein has two TM domains and exists as a homopentamer, in direct conflict with the earlier reports. Follow this link to see the structure in 3D. The soluble intracellular parts of the protein are highly charged, containing 31 positively charged and 53 negatively charged residues. Conversely, the TM domains contain only one charged amino acid, which has been shown to be unimportant in the activity of the transporter. From mutagenesis experiments, it appears that the chemistry of the Mg2+ transport relies on the hydroxyl groups lining the inside of the transport pore; there is also an absolute requirement for the GMN motif (shown in red).
Before the activity of CorA could be studied in vivo, any other Mg2+ transport systems in the bacterial host had to be identified and inactivated or deleted (see below). A strain of S. typhimurium containing a functional CorA gene but lacking MgtA and MgtB was constructed(also see below), and the uptake kinetics of the transporter were analysed. This strain showed nearly normal growth rates on standard media (50 μM Mg2+), but the removal of all three genes created a bacterial strain requiring 100 mM external Mg2+ for normal growth.
Mg2+ is transported into cells containing only the CorA transport system with similar kinetics and cation sensitivities as the Mg2+ uptake described in the earlier papers, and has additionally been quantified(see table). The uptake of Mg2+ was seen to plateau as in earlier studies, and although no actual mechanism for the decrease in transport has been determined, so it has been assumed that the protein is inactivated. Co2+ and Ni2+ are toxic to S. typhimurium cells containing a functional CorA protein and this toxicity stems from the blocking of Mg2+ uptake (competitive inhibition) and the accumulation of these ions inside the cell. Co2+ and Ni2+ have been shown to be transported by CorA by using radioactive tracer analysis, although with lower affinities (km) and velocities (Vmax) than for Mg2+ (see table). The km values for Co2+ and Ni2+ are significantly above those expected to be encountered by the cells in their normal environment, so it is unlikely that the CorA transport system mediates the uptake of these ions under natural conditions. To date, the evidence for Mn2+ transport by CorA is limited to E. coli.
Mg2+ Co2+ Ni2+ km (μM) 15 30 240 Vmax (pmol/min/108 cells) 250 500 360 Ki (μM) - Mg - - 10 Ki (μM) - Co 50 - 20 Ki (μM) - Mn 30 - - Ki (μM) - Ni 300 - 300
The table lists the transport kinetics of the CorA Mg2+ transport system. This table has been compiled from the publications of Snavely et al. (1989b), Gibson et al. (1991) and Smith et al. (1998a) and summarises the kinetic data for the CorA transport protein expressed from the wild type promoter in bacteria lacking MgtA and MgtB. km and Vmax were determined at 20 °C as the uptake of Mg2+ at 37 °C was too rapid to measure accurately.