Mg2+ transporting P-type ATPases of Salmonella typhimurium. Wrong way, wrong place enzymes

Deletion of a cation transporter promotes lysis in Streptococcus pneumoniae

Streptococcus pneumoniae is a significant human pathogen which causes respiratory and serious invasive diseases. Mg(2+) is essential for life, and its concentration varies throughout the human body. Magnesium uptake plays an important role in the virulence of many bacterial pathogens. To study the Mg(2+) uptake of S. pneumoniae strain D39, a mutant was generated in SPD1383, a P-type ATPase with homology to the Salmonella Mg(2+) transporter MgtA, which has also been shown to be a Ca(2+) exporter in strain TIGR4. Under low-Ca(2+) conditions, mutation led to a growth defect in complex medium and the gene was nearly essential for growth under low-Mg(2+) conditions. Addition of Mg(2+) restored the normal growth of the mutant in all cases, but the addition of other divalent cations had no effect. Addition of Ca(2+), Mn(2+), and Zn(2+) in the presence of high Mg(2+) concentrations inhibited restoration of growth. The mutant was unable to proliferate in blood, which was also alleviated by the addition of Mg(2+). The protein was located in the membrane and produced in various S. pneumoniae strains and pathogenic streptococcal species. Surprisingly, mutation of the gene led to an elevated toxicity for endothelial cells. This was caused by an increased amount of pneumolysin in the medium, mediated by elevated lysis of the mutant. Thus, in this study, we uncovered a role for SPD1383 in Mg(2+) uptake and hypothesize that the protein is a Mg(2+/)Ca(2+) antiporter. Furthermore, a disturbance in Mg(2+) homeostasis seems to promote lysis of S. pneumoniae.

Large Mg(2+)-dependent currents are associated with the increased expression of ALR1 in Saccharomyces cerevisiae

Two genes in Saccharomyces cerevisiae, ALR1 and ALR2, encode proteins putatively involved in Mg(2+) uptake. The present study supports this role for ALR1 and provides the first electrophysiological characterisation of this protein. The patch-clamp technique was used to measure whole-cell ion currents in protoplasts prepared from the wild-type strain, the alr1 alr2 double mutant (CM66), and the double mutant over-expressing the ALR1 gene (CM66+ALR1). With 50 mM Mg(2+) in the bathing solution, the inward current in protoplasts of CM66+ALR1 averaged -264+/-48 pA at -150 mV. Inward currents measured in the wild-type and CM66 protoplasts were more than five-fold smaller. When Mg(2+) was the major cation in the pipette solution, time-dependent outward currents were also detected in CM66+ALR1 protoplasts suggesting ALR1 can facilitate Mg(2+) efflux as well as uptake. We conclude that the ALR1 gene encodes a transport protein. The large magnitude of the Mg(2+)-dependent currents suggests that ALR1 could function as a cation channel.

The CorA magnesium transporter gene family

The CorA transport system is the primary Mg2+ influx system of Salmonella typhimurium and Escherichia coli. The CorA protein has no homology to any other known family of proteins. It has an unusual membrane topology, with a large, soluble, highly charged periplasmic N-terminal domain with three transmembrane segments in a shorter, hydrophobic C-terminal domain. Previous phenotypic and molecular data had suggested that this transport system was widespread in the Bacteria. In this report we show that CorA is virtually ubiquitous in the Bacteria and Archaea, forming a distinct family of transport proteins. Genomic sequences to date have revealed at least 22 members of the CorA family in the Bacteria and the Archaea, with 6 more distant members in the yeasts. Only three of the smallest bacterial genomes lack a CorA homologue. Strikingly, phylogenetic analysis does not show clustering by related species or even within kingdom. Several species of Bacteria contain two or even three CorA paralogues. Within species, these paralogues are not closely related, however, and we suggest that they might have distinct transport functions. A multiple alignment suggests three extended consensus regions within the N-terminal soluble domain of CorA, which is predicted to be virtually all alpha-helical. A fourth consensus region includes the last 20 residues of the soluble domain and continues through the entire membrane domain. The first half of this last consensus domain may form an amphipathic alpha-helix that extends from the soluble domain into the first transmembrane segment. The degree of charge in the first transmembrane segment is quite variable, and we suggest that this transport family may include members with only two rather than three transmembrane segments. If so, this would place the N-terminal soluble domain on different sides of the membrane in different members of the family. We suggest that the CorA Mg2+ transport system forms the major Mg2+ uptake system in the Bacteria and Archaea but that some family members may have a function other than Mg2+ transport.

Magnesium and the role of MgtC in growth of Salmonella typhimurium

Salmonella typhimurium has three distinct transport systems for Mg2+: CorA, MgtA, and MgtB. The mgtCB operon encodes two proteins, MgtC, a hydrophobic protein with a predicted molecular mass of 22.5 kDa, and MgtB, a 102-kDa P-type ATPase Mg2+ transport protein. The mgtCB locus has been identified as part of a new Salmonella pathogenicity island, SPI-3. Transcription of mgtCB is regulated by extracellular Mg2+ via the two-component PhoPQ regulatory system important for virulence. To elucidate MgtC's role in a low-Mg2+ environment, we looked at growth and transport in strains lacking the CorA and MgtA Mg2+ transporters but expressing MgtB, MgtC, or both. mgtC mgtB+ and mgtC+ mgtB+ strains exhibited growth in N minimal medium without added Mg2+ with a 1- to 2-h lag phase. An mgtC+ mgtB strain was also able to grow in N minimal medium without added Mg2+ but only after a 24-h lag phase. In N minimal medium containing 10 mM Mg2+, all strains grew after a short lag phase; the mgtC+ mgtB strain grew to a higher optical density at 600 nm than an mgtC+ mgtB+ strain and was comparable to wild type. The lengthy lag phase before growth in an mgtC+ mgtB strain was not due to lack of expression of MgtC. Western blot analysis indicated that substantial MgtC protein is present by 2 h after suspension in N minimal medium. Surprisingly, in an mgtC+ mgtB+ strain, MgtC was undetectable during Mg2+ starvation, although large amounts of MgtB were observed. The lack of expression of MgtC is not dependent on functional MgtB, since a strain carrying a nonfunctional MgtB with a mutation (D379A) also did not make MgtC. Since, during invasion of eukaryotic cells, S. typhimurium appears to be exposed to a low-pH as well as a low-Mg2+ environment, the growth of an mgtC+ mgtB strain was tested at low pH with and without added Mg2+. While significant quantities of MgtC could be detected after suspension at pH 5.2, the mgtC+ mgtB strain was unable to grow at pH 5.2 whether or not Mg2+ was present. Finally, using 63Ni2+ and 57Co2+ as alternative substrates for the unavailable 28Mg2+, cation uptake could not be detected in an mgtC+ mgtB strain after Mg2+ starvation. We conclude that MgtC is not a Mg2+ transporter and that it does not have a primary role in the survival of S. typhimurium at low pH.

Microbial magnesium transport: unusual transporters searching for identity

Mg2+ is unique among biological cations because of its charge density and solution chemistry. This is abundantly reflected in its transport systems, studied primarily in Salmonella typhimurium. The constitutively expressed CorA transport system is the primary Mg2+ influx pathway for the Bacteria and the Archaea. Its structure of a large N-terminal soluble periplasmic domain with three transmembrane segments at the C-terminus is unique among membrane carriers, and its protein sequence bears no resemblance to other known proteins. The MgtE transport system can also mediate Mg2+ uptake, but whether this is its primary function is not known. MgtE also lacks homology to other known proteins. In contrast, the MgtA and MgtB Mg2+ transport systems of enteric bacteria are P-type ATPases by sequence homology, mediating Mg2+ influx with, rather than against, the Mg2+ electrochemical gradient. They are closely related to mammalian Ca2+-ATPases. Expression of MgtA and MgtB is under the control of the PhoPQ two-component regulatory system, important in bacterial virulence. In S. typhimurium, MgtB is encoded by a two-gene operon mgtCB; the function of the MgtC protein is unknown, and it lacks close homologues. The ligand for the PhoQ membrane sensor kinase is Mg2+ and, at decreased extracellular Mg2+ concentrations, transcription of mgtA and mgtCB are enormously induced. All three genes are also induced upon S. typhimurium invasion of epithelial or macrophage cells. Mutation of these genes has no effect on invasion efficiency, but an insertion in mgtC renders S. typhimurium essentially avirulent in the mouse. The physiological roles of the known Mg2+ transport systems are not yet completely defined. Nonetheless, the singular sequence and apparent structure of the CorA and MgtE transport proteins, the complex regulation of MgtA, MgtB and MgtC and their involvement in pathogenesis suggests that further study will be rewarding.

Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci

Salmonella typhimurium has three distinct Mg2+ transport systems, the constitutive high-capacity CorA transporter and two P-type ATPases, MgtA and MgtB, whose transcription is repressed by normal concentrations of Mg2+ in the growth medium. The latter Mg(2+)-transporting ATPase is part of a two-gene operon, mgtCB, with mgtC encoding a 23 kDa protein of unknown function. Transcriptional regulation using fusions of the promoter regions of mgtA and mgtCB to luxAB showed a biphasic time and Mg2+ concentration dependence. Between 1 and 6 h after transfer to nitrogen minimal medium containing defined concentrations of Mg2+, transcription increased about 200-fold for mgtCB and up to 400-fold for mgtA, each with a half-maximal dependence on Mg2+ of 0.5 mM. Continued incubation revealed a second phase of increased transcription, up to 2000-fold for mgtCB and up to 10,000-fold for mgtA. This secondary increase occurred between 6 and 9 h after transfer to defined medium for mgtCB but between 12 and 24 h for mgtA and had a distinct half-maximal dependence for Mg2+ of 0.01 mM. A concomitant increase of at least 1000-fold in uptake of cation was seen between 8 and 24 h incubation with either system, showing that the transcriptional increase was followed by functional incorporation of large amounts of the newly synthesized transporter into the membrane. Regulation of transcription by Mg2+ was not dependent on a functional stationary-phase sigma factor encoded by rpoS, but it was dependent on the presence of a functional phoPQ two-component regulatory system. Whereas mgtCB was completely dependent on regulation via phoPQ, the secondary late Mg(2+)-dependent phase of mgtA transcription was still evident in strains carrying a mutation in either phoP or phoQ, albeit substantially diminished. Several divalent cations blocked the early phase of the increase in transcription elicited by the decrease in Mg2+ concentration, including cations that inhibit Mg2+ uptake (Co2+, Ni2+ and Mn2+) and those which do not (Ca2+ and Zn2+). In contrast, the second later phase of the transcriptional increase was not well blocked by any cation except those which inhibit uptake. Overall, the data suggest that at least two distinct mechanisms for transcriptional regulation of the mgtA and mgtCB loci exist.

Overexpression of the Saccharomyces cerevisiae magnesium transport system confers resistance to aluminum ion

Ionic aluminum (Al3+) is toxic to plants, microbes, fish, and animals, but the mechanism of its toxicity is unknown. We describe the isolation of two yeast genes (ALR1 and ALR2) which confer increased tolerance to Al3+ and Ga3+ ions when overexpressed while increasing strain sensitivity to Zn2+, Mn2+, Ni2+, Cu2+, Ca2+, and La3+ ions. The Alr proteins are homologous to the Salmonella typhimurium CorA protein, a bacterial Mg2+ and Co2+ transport system located in the periplasmic membrane. Yeast strains lacking ALR gene activity required additional Mg2+ for growth, and expression of either ALR1 or ALR2 corrected the Mg(2+)-requiring phenotype. The results suggest that the ALR genes encode the yeast uptake system for Mg2+ and other divalent cations. This hypothesis was supported by evidence that 57Co2+ accumulation was elevated in ALR-overexpressing strains and reduced in strains lacking ALR expression. ALR overexpression also overcame the inhibition of Co2+ uptake by Al3+ ions. The results indicate that aluminum toxicity to yeast occurs as a consequence of reduced Mg2+ influx via the Alr proteins. The molecular identification of the yeast Mg2+ transport system should lead to a better understanding of the regulation of Mg2+ homeostasis in eukaryote cells.

Cloning and characterization of MgtE, a putative new class of Mg2+ transporter from Bacillus firmus OF4

The MM281 strain of Salmonella typhimurium which possesses mutations in each its three known Mg2+ transport systems and requires 100 mM Mg2+ for growth was used to screen a genomic library from the gram-positive alkaliphilic bacterium Bacillus firmus OF4 for clones that could restore the ability to grow without Mg2+ supplementation. Of the clones obtained, five also conferred sensitivity to Co2+, similar to the phenotype of mutants with mutations in the S. typhimurium corA Mg2+ transport locus. All five contained identical inserts by restriction analysis. Using 63Ni2+ as a surrogate for the unavailable 28Mg2+, the plasmid insert was shown to restore cation uptake with properties similar but not identical to those of the S. typhimurium CorA Mg2+ transporter. Sequence analysis of one clone identified a single open reading frame with multiple possible initiation sites. Deletion and mutation analysis identified a minimum open reading frame of 939 bp encoding a polypeptide with a predicted molecular mass of 34 kDa. Disruption of the open reading frame eliminated cation influx activity and restored resistance to Co2+. This putative transporter, designated MgtE, has no sequence similarity to any known protein including CorA and appears to represent a new class of Mg2+ transport system.

Magnesium transport systems: genetics and protein structure (a review)

Magnesium is unique among biological cations. Its volume change from hydrated cation to atomic ion is over an order of magnitude larger than that of any other biological cation. This volume change presents particular problems for a magnesium transport system and suggests that these systems may be significantly different from other classes of ion transport systems. Detailed study of Mg2+ transport in complex organisms is limited by severe technical problems. However, molecular genetic techniques have enabled the isolation of three Mg2+ transport systems from the Gram-negative bacterium Salmonella typhimurium. The MgtA and MgtB transport systems are members of the P-type ATPase superfamily of transporters but possess unique characteristics among members of this family. The CorA transport protein is the first member of an entirely new class of transport proteins. In addition, another completely new family of Mg2+ transport proteins have been identified that is present in both Gram-negative and Gram-positive bacteria. Characterization of these transporters should provide substantial insight into Mg2+ transport and cellular Mg2+ homeostasis.

Polyamine-sensitive magnesium transport in Saccharomyces cerevisiae

In Saccharomyces cerevisiae we found a toxic effect of polyamines, well-known metabolites important for cell proliferation; in magnesium-limited (50 microM Mg2+) synthetic medium, cell growth was severely inhibited by spermine, spermidine and putrescine in descending order. In conjunction with a decrease in the growth rate by the addition of 0.5 mM spermine, the internal Mg2+ content decreased and the spermine content increased. When cell growth ceased, the Mg2+ content had finally decreased to about 40% of the value before the addition of spermine (120-130 nmol/mg dry weight), and the spermine content concomitantly increased 30-fold (from 1 to 30 nmol/mg dry weight); spermine4+ apparently took the internal place of Mg2+ with a probable stoichiometry of 1:2. However, the total amount of Mg2+ retained in the cells remained constant even with the addition of spermine, suggesting that spermine blocks Mg2+ accumulation. In high (2 mM) Mg2+ medium, cell growth was hardly affected by polyamines, and an exchange of spermine and Mg2+ was minimal. Energy-dependent Mg2+ uptake by whole cells was inhibited by spermine, spermidine and putrescine in a similar manner as the growth rates. On the other hand, Mg2+ inhibited spermine uptake. These results suggest that competition takes place between extracellular spermine and Mg2+ for their accumulations. It is thus clear that polyamine-sensitive Mg2+ transport system is indispensable for the physiology of this organism.

Human Menkes X-chromosome disease and the staphylococcal cadmium-resistance ATPase: a remarkable similarity in protein sequences

A search with the proposed amino acid translation product from the new 'candidate gene' for human Menkes disease against protein sequence libraries showed a remarkable similarity to that for the cadmium efflux ATPase from Staphylococcus aureus resistance plasmids. The Menkes sequence appears closer to the CadA Cd2+ sequence than to P-type ATPases from animal sources. Menkes syndrome is an X-chromosome invariably fatal disease that results from aberrant copper metabolism. The gene that is defective in Menkes patients, i.e. the Menkes candidate gene, encodes a P-type ATPase, whose properties satisfactorily explain the phenotype of the disease. P-type ATPases are all cation pumps, either for uptake (e.g. the bacterial Kdp K+ ATPase), for efflux (e.g. the muscle sarcoplasmic reticulum Ca2+ ATPase), or for cation exchange (e.g. the animal cell Na+/K+ ATPase). These enzymes have a conserved aspartate residue that is transiently phosphorylated from ATP during the transport cycle, hence the name 'P-type' ATPase. The Menkes sequence shares with the staphylococcal CadA ATPase those regions common to all P-type ATPases and also an N-terminal dithiol region that was proposed to be a 'metal-binding motif'. There are one or two copies of this motif in the available CadA sequences and six copies in the Menkes sequence.