Membrane topology of a P-type ATPase. The MgtB magnesium transport protein of Salmonella typhimurium

A Translation-Aborting Small Open Reading Frame in the Intergenic Region Promotes Translation of a Mg2+ Transporter in Salmonella Typhimurium

Translation initiation regions in mRNAs that include the ribosome-binding site (RBS) and the start codon are often sequestered within a secondary structure. Therefore, to initiate protein synthesis, the mRNA secondary structure must be unfolded to allow the RBS to be accessible to the ribosome.

Bacterial mRNAs often harbor upstream open reading frames (uORFs) in the 5′ untranslated regions (UTRs). Translation of the uORF usually affects downstream gene expression at the levels of transcription and/or translation initiation. Unlike other uORFs mostly located in the 5′ UTR, we discovered an 8-amino-acid ORF, designated mgtQ, in the intergenic region between the mgtC virulence gene and the mgtB Mg²⁺ transporter gene in the Salmonella mgtCBRU operon. Translation of mgtQ promotes downstream mgtB Mg²⁺ transporter expression at the level of translation by releasing the ribosome-binding sequence of the mgtB gene that is sequestered in a translation-inhibitory stem-loop structure. Interestingly, mgtQ Asp2 and Glu5 codons that induce ribosome destabilization are required for mgtQ-mediated mgtB translation. Moreover, the mgtQ Asp and Glu codons-mediated mgtB translation is counteracted by the ribosomal subunit L31 that stabilizes ribosome. Substitution of the Asp2 and Glu5 codons in mgtQ decreases MgtB Mg²⁺ transporter production and thus attenuates Salmonella virulence in mice, likely by limiting Mg²⁺ acquisition during infection.

Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import

Magnesium cation (Mg2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg2+ is the central ion in chlorophylls, and Mg2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a 'classical' bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the 'typical' bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

Elongation factor P restricts Salmonella's growth by controlling translation of a Mg2+ transporter gene during infection

When a ribosome translates mRNA sequences, the ribosome often stalls at certain codons because it is hard to translate. Consecutive proline codons are such examples that induce ribosome stalling and elongation factor P (EF-P) is required for the stalled ribosome to continue translation at those consecutive proline codons. We found that EF-P is required for translation of the mgtB gene encoding a Mg²⁺ transporter in the mgtCBR virulence operon from the intracellular pathogen Salmonella enterica serovar Typhimurium. Salmonella lacking EF-P decreases MgtB protein levels in a manner dependent on consecutive proline codons located in the mgtB coding region despite increasing transcription of the mgtCBR operon via the mgtP open reading frame in the leader RNA, resulting in an altered ratio between MgtC and MgtB proteins within the operon. Substitution of the consecutive proline codons to alanine codons eliminates EF-P-mediated control of the mgtB gene during infection and thus contributes to Salmonella's survival inside macrophages where Salmonella experiences low levels of EF-P. This suggests that this pathogen utilizes a strategy to coordinate expression of virulence genes by an evolutionarily conserved translation factor.

Magnesium Transport and Magnesium Homeostasis

This review reviews the properties and regulation of the Salmonella enterica serovar Typhimurium and Escherichia coli transporters that mediate Mg2+ influx: CorA and the Mgt P-type ATPases. In addition, potential Mg2+ regulation of transcription and translation, largely via the PhoPQ two component system, is discussed. CorA proteins are a unique class of transporters and are widespread in the Bacteria and Archaea, with rather distant but functional homologs in eukaryotes. The Mgt transporters are highly homologous to other P-type ATPases but are more closely related to the eukaryotic H+ and Ca2+ ATPases than to most prokaryotic ATPases. Hundreds of homologs of CorA are currently known from genomic sequencing. In contrast, only when extracellular and possibly intracellular Mg2+ levels fall significantly is the expression of mgtA and mgtB induced. Topology studies using blaM and lacZ fusions initially indicated that the Salmonella serovar Typhimurium CorA contained three transmembrane (TM) segments; however, subsequent data obtained using a variety of approaches showed that the CorA superfamily of proteins have only two TMs at the extreme C terminus. PhoP-PhoQ is a two-component system consisting of PhoQ, the sensor/receptor histidine kinase, and PhoP, the response regulator/transcriptional activator. The expression of both mgtA and mgtCB in either E. coli or Salmonella serovar Typhimurium is markedly induced in a PhoPQ-dependent manner by low concentrations of Mg2+ in the medium. phoP and phoQ form an operon with two promoters in both E. coli and Salmonella serovar Typhimurium.

Hyperacidification of vacuoles by the combined action of two different P-ATPases in the tonoplast determines flower color

The acidification of endomembrane compartments is essential for enzyme activities, sorting, trafficking, and trans-membrane transport of various compounds. Vacuoles are mildly acidic in most plant cells because of the action of V-ATPase and/or pyrophosphatase proton pumps but are hyperacidified in specific cells by mechanisms that remained unclear. Here, we show that the blue petal color of petunia ph mutants is due to a failure to hyperacidify vacuoles. We report that PH1 encodes a P3B-ATPase, hitherto known as Mg2(+) transporters in bacteria only, that resides in the vacuolar membrane (tonoplast). In vivo nuclear magnetic resonance and genetic data show that PH1 is required and, together with the tonoplast H(+) P3A-ATPase PH5, sufficient to hyperacidify vacuoles. PH1 has no H(+) transport activity on its own but can physically interact with PH5 and boost PH5 H(+) transport activity. Hence, the hyperacidification of vacuoles in petals, and possibly other tissues, relies on a heteromeric P-ATPase pump.

Bacterial Mg2+ homeostasis, transport, and virulence

Organisms must maintain physiological levels of Mg(2+) because this divalent cation is critical for the stabilization of membranes and ribosomes, for the neutralization of nucleic acids, and as a cofactor in a variety of enzymatic reactions. In this review, we describe the mechanisms that bacteria utilize to sense the levels of Mg(2+) both outside and inside the cytoplasm. We examine how bacteria achieve Mg(2+) homeostasis by adjusting the expression and activity of Mg(2+) transporters and by changing the composition of their cell envelope. We discuss the connections that exist between Mg(2+) sensing, Mg(2+) transport, and bacterial virulence. Additionally, we explore the logic behind the fact that bacterial genomes encode multiple Mg(2+) transporters and distinct sensing systems for cytoplasmic and extracytoplasmic Mg(2+). These analyses may be applicable to the homeostatic control of other cations.

Metal acquisition and virulence in Brucella

Similar to other bacteria, Brucella strains require several biologically essential metals for their survival in vitro and in vivo. Acquiring sufficient levels of some of these metals, particularly iron, manganese and zinc, is especially challenging in the mammalian host, where sequestration of these micronutrients is a well-documented component of both the innate and acquired immune responses. This review describes the Brucella metal transporters that have been shown to play critical roles in the virulence of these bacteria in experimental and natural hosts.

Cell organisation, sulphur metabolism and ion transport-related genes are differentially expressed in Paracoccidioides brasiliensis mycelium and yeast cells

BACKGROUND

Mycelium-to-yeast transition in the human host is essential for pathogenicity by the fungus Paracoccidioides brasiliensis and both cell types are therefore critical to the establishment of paracoccidioidomycosis (PCM), a systemic mycosis endemic to Latin America. The infected population is of about 10 million individuals, 2% of whom will eventually develop the disease. Previously, transcriptome analysis of mycelium and yeast cells resulted in the assembly of 6,022 sequence groups. Gene expression analysis, using both in silico EST subtraction and cDNA microarray, revealed genes that were differential to yeast or mycelium, and we discussed those involved in sugar metabolism. To advance our understanding of molecular mechanisms of dimorphic transition, we performed an extended analysis of gene expression profiles using the methods mentioned above.

RESULTS

In this work, continuous data mining revealed 66 new differentially expressed sequences that were MIPS(Munich Information Center for Protein Sequences)-categorised according to the cellular process in which they are presumably involved. Two well represented classes were chosen for further analysis: (i) control of cell organisation - cell wall, membrane and cytoskeleton, whose representatives were hex (encoding for a hexagonal peroxisome protein), bgl (encoding for a 1,3-beta-glucosidase) in mycelium cells; and ags (an alpha-1,3-glucan synthase), cda (a chitin deacetylase) and vrp (a verprolin) in yeast cells; (ii) ion metabolism and transport - two genes putatively implicated in ion transport were confirmed to be highly expressed in mycelium cells - isc and ktp, respectively an iron-sulphur cluster-like protein and a cation transporter; and a putative P-type cation pump (pct) in yeast. Also, several enzymes from the cysteine de novo biosynthesis pathway were shown to be up regulated in the yeast form, including ATP sulphurylase, APS kinase and also PAPS reductase.

CONCLUSION

Taken together, these data show that several genes involved in cell organisation and ion metabolism/transport are expressed differentially along dimorphic transition. Hyper expression in yeast of the enzymes of sulphur metabolism reinforced that this metabolic pathway could be important for this process. Understanding these changes by functional analysis of such genes may lead to a better understanding of the infective process, thus providing new targets and strategies to control PCM.

In silico insight into two rice chromosomal regions associated with submergence tolerance and resistance to bacterial leaf blight and gall midge

Plants respond to both biotic and abiotic stresses through a common signaling system to provide defense and protection against many adverse environments. Many genes/QTLs governing resistance to both biotic and abiotic stresses have been studied and mapped in rice. Sub1, a major QTL for submergence tolerance is collocated with a gene Gm1 for gall midge resistance on chromosome 9 (Region 1). Likewise a bigger region on chromosome 5 (Region 2) has a minor QTL for submergence tolerance collocated with genes for bacterial blight resistance. Utilizing the rice sequence and annotation data (TIGR) and rice genome annotation project database (RAP-DB), we wanted to know the kinds of genes underlying these two chromosomal regions where genes/QTL governing tolerance to both biotic and abiotic stresses are collocated. We also analyzed the pattern of distribution of these genes across the BAC/PAC clones spanning the region so that candidate genes can be short listed for a functional analysis. Genes known to have a role in submergence tolerance were present in both the regions. Region 1, had a unique transcription factor like trithorax protein, which is a positional candidate gene for submergence tolerance. Pyruvate decarboxylase (PDC) gene for alcohol fermentation and cation transporting ATPase c-terminal domain are likely candidates for submergence QTL in Region 2. Genes such as SKP1 and elicitor induced cytochrome p450 associated with tissue necrosis and insect resistance were found in region 1. Multiple copies of ORFs for signal transduction proteins, transcription factors, genes for systemic acquired resistance, Ubiquitin proteins and pathogen elicitor identification and degrading proteins were located as a cluster in Region 2, where bacterial blight resistance genes mapped. Validation of the data obtained from TIGR with other databases (RAP and KOME) confirmed our findings. The functional role of some of the significant candidate genes needs to be established. Allele/gene specific markers can then be designed for use in MAS thus enhancing durable tolerance/resistance faster.

Regulation of salt tolerance by Torulaspora delbrueckii calcineurin target Crz1p

Recently, the academic interest in the yeast Torulaspora delbrueckii has increased notably due to its high resistance to several types of stress, including salt and osmotic imbalance. However, the molecular mechanisms underlying these unusual properties are poorly understood. In Saccharomyces cerevisiae, the high-salt response is mediated by calcineurin, a conserved Ca(2+)/calmodulin-modulated protein phosphatase that regulates the transcriptional factor Crz1p. Here, we cloned the T. delbrueckii TdCRZ1 gene, which encodes a putative zinc finger transcription factor homologue to Crz1p. Consistent with this, overexpression of TdCRZ1 enhanced the salt tolerance of S. cerevisiae wild-type cells and suppressed the sensitivity phenotype of cnb1Delta and crz1Delta mutants to monovalent and divalent cations. However, T. delbrueckii cells lacking TdCrz1p showed phenotypes distinct from those previously observed in S. cerevisiae crz1Delta mutants. Quite remarkably, Tdcrz1-null cells were insensitive to high Na(+) and were more Li(+) tolerant than wild-type cells. Clearly, TdCrz1p was not required for the salt-induced transcriptional activation of the TdENA1 gene, encoding a putative P-type ATPase homologue to the main S. cerevisiae Na(+) pump ENA1. Furthermore, T. delbrueckii cells were insensitive to the immunosuppressive agents FK506 and cyclosporine A, both in the presence and in the absence of NaCl. Signaling through the calcineurin/Crz1 pathway appeared to be essential only on high-Ca(2+)/Mn(2+) media. Hence, T. delbrueckii and S. cerevisiae differ in the regulatory circuits and mechanisms that drive the adaptive response to salt stress.

Heavy metal tolerance and metal homeostasis inPseudomonas putidaas revealed by complete genome analysis

The genome of Pseudomonas putida KT2440 encodes an unexpected capacity to tolerate heavy metals and metalloids. The availability of the complete chromosomal sequence allowed the categorization of 61 open reading frames likely to be involved in metal tolerance or homeostasis, plus seven more possibly involved in metal resistance mechanisms. Some systems appeared to be duplicated. These might perform redundant functions or be involved in tolerance to different metals. In total, P. putida was found to bear two systems for arsenic (arsRBCH), one for chromate (chrA), four to six systems for divalent cations (two cadA and two to four czc chemiosmotic antiporters), two systems for monovalent cations: pacS, cusCBA (plus one cryptic silP gene containing a frameshift mutation), two operons for Cu chelation (copAB), one metallothionein for metal(loid) binding, one system for Te/Se methylation (tpmT) and four ABC transporters for the uptake of essential Zn, Mn, Mo and Ni (one nikABCDE, two znuACB and one mobABC). Some of the metal-related clusters are located in gene islands with atypical genome signatures. The predicted capacity of P. putida to endure exposure to heavy metals is discussed from an evolutionary perspective.

Membrane topology of CadA homologous P-type ATPase of Helicobacter pylori as determined by expression of phoA fusions in Escherichia coli and the positive inside rule

The only experimental data available on the membrane topology of transition metal ATPases are from in vitro studies on two distinct P-type ATPases (CadA and CopA) of a gastric bacterium, Helicobacter pylori, both postulated to contain eight transmembrane domains (H1 to H8). In this study, H. pylori CadA ATPase was subjected to analysis of membrane topology in vivo by expression of ATPase-alkaline phosphatase (AP) hybrid proteins in Escherichia coli using a novel vector, pBADphoA. This vector contains an inducible arabinose promoter and unique restriction sites for fusion of DNA fragments to phoA. The phoA gene lacking sequences encoding its N-terminal signal peptide was linked to the C-terminal regions of the postulated five cytoplasmic and four periplasmic segments of the H. pylori pump. The results obtained by heterologous expression of ATPase-AP hybrid proteins showed consistence with a model of eight transmembrane domains. They also demonstrated that the H. pylori ATPase sequences are well assembled in the cytoplasmic membrane of E. coli, a neutralophilic bacterium. Cloning and amino acid sequence analysis of the homologous ATPase of Helicobacter felis further verified the topological model for the H. pylori pump analyzed here, although the degree of amino acid sequence identity varied between the corresponding transmembrane segments, from 25% for H1 up to 100% for H6. It was found that the topology of ATPase follows the 'positive inside rule'. With respect to the bioenergetic capacities of H. pylori, we discuss here the membrane potential as a possible factor directing insertion of ATPases in the cytoplasmic membrane of gastric bacteria.

Evidence for a salt bridge between transmembrane segments 5 and 6 of the yeast plasma-membrane H+-ATPase

The plasma-membrane H+-ATPase of Saccharomyces cerevisiae, which belongs to the P2 subgroup of cation-transporting ATPases, is encoded by the PMA1 gene and functions physiologically to pump protons out of the cell. This study has focused on hydrophobic transmembrane segments M5 and M6 of the H+-ATPase. In particular, a conserved aspartate residue near the middle of M6 has been found to play a critical role in the structure and biogenesis of the ATPase. Site-directed mutants in which Asp-730 was replaced by an uncharged residue (Asn or Val) were abnormally sensitive to trypsin, consistent with the idea that the proteins were poorly folded, and immunofluorescence confocal microscopy showed them to be arrested in the endoplasmic reticulum. Similar defects are known to occur when either Arg-695 or His-701 in M5 is replaced by a neutral residue (Dutra, M. B., Ambesi, A., and Slayman, C. W. (1998) J. Biol. Chem. 273, 17411-17417). To search for possible charge-charge interactions between Asp-730 and Arg-695 or His-701, double mutants were constructed in which positively and negatively charged residues were swapped or eliminated. Strikingly, two of the double mutants (R695D/D730R and R695A/D730A) regained the capacity for normal biogenesis and displayed near-normal rates of ATP hydrolysis and ATP-dependent H+ pumping. These results demonstrate that neither Arg-695 nor Asp-730 is required for enzymatic activity or proton transport, but suggest that there is a salt bridge between the two residues, linking M5 and M6 of the 100-kDa polypeptide.

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.

Regulation and pH-dependent expression of a bilaterally truncated yeast plasma membrane H+-ATPase

Constitutive, chromosomal expression of yeast pma1 deletion alleles in Saccharomyces cerevisiae yielded functional, truncated forms of the plasma membrane H+-ATPase which were independently capable of supporting wild type yeast growth rates. Deletion of 27 amino-terminal residues affected neither the enzyme's activity nor its responsiveness to changes in glucose metabolism. By contrast, removal of 18 carboxy-terminal amino acids produced an enzyme with a Vmax that was relatively insensitive to glucose-dependent metabolic status and with a Km that was significantly lower than that of the wild type enzyme. These effects were exaggerated when the amino- and carboxy-terminal deletions were combined in a bilaterally truncated H+-ATPase, suggesting that the amino terminus may have a subtle role in modulating ATPase activity. In pma1DeltaDelta cells cultured at pH 6, plasma membrane H+-ATPase levels were much lower than those in cells expressing a wild type ATPase. Increased expression levels could be achieved by growing the pma1DeltaDelta mutant at pH 3, a result that was at least partially due to a sustained, elevated transcription of pma1DeltaDelta mRNA. Our observations suggest that intracellular proton balance can be maintained by regulation of the activity and/or quantity of H+-ATPase in the plasma membrane.

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.

Properties of the P-type ATPases encoded by the copAP operons of Helicobacter pylori and Helicobacter felis

The cop operons of Helicobacter pylori and Helicobacter felis were cloned by gene library screening. Both operons contain open reading frames for a P-type ion pump (CopA) with homology to Cd2+ and Cu2+ ATPases and a putative ion binding protein (CopP), the latter representing a CopZ homolog of the copYZAB operon of Enterococcus hirae. The predicted CopA ATPases contained an N-terminal GMXCXXC ion binding motif and a membrane-associated CPC sequence. A synthetic N-terminal peptide of the H. pylori CopA ATPase bound to Cu2+ specifically, and gene disruption mutagenesis of CopA resulted in an enhanced growth sensitivity of H. pylori to Cu2+ but not to other divalent cations. As determined experimentally, H. pylori CopA contains four pairs of transmembrane segments (H1 to H8), with the ATP binding and phosphorylation domains lying between H6 and H7, as found for another putative transition metal pump of H. pylori (K. Melchers, T. Weitzenegger, A. Buhmann, W. Steinhilber, G. Sachs, and K. P. Schäfer, J. Biol. Chem. 271:446-457, 1996). The corresponding transmembrane segments of the H. felis CopA pump were identified by hydrophobicity analysis and via sequence similarity. To define functional domains, similarly oriented regions of the two enzymes were examined for sequence identity. Regions with high degrees of identity included the N-terminal Cu2+ binding domain, the regions of ATP binding and phosphorylation in the energy transduction domain, and a transport domain consisting of the last six transmembrane segments with conserved cysteines in H4, H6, and H7. The data suggest that H. pylori and H. felis employ conserved mechanisms of ATPase-dependent copper resistance.

Probing of the membrane topology of sarcoplasmic reticulum Ca2+-ATPase with sequence-specific antibodies. Evidence for plasticity of the c-terminal domain

The topology of Ca2+-ATPase in sarcoplasmic reticulum (SR) vesicles was investigated with the aid of sequence-specific antibodies, produced against oligopeptides corresponding to sequences close to the membranous portions of the protein. The antisera in competitive enzyme-linked immunosorbent assays only reacted with intact SR vesicles to a limited extent, but most epitopic regions were exposed by low concentrations of nondenaturing detergent, octaethylene glycol dodecyl ether (C12E8) or after removal of cytosolic regions by proteinase K. In particular, these treatments exposed the loop regions in the C-terminal domain, including L7-8, the loop region located between transmembrane segments M7 and M8, with a putative intravesicular position, which had immunochemical properties very similar to those of the C terminus with a documented cytosolic exposure. In contrast to this, the reactivity of the N-terminal intravesicular loop regions L1-2 and L3-4 was only increased by C12E8 treatment but not by proteinase K proteolysis. Complexation of Ca2+-ATPase with beta,gamma-CrATP stabilized the C-terminal domain of Ca2+-ATPase against proteinase K proteolysis and reaction with most of the antisera, but immunoreactivity was maintained by the L6-7 and L7-8 loops. Immunoelectron microscopic analyses of vesicles following negative staining, thin sectioning, and the SDS-digested freeze-fracture labeling method suggested that the L7-8 epitope, in contrast to L6-7 and the C terminus, can be exposed on either the intravesicular or cytosolic side of the membrane. A preponderant intravesicular location of L7-8 in intact vesicles is suggested by the susceptibility of this region to proteolytic cleavage after disruption of the vesicular barrier with C12E8 and in symmetrically reconstituted Ca2+-ATPase proteoliposomes. In conclusion, our data suggest an adaptable membrane insertion of the C-terminal Ca2+-ATPase domain, which under some conditions permits sliding of M8 through the membrane with cytosolic exposure of L7-8, of possible functional significance in connection with Ca2+ translocation. On the technical side, our data emphasize that extreme caution is needed when using nondenaturing detergents or other treatments like EGTA at alkaline pH to open up vesicles for probing of intravesicular location with antibodies.

Structural aspects of the gastric H,K ATPase

The gastric H,K ATPase is an alpha beta heterodimeric member of the eukaryotic alkali-cation P-type ion-motive ATPase family. The alpha subunit is composed of 1033 amino acids and the beta subunit of 291 amino acids with 6 or 7 potential N-linked glycosylation sites. Much effort has been expended to define the membrane domain of P-type ATPases. A membrane domain of the large subunit consisting of 10 membrane-spanning sequences is suggested by a combination of methods such as (1) tryptic digestion, separation, and sequencing of membrane peptides, (2) labeling with extracytoplasmic reagents, and (3) in vitro translation of hydrophobic segments. The beta subunit has a single transmembrane segment with strong hydrophobic interactions with the alpha subunit. Blue native gel electrophoresis shows that the enzyme is an (alpha-beta)2 dimer. Cross-linking with Cu-phenanthroline provides evidence that association is between the alpha subunits, and the potential SH groups that are Cu sensitive are at cysteine 565 and cysteine 615, in the region of the large cytoplasmic loop between the fourth and fifth transmembrane segments. No cross-linking is observed in the membrane domain. ATP prevents cross-linking because of a conformational change at the surface of the protein induced by ATP or by direct binding of the nucleotide at the site of cross-linking. The WGA binding properties of the beta subunit allow investigation of the region of interaction with the alpha subunit. Thus, digestion of the enzyme by trypsin followed by SDS solubilization and selective elution from a WGA column resulted in coelution of the membrane fragment containing TM7 and TM8. This result demonstrates major hydrophobic interaction between the seventh and eighth transmembrane segments and the beta subunit. An antibody generated against rat parietal cells also recognized shared epitopes in the same region of both the alpha and beta subunits. Biochemical investigation of the arrangement of the transmembrane segments has been hindered by the lack of effective cross-linking reagents probably because of the compact arrangement of this domain, preventing even Cu access. A series of antiulcer drugs has been developed that have a unique chemistry related to their inhibition of the gastric H,K ATPase. They are 2-(substituted pyridyl methylsulfinyl) benzimidazoles, weak bases with a pKa of 4.0. After accumulation in the acidic space generated by the H,K ATPase either in vivo or in vitro, they undergo acid-catalyzed conversion to a tetracyclic sulfenamide which reacts with luminally accessible SH residues to form stable disulfide derivatives. In the particular case of pantoprazole, 2-(3,4-dimethoxy-2-pyridyl-methylsulfinyl)-5-difluoromethoxy benzimidazole, reaction is confined largely to cysteine 813, placed between the fifth and sixth transmembrane segments. The 5 azido analog of pantoprazole provided acid transport-dependent inhibition of the isolated transporting ATPase by this photoactivatable covalent SH reagent. The inhibited enzyme was then photolyzed, cleaved with trypsin, and the membrane fragments compared before and after photolysis. Disappearance of the segment corresponding to TM3,4 and a relative loss of the segment corresponding to TM7,8 suggests close proximity of these two membrane pairs to the loop joining the fifth and sixth transmembrane segments, in particular TM3,4. Use of this type of covalent, photoactivatable site-specific reagent to determine loop proximity can be extended to other acid transporters.

Sites of reaction of the gastric H,K-ATPase with extracytoplasmic thiol reagents

The vesicular gastric H,K-ATPase catalyzes an electroneutral H for K exchange allowing acidification of the intravesicular space. There is a total of 28 cysteines present in the alpha subunit of the gastric H,K-ATPase, of which 10 are found in the predicted transmembrane segments and their connecting loop, and 9 are present in the beta subunit, of which 6 are disulfide-linked. To determine which of these was accessible to extracytoplasmic attack, the enzyme was inhibited by four different substituted 2-pyridylmethylsulfinyl benzimidazoles, 5-methoxy-2-[(4-methoxy-3, 5-dimethyl-2-pyridyl)methylsulfinyl]-1H-benzimidazole (omeprazole), 2-[(4-trifluoroethoxy-3-methyl-2-pyridyl)methylsulfinyl]-1H-ben zimida zole (lansoprazole), 5-difluoromethoxy-2-[3, 4-methoxy-2-pyridyl)methylsulfinyl]-1H-benzimidazole (pantoprazole), and 2-[(4-(3-methoxypropoxy)-3-methyl)-2-pyridyl)methylsulfinyl]-1H-++ +benzi midazole (rabeprazole), under acid transporting conditions. All of these compounds are weak bases that accumulate in the acidic space generated by the pump and undergo an acid catalyzed rearrangement to a cationic sulfenamide, which forms disulfides with accessible cysteines. The relative rates of acid activation of these compounds corresponded to the relative rates of inhibition of ATPase activity and acid transport. Fragmentation of the enzyme by trypsin followed by SDS-polyacrylamide gel electrophoresis showed that omeprazole bound covalently to one of the two cysteines in the domains containing the fifth and sixth transmembrane segments and their extracytoplasmic loop and to cysteine 892 in the loop between the seventh and eighth transmembrane segments, but inhibition correlated with the reaction with cysteines in the fifth and sixth domain. Lansoprazole bound to the cysteines in these two domains as well as to cysteine 321 toward the extracytoplasmic end of the third transmembrane segments. Pantoprazole bound only to either cysteine 813 or 822 in the fifth and sixth transmembrane region. The inhibition of Rabeprazole correlated also with its binding to this part of the protein, but this compound continued to bind after full inhibition, eventually binding also to cysteines 321 and 892. No binding was found to any of the cysteines in the seventh to tenth transmembrane segments. Thermolysin digestion of the isolated omeprazole-labeled fifth and sixth transmembrane pair showed that cysteine 813 was the site of labeling. It is concluded that binding of these sided reagents to cysteine 813 in the loop between transmembrane (TM)5 and TM6 is sufficient for inhibition of ATPase activity and acid transport by the gastric acid pump. Of the 10 cysteines present in the membrane and extracytoplasmic domain, only three are exposed sufficiently to allow reactivity with these cationic thiol reagents. The binding to cysteine 813 defines the location of the extracytoplasmic loop between TM5 and TM6 and places the carboxylic acids 820 and 824 conserved between the gastric H,K- and the Na,K-ATPases in TM6, consistent with their assumed role in cation binding.

Exploring an antifungal target in the plasma membrane H(+)-ATPase of fungi

The plasma membrane H(+)-ATPase is a promising new antifungal target that is readily probed with the sulfhydryl-reactive reagent omeprazole. Inhibition of the H(+)-ATPase by omeprazole is closely linked to cell killing, and it has been suggested that enzyme inhibition may result from a covalent interaction within the first two transmembrane segments (M1 and M2) (Monk et al. (1995) Biochim. Biophys. Acta 1239, 81-90). In this study, the molecular nature of this interaction was examined by screening a series of 26 well-characterized pma1 mutations residing in the first two transmembrane segments of the H(+)-ATPase from Saccharomyces cerevisiae. Only two pma1 mutants, A135G and G158D,G156C, were found to significantly decrease the sensitivity of cells for omeprazole. In contrast, enhanced sensitivity was observed at a number of positions, with D140C(A) and M128C producing the most significant increases in sensitivity. The introduction of cysteine at various locations within this region only marginally affected omeprazole sensitivity, suggesting that this region was not a direct site of covalent modification. Rather, its conformation influences omeprazole binding at some other locus. In order to determine the sidedness of the omeprazole interaction, a novel in vitro assay system was exploited that utilized liposomes co-reconstituted with the H(+)-ATPase and the light-driven proton pump bacteriorhodopsin. Omeprazole was found to completely inhibit proton transport by the H(+)-ATPase at 50 microM in this system. An asymmetrically-distributed chemical trap system involving glutathione was used to demonstrate that this inhibition appears localized to the extracellular portion of the enzyme. This work indicates that omeprazole can inhibit the H(+)-ATPase from its extracellular face, and this inhibition is influenced by changes in the M1, M2 region of the protein.

In vitro translation analysis of integral membrane proteins

A method of in vitro translation scanning was applied to a variety of polytopic integral membrane proteins, a transition metal P type ATPase from Helicobacter pylori, the SERCA 2 ATPase, the gastric H+,K+ ATPase, the CCK-A receptor and the human ileal bile acid transporter. This method used vectors containing the N terminal region of the gastric H+,K+ ATPase or the N terminal region of the CCK-A receptor, coupled via a linker region to the last 177 amino acids of the beta-subunit of the gastric H+,K+ ATPase. The latter contains 5 potential N-linked glycosylation sites. Translation of vectors containing the cDNA encoding one, two or more putative transmembrane domains in the absence or presence of microsomes allowed determination of signal anchor or stop transfer properties of the putative transmembrane domains by the molecular weight shift on SDS PAGE. The P type ATPase from Helicobacter pylori showed the presence of 8 transmembrane segments with this method. The SERCA 2 Ca2+ ATPase with this method had 9 transmembrane co-translational insertion domains and coupled with other evidence these data resulted in a 11 transmembrane segment model. Translation of segments of the gastric H+,K+ ATPase provided evidence for only 7 transmembrane segments but coupled with other data established a 10 membrane segment model. The G7 protein, the CCK-A receptor showed the presence of 6 of the 7 transmembrane segments postulated for this protein. Translation of segments of the human ileal bile acid transporter showed the presence of 8 membrane insertion domains. However, translation of the intact protein provided evidence for an odd number of transmembrane segments, resulting in a tentative model containing 7 or 9 transmembrane segments. Neither G7 type protein appeared to have an arrangement of sequential topogenic signals consistent with the final assembled protein. This method provides a useful addition to methods of determining membrane domains of integral membrane proteins but must in general be utilized with other methods to establish the number of transmembrane alpha-helices.

Influence of pH on Ammonia Accumulation and Toxicity in Halophilic, Methylotrophic Methanogens

We studied the effects of pH and ammonia concentration on the growth of three methanogens. These three halophilic, methylotrophic methanogens, Methanolobus bombayensis, Methanolobus taylorii, and Methanohalophilus zhilinaeae, grew at environmental pH ranges that overlapped with each other and spanned the pH range from 7.0 to 9.5. During growth they had reversed membrane pH gradients ((Delta)pH) at all pH values tested. The (Delta)pH was in the range of -0.4 to -0.9 pH units, with the cytosol being more acidic than the environmental pH. Methanohalophilus zhilinaeae had the most negative (Delta)pH (-0.9 pH units). These negative pH gradients resulted in the accumulation of ammonium (NH(inf4)(sup+)), and when grown at the highest external ammonia concentrations that allowed good growth, cells had cytosolic NH(inf4)(sup+) concentrations as high as 180 mM. The high concentrations of cytosolic NH(inf4)(sup+) were accompanied by greater (Delta)pH and lower concentrations of the major cytosolic cation K(sup+) (compared with cells grown in medium with only 5 mM ammonia). Methanolobus bombayensis and Methanolobus taylorii were more sensitive to total external ammonia at higher external pH values, but the inhibitory concentration of un-ionized ammonia that resulted in a 50% reduction of the growth rate was about 2 to 5 mM, regardless of the pH. This is consistent with growth inhibition by ammonia in other bacteria. However, Methanohalophilus zhilinaeae was more resistant to un-ionized ammonia than any other known organism. It had a 50% inhibitory concentration for un-ionized ammonia of 13 mM at pH 8.5 and 45 mM at pH 9.5. We examined the effects of pH on three ammonia-assimilating activities (glutamine synthetase, glutamate dehydrogenase, and alanine dehydrogenase) in cell lysates and found that the pH ranges were consistent with the observed ranges of intracellular pH.

Probing the cytoplasmic LOOP1 domain of the yeast plasma membrane H(+)-ATPase by targeted factor Xa proteolysis

The cytoplasmic domain linking transmembrane segments 2 and 3 (LOOP1) of the yeast H(+)-ATPase was probed by the introduction of unique factor Xa recognition sites. Three sites, I170EGR, I254EGR and I275EGR, representing different structural regions of the LOOP1 domain, were engineered by site-specific mutagenesis of the PMA1 gene. In each case, multiple amino acid substitutions were required to form the factor Xa sites, which enabled an analysis of clustered mutations. Both I170EGR and I275EGR-containing mutants grew at normal rates, but showed prominent growth resistance to hygromycin B and sensitivity to low external pH. The engineered I254EGR site within the predicted beta-strand region produced a recessive lethal phenotype, indicating that mutations G254I and F257R were not tolerated. Mutant I170EGR- and I275EGR-containing enzymes showed relatively normal Km and Vmax values, but they displayed a strong insensitivity to inhibition by vanadate. An I170EGR/I275EGR double mutant was more significantly perturbed showing a reduced Vmax and pronounced vanadate insensitivity. The I170EGR site within the putative alpha-helical stalk region was cleaved to a maximum of 10% by factor Xa under non-denaturing conditions resulting in a characteristic 81 kDa fragment, whereas the I275EGR site, near the end of the beta-strand region, showed about 30-35% cleavage with the appearance of a 70 kDa fragment. A I170EGR/I275EGR double mutant enzyme showed about 55-60% cleavage. The cleavage profile for the mutant enzymes was enhanced under denaturing conditions, but was unaffected by MgATP or MgATP plus vanadate. Cleavage at the I275EGR position had no adverse effects on ATP hydrolysis or proton transport by the H(+)-ATPase making it unlikely that this localized region of LOOP1 influences coupling. Overall, these results suggest that the local region encompassing I275EGR is accessible to factor Xa, while the region around I170EGR appears buried. Although there is no evidence for gross molecular motion at either site, the effects of multiple amino acid substitutions in these regions suggest that the LOOP1 domain is conformationally active, and that perturbations in this domain affect the distribution of conformational intermediates during steady-state catalysis.

Genetic probing of the first and second transmembrane helices of the plasma membrane H(+)-ATPase from Saccharomyces cerevisiae

Structural features of the putative helical hairpin region comprising transmembrane segments 1 (TM1) and 2 (TM2) of the yeast plasma membrane H(+)-ATPase were probed by site-directed mutagenesis. The importance of phenylalanine residues Phe-116, Phe-119, Phe-120, Phe-126, Phe-144, Phe-159, and Phe-163 was explored by alanine replacement mutagenesis. It was found that substitutions at all positions, except Phe-120 and Phe-144, produced viable enzymes, although a range of cellular growth phenotypes were observed like hygromycin B resistance and low pH sensitivity, which are linked to in vivo action of the H(+)-ATPase. Lethal positions Phe-120 and Phe-144, could be replaced with tryptophan to produce viable enzyme, although the F144W mutant was highly perturbed. ATP hydrolysis measurements showed that Km was not significantly altered for most mutant enzymes, whereas Vmax was moderately reduced with two mutants, F144W and F163A, showing less than 50% of the normal activity. Double Phe-->Ala mutations in TM1 and TM2 were constructed to examine whether such substitutions would result in a higher degree of enzyme destabilization. Mutant F116A/F119A was viable and gave a normal phenotype, while F159A/F163A was not viable. Other double mutants, F116A/F159A and F119AF/159A, which are predicted to lie juxtaposed on TM1 and TM2, produced non-functional enzymes. However, a viable F119V/F159A mutant was isolated and showed hygromycin B resistance. These results suggest that double mutations eliminating 2 phenylalanine residues strongly destabilize the enzyme. A putative proline kink at Gly-122/Pro-123 in TM1 is not essential for enzyme action since these residues could be variously substituted (G122A or G122N; P123A, P123G, or P123F) producing viable enzymes with moderate effects on in vitro ATP hydrolysis or proton transport. However, several substitutions produced prominent growth phenotypes, suggesting that local perturbations were occurring. The location of Pro-123 is important because Gly-122 and Pro-123 could not be exchanged. In addition, a double Pro-Pro created by a G122P mutation was lethal, suggesting that maintenance of an alpha-helical structure is important. Other mutations in the hairpin, including modification of a buried charged residue, E129A, were not critical for enzyme action. These data are consistent with the view that the helical hairpin comprising TM1 and TM2 has important structural determinants that contribute to its overall stability and flexibility.

Cloning and membrane topology of a P type ATPase from Helicobacter pylori

Southern blot screening of a genomic Helicobacter pylori library was employed to find a P type ATPase using a mixture of 16 DNA oligonucleotides coding for the DKTGT(I/L)T consensus sequence specific for the phosphorylation site of this family of ATPases. A positive clone, pRH439, was isolated and sequenced. The inserted 3.4-kb H. pylori DNA contained an intact open reading frame encoding a protein of 686 amino acids carrying the consensus sites for phosphorylation and ATP binding. The amino acid sequence exhibits a 25-30% identity with bacterial Cd2+ and Cu2+ ATPases. Genomic Southern blot analysis showed that this ATPase was present in all H. pylori strains examined, whereas it was not detectable in Campylobacter jejuni and other bacteria. The membrane topology of this ATPase was investigated using in vitro transcription/translation of fusion vectors to find signal anchor and/or stop transfer sequences. Eight regions of the H. pylori ATPase acted as signal anchor and/or stop transfer sequences and were ordered pairwise along the polypeptide chain placing the N and C-terminal amino acids in the cytoplasm. These transmembrane segments are contained between positions 73 and 92 (H1), 98 and 125 (H2), 128 and 148 (H3), 149 and 176 (H4), 309 and 327 (H5), 337 and 371 (H6), 637 and 658 (H7), and 659 and 685 (H8). The membrane domain of the ATPase, therefore, consists of at least four pairs of transmembrane segments with the phosphorylation site and ATP binding domain located in the large cytoplasmic loop between H6 and H7. The cytoplasmic domain contains several histidines and cysteines, perhaps indicative of divalent cation binding sites. There are several charged amino acids (3 Lys, 2 Glu, 2 Asp), predicted to be in the membrane domain mainly in H2, H3, and H4 and a Cys-Pro-Cys putative metal ion site in H6. The extracytoplasmic domain also has several charged amino acids (5 Glu, 1 Asp, 1 Lys, 1 Arg). It is likely that this novel protein is a heavy metal cation transporting ATPase and belongs to a family of P type ATPases containing eight transmembrane segments.

The membrane topology of the rat sarcoplasmic and endoplasmic reticulum calcium ATPases by in vitro translation scanning

The membrane topology of the rat endoplasmic reticulum (ER) and sarcoplasmic reticulum (SR) Ca2+ ATPases were investigated using in vitro transcription/translation of fusion vectors containing DNA sequences encoding putative membrane-spanning domains. The sequences of these Ca2+ ATPases are identical except for the COOH-terminal end, which contains an additional predicted transmembrane segment in the ER ATPase. The M0 and M1 fusion vectors (Bamberg, K., and Sachs, G. (1994) J. Biol. Chem. 269, 16909-16919) encode the NH2-terminal 101 (M0 vector) or 139 (M1 vector) amino acids of the H,K-ATPase alpha subunit followed by a linker region for insertion of putative transmembrane sequences and, finally, the COOH-terminal 177 amino acids of the H,K-ATPase beta subunit containing five N-linked glycosylation consensus sequences. The linker region was replaced by the putative transmembrane domains of the Ca2+ ATPases, either individually or in pairs. Transcription and translation were performed using [35S]methionine in a reticulocyte lysate system in the absence or presence of canine pancreatic microsomes. The translated fusion protein was identified by autoradiography following separation using SDS-polyacrylamide gel electrophoresis. When testing single transmembrane segments, this method detects signal anchor activity with M0 or stop transfer activity with M1. The first four predicted SERCA transmembrane domains acted as both signal anchor and stop transfer sequences. A construct containing the fifth predicted transmembrane segment was able to act only as a stop transfer sequence. The sixth transmembrane segment did not insert cotranslationally into the membrane. The seventh was able to act as both a signal anchor and stop transfer sequence, and the eighth showed stop transfer ability in the M1 vector. The ninth transmembrane segment had both signal anchor and stop transfer capacity, whereas the tenth transmembrane segment showed only stop transfer sequence properties. The eleventh transmembrane sequence, unique to the ER Ca2+ ATPase, had both signal anchor and stop transfer properties. These translation data provide direct experimental evidence for 8 or 9 of the 10 or 11 predicted transmembrane sequences in the current topological models for the SR or ER Ca2+ ATPases, respectively.

Magnesium transport in Salmonella typhimurium: mgtA encodes a P-type ATPase and is regulated by Mg2+ in a manner similar to that of the mgtB P-type ATPase

Salmonella typhimurium has three distinct Mg2+ transport systems: CorA, MgtA, and MgtB, each encoded by its respective gene. corA and mgtB have been previously sequenced and characterized. This report details the sequence and properties of mgtA. Like mgtB, mgtA encodes a P-type ATPase. The mgtA gene encodes a slightly smaller protein than does mgtB, with a predicted molecular mass of about 95 kDa, running at 91 kDa on protein gels, which compares with values of 101 and 102 kDa, respectively, for the MgtB protein. The deduced amino acid sequence of MgtA is only 50% identical to that of MgtB, with a further 25% conservative amino acid substitutions, surprisingly low for such otherwise functionally similar proteins from the same organism. Codon usage for each gene is normal for S. typhimurium, however, indicating that neither gene is the result of a recent acquisition from another organism. A single open reading frame at mgtA encodes MgtA, in contrast to mgtB, which is shown to be an operon encoding (5' to 3') the 22.5-kDa MgtC and the MgtB proteins. Genetic constructs were used to show that deletion of MgtC does not alter the expression or transport properties of MgtB, making the role of the companion MgtC protein unclear. (The S. typhimurium homolog of treR, which encodes a putative repressor for trehalose uptake, is encoded by a gene adjacent to mgtA, and its sequence is also reported. Finally, exteremely strong Mg(2+) regulation of the mgtA and mgtB promoters but not of the corA or treR promoters was demonstrated by cloning the appropriate DNA sequences with luxAB and measuring enhancement of light production as a function of extracellular Mg(2+) concentration. Lowering the extracellular Mg(2+) concentration from 10 mM to 1 or 10 microM elicited a transcriptional response of several thousandfold from both the mgtA and mgtB promoters.

The membrane topology of the carboxyl-terminal third of the Neurospora plasma membrane H(+)-ATPase

To localize transmembrane segments in the carboxyl-terminal third of the Neurospora plasma membrane H(+)-ATPase, we constructed fusion proteins on the cDNA level. These contained DNA fragments encoding hydrophilic residues of the amino and carboxyl termini of the H(+)-ATPase with a DNA fragment encoding the putative transmembrane segment. To report translocation into microsomes, a DNA fragment encoding three consensus N-linked glycosylation sites was engineered carboxyl-terminal to the putative transmembrane segment. Fusion proteins were synthesized in a Neurospora in vitro translation system supplemented with homologous microsomes. By the criteria of glycosylation of fusion proteins by microsomes, sedimentation of products with microsomes after alkaline extraction, and analysis of protected fragments generated from proteinase K digestion of integrated products, we localized six transmembrane segments in the carboxyl-terminal third of the H(+)-ATPase. These results support a 10-segment model of the Neurospora H(+)-ATPase.

Mutations of G158 and their second-site revertants in the plasma membrane H(+)-ATPase gene (pma1) in Saccharomyces cerevisiae

A G158D mutation residing near the cytoplasmic end of transmembrane segment 2 of the H(+)-ATPase from Saccharomyces cerevisiae appears to alter electrogenic proton transport by the proton pump (Perlin et al. (1988) J. Biol. Chem. 263, 18118-18122.) The mutation confers upon whole cells a pronounced growth sensitivity to low pH and a resistance to the antibiotic hygromycin B. The isolated enzyme retains high activity (70% of wild type) but is inefficient at pumping protons in a reconstituted vesicle system, suggesting that this enzyme may be partially uncoupled (Perlin et al. (1989) J. Biol. Chem. 264, 21857-21864). In this study, the acid-sensitive growth phenotype of the pma1-D158 mutant was utilized to isolate second site suppressor mutations in an attempt to probe structural interactions involving amino acid 158. Site-directed mutagenesis of the G158 locus was also performed to explore its local environment. Nineteen independent revertants of pma1-G158D were selected as low pH-resistant colonies. Four were full phenotypic revertants showing both low pH resistance and hygromycin B sensitivity. Of three full revertants analyzed further, one restored the original glycine residue at position 158 while the other two carried compensatory mutations V336A or F830S, in transmembrane segments 4 and 7, respectively. Partial revertants, which could grow on low pH medium but still retained hygromycin B resistance, were identified in transmembrane segments 1 (V127A) and 2 (C148T, G156C), as well as in the cytoplasmic N-terminal domain (E110K) and in the cytoplasmic loop between transmembrane segments 2 and 3 (D170N, L275S). Relative to the G158D mutant, all revertants showed enhanced net proton transport in whole-cell medium acidification assays and/or improved ATP hydrolysis activity. Small polar amino acids (Asp and Ser) could be substituted for glycine at the 158 position to produce active, albeit somewhat defective, enzymes; larger hydrophobic residues (Leu and Val) produced more severe phenotypes. These results suggest that G158 is likely to reside in a tightly packed polar environment which interacts, either directly or indirectly, with transmembrane segments 1, 4 and 7. The revertant data are consistent with transmembrane segments 1 and 2 forming a conformationally sensitive helical hairpin structure.

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.

Distribution of the CorA Mg2+ transport system in gram-negative bacteria

The CorA Mg2+ transport system is the dominant constitutive uptake mechanism in Salmonella typhimurium and Escherichia coli. Southern blot hybridization and PCR techniques were used to screen a panel of 18 additional gram-negative bacterial species for corA homologs. Virtually all strains tested positive for the presence of corA. Thus, corA appears to be ubiquitous within gram-negative bacteria and is likely their major Mg2+ influx system.

Topological analyses of the L-fucose-H+ symport protein, FucP, from Escherichia coli

The transport of L-fucose into Escherichia coli is mediated by the L-fucose-H+ symport protein (FucP). The fucP gene has been sequenced and encodes a hydrophobic protein that contains 438 amino acid residues, with a predicted M(r) of 47,773. The hydropathic profile of FucP indicates 10 to 12 hydrophobic regions that could span the membrane as alpha-helices. A 12-helix model with the N- and C-termini located in the cytoplasm was derived from the hydropathic profile and from application of the 'positive inside' rule. This model was tested using beta-lactamase fusion technology. Analyses of 62 different FucP-beta-lactamase fusions suggested that the FucP protein crosses the cytoplasmic membrane of E. coli 12 times, with the N- and C-termini in the cytoplasm. From measurements of [14C]-L-fucose uptake, it was deduced that the last putative transmembrane region must be complete for transport activity to be retained and that the four C-terminal residues were unnecessary for transport activity. Fourier transform analyses show that all the predicted helices contain a periodicity that enables hydrophobic/hydrophilic faces to be identified; these were particularly evident in putative helices 1, 3, 4, 5, 6, 10 and 11.

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.

Structure, transmembrane topology and helix packing of P-type ion pumps

Electron microscopy has recently provided improved structures for P-type ion pumps. In the case of Ca(2+)-ATPase, the use of unstained specimens revealed the structure of the transmembrane domain. The composition of this domain has been controversial due to the variety of methods used to study the number and exact locations of transmembrane crossings within the sequence. After reviewing the results from several members of the family, we found a consensus for 10 transmembrane segments, and also that 10 helices fitted well into the structure of Ca(2+)-ATPase. Thus, we present the most detailed model for transmembrane structure so far, in the hope of stimulating more precise experimental strategies.

The roles of magnesium in biotechnology

This review highlights the important roles played by magnesium in the growth and metabolic functions of microbial and animal cells, and therefore assigns a key role for magnesium ions in biotechnology. The fundamental biochemical and physiological actions of magnesium as a regulatory cation are outlined. Such actions are deemed to be relevant in an applied sense, because Mg2+ availability in cell culture and fermentation media can dramatically influence growth and metabolism of cells. Manipulation of extracellular and intracellular magnesium ions can thus be envisaged as a relatively simplistic, but nevertheless versatile, means of physiological cell engineering. In addition, biological antagonism between calcium and magnesium at the molecular level may have profound consequences for the optimization of biotechnological processes that exploit cells. In fermentation, for example, it is argued that the efficiency of microbial conversion of substrate to product may be improved by altering Mg:Ca concentration ratios in industrial feedstocks in a way that makes more magnesium available to the cells. With particular respect to yeast-based biotechnologies, magnesium availability is seen as being crucially important in governing central pathways of carbohydrate catabolism, especially ethanolic fermentation. It is proposed that such influences of magnesium ions are expressed at the combined levels of key enzyme activation and cell membrane stabilization. The former ensures optimum flow of substrate to ethanol and the latter acts to protect yeasts from physical and chemical stress.