MgtA and MgtB: prokaryotic P-type ATPases that mediate Mg2+ influx

ERMA (TMEM94) is a P-type ATPase transporter for Mg2+ uptake in the endoplasmic reticulum

Intracellular Mg2+ (iMg2+) is bound with phosphometabolites, nucleic acids, and proteins in eukaryotes. Little is known about the intracellular compartmentalization and molecular details of Mg2+ transport into/from cellular organelles such as the endoplasmic reticulum (ER). We found that the ER is a major iMg2+ compartment refilled by a largely uncharacterized ER-localized protein, TMEM94. Conventional and AlphaFold2 predictions suggest that ERMA (TMEM94) is a multi-pass transmembrane protein with large cytosolic headpiece actuator, nucleotide, and phosphorylation domains, analogous to P-type ATPases. However, ERMA uniquely combines a P-type ATPase domain and a GMN motif for ERMg2+ uptake. Experiments reveal that a tyrosine residue is crucial for Mg2+ binding and activity in a mechanism conserved in both prokaryotic (mgtB and mgtA) and eukaryotic Mg2+ ATPases. Cardiac dysfunction by haploinsufficiency, abnormal Ca2+ cycling in mouse Erma+/- cardiomyocytes, and ERMA mRNA silencing in human iPSC-cardiomyocytes collectively define ERMA as an essential component of ERMg2+ uptake in eukaryotes.

Flipping the switch: dynamic modulation of membrane transporter activity in bacteria

The controlled entry and expulsion of small molecules across the bacterial cytoplasmic membrane is essential for efficient cell growth and cellular homeostasis. While much is known about the transcriptional regulation of genes encoding transporters, less is understood about how transporter activity is modulated once the protein is functional in the membrane, a potentially more rapid and dynamic level of control. In this review, we bring together literature from the bacterial transport community exemplifying the extensive and diverse mechanisms that have evolved to rapidly modulate transporter function, predominantly by switching activity off. This includes small molecule feedback, inhibition by interaction with small peptides, regulation through binding larger signal transduction proteins and, finally, the emerging area of controlled proteolysis. Many of these examples have been discovered in the context of metal transport, which has to finely balance active accumulation of elements that are essential for growth but can also quickly become toxic if intracellular homeostasis is not tightly controlled. Consistent with this, these transporters appear to be regulated at multiple levels. Finally, we find common regulatory themes, most often through the fusion of additional regulatory domains to transporters, which suggest the potential for even more widespread regulation of transporter activity in biology.

Complete genome assembly of Hawai'i environmental nontuberculous mycobacteria reveals unexpected co-isolation with methylobacteria

Nontuberculous mycobacteria (NTM) are ubiquitous environmental opportunistic pathogens that can cause chronic lung disease. Within the United States, Hawai'i has the highest incidence of NTM lung disease, though the precise reasons are yet to be fully elucidated. One possibility is the high prevalence of NTM in the Hawai'i environment acting as a potential reservoir for opportunistic NTM infections. Through our previous initiatives to collect and characterize NTM in Hawai'i, community scientists of Hawai'i have collected thousands of environmental samples for sequencing. Here, these community scientists were invited for the first time into a high school lab in O'ahu for a genomic sequencing workshop, where participants sequenced four of the collected isolate genomic samples using the Oxford Nanopore Technologies MinION sequencer. Participants generated high quality long read data that when combined with short read Illumina data yielded complete bacterial genomic assemblies suitable for in-depth analysis. The gene annotation analysis identified a suite of genes that might help NTM thrive in the Hawai'i environment. Further, we found evidence of co-occurring methylobacteria, revealed from the sequencing data, suggesting that in some cases methylobacteria and NTM may coexist in the same niche, challenging previously accepted paradigms. The sequencing efforts presented here generated novel insights regarding the potential survival strategies and microbial interactions of NTM in the geographic hot spot of Hawai'i. We highlight the contributions of community scientists and present an activity that can be reimplemented as a workshop or classroom activity by other research groups to engage their local communities.

Bacterial redox response factors in the management of environmental oxidative stress

Bacteria evolved to survive in the available environmental chemosphere via several cellular mechanisms. A rich pool of antioxidants and stress regulators plays a significant role in the survival of bacteria in unfavorable environmental conditions. Most of the microbes exhibit resistant phenomena in toxic environment niches. Naturally, bacteria possess efficient thioredoxin reductase, glutaredoxin, and peroxiredoxin redox systems to handle environmental oxidative stress. Further, an array of transcriptional regulators senses the oxidative stress conditions. Transcription regulators, such as OxyR, SoxRS, PerR, UspA, SsrB, MarA, OhrR, SarZ, etc., sense and transduce bacterial oxidative stress responses. The redox-sensitive transcription regulators continuously recycle the utilized antioxidant enzymes during oxidative stress. These regulators promote the expression of antioxidant enzymes such as superoxide dismutase, catalase, and peroxides that overcome oxidative insults. Therefore, the transcriptional regulations maintain steady-state activities of antioxidant enzymes representing the resistance against host cell/environmental oxidative insults. Further, the redox system provides reducing equivalents to synthesize biomolecules, thereby contributing to cellular repair mechanisms. The inactive transcriptional regulators in the undisturbed cells are activated by oxidative stress. The oxidized transcriptional regulators modulate the expression of antioxidant and cellular repair enzymes to survive in extreme environmental conditions. Therefore, targeting these antioxidant systems and response regulators could alter cellular redox homeostasis. This review presents the mechanisms of different redox systems that favor bacterial survival in extreme environmental oxidative stress conditions.

Ion selectivity and gating behavior of the CorA-type channel Bpss1228

Magnesium is an essential element to sustain all forms of life. Total intracellular magnesium content is determined by the balance of magnesium influx and efflux. CorA is a divalent selective channel in the metal ion transport superfamily and is the major Mg²⁺ uptake pathway in prokaryotes and eukaryotic mitochondria. Previous studies have demonstrated that CorA showed distinct magnesium bound closed conformation and Mg²⁺-free states. In addition, CorA is regulated by cytoplasmic magnesium ions and its gating mechanism has been investigated by electron paramagnetic resonance technique and molecular dynamic simulations. Here, we report a study of the putative CorA-type channel Bpss1228 from Burkholderia pseudomallei, which has been shown to be significantly associated with pseudomallei infection. We expressed and purified the Bpss1228 in full-length. Subsequently, electrophysiological experiments further investigated the electrical characteristics of Bpss1228 and revealed that it was a strictly cation-selective channel. We also proved that Bpss1228 not only possessed magnesium-mediated regulatory property a remarkable ability to be modulated by magnesium ions. Finally, we observed the three-step gating behavior of Bpss1228 on planar lipid bilayer, and further proposed a synergistic gating mechanism by which CorA family channels control intracellular magnesium homeostasis.

Structural and functional properties of a magnesium transporter of the SLC11/NRAMP family

Members of the ubiquitous SLC11/NRAMP family catalyze the uptake of divalent transition metal ions into cells. They have evolved to efficiently select these trace elements from a large pool of Ca²⁺ and Mg²⁺, which are both orders of magnitude more abundant, and to concentrate them in the cytoplasm aided by the cotransport of H⁺ serving as energy source. In the present study, we have characterized a member of a distant clade of the family found in prokaryotes, termed NRMTs, that were proposed to function as transporters of Mg²⁺. The protein transports Mg²⁺ and Mn²⁺ but not Ca²⁺ by a mechanism that is not coupled to H⁺. Structures determined by cryo-EM and X-ray crystallography revealed a generally similar protein architecture compared to classical NRAMPs, with a restructured ion binding site whose increased volume provides suitable interactions with ions that likely have retained much of their hydration shell.

Current Understandings on Magnesium Deficiency and Future Outlooks for Sustainable Agriculture

The productivity of agricultural produce is fairly dependent on the availability of nutrients and efficient use. Magnesium (Mg2+) is an essential macronutrient of living cells and is the second most prevalent free divalent cation in plants. Mg2+ plays a role in several physiological processes that support plant growth and development. However, it has been largely forgotten in fertilization management strategies to increase crop production, which leads to severe reductions in plant growth and yield. In this review, we discuss how the Mg2+ shortage induces several responses in plants at different levels: morphological, physiological, biochemical and molecular. Additionally, the Mg2+ uptake and transport mechanisms in different cellular organelles and the role of Mg2+ transporters in regulating Mg2+ homeostasis are also discussed. Overall, in this review, we critically summarize the available information about the responses of Mg deficiency on plant growth and development, which would facilitate plant scientists to create Mg2+-deficiency-resilient crops through agronomic and genetic biofortification.

A Bordetella pertussis MgtC homolog plays a role in the intracellular survival

Bordetella pertussis, the causative agent of whooping cough, has the capability to survive inside the host cells. This process requires efficient adaptation of the pathogen to the intracellular environment and the associated stress. Among the proteins produced by the intracellular B. pertussis we identified a protein (BP0414) that shares homology with MgtC, a protein which was previously shown to be involved in the intracellular survival of other pathogens. To explore if BP0414 plays a role in B. pertussis intracellular survival a mutant strain defective in the production of this protein was constructed. Using standard in vitro growth conditions we found that BP0414 is required for B. pertussis growth under low magnesium availability or low pH, two environmental conditions that this pathogen might face within the host cell. Intracellular survival studies showed that MgtC is indeed involved in B. pertussis viability inside the macrophages. The use of bafilomycin A1, which inhibits phagosome acidification, abolished the survival defect of the mgtC deficient mutant strain suggesting that in intracellular B. pertussis the role of MgtC protein is mainly related to the bacterial adaptation to the acidic conditions found inside the of phagosomes. Overall, this work provides an insight into the importance of MgtC in B. pertussis pathogenesis and its contribution to bacterial survival within immune cells.

Asymmetry in structural response of inner and outer transmembrane segments of CorA protein by a coarse-grain model

Structure of CorA protein and its inner (i.corA) and outer (o.corA) transmembrane (TM) components are investigated as a function of temperature by a coarse-grained Monte Carlo simulation. Thermal response of i.corA is found to differ considerably from that of the outer component, o.corA. Analysis of the radius of gyration reveals that the inner TM component undergoes a continuous transition from a globular conformation to a random coil structure on raising the temperature. In contrast, the outer transmembrane component exhibits an abrupt (nearly discontinuous) thermal response in a narrow range of temperature. Scaling of the structure factor shows a globular structure of i.corA at a low temperature with an effective dimension D ∼ 3 and a random coil at a high temperature with D ∼ 2. The residue distribution in o.corA is slightly sparser than that of i.corA in a narrow thermos-responsive regime. The difference in thermos-response characteristics of these components (i.corA and o.corA) may reflect their unique transmembrane functions.

Critical Issues in the Study of Magnesium Transport Systems and Magnesium Deficiency Symptoms in Plants

Magnesium (Mg) is the second most abundant cation in living cells. Over 300 enzymes are known to be Mg-dependent, and changes in the Mg concentration significantly affects the membrane potential. As Mg becomes deficient, starch accumulation and chlorosis, bridged by the generation of reactive oxygen species, are commonly found in Mg-deficient young mature leaves. These defects further cause the inhibition of photosynthesis and finally decrease the biomass. Recently, transcriptome analysis has indicated the transcriptinal downregulation of chlorophyll apparatus at the earlier stages of Mg deficiency, and also the potential involvement of complicated networks relating to hormonal signaling and circadian oscillation. However, the processes of the common symptoms as well as the networks between Mg deficiency and signaling are not yet fully understood. Here, for the purpose of defining the missing pieces, several problems are considered and explained by providing an introduction to recent reports on physiological and transcriptional responses to Mg deficiency. In addition, it has long been unclear whether the Mg deficiency response involves the modulation of Mg2+ transport system. In this review, the current status of research on Mg2+ transport and the relating transporters are also summarized. Especially, the rapid progress in physiological characterization of the plant MRS2 gene family as well as the fundamental investigation about the molecular mechanism of the action of bacterial CorA proteins are described.

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.

Structural dynamics of the magnesium-bound conformation of CorA in a lipid bilayer

The transmembrane conformation of Thermotoga maritima CorA, a magnesium transport system, has been studied in its Mg(2+)-bound form by site-directed spin labeling and electron paramagnetic resonance spectroscopy. Probe mobility together with accessibility data were used to evaluate the overall dynamics and relative arrangement of individual transmembrane segments TM1 and TM2. TM1 extends toward the cytoplasmic side creating a water-filled cavity, while TM2 is located in the periphery of the oligomer, contacting the lipid bilayer. A structural model for the conserved extracellular loop was generated based on EPR data and MD simulations, in which residue E316 is located toward the five-fold symmetry axis in position to electrostatically influence divalent ion translocation. Electrostatic analysis of our model suggest that, in agreement with the crystal structure, Mg(2+) -bound CorA is in a closed conformation. The present results suggest that long-range structural rearrangements are necessary to allow Mg(2+) translocation.

A bacterial mRNA leader that employs different mechanisms to sense disparate intracellular signals

Bacterial mRNAs often contain leader sequences that respond to specific metabolites or ions by altering expression of the associated downstream protein-coding sequences. Here we report that the leader RNA of the Mg(2+) transporter gene mgtA of Salmonella enterica, which was previously known to function as a Mg(2+)-sensing riboswitch, harbors an 18 codon proline-rich open reading frame-termed mgtL-that permits intracellular proline to regulate mgtA expression. Interfering with mgtL translation by genetic, pharmacological, or environmental means was observed to increase the mRNA levels from the mgtA coding region. Substitution of the mgtL proline codons by other codons abolished the response to proline and to hyperosmotic stress but not to Mg(2+). Our findings show that mRNA leader sequences can consist of complex regulatory elements that utilize different mechanisms to sense separate signals and mediate an appropriate cellular response.

The CorA Mg2+ channel is required for the virulence of Salmonella enterica serovar typhimurium

CorA is the primary Mg(2+) channel in Salmonella enterica serovar Typhimurium. A corA mutant is attenuated in mice and defective for invasion of and replication within epithelial cells. Microarray studies show that several virulence effectors are repressed in a corA mutant strain, which ultimately manifests itself as a decrease in virulence.

Crystallization and preliminary X-ray diffraction analysis of the cytosolic domain of the Mg2+ transporter MgtE

The MgtE family of Mg(2+) transporters is ubiquitously conserved in all domains of life. The cytosolic domains of the MgtE Mg(2+) transporters include a cystathionine-beta-synthase (CBS) domain which is known to play a regulatory function in transporter proteins. The cytosolic domain of MgtE from Thermus thermophilus was overexpressed, purified and crystallized in the presence and absence of Mg(2+). The crystals formed in the presence of Mg(2+) diffracted X-rays to 2.3 A resolution using synchrotron radiation, belong to space group P6(5)22 with unit-cell parameters a = b = 57.7, c = 317.6 A and are expected to contain one molecule in the asymmetric unit. The crystals formed in the absence of Mg(2+) diffracted X-rays to 3.5 A resolution using synchrotron radiation, belong to space group P2(1)2(1)2(1) with unit-cell parameters a = 77.0, b = 100.3, c = 100.3 A and are expected to contain two molecules in the asymmetric unit.

Contribution of the PhoP-PhoQ and PmrA-PmrB two-component regulatory systems to Mg2+-induced gene regulation in Pseudomonas aeruginosa

When grown in divalent cation-limited medium, Pseudomonas aeruginosa becomes resistant to cationic antimicrobial peptides and polymyxin B. This resistance is regulated by the PhoP-PhoQ and PmrA-PmrB two-component regulatory systems. To further characterize Mg(2+) regulation in P. aeruginosa, microarray transcriptional profiling was conducted to compare wild-type P. aeruginosa grown under Mg(2+)-limited and Mg(2+)-replete conditions to isogenic phoP and pmrA mutants grown under Mg(2+)-limited conditions. Under Mg(2+)-limited conditions (0.02 mM Mg(2+)), approximately 3% of the P. aeruginosa genes were differentially expressed compared to the expression in bacteria grown under Mg(2+)-replete conditions (2 mM Mg(2+)). Only a modest subset of the Mg(2+)-regulated genes were regulated through either PhoP or PmrA. To determine which genes were directly regulated, a bioinformatic search for conserved binding motifs was combined with confirmatory reverse transcriptase PCR and gel shift promoter binding assays, and the results indicated that very few genes were directly regulated by these response regulators. It was found that in addition to the previously known oprH-phoP-phoQ operon and the pmrHFIJKLM-ugd operon, the PA0921 and PA1343 genes, encoding small basic proteins, were regulated by Mg(2+) in a PhoP-dependent manner. The number of known PmrA-regulated genes was expanded to include the PA1559-PA1560, PA4782-PA4781, and feoAB operons, in addition to the previously known PA4773-PA4775-pmrAB and pmrHFIJKLM-ugd operons.

Novel genes affecting urease acivity in Actinobacillus pleuropneumoniae

Characterization of a series of urease-negative transposon mutations of Actinobacillus pleuropneumoniae revealed that many of the mutants had insertions 2 to 4 kbp upstream of the urease gene cluster. A 5-kbp upstream region of DNA was sequenced and found to contain six open reading frames (ORFs) transcribed in the same orientation as the urease genes. As well, a partial ORF, apuR, 202 bp upstream of these six ORFs, is transcribed in the opposite orientation. The predicted product of this partial ORF shows homology with many members of the LysR family of transcriptional regulators. Five of the ORFs (cbiKLMQO) appear to form an operon encoding a putative nickel and cobalt periplasmic permease system. The cbiM and cbiQ genes encode proteins that have sequence similarity with known cobalt transport membrane proteins, and the cbiO gene encodes a cobalt transport ATP-binding protein homologue. The product of the cbiK gene is predicted to be the periplasmic-binding-protein component of the system, though it does not show any sequence similarity with CbiN, the cobalt-binding periplasmic protein. Escherichia coli clones containing this putative transport operon together with the urease genes of A. pleuropneumoniae were urease positive when grown in unsupplemented Luria-Bertani broth, whereas a clone containing only the minimal urease gene cluster required the addition of high concentrations of NiCl(2) for full urease activity. This result supports the hypothesis that nickel is a substrate for this permease system. The sixth ORF, utp, appears to encode an integral membrane protein which has significant sequence identity with mammalian urea transport proteins, though its function in A. pleuropneumoniae remains to be determined.

Characterization of a gene encoding a Ca(2+)-ATPase-like protein in the plastid envelope

By screening an Arabidopsis expression library with an antiserum against chloroplast envelope proteins, we have isolated a partial cDNA with an open reading frame that encodes a polypeptide similar to P-type cation-transporting ATPases. The corresponding genomic clone was isolated and the complete coding sequence was deduced after identification and mapping of introns. The gene has been designated PEA1 (plastid envelope ATPase) and the predicted polypeptide PEA1p. PEA1p has 946 amino acids and a molecular mass of 104 kDa. This protein is 40-44% identical to various mammalian plasma membrane Ca(2+)-ATPases but lacks the C-terminal calmodulin binding domain present in the mammalian polypeptides. When aligned with mammalian plasma membrane Ca(2+)-ATPases, PEA1p has a 70- to 80-amino acid N-terminal region that extends beyond the N terminus of these enzymes. This extension has some similarity to the transit peptide of the plastid envelope phosphate translocator and may function to target the protein to the plastid. Antibodies raised against a portion of PEA1p recognize a single 90- to 95-kDa polypeptide in chloroplast inner envelope preparations. Transcript abundance as determined by RNase protection was found to be 7- to 9-fold higher in roots than in leaves. Possible roles for a plastid envelope calcium pump are suggested.