Mrs2p forms a high conductance Mg2+ selective channel in mitochondria

Molecular cloning and functional characterization of mitochondrial RNA splicing 2 in fish Megalobrama amblycephala, and its potential roles in magnesium homeostasis and mitochondrial function

Mitochondrial function can be regulated by ion channels. Mitochondrial RNA splicing 2 (Mrs2) is a magnesium ion (Mg2+) channel located in the inner mitochondrial membrane, thereby mediating the Mg2+ influx into the mitochondrial matrix. However, its potential role in regulating the Mg homeostasis and mitochondrial function in aquatic species is still unclear. This study molecularly characterizes the gene encoding Mrs2 in fish M. amblycephala with its functions in maintaining the Mg homeostasis and mitochondrial function verified. The mrs2 gene is 2133 bp long incorporating a 1269 bp open reading frame, which encodes 422 amino acids. The Mrs2 protein includes two transmembrane domains and a conserved tripeptide Gly-Met-Asn, and has a high homology (65.92-97.64%) with those of most vertebrates. The transcript of mrs2 was relatively high in the white muscle, liver and kidney. The inhibition of mrs2 reduces the expressions of Mg2+ influx/efflux-related proteins, mitochondrial Mg content, and the activities of mitochondrial complex I and V in hepatocytes. However, the over-expression of mrs2 increases the expressions of Mg2+ influx/efflux-related proteins, mitochondrial Mg content, and the complex V activity, but decreases the activities of mitochondrial complex III and IV and citrate synthase in hepatocytes. Collectively, Mrs2 is highly conserved among different species, and is prerequisite for maintaining Mg homeostasis and mitochondrial function in fish.

N-Glycosylation of MRS2 balances aerobic and anaerobic energy production by reducing rapid mitochondrial Mg2+ influx in conditions of high glucose or impaired respiratory chain function

N-linked glycoproteins function in numerous biological processes, modulating enzyme activities as well as protein folding, stability, oligomerization, and trafficking. While N-glycosylation of mitochondrial proteins has been detected by untargeted MS-analyses, the physiological existence and roles of mitochondrial protein N-linked glycosylation remain under debate. Here, we report that MRS2, a mitochondrial inner membrane protein that functions as the high flux magnesium transporter, is N-glycosylated to various extents depending on cellular bioenergetic status. Both N-glycosylated and unglycosylated isoforms were consistently detected in mitochondria isolated from mouse liver, rat and mouse liver fibroblast cells (BRL 3A and AFT024, respectively) as well as human skin fibroblast cells. Immunoblotting of MRS2 showed it was bound to, and required stringent elution conditions to remove from, lectin affinity columns with covalently bound concanavalin A or Lens culinaris agglutinin. Following peptide:N-glycosidase F (PNGase F) digestion of the stringently eluted proteins, the higher Mr MRS2 bands gel-shifted to lower Mr and loss of lectin affinity was seen. BRL 3A cells treated with two different N-linked glycosylation inhibitors, tunicamycin or 6-diazo-5-oxo-l-norleucine, resulted in decreased intensity or loss of the higher Mr MRS2 isoform. To investigate the possible functional role of MRS2 N- glycosylation, we measured rapid Mg2+ influx capacity in intact mitochondria isolated from BRL 3A cells in control media or following treatment with tunicamycin or 6-diazo-5-oxo-l-norleucine. Interestingly, rapid Mg2+ influx capacity increased in mitochondria isolated from BRL 3A cells treated with either N-glycosylation inhibitor. Forcing reliance on mitochondrial respiration by treatment with either galactose media or the glycolytic inhibitor 2-deoxyglucose or by minimizing glucose concentration similarly reduced the N-glycosylated isoform of MRS2, with a correlated concomitant increase in rapid Mg2+ influx capacity. Conversely, inhibiting mitochondrial energy production in BRL 3A cells with either rotenone or oligomycin resulted in an increased fraction of N-glycosylated MRS2, with decreased rapid Mg2+ influx capacity. Collectively, these data provide strong evidence that MRS2 N-glycosylation is directly involved in the regulation of mitochondrial matrix Mg2+, dynamically communicating relative cellular nutrient status and bioenergetic capacity by serving as a physiologic brake on the influx of mitochondrial matrix Mg2+ under conditions of glucose excess or mitochondrial bioenergetic impairment.

Rare earth elements perturb root architecture and ion homeostasis in Arabidopsis thaliana

Rare earth elements (REEs) are crucial elements for current high-technology and renewable energy advances. In addition to their increasing usage and their low recyclability leading to their release into the environment, REEs are also used as crop fertilizers. However, little is known regarding the cellular and molecular effects of REEs in plants, which is crucial for better risk assessment, crop safety and phytoremediation. Here, we analysed the ionome and transcriptomic response of Arabidopsis thaliana exposed to a light (lanthanum, La) and a heavy (ytterbium, Yb) REE. At the transcriptome level, we observed the contribution of ROS and auxin redistribution to the modified root architecture following REE exposure. We found indications for the perturbation of Fe homeostasis by REEs in both roots and leaves of Arabidopsis suggesting competition between REEs and Fe. Furthermore, we propose putative ways of entry of REEs inside cells through transporters of microelements. Finally, similar to REE accumulating species, organic acid homeostasis (e.g. malate and citrate) appears critical as a tolerance mechanism in response to REEs. By combining ionomics and transcriptomics, we elucidated essential patterns of REE uptake and toxicity response of Arabidopsis and provide new hypotheses for a better evaluation of the impact of REEs on plant homeostasis.

Cryo-EM structures of human magnesium channel MRS2 reveal gating and regulatory mechanisms

Magnesium ions (Mg2+) play an essential role in cellular physiology. In mitochondria, protein and ATP synthesis and various metabolic pathways are directly regulated by Mg2+. MRS2, a magnesium channel located in the inner mitochondrial membrane, mediates the influx of Mg2+ into the mitochondrial matrix and regulates Mg2+ homeostasis. Knockdown of MRS2 in human cells leads to reduced uptake of Mg2+ into mitochondria and disruption of the mitochondrial metabolism. Despite the importance of MRS2, the Mg2+ translocation and regulation mechanisms of MRS2 are still unclear. Here, using cryo-EM we report the structures of human MRS2 in the presence and absence of Mg2+ at 2.8 Å and 3.3 Å, respectively. From the homo-pentameric structures, we identify R332 and M336 as major gating residues, which are then tested using mutagenesis and two cellular divalent ion uptake assays. A network of hydrogen bonds is found connecting the gating residue R332 to the soluble domain, potentially regulating the gate. Two Mg2+-binding sites are identified in the MRS2 soluble domain, distinct from the two sites previously reported in CorA, a homolog of MRS2 in prokaryotes. Altogether, this study provides the molecular basis for understanding the Mg2+ translocation and regulatory mechanisms of MRS2.

Cryo-EM structures of human magnesium channel MRS2 reveal gating and regulatory mechanisms

Magnesium ions (Mg2+) play an essential role in cellular physiology. In mitochondria, protein and ATP synthesis and various metabolic pathways are directly regulated by Mg2+. MRS2, a magnesium channel located in the inner mitochondrial membrane, mediates the influx of Mg2+ into the mitochondrial matrix and regulates Mg2+ homeostasis. Knockdown of MRS2 in human cells leads to reduced uptake of Mg2+ into mitochondria and disruption of the mitochondrial metabolism. Despite the importance of MRS2, the Mg2+ translocation and regulation mechanisms of MRS2 are still unclear. Here, using cryo-EM we determined the structure of human MRS2 in the presence and absence of Mg2+ at 2.8 Å and 3.3 Å, respectively. From the homo-pentameric structures, we identified R332 and M336 as major gating residues, which were then tested using mutagenesis and two cellular divalent ion uptake assays. A network of hydrogen bonds was found connecting the gating residue R332 to the soluble domain, potentially regulating the gate. Two Mg2+-binding sites were identified in the MRS2 soluble domain, distinct from the two sites previously reported in CorA, a homolog of MRS2 in prokaryotes. Altogether, this study provides the molecular basis for understanding the Mg2+ translocation and regulatory mechanisms of MRS2.

Structure and function of the human mitochondrial MRS2 channel

The human Mitochondrial RNA Splicing 2 protein (MRS2) has been implicated in Mg2+ transport across mitochondrial inner membranes, thus playing an important role in Mg2+ homeostasis critical for mitochondrial integrity and function. However, the molecular mechanisms underlying its fundamental channel properties such as ion selectivity and regulation remain unclear. Here, we present structural and functional investigation of MRS2. Cryo-electron microscopy structures in various ionic conditions reveal a pentameric channel architecture and the molecular basis of ion permeation and potential regulation mechanisms. Electrophysiological analyses demonstrate that MRS2 is a Ca2+-regulated, non-selective channel permeable to Mg2+, Ca2+, Na+ and K+, which contrasts with its prokaryotic ortholog, CorA, operating as a Mg2+-gated Mg2+ channel. Moreover, a conserved arginine ring within the pore of MRS2 functions to restrict cation movements, likely preventing the channel from collapsing the proton motive force that drives mitochondrial ATP synthesis. Together, our results provide a molecular framework for further understanding MRS2 in mitochondrial function and disease.

Molecular basis of Mg2+ permeation through the human mitochondrial Mrs2 channel

Mitochondrial RNA splicing 2 (Mrs2), a eukaryotic CorA ortholog, enables Mg2+ to permeate the inner mitochondrial membrane and plays an important role in mitochondrial metabolic function. However, the mechanism by which Mrs2 permeates Mg2+ remains unclear. Here, we report four cryo-electron microscopy (cryo-EM) reconstructions of Homo sapiens Mrs2 (hMrs2) under various conditions. All of these hMrs2 structures form symmetrical pentamers with very similar pentamer and protomer conformations. A special structural feature of Cl--bound R-ring, which consists of five Arg332 residues, was found in the hMrs2 structure. Molecular dynamics simulations and mitochondrial Mg2+ uptake assays show that the R-ring may function as a charge repulsion barrier, and Cl- may function as a ferry to jointly gate Mg2+ permeation in hMrs2. In addition, the membrane potential is likely to be the driving force for Mg2+ permeation. Our results provide insights into the channel assembly and Mg2+ permeation of hMrs2.

Post-translational modifications and protein quality control of mitochondrial channels and transporters

Mitochondria play a critical role in energy metabolism and signal transduction, which is tightly regulated by proteins, metabolites, and ion fluxes. Metabolites and ion homeostasis are mainly mediated by channels and transporters present on mitochondrial membranes. Mitochondria comprise two distinct compartments, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM), which have differing permeabilities to ions and metabolites. The OMM is semipermeable due to the presence of non-selective molecular pores, while the IMM is highly selective and impermeable due to the presence of specialized channels and transporters which regulate ion and metabolite fluxes. These channels and transporters are modulated by various post-translational modifications (PTMs), including phosphorylation, oxidative modifications, ions, and metabolites binding, glycosylation, acetylation, and others. Additionally, the mitochondrial protein quality control (MPQC) system plays a crucial role in ensuring efficient molecular flux through the mitochondrial membranes by selectively removing mistargeted or defective proteins. Inefficient functioning of the transporters and channels in mitochondria can disrupt cellular homeostasis, leading to the onset of various pathological conditions. In this review, we provide a comprehensive overview of the current understanding of mitochondrial channels and transporters in terms of their functions, PTMs, and quality control mechanisms.

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg²⁺ transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Alzheimer's Disease-Associated SNP rs708727 in SLC41A1 May Increase Risk for Parkinson's Disease: Report from Enlarged Slovak Study

SLC41A1 (A1) SNPs rs11240569 and rs823156 are associated with altered risk for Parkinson's disease (PD), predominantly in Asian populations, and rs708727 has been linked to Alzheimer's disease (AD). In this study, we have examined a potential association of the three aforementioned SNPs and of rs9438393, rs56152218, and rs61822602 (all three lying in the A1 promoter region) with PD in the Slovak population. Out of the six tested SNPs, we have identified only rs708727 as being associated with an increased risk for PD onset in Slovaks. The minor allele (A) in rs708727 is associated with PD in dominant and completely over-dominant genetic models (OR_D = 1.36 (1.05-1.77), p = 0.02, and OR_COD = 1.34 (1.04-1.72), p = 0.02). Furthermore, the genotypic triplet GG_(rs708727) + AG_(rs823156) + CC_(rs61822602) might be clinically relevant despite showing a medium (h ≥ 0.5) size difference (h = 0.522) between the PD and the control populations. RandomForest modeling has identified the power of the tested SNPs for discriminating between PD-patients and the controls to be essentially zero. The identified association of rs708727 with PD in the Slovak population leads us to hypothesize that this A1 polymorphism, which is involved in the epigenetic regulation of the expression of the AD-linked gene PM20D1, is also involved in the pathoetiology of PD (or universally in neurodegeneration) through the same or similar mechanism as in AD.

Mitochondrial Metal Ion Transport in Cell Metabolism and Disease

Mitochondria are vital to life and provide biological energy for other organelles and cell physiological processes. On the mitochondrial double layer membrane, there are a variety of channels and transporters to transport different metal ions, such as Ca²⁺, K⁺, Na⁺, Mg²⁺, Zn²⁺ and Fe²⁺/Fe³⁺. Emerging evidence in recent years has shown that the metal ion transport is essential for mitochondrial function and cellular metabolism, including oxidative phosphorylation (OXPHOS), ATP production, mitochondrial integrity, mitochondrial volume, enzyme activity, signal transduction, proliferation and apoptosis. The homeostasis of mitochondrial metal ions plays an important role in maintaining mitochondria and cell functions and regulating multiple diseases. In particular, channels and transporters for transporting mitochondrial metal ions are very critical, which can be used as potential targets to treat neurodegeneration, cardiovascular diseases, cancer, diabetes and other metabolic diseases. This review summarizes the current research on several types of mitochondrial metal ion channels/transporters and their functions in cell metabolism and diseases, providing strong evidence and therapeutic strategies for further insights into related diseases.

The antagonistic effect of magnesium hydroxide particles on vascular endothelial activation induced by acidic PLGA degradation products

Although drug-eluting stents (DESs) are mainly coated with biodegradable polymers such as PLGA and PLLA, their acidic degradation products can alter the local microenvironment and affect the homeostasis of adjacent tissue. Previously, we developed anti-inflammatory PLGA-based materials including magnesium hydroxide (MH) to relieve the side effects caused by PLGA degradation. However, the underlying molecular mechanism of its protective effects has not yet been clarified. Here, we demonstrated the pathological mechanism of vascular endothelial activation caused by PLGA by-products. The PLGA by-products accumulated in HCAECs through MCT1, followed by oxidative stress and the activation of the MAPK/NF-κB signaling pathway. Finally, the PLGA by-products increased the expression of VCAM-1 as well as the secretion of proinflammatory cytokines. However, the addition of MH particles significantly diminished the activation of this molecular pathway and the expression of inflammation-related factors induced by acidic PLGA degradation products. Furthermore, Mg2+ released from MH particles restored endothelial function in both intracellular and extracellular spaces. Taken together, MH particles prevent the accumulation of PLGA degradation products in HCAECs, thereby repressing the associated vascular endothelial activation. These findings on the biochemical mechanisms are expected to provide important clues for addressing the safety issues in nearly all biodegradable polymer-based implants.

Asymmetric CorA Gating Mechanism as Observed by Molecular Dynamics Simulations

The CorA family of proteins plays a housekeeping role in the homeostasis of divalent metal ions in many bacteria and archaea as well as in mitochondria of eukaryotes, rendering it an important target to study the mechanisms of divalent transport and regulation across different life domains. Despite numerous studies, the mechanistic details of the channel gating and the transport of the metal ions are still not entirely understood. Here, we use all-atom and coarse-grained molecular dynamics simulations combined with in vitro experiments to investigate the influence of divalent cations on the function of CorA. Simulations reveal pronounced asymmetric movements of monomers that enable the rotation of the α7 helix and the cytoplasmic subdomain with the subsequent formation of new interactions and the opening of the channel. These computational results are functionally validated using site-directed mutagenesis of the intracellular cytoplasmic domain residues and biochemical assays. The obtained results infer a complex network of interactions altering the structure of CorA to allow gating. Furthermore, we attempt to reconcile the existing gating hypotheses for CorA to conclude the mechanism of transport of divalent cations via these proteins.

Lactate Elicits ER-Mitochondrial Mg2+ Dynamics to Integrate Cellular Metabolism

Mg2+ is the most abundant divalent cation in metazoans and an essential cofactor for ATP, nucleic acids, and countless metabolic enzymes. To understand how the spatio-temporal dynamics of intracellular Mg2+ (iMg2+) are integrated into cellular signaling, we implemented a comprehensive screen to discover regulators of iMg2+ dynamics. Lactate emerged as an activator of rapid release of Mg2+ from endoplasmic reticulum (ER) stores, which facilitates mitochondrial Mg2+ (mMg2+) uptake in multiple cell types. We demonstrate that this process is remarkably temperature sensitive and mediated through intracellular but not extracellular signals. The ER-mitochondrial Mg2+ dynamics is selectively stimulated by L-lactate. Further, we show that lactate-mediated mMg2+ entry is facilitated by Mrs2, and point mutations in the intermembrane space loop limits mMg2+ uptake. Intriguingly, suppression of mMg2+ surge alleviates inflammation-induced multi-organ failure. Together, these findings reveal that lactate mobilizes iMg2+ and links the mMg2+ transport machinery with major metabolic feedback circuits and mitochondrial bioenergetics.

General Aspects of Metal Ions as Signaling Agents in Health and Disease

This review focuses on the current knowledge on the involvement of metal ions in signaling processes within the cell, in both physiological and pathological conditions. The first section is devoted to the recent discoveries on magnesium and calcium-dependent signal transduction-the most recognized signaling agents among metals. The following sections then describe signaling pathways where zinc, copper, and iron play a key role. There are many systems in which changes in intra- and extra-cellular zinc and copper concentrations have been linked to important downstream events, especially in nervous signal transduction. Iron signaling is mostly related with its homeostasis. However, it is also involved in a recently discovered type of programmed cell death, ferroptosis. The important differences in metal ion signaling, and its disease-leading alterations, are also discussed.

Cation permeability in CorA family of proteins

CorA proteins belong to 2-TM-GxN family of membrane proteins, and play a major role in Mg²⁺ transport in prokaryotes and eukaryotic mitochondria. The selection of substrate is believed to occur via the signature motif GxN, however there is no consensus how strict this selection within the family. To answer this question, we employed fluorescence-based transport assays on three different family members, namely CorA from bacterium Thermotoga maritima, CorA from the archeon Methanocaldococcus jannaschii and ZntB from bacterium Escherichia coli, reconstituted into proteoliposomes. Our results show that all three proteins readily transport Mg²⁺, Co²⁺, Ni²⁺ and Zn²⁺, but not Al³⁺. Despite the similarity in cation specificity, ZntB differs from the CorA proteins, as in the former transport is stimulated by a proton gradient, but in the latter by the membrane potential, confirming the hypothesis that CorA and ZntB proteins diverged to different transport mechanisms within the same protein scaffold.

Real time dynamics of Gating-Related conformational changes in CorA

CorA, a divalent-selective channel in the metal ion transport superfamily, is the major Mg²⁺-influx pathway in prokaryotes. CorA structures in closed (Mg²⁺-bound), and open (Mg²⁺-free) states, together with functional data showed that Mg²⁺-influx inhibits further Mg²⁺-uptake completing a regulatory feedback loop. While the closed state structure is a symmetric pentamer, the open state displayed unexpected asymmetric architectures. Using high-speed atomic force microscopy (HS-AFM), we explored the Mg²⁺-dependent gating transition of single CorA channels: HS-AFM movies during Mg²⁺-depletion experiments revealed the channel’s transition from a stable Mg²⁺-bound state over a highly mobile and dynamic state with fluctuating subunits to asymmetric structures with varying degree of protrusion heights from the membrane. Our data shows that at Mg²⁺-concentration below K_d, CorA adopts a dynamic (putatively open) state of multiple conformations that imply structural rearrangements through hinge-bending in TM1. We discuss how these structural dynamics define the functional behavior of this ligand-dependent channel.

Harnessing Metal Homeostasis Offers Novel and Promising Targets Against Candida albicans

Fungal infections, particularly of Candida species, which are the commensal organisms of human, are one of the major debilitating diseases in immunocompromised patients. The limited number of antifungal drugs available to treat Candida infections, with the concomitant increasing incidence of multidrug-resistant (MDR) strains, further worsens the therapeutic options. Thus, there is an urgent need for the better understanding of MDR mechanisms, and their reversal, by employing new strategies to increase the efficacy and safety profiles of currently used therapies against the most prevalent human fungal pathogen, Candida albicans. Micronutrient availability during C. albicans infection is regarded as a critical factor that influences the progression and magnitude of the disease. Intracellular pathogens colonize a variety of anatomical locations that are likely to be scarce in micronutrients, as a defense strategy adopted by the host, known as nutritional immunity. Indispensable critical micronutrients are required both by the host and by C. albicans, especially as a cofactor in important metabolic functions. Since these micronutrients are not freely available, C. albicans need to exploit host reservoirs to adapt within the host for survival. The ability of pathogenic organisms, including C. albicans, to sense and adapt to limited micronutrients in the hostile environment is essential for survival and confers the basis of its success as a pathogen. This review describes that micronutrients availability to C. albicans is a key attribute that may be exploited when one considers designing strategies aimed at disrupting MDR in this pathogenic fungi. Here, we discuss recent advances that have been made in our understanding of fungal micronutrient acquisition and explore the probable pathways that may be utilized as targets.

The rise and fall of novel renal magnesium transporters

Body Mg2+ balance is finely regulated in the distal convoluted tubule (DCT), where a tight interplay among transcellular reabsorption, mitochondrial exchange, and basolateral extrusion takes place. In the last decades, several research groups have aimed to identify the molecular players in these processes. A multitude of proteins have been proposed to function as Mg2+ transporter in eukaryotes based on phylogenetic analysis, differential gene expression, and overexpression studies. However, functional evidence for many of these proteins is lacking. The aim of this review is, therefore, to critically reconsider all putative Mg2+ transporters and put their presumed function in context of the renal handling of Mg2+. Sufficient experimental evidence exists to acknowledge transient receptor potential melastatin (TRPM) 6 and TRPM7, solute carrier family 41 (SLC41) A1 and SLC41A3, and mitochondrial RNA splicing 2 (MRS2) as Mg2+ transporters. TRPM6/7 facilitate Mg2+ influx, SLC41A1 mediates Mg2+ extrusion, and MRS2 and SLC41A3 are implicated in mitochondrial Mg2+ homeostasis. These proteins are highly expressed in the DCT. The function of cyclin M (CNNM) proteins is still under debate. For the other proposed Mg2+ transporters including Mg2+ transporter subtype 1 (MagT1), nonimprinted in Prader-Willi/Angelman syndrome (NIPA), membrane Mg2+ transport (MMgT), Huntingtin-interacting protein 14 (HIP14), and ATP13A4, functional evidence is limited, or functions alternative to Mg2+ transport have been suggested. Additional characterization of their Mg2+ transport proficiency should be provided before further claims about their role as Mg2+ transporter can be made.

In vitro reconstitution, functional dissection, and mutational analysis of metal ion transport by mitoferrin-1

Iron is universally important to cellular metabolism, and mitoferrin-1 and -2 have been proposed to be the iron importers of mitochondria, the cell's assembly plant of heme and iron-sulfur clusters. These iron-containing prosthetic groups are critical for a host of physiological processes ranging from oxygen transport and energy consumption to maintaining protein structural integrity. Mitoferrin-1 (Mfrn1) belongs to the mitochondrial carrier (MC) family and is atypical given its putative metallic cargo; most MCs transport nucleotides, amino acids, or other small- to medium-size metabolites. Despite the clear importance of Mfrn1 in iron utilization, its transport activity has not been demonstrated unambiguously. To bridge this knowledge gap, we have purified recombinant Mfrn1 under non-denaturing conditions and probed its metal ion-binding and transport functions. Isothermal titration calorimetry indicates that Mfrn1 has micromolar affinity for Fe(II), Mn(II), Co(II), and Ni(II). Mfrn1 was incorporated into defined liposomes, and iron transport was reconstituted in vitro, demonstrating that Mfrn1 can transport iron. Mfrn1 can also transport manganese, cobalt, copper, and zinc but discriminates against nickel. Experiments with candidate ligands for cellular labile iron reveal that Mfrn1 transports free iron and not a chelated iron complex and selects against alkali divalent ions. Extensive mutagenesis identified multiple residues that are crucial for metal binding, transport activity, or both. There is a clear abundance of residues with side chains that can coordinate first-row transition metal ions, suggesting that these could form primary or auxiliary metal-binding sites during the transport process.

Magnesium Extravaganza: A Critical Compendium of Current Research into Cellular Mg2+ Transporters Other than TRPM6/7

Magnesium research has boomed within the last 20 years. The real breakthrough came at the start of the new millennium with the discovery of a plethora of possible Mg homeostatic factors that, in particular, included putative Mg²⁺ transporters. Until that point, Mg research was limited to biochemical and physiological work, as no target molecular entities were known that could be used to explore the molecular biology of Mg homeostasis at the level of the cell, tissue, organ, or organism and to translate such knowledge into the field of clinical medicine and pharmacology. Because of the aforementioned, Mg²⁺ and Mg homeostasis, both of which had been heavily marginalized within the biomedical field in the twentieth century, have become overnight a focal point of many studies ranging from primary biomedical research to translational medicine.The amount of literature concerning cellular Mg²⁺ transport and cellular Mg homeostasis is increasing, together with a certain amount of confusion, especially about the function(s) of the newly discovered and, in the majority of instances, still only putative Mg²⁺ transporters/Mg²⁺ homeostatic factors. Newcomers to the field of Mg research will thus find it particularly difficult to orient themselves.Here, we briefly but critically summarize the status quo of the current understanding of the molecular entities behind cellular Mg²⁺ homeostasis in mammalian/human cells other than TRPM6/7 chanzymes, which have been universally accepted as being unspecific cation channel kinases allowing the flux of Mg²⁺ while constituting the major gateway for Mg²⁺ to enter the cell.

Structure and Cooperativity of the Cytosolic Domain of the CorA Mg2+ Channel from Escherichia coli

Structures of the Mg²⁺ bound (closed) and apo (open) states of CorA suggests that channel gating is accomplished by rigid-body motions between symmetric and asymmetric assemblies of the cytosolic portions of the five subunits in response to ligand (Mg²⁺) binding/unbinding at interfacial sites. Here, we structurally and biochemically characterize the isolated cytosolic domain from Escherichia coli CorA. The data reveal an Mg²⁺-ligand binding site located in a novel position between each of the five subunits and two Mg²⁺ ions trapped inside the pore. Soaking experiments show that cobalt hexammine outcompetes Mg²⁺ at the pore site closest to the membrane. This represents the first structural information of how an analog of hexa-hydrated Mg²⁺ (and competitive inhibitor of CorA) associates to the CorA pore. Biochemical data on the isolated cytoplasmic domain and full-length protein suggests that gating of the CorA channel is governed cooperatively.

Comparative genomic analyses of transport proteins encoded within the red algae Chondrus crispus, Galdieria sulphuraria, and Cyanidioschyzon merolae11

Galdieria sulphuraria and Cyanidioschyzon merolae are thermo-acidophilic unicellular red algal cousins capable of living in volcanic environments, although the former can additionally thrive in the presence of toxic heavy metals. Bioinformatic analyses of transport systems were carried out on their genomes, as well as that of the mesophilic multicellular red alga Chondrus crispus (Irish moss). We identified transport proteins related to the metabolic capabilities, physiological properties, and environmental adaptations of these organisms. Of note is the vast array of transporters encoded in G. sulphuraria capable of importing a variety of carbon sources, particularly sugars and amino acids, while C. merolae and C. crispus have relatively few such proteins. Chondrus crispus may prefer short chain acids to sugars and amino acids. In addition, the number of encoded proteins pertaining to heavy metal ion transport is highest in G. sulphuraria and lowest in C. crispus. All three organisms preferentially utilize secondary carriers over primary active transporters, suggesting that their primary source of energy derives from electron flow rather than substrate-level phosphorylation. Surprisingly, the percentage of inorganic ion transporters encoded in C. merolae more closely resembles that of C. crispus than G. sulphuraria, but only C. crispus appears to signal via voltage-gated cation channels and possess a Na⁺ /K⁺ -ATPase and a Na⁺ exporting pyrophosphatase. The results presented in this report further our understanding of the metabolic potential and toxic compound resistances of these three organisms.

Identification of Yeast Mutants Exhibiting Altered Sensitivity to Valinomycin and Nigericin Demonstrate Pleiotropic Effects of Ionophores on Cellular Processes

Ionophores such as valinomycin and nigericin are potent tools for studying the impact of ion perturbance on cellular functions. To obtain a broader picture about molecular components involved in mediating the effects of these drugs on yeast cells under respiratory growth conditions, we performed a screening of the haploid deletion mutant library covering the Saccharomyces cerevisiae nonessential genes. We identified nearly 130 genes whose absence leads either to resistance or to hypersensitivity to valinomycin and/or nigericin. The processes affected by their protein products range from mitochondrial functions through ribosome biogenesis and telomere maintenance to vacuolar biogenesis and stress response. Comparison of the results with independent screenings performed by our and other laboratories demonstrates that although mitochondria might represent the main target for both ionophores, cellular response to the drugs is very complex and involves an intricate network of proteins connecting mitochondria, vacuoles, and other membrane compartments.

Structural Insights into Mitochondrial Calcium Uniporter Regulation by Divalent Cations

Calcium (Ca(2+)) flux into the matrix is tightly controlled by the mitochondrial Ca(2+) uniporter (MCU) due to vital roles in cell death and bioenergetics. However, the precise atomic mechanisms of MCU regulation remain unclear. Here, we solved the crystal structure of the N-terminal matrix domain of human MCU, revealing a β-grasp-like fold with a cluster of negatively charged residues that interacts with divalent cations. Binding of Ca(2+) or Mg(2+) destabilizes and shifts the self-association equilibrium of the domain toward monomer. Mutational disruption of the acidic face weakens oligomerization of the isolated matrix domain and full-length human protein similar to cation binding and markedly decreases MCU activity. Moreover, mitochondrial Mg(2+) loading or blockade of mitochondrial Ca(2+) extrusion suppresses MCU Ca(2+)-uptake rates. Collectively, our data reveal that the β-grasp-like matrix region harbors an MCU-regulating acidic patch that inhibits human MCU activity in response to Mg(2+) and Ca(2+) binding.

Mitochondrial Mg(2+) homeostasis decides cellular energy metabolism and vulnerability to stress

Cellular energy production processes are composed of many Mg²⁺ dependent enzymatic reactions. In fact, dysregulation of Mg²⁺ homeostasis is involved in various cellular malfunctions and diseases. Recently, mitochondria, energy-producing organelles, have been known as major intracellular Mg²⁺ stores. Several biological stimuli alter mitochondrial Mg²⁺ concentration by intracellular redistribution. However, in living cells, whether mitochondrial Mg²⁺ alteration affect cellular energy metabolism remains unclear. Mg²⁺ transporter of mitochondrial inner membrane MRS2 is an essential component of mitochondrial Mg²⁺ uptake system. Here, we comprehensively analyzed intracellular Mg²⁺ levels and energy metabolism in Mrs2 knockdown (KD) cells using fluorescence imaging and metabolome analysis. Dysregulation of mitochondrial Mg²⁺ homeostasis disrupted ATP production via shift of mitochondrial energy metabolism and morphology. Moreover, Mrs2 KD sensitized cellular tolerance against cellular stress. These results indicate regulation of mitochondrial Mg²⁺via MRS2 critically decides cellular energy status and cell vulnerability via regulation of mitochondrial Mg²⁺ level in response to physiological stimuli.

Magnesium Uptake by CorA Transporters Is Essential for Growth, Development and Infection in the Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae, the causative organism of rice blast, infects cereal crops and grasses at various stages of plant development. A comprehensive understanding of its metabolism and the implications on pathogenesis is necessary for countering this devastating crop disease. We present the role of the CorA magnesium transporters, MoAlr2 and MoMnr2, in development and pathogenicity of M. oryzae. The MoALR2 and MoMNR2 genes individually complement the Mg²⁺ uptake defects of a S. cerevisiae CorA transporter double mutant. MoALR2 and MoMNR2 respond to extracellular Mg²⁺ and Ca²⁺ levels and their expression is elevated under Mg²⁺ scarce conditions. RNA silencing mediated knockdown of MoALR2 (WT+siALR2, Δmnr2+siALR2 and ALR2+MNR2 simultaneous silencing) drastically alters intracellular cation concentrations and sensitivity to metal ions. MoALR2 silencing is detrimental to vegetative growth and surface hydrophobicity of mycelia, and the transformants display loss of cell wall integrity. MoALR2 is required for conidiogenesis and appressorium development, and is essential for infection. Investigation of knockdown transformants reveal low cAMP levels and altered expression of genes encoding proteins involved in MoMps1 cell wall integrity and cAMP MoPmk1 driven MAP Kinase signaling pathways. In contrast to MoALR2 knockdowns, the MoMNR2 deletion (Δmnr2) shows increased sensitivity to CorA inhibitors as well as altered cation sensitivity, but has limited effect on surface hydrophobicity and severity of plant infection. Interestingly, MoALR2 expression is elevated in Δmnr2. Impairment of development and infectivity of knockdown transformants and altered intracellular cation composition suggest that CorA transporters are essential for Mg²⁺ homeostasis within the cell, and are crucial to maintaining normal gene expression associated with cell structure, signal transduction and surface hydrophobicity in M. oryzae. We suggest that CorA transporters, and especially MoALR2, constitute an attractive target for the development of antifungal agents against this pathogen.

Solute carrier 41A3 encodes for a mitochondrial Mg(2+) efflux system

The important role of magnesium (Mg²⁺) in normal cellular physiology requires flexible, yet tightly regulated, intracellular Mg²⁺ homeostasis (IMH). However, only little is known about Mg²⁺ transporters of subcellular compartments such as mitochondria, despite their obvious importance for the deposition and reposition of intracellular Mg²⁺ pools. In particular, knowledge about mechanisms responsible for extrusion of Mg²⁺ from mitochondria is lacking. Based on circumstantial evidence, two possible mechanisms of Mg²⁺ release from mitochondria were predicted: (1) Mg²⁺ efflux coupled to ATP translocation via the ATP-Mg/Pi carrier, and (2) Mg²⁺ efflux via a H⁺/Mg²⁺ exchanger. Regardless, the identity of the H⁺-coupled Mg²⁺ efflux system is unknown. We demonstrate here that member A3 of solute carrier (SLC) family 41 is a mitochondrial Mg²⁺ efflux system. Mitochondria of HEK293 cells overexpressing SLC41A3 exhibit a 60% increase in the extrusion of Mg²⁺ compared with control cells. This efflux mechanism is Na⁺-dependent and temperature sensitive. Our data identify SLC41A3 as the first mammalian mitochondrial Mg²⁺ efflux system, which greatly enhances our understanding of intracellular Mg²⁺ homeostasis.

MLKL forms cation channels

The mixed lineage kinase domain-like (MLKL) protein is a key factor in tumor necrosis factor-induced necroptosis. Recent studies on necroptosis execution revealed a commitment role of MLKL in membrane disruption. However, our knowledge of how MLKL functions on membrane remains very limited. Here we demonstrate that MLKL forms cation channels that are permeable preferentially to Mg²⁺ rather than Ca²⁺ in the presence of Na⁺ and K⁺. Moreover, the N-terminal domain containing six helices (H1-H6) is sufficient to form channels. Using the substituted cysteine accessibility method, we further determine that helix H1, H2, H3, H5 and H6 are transmembrane segments, while H4 is located in the cytoplasm. Finally, MLKL-induced membrane depolarization and cell death exhibit a positive correlation to its channel activity. The Mg²⁺-preferred permeability and five transmembrane segment topology distinguish MLKL from previously identified Mg²⁺-permeable channels and thus establish MLKL as a novel class of cation channels.

The Saccharomyces cerevisiae mitochondrial unselective channel behaves as a physiological uncoupling system regulated by Ca2+, Mg2+, phosphate and ATP

It is proposed that the Saccharomyces cerevisiae the Mitochondrial Unselective Channel ((Sc)MUC) is tightly regulated constituting a physiological uncoupling system that prevents overproduction of reactive oxygen species (ROS). Mg(2+), Ca(2+) or phosphate (Pi) close (Sc)MUC, while ATP or a high rate of oxygen consumption open it. We assessed (Sc)MUC activity by measuring in isolated mitochondria the respiratory control, transmembrane potential (ΔΨ), swelling and production of ROS. At increasing [Pi], less [Ca(2+)] and/or [Mg(2+)] were needed to close (Sc)MUC or increase ATP synthesis. The Ca(2+)-mediated closure of (Sc)MUC was prevented by high [ATP] while the Mg(2+) or Pi effect was not. When Ca(2+) and Mg(2+) were alternatively added or chelated, (Sc)MUC opened and closed reversibly. Different effects of Ca(2+) vs Mg(2+) effects were probably due to mitochondrial Mg(2+) uptake. Our results suggest that (Sc)MUC activity is dynamically controlled by both the ATP/Pi ratio and divalent cation fluctuations. It is proposed that the reversible opening/closing of (Sc)MUC leads to physiological uncoupling and a consequent decrease in ROS production.

The Cryo-EM structure of the CorA channel from Methanocaldococcus jannaschii in low magnesium conditions

CorA channels are responsible for the uptake of essential magnesium ions by bacteria. X-ray crystal structures have been resolved for two full-length CorA channels, each in a non-conducting state with magnesium ions bound to the protein: These structures reveal a homo-pentameric quaternary structure with approximate 5-fold rotational symmetry about a central pore axis. We report the structure of the detergent solubilized Methanocaldococcus jannaschii CorA channel determined by Cryo-Electron Microscopy and Single Particle Averaging, supported by Small Angle X-ray Scattering and X-ray crystallography. This structure also shows a pentameric channel but with a highly asymmetric domain structure. The asymmetry of the domains includes differential separations between the trans-membrane segments, which reflects mechanical coupling of the cytoplasmic domain to the trans-membrane domain. This structure therefore reveals an important aspect of the gating mechanism of CorA channels by providing an indication of how the absence of magnesium ions leads to major structural changes.

Mg2+, K+, and the ribosome

Mg(2+) and K(+) are the prevalent di- and monovalent cations inside the cells in all three domains, playing a dominant role in structure and function of biological macromolecules. Ribosomes bind a substantial fraction of total Mg(2+) and K(+) cations. In this issue of the Journal of Bacteriology, Akanuma and coworkers (G. Akanuma et al., J. Bacteriol. 196:3820-3830, 2014, doi:10.1128/JB.01896-14) report a surprising genetic link between ribosome amounts per cell and the intracellular Mg(2+) concentrations.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

A repulsion mechanism explains magnesium permeation and selectivity in CorA

Magnesium (Mg(2+)) plays a central role in biology, regulating the activity of many enzymes and stabilizing the structure of key macromolecules. In bacteria, CorA is the primary source of Mg(2+) uptake and is self-regulated by intracellular Mg(2+). Using a gating mutant at the divalent ion binding site, we were able to characterize CorA selectivity and permeation properties to both monovalent and divalent cations under perfused two-electrode voltage clamp. The present data demonstrate that under physiological conditions, CorA is a multioccupancy Mg(2+)-selective channel, fully excluding monovalent cations, and Ca(2+), whereas in absence of Mg(2+), CorA is essentially nonselective, displaying only mild preference against other divalents (Ca(2+) > Mn(2+) > Co(2+) > Mg(2+) > Ni(2)(+)). Selectivity against monovalent cations takes place via Mg(2+) binding at a high-affinity site, formed by the Gly-Met-Asn signature sequence (Gly312 and Asn314) at the extracellular side of the pore. This mechanism is reminiscent of repulsion models proposed for Ca(2+) channel selectivity despite differences in sequence and overall structure.

The G-M-N motif determines ion selectivity in the yeast magnesium channel Mrs2p

The highly conserved G-M-N motif of the CorA-Mrs2-Alr1 family of Mg(2+) channels has been shown to be essential for Mg(2+) transport. We performed random mutagenesis of the G-M-N sequence of Saccharomyces cerevisiae Mrs2p in an unbiased genetic screen. A large number of mutants still capable of Mg(2+) influx, albeit below the wild-type level, were generated. Growth complementation assays, performed in media supplemented with Ca(2+) or Co(2+) or Mn(2+) or Zn(2+) at varying concentrations, lead to identification of mutants with reduced growth in the presence of Mn(2+) and Zn(2+). We hereby conclude that (1) at least two, but predominantly all three amino acids of the G-M-N motif must be replaced by certain combinations of other amino acids to remain functional, (2) replacement of any single amino acid within the G-M-N motif always impairs the function of Mrs2p, and (3) we show that the G-M-N motif determines ion selectivity, likely in concurrence with the negatively charged loop at the entrance of the channel thereby forming the Mrs2p selectivity filter.

Regulation of cation balance in Saccharomyces cerevisiae

All living organisms require nutrient minerals for growth and have developed mechanisms to acquire, utilize, and store nutrient minerals effectively. In the aqueous cellular environment, these elements exist as charged ions that, together with protons and hydroxide ions, facilitate biochemical reactions and establish the electrochemical gradients across membranes that drive cellular processes such as transport and ATP synthesis. Metal ions serve as essential enzyme cofactors and perform both structural and signaling roles within cells. However, because these ions can also be toxic, cells have developed sophisticated homeostatic mechanisms to regulate their levels and avoid toxicity. Studies in Saccharomyces cerevisiae have characterized many of the gene products and processes responsible for acquiring, utilizing, storing, and regulating levels of these ions. Findings in this model organism have often allowed the corresponding machinery in humans to be identified and have provided insights into diseases that result from defects in ion homeostasis. This review summarizes our current understanding of how cation balance is achieved and modulated in baker's yeast. Control of intracellular pH is discussed, as well as uptake, storage, and efflux mechanisms for the alkali metal cations, Na(+) and K(+), the divalent cations, Ca(2+) and Mg(2+), and the trace metal ions, Fe(2+), Zn(2+), Cu(2+), and Mn(2+). Signal transduction pathways that are regulated by pH and Ca(2+) are reviewed, as well as the mechanisms that allow cells to maintain appropriate intracellular cation concentrations when challenged by extreme conditions, i.e., either limited availability or toxic levels in the environment.

The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.

Identification and proximal tubular localization of the Mg²⁺ transporter, Slc41a1, in a seawater fish

The second most abundant cation in seawater (SW), Mg²⁺, is present at concentrations of ~53 mM. Marine teleosts maintain plasma Mg²⁺ concentration at 1-2 mM by excreting Mg²⁺ into the urine. Urine Mg²⁺ concentrations of SW teleosts exceed 70 mM, most of which is secreted by the renal tubular epithelial cells. However, molecular mechanisms of the Mg²⁺ secretion have yet to be clarified. To identify transporters involved in Mg²⁺ secretion, we analyzed the expression of fish homologs of the Slc41 Mg²⁺ transporter family in various tissues of SW pufferfish torafugu (Takifugu rubripes) and its closely related euryhaline species mefugu (Takifugu obscurus). Takifugu genome contained five members of Slc41 genes, and only Slc41a1 was highly expressed in the kidney. Renal expression of Slc41a1 was markedly elevated when mefugu were transferred from fresh water (FW) to SW. In situ hybridization analysis and immunohistochemistry at the light and electron microscopic levels revealed that Slc41a1 is localized to vacuoles in the apical cytoplasm of the proximal tubules. These results suggest that pufferfish Slc41a1 is a Mg²⁺ transporter involved in renal tubular transepithelial Mg²⁺ secretion by mediating Mg²⁺ transport from the cytosol to the vacuolar lumen, and support the hypothesis that Mg²⁺ secretion is mediated by exocytosis of Mg²⁺-rich vacuoles to the lumen.

Extra-matrix Mg2+ limits Ca2+ uptake and modulates Ca2+ uptake-independent respiration and redox state in cardiac isolated mitochondria

Cardiac mitochondrial matrix (m) free Ca(2+) ([Ca(2+)]m) increases primarily by Ca(2+) uptake through the Ca(2+) uniporter (CU). Ca(2+) uptake via the CU is attenuated by extra-matrix (e) Mg(2+) ([Mg(2+)]e). How [Ca(2+)]m is dynamically modulated by interacting physiological levels of [Ca(2+)]e and [Mg(2+)]e and how this interaction alters bioenergetics are not well understood. We postulated that as [Mg(2+)]e modulates Ca(2+) uptake via the CU, it also alters bioenergetics in a matrix Ca(2+)-induced and matrix Ca(2+)-independent manner. To test this, we measured changes in [Ca(2+)]e, [Ca(2+)]m, [Mg(2+)]e and [Mg(2+)]m spectrofluorometrically in guinea pig cardiac mitochondria in response to added CaCl2 (0-0.6 mM; 1 mM EGTA buffer) with/without added MgCl2 (0-2 mM). In parallel, we assessed effects of added CaCl2 and MgCl2 on NADH, membrane potential (ΔΨm), and respiration. We found that ≥0.125 mM MgCl2 significantly attenuated CU-mediated Ca(2+) uptake and [Ca(2+)]m. Incremental [Mg(2+)]e did not reduce initial Ca(2+)uptake but attenuated the subsequent slower Ca(2+) uptake, so that [Ca(2+)]m remained unaltered over time. Adding CaCl2 without MgCl2 to attain a [Ca(2+)]m from 46 to 221 nM enhanced state 3 NADH oxidation and increased respiration by 15 %; up to 868 nM [Ca(2+)]m did not additionally enhance NADH oxidation or respiration. Adding MgCl2 did not increase [Mg(2+)]m but it altered bioenergetics by its direct effect to decrease Ca(2+) uptake. However, at a given [Ca(2+)]m, state 3 respiration was incrementally attenuated, and state 4 respiration enhanced, by higher [Mg(2+)]e. Thus, [Mg(2+)]e without a change in [Mg(2+)]m can modulate bioenergetics independently of CU-mediated Ca(2+) transport.

Structural asymmetry in the magnesium channel CorA points to sequential allosteric regulation

Magnesium ions (Mg(2+)) are essential for life, but the mechanisms regulating their transport into and out of cells remain poorly understood. The CorA-Mrs2-Alr1 superfamily of Mg(2+) channels represents the most prevalent group of proteins enabling Mg(2+) ions to cross membranes. Thermotoga maritima CorA (TmCorA) is the only member of this protein family whose complete 3D fold is known. Here, we report the crystal structure of a mutant in the presence and absence of divalent ions and compare it with previous divalent ion-bound TmCorA structures. With Mg(2+) present, this structure shows binding of a hydrated Mg(2+) ion to the periplasmic Gly-Met-Asn (GMN) motif, revealing clues of ion selectivity in this unique channel family. In the absence of Mg(2+), TmCorA displays an unexpected asymmetric conformation caused by radial and lateral tilts of protomers that leads to bending of the central, pore-lining helix. Molecular dynamics simulations support these movements, including a bell-like deflection. Mass spectrometric analysis confirms that major proteolytic cleavage occurs within a region that is selectively exposed by such a bell-like bending motion. Our results point to a sequential allosteric model of regulation, where intracellular Mg(2+) binding locks TmCorA in a symmetric, transport-incompetent conformation and loss of intracellular Mg(2+) causes an asymmetric, potentially influx-competent conformation of the channel.

The periplasmic loop provides stability to the open state of the CorA magnesium channel

Crystal structures of the CorA Mg(2+) channel have suggested that metal binding in the cytoplasmic domain stabilizes the pentamer in a closed conformation. The open "metal free" state of the channel is, however, still structurally uncharacterized. Here, we have attempted to map conformational states of CorA from Thermotoga maritima by determining which residues support the pentameric structure in the presence or absence of Mg(2+). We find that when Mg(2+) is present, the pentamer is stabilized by the putative gating sites (M1/M2) in the cytoplasmic domain. Strikingly however, we find that the conserved and functionally important periplasmic loop is vital for the integrity of the pentamer when Mg(2+) is absent from the M1/M2 sites. Thus, although the periplasmic loops were largely disordered in the x-ray structures of the closed channel, our data suggests a prominent role for the loops in stabilizing the open conformation of the CorA channels.

Newly developed Mg2+-selective fluorescent probe enables visualization of Mg2+ dynamics in mitochondria

Mg(2+) plays important roles in numerous cellular functions. Mitochondria take part in intracellular Mg(2+) regulation and the Mg(2+) concentration in mitochondria affects the synthesis of ATP. However, there are few methods to observe Mg(2+) in mitochondria in intact cells. Here, we have developed a novel Mg(2+)-selective fluorescent probe, KMG-301, that is functional in mitochondria. This probe changes its fluorescence properties solely depending on the Mg(2+) concentration in mitochondria under physiologically normal conditions. Simultaneous measurements using this probe together with a probe for cytosolic Mg(2+), KMG-104, enabled us to compare the dynamics of Mg(2+) in the cytosol and in mitochondria. With this method, carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP)-induced Mg(2+) mobilization from mitochondria to the cytosol was visualized. Although a FCCP-induced decrease in the Mg(2+) concentration in mitochondria and an increase in the cytosol were observed both in differentiated PC12 cells and in hippocampal neurons, the time-courses of concentration changes varied with cell type. Moreover, the relationship between mitochondrial Mg(2+) and Parkinson's disease was analyzed in a cellular model of Parkinson's disease by using the 1-methyl-4-phenylpyridinium ion (MPP(+)). A gradual decrease in the Mg(2+) concentration in mitochondria was observed in response to MPP(+) in differentiated PC12 cells. These results indicate that KMG-301 is useful for investigating Mg(2+) dynamics in mitochondria. All animal procedures to obtain neurons from Wistar rats were approved by the ethical committee of Keio University (permit number is 09106-(1)).

Regulation of Alr1 Mg transporter activity by intracellular magnesium

Mg homeostasis is critical to eukaryotic cells, but the contribution of Mg transporter activity to homeostasis is not fully understood. In yeast, Mg uptake is primarily mediated by the Alr1 transporter, which also allows low affinity uptake of other divalent cations such as Ni(2+), Mn(2+), Zn(2+) and Co(2+). Using Ni(2+) uptake to assay Alr1 activity, we observed approximately nine-fold more activity under Mg-deficient conditions. The mnr2 mutation, which is thought to block release of vacuolar Mg stores, was associated with increased Alr1 activity, suggesting Alr1 was regulated by intracellular Mg supply. Consistent with a previous report of the regulation of Alr1 expression by Mg supply, Mg deficiency and the mnr2 mutation both increased the accumulation of a carboxy-terminal epitope-tagged version of the Alr1 protein (Alr1-HA). However, Mg supply had little effect on ALR1 promoter activity or mRNA levels. In addition, while Mg deficiency caused a seven-fold increase in Alr1-HA accumulation, the N-terminally tagged and untagged Alr1 proteins increased less than two-fold. These observations argue that the Mg-dependent accumulation of the C-terminal epitope-tagged protein was primarily an artifact of its modification. Plasma membrane localization of YFP-tagged Alr1 was also unaffected by Mg supply, indicating that a change in Alr1 location did not explain the increased activity we observed. We conclude that variation in Alr1 protein accumulation or location does not make a substantial contribution to its regulation by Mg supply, suggesting Alr1 activity is directly regulated via as yet unknown mechanisms.

X-ray crystallography and isothermal titration calorimetry studies of the Salmonella zinc transporter ZntB

The ZntB Zn(2+) efflux system is important for maintenance of Zn(2+) homeostasis in Enterobacteria. We report crystal structures of ZntB cytoplasmic domains from Salmonella enterica serovar Typhimurium (StZntB) in dimeric and physiologically relevant homopentameric forms at 2.3 Å and 3.1 Å resolutions, respectively. The funnel-like structure is similar to that of the homologous Thermotoga maritima CorA Mg(2+) channel and a Vibrio parahaemolyticus ZntB (VpZntB) soluble domain structure. However, the central α7 helix forming the inner wall of the StZntB funnel is oriented perpendicular to the membrane instead of the marked angle seen in CorA or VpZntB. Consequently, the StZntB funnel pore is cylindrical, not tapered, which may represent an "open" form of the ZntB soluble domain. Our crystal structures and isothermal titration calorimetry data indicate that there are three Zn(2+) binding sites in the full-length ZntB, two of which could be involved in Zn(2+) transport.

Elongation factor 4 (EF4/LepA) accelerates protein synthesis at increased Mg2+ concentrations

Elongation factor 4 (EF4) is one of the most conserved proteins present in bacteria as well as in mitochondria and chloroplasts of eukaryotes. Although EF4 has the unique ability to catalyze the back-translocation reaction on posttranslocation state ribosomes, the physiological role of EF4 remains unclear. Here we demonstrate that EF4 is stored at the membrane of Escherichia coli cells and released into the cytoplasm upon conditions of high ionic strength or low temperature. Under such conditions, wild-type E. coli cells overgrow mutant cells lacking the EF4 gene within 5-10 generations. Elevated intracellular Mg(2+) concentrations or low temperature retard bacterial growth and inhibit protein synthesis, probably because of formation of aberrant elongating ribosomal states. We suggest that EF4 binds to these stuck ribosomes and remobilizes them, consistent with the EF4-dependent enhancement (fivefold) in protein synthesis observed under these unfavorable conditions. The strong selective advantage conferred by the presence of EF4 at high intracellular ionic strength or low temperatures explains the ubiquitous distribution and high conservation of EF4.

A mutation in the gene encoding mitochondrial Mg²+ channel MRS2 results in demyelination in the rat

The rat demyelination (dmy) mutation serves as a unique model system to investigate the maintenance of myelin, because it provokes severe myelin breakdown in the central nervous system (CNS) after normal postnatal completion of myelination. Here, we report the molecular characterization of this mutation and discuss the possible pathomechanisms underlying demyelination. By positional cloning, we found that a G-to-A transition, 177 bp downstream of exon 3 of the Mrs2 (MRS2 magnesium homeostasis factor (Saccharomyces cerevisiae)) gene, generated a novel splice acceptor site which resulted in functional inactivation of the mutant allele. Transgenic rescue with wild-type Mrs2-cDNA validated our findings. Mrs2 encodes an essential component of the major Mg²⁺ influx system in mitochondria of yeast as well as human cells. We showed that the dmy/dmy rats have major mitochondrial deficits with a markedly elevated lactic acid concentration in the cerebrospinal fluid, a 60% reduction in ATP, and increased numbers of mitochondria in the swollen cytoplasm of oligodendrocytes. MRS2-GFP recombinant BAC transgenic rats showed that MRS2 was dominantly expressed in neurons rather than oligodendrocytes and was ultrastructurally observed in the inner membrane of mitochondria. Our observations led to the conclusion that dmy/dmy rats suffer from a mitochondrial disease and that the maintenance of myelin has a different mechanism from its initial production. They also established that Mg²⁺ homeostasis in CNS mitochondria is essential for the maintenance of myelin.

The myelin sheath that surrounds the axon of a neuron acts as a biological insulator. Its major function is to increase the speed at which impulses propagate along myelinated fibers in the central nervous system, as well as the peripheral nervous system. Alterations or damage affecting this structure (demyelination) result in the disruption of signals between the brain and other parts of the body. In the rat, mutations producing demyelination have been frequently identified and characterized and have contributed to a better understanding of the genetics of myelin development, physiology, and pathology. This paper reports the molecular characterization of a recessive allele responsible for the progressive disruption of myelin that was initially observed in mutant rats, previously named demyelination (dmy). This mutation generates an additional splicing acceptor site in an intron of the mitochondrial Mg²⁺ transporter gene (Mrs2), resulting in the insertion of a 83-bp genomic DNA segment into the Mrs2 transcript and complete functional inactivation of the mutant allele. We firstly defined the biological function of MRS2 in mammals and demonstrated the crucial and unexpected role of MRS2 in myelin physiology. Our findings might be helpful in the development of new therapeutic strategies for demyelinating syndromes.

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.