The B. subtilis MgtE magnesium transporter can functionally compensate TRPM7-deficiency in vertebrate B-cells

Characterization of a novel MgtE homolog and its structural dynamics in membrane mimetics

Magnesium (Mg2+) is the most abundant divalent cation in the cell and is critical for numerous cellular processes. Despite its importance, the mechanisms of intracellular Mg2+ transport and its regulation are poorly understood. MgtE is the main Mg2+ transport system in almost half of bacterial species and is an ortholog of mammalian SLC41A1 transporters, which are implicated in neurodegenerative diseases and cancer. To date, only MgtE from Thermus thermophilus (MgtETT) has been extensively characterized, mostly in detergent micelles, and gating-related structural dynamics in biologically relevant membranes are scarce. The MgtE homolog from Bacillus firmus (MgtEBF) is unique since it lacks the entire Mg2+-sensing N-domain but has conserved structural motifs in the TM-domain for Mg2+ transport. In this work, we have successfully purified this novel homolog in a stable and functional form, and ColabFold structure prediction analysis suggests a homodimer. Further, microscale thermophoresis experiments show that MgtEBF binds Mg2+ and ATP, similar to MgtETT. Importantly, we show that, despite lacking the N-domain, MgtEBF mediates Mg2+ transport function in the presence of an inwardly directed Mg2+ gradient in reconstituted proteoliposomes. Furthermore, comparison of the organization and dynamics of Trp residues in the TM-domain of MgtEBF in membrane mimetics, in apo- and Mg2+-bound forms, suggests that the cytoplasmic binding of Mg2+ might involve modest gating-related conformational changes at the TM-domain. Overall, our results show that the gating-related structural dynamics (hydration dynamics, conformational heterogeneity) of the full-length MgtEBF is significantly changed in functionally pertinent membrane environment, emphasizing the importance of lipid-protein interactions in MgtE gating 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.

Gating-related Structural Dynamics of the MgtE Magnesium Channel in Membrane-Mimetics Utilizing Site-Directed Tryptophan Fluorescence

Magnesium is the most abundant divalent cation present in the cell, and an abnormal Mg²⁺ homeostasis is associated with several diseases in humans. However, among ion channels, the mechanisms of intracellular regulation and transport of Mg²⁺ are poorly understood. MgtE is a homodimeric Mg²⁺-selective channel and is negatively regulated by high intracellular Mg²⁺ concentration where the cytoplasmic domain of MgtE acts as a Mg²⁺ sensor. Most of the previous biophysical studies on MgtE have been carried out in detergent micelles and the information regarding gating-related structural dynamics of MgtE in physiologically-relevant membrane environment is scarce. In this work, we monitored the changes in gating-related structural dynamics, hydration dynamics and conformational heterogeneity of MgtE in micelles and membranes using the intrinsic site-directed Trp fluorescence. For this purpose, we have engineered six single-Trp mutants in the functional Trp-less background of MgtE to obtain site-specific information on the gating-related structural dynamics of MgtE in membrane-mimetic systems. Our results indicate that Mg²⁺-induced gating might involve the possibility of a 'conformational wave' from the cytosolic N-domain to transmembrane domain of MgtE. Although MgtE is responsive to Mg²⁺-induced gating in both micelles and membranes, the organization and dynamics of MgtE is substantially altered in physiologically important phospholipid membranes compared to micelles. This is accompanied by significant changes in hydration dynamics and conformational heterogeneity. Overall, our results highlight the importance of lipid-protein interactions and are relevant for understanding gating mechanism of magnesium channels in general, and MgtE in particular.

Transcriptional Profiling Analysis of Bacillus subtilis in Response to High Levels of Fe(3.)

Iron is essential to microorganisms for its important biological function but could be highly toxic in excess. We have used genome-wide transcriptional analysis in Fe(3+)-treated (4 mM) Bacillus subtilis to reveal the effect of excess Fe(3+) on B. subtilis and characterized the potential pathways involved in Fe(3+) stress tolerance. A total of 366 and 400 genes were identified as significantly up-regulated and down-regulated, respectively. We found excess Fe(3+) had four major influences on B. subtilis: Fe(3+) resulted in oxidative stress and induced genes involved in oxidative stress resistance including the SigB-regulated genes, but the PerR regulon was not inducible in Fe(3+)-mediated oxidative stress except zosA; Fe(3+) significantly disturbed homeostasis of Mn(2+) and Zn(2+), and the mechanism was proposed in this article; the acidity of Fe(3+)-induced genes involved in acid consuming and production of bases and shifted B. subtilis to carbon starvation state; Fe(3+)-induced genes related to membrane remodeling (bkd operon), which prevents Fe(3+)'s incorporation to membrane lipids. Moreover, Fe(3+) repressed the stringent control response, consistent with the induction of stringent control in iron limitation, demonstrating that iron might be a signal in stringent control of B. subtilis. This study was the first to provide a comprehensive overview of the genetic response of B. subtilis to ecxess Fe(3+).

Identification of a Mg2+-sensitive ORF in the 5'-leader of TRPM7 magnesium channel mRNA

TRPM7 is an essential and ubiquitous channel-kinase regulating cellular influx of Mg²⁺. Although TRPM7 mRNA is highly abundant, very small amount of the protein is detected in cells, suggesting post-transcriptional regulation of trpm7 gene expression. We found that TRPM7 mRNA 5′-leader contains two evolutionarily conserved upstream open reading frames that act together to drastically inhibit translation of the TRPM7 reading frame at high magnesium levels and ensure its optimal translation at low magnesium levels, when the activity of the channel-kinase is most required. The study provides the first example of magnesium channel synthesis being controlled by Mg²⁺ in higher eukaryotes.