TCDB is operated by the Saier Lab Bioinformatics Group

1.A.35 The CorA/Mrs2 Metal Ion Transporter (MIT) Family

The MIT (CorA/Mrs2) family, also called the CorA or Mrs2 family, is a large and diverse family with sequenced members in Gram-positive and Gram-negative bacteria, blue-green bacteria, archaea, plants, animals, yeast, slime molds, Guillardia theta, and Plasmodium. Functionally characterized proteins include the Mg2+-Co2+-Ni2+ CorA permeases of Salmonella typhimurium and E. coli, a Zn2+-Cd2+ effluxing system in S. typhimurium, a divalent cation transporting CorA homologue in Methanococcus janaschii, aluminum-resistance Mg2+ transport permeases (AlR1p and AlR2p) of Saccharomyces cerevisiae and a putative manganese-resistance magnesium transport permease (MnR2p) of S. cerevisiae. Most members of the MIT family are between 300 and 400 amino acyl residues in length and possess two (or three) putative transmembrane α-helical spanners (TMSs). It seems likely that some homologues have two and others have three TMSs. The greatest degree of conservation between homologues is found in TMSs 1 and 2 of the Thermotoga maritima protein. The yeast metal resistance proteins, which are 850-900 amino acyl residues in length, also exhibit two or three putative TMSs. Overexpression of the yeast proteins, AlR1p and MnR2p, overcomes toxicity to aluminum and manganese, respectively.

The crystal structure of the CorA homologue from Thermotoga maritima has been solved at 3.9 Å resolution for the full-length protein and at 1.85 Å resolution for the cytoplasmic domain (Lunin et al., 2006). It is a funnel-shaped homopentamer with 2 TMSs per monomer. The channel is formed by an inner group of five helices and putatively gated by bulky hydrophobic residues. The large cytoplasmic domain forms a funnel whose wide mouth points into the cell and whose walls are formed by five long helices that are extensions of the transmembrane helices. The cytoplasmic neck of the pore is surrounded, on the outside of the funnel, by a ring of highly conserved positively charged residues. Two negatively charged helices in the cytoplasmic domain extend back towards the membrane on the outside of the funnel and abut the ring of positive charge. An apparent Mg2+ ion was bound between monomers at a conserved site in the cytoplasmic domain suggesting a mechanism to link gating of the pore to the intracellular concentration of Mg2+. These results contrast with those of Wang et al. (2006) who identified a soluble oligomeric N-terminal domain of the E. coli CorA that appeared to be tetrameric and to bind its substrates with the same affinity of native CorA (Wang et al., 2006).

The intracellular funnel domain of CorA constitutes an allosteric regulatory module that can be engineered to promote an activated or closed state (Payandeh et al., 2008). A periplasmic gate is centered about a proline-induced kink of the pore-lining helix. 'Helix-straightening' mutations result in gain-of-function. The narrowest constriction along the pore is a hydrophobic gate, likely forming an energetic barrier to ion flux. Highly conserved acidic residues found in the short periplasmic loop are not essential for CorA function or Mg2+ selectivity but may be required for proper protein folding and stability. Cation selectivity by the CorA Mg2+ channel requires a fully hydrated cation (Moomaw and Maguire, 2010).  The structure, mechanism and regulation of divalent ion transport via MIT family channels have been reviewed (Guskov and Eshaghi 2012; Payandeh et al. 2013).

The CorA permeases of S. typhimurium and E. coli mediate both influx and efflux of Mg2+. They transport Mg2+, Co2+ and Ni2+ but not Fe2+ (Papp and Maguire, 2004). Mg2+ is transported with an apparent KM of 20-30 μM. The archaeal CorA protein is functionally similar to the CorA homologues of enteric bacteria. The yeast proteins appear to exhibit broad specificity transporting a wide range of di- and trivalent metal cations. In this respect, and also with respect to topology, MIT family members resemble channel proteins. The ZntB protein is a very distant homologue showing greatest similarity to the M. janaschii protein but catalyzing efflux of Zn2+ and Ca2+ (Worlock and Smith, 2002).

The CorA proteins of E. coli and S. typhimurium are each 316 amino acyl residues in length. Hydropathy analysis had predicted two transmembrane α-helical spanners (TMSs) in the C-terminal regions of these proteins. Site-specific mutagenesis studies of three conserved residues in TMS 3 suggest that they contribute to the Mg2+ transport pathway. In prokaryotes, cellular Mg2+ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg2+ transporters (Franken et al. 2022). For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore, are conserved during evolution. Thus, 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 (Franken et al. 2022).

The transport mode and energy coupling mechanism(s) are poorly understood. A homopentameric structure for several CorA homologues has been established (Lunin et al., 2006), a channel-type mechanism rather than a carrier-type mechanism is operative, based on its high capacity and its 'gating' properties. CorA of S. typhimurium is a high capacity transporter that is constitutively synthesized. Some MIT family members may serve as the major Mg2+ influx systems in several prokaryotes, but others may catalyze divalent cation efflux.

Mrs2 of Saccharomyces cerevisiae is essential for the splicing of group II introns from RNA in mitochondria, and independently, for the maintenance of a functional respiratory system. Mrs2 is an integral protein of the inner mitochondrial membrane. It exhibits two adjacent putative TMSs in its C-terminal half. The large N-terminal domain and a shorter C-terminal domain are probably localized to the mitochondrial matrix. Mrs2 is distantly related to the E. coli CorA protein and other members of the MIT family. Null mutations in the mrs2 gene give mitochondria with low internal Mg2+ concentrations, and overexpression of the E. coli corA gene in yeast partially suppresses the mrs2 phenotype; CorA also partially restores intramitochondrial Mg2+ concentrations. More recently, it has been shown directly that Mrs2 is the electrophoretic mitochondrial Mg2+ uptake system, involved in Mg2+ homeostasis. It catalyzes Mg2+ uptake in the presence of a membrane potential and Mg2+ efflux in its absence (Kolisek et al., 2003). Thus, it functions like a Mg2+ channel as expected for a MIT family member. Mrs2 is demonstrably homologous to another S. cerevisiae ORF (YPL060w), as well as ORFs from other yeast, fungi, protozoans, plants and animals. 

Magnesium transporters (MGTs) regulate magnesium absorption, transport, and redistribution in higher plants. In Oryza sativa there are 23 non-redundant MGT transporters that function in Mg2+ homeostasis, particularly under salt stress (Mohamadi et al. 2023). These authors analyzed the protein properties, gene structures, phylogenetic relationships, synteny patterns, expression, and co-expression networks of these OsMGT. The evolutionary relationship of the OsMGTs was fully consistent with their functional domains, and were divided into three main familiess based on their conserved domains: MMgT with 2 N-terminal TMSs (TC 1.A.67), CorA-like with 2 C-terminal TMSs (TC# 1.A.35), and NIPA with 9 or 10 TMSs each (TC# 2.A.7.35). These last group of proteins are members of the DMT superfamily.

The transport reaction probably catalyzed by Mrs2 is:

Mg2+ (cytoplasm) Mg2+ (mitochondrial matrix)

The transport reaction generally catalyzed by MIT family members is:

M2+(in) (or possible M3+(in)) M2+(out) (or possibly M3+(out))
M2+ = a divalent heavy metal ion

References associated with 1.A.35 family:

Bui, D.M., J. Gregan, E. Jarosch, A. Ragnini and R.J. Schweyen. (1999). The bacterial magnesium transporter CorA can functionally substitute for its putative homologue Mrs2p in the yeast inner mitochondrial membrane. J. Biol. Chem. 274: 20438-20443. 10400670
Cleverley, R.M., J. Kean, C.A. Shintre, C. Baldock, J.P. Derrick, R.C. Ford, and S.M. Prince. (2015). The Cryo-EM structure of the CorA channel from Methanocaldococcus jannaschii in low magnesium conditions. Biochim. Biophys. Acta. 1848: 2206-2215. 26051127
Da Costa, B.M., K. Cornish, and J.D. Keasling. (2007). Manipulation of intracellular magnesium levels in Saccharomyces cerevisiae with deletion of magnesium transporters. Appl. Microbiol. Biotechnol. 77: 411-425. 17926032
Dalmas, O., P. Sompornpisut, F. Bezanilla, and E. Perozo. (2014). Molecular mechanism of Mg2+-dependent gating in CorA. Nat Commun 5: 3590. 24694723
Franken, G.A.C., M.A. Huynen, L.A. Martínez-Cruz, R.J.M. Bindels, and J.H.F. de Baaij. (2022). Structural and functional comparison of magnesium transporters throughout evolution. Cell Mol Life Sci 79: 418. 35819535
Graschopf, A., J.A. Stadler, M.K. Hoellerer, S. Eder, M. Sieghardt, S.D. Kohlwein, and R.J. Schweyen. (2001). The yeast plasma membrane protein Alr1 controls Mg2+ homeostasis and is subject to Mg2+-dependent control of its synthesis and degradation. J. Biol. Chem. 276: 16216-16222. 11279208
Guskov, A. and S. Eshaghi. (2012). The Mechanisms of Mg2+ and Co2+ Transport by the CorA Family of Divalent Cation Transporters. Curr Top Membr 69: 393-414. 23046658
Hantke, K. (1997). Ferrous iron uptake by a magnesium transport system is toxic for Escherichia coli and Salmonella typhimurium. J. Bacteriol. 179: 6201-6204. 9324273
Hu, J., M. Sharma, H. Qin, F.P. Gao, and T.A. Cross. (2009). Ligand binding in the conserved interhelical loop of CorA, a magnesium transporter from Mycobacterium tuberculosis. J. Biol. Chem. 284: 15619-15628. 19346249
Ishijima, S., M. Uda, T. Hirata, M. Shibata, N. Kitagawa, and I. Sagami. (2015). Magnesium uptake of Arabidopsis transporters, AtMRS2-10 and AtMRS2-11, expressed in Escherichia coli mutants: Complementation and growth inhibition by aluminum. Biochim. Biophys. Acta. 1848: 1376-1382. 25772503
Kehres, D.G., C.H. Lawyer, and M.E. Maguire. (1998). The CorA magnesium transporter gene family. Microbial Comp. Genom. 3: 151-169. 9775386
Kern, A.L., D. Bonatto, J.F. Dias, M.L. Yoneama, M. Brendel, and J.A. Pêgas Henriques. (2005). The function of Alr1p of Saccharomyces cerevisiae in cadmium detoxification: insights from phylogenetic studies and particle-induced X-ray emission. Biometals 18: 31-41. 15865408
Khan, M.B., G. Sponder, B. Sjöblom, S. Svidová, R.J. Schweyen, O. Carugo, and K. Djinović-Carugo. (2013). Structural and functional characterization of the N-terminal domain of the yeast Mg2+ channel Mrs2. Acta Crystallogr D Biol Crystallogr 69: 1653-1664. 23999289
Kolisek, M., G. Zsurka, J. Samaj, J. Weghuber, R.J. Schweyen, and M. Schweigel. (2003). Mrs2p is an essential component of the major electrophoretic Mg2+ influx system in mitochondria. EMBO J. 22: 1235-1244. 12628916
Kowatz, T. and M.E. Maguire. (2018). Loss of cytosolic Mg binding sites in the Thermotoga maritima CorA Mg channel is not sufficient for channel opening. Biochim. Biophys. Acta. Gen Subj 1863: 25-30. [Epub: Ahead of Print] 30293964
Lee, J.M. and R.C. Gardner. (2006). Residues of the yeast ALR1 protein that are critical for magnesium uptake. Curr. Genet. 49: 7-20. 16328501
Lunin, V.V., E. Dobrovetsky, G. Khutoreskaya, R. Zhang, A. Joachimiak, D.A. Doyle, A. Bochkarev, M.E. Maguire, A.M. Edwards, and C.M. Koth. (2006). Crystal structure of the CorA Mg2+ transporter. Nature 440: 833-837. 16598263
Ma, Q., Y. Feng, S. Luo, L. Cheng, W. Tong, X. Lu, Y. Li, and P. Zhang. (2023). The aquaporin MePIP2;7 improves MeMGT9-mediated Mg acquisition in cassava. J Integr Plant Biol. [Epub: Ahead of Print] 37548108
MacDiarmid, C.W. and R.C. Gardner. (1998). Overexpression of the Saccharomyces cerevisiae magnesium transport system confers resistance to aluminum ion. J. Biol. Chem. 273: 1727-1732. 9430719
Maguire, M.E. (2006). The structure of CorA: a Mg2+-selective channel. Curr. Opin. Struct. Biol. 16: 432-438. 16828282
Mohamadi, S.F., N. Babaeian Jelodar, N. Bagheri, G. Nematzadeh, and S.H. Hashemipetroudi. (2023). New insights into comprehensive analysis of magnesium transporter () gene family in rice ( L.). 3 Biotech 13: 322. 37649592
Moomaw, A.S. and M.E. Maguire. (2010). Cation selectivity by the CorA Mg2+ channel requires a fully hydrated cation. Biochemistry 49: 5998-6008. 20568735
Nordin, N., A. Guskov, T. Phua, N. Sahaf, Y. Xia, S. Lu, H. Eshaghi, and S. Eshaghi. (2013). Exploring the structure and function of Thermotoga maritima CorA reveals the mechanism of gating and ion selectivity in Co2+/Mg2+ transport. Biochem. J. 451: 365-374. 23425532
Papp, K.M. and M.E. Maguire. (2004). The CorA Mg2+ transporter does not transport Fe2+. J. Bacteriol. 186: 7653-7658. 15516579
Payandeh, J., C. Li, M. Ramjeesingh, E. Poduch, C.E. Bear, and E.F. Pai. (2008). Probing structure-function relationships and gating mechanisms in the CorA Mg2+ transport system. J. Biol. Chem. 283: 11721-11733. 18276588
Payandeh, J., R. Pfoh, and E.F. Pai. (2013). The structure and regulation of magnesium selective ion channels. Biochim. Biophys. Acta. 1828: 2778-2792. 23954807
Pohland, A.C. and D. Schneider. (2019). Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import. Biol Chem. [Epub: Ahead of Print] 30913030
Schäffers, O.J.M., J.G.J. Hoenderop, R.J.M. Bindels, and J.H.F. de Baaij. (2018). The rise and fall of novel renal magnesium transporters. Am. J. Physiol. Renal Physiol 314: F1027-F1033. 29412701
Schindl, R., J. Weghuber, C. Romanin, and R.J. Schweyen. (2007). Mrs2p forms a high conductance Mg2+ selective channel in mitochondria. Biophys. J. 93: 3872-3883. 17827224
Schmitz J., Tierbach A., Lenz H., Meschenmoser K. and Knoop V. (2013). Membrane protein interactions between different Arabidopsis thaliana MRS2-type magnesium transporters are highly permissive. Biochim Biophys Acta. 1828(9):2032-40. 23732234
Smith, R.L. and M.E. Maguire. (1998). Microbial magnesium transport: unusual transporters searching for identity. Mol. Microbiol. 28: 217-226. 9622348
Smith, R.L., E. Gottlieb, L.M. Kucharski, and M.E. Maguire. (1998). Functional similarity between archaeal and bacterial CorA magnesium transporters. J. Bacteriol. 180: 2788-2791. 9573171
Smith, R.L., J.L. Banks, M.D. Snavely, and M.E. Maguire. (1993). Sequence and topology of the CorA magnesium transport systems of Salmonella typhimurium and Escherichia coli. Identification of a new class of transport protein. J. Biol. Chem. 268: 14071-14080. 8314774
Smith, R.L., M.A. Szegedy, L.M. Kucharski, C. Walker, R.M. Wiet, A. Redpath, M.T. Kaczmarek, and M.E. Maguire. (1998). The CorA Mg2+ transport protein of Salmonella typhimurium. Mutagenesis of conserved residues in the third membrane domain identifies a Mg2+ pore. J. Biol. Chem. 273: 28663-28669. 9786860
Sponder, G., S. Svidová, M.B. Khan, M. Kolisek, R.J. Schweyen, O. Carugo, and K. Djinović-Carugo. (2013). The G-M-N motif determines ion selectivity in the yeast magnesium channel Mrs2p. Metallomics 5: 745-752. 23686104
Tan, K., A. Sather, J.L. Robertson, S. Moy, B. Roux, and A. Joachimiak. (2009). Structure and electrostatic property of cytoplasmic domain of ZntB transporter. Protein. Sci. 18: 2043-2052. 19653298
ter Huurne, A.A., S. Muir, M. van Houten, B.A. van der Zeijst, W. Gaastra, and J.G. Kusters. (1994). Characterization of three putative Serpulina hyodysenteriae hemolysins. Microb Pathog. 16: 269-282. 7968456
Uthayabalan, S., N. Vishnu, M. Madesh, and P.B. Stathopulos. (2023). The human MRS2 magnesium-binding domain is a regulatory feedback switch for channel activity. Life Sci Alliance 6:. 36754568
Wan, Q., M.F. Ahmad, J. Fairman, B. Gorzelle, M. de la Fuente, C. Dealwis, and M.E. Maguire. (2011). X-ray crystallography and isothermal titration calorimetry studies of the Salmonella zinc transporter ZntB. Structure 19: 700-710. 21565704
Wang, S.Z., Y. Chen, Z.H. Sun, Q. Zhou, and S.F. Sui. (2006). Escherichia coli CorA periplasmic domain functions as a homotetramer to bind substrate. J. Biol. Chem. 281: 26813-26820. 16835234
Warren, M.A., L.M. Kucharski, A. Veenstra, L. Shi, P.F. Grulich, and M.E. Maguire. (2004). The CorA Mg2+ transporter is a homotetramer. J. Bacteriol. 186: 4605-4612. 15231793
Weghuber, J., F. Dieterich, E.M. Froschauer, S. Svidovà, and R.J. Schweyen. (2006). Mutational analysis of functional domains in Mrs2p, the mitochondrial Mg2+ channel protein of Saccharomyces cerevisiae. FEBS J. 273: 1198-1209. 16519685
Worlock, A.J. and R.L. Smith. (2002). ZntB is a novel Zn2+ transporter in Salmonella enterica serovar typhimurium. J. Bacteriol. 184: 4369-4373. 12142406
Wunderlich, J. (2022). Updated List of Transport Proteins in. Front Cell Infect Microbiol 12: 926541. 35811673
Zhu, Y., A. Davis, B.J. Smith, J. Curtis, and E. Handman. (2009). Leishmania major CorA-like magnesium transporters play a critical role in parasite development and virulence. Int J Parasitol 39: 713-723. 19136005