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

The MIT family, also called the CorA 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.

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.

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



This family belongs to the .

 

References:

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.

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.

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.

Dalmas, O., P. Sompornpisut, F. Bezanilla, and E. Perozo. (2014). Molecular mechanism of Mg2+-dependent gating in CorA. Nat Commun 5: 3590.

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.

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.

Hantke, K. (1997). Ferrous iron uptake by a magnesium transport system is toxic for Escherichia coli and Salmonella typhimurium. J. Bacteriol. 179: 6201-6204.

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.

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.

Kehres, D.G., C.H. Lawyer, and M.E. Maguire. (1998). The CorA magnesium transporter gene family. Microbial Comp. Genom. 3: 151-169.

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.

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.

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.

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]

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.

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.

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.

Maguire, M.E. (2006). The structure of CorA: a Mg2+-selective channel. Curr. Opin. Struct. Biol. 16: 432-438.

Moomaw, A.S. and M.E. Maguire. (2010). Cation selectivity by the CorA Mg2+ channel requires a fully hydrated cation. Biochemistry 49: 5998-6008.

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.

Papp, K.M. and M.E. Maguire. (2004). The CorA Mg2+ transporter does not transport Fe2+. J. Bacteriol. 186: 7653-7658.

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.

Payandeh, J., R. Pfoh, and E.F. Pai. (2013). The structure and regulation of magnesium selective ion channels. Biochim. Biophys. Acta. 1828: 2778-2792.

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]

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.

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.

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.

Smith, R.L. and M.E. Maguire. (1998). Microbial magnesium transport: unusual transporters searching for identity. Mol. Microbiol. 28: 217-226.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Worlock, A.J. and R.L. Smith. (2002). ZntB is a novel Zn2+ transporter in Salmonella enterica serovar typhimurium. J. Bacteriol. 184: 4369-4373.

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.

Examples:

TC#NameOrganismal TypeExample
1.A.35.1.1

Divalent cation (Mg2+, Co2+ and Ni2+) transport system, CorA.  Helical tilting and rotation in TM1 generates an iris-like motion that increases the diameter of the permeation pathway, triggering ion conduction, thus defining the gating mechanism (Dalmas et al. 2014).

Bacteria

CorA of E. coli (P0ABI4)

 
1.A.35.1.2

Divalent cation (Mg2+, Co2+ and Ni2+) transport system, CorA

Bacteria

CorA of Salmonella typhimurium (P0A2R8)

 
1.A.35.1.3

Magnesium transport protein CorA

Bacteria

CorA of Bacillus subtilis

 
Examples:

TC#NameOrganismal TypeExample
1.A.35.2.1

Aluminum resistance protein AlR1p; Alr1; ALR1, LLR1; MNR1 (Mg2+ homeostasis transporter [Mg2+ regulated]) AlR1p and AlR2p (P43553; a close paralogue) both catalyze uptake of Mg2+ and a variety of heavy metals (da Costa et al., 2007). Truncation of Alr1 showed that the N-terminal 239 amino acids and the C-terminal 53 amino acids are not essential for magnesium uptake (Lee and Gardner 2006). Mutations in the C-terminal part of ALR1 that is homologous to bacterial CorA magnesium transporters that gave severe phenotypes had amino acid changes in the small region containing the TMSs. Eighteen single amino acid mutants in this region were classified into three categories for magnesium uptake: no, low and moderate activity. Conservative mutations that reduced or inactivated uptake led to the identification of Ser(729), Ile(746) and Met(762) (part of the conserved GMN motif) as critical residues. High expression of inactive mutants inhibited the capability of wild-type Alr1 to transport magnesium, consistent with Alr1 forming homo-oligomers (Lee and Gardner 2006). Alr1 may play a role in cadmium resistance (Kern et al. 2005).

Yeast

Al1Rp of Saccharomyces cerevisiae

 
1.A.35.2.2

Inner mitochondrial membrane manganese channel protein MnR2p or MRS2-11.  The regulatory domain of  Mrs2 from the yeast inner mitochondrial membrane is similar to the E. coli regulatory domain of CorA with Met309 serving the same function as Met291 in CorA (Khan et al. 2013).

Yeast

MnR2p of Saccharomyces cerevisiae

 
1.A.35.2.3

Putative metal ion transporter C27B12.12c

Yeast

C27B12.12c of Schizosaccharomyces pombe

 
Examples:

TC#NameOrganismal TypeExample
1.A.35.3.1

Divalent metal ion (Mg2+, Ca2+, Ni2+, etc.) transporter of 317 aas and 3 TMSs.  The cryo-EM structure shows a pentameric channel with an asymmetric domain structure and featuring differential separations between the trans-segments, probably reflecting mechanical coupling of the cytoplasmic domain to the transmembrane domain and suggesting a gating mechanism (Cleverley et al. 2015).

Archaea

CorA of Methanocaldococcus jannaschii (Methanococcus jannaschii)

 
1.A.35.3.2

Magnesium transport protein, CorA.  The structure at 2.7 Å resolution is known. The CorA monomer has a C-terminal membrane domain containing two transmembrane segments and a large N-terminal cytoplasmic soluble domain. In the membrane, CorA forms a homopentamer shaped like a funnel which binds fully hydrated Mg2+ in the periplasm (Maguire 2006). A ring of positive charges are external to the ion-conduction pathway at the cytosolic membrane interface, and highly negatively charged helices in the cytosolic domain  appear to interact with the ring of positive charge to facilitate Mg2+ entry. Mg2+ ions are present in the cytosolic domain that are well placed to control the interaction of the ring of positive charge and the negatively charged helices, and thus, control Mg2+ entry (Maguire 2006).  Gating is achieved by helical rotation upon the binding of a metal ion substrate to the regulatory binding sites. The preference for Co2+ over Mg2+ has been reported to be determined by the presence of threonine side chains in the channel (Nordin et al. 2013), but more recently, Kowatz and Maguire 2018 showed that Co2+ is not a substrate, and that the intersubunit bound Mg2+ is not required for function and does not control the open versus closed states.

Bacteria

CorA of Thermotoga maritima

 
1.A.35.3.3Putative metal ion transporter YfjQBacilli

YfjQ of Bacillus subtilis

 
1.A.35.3.4

Putative CorA protein of 302 aas

Firmictues

CorA of Streptococcus sanguinis

 
1.A.35.3.5

MIP family protein of 366 aas. Mg2+,Co2+ and the CorA-specific inhibitor (Co(III) hexamine chloride) bind in the loop at the same binding site. This site includes the glutamic acid residue from the conserved "MPEL" motif (Hu et al. 2009).

Actinobacteria

CorA of Mycobacterium tuberculosis

 
1.A.35.3.6

The CorA-homologous magnesium ion transporter of 831 aas, Mgt1.  Is critical for parasite development and virulence (Zhu et al. 2009).

Euglenozoa

Mgt1 of Leishmania major

 
1.A.35.3.7

Putative Mg2+ transporter of 322 aas and 2 or 3 TMSs.

CorA protein of Lactobacillus phage LfeInf

 
1.A.35.3.8

Uncharacterized CorA homologue of 373 aas and 3 TMSs (Pohland and Schneider 2019).

CorA of Acaryochloris marina

 
Examples:

TC#NameOrganismal TypeExample
1.A.35.4.1

Zn2+/Cd2+ efflux system, ZntB. Mg2+ is not transported. Wan et al. 2011 reported crystal structures 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 Mg2+ channel and a Vibrio parahaemolyticus ZntB (VpZntB). 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. There are three Zn2+ binding sites in the full-length ZntB, two of which could be involved in Zn2+ transport.

Bacteria

ZntB of Salmonella enterica serovar Typhimurium

 
1.A.35.4.2

The ZntB Zn2+/Cd2+ transporter. The 1.9Å structure of the N-terminal cytoplasmic domain of ZntB has been solved (Tan et al., 2009).

Bacteria

ZntB of Vibrio parahaemolyticus (Q87M69)

 
Examples:

TC#NameOrganismal TypeExample
1.A.35.5.1

Mitochondrial inner membrane Mg2+ channel protein, Mrs2 (Schindl et al., 2007).  Mutational analyses have been carried out, suggesting that internal Mg2+ affects intron splicing (Weghuber et al. 2006). MRS2 is involved in mitochondrial Mg2+ homeostasis (Schäffers et al. 2018). The G-M-N motif determines the ion selectivity, likely together with the negatively charged loop at the entrance of the channel, thereby forming the Mrs2p selectivity filter (Sponder et al. 2013).

Yeast, plant & animals

Mrs2 of Saccharomyces cerevisiae (Q01926)

 
1.A.35.5.2

High affinity root Mg2+ transporter, Mrs2/MGT1/MRS2-10.  Plants have at least  11 Mrs2 homologues, and they can form homo as well as heterooligomeric channels (Schmitz et al. 2013). Expression in an E. coli triple mutant, corA mgtA yhiD, which required high (>10 mM) Mg2+ for growth, allowed growth on >1 mM Mg2+ and resulted in Al3+ sensitivity (Ishijima et al. . 2015). It therefore appears that MRS2-10 catalyzes Mg2+ and Al 3+ uptake.

Plants

Mrs2-10 of Arabidposis thaliana (Q9SAH0)

 
1.A.35.5.3

Magnesium transporter MRS2-11, MRS2B, chloroplastic (Magnesium Transporter 10) (AtMGT10).  Expression in an E. coli triple mutant, corA mgtA yhiD, which required high (>10 mM) Mg2+ for growth, allowed growth on >1 mM Mg2+ and resulted in Al3+ sensitivity (Ishijima et al. 2015).

Plants

MGT10 of Arabidopsis thaliana

 
1.A.35.5.4Magnesium transporter MRS2-4 (Magnesium Transporter 6) (AtMGT6)PlantsMRS2-4 of Arabidopsis thaliana
 
1.A.35.5.5

Mitochondrial inner membrane magnesium transporter MFM1; LPE10 (MRS2 function modulating factor 1)

FungiMFM1 of Saccharomyces cerevisiae
 
1.A.35.5.6Magnesium transporter MRS2-5 (Magnesium Transporter 3) (AtMGT3)PlantsMRS2-5 of Arabidopsis thaliana
 
1.A.35.5.7

MRS2 (HPTm MRS2L) of 443 aas and 2 C-terminal TMSs. Implicated in mitochondrial Mg2+ homeostasis (Schäffers et al. 2018).

MRS2 of Homo sapiens