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
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).
CorA of E. coli (P0ABI4)
Divalent cation (Mg2+, Co2+ and Ni2+) transport system, CorA
CorA of Salmonella typhimurium (P0A2R8)
Magnesium transport protein CorA
CorA of Bacillus subtilis
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).
Al1Rp of Saccharomyces cerevisiae
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).
MnR2p of Saccharomyces cerevisiae
Putative metal ion transporter C27B12.12c
C27B12.12c of Schizosaccharomyces pombe
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).
CorA of Methanocaldococcus jannaschii (Methanococcus jannaschii)
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.
CorA of Thermotoga maritima
YfjQ of Bacillus subtilis
Putative CorA protein of 302 aas
CorA of Streptococcus sanguinis
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).
CorA of Mycobacterium tuberculosis
The CorA-homologous magnesium ion transporter of 831 aas, Mgt1. Is critical for parasite development and virulence (Zhu et al. 2009).
Mgt1 of Leishmania major
Putative Mg2+ transporter of 322 aas and 2 or 3 TMSs.
CorA protein of Lactobacillus phage LfeInf
Uncharacterized CorA homologue of 373 aas and 3 TMSs (Pohland and Schneider 2019).
CorA of Acaryochloris marina
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.
ZntB of Salmonella enterica serovar Typhimurium
The ZntB Zn2+/Cd2+ transporter. The 1.9Å structure of the N-terminal cytoplasmic domain of ZntB has been solved (Tan et al., 2009).
ZntB of Vibrio parahaemolyticus (Q87M69)
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)
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.
Mrs2-10 of Arabidposis thaliana (Q9SAH0)
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).
MGT10 of Arabidopsis thaliana
Mitochondrial inner membrane magnesium transporter MFM1; LPE10 (MRS2 function modulating factor 1)
MRS2 (HPTm MRS2L) of 443 aas and 2 C-terminal TMSs. Implicated in mitochondrial Mg2+ homeostasis (Schäffers et al. 2018).
MRS2 of Homo sapiens