TCDB is operated by the Saier Lab Bioinformatics Group

2.A.37 The Monovalent Cation:Proton Antiporter-2 (CPA2) Family

The CPA2 family is a moderately large from bacteria, archaea and eukaryotes. Among the functionally well-characterized members of the family are (1) the KefB/KefC K+ efflux proteins of E. coli which may be capable of catalyzing both K+/H+ antiport and K+ uniport, depending on conditions (Bakker et al., 1987; Booth et al., 1996; Munro et al., 1991), (2) the Na+/H+ antiporter of Enterococcus hirae (Waser et al., 1992) and (3) the K+/H+ antiporter of S. cerevisiae. It has been proposed that under normal physiological conditions, these proteins may function by essentially the same mechanism (Reizer et al., 1992).

KefC and KefB of E. coli are responsible for glutathione-gated K+ efflux (Ferguson et al., 1993, 1997). Each of these proteins consists of a transmembrane hydrophobic N-terminal domain, and a less well-conserved C-terminal hydrophilic domain. Each protein interacts with a second protein encoded by genes that overlap the gene encoding the primary transporter. The KefC ancillary protein is YabF while the KefB ancillary protein is YheR. These ancillary proteins stimulate transport activity about 10-fold (Miller et al., 2000). These proteins are important for cell survival during exposure to toxic metabolites, possibly because they can release K+, allowing H+ uptake. Activation of the KefB or KefC K+ efflux system only occurs in the presence of glutathione and a reactive electrophile such as methylglyoxal or N-ethylmaleimide. Formation of the methylglyoxal-glutathione conjugate, S-lactoylglutathione, is catalyzed by glyoxalase I, and S-lactoylglutathione activates KefB and KefC (MacLean et al., 1998). H+ uptake (acidification of the cytoplasm) accompanying or following K+ efflux may serve as a further protective mechanism against electrophile toxicity (Booth et al., 1996; Ferguson et al., 1993, 1997; Stumpe et al., 1996). Inhibition of transport by glutathione was enhanced by NADH (Fujisawa et al., 2007).

Gram negative bacteria are protected against toxic electrophilic compounds by glutathione-gated potassium efflux systems (Kef) that modulate cytoplasmic pH. Roosild et al. (2010) have elucidated the mechanism of gating through structural and functional analysis of the E. coli KefC. The revealed mechanism can explain how subtle chemical differences in glutathione derivatives can produce opposite effects on channel function. Kef channels are regulated by potassium transport and NAD-binding (KTN) domains that sense both reduced glutathione, which inhibits Kef activity, and glutathione adducts that form during electrophile detoxification and activate Kef. Roosild et al. (2010) found that reduced glutathione stabilizes an interdomain association between two KTN folds, whereas large adducts sterically disrupt this interaction. F441 is identified as the pivotal residue discriminating between reduced glutathione and its conjugates. They demonstrated a major structural change on the binding of an activating ligand to a KTN-domain protein.

The MagA protein of Magnetospirillum sp. strain AMB-1 has been reported to be required for synthesis of bacterial magnetic particles. The magA gene is subject to transcriptional activation by an iron deficiency (Nakamura et al., 1995).  However, are more recent report has shown that magA mutants of both Magnetospirillum magneticum AMB-1 and M. gryphiswaldense MSR-1 formed wild-type-like magnetosomes without a growth defect (Uebe et al. 2012). Its transport function is not known. The GerN and GrmA proteins of Bacillus cereus and Bacillus megaterium, respectively, are spore germination proteins that can exchange Na+ for H+ and/or K+ (Southworth et al., 2001). The AmhT homologue of Bacillus pseudofirmus transports both K+ and NH4+, influences ammonium homeostasis, and is required for normal sporulation and germination. The identification of these proteins as members of the CPA2 family reveals that monovalent cation transport is required for Bacillus spore formation and germination (Tani et al., 1996).

The proteins of the CPA2 family consist of between 333 and 900 amino acyl residues. They exhibit 10-14 transmembrane α-helical spanners (TMSs). Several organisms possess multiple CPA2 paralogues. Thus, E. coli has three, Methanococcus jannaschii has four and Synechocystis sp. has five paralogues. The potassium efflux system, Kef, protects bacteria against the detrimental effects of electrophilic compounds via acidification of the cytoplasm. Kef is inhibited by glutathione (GSH) but activated by glutathione-S-conjugates (GS-X) formed in the presence of electrophiles. GSH and GS-X bind to overlapping sites on Kef, which are located in a cytosolic regulatory domain (Healy et al. 2014).

 

The generalized transport reaction catalyzed by members of the CPA2 family is:

M+ (in) + nH+ (out) ⇌ M+ (out) + nH+ (in).

(The carrier-mediated mode)

Some members may also catalyze:

M+ (in) ⇌ M+ (out).

(The channel-mediated mode)

 

This family belongs to the: CPA Superfamily.

References associated with 2.A.37 family:

Aranda-Sicilia, M.N., O. Cagnac, S. Chanroj, H. Sze, M.P. Rodríguez-Rosales, and K. Venema. (2012). Arabidopsis KEA2, a homolog of bacterial KefC, encodes a K+/H+ antiporter with a chloroplast transit peptide. Biochim. Biophys. Acta. 1818: 2362-2371. 22551943
Bakker, E.P., A. Borchard, M. Michels, K. Altendorf, and A. Siebers. (1987). High-affinity potassium uptake system in Bacillus acidocaldarius showing immunological cross-reactivity with the Kdp system from Escherichia coli. J. Bacteriol. 169: 4342-4348. 2957359
Bock, K.W., D. Honys, J.M. Ward, S. Padmanaban, E.P. Nawrocki, K.D. Hirschi, D. Twell, and H. Sze. (2006). Integrating membrane transport with male gametophyte development and function through transcriptomics. Plant Physiol. 140: 1151-1168. 16607029
Booth, I.R., M.A. Jones, D. McLaggan, Y. Nikolaev, L.S. Ness, C.M. Wood, S. Miller, S. Tötemeyer, and G.P. Ferguson. (1996). Bacterial ion channels. In Transport Processes in Eukaryotic and Prokaryotic Organisms, Vol. 2 (W.N. Konings, H.R. Kaback and J.S. Lolkema, eds.), Elsevier Press, New York, pp. 693-729.
Chanroj, S., Y. Lu, S. Padmanaban, K. Nanatani, N. Uozumi, R. Rao, and H. Sze. (2011). Plant-specific cation/H+ exchanger 17 and its homologs are endomembrane K+ transporters with roles in protein sorting. J. Biol. Chem. 286: 33931-33941. 21795714
Ferguson, G.P., A.W. Munro, R.M. Douglas, D. McLaggan, and I.R. Booth. (1993). Activation of potassium channels during metabolite detoxification in Escherichia coli. Mol. Microbiol. 9: 1297-1303. 7934942
Ferguson, G.P., S. Tötemeyer, M.J. MacLean, and I.R. Booth. (1998). Methylglyoxal production in bacteria: suicide or survival? Arch. Microbiol. 170: 209-219. 9732434
Ferguson, G.P., Y. Nikolaev, D. McLaggan, M. MacLean, and I.R. Booth. (1997). Survival during exposure to the electrophilic reagent N-ethylmaleimide in Escherichia coli: role of KefB and KefC potassium channels. J. Bacteriol. 179: 1007-1012. 9023177
Fujisawa, M., M. Ito, and T.A. Krulwich. (2007). Three two-component transporters with channel-like properties have monovalent cation/proton antiport activity. Proc. Natl. Acad. Sci. USA 104: 13289-13294. 17679694
Healy, J., S. Ekkerman, C. Pliotas, M. Richard, W. Bartlett, S.C. Grayer, G.M. Morris, S. Miller, I.R. Booth, S.J. Conway, and T. Rasmussen. (2014). Understanding the structural requirements for activators of the Kef bacterial potassium efflux system. Biochemistry 53: 1982-1992. 24601535
Inaba, M., A. Sakamoto, and N. Murata. (2001). Functional expression in Escherichia coli of low-affinity and high-affinity Na+(Li+)/H+ antiporters of Synechocystis. J. Bacteriol. 183: 1376-1384. 11157951
MacLean, M.J., L.S. Ness, G.P. Ferguson, and I.R. Booth. (1998). The role of glyoxalase I in the detoxification of methylglyoxal and in the activation of the KefB K+ efflux system in Escherichia coli. Mol. Microbiol. 27: 563-571. 9489668
Miller, S., L.S. Ness, C.M. Wood, B.C. Fox, and I.R. Booth. (2000). Identification of an ancillary protein, YabF, required for activity of the KefC glutathione-gated potassium efflux system in Escherichia coli. J. Bacteriol. 182: 6536-6540. 11053405
Miller, S., R.M. Douglas, P. Carter, and I.R. Booth. (1997). Mutations in the glutathione-gated KefC K+ efflux system of Escherichia coli that cause constitutive activation. J. Biol. Chem. 272: 24942-24947. 9312097
Munro, A.W., G.Y. Ritchie, A.J. Lamb, R.M. Douglas, and I.R. Booth. (1991). The cloning and DNA sequence of the gene for the glutathione-regulated potassium-efflux system KefC of Escherichia coli. Mol. Microbiol. 5: 607-616. 2046548
Nakamura, C., T. Kikuchi, J.G. Burgess, and T. Matsunaga. (1995). Iron-regulated expression and membrane localization of the MagA protein in Magnetospirillum sp. strain AMB-1. J. Biochem. 118: 23-27. 8537318
Ness, L.S. and I.R. Booth. (1999). Different foci for the regulation of the activity of the KefB and KefC glutathione-gated K+ efflux systems. J. Biol. Chem. 274: 9524-9530. 10092637
Radchenko, M.V., R. Waditee, S. Oshimi, M. Fukuhara, T. Takabe, and T. Nakamura. (2006). Cloning, functional expression and primary characterization of Vibrio parahaemolyticus K+/H+ antiporter genes in Escherichia coli. Mol. Microbiol. 59: 651-663. 16390457
Ramírez, J., O. Ramírez, C. Saldańa, R. Coria, and A. Peńa. (1998). A Saccharomyces cerevisiae mutant lacking a K+/H+ exchanger. J. Bacteriol. 180: 5860-5865. 9811642
Reizer, J., A. Reizer, and M.H. Saier, Jr. (1992). The putative Na+/H+ antiporter (NapA) of Enterococcus hirae is homologous to the putative K+/H+ antiporter (KefC) of Escherichia coli. FEMS Microbiol. Lett. 94: 161-164. 1325937
Roosild, T.P., S. Castronovo, J. Healy, S. Miller, C. Pliotas, T. Rasmussen, W. Bartlett, S.J. Conway, and I.R. Booth. (2010). Mechanism of ligand-gated potassium efflux in bacterial pathogens. Proc. Natl. Acad. Sci. USA 107: 19784-19789. 21041667
Saier, M.H., Jr., B.H. Eng, S. Fard, J. Garg, D.A. Haggerty, W.J. Hutchinson, D.L. Jack, E.C. Lai, H.J. Liu, D.P. Nusinew, A.M. Omar, S.S. Pao, I.T. Paulsen, J.A. Quan, M. Sliwinski, T.-T. Tseng, S. Wachi, and G.B. Young. (1999). Phylogenetic characterization of novel transport protein families revealed by genome analyses. Biochim. Biophys. Acta 1422: 1-56. 10082980
Southworth, T.W., A.A. Guffanti, A. Moir, and T.A. Krulwich. (2001). GerN, an endospore germination protein of Bacillus cereus, is an Na+/H+-K+ antiporter. J. Bacteriol. 183: 5896-5903. 11566988
Stumpe, S., A. Schlösser, M. Schleyer, and E.P. Bakker. (1996). K+ circulation across the prokaryotic cell membrane: K+-uptake systems. In Transport Processes in Eukaryotic and Prokaryotic Organisms, Vol. 2 (W.N. Konings, H.R. Kaback and J.S. Lolkema, eds.), Elsevier Press, New York, pp. 473-499.
Tani, K., T. Watanabe, H. Matsuda, M. Nasu, and M. Kondo. (1996). Cloning and sequencing of the spore germination gene of Bacillus megaterium ATCC 12872: similarities to the NaH-antiporter gene of Enterococcus hirae. Microbiol. Immunol. 40: 99-105. 8867604
Tsunekawa K., Shijuku T., Hayashimoto M., Kojima Y., Onai K., Morishita M., Ishiura M., Kuroda T., Nakamura T., Kobayashi H., Sato M., Toyooka K., Matsuoka K., Omata T. and Uozumi N. (2009). Identification and characterization of the Na+/H+ antiporter Nhas3 from the thylakoid membrane of Synechocystis sp. PCC 6803. J Biol Chem. 284(24):16513-21. 19372598
Uebe, R., V. Henn, and D. Schüler. (2012). The MagA protein of Magnetospirilla is not involved in bacterial magnetite biomineralization. J. Bacteriol. 194: 1018-1023. 22194451
Waser, M., D. Hess-Bienz, K. Davies, and M. Solioz. (1992). Cloning and disruption of a putative NaH-antiporter gene of Enterococcus hirae. J. Biol. Chem. 267: 5396-5400. 1312090
Wei, Y., J. Liu, Y. Ma, and T.A. Krulwich. (2007). Three putative cation/proton antiporters from the soda lake alkaliphile Alkalimonas amylolytica N10 complement an alkali-sensitive Escherichia coli mutant. Microbiology. 153: 2168-2179. 17600061
Wei, Y., T.W. Southworth, H. Kloster, M. Ito, A.A. Guffanti, A. Moir, and T.A. Krulwich. (2003). Mutational loss of a K+ and NH4+ transporter affects the growth and endospore formation of alkaliphilic Bacillus pseudofirmus OF4. J. Bacteriol. 185: 5133-5147. 12923086
Zhao, J., N.H. Cheng, C.M. Motes, E.B. Blancaflor, M. Moore, N. Gonzales, S. Padmanaban, H. Sze, J.M. Ward, and K.D. Hirschi. (2008). AtCHX13 is a plasma membrane K+ transporter. Plant Physiol. 148: 796-807. 18676662