Biology:Monovalent cation:proton antiporter-3
The Monovalent Cation (K+ or Na+):Proton Antiporter-3 (CPA3) Family (TC# 2.A.63) is a member of the Na+ transporting Mrp superfamily. The CPA3 family consists of bacterial multicomponent K+:H+ and Na+:H+ antiporters. The best characterized systems are the PhaABCDEFG system of Sinorhizobium meliloti (TC# 2.A.63.1.1) that functions in pH adaptation and as a K+ efflux system, and the MnhABCDEFG system of Staphylococcus aureus (TC# 2.A.63.1.3) that functions as a Na+ efflux Na+:H+ antiporter.[1]
Homology
A homologous, but only partially sequenced, system was earlier reported to catalyze Na+:H+ antiport in an alkalophilic Bacillus strain. PhaA and PhaD are respectively homologous to the ND5 and ND4 subunits of the H+-pumping NADH:ubiquinone oxidoreductase (TC #3.D.1). Homologous protein subunits from E. coli NADH:quinone oxidoreductase can functionally replace MrpA and MrpD in Bacillus subtilis.[2]
Homologues of PhaA, B, C and D and Nha1, 2, 3 and 4 of an alkalophilic Bacillus strain are the Yuf(Mrp)T, U, V and D genes of Bacillus subtilis. In this system, YufT is believed to be responsible for Na+:H+ antiporter activity, but it does not have activity in the absence of other constituents of the operon.[3]
Structure
The seven Pha proteins are of the following sizes (in #aas) and exhibit the following putative numbers of transmembrane α-helical spanners (TMSs):
- PhaA - 725 and 17
- PhaB - 257 and 5
- PhaC - 115 and 3
- PhaD - 547 and 13
- PhaE - 161 and 3
- PhaF - 92 and 3
- PhaG - 120 and 3
All are predicted to be integral membrane proteins.
Corresponding values for the S. aureus Mnh system are:[4]
- MnhA - 801 and 18
- MnhB - 142 and 4
- MnhC - 113 and 3
- MnhD - 498 and 13
- MnhE - 159 and 4
- MnhF - 97 and 3
- MnhG - 118 and 3
In view of the complexity of the system, large variation in subunit structure, and the homology with NDH family protein constituents, a complicated energy coupling mechanism, possibly involving a redox reaction, cannot be ruled out.
Function
Na+ or Li+ does, but K+, Ca2+, and Mg2+ do not, support significant antiport by the Gram-positive bacterial systems (TC# 2.A.6.3.1.2 and TC# 2.A.6.3.1.3).[5] Na+(Li+)/H+ antiporters have alkaline pH optima and apparent Km values for Na+ that are among the lowest reported for bacterial Na+/H+ antiporters. Na+/H+antiport consumes the pmf and therefore is probably electrogenic.[5]
YufF (MrpF) appears to catalyze cholate efflux, possibly by a Na+ symport mechanism.[6] It plays a major role in Na+ extrusion and is required for initiation of sporulation.[3][7][8] Additionally, another component of the operon, MrpF (equivalent to PhaF of R. meliloti) has been implicated in choline and Na+ efflux.[8] The MrpA-G proteins of B. subtilis have been shown to be present in a single multicomponent complex.[9] They provide Na+/H+ antiport activity and function in multiple compound resistance and pH homeostasis.[3]
Transport Reaction
The generalized reaction believed to be catalyzed by CPA3 family members is:
[K+ or Na+] (in) + H+ (out) ⇌ [K+ or Na+] (out) + H+ (in).
See also
- Sodium-Proton antiporter
- Monovalent Cation (K+ or Na+):Proton Antiporter-1
- Monovalent Cation (K+ or Na+):Proton Antiporter-2
- Transporter Classification Database
References
- ↑ "2.A.63 The Monovalent Cation (K+ or Na+):Proton Antiporter-3 (CPA3) Family". http://www.tcdb.org/search/result.php?tc=2.A.63.
- ↑ "Homologous protein subunits from Escherichia coli NADH:quinone oxidoreductase can functionally replace MrpA and MrpD in Bacillus subtilis". Biochimica et Biophysica Acta (BBA) - Bioenergetics 1807 (4): 427–36. April 2011. doi:10.1016/j.bbabio.2011.01.005. PMID 21236240.
- ↑ 3.0 3.1 3.2 "Analyses of a Bacillus subtilis homologue of the Na+/H+ antiporter gene which is important for pH homeostasis of alkaliphilic Bacillus sp. C-125". Biochimica et Biophysica Acta (BBA) - Bioenergetics 1409 (3): 171–5. January 1999. doi:10.1016/s0005-2728(98)00157-1. PMID 9878723.
- ↑ "A putative multisubunit Na+/H+ antiporter from Staphylococcus aureus". Journal of Bacteriology 180 (24): 6642–8. December 1998. doi:10.1128/JB.180.24.6642-6648.1998. PMID 9852009.
- ↑ 5.0 5.1 "Catalytic properties of Staphylococcus aureus and Bacillus members of the secondary cation/proton antiporter-3 (Mrp) family are revealed by an optimized assay in an Escherichia coli host". Journal of Bacteriology 189 (8): 3081–90. April 2007. doi:10.1128/JB.00021-07. PMID 17293423.
- ↑ "Effects of nonpolar mutations in each of the seven Bacillus subtilis mrp genes suggest complex interactions among the gene products in support of Na(+) and alkali but not cholate resistance". Journal of Bacteriology 182 (20): 5663–70. October 2000. doi:10.1128/jb.182.20.5663-5670.2000. PMID 11004162.
- ↑ "Function of a principal Na(+)/H(+) antiporter, ShaA, is required for initiation of sporulation in Bacillus subtilis". Journal of Bacteriology 182 (4): 898–904. February 2000. doi:10.1128/jb.182.4.898-904.2000. PMID 10648512.
- ↑ 8.0 8.1 "mrp, a multigene, multifunctional locus in Bacillus subtilis with roles in resistance to cholate and to Na+ and in pH homeostasis". Journal of Bacteriology 181 (8): 2394–402. April 1999. doi:10.1128/JB.181.8.2394-2402.1999. PMID 10198001.
- ↑ "Complex formation by the mrpABCDEFG gene products, which constitute a principal Na+/H+ antiporter in Bacillus subtilis". Journal of Bacteriology 189 (20): 7511–4. October 2007. doi:10.1128/JB.00968-07. PMID 17693497.
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