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 K_m 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

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