Differences in the phenotypic effects of mutations in homologous MrpA and MrpD subunits of the multi-subunit Mrp-type Na+/H+ antiporter

Abstract

Mrp antiporters are the sole antiporters in the Cation/Proton Antiporter 3 family of transporter databases because of their unusual structural complexity, 6-7 hydrophobic proteins that function as a hetero-oligomeric complex. The two largest and homologous subunits, MrpA and MrpD, are essential for antiport activity and have direct roles in ion transport. They also show striking homology with proton-conducting, membrane-embedded Nuo subunits of respiratory chain complex I of bacteria, e.g., Escherichia coli. MrpA has the closest homology to the complex I NuoL subunit and MrpD has the closest homology to the complex I NuoM and N subunits. Here, introduction of mutations in MrpD, in residues that are also present in MrpA, led to defects in antiport function and/or complex formation. No significant phenotypes were detected in strains with mutations in corresponding residues of MrpA, but site-directed changes in the C-terminal region of MrpA had profound effects, showing that the MrpA C-terminal region has indispensable roles in antiport function. The results are consistent with a divergence in adaptations that support the roles of MrpA and MrpD in secondary antiport, as compared to later adaptations supporting homologs in primary proton pumping by the respiratory chain complex I.

Sodium antiporters of Pseudomonas aeruginosa in challenging conditions: effects on growth, biofilm formation, and swarming motility

“…Mrp and Complex I seem to have some evolutionary relationship based on the homology between MrpC and NuoK [31], MrpA and NuoL, and MrpD and NuoMN [33]. Interestingly, the NuoL subunit has been suggested to transport sodium in E. coli [17,22,43].…”

Functional Role of MrpA in the MrpABCDEFG Na + /H + Antiporter Complex from the Archaeon Methanosarcina acetivorans

“…The MrpA model has been shown to accommodate the antiporter-like NuoN subunit of complex I, which is homologous to MrpD (21). A role for the MrpA_C domain is consistent with an essential role for the domain in the antiport activity of the Bacillus pseudofirmus Mrp complex (43).…”

A Hydrophobic Small Protein, BpOF4_01690, Is Critical for Alkaliphily of Alkaliphilic Bacillus pseudofirmus OF4

“…Reportedly, Na + in the medium is utilized as a source of coupling ions for flagellar rotation, uptake of various substrates, Na + /H + antiporters, voltage-gated sodium channel, etc. (Krulwich and Ito, 2013; Preiss et al, 2015; Ito et al, 2017; Morino et al, 2017). Therefore, ensuring optimal Na + concentration is critical for the provision of favorable growth conditions.…”