Abstract
Mrp systems are widely distributed and structurally complex cation/proton antiporters. Antiport activity requires hetero-oligomeric complexes of all six or seven hydrophobic Mrp proteins (MrpA-MrpG). Here, a panel of site-directed mutants in conserved or proposed motif residues was made in the Mrp Na(+)(Li(+))/H(+) antiporter from an alkaliphilic Bacillus. The mutant operons were expressed in antiporter-deficient Escherichia coli KNabc and assessed for antiport properties, support of sodium resistance, membrane levels of each Mrp protein, and presence of monomeric and dimeric Mrp complexes. Antiport did not depend on a VFF motif or a conserved tyrosine pair, but a role for a conserved histidine in a potential quinone binding site of MrpA was supported. The importance of several acidic residues for antiport was confirmed, and the importance of additional residues was demonstrated (e.g. three lysine residues conserved across MrpA, MrpD, and membrane-bound respiratory Complex I subunits (NuoL/M/N)). The results extended indications that MrpE is required for normal membrane levels of other Mrp proteins and for complex formation. Moreover, mutations in several other Mrp proteins lead to greatly reduced membrane levels of MrpE. Thus, changes in either of the two Mrp modules, MrpA-MrpD and MrpE-MrpG, influence the other. Two mutants, MrpB-P37G and MrpC-Q70A, showed a normal phenotype but lacked the MrpA-MrpG monomeric complex while retaining the dimeric hetero-oligomeric complex. Finally, MrpG-P81A and MrpG-P81G mutants exhibited no antiport activity but supported sodium resistance and a low [Na(+)](in). Such mutants could be used to screen hypothesized but uncharacterized sodium efflux functions of Mrp apart from Na(+) (Li(+))/H(+) antiport.
Highlights
Introduction of theSite-directed Mutations in the mrp Operon—pGEMmrpTFCHS7, in which different epitope tags are linked to the C terminus of MrpA (T7 tag), MrpB (FLAG tag), MrpC (c-myc tag), MrpD (His7 tag), and MrpG (S tag), respectively, was constructed previously [22]
Computer Analysis of Mrp Protein Topologies and Selection of Residues for Mutation—The secondary structure of each Mrp protein was predicted by ConPred II, HMMTOP, and TMHMM
All three programs predicted the number of transmembrane segments (TMSs) of MrpB, MrpC, MrpD, MrpF, and MrpG to be 4, 3, 14, 3, and 3, FIGURE 1
Summary
Introduction of theSite-directed Mutations in the mrp Operon—pGEMmrpTFCHS7, in which different epitope tags are linked to the C terminus of MrpA (T7 tag), MrpB (FLAG tag), MrpC (c-myc tag), MrpD (His7 tag), and MrpG (S tag), respectively, was constructed previously [22]. Dark gray highlights indicate mutant phenotypes that differ markedly from the wild-type transformant
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