Trans membrane domain IV is involved in ion transport activity and pH regulation of the NhaA-Na⁺/H⁺ antiporter of Escherichia coli

We have previously shown that the activity of NhaA is regulated by pH and found mutations that affect dramatically the pH dependence of the rate but not the K_m (for Na⁺ and Li⁺) of NhaA. In the present work, we found that helix IV is involved both in ion translocation as well as in pH regulation of NhaA. Two novel types of NhaA mutants were found clustered in trans membrane segment (TMS) IV: One type (D133C, T132C, and P129L) affects the apparent K_m of NhaA to the cations with no significant effect on the pH profile of the antiporter; no shift of the pH profile was found when the activity of these mutants was measured at saturating Na⁺ concentration. In contrast, the other type of mutations (A127V and A127T) was found to affect both the K_m and the pH dependence of the rate of NhaA whether tested at saturating Na⁺ concentration or not. These results imply that residues involved in the ion translocation of NhaA may (A127) or may not (D133, T132, and P129) overlap with those affecting the pH response of the antiporter. All mutants cluster in the N-terminal half of the putative α-helix IV, one type on one face, the other on the opposite. Cys accessibility test demonstrated that although D133C is located in the middle of TMS IV, it is inhibited by N-ethylmaleimide and is exposed to the cytoplasm.