The pH sensor and ion binding of NhaD Na⁺/H⁺ antiporter from IT superfamily

Sodium-proton (Na+/H+) antiporters from the ion transporter (IT) superfamily play a vital role in controlling the pH and electrolyte homeostasis. However, very limited information regarding their structural functions is available to date. In this study, the structural model of the NhaD antiporter was proposed as a typical hairpin structure of IT proteins, with two symmetrically conserved scaffold domains that frame the core substrate-binding sites, and four motifs were identified. Furthermore, 25 conserved sites involving these domains were subjected to site-directed mutagenesis, and all mutations resulted in an impact on transport abilities. In particular, as candidates for Na+-binding sites, D166 and D405 mutations at hairpin discontinuities were detrimental to transport activities and were found to induce pronounced conformational changes using fluorescence resonance energy transfer (FRET) assays. In addition, as observed in the NhaA structure, some charged residues, for example, E64, E65, R454, and R464, are predicted to be involved in the net charge switch of NhaD activation, by collectively form a "pH sensor" at the entrance of the cytoplasmic funnel. Mutations encompassing these residues were detrimental to the transport activity of NhaD or lost the capacity to respond to pH signals and trigger conformational changes for Na+ translocation.