Na+/H+ antiporters

Na+/H+ antiporters are membrane proteins that play a major role in pH and N a+ homeostasis of cells throughout the biological kingdom, from bacteria to humans and higher plants. The emerging genomic sequence projects already h ave started to reveal that the Na+/H+ antiporters cluster in several famili es. Structure and function studies of a purified antiporter protein have as yet been conducted mainly with NhaA, the key Na+/H+ antiporter of Escheric hia coli. This antiporter has been overexpressed, purified and reconstitute d in a functional form in proteoliposomes. It has recently been crystallize d in both 3D as well as 2D crystals. The NhaA 2D crystals were analyzed by cryoelectron microscopy and a density map at 4 Angstrom resolution was obta ined and a 3D map was reconstructed. NhaA is shown to exist in the 2D cryst als as a dimer of monomers each composed of 12 transmembrane segments with an asymmetric helix packing. This is the first insight into the structure o f a polytopic membrane protein. Many Na+/H+ antiporters are characterized b y very dramatic sensitivity to pH, a property that corroborates their role in pH homeostasis. The molecular mechanism underlying this pH sensitivity h as been studied in NhaA. Amino acid residues involved in the pH response ha ve been identified. Conformational changes transducing the pH change into a change in activity were found in loop VIII-IX and at the N-terminus by pro bing trypsin digestion or binding of a specific monoclonal antibody respect ively. Regulation by pH of the eukaryotic Na+/H+ antiporters involves an in tricate signal transduction pathway (recently reviewed by Yun et al., Am. J . Physiol. 269 (1995) G1-G11). The transcription of NhaA has been shown to be regulated by a novel Na+-specific regulatory network. It is envisaged th at interdisciplinary approaches combining structure, molecular and cell bio logy as well as genomics should be applied in the future to the study of th is important group of transporters. (C) 2001 Elsevier Science B.V. All righ ts reserved.