Comparative molecular analysis of Na+/H+ exchangers: a unified model for Na+/H+ antiport?

Despite 30 years of study on Na+/H+ exchange, the molecular mechanisms of antiport remain obscure. Most challenging, the identity of amino acids involved in binding transported cations is still unknown. We review data examining the identity of residues that are involved in cation binding and translocation of prokaryotic and eukaryotic Na+/H+ antiporters. Several polar residues specifically distributed within or immediately adjacent to membrane spanning regions are implicated as being important. These key amino acids are conserved in prokaryotes and in some lower eukaryotic forms of the Na+/ H+ antiporter, despite their being dispersed throughout the protein and despite an overall low similarity in the linear sequence of these Na+/H+ antiporters. We suggest that this conservation of isolated residues (together with distances between them) reflects a general physicochemical mechanism of cation binding by exchangers. The binding could be based on coordination of the substrate cation by a crown ether-like cluster of polar atomic groups amino acids, as has been hypothesized by Boyer. Traditional screening for the extended, highly conserved linear protein sequences might not be applicable when searching for functional domains of ion transporters. Three-dimensional constellations of polar residues (3D-motifs) may be evolutionary conserved rather than linear primary sequence.