Defining topological similarities among ion transport proteins with anti-amiloride antibodies.

The structural features of amiloride binding sites on amiloride-sensitive transport proteins have received limited characterization. An antibody that recognizes limited regions of amiloride and can mimic, in binding specificity, certain amiloride-sensitive transport proteins was used as a model to elucidate potential amino acid residue relationships that might define putative amiloride contact sites. Analysis of the structure of this antibody has allowed us to identify sequence relationships among several Na+ selective transport proteins. A structure-based relational database was employed to re-examine sequence homologies among these ion transport proteins. A search of the protein sequence databank identified representative amino acid tracts among amiloride sensitive proteins involving planar residues that might be involved in interacting with amiloride. Computer models of sites within transmembrane domains of NHE1 and NHE2 isoforms of the Na+/H+ exchanger reflective of these planar tracts indicate that amiloride probably spans two helices for interaction with the Na+/H+ exchanger. Structural analysis of this monoclonal anti-amiloride antibody appears to mimic some of the salient features of amiloride binding sites on these proteins.