DEFINING TOPOLOGICAL SIMILARITIES AMONG ION-TRANSPORT PROTEINS WITH ANTI-AMILORIDE ANTIBODIES

The structural features of amiloride binding sites an amiloride-sensit ive transport proteins have received limited characterization. An anti body that recognizes limited regions of amiloride and can mimic, in bi nding specificity, certain amiloride-sensitive transport proteins was used as a model to elucidate potential amino acid residue relationship s that might define putative amiloride contact sites. Analysis of the structure of this antibody has allowed us to identify sequence relatio nships among several Na+ selective transport proteins. A structure-bas ed relational database was employed to re-examine sequence homologies among these ion transport proteins. A search of the protein sequence d atabank identified representative amino acid tracts among amiloride se nsitive proteins involving planar residues that might be involved in i nteracting with amiloride. Computer models of sites within transmembra ne domains of NHE1 and MHE2 isoforms of the Na+/H+ exchanger reflectiv e of these planar tracts indicate that amiloride probably spans two he lices for interaction with the Na+/H+ exchanger. Structural analysis o f this monoclonal anti-amiloride antibody appears to mimic some of the salient features of amiloride binding sites on these proteins.