Identification of the bovine γ-aminobutyric acid type A receptor α subunit residues photolabeled by the imidazobenzodiazepine [3H]Ro15-4513

Ligands binding to the benzodiazepine-binding site in gamma-aminobutyric acid type A (GABA(A)) receptors may allosterically modulate function. Depending upon the ligand, the coupling can either be positive (flunitrazepam), negative (Ro15-4513), or neutral (flumazenil). Specific amino acid determinants of benzodiazepine binding affinity and/or allosteric coupling have been identified within GAEA(A) receptor alpha and gamma subunits that localize the binding site at the subunit interface. Previous photolabeling studies with [H-3]flunitrazepam identified a primary site of incorporation at alpha(1)His-102, whereas studies with [H-3]Ro15-4513 suggested incorporation into the a, subunit at unidentified amino acids C-terminal to alpha(1)His-102. To determine the site(s) of photoincorporation by Ro15-4513, we affinity-purified (similar to200-fold) GABAA receptor from detergent extracts of bovine cortex, photolabeled it with [H-3]Ro15-4513, and identified H-3-labeled amino acids by N-terminal sequence analysis of subunit fragments generated by sequential digestions with a panel of proteases. The patterns of 3 H release seen after each digestion of the labeled fragments determined the number of amino acids between the cleavage site and labeled residue, and the use of sequential proteolytic fragmentation identified patterns of cleavage sites unique to the different a subunits. Based upon this radiochemical sequence analysis, [H-3]Ro15-4513 was found to selectively label the homologous tyrosines alpha(1)Tyr-210, alpha(2)Tyr-209, and alpha(3)Tyr-234, in GABAA receptors containing those subunits. These results are discussed in terms of a homology model of the benzodiazepine-binding site based on the molluscan acetylcholine-binding protein structure.