γ-aminobutyric acid/benzodiazepine receptor protein carries binding sites for both ligands on both two major peptide subunits

Summary Affinity column-purified GABA-benzodiazepine receptor proteins from human, cow, and rat brain were photoaffinity labeled with both [ 3 H]flunitrazepam and [ 3 H]muscimol and examined by gel electrophoresis in sodium dodecyl sulfate. Using high receptor protein concentrations (1 μM), the benzodiazepine ligand [ 3 H]flunitrazepam was incorporated covalently primarily into the expected 52 kiloDalton major subunit but also significantly into a second 57 kiloDalton peptide. Likewise the GABA ligand [ 3 H]muscimol photolabeled primarily the 57 kiloDalton peptide but also to some extent the 52 kiloDalton peptide. This cross-labeling suggests strongly that both major subunits carry binding sites for both GABA and benzodiazepine.