Progress Towards the Understanding of the Gaba_a Receptor Structure

Abstract

The GABA_A receptor of mammalian brain is a ligand-gated channel protein with allosteric binding sites for the benzodiazepines and barbiturate drugs. The receptor is an acidic oligomeric membrane glycoprotein and it has been purified to homogeneity from bovine cerebral cortex, bovine cerebellum and rat cerebral cortex by benzodiazepine affinity chromatography. In each case, extraction and purification with the zwitterionic detergent CHAPS and exogenous phospholipid has demonstrated the coexistence of GABA, benzodiazepine and cage convulsant ligand binding sites on a single protein complex; in addition the allosteric interactions between these sites are preserved in the isolated protein. The receptor has a heterologous structure that is conserved at the subunit level between the aforementioned mammalian species and brain regions. SDS-PAGE has shown that the receptor consists of two subunits, α (M_r 53000) and β (M_r 57000) present in equal stoichiometry. A model consistent with the determination of the molecular weight of the native protein, i.e., M_r 230,000, is that of a tetramer α₂β₂. [³H] Flunitrazepam and [³H]muscimol have been employed as photoaffinity labels to map the benzodiazepine and GABA binding polypeptides respectively. Polyclonal and monoclonal antibodies have been raised to the native bovine GABA_A receptor and these have been employed for the further characterisation of the receptor protein.