Identification of an Amino Acid Defining the Distinct Properties of Murine β1 and β3 Subunit-Containing GABAA Receptors

Abstract

Murine γ-aminobutyric acid (GABA) type A homomeric receptors made of β₁ subunits are profoundly different, when expressed in Xenopus oocytes, from β₃ homomeric receptors. Application of the intravenous general anesthetic pentobarbital, etomidate, or propofol to β₃ homomeric receptors allows current flow. In contrast, β₁ homomers do not respond to any of these agents. Through construction of chimeric β₁/β₃ receptors, we identified a single amino acid that determines the pharmacological difference between the two β subunits. When the serine residue present in the wild-type nonresponsive β₁ subunit is replaced by an asparagine found in the same position in the β₃ subunit, the resulting point-mutated β₁S265N forms receptors responsive to intravenous general anesthetics, like the wild-type β₃ subunits. Conversely, after mutation of the wild-type β₃ to β₃N265S, the homomeric receptor loses its ability to respond to these same general anesthetics. Wild-type-to-mutant titration experiments showed that the nonresponsive phenotype is dominant: A single nonresponsive residue within a pentameric receptor is sufficient to render the receptor nonresponsive. In α₁β_x or α₁β_xγ₂ heteromeric receptors, the same residue manifests as a partial determinant of the degree of potentiation of the GABA-induced current by some general anesthetics. The location of this amino acid at the extracellular end of the second transmembrane segment, its influence in both homomeric and heteromeric receptor function, and its dominant behavior suggest that this residue of the β subunit is involved in an allosteric modulation of the receptor.