Identification of an amino acid defining the distinct properties of murine β1 and β3 subunit-containing GABA(A) 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 β1 S265N 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(x)γ2 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 subunit is involved in an allosteric modulation of the receptor.