Identification of an amino acid-defining the distinct properties of murine B1 and B3 subunit-containing GABAA receptors

Murine gamma-aminobutyric acid (GABA) type A homomeric receptors made of beta(1) subunits are profoundly different, when expressed in Xenopus oocytes, from beta(3) homomeric receptors. Application of the intravenous general anesthetic pentobarbital, etomidate, or propofol to beta(3) homomeric receptors allows current flow. In contrast, beta(1) homomers do not respond to any of these agents. Through construction of chimeric beta(1)/beta(3) receptors, we identified a single amino acid that determines the pharmacological difference between the two beta subunits. When the serine residue present in the wild-type nonresponsive beta(1) subunit is replaced by an asparagine found in the same position in the beta(3) subunit, the resulting point-mutated beta(1)S265N forms receptors responsive to intravenous general anesthetics, like the wild-type beta(3) subunits. Conversely, after mutation of the wild-type beta(3) to beta(3)N265S, 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 alpha(1)beta(x) or alpha(1)beta(x)gamma(2) 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 beta subunit:is involved in an allosteric modulation of the receptor.