Enhancement of glycine receptor function by ethanol is inversely correlated with molecular volume at position α267

Glycine and gamma-aminobutyric acid (GABA)(A) receptors are members of the "superfamily" of ion channels, and are sensitive to allosteric modulation by n-alcohols such as ethanol and butanol. We recently demonstrated that the mutation of Ser-267 to Ile in the alpha 1 subunit abolished ethanol regulation of glycine receptors (Gly-R). In the present study, a pair of chimeric receptors was studied, in which a 45-amino acid domain comprising transmembrane domains 2 and 3 was exchanged between the Gly-R alpha 1 and gamma-aminobutyric acid rho 1 subunits. Detailed pharmacologic analysis of these chimeras confirmed that this domain of the Gly-R confers enhancement of recep tor function by ethanol and butanol. An extensive series of mutations at Ser-267 in the Gly-R alpha 1 subunit was also prepared, and the resulting homomeric receptors were expressed and tested for sensitivity to glycine, and allosteric modulation by alcohols. All of the mutant receptors expressed successfully in Xenopus oocytes. Mutation of Ser-267 to small amino acid residues such as Gly or Ala produced receptors in which glycine responses were potentiated by ethanol. As we have reported previously, the mutant Gly-R alpha 1 (Ser-267 --> Ile) was completely insensitive to ethanol; mutation of Ser-267 to Val had a similar effect. Mutation of Ser-267 to large residues such as His, Cys, or Tyr resulted in inhibition of Gly-R function by ethanol. These results demonstrate that the size of the amino acid residue at position alpha 267 plays a crucial role in determining the functional consequences of allosteric modulation of the Gly-R by alcohols.