A TM2 residue in the β1 subunit determines spontaneous opening of homomeric and heteromeric γ-aminobutyric acid-gated ion channels

gamma-Aminobutyric acid type A (GABA(A)) receptors are major inhibitory neurotransmitter-gated ion channels in the central nervous system. GABA(A) receptors consist of multiple subunits and exhibit distinct pharmacological and channel properties. Of all GABA(A) receptor subunits, the beta subunit is thought to be a key component for the functionality of the receptors. Certain types of GABA(A) receptors have been found to be constitutively active. However, the molecular basis for spontaneous opening of channels of these receptors is not totally understood. In this study, we showed that channels that contain the beta1 but not beta3 subunits opened spontaneously when these subunits were expressed homomerically or co-expressed with other types of GABA(A) receptor subunits in Xenopus oocytes. Using subunit chimeras and site-directed mutagenesis, we localized a key amino acid residue, a serine at position 265, that is critical in conferring an open state of the beta1 subunit-containing GABA(A) receptors in the absence of agonist. Moreover, some point mutations of Ser-265 also produced constitutively active channels. The magnitude of spontaneous activity of these receptors was correlated with the molecular volume of the residue at 265 for both homomeric and heteromeric GABA(A) receptors, suggesting that the spontaneous activity of the beta1 subunit-containing GABA(A) receptors may be mediated through a similar molecular mechanism that is dependent on the molecular volume of the residue at 265.