A mutant residue in the third transmembrane region of the GABAA alpha1 subunit causes increased agonistic neurosteroid responses

Pregnane derived steroids have agonistic and antagonistic actions at GABAA receptors. Putative binding sites for agonistic neurosteroids are located within the transmembrane (TM) regions. A mutation within the rat α1 TM3 region, S299C, caused the expressed receptors to have unusual and extreme sensitivity to agonistic neurosteroids. For mutant α1S299C receptors, with wild type β and γ subunits, expressed in Xenopus oocytes, steroids activated the GABAA receptors in the absence of GABA. Maximal steroid induced currents were about half of maximal GABA currents. The steroid activation was biphasic with EC50's much lower than wild type, in subnanomolar and nanomolar concentrations, while the wild type had only one activation peak with near micromolar EC50. These currents could be blocked by both picrotoxin and an antagonist neurosteroid. The steroids did not seem to potentiate significantly submaximal GABA currents. The α1S299C mutation did not affect responses to the extracellularly acting partial agonist piperidine-4-sulfate. Substituted cysteine experiments indicate that this mutant can be modified by pCMBS− when the sulfhydryl reagent is added with the higher steroid concentration for activation but not the lower steroid concentration. The pCMBS− will also immediately block the high concentration steroid current. Taken together the data suggest that α1S299 is important in at least the in transduction of the steroid binding to the rest of the receptor.

► A mutation in the third transmembrane region of the alpha1 subunit of the GABAA receptor causes extreme sensitivity to neurosteroids.
► The mutation at S299 to C causes the receptors to show biphasic activation by neurosteroids but little potentiation.
► The current induced by the higher concentration can be directly blocked and then subsequently inhibited by pCMBS−.
► The mutant does not have significant differences in responses to other ligands.
► The residue may be involved in steroid access or in transduction of the steroid binding.