Functional asymmetry of the conserved cystine loops in αβγ GABAA receptors revealed by the response to GABA activation and drug potentiation
- 作者：M. Louise Tierney ; Tien Luu ; Peter W. Gage
- 关键词：GABAA receptor ; Cys-loop ; Gating ; Electrophysiology ; Diazepam
- 刊名：The International Journal of Biochemistry and Cell Biology
- 出版年：2008
- 期刊代码：127_13572725
- 类别：med
- 出版时间：2008
- 卷：40
- 期：5
- 页码：968-979
- 文件大小：718 K
摘要
Ligand-gated ion channels respond to specific neurotransmitters by transiently opening an integral membrane ion-selective pore, allowing ions to move down their electrochemical gradient. A distinguishing feature of all members of the ligand-gated ion channel superfamily is the presence of a 13-amino acid disulfide loop (Cys-loop) in the extracellular ligand-binding domain. Structural data derived from the acetylcholine receptor place this loop at the interface between the ligand-binding domain and the transmembrane pore-forming domain where it is ideally located to participate in coupling ligand binding to channel opening. We have introduced specific mutations into a conserved motif at the mid-point of the Cys-loop of the GABAA receptor subunits 
1, β2 and γ2S where the sequence reads aromatic, proline, aliphatic (ArProAl motif). Receptors carrying a mutation in the Cys-loop of one of their subunits were expressed in L929 cells and responses to both GABA and drugs were assessed using the whole-cell patch clamp technique. Drug potentiation and direct activation were significantly enhanced by mutations in this Cys-loop but these effects were subunit-dependent. Currents in response to agonists were larger when mutations were carried in the and β subunits but not in the γ subunit. In contrast, potentiation of current responses by diazepam, etomidate and pentobarbital were all enhanced when mutations were carried in the and γ subunits, but not the β subunit. Since the disruption of interactions mediated through the ArProAl motif enhances the mutant receptor's response to both agonist and drugs we suggest that this motif in the Cys-loop of the wild-type receptor participates in interactions that create activation barriers to conformational changes during channel gating.
1, β2 and γ2S where the sequence reads aromatic, proline, aliphatic (ArProAl motif). Receptors carrying a mutation in the Cys-loop of one of their subunits were expressed in L929 cells and responses to both GABA and drugs were assessed using the whole-cell patch clamp technique. Drug potentiation and direct activation were significantly enhanced by mutations in this Cys-loop but these effects were subunit-dependent. Currents in response to agonists were larger when mutations were carried in the and β subunits but not in the γ subunit. In contrast, potentiation of current responses by diazepam, etomidate and pentobarbital were all enhanced when mutations were carried in the and γ subunits, but not the β subunit. Since the disruption of interactions mediated through the ArProAl motif enhances the mutant receptor's response to both agonist and drugs we suggest that this motif in the Cys-loop of the wild-type receptor participates in interactions that create activation barriers to conformational changes during channel gating.