MRC Laboratory of Molecular Biology, Cambridge Biomedical Campus, Cambridge, UK.
Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA.
Nature. 2019 Jan;565(7740):516-520. doi: 10.1038/s41586-018-0833-4. Epub 2019 Jan 2.
Type A γ-aminobutyric acid (GABA) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the α1β2/3γ2 GABA receptors. The β3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in β1-β3 triple knockout neurons. So far, efforts to generate accurate structural models for heteromeric GABA receptors have been hampered by the use of engineered receptors and the presence of detergents. Notably, some recent cryo-electron microscopy reconstructions have reported 'collapsed' conformations; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor, the large body of structural work on homologous homopentameric receptor variants and the logic of an ion-channel architecture. Here we present a high-resolution cryo-electron microscopy structure of the full-length human α1β3γ2L-a major synaptic GABA receptor isoform-that is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator 'megabody' and is in a desensitized conformation. Each GABA receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of α1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABA receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.
A型 γ-氨基丁酸（GABA）受体是五聚体配体门控离子通道，是脊椎动物神经系统中快速抑制性神经传递的主要驱动因素。它们的功能障碍与一系列神经疾病有关，包括抑郁症、癫痫和精神分裂症。在理论上可能存在的众多组装体中，大脑中最常见的是α1β2/3γ2 GABA 受体。β3 亚基在维持抑制性张力方面起着重要作用，仅表达该亚基就足以挽救β1-β3 三重敲除神经元中的抑制性突触传递。到目前为止，由于使用工程受体和去污剂的存在，为异源 GABA 受体生成准确结构模型的努力受到了阻碍。值得注意的是，一些最近的低温电子显微镜重建报告了“塌陷”构象；然而，这些与原型五聚体配体门控离子通道河豚乙酰胆碱受体的结构、同源同五聚体受体变体的大量结构工作以及离子通道结构的逻辑不一致。在这里，我们展示了全长人α1β3γ2L-一种主要的突触 GABA 受体同工型的高分辨率低温电子显微镜结构，该结构在脂质纳米盘中功能重建。该受体与正变构调节剂“megabody”结合，并处于脱敏构象。每个 GABA 受体五聚体包含两个磷脂酰肌醇-4,5-二磷酸分子，其头部占据α1 亚基细胞内近膜区带正电荷的口袋。在这个水平之外，细胞内 M3-M4 环大部分是无序的，可能是因为没有存在相互作用的突触后蛋白。该结构说明了异源 GABA 受体组织的分子原理，并为 GABA 能信号转导和药理学的未来机制研究提供了参考框架。
