Metabotropic GABA_B G protein-coupled receptor functions as a mandatory heterodimer of GB1 and GB2 subunits and mediates inhibitory neurotransmission in the central nervous system. Each subunit is composed of the extracellular Venus flytrap (VFT) domain and transmembrane (TM) domain. Here we present cryo-EM structures of full-length human heterodimeric GABA_B receptor in the antagonist-bound inactive state and in the active state complexed with an agonist and a positive allosteric modulator in the presence of G_i1 protein at a resolution range of 2.8–3.0 Å. Our structures reveal that agonist binding stabilizes the closure of GB1 VFT, which in turn triggers a rearrangement of TM interfaces between the two subunits from TM3-TM5/TM3-TM5 in the inactive state to TM6/TM6 in the active state and finally induces the opening of intracellular loop 3 and synergistic shifting of TM3, 4 and 5 helices in GB2 TM domain to accommodate the α5-helix of G_i1. We also observed that the positive allosteric modulator anchors at the dimeric interface of TM domains. These results provide a structural framework for understanding class C GPCR activation and a rational template for allosteric modulator design targeting the dimeric interface of GABA_B receptor.

代谢型 GABA _B G 蛋白偶联受体作为 GB1 和 GB2 亚基的强制异二聚体发挥作用，介导中枢神经系统中的抑制性神经传递。每个亚基由细胞外捕蝇草 (VFT) 域和跨膜 (TM) 域组成。在这里，我们展示了处于拮抗剂结合的非活性状态和与激动剂和正变构调节剂在 G _i1存在下复合的活性状态的全长人异二聚体 GABA _B受体的冷冻电镜结构蛋白质分辨率范围为 2.8–3.0 Å。我们的结构表明，激动剂结合稳定了 GB1 VFT 的闭合，这反过来触发了两个亚基之间的 TM 界面的重排，从处于非活动状态的 TM3-TM5/TM3-TM5 到处于活动状态的 TM6/TM6，并最终诱导细胞内环 3 的打开和 GB2 TM 结构域中 TM3、4 和 5 螺旋的协同移位以适应 G _i1的α5-螺旋。我们还观察到正变构调节剂锚定在 TM 结构域的二聚体界面。这些结果为理解 C 类 GPCR 激活提供了结构框架，并为针对 GABA _B受体二聚体界面的变构调节剂设计提供了合理模板。