G protein-coupled receptors (GPCRs) relay information across cell membranes through conformational coupling between the ligand-binding domain and cytoplasmic signaling domain. In dimeric class C GPCRs, the mechanism of this process, which involves propagation of local ligand-induced conformational changes over 12 nm through three distinct structural domains, is unknown. Here, we used single-molecule FRET and live-cell imaging and found that metabotropic glutamate receptor 2 (mGluR2) interconverts between four conformational states, two of which were previously unknown, and activation proceeds through the conformational selection mechanism. Furthermore, the conformation of the ligand-binding domains and downstream domains are weakly coupled. We show that the intermediate states act as conformational checkpoints for activation and control allosteric modulation of signaling. Our results demonstrate a mechanism for activation of mGluRs where ligand binding controls the proximity of signaling domains, analogous to some receptor kinases. This design principle may be generalizable to other biological allosteric sensors.

G 蛋白偶联受体 (GPCR) 通过配体结合域和细胞质信号域之间的构象耦合跨细胞膜传递信息。在二聚体 C 类 GPCR 中，这一过程的机制是未知的，该过程涉及局部配体诱导的构象变化在 12 nm 上通过三个不同的结构域传播。在这里，我们使用单分子 FRET 和活细胞成像，发现代谢型谷氨酸受体 2 (mGluR2) 在四种构象状态之间相互转换，其中两种是以前未知的，并且通过构象选择机制进行激活。此外，配体结合域和下游域的构象是弱耦合的。我们表明，中间状态充当激活和控制信号变构调制的构象检查点。我们的结果证明了一种激活 mGluR 的机制，其中配体结合控制信号域的接近度，类似于一些受体激酶。这种设计原理可以推广到其他生物变构传感器。