G protein-coupled receptors (GPCRs) modulate every aspect of physiological functions mainly through activating heterotrimeric G proteins. A majority of GPCRs promiscuously couple to multiple G protein subtypes. Here we validate that in addition to the well-known G_i/o pathway, somatostatin receptor 2 and 5 (SSTR2 and SSTR5) couple to the G_q/11 pathway and show that smaller ligands preferentially activate the G_i/o pathway. We further determined cryo-electron microscopy structures of the SSTR2‒G_o and SSTR2‒G_q complexes bound to octreotide and SST-14. Structural and functional analysis revealed that G protein selectivity of SSTRs is not only determined by structural elements in the receptor–G protein interface, but also by the conformation of the agonist-binding pocket. Accordingly, smaller ligands fail to stabilize a broader agonist-binding pocket of SSTRs that is required for efficient G_q/11 coupling but not G_i/o coupling. Our studies facilitate the design of drugs with selective G protein signaling to improve therapeutic efficacy.

G 蛋白偶联受体 (GPCR) 主要通过激活异三聚体 G 蛋白来调节生理功能的各个方面。大多数 GPCRs 混杂地与多种 G 蛋白亚型结合。在这里，我们验证除了众所周知的 G _i/o通路外，生长抑素受体 2 和 5（SSTR2 和 SSTR5）与 G _q/11通路偶联，并表明较小的配体优先激活 G _i/o通路。_{我们进一步确定了 SSTR2-G o}和 SSTR2-G _q的冷冻电子显微镜结构与奥曲肽和 SST-14 结合的复合物。结构和功能分析表明，SSTR 的 G 蛋白选择性不仅取决于受体-G 蛋白界面中的结构元素，还取决于激动剂结合口袋的构象。因此，较小的配体无法稳定更广泛的 SSTR 激动剂结合口袋，这是有效 G _q/11耦合而非 G _i/o耦合所需的。我们的研究有助于设计具有选择性 G 蛋白信号的药物，以提高治疗效果。