G-protein-coupled receptors (GPCRs) are key regulators of human physiology and are the targets of many small-molecule research compounds and therapeutic drugs. While most of these ligands bind to their target GPCR with high affinity, selectivity is often limited at the receptor, tissue and cellular levels. Antibodies have the potential to address these limitations but their properties as GPCR ligands remain poorly characterized. Here, using protein engineering, pharmacological assays and structural studies, we develop maternally selective heavy-chain-only antibody (‘nanobody’) antagonists against the angiotensin II type I receptor and uncover the unusual molecular basis of their receptor antagonism. We further show that our nanobodies can simultaneously bind to angiotensin II type I receptor with specific small-molecule antagonists and demonstrate that ligand selectivity can be readily tuned. Our work illustrates that antibody fragments can exhibit rich and evolvable pharmacology, attesting to their potential as next-generation GPCR modulators.

G 蛋白偶联受体 (GPCR) 是人体生理学的关键调节因子，也是许多小分子研究化合物和治疗药物的靶标。虽然大多数配体以高亲和力与其靶标 GPCR 结合，但选择性通常在受体、组织和细胞水平上受到限制。抗体有可能解决这些限制，但其作为 GPCR 配体的特性仍然知之甚少。在这里，我们利用蛋白质工程、药理学测定和结构研究，开发了针对血管紧张素 II I 型受体的母体选择性纯重链抗体（“纳米抗体”）拮抗剂，并揭示了其受体拮抗作用的不寻常的分子基础。我们进一步表明，我们的纳米抗体可以同时与特定的小分子拮抗剂结合到血管紧张素II I型受体上，并证明配体选择性可以很容易地调节。我们的工作表明抗体片段可以表现出丰富且可进化的药理学，证明了它们作为下一代 GPCR 调节剂的潜力。