Structural basis of μ-opioid receptor targeting by a nanobody antagonist

μ-opioid receptor targeting by a nanobody antagonist is explained using structure and functional data. Researchers describe NbE, a nanobody that selectively binds the extracellular side of the μ-opioid receptor (μOR) with nanomolar affinity (reported Kd 56 nM) and acts as an antagonist rather than an agonist. NbE binding is reduced by naloxone, consistent with overlap at or near the orthosteric ligand pocket, and NbE diminishes μOR signaling elicited by DAMGO and morphine. Cryo-EM structural work shows how NbE achieves antagonism: a β-hairpin loop from NbE inserts deeply into the μOR orthosteric pocket, and additional contacts with two extracellular receptor loops (ECLs) contribute to binding strength and μOR selectivity. The study then uses NbE’s CDR3 sequence as a blueprint to design linear and cyclic peptide mimetics that recreate antagonism, offering a route toward lower molecular weight μOR antagonists inspired by nanobody binding modes.