The µ-opioid receptor (µOR) is an important target for pain management 1 and molecular
understanding of drug action on µOR will facilitate the development of better therapeutics.
Here we show, using double electron–electron resonance and single-molecule fluorescence
resonance energy transfer, how ligand-specific conformational changes of µOR translate
into a broad range of intrinsic efficacies at the transducer level. We identify several
conformations of the cytoplasmic face of the receptor that interconvert on different
timescales, including a pre-activated conformation that is capable of G-protein binding,
and a fully activated conformation that markedly reduces GDP affinity within the ternary
complex. Interaction of β-arrestin-1 with the μOR core binding site appears less specific
and occurs with much lower affinity than binding of G i .
