Article

Molecular control of δ-opioid receptor signalling

Nature (Impact Factor: 42.35). 01/2014; 506(7487). DOI: 10.1038/nature12944
Source: PubMed

ABSTRACT Opioids represent widely prescribed and abused medications, although their signal transduction mechanisms are not well understood. Here we present the 1.8 Å high-resolution crystal structure of the human δ-opioid receptor (δ-OR), revealing the presence and fundamental role of a sodium ion in mediating allosteric control of receptor functional selectivity and constitutive activity. The distinctive δ-OR sodium ion site architecture is centrally located in a polar interaction network in the seven-transmembrane bundle core, with the sodium ion stabilizing a reduced agonist affinity state, and thereby modulating signal transduction. Site-directed mutagenesis and functional studies reveal that changing the allosteric sodium site residue Asn 131 to an alanine or a valine augments constitutive β-arrestin-mediated signalling. Asp95Ala, Asn310Ala and Asn314Ala mutations transform classical δ-opioid antagonists such as naltrindole into potent β-arrestin-biased agonists. The data establish the molecular basis for allosteric sodium ion control in opioid signalling, revealing that sodium-coordinating residues act as 'efficacy switches' at a prototypic G-protein-coupled receptor.

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    • "The insertion of b 562 RIL into ICL3 of the smoothened receptor has also been proposed as a reason for the lack of structural rearrangements at the cytoplasmic surface upon agonist binding (Wang et al., 2013b). Finally, comparison of the murine d-opioid receptor structure solved with an ICL3 T4L fusion (Granier et al., 2012) and the human d-opioid receptor with an N-terminal b 562 RIL fusion (Fenalti et al., 2014) shows a high degree of structural similarity, with the main deviations occurring proximal to the sites of fusion. "
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    • "The several recent crystal structures that show interacting parallel receptors in the crystal unit cell (Fig. 3) have been used as an argument in favor of GPCR dimerization, although the possibility exists that these are crystallographic artifacts and/or they do not necessarily represent physiologically relevant interfaces. Two of the five available opioid receptor crystal structures (Fenalti et al., 2014; Granier et al., 2012; Manglik et al., 2012; Thompson et al., 2012; Wu et al., 2012), specifically the structures of μ (Manglik et al., 2012) and κ (Wu et al., 2012) receptors, also reveal parallel arrangements of interacting receptors. As shown in Fig. 3, these correspond to two different interfaces in the case of μ receptor, one of which is also seen in the κ receptor crystal "
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    • "This sodium site appears to collapse in active-state structures, suggesting that it plays a key role in constraining GPCRs in an inactive state (Wootten et al., 2013b; Katritch et al., 2014). Interestingly, the sodium site has also been recently implicated in the regulation of biased agonism by the d-opioid receptor (Fenalti et al., 2014) and in the actions of synthetic small-molecule allosteric ligands for the m-opioid receptor (Livingston and Traynor, 2014). It should be noted, however, that a subset of class A GPCRs does not possess the requisite acidic residue at the 2.50 position. "
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