Fig. 4. Sodium-coordinating residues form an efficacy switch regulating biased signaling.

Mutation of the sodium-anchoring residues Asp95, Asn310 and Asn314 promotes efficacy switching of the cyclopentene-containing antagonist naltrindole into a potent β-arrestin-biased agonist. Normalized concentration-responses of δ-OR-mediated Gα_i signaling induced by (a) BW373U86 and (c) naltrindole, and δ-OR mediated β-arrestin recruitment with (b) BW373U86 and (d) naltrindole were quantified as in Methods. Results represent average ± SEM of four independent experiments each in quadruplicate and are presented as % of activation by BW373U86. Receptors were all transfected with 15 µg DNA revealing weak partial agonist activity of naltrindole at Gα_i signaling as previously described^29,30.