Conformational dynamics of KIR3.1/KIR3.2 channel activation via delta-opioid receptors (DORs)

This study assessed how conformational information encoded by ligand binding to δ-opioid receptors (DORs) is transmitted to Kir3.1/Kir3.2 channels. HEK293 cells were transfected with bioluminescence resonance energy transfer (BRET) donor/acceptor pairs that allowed us to independently evaluate reciprocal interactions among signaling partners. These and co-immunoprecipitation studies indicated that DORs, Gβγ and Kir3 subunits constitutively interacted with one another. GαoA associated with DORs and Gβγ but, despite being part of the complex, no evidence of its direct association with the channel was obtained. DOR activation by different ligands left DOR-Kir3 interactions unmodified, but modulated BRET between DOR-GαoA, DOR-Gβγ, GαoA-Gβγ and Gβγ-Kir3 interfaces. Ligand-induced BRET changes assessing Gβγ-Kir3.1 subunit interaction: i) followed similar kinetics as those monitoring the GαoA-Gβγ interface, ii) displayed same order of efficacy as those observed at the DOR-Gβγ interface, iii) were sensitive to pertussis toxin and iv) were predictive of whether a ligand could evoke channel currents. Conformational changes at the Gβγ/Kir3 interface were lost when Kir3.1 subunits were replaced by a mutant lacking essential sites for Gβγ-mediated activation. Thus, conformational information encoded by agonist binding to the receptor is relayed to the channel via structural rearrangements that involve repositioning of Gαβγ with respect to DORs, GαoA and channel subunits. Further, the fact that BRET changes at the Gβγ-Kir3 interface are predictive of ligand ability to induce channel currents points to these conformational biosensors as screening tools for identifying GPCR ligands that induce Kir3 channel activation.