Conformational dynamics of Kir3.1/Kir3.2 channel activation via δ-opioid receptors.

This study assessed how conformational information encoded by ligand binding to δ-opioid receptors (DORs) is transmitted to Kir3.1/Kir3.2 channels. Human embryonic kidney 293 cells were transfected with bioluminescence resonance energy transfer (BRET) donor/acceptor pairs that allowed us to evaluate independently reciprocal interactions among signaling partners. These and coimmunoprecipitation studies indicated that DORs, Gßγ, and Kir3 subunits constitutively interacted with one another. GαoA associated with DORs and Gßγ, 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-Gßγ, GαoA-Gßγ, and Gßγ-Kir3 interfaces. Ligand-induced BRET changes assessing Gßγ-Kir3.1 subunit interaction 1) followed similar kinetics to those monitoring the GαoA-Gßγ interface, 2) displayed the same order of efficacy as those observed at the DOR-Gßγ interface, 3) were sensitive to pertussis toxin, and 4) were predictive of whether a ligand could evoke channel currents. Conformational changes at the Gßγ/Kir3 interface were lost when Kir3.1 subunits were replaced by a mutant lacking essential sites for Gßγ-mediated activation. Thus, conformational information encoded by agonist binding to the receptor is relayed to the channel via structural rearrangements that involve repositioning of Gßγ with respect to DORs, GαoA, and channel subunits. Further, the fact that BRET changes at the Gßγ-Kir3 interface are predictive of a ligand's ability to induce channel currents points to these conformational biosensors as screening tools for identifying GPCR ligands that induce Kir3 channel activation.