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Notes: Low-density membrane fragments (domains) were separated from the bulk of plasma membranes of human embryonic kidney (HEK)293 cells expressing a δ-opioid (DOP) receptor-Gi1α fusion protein by drastic homogenization and flotation on equilibrium sucrose density gradients. The functional activity of trimeric G proteins and capacity of the DOP receptor to stimulate both the fusion protein-linked Gi1α and endogenous pertussis-toxin sensitive G proteins was measured as d-Ala2, d-Leu5-enkephalin stimulated high-affinity GTPase or guanosine-5′-[γ-35S]triphosphate ([35S]GTPγS) binding. The maximum d-Ala2-d-Leu5 enkephalin (DADLE)-stimulated GTPase was two times higher in low-density membrane fragments than in bulk of plasma membranes; 58 and 27 pmol/mg/min, respectively. The same difference was obtained for [35S]GTPγS binding. Contrarily, the low-density domains contained no more than half the DOP receptor binding sites (Bmax = 6.6 pmol/mg versus 13.6 pmol/mg). Thus, when corrected for expression levels of the receptor, low-density domains exhibited four times higher agonist-stimulated GTPase and [35S]GTPγS binding than the bulk plasma membranes. The regulator of G protein signaling RGS1, enhanced further the G protein functional activity but did not remove the difference between domain-bound and plasma membrane pools of G protein. The potency of the agonist in functional studies and the affinity of specific [3H]DADLE binding to the receptor were, however, the same in both types of membranes – EC50 = 4.5 ± 0.1 × 10−8 and 3.2 ± 1.4 × 10−8 m for GTPase; Kd = 1.2 ± 0.1 and 1.3 ± 0.1 nm for [3H]DADLE radioligand binding assay. Similar results were obtained when sodium bicarbonate was used for alkaline isolation of membrane domains. By contrast, detergent-insensitive membrane domains isolated following treatment of cells with Triton X100 exhibited no DADLE-stimulated GTPase or GTPγS binding. Functional coupling between the DOP receptor and cognate G proteins was also blocked by high-energy ultrasound and repeated freezing-thawing. Our data indicate, for the first time, that membrane domains isolated using ‘detergent-free’ procedures exhibit higher efficiency of coupling between a G protein-coupled receptor and its corresponding G protein(s) than bulk plasma membranes. Detergent-extraction diminishes these interactions, even when the receptor and G proteins are physically tethered together.