Stimulation of phospholipase C by the cloned μ, δ and κ opioid receptors via chimeric Gαqmutants

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Abstract
Opioid receptors (μ, δ and κ) are known to regulate diverse physiological functions and yet, at the molecular level, they are coupled to a seemingly identical set of G proteins. A recent study has discerned subtle differences between the opioid receptors in their ability to activate the pertussis toxin-insensitive G16. Differences in microarchitecture might be magnified when these receptors are provided with ‘non-native’ partners. Here, we examined whether the opioid receptors can interact productively with a set of chimeric Gαq subunits which are known to link many Gi-coupled receptors to phosphoinositide-specific phospholipase C (PI-PLC). The qi5, qo5 and qz5 chimeras have the last five residues of Gαq replaced by those of Gαi, Gαo and Gαz, respectively. Except for μ-receptor and qo5, each pair of opioid receptor and Gαq chimera allowed opioid agonists to stimulate PI-PLC in transfected COS-7 cells. The Gαq chimera-mediated responses were ligand selective, agonist dose dependent and saturable. The most robust responses were obtained with κ-receptor and qi5 or qz5, whereas the coupling of δ- and μ-receptors to Gαq chimeras produced much weaker responses. Among the Gαq chimeras, qo5 was less efficiently coupled to the opioid receptors. As revealed by radioligand binding assays and immunoblot analysis, differences in the efficiency of coupling were not due to variations in the expression of receptors and Gαq chimeras. Differences in the magnitude of PI-PLC responses are thus likely to represent structural incompatibility between opioid receptors and Gαq chimeras, suggesting that each opioid receptor possesses unique structural surfaces for the binding of G proteins.