Stimulation of phospholipase C by the cloned mu, delta and kappa opioid receptors via chimeric G alpha(q) mutants

Opioid receptors (mu, delta and kappa) are known to regulate diverse physio logical functions and yet, at the molecular level, they are coupled to a se emingly identical set of G proteins. A recent study has discerned subtle di fferences between the opioid receptors in their ability to activate the per tussis toxin-insensitive G(16) Differences in microarchitecture might be ma gnified when these receptors are provided with 'non-native' partners. Here, we examined whether the opioid receptors can interact productively with a set of chimeric G alpha(q) subunits which are known to link many G(i)-coupl ed receptors to phosphoinositide-specific phospholipase C (PI-PLC). The qi5 , qo5 and qz5 chimeras have the last five residues of G alpha(q) replaced b y those of G alpha(i), G alpha(o), and G alpha(z), respectively. Except for mu-receptor and qo5, each pair of opioid receptor and G alpha q chimera al lowed opioid agonists to stimulate PI-PLC in transfected COS-7 cells. The G alpha(q) chimera-mediated responses were ligand selective, agonist dose de pendent and saturable. The most robust responses were obtained with kappa-r eceptor and qi5 or qz5, whereas the coupling of delta- and mu-receptors to G alpha(q) chimeras produced much weaker responses. Among the G alpha(q) ch imeras, qo5 was less efficiently coupled to the opioid receptors. As reveal ed by radioligand binding assays and immunoblot analysis, differences in th e efficiency of coupling were not due to variations in the expression of re ceptors and G alpha(q) chimeras. Differences in the magnitude of PI-PLC res ponses are thus likely to represent structural incompatibility between opio id receptors and G alpha(q) chimeras, suggesting that each opioid receptor possesses unique structural surfaces for the binding of G proteins.