COEXPRESSION STUDIES WITH MUTANT MUSCARINIC ADRENERGIC-RECEPTORS PROVIDE EVIDENCE FOR INTERMOLECULAR CROSS-TALK BETWEEN G-PROTEIN-LINKED RECEPTORS

We have tested the hypothesis that guanine-nucleotide-binding-protein-coupled receptors may be able to interact with each other at a molecular level. To address this question, we have initially created two chimeric receptors, alpha2/m3 and m3/alpha2, in which the C-terminal receptor portions (containing transmembrane domains VI and VII) were exchanged between the alpha2C-adrenergic and the m3 muscarinic receptor. Transfection of COS-7 cells with either of the two chimeric constructs alone did not result in any detectable binding activity for the muscarinic ligand N-[H-3]methylscopolamine or the adrenergic ligand [H-3]rauwolscine. However, cotransfection with alpha2/m3 and m3/alpha2 resulted in the appearance of specific binding sites (30-35 fmol/mg of membrane protein) for both radioligands. These sites displayed ligand binding properties similar to those of the two wild-type receptors. Furthermore, COS-7 cells cotransfected with alpha2/m3 and m3/alpha2 were able to mediate a pronounced stimulation of phosphatidylinositol hydrolysis upon stimulation with the muscarinic agonist carbachol (E(max) almost-equal-to 40-50% of wild-type m3). A mutant m3 receptor (containing 16 amino acids of m2 receptor sequence at the N terminus of the third cytoplasmic loop) that was capable of binding muscarinic ligands but was virtually unable to stimulate phosphatidylinositol hydrolysis was also used in various cotransfection experiments. Coexpression of this chimeric receptor with other functionally impaired mutant muscarinic receptors (e.g., with an m3 receptor containing a Pro --> Ala point mutation in transmembrane region VII) resulted in a considerable stimulation of phosphatidylinositol breakdown after carbachol treatment (E(max) almost-equal-to 40-50% of wild-type m3). Thus, these data suggest that guanine-nucleotide-binding-protein-coupled receptors can interact with each other at a molecular level. One may speculate that the formation of receptor dimers involving the intermolecular exchange of N- and C-terminal receptor domains (containing transmembrane domains I-V and VI and VII, respectively) may underlie this phenomenon.