Chimeric m2/m3 muscarinic receptors: role of carboxyl terminal receptor domains in selectivity of ligand binding and coupling to phosphoinositide hydrolysis

J Wess, TI Bonner and MR Brann

Laboratory of Molecular Biology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892.

The cloning and expression of five mammalian muscarinic receptor genes (m1-m5) have shown that the individual receptor subtypes differ in their functional and ligand-binding properties. To study the role of the carboxyl terminal receptor domains in this pharmacological diversity, we constructed chimeric m2/m3 receptors in which a region comprising part of transmembrane domain VI, the third extracellular loop, transmembrane region VII, and the cytoplasmic tail (collectively referred to as C-terminal domains) was exchanged between the human m2 and the rat m3 receptor. The ability of the cloned receptors to mediate stimulation of phosphoinositide hydrolysis and to bind subtype- selective muscarinic ligands was studied after their transient expression in COS-7 cells. Whereas wild-type m3 strongly stimulated phosphoinositide breakdown, wild-type m2 gave only a poor response. Exchange of the C-terminal domains between m2 and m3 had no significant effect on the magnitude of these responses. In N-[3H]methylscopolamine competition binding studies, the muscarinic antagonists AF-DX 116 and methoctramine showed 11- and 23-fold higher affinities, respectively, for m2 than for m3, whereas hexahydro-silad-ifenidol (HHSiD) and 4- diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) displayed the reverse selectivity profile, having approximately 10-fold higher affinities for m3. In comparison with wild-type m3, the mutant m3 receptor containing the C-terminal domains of m2 displayed 2.5- and 8- fold higher affinities for AF-DX 116 and methoctramine but 7- and 3- fold lower affinities for HHSiD and 4-DAMP, respectively. The mutant m2 receptor with the C-terminal domains of m3 showed 2-3-fold lower affinities for AF-DX 116 and methoctramine but 2-3-fold higher affinities for HHSiD and 4-DAMP, as compared with wild-type m2. These data suggest that the C-terminal domains of the muscarinic receptors are not involved in conferring selectivity of coupling to phosphoinositide hydrolysis but contain major structural determinants of antagonist binding selectivity.

Volume 38, Issue 6, pp. 872-877, 12/01/1990
Copyright © 1990 by American Society for Pharmacology and Experimental Therapeutics

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