Delineation of muscarinic receptor domains conferring selectivity of coupling to guanine nucleotide-binding proteins and second messengers.

The cloning and functional expression of five mammalian muscarinic acetylcholine receptor genes (m1-m5) has revealed that m1, m3, and m5 primarily couple to stimulation of phosphoinositide (PI) turnover, whereas m2 and m4 are strongly linked to inhibition of adenylate cyclase, albeit not exclusively. To identify the structural domains responsible for this functional specificity, cDNAs encoding chimeric m2/m3 receptors were constructed. The abilities of these receptors to mediate stimulation of PI hydrolysis and inhibition of prostaglandin E2-stimulated cAMP accumulation, as well as the pertussis toxin (PTX) sensitivity of these responses, were examined after stable expression in mouse A9 L cells. Substitution of the putative third cytoplasmic loop (i3) of m2 with the corresponding m3 sequence resulted in a chimeric receptor that, similar to m3, stimulated PI breakdown by a PTX-insensitive mechanism but did not inhibit adenylate cyclase. Conversely, a chimeric m3 receptor containing the i3 domain of m2 showed the same functional profile as m2 (i.e., inhibition of adenylate cyclase and weak stimulation of PI turnover by a PTX-sensitive mechanism), indicating that the i3 loop is sufficient to determine coupling selectivity. Similarly, exchange of a short N-terminal segment of i3 (16 or 17 amino acids) between m2 and m3 resulted in chimeric receptors that gained the ability to mediate the functional responses of the wild-type receptor from which the segment was derived, although with substantially reduced efficiency. However, the chimeric m2 receptor containing the 17-amino acid m3 sequence in the N-terminal portion of i3 retained its ability to inhibit adenylate cyclase. Carbachol binding studies involving the use of the GTP analog 5'-guanylyl imidodiphosphate and PTX-pretreated cells generally correlated well with the functional findings. Our data indicate that the N-terminal portion of i3 is a sufficient but not the exclusive determinant of coupling selectivity displayed by the various muscarinic receptors.