THE FUNCTION OF A HIGHLY-CONSERVED ARGININE RESIDUE IN ACTIVATION OF THE MUSCARINIC M(1) RECEPTOR

Arg(123) in the rat muscarinic M(1) receptor is part of the highly conserved triad Asp-Arg-Tyr found at the junction of transmembrane helix 3 with the second intracellular loop. Mutation of Arg(123) to Lys, Ala, Leu, Glu and Gin had no effect on levels of receptor expression in COS-7 cells, or on affinities for the antagonist N-methylscopolamine. Acetylcholine stimulation of the Lys(123) receptor evoked the same maximum phosphoinositide response as the wild type, although the potency was reduced sis-fold, but mutation to other residues strongly disrupted receptor function. Mutation of Arg(123) always decreased the ratio of the high affinity to the low affinity agonist binding constant, but the absolute effect on the latter varied from a it-fold increase for the Lys(123) to a small decrease for the Leu(123) mutation. These results suggest that a positive charge at position 123 is central to the activation of G-proteins by the muscarinic M(1) receptor.