Functional role in ligand binding and receptor activation of an asparagine residue present in the sixth transmembrane domain of all muscarinic acetylcholine receptors.

The molecular mechanisms through which muscarinic receptors are activated upon binding of the neurotransmitter acetylcholine (ACh) are still poorly understood. Classical structure-function relationship studies have previously established that the ACh ester moiety plays a key role in muscarinic receptor recognition and activation. Consistent with this notion, all recently proposed three-dimensional muscarinic receptor models predict that an asparagine residue present in transmembrane domain VI of all muscarinic receptors is critically involved in the binding of the ACh ester moiety by means of hydrogen bonding. To test the correctness of this hypothesis, we created several mutant m3 muscarinic receptors in which this residue (Asn507) was replaced with alanine, serine, or aspartic acid. Radioligand binding studies with transfected COS-7 cells showed that, in contrast to the predictions made based on molecular modeling studies, all three mutant receptors were able to bind ACh and the structurally related muscarinic agonist, carbachol, with high affinities which differed from the corresponding wild type values by less than 5-fold. However, all three mutations led to dramatic reductions (235-28,300-fold) in binding affinities for certain subclasses of muscarinic antagonists including atropine-like agents and pirenzepine. The m3(Asn507-->Ala) and m3(Asn507-->Asp) mutant receptors were able to mediate carbachol-induced phosphatidylinositol hydrolysis in a fashion similar to that of the wild type receptor. Interestingly, the m3(Asn507-->Ser) mutant receptor displayed about 2-fold increased basal inositol phosphate levels, raising the possibility that it is constitutively active. In conclusion, our data suggest that the asparagine residue present in transmembrane domain VI of all muscarinic receptors is not critical for ACh binding and agonist-induced receptor activation, but plays a key role in the binding of certain subclasses of muscarinic antagonists.