G protein-linked receptors: pharmacological evidence for the formation of heterodimers.

By means of the expression of two chimeric receptors, alpha(2)/M(3) and M(3)/alpha(2), in which the carboxy-terminal receptor portions, containing transmembrane domains VI and VII, were exchanged between the alpha(2C)-adrenergic and the M(3) muscarinic receptor, it has been shown that G protein-coupled receptors are able to interact functionally with each other at the molecular level to form (hetero)dimers. In the present study, we tested the hypothesis that interaction between two different muscarinic receptor subtypes can lead to the formation of a heterodimeric muscarinic receptor with a new pharmacological profile. Initially, muscarinic M(2) or M(3) wild-type receptors were expressed together with gene fragments originating from M(3) or M(2) receptors, respectively. Antagonist binding, performed with pirenzepine and tripitramine, revealed the presence of two populations of binding sites: one represents the wild-type M(2) or M(3) receptors, the other the heterodimeric M(2)/M(3) receptor. In another set of experiments, we constructed a point mutant M(2) receptor M(2) (Asn404-->Ser), in which asparagine 404 was replaced by serine. Although this receptor alone did not show any binding for N-[(3)H]methylscopolamine (up to 2 nM), when cotransfected with M(3), it resulted in the rescue of a high-affinity binding for tripitramine. These findings demonstrate that M(2) and M(3) muscarinic receptor subtypes can cross-interact with each other and form a new pharmacological heterodimeric receptor.