Cleavage-resistant fusion proteins of the M2 muscarinic receptor and Gαi1. Homotropic and heterotropic effects in the binding of ligands

► Fusion proteins of the M2 muscarinic receptor and Gαi1 undergo proteolysis near the point of fusion. ► The cleavage was prevented by deletion of the first 17 amino acids from the N-terminus of Gαi1. ► The stable adduct from Sf9 cells exhibits GMP-PNP-sensitive heterogeneity toward agonists. ► Signaling therefore proceeds via a stable complex of two or more receptors and G proteins. ► Cleavage may be common to fusion proteins, thereby rendering the data ambiguous or misleading.