Coupling of the muscarinic m2 receptor to G protein-activated K⁺ channels via Gα(z) and a receptor-Gα(z) fusion protein. Fusion between the receptor and Gα(z) eliminates catalytic (collision) coupling

G protein-activated K⁺ channel (GIRK), which is activated by the G(βγ) subunit of heterotrimeric G proteins, and muscarinic m2 receptor (m2R) were coexpressed in Xenopus oocytes. Acetylcholine evoked a K⁺ current, I(ACh), via the endogenous pertussis toxin (PTX)-sensitive G(i/o) proteins. Activation of I(ACh) was accelerated by increasing the expression of m2R, suggesting a collision coupling mechanism in which one receptor catalytically activates several G proteins. Coexpression of the α subunit of the PTX-insensitive G protein G(z), Gα(z), induced a slowly activating PTX- insensitive I(ACh), whose activation kinetics were also compatible with the collision coupling mechanism. When GIRK was coexpressed with an m2R·Gα(z) fusion protein (tandem), in which the C terminus of m2R was tethered to the N terminus of Gα(z), part of I(ACh) was still eliminated by PTX. Thus, the m2R of the tandem activates the tethered Gα(z) but also the nontethered G(i/o) proteins. After PTX treatment, the speed of activation of the m2R·Gα(z)- mediated response did not depend on the expression level of m2R·Gα(ζ) and was faster than when m2R and Gα(z) were coexpressed as separate proteins. These results demonstrate that fusing the receptor and the Gα strengthens their coupling, support the collision-coupling mechanism between m2R and the G proteins, and suggest a noncatalytic (stoichiometric) coupling between the G protein and GIRK in this model system.