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

G protein-activated K+ channel (GIRK), which is activated by the Gp, subuni t of heterotrimeric G proteins, and muscarinic m2 receptor (m2R) were coexp ressed in Xenopus oocytes, Acetylcholine evoked a K+ current, I-ACh, via th e endogenous pertussis toxin (PTX)-sensitive G(z), proteins. Activation of I-ACh was accelerated by increasing the expression of m2R, suggesting a col lision coupling mechanism in which one receptor catalytically activates sev eral G proteins. Coexpression of the alpha subunit of the PTX-insensitive G protein G(z), G(alpha z), induced a slowly activating PTX-insensitive I-AC h, whose activation kinetics were also compatible with the collision coupli ng mechanism. When GIRK was coexpressed with an m2R.G alpha(z) fusion prote in (tandem), in which the C terminus of m2R was tethered to the N terminus of G(alpha z), part of IACh Was still eliminated by PTX. Thus, the m2R of t he tandem activates the tethered Ga, but also the nontethered G(i/o) protei ns. After PTX treatment, the speed of activation of the m2R.G alpha(z)-medi ated response did not depend on the expression level of m2R.G alpha(z), and was faster than when m2R and Ga, were coexpressed as separate proteins. Th ese results demonstrate that fusing the receptor and the G alpha strengthen s their coupling, support the collision-coupling mechanism between m2R and the G proteins, and suggest a noncatalytic (stoichiometric) coupling betwee n the G: protein and GIRK in this model system.