ROLE OF AMINO-TERMINAL AND CARBOXYL-TERMINAL REGIONS OF G(ALPHA-Z) INTHE RECOGNITION OF G(I)-COUPLED RECEPTORS

Many G(i)-coupled receptors are known to interact with the pertussis t oxin (PTX)-insensitive G(z) protein. Given that the alpha subunits of G(i) and G(z) share only 60% identity in their amino acid sequences, t heir receptor-interacting domains must be highly similar. By swapping the carboxyl termini of alpha i2 and alpha z with each other or with t hose of alpha t, alpha 12, and alpha 13, we examined the relative cont ributions of the carboxyl-end 36 amino acids of the alpha chains towar d receptor recognition. Chimeric alpha chains lacking the site for PTX -catalyzed ADP-ribosylation were coexpressed with the type II adenylyl cyclase (AC II) and one of several G(i)-coupled receptors (formyl pep tide, dopamine D-2, and delta-opioid receptors) in human embryonic kid ney 293 cells. The alpha i2/alpha z chimera was able to interact with both aminergic and peptidergic receptors, resulting in beta gamma-medi ated stimulation of AC II in the presence of agonists and PTX. Functio nal and mutational analyses of alpha i2/alpha z revealed that this chi mera can inhibit the endogenous ACs of 293 cells. Similarly, the alpha z/alpha i2 chimera seemed to retain the abilities to interact with re ceptors and inhibit cAMP accumulation. Fusion of the carboxyl-terminal 36 amino acids of alpha z to a backbone of alpha t1 produced a chimer a, alpha t1/alpha z, that did not couple to any of the G(i)-coupled re ceptors tested. Interestingly, an alpha 13/alpha z chimera (with only the last five amino acids switched) displayed differential abilities t o interact with receptors. Signals from aminergic, but not peptidergic , receptors were transduced by alpha 13/alpha z. A similar construct, alpha 12/ alpha z, behaved just like alpha 13/alpha z. These results i ndicated that ''alpha i-like'' or ''alpha z-like'' sequences at the ca rboxyl termini of alpha subunits are not always necessary or sufficien t for specifying interaction with G(i)-coupled receptors.