Role of amino- and carboxyl-terminal regions of G(alphaZ) in the recognition of Gi-coupled receptors.

Many Gi-coupled receptors are known to interact with the pertussis toxin (PTX)-insensitive Gz protein. Given that the alpha subunits of Gi and Gz share only 60% identity in their amino acid sequences, their receptor-interacting domains must be highly similar. By swapping the carboxyl termini of alpha i2 and alpha z with each other or with those of alpha t, alpha12, and alpha13, we examined the relative contributions of the carboxyl-end 36 amino acids of the alpha chains toward 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 Gi-coupled receptors (formyl peptide, dopamine D2, and delta-opioid receptors) in human embryonic kidney 293 cells. The alpha i2/alpha z chimera was able to interact with both aminergic and peptidergic receptors, resulting in betagamma-mediated stimulation of AC II in the presence of agonists and PTX. Functional and mutational analyses of alpha i2/alpha z revealed that this chimera can inhibit the endogenous ACs of 293 cells. Similarly, the alpha z/alpha i2 chimera seemed to retain the abilities to interact with receptors and inhibit cAMP accumulation. Fusion of the carboxyl-terminal 36 amino acids of alpha z to a backbone of alpha t1 produced a chimera, alpha t1/alpha z, that did not couple to any of the Gi-coupled receptors tested. Interestingly, an alpha13/alpha z chimera (with only the last five amino acids switched) displayed differential abilities to interact with receptors. Signals from aminergic, but not peptidergic, receptors were transduced by alpha13/alpha z. A similar construct, alpha12/alpha z, behaved just like alpha13/alpha z. These results indicated that "alpha i-like" or "alpha z-like" sequences at the carboxyl termini of alpha subunits are not always necessary or sufficient for specifying interaction with Gi-coupled receptors.