Abstract

Many G_i-coupled receptors are known to interact with the pertussis toxin (PTX)-insensitive G_z protein. Given that the α subunits of G_i and G_z share only 60% identity in their amino acid sequences, their receptor-interacting domains must be highly similar. By swapping the carboxyl termini of αi2 and αz with each other or with those of αt, α12, and α13, we examined the relative contributions of the carboxyl-end 36 amino acids of the α chains toward receptor recognition. Chimeric α 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 peptide, dopamine D₂, and δ-opioid receptors) in human embryonic kidney 293 cells. The αi2/αz chimera was able to interact with both aminergic and peptidergic receptors, resulting in βγ-mediated stimulation of AC II in the presence of agonists and PTX. Functional and mutational analyses of αi2/αz revealed that this chimera can inhibit the endogenous ACs of 293 cells. Similarly, the αz/αi2 chimera seemed to retain the abilities to interact with receptors and inhibit cAMP accumulation. Fusion of the carboxyl-terminal 36 amino acids of αz to a backbone of αt1 produced a chimera, αt1/αz, that did not couple to any of the G_i-coupled receptors tested. Interestingly, an α13/αz 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 α13/αz. A similar construct, α12/αz, behaved just like α13/αz. These results indicated that “αi-like” or “αz-like” sequences at the carboxyl termini of α subunits are not always necessary or sufficient for specifying interaction with G_i-coupled receptors.