The Amino Terminus of Gαzis Required for Receptor Recognition, Whereas its α4/β6 Loop Is Essential for Inhibition of Adenylyl Cyclase

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Abstract
Gz couples to most of the known Gi-linked receptors and its α subunit (Gαz) inhibits adenylyl cyclases as efficiently as Gαi subtypes. A series of chimeric Gα subunits with different portions of Gαz and Gαt1 (a regulator of cGMP phosphodiesterase) were constructed to study the essential structural elements of Gαz that determine receptor coupling and effector interaction. The receptor-mediated functions of the chimeras were assessed in two aspects: 1) stimulation of type 2 adenylyl cyclase through the release of βγ subunits from the chimeras, and 2) inhibition of isoproterenol-stimulated adenylyl cyclase by the chimeric Gα subunits. The results suggested that the presence of both termini of Gαz were critical for coupling to δ-opioid receptor, with the N-terminal region being more important. Moreover, a stretch of amino acids (295–319) corresponding to the α4/β6 loop was identified as one of the adenylyl cyclase inhibitory domains of Gαz.