The NH2‐terminal α subunit attenuator domain confers regulation of G protein activation by βγ complexes

Abstract

G_s and G_i, respectively, activate and inhibit the enzyme adenylyl cyclase. Regulation of adenylyl cyclase by the heterotrimeric G_s and G_i proteins requires the dissociation of GDP and binding of GTP to the α_s or α_i subunit. The βγ subunit complex of G_s and G_i functions, in part, to inhibit GDP dissociation and α subunit activation by GTP. Multiple β and γ polypeptides are expressed in different cell types, but the functional significance for this heterogeneity is unclear. The βγ complex from retinal rod outer segments (βγ_t) has been shown to discriminate between α_i and α_s subunits (Helman et al: Eur J Biochem 169:431–439, 1987). βγ_t efficiently interacts with α_i‐like G protein subunits, but poorly recognizes the α_s subunit. βγ_t was, therefore, used to define regions of the α_i subunit polypeptide that conferred selective regulation compared to the α_s polypeptide. A series of α subunit chimeras having NH₂‐terminal α_i and COOH‐terminal α_s sequences were characterized for their regulation by βγ_t, measured by the kinetics of GTPγS activation of adenylyl cyclase. A 122 amino acid NH₂‐terminal region of the α_i polypeptide encoded within an α_i/α_s chimera was sufficient for βγ_t to discriminate the chimera from α_s. A shorter 54 amino acid α_i sequence substituted for the corresponding NH₂‐terminal region of α_s was insufficient to support the α_i‐like interaction with βγ_t. The findings are consistent with our previous observation (Osawa et al: Cell 63:697–706, 1990) that a region in the NH₂‐terminal moiety functions as an attenuator domain controlling GDP dissociation and GTP activation of the α subunit polypeptide and that the attenuator domain is involved in functional recognition and regulation by βγ complexes.