Expression of a G alpha s/G alpha i chimera that constitutively activates cyclic AMP synthesis.

A chimeric G alpha subunit cDNA, referred to as G alpha s/i(38), was constructed containing the complete 5'-untranslated region of G alpha s, the first 356 codons of the rat G alpha s and the last 36 codons and 428 base pairs of the 3'-untranslated region of the rat G alpha i cDNA. Transient expression of the G alpha s/i(38) protein in COS cells allowed detection of a chimeric protein which was recognized by antibodies generated against an internal G alpha s sequence as well as antibodies recognizing the carboxyl terminus of G alpha i2. Chinese hamster ovary cell clones stably expressing the chimeric G-protein alpha subunit transcript (G alpha s/i(38] demonstrated 1.5-2.5-fold constitutively elevated cyclic AMP levels and a 3-4-fold increase in the activity ratio of cyclic AMP-dependent protein kinase, although expression of the chimeric polypeptide could not be demonstrated presumably because of low expression of the mutant alpha s. Expression of the rat G alpha s transcript yielded clones that were similar to wild-type Chinese hamster ovary cells in regard to cyclic AMP levels and protein kinase activity. In the presence of methyl isobutylxanthine, a cyclic AMP phosphodiesterase inhibitor, cyclic AMP levels in clones expressing the G alpha s/i(38) transcript were 10-15-fold higher than G alpha s expressing clones. Adenylyl cyclase activation by guanosine 5'-O-(thiotriphosphate) (GTP gamma S) in membranes from clones expressing the G alpha s/i(38) transcript demonstrated a diminished lag time for maximal activation, indicating an increased relative GDP dissociation rate for the chimeric G alpha subunit and an increase in total adenylyl cyclase activity relative to wild-type G alpha s expressing clones. Cholate extracts from membranes of G alpha s/i(38) expressing clones, when mixed with cyc- S49 membranes, reconstituted an increased GTP gamma S-stimulated adenylyl cyclase activity and a diminished lag time for maximal activation compared to cholate extracts prepared from G alpha s-expressing clones. The G alpha s/i(38) construct confers a dominant constitutive activation of adenylyl cyclase when expressed in cells in the presence of a background of wild-type G alpha s.

Receptor activation of G proteins.

G proteins are a highly conserved family of membrane-associated proteins composed of alpha, beta, and gamma subunits. The alpha subunit, which is unique for each G protein, binds GDP or GTP. Receptors such as those for beta- and alpha-adrenergic catecholamines, muscarinic agonists, and the retinal photoreceptor rhodopsin, catalyze the exchange of GDP for GTP binding to the alpha subunit of a specific G protein. G alpha.GTP regulates appropriate effector enzymes such as adenylyl cyclase or the cyclic GMP phosphodiesterase. The beta gamma-subunit complex of G proteins is required for efficient receptor-catalyzed alpha subunit guanine nucleotide exchange and also functions as an attenuator of alpha subunit activation of effector enzymes. Recent elucidation of both receptor and G protein primary sequence has allowed structural predictions and new experimental approaches to study the mechanism of receptor-catalyzed G protein regulation of specific effector systems and the control of cell function including metabolism, secretion, and growth.