Inhibitory and stimulatory G proteins of adenylate cyclase: cDNA and amino acid sequences of the alpha chains.

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Abstract
The G protein family of signal transducers includes five heterotrimers, which are most clearly distinguished by their different .alpha. chains. The family included Gs and Gi, the stimulatory and inhibitory GTP-binding regulators of adenylate cyclase; GO, a protein of unknown function abundant in brain; and transducin 1 and transducin 2, proteins involved in retinal phototransduction. Using a bovine .alpha.t1 cDNA as a hybridization probe, we have isolated mouse cDNAs that encode .alpha. chains of two G proteins. One encodes a polypeptide of 377 amino acids (Mr 43,856), identified as .alpha.s because it specifically fails to hybridize with any transcript in an .alpha.s-deficient S49 mouse lymphoma mutant, cyc-; the other encodes a polypeptide of 355 amino acids (Mr 40,482), presumed to be .alpha.i. These .alpha. chains and those of the retinal transducins exhibit impressive sequence homology. Of the four, .alpha.t1 and .alpha.t2 are most alike (81% identical amino acid residues), whereas the presumptive .alpha.i is more similar than .alpha.s to .alpha.t1 (63% vs. 38% identical residues). Sequence homologies with p21ras and elongation factor Tu identify regions of the .alpha. chains that form the site for GTP binding and hydrolysis. Further comparison of the .alpha.-chain sequences suggests additional regions that may contribute to interactions with .beta..gamma. subunits and the receptor and effector components of different signal transduction systems.