Biochemical characterization of three stimulatory GTP-binding proteins. The large and small forms of G(s) and the olfactory-specific G-protein, G(olf)

The biochemical properties of three stimulatory guanine nucleotide-binding protein (G-protein) α subunits, the large and small forms of G(s), G(s-l) (52 kDa) and G(s-s) (45 kDa), and the olfactory specific G-protein, G(olf), have been compared. Complementary DNAs (cDNAs) encoding each α subunit were independently expressed in a mammalian cell line deficient in endogenous stimulatory G-proteins (S49 cyc^-kin^-). G(s-l) and G(s-s) respond similarly to activation by the β-adrenergic agonist isoproterenol (EC₅₀ = 80 and 60 nM, respectively) and the receptor-independent G-protein activators guanosine 5-O-3-(thio)triphosphate) (GTPγS) and A1F₄^-. The ability of G(olf) to interact with the β-adrenergic receptor was also examined. Surprisingly, G(olf) interacts with β-adrenergic receptors and is activated by isoproterenol (EC₅₀ = 120 nM). All three G-proteins respond similarly to treatment with different α, β, and γ thiophosphoryl analogs of GTP. Specifically, (R)-GTPαS and GTPγS activate each G-protein, whereas (S)-GTPαS and (R)- or (S)-GTPβS are inactive. In addition, similar to G(sα), G(olfα) is covalently modified and constitutively activated by cholera toxin. These studies demonstrate that all three stimulatory G-proteins are functionally and structurally similar, however, subtle differences between G(olf) and the two forms of G(s) appear to modulate their interactions with receptors.