Biochemical characterization of three stimulatory GTP-binding proteins. The large and small forms of Gs and the olfactory-specific G-protein, Golf.

Abstract

The biochemical properties of three stimulatory guanine nucleotide-binding protein (G-protein) alpha subunits, the large and small forms of Gs, Gs-l (52 kDa) and Gs-s (45 kDa), and the olfactory specific G-protein, Golf, have been compared. Complementary DNAs (cDNAs) encoding each alpha subunit were independently expressed in a mammalian cell line deficient in endogenous stimulatory G-proteins (S49 cyc-kin-). Gs-l and Gs-s respond similarly to activation by the beta-adrenergic agonist isoproterenol (EC50 = 80 and 60 nM, respectively) and the receptor-independent G-protein activators guanosine 5-O-3-(thio)triphosphate) (GTP gamma S) and AlF-4. The ability of Golf to interact with the beta-adrenergic receptor was also examined. Surprisingly, Golf interacts with beta-adrenergic receptors and is activated by isoproterenol (EC50 = 120 nM). All three G-proteins respond similarly to treatment with different alpha, beta, and gamma thiophosphoryl analogs of GTP. Specifically, (R)-GTP alpha S and GTP gamma S activate each G-protein, whereas (S)-GTP alpha S and (R)- or (S)-GTP beta S are inactive. In addition, similar to Gs alpha, Golf alpha 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 Golf and the two forms of Gs appear to modulate their interactions with receptors.