Distinct interactions of G(salpha-long), G(salpha-short), and G(alphaolf) with GTP, ITP, and XTP

The G(s)-proteins G(salpha-short) (G(salphaS)) and G(salpha-long) (G(salphaL)), and the olfactory G(s) protein (G(alphaolf)) mediate activation of adenylyl cyclase by the beta(2)-adrenoceptor (beta(2)AR). Early studies showed that the purine nucleotides GTP, ITP, and XTP differentially support receptor-mediated adenylyl cyclase activation in various native membrane systems, but those findings have remained unexplained thus far. We systematically analyzed the effects of GTP, ITP, and XTP on the coupling of the beta(2)AR to G(salphaS), G(salphaL), and G(alphaolf), respectively, using fusion proteins expressed in Sf9 insect cells. Fusion proteins ensure defined receptor/G-protein stoichiometry and efficient coupling. At all three fusion proteins, GTP, ITP, and XTP exhibited unique profiles with respect to their potency and efficacy at disrupting high-affinity agonist binding and supporting adenylyl cyclase activation by partial and full agonists. Our data can be interpreted in two ways: (i) GTP, ITP, and XTP may stabilize different active conformations in various G(s)-proteins, or (ii) GTP, ITP, and XTP may differ from one another in the kinetics of interaction with various G(s)-proteins. Regardless of which of the two explanations is correct, our present data demonstrate that GTP, ITP, and XTP are highly efficient regulators of signal transduction mediated through a specific G-protein. Also discussed is the possibility that G-protein activation by ITP and XTP may be of relevance in Lesch-Nyhan syndrome, a defect of the purine salvage pathway associated with abnormalities in various neurotransmitter systems.