The effect of GTP and Mg²⁺ on the GTPase activity and the fluorescent properties of G(o)

The structures of the guanosine 5'-O-(3-thio)triphosphate (GTPγS)-containing guanine nucleotide-binding regulatory proteins (G proteins) are distinct from those of the GDP-containing forms. One indication of the conformational change caused by GTPγS is a Mg²⁺-sensitive increase in the intensity of the proteins' tryptophan fluorescence, (Higashijima, T., Ferguson, K.M., Sternweis, P.C., Ross, E.M., Smigel, M.D., Gilman, A.G. (1987), J. Biol. Chem., 262, 762-766). GTP causes a similar change in the fluorescence of G(o), a G protein from bovine brain. When Mg²⁺ is also present, the increase in fluorescence is transient, and the rate of decline in the intensity of the fluorescence is the same as the rate of GTP hydrolysis by the protein. The steady-state rate of hydrolysis of GTP by G(o) (0.3-0.4/min) is slower than the catalytic rate of the protein (2/min), because the rate-limiting step in the reaction is the release of GDP.