The effect of GTP and Mg2+ on the GTPase activity and the fluorescent properties of Go.

The structures of the guanosine 5'O-(3-thio)triphosphate (GTP gamma S)-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 GTP gamma S 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 Go, 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 Go (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.