N-methyl-3'-O-anthranoyl (MANT) guanine nucleotide analogs are useful environmentally sensitive fluorescent probes for studying G protein mechanisms. Previously, we showed that MANT fluorescence intensity when bound to G protein was related to the degree of G protein activation where MANT-guanosine-5'-O-(3-thiotriphosphate) (mGTP gammaS) had the highest fluorescence followed by mGTP and mGDP, respectively (Remmers, A. E., Posner, R., and Neubig, R. R. (1994) J. Biol. Chem. 269, 13771-13778). To directly examine G protein conformations with nucleotide triphosphates bound, we synthesized several nonhydrolyzable MANT-labeled guanine nucleotides. The relative maximal fluorescence levels observed upon binding to recombinant myristoylated Goalpha (myrGoalpha) and myrGialpha1 were: mGTPgammaS > MANT-5'-guanylyl-imidodiphosphate > MANT-guanylyl-(beta,gamma-methylene)-diphosphonate > MANT-guanosine 5'-O-2-(thio)diphosphate. Using protection against tryptic digestion as a measure of the activated conformation, the ability of the MANT guanine nucleotides to maximally activate myrGo alpha correlated with maximal fluorescence. Biphasic dissociation kinetics were observed for all of the MANT guanine nucleotides. The data were consistent with the following model, [formula: see text] where G protein activation (G*-GXP) is determined by a conformational equilibrium between two triphosphate bound states as well as by the balance between binding and hydrolysis of the nucleotide triphosphate. Compared with myrGialpha1, maximal mGTP fluorescence was only 2-fold higher for the myrGialpha1 Q204L mutant, a mutant with greatly reduced GTPase activity, and only 24% that of mGTPgammaS, indicating that partial activation by mGTP was not just due to hydrolysis of mGTP. These results extend our previous conclusion that GTP analogs do not fully activate G protein.