The effect of activating ligands on the intrinsic fluorescence of guanine nucleotide-binding regulatory proteins.

The intensity of the tryptophan fluorescence of the alpha subunits of guanine nucleotide-binding regulatory proteins increases when they bind guanosine 5'-O-(3-thio)triphosphate (GTY gamma S). The kinetics of the fluorescence enhancement and of the measured binding of [35S]GTP gamma S are well correlated. The addition of Mg2+ to the nucleotide-bound proteins causes a further, rapid increase in the fluorescence intensity. Similar effects result from exposure of the proteins to F- and Mg2+, and the required concentration of F- is reduced by the inclusion of Al3+. It is presumed that the more highly fluorescent state of the G protein alpha subunits represents their active conformation.