CHARACTERIZATION OF THE ALUMINUM AND BERYLLIUM FLUORIDE SPECIES WHICHACTIVATE TRANSDUCIN - ANALYSIS OF THE BINDING AND DISSOCIATION KINETICS

Aluminofluoride and beryllofluoride complexes can activate the heterot rimeric G-proteins by binding next to GDP in the nucleotide site of th eir G-alpha subunit and acting as analogs of the gamma-phosphate of a GTP. However, the exact structures of the activatory complexes in solu tion as well as those of the bound complexes in the nucleotide site ar e still disputed. We have studied, by monitoring the activation-depend ent tryptophan fluorescence of transducin T-alpha subunit, the pF (-lo g[F-]) and pH dependencies of the kinetics of activation and deactivat ion of T-alpha-GDP in the presence of NaF and aluminum or beryllium sa lts. Comparisons were made with the calculated pF and pH dependencies of the distribution of the metallofluoride complexes, in order to iden tify the activating species. We observed that the contribution of a ma gnesium-dependent mechanism of activation by fluoride (Antonny, B., Bi gay, J., and Chabre, M. (1990) FEBS Lett. 268, 277-280) and effects du e to slow equilibration kinetics between various aluminofluoride compl exes could give rise to puzzling kinetics that had caused misinterpret ations of previous results. Once corrected for theses effects, our res ults suggest that with aluminum AlF3(OH)- is, rather than AlF4-, the m ain activating species and that the bound form of the complex is tetra coordinated GDP-AlF3. Deactivation kinetics depend on the free fluorid e concentration in the medium, suggesting that the simple bimolecular scheme: T-alpha-GDP-AlF3 if and only if T-alpha-GDP + AlF3(OH) does no t fully describe the interaction. Fluorides in the bound complexes can also exchange with free F- ions in solution. With beryllium, two comp lexes are activatory: BeF3-.H2O and BeF2(OH)-.H2O. In the nucleotide s ite these give two tetracoordinated complexes, GDP.BeF3 and GDP.BeF2(O H), as shown by their different dissociation rates.