Interaction of guanosine nucleotides with elongation factor 2. II. Effect of ribosomes and magnesium ions on guanosine diphosphate and guanosine triphosphate binding to the enzyme.

The effects of ribosomes and Mg-2plus on the binding of GDP and GTP to elongation factor 2 (EF-2) have been studied by an improved filter-binding assay. Both ribosomes and Mg-2plus strongly inhibit the binding of GDP but have apparently no effect on the GTP binding to the enzyme. An apparent stimulation by ribosomes of GTP binding to EF-2 is time-dependent and parallels a concomitant increase of the GDP concentration in the incubation mixture. Based on these results and evidence obtained by other investigators it is suggested that changes in the GTP:GDP ratio associated with the elongation and termination reactions of protein synthesis cause conformational changes of the respective factors which consequently will modulate the binding and dissociation of the enzymes from ribosomes. Further evidence of the role GDP may play as a modulator of protein synthesis might possibly be provided by studies of the GTP-GDP transphosphorylase activity which is present as an impurity in highly purified preparations of EF-2 as well as in ribosome preparations. It is demonstrated that relatively high concentrations of GDP in the presence of GTP completely block the ribosome-dependent GTPase activity of EF-2. Instead, the transphosphorylase enzyme(s) catalyzes an exchange reaction between GTP and GDP during which GDP remains bound to EF-2 and the relative concentrations of the two nucleotides do not change.