Mechanism of Elongation Factor (EF)-Ts-catalyzed Nucleotide Exchange in EF-Tu CONTRIBUTION OF CONTACTS AT THE GUANINE BASE*

Nucleotide exchange in elongation factor Tu (EF-Tu) is catalyzed by elongation factor Ts (EF-Ts). Similarly to other GTP-binding proteins, the structural changes in the P loop and the Mg2 binding site are known to be impor-tant for nucleotide release from EF-Tu. In the present paper, we determine the contribution of the contacts be-tween helix D of EF-Tu at the base side of the nucleotide and the N-terminal domain of EF-Ts to the catalysis. The rate constants of the multistep reaction between Esche-richia coli EF-Tu, EF-Ts, and GDP were determined by stopped-flow kinetic analysis monitoring the fluores-cence of either Trp-184 in EF-Tu or mant-GDP. Mutational analysis shows that contacts between helix D of EF-Tu and the N-terminal domain of EF-Ts are important for both complex formation and the acceleration of GD