Elongation factor Ts (EF-Ts) functions as a nucleotide-exchange factor by binding elongation factor Tu (EF-Tu) and accelerating the GDP dissociation from EF-Tu; thus EF-Ts promotes the transition of EF-Tu from the inactive GDP form to the active GTP form. Thermus thermophilus EF-Ts exists as a stable dimer in solution which binds two molecules of EF-Tu to form a (EF-Tu·EF- Ts)2 heterotetramer. Here we report the crystal structure of the dimerization domain of EF-Ts from T. thermophilus refined to 1.7 Å resolution. A three-stranded antiparallel β-sheet from each subunit interacts to form a β-sandwich that serves as an extensive dimer interface tethered by a disulfide bond. This interface is distinctly different from the predominantly α-helical one that stabilizes the EF-Ts dimer from Escherichia coli [Kawashima, T., el al. (1996) Nature 379, 511-518]. To test whether the homodimeric form of T. thermophilus EFTs is necessary for catalyzing nucleotide exchange, the present structure was used to design mutational changes within the dimer interface that disrupt the T. thermophilus EF-Ts dimer but not the tertiary structure of the subunits. Surprisingly, EF-Ts monomers created in this manner failed to catalyze nucleotide exchange in EF- Tu, indicating that, in vitro, T. thermophilus EF-Ts functions only as a homodimer.
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