STRUCTURE AND IMPORTANCE OF THE DIMERIZATION DOMAIN IN ELONGATION-FACTOR TS FROM THERMUS-THERMOPHILUS

Elongation factor Ts (EF-Ts) functions as a nucleotide-exchange factor by binding elongation factor Tu (EF-Tu) and accelerating the GDP diss ociation 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 E F-Tu to form a (EF-Tu . EF-Ts)(2) heterotetramer. Here we report the c rystal structure of the dimerization domain of EF-Ts from T. thermophi lus refined to 1.7 Angstrom resolution. A three-stranded antiparallel beta-sheet from each subunit interacts to form a beta-sandwich that se rves as an extensive dimer interface tethered by a disulfide bond. Thi s interface is distinctly different from the predominantly alpha-helic al one that stabilizes the EF-Ts dimer from Escherichia coli [Kawashim a, T., et al. (1996) Nature 379, 511-518]. To test whether the homodim eric form of T. thermophilus EF-Ts is necessary for catalyzing nucleot ide exchange, the present structure was used to design mutational chan ges 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 exch ange in EF-Tu, indicating that, in vitro, T. thermophilus EF-Ts functi ons only as a homodimer.