Elongation factor Ts can act as a steric chaperone by increasing the solubility of nucleotide binding-impaired elongation factor-Tu

Several elongation factor (EF) Tu mutants (T25A, H22Y/T25S, D80N, D138N) th at have impaired nucleotide binding show decreased solubility on overexpres sion in the E. coli cell, an indication that they do not fold correctly. Mo reover, EF-Tu[T25A] and EF-Tu[D80N] were shown to inhibit cell growth on ex pression, an effect attributed to their sequestration of EF-Ts [Krab, I. M. , and Parmeggiani, A. (1999) J. Biol. Chem. 274, 11132-11138; Krab, I. M., and Parmeggiani, A. (1999) Biochemistry 38, 13035-13041]. We present here r esults showing that the co-overexpression of EF-Ts at a 1:1 ratio dramatica lly improves the solubility of mutant EF-Tu, although in the case of EF-Tu[ D138N]-which cannot at all bind the nucleotides available in the cell-this is a slow process. Moreover, with co-overexpression of EF-Ts, the mentioned growth inhibition is relieved. We conclude that for the formation of a cor rect EF-Tu structure the nucleotide plays an important role as a "folding n ucleus", and also that in its absence EF-Ts can act as a folding template o r steric chaperone for the correct folding of EF-Tu.