Transient non-native interactions in early folding intermediates do not influence the folding kinetics of Escherichia coli tryptophan synthase beta 2 subunits.

Local structures formed in early intermediates are thought to play a key role in orientating the protein folding pathway. Here, we test whether non-native interactions formed by eight N-terminal residues in early folding intermediates of tryptophan synthase beta chains [Navon, A., Schulze, A. J., Guillou, Y., Zylinksii, C. A., Baleux, F., Expert-Bezançon, N., Friguet, B., Djavadi-Ohaniance, L. & Goldberg, M. E. (1995) J. Biol. Chem. 270, 4255-4261] influence the folding kinetics. The kinetics of in vitro renaturation of wild-type beta chains and truncated beta chains lacking residues 2-9 were compared. Each step analyzed (molten globule formation, tryptophan shielding, closing up of distant residues, and complete renaturation) occurred with similar kinetics in the normal and truncated proteins. Thus, non-native interactions formed early during the folding of beta chains seem to influence neither the rate and efficiency of the complete renaturation, nor the kinetics of the early folding steps, which suggests that such non-native early intermediates are poorly stable and highly dynamic. This observation is consistent with the current view that productive folding should avoid the formation of stable intermediates.