A possible initial folding intermediate: the C-terminal proteolytic domain of tryptophan synthase .beta. chains folds in less than 4 milliseconds into a condensed state with non-native-like secondary structure

The isolated F2-V8 peptide corresponding to the 101 C-terminal residues of Escherichia coli tryptophan synthase beta chains folds into a heat-stable, yet fluctuating, condensed state that contains a lot of secondary structure. However, this state has non-native-like secondary and supersecondary structures [Chaffotte, A., Guillou, Y., Delepierre, M., Hinz, H.-J., & Goldberg, M. E. (1991) Biochemistry 30, 8067-8074]. To characterize the rate of appearance of this state, stopped-flow studies on the far-ultraviolet circular dichroism (CD) and on the binding of 1-anilino-8-naphthalenesulfonate (ANS) have been conducted during the folding of guanidine-unfolded F2-V8. It was shown that both the CD signal at 222 nm and the ANS binding properties of folded isolated F2-V8 were regained, at 20 degrees C, within the dead time of the stopped-flow apparatus, which was 4 ms. At 12 degrees C, the binding of ANS was also completed within this dead time, but the ellipticity showed some minor later changes. After a rapid overshoot of the CD signal that occurred during the 4-ms dead time, a small readjustment of the ellipticity to the final value occurred more slowly and was completed after about 25 ms. Thus, even at 12 degrees C, the hydrophobic core and most of the secondary structure of folded F2-V8 were formed in less than 4 ms. These observations strongly suggest that the previously described condensed non-native-like state of F2-V8 results from a very rapid, nonspecific, hydrophobic collapse. It is proposed that such a state may be a general early intermediate in protein folding.