Molecular dissection of the folding mechanism of the a subunit of tryptophan synthase: an amino-terminal autonomous folding unit controls several rate-limiting steps in the folding of a single domain protein
Zitzewitz, J.A.; Matthews, C.Robert
Biochemistry (American Chemical Society) 38(31): 205-14
The a subunit of tryptophan synthase (aTS) from Escherichia coli is a 268-residue 8-stranded b/a barrel protein. Two autonomous folding units, comprising the first six strands (residues 1-188) and the last two strands (residues 189-268), have been previously identified in this single structural domain protein by tryptic digestion [Higgins, W., Fairwell, T., and Miles, E. W. (1979) Biochemistry 18, 4827-4835]. The larger, amino-terminal fragment, aTS(1-188), was overexpressed and independently purified, and its equilibrium and kinetic folding properties were studied by absorbance, fluorescence, and near- and far-UV dichroism spectroscopies. The native state of the fragment unfolds cooperatively in an apparent two-state transition with a stability of 3.98 [plus or minus] 0.19 kcal mol-1 in the absence of denaturant and a corresponding m value of 1.07 [plus or minus] 0.05 kcal mol-1 M-1. Similar to the full-length protein, the unfolding of the fragment shows two kinetic phases which arise from the presence of two discrete native state populations. Additionally, the fragment exhibits a significant burst phase in unfolding, indicating that a fraction of the folded state ensemble under native conditions has properties similar to those of the equilibrium intermediate populated at 3 M urea in full-length aTS. Refolding of aTS(1-188) is also complex, exhibiting two detectable kinetic phases and a burst phase that is complete within 5 ms. The two slowest isomerization phases observed in the refolding of the full-length protein are absent in the fragment, suggesting that these phases reflect contributions from the carboxy-terminal segment. The folding mechanism of (aTS(1-188) appears to be a simplified version of the mechanism for the full-length protein [Bilsel, O., Zitzewitz, J. A., Bowers, K. E., and Matthews, C. R. (1999) Biochemistry 38, 1018-1029]. Four parallel channels in the full-length protein are reduced to a pair of channels that most likely reflect a cis/trans proline isomerization reaction in the amino-terminal fragment. The off- and on-pathway intermediates that exist for both full-length aTS and aTS(1-188) may reflect the preponderance of local interactions in the b/a barrel motif. Copyright 1999, American Chemical Society.