Partial NMR assignments and secondary structure mapping of the isolated alpha subunit of Escherichia coli tryptophan synthase, a 29-kD TIM barrel protein.

The alpha subunit of tryptophan synthase (alphaTS) from S. typhimurium belongs to the triosephosphate isomerase (TIM) or the (beta/alpha)(8) barrel fold, one of the most common structures in biology. To test the conservation of the global fold in the isolated Escherichia coli homolog, we have obtained a majority of the backbone assignments for the 29-kD alphaTS by using standard heteronuclear multidimensional NMR methods on uniformly (15)N- and (15)N/(13)C-labeled protein and on protein selectively (15)N-labeled at key hydrophobic residues. The secondary structure mapped by chemical shift index, nuclear Overhauser enhancements (NOEs), and hydrogen-deuterium (H-D) exchange, and several abnormal chemical shifts are consistent with the conservation of the global TIM barrel fold of the isolated E. coli alphaTS. Because most of the amide protons that are slow to exchange with solvent correspond to the beta-sheet residues, the beta-barrel is likely to play an important role in stabilizing the previously detected folding intermediates for E. coli alphaTS. A similar combination of uniform and selective labeling can be extended to other TIM barrel proteins to obtain insight into the role of the motif in stabilizing what appear to be common partially folded forms.