Interaction between Two Discontiguous Chain Segments from the β-Sheet of Escherichia coli Thioredoxin Suggests an Initiation Site for Folding

The approach of comparing folding and folding/binding processes is exquisitely poised to narrow down the regions of the sequence that drive protein folding. We have dissected the small single α/β domain of oxidized Escherichia coli thioredoxin (Trx) into three complementary fragments (N, residues 1−37; M, residues 38−73; and C, residues 74−108) to study them in isolation and upon recombination by far-UV CD and NMR spectroscopy. The isolated fragments show a minimum of ellipticity of ca. 197 nm in their far-UV CD spectra without concentration dependence, chemical shifts of Hα that are close to the random coil values, and no medium- and long-range NOE connectivities in their three-dimensional NMR spectra. These fragments behave as disordered monomers. Only the far-UV CD spectra of binary or ternary mixtures that contain N- and C-fragments are different from the sum of their individual spectra, which is indicative of folding and/or binding of these fragments. Indeed, the cross-peaks corresponding to the rather hydrophobic β2 and β4 regions of the β-sheet of Trx disappear from the 1H−15N HSQC spectra of isolated labeled N- and C-fragments, respectively, upon addition of the unlabeled complementary fragments. The disappearing cross-peaks indicate interactions between the β2 and β4 regions, and their reappearance at lower temperatures indicates unfolding and/or dissociation of heteromers that are predominantly held by hydrophobic forces. Our results argue that the folding of Trx begins by zippering two discontiguous and rather hydrophobic chain segments (β2 and β4) corresponding to neighboring strands of the native β-sheet.