Characterization of an unfolding intermediate and kinetic analysis of guanidine hydrochloride-induced denaturation of the colicin E1 channel peptide

The equilibrium unfolding pathway of the colicin E1 channel peptide was shown in a previous study to involve an unfolding intermediate, stable in approximately 4 M guanidine hydrochloride, which comprised primarily the C-terminal hydrophobic alpha-helical hairpin segment of the peptide [Steer, B. A., & Merrill, A. R. (1995) Biochemistry 34, 7225-7233]. In this study, the structural nature of this unfolding intermediate was investigated further, and it was found that the intermediate primarily consists of a dimer species and is comprised of two partially denatured monomeric peptides, which appear to be associated by hydrophobic interactions. The dimerized structure was detected by size-exclusion high-performance liquid chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, chemical cross-linking, and intermolecular fluorescence energy transfer. Using stopped-flow fluorescence spectroscopy, the kinetics of the denaturation and dimerization of the colicin E1 channel peptide in 4 M guanidine hydrochloride were examined. Denaturation kinetics were also investigated by wild-type peptide Trp fluorescence and 1-anilinonaphthalene-8-sulfonic acid binding. The kinetics of dimer formation were examined by monitoring the time dependence of intermolecular Trp to 5-[[2-[(iodoacetyl)amino]ethyl]amino]napthalene-1-sulfonic acid fluorescence resonance energy transfer upon denaturation in 4 M guanidine hydrochloride. In addition, single Trp mutant peptides were employed as site-specific fluorescent probes of unfolding kinetics and reported diverse and characteristic unfolding kinetics. However, it was shown that following a rapid and major unfolding transition the peptide's core residues cluster slowly, by hydrophobic association, forming an intermediate species which is a prerequisite to dimerization. These equilibrium and kinetic unfolding data describe a unique unfolding mechanism where the channel peptide forms a partially unfolded dimerized structure in 4 M guanidine hydrochloride.