title: Exploring one-state downhill protein folding in single molecules.
abstract: A one-state downhill protein folding process is barrierless at all conditions, resulting in gradual melting of native structure that permits resolving folding mechanisms step-by-step at atomic resolution. Experimental studies of one-state downhill folding have typically focused on the thermal denaturation of proteins that fold near the speed limit (ca. 10(6) s(-1)) at their unfolding temperature, thus being several orders of magnitude too fast for current single-molecule methods, such as single-molecule FRET. An important open question is whether one-state downhill folding kinetics can be slowed down to make them accessible to single-molecule approaches without turning the protein into a conventional activated folder. Here we address this question on the small helical protein BBL, a paradigm of one-state downhill thermal (un)folding. We decreased 200-fold the BBL folding-unfolding rate by combining chemical denaturation and low temperature, and carried out free-diffusion single-molecule FRET experiments with 50-μs resolution and maximal photoprotection using a recently developed Trolox-cysteamine cocktail. These experiments revealed a single conformational ensemble at all denaturing conditions. The chemical unfolding of BBL was then manifested by the gradual change of this unique ensemble, which shifts from high to low FRET efficiency and becomes broader at increasing denaturant. Furthermore, using detailed quantitative analysis, we could rule out the possibility that the BBL single-molecule data are produced by partly overlapping folded and unfolded peaks. Thus, our results demonstrate the one-state downhill folding regime at the single-molecule level and highlight that this folding scenario is not necessarily associated with ultrafast kinetics.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
authors.forename: Jianwei
authors.initials: J
authors.lastname: Liu
authors.forename: Luis A
authors.initials: LA
authors.lastname: Campos
authors.forename: Michele
authors.initials: M
authors.lastname: Cerminara
authors.forename: Xiang
authors.initials: X
authors.lastname: Wang
authors.forename: Ravishankar
authors.initials: R
authors.lastname: Ramanathan
authors.forename: Douglas S
authors.initials: DS
authors.lastname: English
authors.forename: Victor
authors.initials: V
authors.lastname: Muñoz
mesh_terms: D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism*; D031541:Fluorescence Resonance Energy Transfer / Q000379:methods*; D011489:Protein Denaturation; D017510:Protein Folding; D017434:Protein Structure, Tertiary
chemical_list: D029968:Escherichia coli Proteins