PT  - JOURNAL ARTICLE
AU  - Jörg Schönfelder
AU  - David De Sancho
AU  - Ronen Berkovich
AU  - Robert B. Best
AU  - Victor Muñoz
AU  - Raul Perez-Jimenez
TI  - Reversible two-state folding of the ultrafast protein gpW under mechanical force
AID  - 10.1101/314583
DP  - 2018 Jan 01
TA  - bioRxiv
PG  - 314583
4099  - http://biorxiv.org/content/early/2018/05/05/314583.short
4100  - http://biorxiv.org/content/early/2018/05/05/314583.full
AB  - Ultrafast folding proteins have limited cooperativity and thus are excellent models to resolve, via single-molecule experiments, the fleeting molecular events that proteins undergo during folding. Here we report single-molecule atomic force microscopy (AFM) experiments on gpW, a protein that, in bulk, folds in a few microseconds over a marginal folding barrier (~1 kBT). Applying pulling forces of only 5 pN we maintain gpW in quasi-equilibrium near its mechanical unfolding midpoint, and detect how it interconverts stochastically between the folded and an extended state. This binary pattern indicates that, under an external force, gpW (un)folds over a significant free energy barrier. Using molecular simulations and a theoretical model we rationalize how force induces such barrier in an otherwise downhill free energy surface. Force-induced folding barriers are likely a general occurrence for ultrafast folding biomolecules studied with single molecule force spectroscopy.