Figure 1: Optical trapping experiments reveal that FL is mechanically labile.

(a) The predicted three-dimensional structure of the FL domain (left) and its schematic representation (right). The FL domain assumes the classic ferredoxin-like fold with the signature βαββαβ secondary structure arrangement along its backbone. In the schematic, open arrows represent β strands, green boxes represent α-helices and black lines indicate loops between secondary structure elements. (b,c). Representative force–extension curves of a single-FL domain (b: pulling velocity of 100 nm s⁻¹; c: pulling velocity of 500 nm s⁻¹). The distinct unfolding force peak, occurring at ∼5 pN with a contour length increment ΔLc of ∼27 nm, corresponds to the unfolding of a FL domain. Red lines correspond to fits to the experimental data using the extendable worm-like chain model of polymer elasticity. The relaxation curves show that FL can refold when the stretching force is relaxed. Inset: schematic of the optical trapping geometry with the FL domain linked via DNA handles to functionalized silica beads. The predicted three-dimensional structure of the FL domain shows an α/β structure that belongs to the ferredoxin-like fold. As the N, C-termini of FL is close to each other, we added a mechanically stable GB1 domain (grey ellipsoid) at the N terminus of FL to facilitate the attachment of DNA handles to FL via cysteine cross-linking. The schematic is not drawn to scale.