Fig. 3.

Peptide binding increases mechanical stability of the subdomain and tightens subdomain interface. (A) Three-dimensional structure of the SBD with the bound NR peptide (PDB ID code 4EZW; ref. 11). Schematic below illustrates the variant used for single-molecule experiments. (B) Consecutive force extension pulling curves of the SBD in the presence of 250 µM NR peptide and corresponding distribution of unfolding forces. (C) Scatter plot of unfolding forces vs. contour lengths of the β-subdomain for the apo state (empty symbols) and in the presence of the NR peptide (gray). (D and E) Block-averaged mean of the contour-length increase as a function of force for α^Fβ^F and β^F type of fluctuations in the absence (open symbols) or presence (filled symbols) of the NR peptide. Solid lines are corresponding fits of a two-state open/close equilibrium model. (F) Structural interpretation (helical part in red, β-subdomain in green) of the upper (closed state) and lower states (force open state) and associated free energies with α^Fβ^F and β^F fluctuations in the presence peptides.