Figure 2.

Single-molecule force spectroscopy of the ARM region and full-length β-catenin. (A) Force-extension recordings from the mechanical unfolding of the three constructs from Fig. 1C: ARMI27 (top, from L_C⁰ to L_C^β: ARM region; from L_C^β to the last force peak: I27 modules), the βI27 (middle, from L_C⁰ to L_C^β: full-length β-catenin region; from L_C^β to the last force peak: I27 modules) and β-catenin (bottom). Peaks were fitted using the worm-like chain model of polymer elasticity (smooth lines (26)), which allows to calculate contour lengths (L_C⁰, ΔL_C, and L_C^β). The region under study in the three constructs unfolded in a similar way, with weak forces and variable ΔL_C values (several traces from ARMI27 construct are shown as a representative example, Fig. S6). The I27 marker shows an ΔL_C of 28 nm and a F_U of around 200 pN, in accordance with previous reports (25). (B) F_U histograms. Lines correspond to log-normal fittings (χ² are 1.5 × 10⁻⁵, 8.5 × 10⁻⁵, and 2.3 × 10⁻⁵ for ARMI27, βI27 and β-catenin, respectively). Only 0.4% of the data points lie above 100 pN for ARMI27 (top), being 10% for βI27 (middle), and β-catenin (bottom) (containing unstructured terminal regions). This suggests that the ARM region is somehow stabilized by the outer regions. (C) The ΔL_C histograms for the proteins show similar (though slightly shifted) multimodal distributions with the mean values close to the length of one or multiple ARM repeats (see details in the main text). ARMI27 (top), βI27 (middle), and β-catenin (bottom).