Figure 2.

Mechanical properties of C2AB. (A) Typical force-extension curve obtained after stretching a I27₂-C2AB-I27₂ protein. The I27 domains unfold at forces of ∼190 pN and produces an increase in contour length (ΔL_c) of ∼28 nm. These values were identified by measuring the spacing between force peaks using the WLC equation (thin black lines). In this recording, four I27 domains are unfolded, and the first force peaks must correspond to the unfolding of C2 domains. (B) For C2 domains, we measured a ΔL_c of ∼43 ± 8 nm (n = 98). (C) An unfolding force histogram shows that C2 domains must unfold at forces of 62 ± 30 pN (n = 279; red), a value that is threefold lower than that for I27 (188 ± 29 pN, n = 264; black). (D) A plot of unfolding force versus the loading rate shows that the threefold difference between C2 and I27 domains is maintained over a wide range of loading rates.