FIGURE 1.

The Z1 and Z2 domains of titin exhibit distinct mechanical stability. A and B, diagram of the polyprotein (Z1)₈ and (Z2)₈ constructs and typical force-extension trajectories, where all the monomers in the chain unfold. C, distribution of unfolding forces for the (Z1)₈ construct, yielding an average force 125 ± 23 pN, n = 108. D, fitting the data to the WLC model of polymer elasticity (red lines) yields an average increment in contour length of ΔL = 30.8 ± 0.6 nm. E, distribution of unfolding forces for the (Z2)₈ construct, yielding an average force of 174 ± 40 pN, n = 148 (F) and an associated contour length increase of ΔL 30.8 ± 0.7 nm (F).