Figure 2.

Characteristic fingerprint of I27 domain unfolding by a stretching force. (A) Stretching of an I27₁₂ polyprotein produces a force-extension curve showing the characteristic sawtooth pattern of unfolding. The force-extension curves show different number of sawteeth, depending on the number of modules picked up by the AFM tip. The red squares represent the modules being picked up in this particular experiment. The solid lines are fits of the data to the WLC model of polymer elasticity. L₀ is the contour length of the fully folded polyprotein; upon module unfolding at about ≈200 pN, an additional 28.1 nm will be added to the contour length of the protein. (B) Relationship between L₀, determined from the fit to the first sawtooth, and the number of modules picked up by the AFM tip, determined by the number of sawteeth. The solid line is a linear regression of the data with a slope of 4.3 nm/module.