Figure 2.

Force-extension curves for a heteropolyprotein constructed as (I27-I28)₄. The force-extension curves always show two distinct levels of unfolding forces (dashed lines). However, the number of modules unfolding in each case varies because the AFM tip picks up the proteins at a random location resulting in the stretch unfolding of different number of modules every time a new molecule is picked up. The diagram on the left provides an explanation for the recordings and marks the number of modules that will unfold in each case (circles represent I28 modules, squares represent I27 modules). Some exclusion rules apply; for example, it is not possible to observe three or more unfolding events of one kind and less than two of the other kind (see Table 1).