Figure 2.

Mechanical clamps involving shear or zipping in localized regions. Except for the zipper motif, all other motifs involve shear between secondary structures. The black arrows indicate β-strands. More generically, however, they may also depict α-helices. The lighter arrows indicate the sense of motion that is induced by pulling. The ‘delocalized’ clamp consists of multiple local shearing regions. Unlike the motifs shown in Figure 3, its operation does not rely on motion of any backbone loops. This is the reason why motif D is lamped together with the clamps displayed in the current Figure 3.