Fig. 4.

A comparison of IL-1β, HYB^β and HYB^β,L. (A) The aligned structures of HYB^β and HYB^β,L. Common regions are in gray. The tight β-turn common to both hybrids is shown at the bottom right as a blue tube. The blue β11–β12 connecting loop from IL-1Ra replaces the one residue shorter red loop of HYB^β. Residues of IL-1Ra that make contact with residue 145 (pale blue) are shown as blue spheres. A residue whose contact is present in HYB^β but not in IL-1Ra is depicted by red spheres. (B) Contact maps of HYB^β (384 contacts) and HYB^β,L (398 contacts). Gray contacts are present in both structures. The red contacts are present only in HYB^β, and the blue ones are present only in HYB^β,L. The largest differences in the contacts are in the third or C-terminal trefoil. (C) The HYB^β data are reproduced from Fig. 3D. (D) HYB^β,L. The central trefoil (red) forms and stabilizes earlier than in HYB^β. The C-terminal region (green) forms late along with the N-terminal trefoil (blue). The stiffer C-terminal trefoil is harder to fold to. (E) Free-energy barriers for IL-1β and the two hybrids. Removal of the β-bulge reduces the barrier, whereas the addition of the stiffer third trefoil restores it.