Correction: Ca²⁺ Binding Enhanced Mechanical Stability of an Archaeal Crystallin

Figure 1 is incorrectly printed in black and white. Please see the correct Figure 1 here.

Figure 1. Structure and 2D topology diagram of two β-sandwich proteins with Greek key motifs used in this study.

The pulling direction used in the single-molecule force spectroscopy (SMFS) experiments is shown by arrows. (A) NMR structure of I27 (PDB ID: 1TIT). Terminal β-strands A′ and G are directly connected by H-bonding, shearing this “mechanical-clamp” results in the mechanical unfolding of the protein. The rupture of H-bonds between A and B strands constitutes the less stable mechanical intermediate. (B) 2D topology diagram of I27. The five-stranded (BCDEF) ‘double’ Greek key (3,2)₃ formed by overlapping (3,1)_N and (2,2)_C Greek keys (as defined by Hutchinson and Thornton [53]). (C) NMR structure of M-crystallin (PDB ID: 2K1W) bound to two Ca²⁺ ions (shown as black spheres). The terminal β-strands A and H are not directly bonded and they need to be “peeled” away from each other to unfold the protein. (D) 2D topology diagram of M-crystallin showing the two (3,1)_C Greek keys formed by ABCD and EFGH. In both cases, the backbone H-bonding around the terminal strands is shown.