Internal symmetry in crystallin proteins.

A) An archaean βγ-crystallin with two repeats per chain (3HZ2). The full chain is displayed along its 2-fold axis, followed by a superposition of the repeats. The conserved calcium binding motif N/D-N/D-#-I-S/T-S is highlighted in yellow throughout. B) Human γ-D crystallin structure with four repeats per chain (1HK0). Two levels of symmetry exist: a C2 symmetry within each globular domain, and an additional C2 axis relating the two domains. The calcium binding motif has been lost (red bar below sequence), but other conserved positions (blue and magenta in the sequence) show the homology between the repeats.