Structure of Bovine Eye Lens γD (γIIIb)-Crystallin at 1.95 Å

The crystal structure of bovine lens γIIIb-crystallin at 2.5 Å resolution previously reported was interpreted using a consensus sequence derived from related vertebrate sequences on the assumption that γIIIb-crystallin derived from the γC-crystallin gene. It has recently been shown that γIIIb is a product of the bovine γD gene. The structure of γIIIb has now been refined with the bovine γD sequence using new 1.95 Å resolution synchrotron data. The crystallographic R factor was 20.4% for all 33 104 reflection data between 8.0 and 1.95 Å measured at 277(1) K. The electron density fully supported the assignment of the γD sequence to γIIIb. The crystal belongs to space group P2₁2₁2₁ with two molecules of molecular mass 20 749 Da in the asymmetric unit in which 219 water molecules were located. The two-domain four-Greek-key motif highly symmetrical protein is very similar in structure to γB-crystallin (81% sequence identity). There is a single amino-acid deletion in γD in the linker region connecting the two domains. The intermolecular oganization in the crystal lattice is quite different from γB as a result of key mutations involving surface residues Leu51, Ile103 and His155. These point mutations will contribute to the intermolecular behaviour of the γ-crystallins in the eye lens, where they are major components of the densely packed, high refractive index regions of the lens.