Structure of the bovine eye lens gamma D (gamma IIIb)-crystallin at 1.95 angstrom

The crystal structure of bovine lens gamma IIIb-crystallin at 2.5 Angstrom resolution previously reported was interpreted using a consensus sequence derived from related vertebrate sequences on the assumption that gamma IIIb-crystallin derived from the gamma C-crystallin gene. It has recently been shown that gamma IIIb is a product of the bovine gamma D gene. The structure of gamma IIIb has now been refined with the bovine gamma D sequence using new 1.95 Angstrom resolution synchrotron data. The crystallographic R factor was 20.4% for all 33 104 reflection data between 8.0 and 1.95 Angstrom measured at 277 (1) K. The electron density fully supported the assignment of the gamma D sequence to gamma IIIb. The crystal belongs to space group P2(1)2(1)2(1) 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 gamma B-crystallin (81% sequence identity). There is a single amino-acid deletion in gamma D in the linker region connecting the two domains. The intermolecular oganization in the crystal lattice is quite different from gamma B as a result of key mutations involving surface residues Leu51, Ile103 and His155. These point mutations will contribute to the intermolecular behaviour of the gamma-crystallins in the eye lens, where they are major components of the densely packed, high refractive index regions of the lens.