RESUMEN
The crystal structure of gamma-crystallin IIIb (gamma C) from calf eye lens has been refined at 2.5 A resolution. The molecule of about 21 kDa consists of two similar domains. Each domain is composed of two motifs with the 'Greek key' topology which form a pair of four-stranded beta-sheets with an antiparallel packing. The molecule has three hydrophobic cores: one within each domain and one between them. Six of the eight functionally important cysteines are located within the N-domain, and only two in the C-domain. Several large clusters of charged residues are at the surface of the molecule. Surface residues Val 101, Met 103 and Leu 155 are important for packing of molecules in crystal medium and possibly in the lens. Features of the gamma-crystallin IIIb molecule which may be related to its function in the vertebrate eye lens are briefly discussed. An attempt has been made to correlate molecular characteristics with some general properties of the eye lens such as high density and refractive index gradients and strong stability of the lens during an organism's lifetime.