Structural variation in mammalian gamma-crystallins based on computer graphics analyses of human, rat and calf...

A comparison of mammalian gamma-crystallins has been made by computer-graphics model building of several gamma-crystallin sequences based on the atomic co-ordinates of the X-ray determined structure of calf gamma-II crystallin. The complete family of rat gamma-crystallins is compared together with the orthologous protein, gamma 1-2 crystallin, from rat, human and calf lens, and the orthologous protein, gamma 2-1 crystallin, from rat and human lens. In human gamma-crystallins, a major structural difference, the replacement of an arginine by a cysteine, occurs in one of the four-fold repeated folded hairpins, which may affect stability. Sequence variations involving buried residues were observed, leading to small differences in core packing of the different sequences which may be related to their regional location in the lens. Model-building studies also indicate that the surfaces of the different gamma-crystallins vary in number of exposed hydrophobic residues and ion pairs. These differences would affect protein-water interactions and therefore contribute to refractive index. A major variable region of the gamma-crystallin structures involves polar residues surrounding the inter-domain contact and the length of the polypeptide connecting the two domains. An attempt is made to correlate bovine gamma-crystallins which are known to be responsible for cold cataract with the corresponding sequences from rat lens.