Predicted secondary and tertiary structures of carp γ-crystallins with high methionine content: Role of methionine residues in the protein stability

A systematic structural comparison of several carp γ-crystalline with high methionine contents was made by the secondary-structure prediction together with computer model-building based on the established X-ray structure of calf γ-II crystallin. The overall surface hydrophilicity profile and the distribution of helices, β-sheets, and β-turns along the polypeptide chains are very similar among these carp γ-crystallins. In addition, their general polypeptide packing is close to the characteristic 2 domain/4 motif Greek key three-dimensional conformation depicted for the calf γ-II crystallin. Interestingly, most hydrophobic methionine residues are located on the protein surface with only a few buried inside the protein surface or in the interface between two motifs of each domain. The exposed hydrophobic and polarizable methionine cluster on the protein surface may have a bearing on the crystallin stability and dense packing in the piscine species, and probably also provides a malleable nonpolar surface for the interaction with other crystallin components for the maintenance of a clear and transparent lens.