Structure/function studies of dogfish alpha-crystallin, comparison with bovine alpha-crystallin

alpha-Crystallin is the major protein of the mammalian lens where it contributes to the refractive properties needed for vision and possibly to the stability of the tissue. The aim of this study was to determine whether the properties of alpha-crystallin have changed during the course of evolution. Dogfish alpha-crystallin, which appeared over 420 million years ago, has been contrasted with bovine alpha-crystallin, which emerged around 160 million years later, by comparing their sizes, the microenvironments of their cysteine and tryptophan residues, their chaperone-like activities and the flexibility of their COOH-terminal extensions. Dogfish alpha-crystallin consists of alphaA- and alphaB-polypeptides, in a 1:5 ratio, and has a molecular mass of around 400 kDa. By contrast, the bovine protein is around 600-800 kDa in mass and has a 3:1 subunit ratio. Cysteine residues in the proteins were equally accessible to reaction with 5,5'-dithiobis-(2-nitrobenzoic acid). Quenching of fluorescence with acrylamide indicated tryptophan residues in the two proteins were in similar environments. The chaperone activity of dogfish alpha-crystallin was comparable to that of bovine alpha-crystallin in preventing the heat-induced precipitation of beta(L)-crystallin but the dogfish protein was three times more effective at preventing insulin precipitation after reduction at 37 C. (1)H nuclear magnetic resonance spectroscopic studies showed that the last 17 amino acids of the dogfish alphaB polypeptide (V162-K178) have great conformational flexibility, are highly exposed to solvent and adopt little ordered conformation. This is comparable to, but slightly longer in length, than the COOH-terminal extension observed in mammalian alpha-crystallins. The structure and properties of alpha-crystallin have changed relatively little during the evolutionary period from the emergence of sharks and mammals.