CONFORMATIONAL STUDIES ON DELTA-CRYSTALLIN, THE CORE PROTEIN OF THE BIRD EYE LENS

Proteins that perform other functions elsewhere appear to be recruited for structural purposes in the eye lens. The lens being a tissue with very little metabolic activity and little or no turnover, the lens pr oteins, crystallins, are long lived. In an effort to understand whethe r their recruitment might be related to their conformation and structu ral stability, we have examined these features of the avian lens prote in delta-crystallin. The native molecule is a tetramer (molecular mass 200 kDa) that is highly alpha-helical in conformation, and with an un usually blue tryptophan fluorescence (315, 325 nm), which is only part ially quenched by conventional quenchers. We show that the fluorescenc e doublet arises due to Trp residues that are effectively buried insid e the rigid hydrophobic core of the tetrameric aggregate. The protein is heat stable up to 91 degrees C. Guanidinium chloride (GuHCl) effect s the complete denaturation of delta-crystallin, whereas heat or urea treatment results in only partial unfolding or dissociation. The initi al transition is the disruption of the quaternary structure by perturb ing the intersubunit interactions, leading to exposure of hydrophobic contact surfaces (as monitored by extrinsic probe fluorescence). This initial transition is seen upon heating to 60 degrees C as well as in 1 M GuHCl and 4 M urea. We show that in 2.2 M GuHCl the molecule is sw ollen but is still largely helical with the Trp residues being present in a somewhat more polar environment than in the native molecule. Bey ond 4 M GuHCl there is a gradual unfolding of the molecule, which is c omplete in 6 M GuHCl. This structural robustness of delta-crystallin m ight be important in its recruitment as the core protein of the avian lens.