Conformational studies on δ-crystallin, the core protein of the bird eye lens

Abstract

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 proteins, crystallins, are long lived. In an effort to understand whether their recruitment might be related to their conformation and structural stability, we have examined these features of the avian lens protein δ-crystallin. The native molecule is a tetramer (molecular mass 200 kDa) that is highly α-helical in conformation, and with an unusually blue tryptophan fluorescence (315,325 nm), which is only partially quenched by conventional quenchers. We show that the fluorescence doublet arises due to Trp residues that are effectively buried inside the rigid hydrophobic core of the tetrameric aggregate. The protein is heat stable up to 91°C. Guanidinium chloride (GuHCl) effects the complete denaturation of δ-crystallin, whereas heat or urea treatment results in only partial unfolding or dissociation. The initial transition is the disruption of the quaternary structure by perturbing 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°C as well as in 1 M GuHCl and 4 M urea. We show that in 2.2 M GuHCl the molecule is swollen but is still largely helical with the Trp residues being present in a somewhat more polar environment than in the native molecule. Beyond 4 M GuHCl there is a gradual unfolding of the molecule, which is complete in 6 M GuHCl. This structural robustness of δ-crystallin might be important in its recruitment as the core protein of the avian lens.