The structural differences between bovine lens alphaA- and alphaB-crystallin.

Abstract

Lens alphaA- and alphaB-crystallin have been reported to act differently in their protection against nonthermal destabilization of proteins. The nature of this difference, however, is not completely understood. Therefore we used a combination of thermally and solvent-induced structural changes to investigate the difference in the secondary, tertiary and quaternary structures of alphaA- and alphaB-crystallin. We demonstrate the relationship between the changes in the tertiary and quaternary structures for both polypeptides. Far-ultraviolet circular dichroism revealed that the secondary structure of alphaB-crystallin is more stable than that of alphaA-crystallin, and the temperature-induced secondary structure changes of both polypeptides are partially reversible. Tryptophan fluorescence revealed two distinct transitions for both alphaA- and alphaB-crystallin. Compared to alphaB-crystallin, both transitions of alphaA-crystallin occurred at higher temperature. The changes in the hydrophobicity are accompanied by changes in the quaternary structure and are biphasic, as shown by bis-1-anilino-8-naphthalenesulfonate fluorescence and sedimentation velocity. These phenomena explain the difference in the chaperone capacity of alphaA- and alphaB-crystallin carried out at different temperatures. The quaternary structure of alpha-crystallin is more stable than that of alphaA- and alphaB-crystallin. The latter has a strong tendency to dissociate under thermal or solvent destabilization. This phenomenon is related to the difference in subunit organization of alphaA- and alphaB-crystallin where both hydrophobic and ionic interactions are involved. We find that an important subunit rearrangement of alphaA-crystallin takes place once the molecule is destabilized. This subunit rearrangement is a requisite phenomenon for maintaining alpha-crystallin in its globular form and as a stable complex. On the base of our results, we suggest a four-state model describing the folding and dissociation of alphaA- and alphaB-crystallin better than a three-state model [Sun et al. (1999) J. Biol. Chem. 274, 34067-34071].