The structural differences between bovine lens alpha A- and alpha B-crystallin

Lens alpha A- and alpha B-crystallin have been reported to act differently in their protection against nonthermal destabilization of proteins. The nat ure 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 struct ures of alpha A- and alpha B-crystallin. We demonstrate the relationship be tween the changes in the tertiary and quaternary structures far both polype ptides. Far-ultraviolet circular dichroism revealed that the secondary stru cture of alpha B-crystallin is more stable than that of alpha A-crystallin, and the temperature-induced secondary structure changes of both polypeptid es are partially reversible. Tryptophan fluorescence revealed two distinct transitions for both alpha A- and alpha B-crystallin. Compared to alpha B-c rystallin, both transitions of alpha A-crystallin occurred at higher temper ature. The changes in the hydrophobicity are accompanied by changes in the quaternary structure and are biphasic, as shown by bis-1-anilino-8-naphthal enesulfonate fluorescence and sedimentation velocity. These phenomena expla in the difference in the chaperone capacity of alpha A- and alpha B-crystal lin carried out at different temperatures. The quaternary structure of alph a-crystallin is more stable than that of alpha A- and alpha B-crystallin. T he latter has a strong tendency to dissociate under thermal or solvent dest abilization. This phenomenon is related to the difference in subunit organi zation of alpha A- and alpha B-crystallin when both hydrophobic and ionic i nteractions are involved. We find that an important subunit rearrangement o f alpha A-crystallin takes place once the molecule is destabilized. This su bunit rearrangement is a requisite phenomenon for maintaining alpha-crystal lin in its globular form and as a stable complex. On the base of our result s, we suggest a four-state model describing the folding and dissociation of alpha A- and alpha B-crystallin better than a three-state model [Sun et al . (1999) J. Biol. Chern. 274, 34067-34071].