Chaperone activity in the lens

alpha-crystallin, the major protein of the eye lens, is a molecular chaperone that is able to prevent the precipitation of denatured proteins. This activity is thought to be important for the maintenance of lens transparency. Loss of the activity has been postulated to contribute to the development of cataract. The purpose of this study was to determine how chaperone activity was affected by growth and ageing of the lens.

alpha-crystallins were purified from nine concentric tissue layers removed from an adult bovine lens. The ability to inhibit the precipitation of beta(L)-crystallin, following thermal denaturation, was used to assess the chaperone activity of these proteins. The molar ratio of alpha-crystallin/beta(L)-crystallin required to inhibit the precipitation of beta(L)-crystallin by 50 per cent was used as a measure of the affinity of the chaperone for denatured protein.

As evidenced by a gradual increase in the ratio, from 0.52 to 1.24, the protective ability of alpha-crystallin decreased from the outside of the lens into the centre. alpha-crystallin from the cortex of the lens provided greater protection against precipitation of proteins than older alpha-crystallin from the nucleus. The reasons for this were investigated. Gel electrophoresis of the proteins from each concentric layer revealed an increase in degraded polypeptides from approximately one per cent in the cortex to more than nine per cent in the centre of the lens. This increase appears to be correlated with the decrease in chaperone ability. Renaturing alpha-crystallin obtained from the nucleus did not increase its chaperone activity, indicating conformational changes were not responsible for the decreased activity. Phosphorylation did not appear to have any significant effect on the chaperone activity.

The loss of chaperone activity, accompanying fibre cell compression into the centre of the lens, can be attributed to degradation of the alpha-crystallin polypeptides.