Physiological role of the association complexes of α-crystallin and its substrates on the chaperone activity

Previous reports on the chaperone activity of α-crystallin to prevent protein denaturation and thermal aggregation have suggested that partially denatured proteins can bind α-crystallin in its central region. Likewise, β- and γ-crystallin can also be localized to the central cavity of α-crystallin particle in vivo, which provides indirect evidence that α-crystallin can function as a chaperone in the intact lens. In this study, we have further demonstrated that the binding of the substrate proteins to α-crystallin by short-term preincubation may mimic the in vivo conditions of crystallin association. Preheating of α-crystallin with its substrate proteins at 60°C for 20 min resulted in the formation of stable complexes between α-crystallin and its substrates (8.0% of insulin or 5.3% of γ-crystallin was involved in complex formation). Under such conditions, the chaperone activity of α-crystallin to inhibit dithiothreitol-, ultraviolet-, or oxidation-induced protein aggregation can be greatly enhanced. Since UV-irradiation and oxidative stress are common insults to eye lenses under normal physiological conditions, the presence of α/γ and α/β complex in vivo may play an important role to maintain the lens in a transparent state.