Association of partially folded lens βB2-crystallins with the α-crystallin molecular chaperone

Age-related cataract is a result of crystallins, the predominant lens proteins, forming light- scattering aggregates. In the low protein turnover environment of the eye lens, the crystallins are susceptible to modifications that can reduce stability, increasing the probability of unfolding and aggregation events occurring. It is hypothesized that the alpha-crystallin molecular chaperone system recognizes and binds these proteins before they can form the light-scattering centres that result in cataract, thus maintaining the long-term transparency of the lens. In the present study, we investigated the unfolding and aggregation of (wild-type) human and calf beta B2-crystallins and the formation of a complex between alpha-crystallin and beta B2-crystallins under destabilizing conditions. Human and calf beta B2-crystallin unfold through a structurally similar pathway, but the increased stability of the C-terminal domain of human beta B2-crystallin relative to calf beta B2-crystallin results in the increased population of a partially folded intermediate during unfolding. This intermediate is aggregation-prone and prevents constructive refolding of human beta B2-crystallin, while calf beta B2-crystallin can refold with high efficiency. alpha-Crystallin can effectively chaperone both human and calf beta B2-crystallins from thermal aggregation, although chaperone-bound beta B2-crystallins are unable to refold once returned to native conditions. Ordered secondary structure is seen to increase in alpha-crystallin with elevated temperatures up to 60 degrees C; structure is rapidly lost at temperatures of 70 degrees C and above. Our experimental results combined with previously reported observations of alpha-crystallin quaternary structure have led us to propose a structural model of how activated alpha-crystallin chaperones unfolded beta B2-crystallin.