Association of partially-folded lens {beta}B2-crystallins with the {alpha}-crystallin molecular chaperone
Résumé
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 hypothesised that the α-crystallin molecular chaperone system recognises 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 this study we investigated the unfolding and aggregation of (wildtype) human and calf βB2-crystallins, and the formation of a complex between α-crystallin and βB2-crystallins under destabilising conditions. Human and calf βB2-crystallin unfold through a structurally similar pathway, but the increased stability of the C-terminal domain of human βB2-crystallin relative to calf βB2-crystallin results in the increased population of a partially folded intermediate during unfolding. This intermediate is aggregation prone, and prevents constructive refolding of human βB2-crystallin, whilst calf βB2-crystallin can refold with high efficiency. α-crystallin can effectively chaperone both human and calf βB2-crystallin from thermal aggregation, though chaperone-bound βB2-crystallins are unable to refold once returned to native conditions. Ordered secondary structure is seen to increase in α-crystallin with elevated temperatures up to 60°C; structure is rapidly lost at temperatures of 70°C and above. Our experimental data combined with previously reported observations of α-crystallin quaternary structure have lead us to propose a structural model of how activated α-crystallin chaperones unfolded βB2-crystallin.
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