The role of the C-terminal extension of αB-crystallin upon structure and function and the relationship with disease

Abstract

The eye lens protein αB-crystallin plays an important role in maintaining the refractive index of the lens, however is also found in many non-lenticular tissues suggest that it has general cellular functions over and above its role in light refraction. The role of the C-terminal extension of αB -cryStellin upon solubility, structure and function and the relationship with disease has not been investigated. In this study, the systematic analysis of the role of the C-terminal extension of αB -crystallin aimed to investigate the hypothesis that the C-terminal region in αB -crystallm is important for solubility, structure and function of the protein. This study also aimed to find a link between the role of the C-terminal and the symptoms of disease, thus providing a potential explanation of whether the three congenital mutations (450delA, Q151X and 464delCT), which alter the C-terminal extension of αB -crystallin, cause the various diseases via the loss of chaperone function or perhaps by a different mechanism. Further more this study aimed to elucidate the mechanism of phenotypic heterogeneity associated with these αB -crystallin mutations.I compared the three C-terminal αB -crystallin mutants (450delA, Q151X and 464delCT) to a series of C-terminal truncations (E164X, E165X, K174X and A171X) and found that the C-terminal extension was essential for oligomerisation but not chaperone function, infact the removal of the entire C-terminal extension actually enhanced chaperone activity, however significantly destabilized αB -crystallin causing it to self-aggregate. This instability was supported by refolding analysis, where the 450delA and 464delCT mutants could only be refolded and assayed as a complex with wild type αB -crystallin, however the Q151X αB -crystallin could be refolded alone. From these studies, I conclude that all three disease-causing mutations (450delA, 464delCT and Q151X) in the C-terminal extension destabilise αB -crystฝlin and increase its tendency to self-aggregate. I propose that it is this, rather than a catastrophic loss of chaperone activity, that is a major factor in disease development.