Truncation of αB-crystallin by the myopathy-causing Q151X mutation significantly destabilizes the protein leading to aggregate formation in transfected cells

Here we investigate the effects of a myopathy-causing mutation in alpha B-crystallin, Q151X, upon its structure and function. This mutation removes the C-terminal domain of alpha B-crystallin, which is expected to compromise both its oligomerization and chaperone activity. We compared this to two other alpha B-crystallin mutants (450delA, 464delCT) and also to a series of C-terminal truncations (E164X, E165X, K174X, and A171X). We find that the effects of the Q151X mutation were not always as predicted. Specifically, we have found that although the Q151X mutation decreased oligomerization of alpha B-crystallin and even increased some chaperone activities, it also significantly destabilized alpha B-crystallin causing it to self-aggregate. This conclusion was supported by our analyses of both the other disease-causing mutants and the series of C-terminal truncation constructs of alpha B-crystallin. The 450delA and 464delCT mutants could only be refolded and assayed as a complex with wild type alpha B-crystallin, which was not the case for Q151X alpha B-crystallin. From these studies, we conclude that all three disease-causing mutations (450delA, 464delCT, and Q151X) in the C-terminal extension destabilize alpha B-crystallin and increase its tendency to self-aggregate. We propose that it is this, rather than a catastrophic loss of chaperone activity, which is a major factor in the development of the reported diseases for the three disease-causing mutations studied here. In support of this hypothesis, we show that Q151X alpha B-crystallin is found mainly in the insoluble fraction of cell extracts from transient transfected cells, due to the formation of cytoplasmic aggregates.