Lens alpha-crystallin: function and structure.

alpha-Crystallin is a major lens protein, comprising up to 40% of total lens proteins, where its structural function is to assist in maintaining the proper refractive index in the lens. In addition to its structural role, it has been shown to function in a chaperone-like manner. The chaperone-like function of alpha-crystallin will help prevent the formation of large light-scattering aggregates and possibly cataract. In the lens, alpha-crystallin is a polydisperse molecule consisting of a 3:1 ratio of alpha A to alpha B subunits. In this study, we expressed recombinant alpha A- and alpha B-crystallin in E. coli and compared the polydispersity, structure and aggregation state between each other and native bovine lens alpha-crystallin. Using gel permeation chromatography to assay for polydispersity, we found native alpha-crystallin to be significantly more polydisperse than either recombinant alpha A- or alpha B-crystallin, with alpha B-crystallin having the most homogeneous structure of the three. Reconstructed images of alpha B-crystallin obtained with cryo-electron microscopy support the concept that alpha B-crystallin is an extremely dynamic molecule and demonstrated that it has a hollow interior. Interestingly, we present evidence that native alpha-crystallin is significantly more thermally stable than either alpha A- or alpha B-crystallin alone. In fact, our experiments suggest that a 3:1 ratio of alpha A to alpha B subunit composition in an alpha-crystallin molecule is optimal in terms of thermal stability. This fascinating result explains the stoichiometric ratios of alpha A- and alpha B-crystallin subunits in the mammalian lens.