Lens alpha-crystallin: function and structure

alpha-Crystallin is a major lens protein, comprising up to 40% of total len s proteins, where its structural function is to assist in maintaining the p roper 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 f unction 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-crys tallin in E. coli and compared the polydispersity, structure and aggregatio n state between each other and native bovine lens alpha-crystallin. Using g el permeation chromatography to assay for polydispersity, we found native a lpha-crystallin to be significantly more polydisperse than either recombina nt alpha A- or alpha B-crystallin, with alpha B-crystallin having the most homogeneous structure of the three. Reconstructed images of alpha B-crystal lin 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-crys tallin 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 a lpha A to alpha B subunit composition in an alpha-crystallin molecule is op timal in terms of thermal stability. This fascinating result explains the s toichiometric ratios of alpha A- and alpha B-crystallin subunits in the mam malian lens.