SDS induced structural changes in α-crystallin and it's effect on refolding

alpha-Crystallin, a major eye lens protein and a key member of the small heat shock protein family, acts like a chaperone by preventing aggregation of substrate proteins. One of the hallmarks of most small heat shock proteins is their existence as a large oligomer, the role of which in its function is not understood at present. We have studied the role of the oligomer in the stability of its structure against SDS induced destabilization by CD measurements. alpha-Crystallin from bovine source as well as recombinant preparation was used for this purpose. As SDS concentration was gradually increased, the beta-sheet structure was diminished followed by concomitant increase in the alpha-helical structure. The quaternary structural changes in presence of SDS were also monitored by light scattering, polarization and anisotropy measurements. It was found that the breakdown of the oligomeric structure was nearly complete above 1 mM SDS concentration. The results were compared with that of a monomeric gamma-crystallin, which is also a major beta-sheet protein like alpha-crystallin. When alpha-crystallin was first converted into monomeric random coil structure in presence of 6 M urea and allowed to refold in SDS solution, amount of alpha-helix was more than that incubated directly in the same concentration of SDS. The results show that alpha-crystallin attains extra structural stability against external stress due to its oligomeric structure. The implication for the extra stability is discussed in reference to its function as molecular chaperone.