Differential domain folding/unfolding of gamma-crystallins: existence of two distinct groups

High resolution calorimetry and spectral measurements have been employed to examine folding/unfolding behaviour of gamma-crystallins which are known to contain two homologous domains. Results have been analyzed in terms of selective unfolding of domains, interdomain interactions, conformational stability and the existence of intermediates in the order-disorder transition equilibrium. Both spectral and thermotropic data indicate that, in terms of structural hierarchy, these proteins can be divided into two distinct groups, gamma II and gamma IIIB belonging to one and gamma IIIA and gamma IVA to the other. The unfolding/folding characteristics of these two groups are distinctly different. Equilibrium unfolding of gamma II and gamma IIIB is biphasic indicating the existence of an intermediate in which one domain unfolds and the other remains in the native form. The absence of a cooperative transition in gamma IIIA and gamma IVA in acidic urea has also been attributed to a structured intermediate, most likely a molten globule, which may not be thermodynamically as distinct as of the former group. The difference in the equilibrium folding/unfolding transition of these two groups has been explained by subtle differences in the packing arrangement of their two domains and interactions between them. Since the intermediates, under certain circumstances, are known to aggregate, they are likely to play a critical role in the etiology of human cataract formation.