Structure and stability of gamma-crystallins. Denaturation and proteolysis behavior

The denaturation behavior of bovine lens gamma-crystallin fractions II, III, and IV and their susceptibility to proteolysis in vitro was compared to determine whether differences in their stability could play a role in cataract formation. Tertiary and secondary structure changes induced by increasing concentrations of urea, guanidine hydrochloride, and sodium dodecyl sulfate and by increasingly alkaline pH were followed by near-UV and far-UV circular dichroism, Trp fluorescence emission, and exposure of sulfhydryl groups. Major differences were found in the denaturation and proteolysis behavior of the three gamma-crystallin fractions. In general, the unfolding of gamma-II and gamma-III crystallins is rather gradual, suggesting the presence of intermediate unfolding states; in contrast, the order-disorder transition of gamma-IV crystallin is abrupt. The gamma-IV crystallin fraction is the most stable in urea and guanidine hydrochloride, but is most susceptible to nonspecific proteolysis and alkaline pH denaturation. Differences in denaturation and proteolysis behavior are attributed to the inherent differences in the tertiary structures of these crystallins.