Effect of heat-induced structural perturbation of secondary and tertiary structures on the chaperone activity of alpha-crystallin.

alpha-Crystallin, a major protein of the lens, is known to have chaperone activity to protect other proteins against thermal aggregation. Heat-induced structural change of alpha-crystallin was previously shown to increase its chaperone activity. In this report, we studied the thermal reversibility of alpha-crystallin and the effect of change in secondary structure on its chaperone function in vitro. The heat-induced conformational changes in the aromatic region of near-UV CD spectra showed only a small degree of reversibility. The structural transitions from 50 to 70 degrees C were largely reversible if the incubation time was short. However, the protective ability to inhibit thermal aggregation of alcohol dehydrogenase by alpha-crystallin was essentially similar at 48 and 70 degrees C. Under long-term heating at high temperatures, there was a time-dependent irreversibility of structural change in alpha-crystallin as revealed by CD spectroscopy. Such denatured alpha-crystallin by long-term heating can still preserve its ability to prevent UV-induced aggregation of gamma-crystallin at room temperature, indicating relatively little effect of heat-induced changes in secondary structure on the chaperone activity of alpha-crystallin.