Chaperone-like activity and temperature-induced structural changes of α-crystallin

&#945;-Crystallin is known to exhibit chaperone-like activity. We have studied its chaperone-like activity toward the aggregation of &#946;<SUB>L</SUB>-crystallin upon refolding of this protein from its unfolded state in guanidinium chloride. The chaperone-like activity of &#945;-crystallin is less pronounced below 30°C and is enhanced above this temperature. The plot of percentage protection as a function of temperature shows two transitions; one at 30°C and another at around 55°C. We have performed steady state fluorescence, fluorescence polarization, fluorescence quenching, circular dichroism, sedimentation analysis, and gel filtration chromatography to probe the temperature-induced structural changes of &#945;-crystallin. Our results show that at above 50°C, &#945;-crystallin undergoes a transition to a multimeric molten globule-like state. Above 30°C, a minor but detectable perturbation in its tertiary structure occurs that might lead to the observed exposure of its hydrophobic surfaces. These results support our earlier hypothesis that &#945;-crystallin prevents the aggregation of other proteins by providing appropriately placed hydrophobic surfaces; a structural transition above 30°C involving enhanced or reorganized hydrophobic surfaces of &#945;-crystallin is important for its chaperone-like activity. It is possible that a structural alteration induced by temperature forms a part of the general mechanism of chaperone function, because they are required to function more effectively at nonpermissible temperatures