alpha-Crystallin has been shown to function as a molecular chaperone in preventing thermal aggregation of crystallins and other proteins. The molecular mechanism of this protection is not yet clear. gamma-Crystallin aggregates upon exposure to UV light. We have investigated the effect of the presence of alpha-crystallin in the photoaggregation process and find that alpha-crystallin does not prevent photoaggregation at low temperatures. The protection starts around 30 degrees C and steeply increases with temperature. The plot of protection ability versus temperature is sigmoidal, indicating a structural transition. Perturbation of the quaternary structure of alpha by non-thermal mode, such as 3 M urea, also results in enhanced protection. Pyrene, a hydrophobic fluorophore, is sparingly soluble in water. alpha-Crystallin enhances the solubility of pyrene by severalfold. Temperature dependence of this solubilization shows a transition around 30 degrees C (another at about 50 degrees C). Fluorescence intensity ratio of third and first peaks of pyrene emission (I3/I1,), indicative of hydrophobicity of the reporting area, also shows similar transitions suggesting enhanced hydrophobicity. Gel filtration experiments of irradiated samples indicate the complex formation between gamma- and alpha-crystallins. alpha-Crystallin does not prevent cold precipitation of gamma-crystallin. On the basis of these results, we hypothesize that alpha-crystallin prevents aggregation of non-native structures by providing appropriately placed hydrophobic surfaces. A structural transition above 30 degrees C enhances the protective ability, perhaps by increasing or reorganizing the hydrophobic surfaces. A similar temperature dependence has been reported for GroEL. Whether a structural switch, either activated by temperature, solvent conditions, or small molecule binding, forms a part of the general mechanism of chaperone activity needs to be investigated.