Studies on the alpha-crystallin target protein binding sites: sequential binding with two target proteins.

Purpose: alpha-Crystallin belongs to a class of small heat shock proteins and is shown to prevent aggregation of several proteins. We have shown that the temperature-induced structural perturbation leads to several fold enhanced activity. The purpose of this study was to investigate the availability and specificity of the hydrophobic sites that might become available at elevated temperatures. Specifically, we address the following question: Is there an increased exposure of fixed number of hydrophobic sites as a function of temperature or does a new set of sites become available at elevated temperatures?
Methods: alpha-Crystallin target protein complexes were made at two different temperatures and this complex was investigated for its chaperone-like activity towards the same target protein and also other target proteins. DTT-induced aggregation of insulin, alpha-lactalbumin, thermal aggregation of betaL- and gamma-crystallin, and photo-aggregation of gamma-crystallin were used as model systems. Increased light scattering was used to monitor the progress of aggregation.
Results: alpha-Crystallin target protein complex prepared at 37 degrees C temperature was effective against thermal aggregation of betaL-crystallin as well as non-thermal aggregation at elevated temperatures. However, the complex prepared at high temperature was ineffective at lower temperatures as well as with other target proteins at both temperatures.
Conclusions: More target protein binding sites become available at elevated temperatures. The sites available at low temperature are a subset of the total sites available at elevated temperatures.