Temperature dependent chaperone-like activity of alpha-crystallin.

Alpha-crystallin, a multimeric protein present in the eye lens, is known to have chaperone-like activity in preventing the aggregation of enzymes and other crystallins. We have studied the chaperone-like activity of this protein towards the aggregation of insulin B chain, induced by reducing the interchain disulphide bond with dithiothreitol. At room temperature, there is no detectable protection (at a 1:1 (w/w) ratio of insulin: alpha-crystallin) against the aggregation of insulin B chain by alpha-crystallin, whereas it completely prevents this aggregation at 40 degrees C. We have monitored the temperature dependence of the protection of aggregation by alpha-crystallin; the protection increases sharply above 30 degrees C and reaches almost 100% by 41 degrees C. Probing the hydrophobic surfaces of alpha-crystallin with the hydrophobic fluorphore 8-anilino-1 naphthalene sulfonate suggests that the hydrophobic surfaces of alpha-crystallin are exposed to a greater extent above 30 degrees C. A complete prevention of the aggregation is achieved at 27.6 degrees C by increasing the concentration of alpha-crystallin by more than 8 fold. Similar temperature dependent chaperone-like activity of alpha-crystallin is observed towards the aggregation of zeta-crystallin, an enzyme crystallin from guinea pig. We have earlier shown that alpha-crystallin exposes hydrophobic surface(s) at temperatures above 30 degrees C. These results support our earlier hypothesis [Raman, B. and Rao, Ch.M. (1994) J. Biol. Chem. 269, 27264-27268] that the chaperone-like activity of alpha-crystallin is more pronounced in its structurally perturbed state.