Temperature dependent chaperone‐like activity of alpha‐crystallin

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Abstract
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: α‐crystallin) against the aggregation of insulin B chain by α‐crystallin, whereas it completely prevents this aggregation at 40°C. We have monitored the temperature dependence of the protection of aggregation by α‐crystallin; the protection increases sharply above 30°C and reaches almost 100% by 41°C. Probing the hydrophobic surfaces of α‐crystallin with the hydrophobic fluorphore 8‐anilino‐1 naphthalene sulfonate suggests that the hydrophobic surfaces of α‐crystallin are exposed to a greater extent above 30°C. A complete prevention of the aggregation is achieved at 27.6°C by increasing the concentration of α‐crystallin by more than 8 fold. Similar temperature dependent chaperone‐like activity of α‐crystallin is observed towards the aggregation of zetacrystallin, an enzyme crystallin from guinea pig. We have earlier shown that α‐crystallin exposes hydrophobic surface(s) at temperatures above 30°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 α‐crystallin is more pronounced in its structurally perturbed state.

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