Differential Temperature-dependent Chaperone-like Activity of αA- and αB-crystallin Homoaggregates*

Abstract

α-Crystallin, a heteromultimeric protein made up of αA- and αB-crystallins, functions as a molecular chaperone in preventing the aggregation of proteins. We have shown earlier that structural perturbation of α-crystallin can enhance its chaperone-like activity severalfold. The two subunits of α-crystallin have extensive sequence homology and individually display chaperone-like activity. We have investigated the chaperone-like activity of αA- and αB-crystallin homoaggregates against thermal and nonthermal modes of aggregation. We find that, against a nonthermal mode of aggregation, αB-crystallin shows significant protective ability even at subphysiological temperatures, at which αA-crystallin or heteromultimeric α-crystallin exhibit very little chaperone-like activity. Interestingly, differences in the protective ability of these homoaggregates against the thermal aggregation of β_L-crystallin is negligible. To investigate this differential behavior, we have monitored the temperature-dependent structural changes in both the proteins using fluorescence and circular dichroism spectroscopy. Intrinsic tryptophan fluorescence quench-ing by acrylamide shows that the tryptophans in αB-crystallin are more accessible than the lone tryptophan in αA-crystallin even at 25 °C. Protein-bound 8-anilinonaphthalene-1-sulfonate fluorescence demonstrates the higher solvent accessibility of hydrophobic surfaces on αB-crystallin. Circular dichroism studies show some tertiary structural changes in αA-crystallin above 50 °C. αB-crystallin, on the other hand, shows significant alteration of tertiary structure by 45 °C. Our study demonstrates that despite a high degree of sequence homology and their generally accepted structural similarity, αB-crystallin is much more sensitive to temperature-dependent structural perturbation than αA- or α-crystallin and shows differences in its chaperone-like properties. These differences appear to be relevant to temperature-dependent enhancement of chaperone-like activity of α-crystallin and indicate different roles for the two proteins both in α-crystallin heteroaggregate and as separate proteins under stress conditions.