Differential temperature-dependent chaperone-like activity of alpha A- andalpha B-crystallin homoaggregates

alpha-Crystallin, a heteromultimeric protein made up of alpha A- and alpha B-crystallins, functions as a molecular chaperone in preventing the aggrega tion of proteins. We have shown earlier that structural perturbation of alp ha-crystallin can enhance its chaperone-like activity severalfold. The two subunits of alpha-crystallin have extensive sequence homology and individua lly display chaperone-like activity. We have investigated the chaperone-lik e activity of alpha A- and alpha B-crystallin homoaggregates against therma l and nonthermal modes of aggregation. me find that, against a nonthermal m ode of aggregation, alpha B-crystallin shows significant protective ability even at subphysiological temperatures, at which alpha A-crystallin or hete romultimeric alpha-crystallin exhibit very little chaperone-like activity. Interestingly, differences in the protective ability of these homoaggregate s against the thermal aggregation of beta(L)-crystallin is negligible. To i nvestigate this differential behavior, me have monitored the temperature-de pendent structural changes in both the proteins using fluorescence and circ ular dichroism spectroscopy. Intrinsic tryptophan fluorescence quenching by acrylamide shows that the tryptophans in alpha B-crystallin are more acces sible than the lone tryptophan in alpha A-crystallin even at 25 degrees C. Protein-bound 8-anilinon-aphthalene-1-sulfonate fluorescence demonstrates t he higher solvent accessibility of hydrophobic surfaces on alpha B-crystall in. Circular dichroism studies show some tertiary structural changes in alp ha A-crystallin above 50 degrees C, alpha B-crystallin, on the other hand, shows significant alteration of tertiary structure by 45 degrees C, Our stu dy demonstrates that despite a high degree of sequence homology and their g enerally accepted structural similarity, alpha B-crystallin is much more se nsitive to temperature-dependent structural perturbation than alpha A- or a lpha-crystallin and shows differences in its chaperone-like properties. The se differences appear to be relevant to temperature-dependent enhancement o f chaperone-like activity of alpha-crystallin and indicate different roles for the two proteins both in alpha-crystallin heteroaggregate and as separa te proteins under stress conditions.