Mini-αB-crystallin:: A functional element of αB-crystallin with chaperone-like activity

alpha-Crystallin is a member of the family of small heat-shock proteins (sHSP) and is composed of two subunits, alpha A-crystallin and alpha B-crystallin, which exhibit molecular chaperone-like properties. In a previous study, we found that residues 70-88 in alpha A-crystallin can function like a molecular chaperone by preventing the aggregation and precipitation of denaturing substrate proteins [Sharma, K. K., et al. (2000) J. Biol. Chem. 275, 3767-3771]. In this study, we show that the complementary sequence in alpha B-crystallin, residues 73-92 (DRFSVNLDVKHFSPEELKVK), is the functional chaperone site of alpha B-crystallin. Like the mini-alpha A-crystallin chaperone, the mini-alpha B-crystallin chaperone interacts with 1,1'-bi(4-anilino) naphthalene-5,5'-disulphonic acid (bis-ANS) and also possesses significant beta-sheet and random coil structure. Deletion of four residues (DRFS) from the N-terminus or deletion of C-terminus LKVK residues from the 73-92 peptide abolishes the chaperone-like activity against denaturing alcohol dehydrogenase. However, removal of DRFS or HFSPEELKVK is necessary to completely abolish the antiaggregation property of the peptide in insulin reduction assay. Substitution of Asp at a site corresponding to D80 in alpha B-crystallin with D-Asp or beta-Asp results in a significant loss of chaperone-like activity. Kynurenine modification of His in the peptide abolishes the antiaggregation property of the mini-chaperone. These data suggest that the 73-92 region in alpha B-crystallin is one of the substrate binding sites during chaperone activity.