αB-crystalin, a small heat shock protein and a component of α-crystalin, is a molecular chaperone playing an important role in preventing the formation of cataracts. It has been reported that His18 is an important site for Cu2+ to bind with to form a stable metal complex and thus to enhance this chaperone-like activity of human αB-crystalin. In this work, we used site-directed mutagenesis to clone and express H18G rat lens αB-crystalin in order to investigate the role of His18 in chaperoning activity.We found that 1 mMof Cu2+, or Zn2+, rather than Mg2+, significantly enhanced the chaperone-like activity of wild type αBcrystalin. Whereas, it is Zn2+ and Mg2+, not Cu2+, that significantly reduced this activity of H18G αBcrystalin. In the absence of cation, H18G showed better activity compared to the wild type αB-crystalin. ANS fluorescence measurement showed there was no linear relationship between chaperone-like activity and surface hydrophobicity, indicating that surface hydrophobicity is not a prerequisite for chaperone-like activity. An HPLC size-exclusion chromatography study showed that in the presence of metal ions, wild type αB-crystalin tended to aggregate via dissociation and re-association into a high molecular aggregate with a molecular weight higher than 1400 kDa and then precipitated, suggesting that the presence of metal ions is a factor leading to the formation of cataracts. Both the near and far UV-CD spectra suggested that the wild type αB-crystalin reflected more β-sheet structural characteristics; whereas the H18G reflected more random coil characteristics. The H18G induced structural alterations as to developmore random coil characteristics and more micro-environmental changes around the tryptophan residues. This work suggested that His18 may not be a crucial binding site for Cu2+, but rather that it may be an important binding site for Zn2+ in terms of chaperone-like activity and the process of metal induced self-aggregation is prerequisite for chaperone-like activity to occur.
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