Selective Cu2+ binding, redox silencing, and cytoprotective effects of the small heat shock proteins alphaA- and alphaB-crystallin.

Abstract

Oxidative stress and Cu(2+) have been implicated in several neurodegenerative diseases and in cataract. Oxidative stress, as well as Cu(2+), is also known to induce the expression of the small heat shock proteins alpha-crystallins. However, the role of alpha-crystallins in oxidative stress and in Cu(2+)-mediated processes is not clearly understood. We demonstrate using fluorescence and isothermal titration calorimetry that alpha-crystallins (alphaA- and alphaB-crystallin and its phosphorylation mimic, 3DalphaB-crystallin) bind Cu(2+) with close to picomolar range affinity. The presence of other tested divalent cations such as Zn(2+), Mg(2+), and Ca(2+) does not affect Cu(2+) binding, indicating selectivity of the Cu(2+)-binding site(s) in alpha-crystallins. Cu(2+) binding induces structural changes and increase in the hydrodynamic radii of alpha-crystallins. Cu(2+) binding increases the stability of alpha-crystallins towards guanidinium chloride-induced unfolding. Chaperone activity of alphaA-crystallin increases significantly upon Cu(2+) binding. Alpha-crystallins rescue amyloid beta peptide, Abeta(1-40), from Cu(2+)-induced aggregation in vitro. Alpha-crystallins inhibit Cu(2+)-induced oxidation of ascorbate and, hence, prevent the generation of reactive oxygen species. Interestingly, alpha-synuclein, a Cu(2+)-binding protein, does not inhibit this oxidation process significantly. We find that the Cu(2+)-sequestering (or redox-silencing) property of alpha-crystallins confers cytoprotection. To the best of our knowledge, this is the first study to reveal high affinity (close to picomolar) for Cu(2+) binding and redox silencing of Cu(2+) by any heat shock protein. Thus, our study ascribes a novel functional role to alpha-crystallins in Cu(2+) homeostasis and helps in understanding their protective role in neurodegenerative diseases and cataract.

• Authors: Md Faiz Ahmad, Devendra Singh, Aftab Taiyab, Tangirala Ramakrishna, Bakthisaran Raman, Ch Mohan Rao
• Journal: Journal of molecular biology (J Mol Biol) Vol. 382 Issue 3 Pg. 812-24 (Oct 10 2008) ISSN: 1089-8638 [Electronic] Netherlands
• PMID: 18692065 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Antioxidants
• Heat-Shock Proteins, Small
• Reactive Oxygen Species
• alpha-Crystallin A Chain
• alpha-Crystallin B Chain
• Copper
• Ascorbic Acid

Topics

• Antioxidants (metabolism)
• Ascorbic Acid (metabolism)
• Binding Sites
• Calorimetry
• Cell Line
• Copper (chemistry, metabolism)
• Heat-Shock Proteins, Small (chemistry, genetics, metabolism)
• Humans
• Oxidation-Reduction
• Oxidative Stress
• Reactive Oxygen Species (chemistry, metabolism)
• Spectrometry, Fluorescence
• alpha-Crystallin A Chain (chemistry, genetics, metabolism)
• alpha-Crystallin B Chain (chemistry, genetics, metabolism)