Selective Cu2+ binding, redox silencing, and cytoprotective effects of the small heat shock proteins alphaA- and alphaB-crystallin.
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.
Ahmad
Md Faiz
MF
Centre for Cellular and Molecular Biology, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500 007, India.
Singh
Devendra
D
Taiyab
Aftab
A
Ramakrishna
Tangirala
T
Raman
Bakthisaran
B
Rao
Ch Mohan
ChM
D000975:Antioxidants / Q000378:metabolism; D001205:Ascorbic Acid / Q000378:metabolism; D001665:Binding Sites; D002151:Calorimetry; D002460:Cell Line; D003300:Copper / Q000737:chemistry / Q000378:metabolism*; D050888:Heat-Shock Proteins, Small / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D010084:Oxidation-Reduction; D018384:Oxidative Stress; D017382:Reactive Oxygen Species / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D038202:alpha-Crystallin A Chain / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D038203:alpha-Crystallin B Chain / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D000975:Antioxidants; D050888:Heat-Shock Proteins, Small; D017382:Reactive Oxygen Species; D038202:alpha-Crystallin A Chain; D038203:alpha-Crystallin B Chain; D003300:Copper; D001205:Ascorbic Acid