title: 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.affiliation: Centre for Cellular and Molecular Biology, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500 007, India.
authors.forename: Md Faiz
authors.initials: MF
authors.lastname: Ahmad
authors.forename: Devendra
authors.initials: D
authors.lastname: Singh
authors.forename: Aftab
authors.initials: A
authors.lastname: Taiyab
authors.forename: Tangirala
authors.initials: T
authors.lastname: Ramakrishna
authors.forename: Bakthisaran
authors.initials: B
authors.lastname: Raman
authors.forename: Ch Mohan
authors.initials: ChM
authors.lastname: Rao
mesh_terms: 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*
chemical_list: 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