title: Calcium-induced decrease of the thermal stability and chaperone activity of alpha-crystallin.
abstract: Alpha-crystallin, one of the major proteins in the vertebrate eye lens, acts as a molecular chaperone, like the small heat-shock proteins, by protecting other proteins from denaturing under stress or high temperature conditions. alpha-Crystallin aggregation is involved in lens opacification, and high [Ca(2+)] has been associated with cataract formation, suggesting a role for this cation in the pathological process. We have investigated the effect of Ca(2+) on the thermal stability of alpha-crystallin by UV and Fourier-transform infrared (FTIR) spectroscopies. In both cases, a Ca(2+)-induced decrease in the midpoint of the thermal transition is detected. The presence of high [Ca(2+)] results also in a marked decrease of its chaperone activity in an insulin-aggregation assay. Furthermore, high Ca(2+) concentration decreases Cys reactivity towards a sulfhydryl reagent. The results obtained from the spectroscopic analysis, and confirmed by circular dichroism (CD) measurements, indicate that Ca(2+) decreases both secondary and tertiary-quaternary structure stability of alpha-crystallin. This process is accompanied by partial unfolding of the protein and a clear decrease in its chaperone activity. It is concluded that Ca(2+) alters the structural stability of alpha-crystallin, resulting in impaired chaperone function and a lower protective ability towards other lens proteins. Thus, alpha-crystallin aggregation facilitated by Ca(2+) would play a role in the progressive loss of transparency of the eye lens in the cataractogenic process.
authors.affiliation: Departament d'Enginyeria Química, Universitat Politècnica de Catalunya, Colom 1, 08222 Terrassa, Catalonia, Spain.
authors.forename: Luis J
authors.initials: LJ
authors.lastname: del Valle
authors.forename: Cristina
authors.initials: C
authors.lastname: Escribano
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Pérez
authors.forename: Pere
authors.initials: P
authors.lastname: Garriga
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002417:Cattle; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D006358:Hot Temperature; D007908:Lens, Crystalline / Q000737:chemistry; D018832:Molecular Chaperones / Q000737:chemistry / Q000187:drug effects; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D013053:Spectrophotometry; D017550:Spectroscopy, Fourier Transform Infrared; D013816:Thermodynamics; D038201:alpha-Crystallins / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism
chemical_list: D018832:Molecular Chaperones; D038201:alpha-Crystallins; D004228:Dithionitrobenzoic Acid; D002118:Calcium