Identification and characterization of a copper-binding site in αA-crystallin

Previous studies have shown that both αA- and αB-crystallins bind Cu2+, suppress the formation of Cu2+-mediated active oxygen species, and protect ascorbic acid from oxidation by Cu2+. αA- and αB-crystallins are small heat shock proteins with molecular chaperone activity. In this study we show that the mini-αA-crystallin, a peptide consisting of residues 71-88 of αA-crystallin, prevents copper-induced oxidation of ascorbic acid. Evaluation of binding of copper to mini-αA-crystallin showed that each molecule of mini-αA-crystallin binds one copper molecule. Isothermal titration calorimetry and nanospray mass spectrometry revealed dissociation constants of 10.72 and 9.9 μM, respectively. 1,1′-Bis(4-anilino)naphthalene-5,5′-disulfonic acid interaction with mini-αA-crystallin was reduced after binding of Cu2+, suggesting that the same amino acids interact with these two ligands. Circular dichroism spectrometry showed that copper binding to mini-αA-crystallin peptide affects its secondary structure. Substitution of the His residue in mini-αA-crystallin with Ala abolished the redox-suppression activity of the peptide. During the Cu2+-induced ascorbic acid oxidation assay, a deletion mutant, αAΔ70-77, showed about 75% loss of ascorbic acid protection compared to the wild-type αA-crystallin. This difference indicates that the 70-77 region is the primary Cu2+-binding site(s) in human native full-size αA-crystallin. The role of the chaperone site in Cu2+ binding in native αA-crystallin was confirmed by the significant loss of chaperone activity by the peptide after Cu2+ binding.