Zn2+ enhances the molecular chaperone function and stability of α-crystallin

Abstract

α-Crystallin, the major eye lens protein, is a molecular chaperone that plays a crucial role in the suppression of protein aggregation and thus in the long-term maintenance of lens transparency. Zinc is a micronutrient of the eye, but its molecular interaction with α-crystallin has not been studied in detail. In this paper, we present results of in vitro experiments that show bivalent zinc specifically interacts with α-crystallin with a dissociation constant in the submillimolar range (Kd ∼ 0.2-0.4 mM). We compared the effect of Zn2+ with those of Ca2+, Cu 2+, Mg2+, Cd2+, Pb2+, Ni 2+, Fe2+, and Co2+ at 1 mM on the structure and chaperoning ability of α-crystallin. An insulin aggregation assay showed that among the bivalent metal ions, only 1 mM Zn2+ improved the chaperone function of α-crystallin by 30% compared to that in the absence of bivalent metal ions. Addition of 1 mM Zn2+ increased the yield of α-crystallin-assisted refolding of urea-treated LDH to its native state from 33 to 38%, but other bivalent ions had little effect. The surface hydrophobicity of α-crystallin was increased by 50% due to the binding of Zn2+. In the presence of 1 mM Zn2+, the stability of α-crystallin was enhanced by 36 kJ/mol, and it became more resistant to tryptic cleavage. The implications of enhanced stability and molecular