UV-A-induced structural and functional changes in human lens deamidated αB-crystallin

PURPOSE: To determine comparative effects of ultraviolet (UV)-A irradiation on structural and functional properties of wild type (WT) αB-crystallin and its three deamidated mutant proteins (αB-Asn78Asp, αB-Asn146Asp, and αB-Asn78/146Asp). METHODS: Three deamidated mutants previously generated from recombinant WT αB-crystallin, using a site-specific mutagenesis procedure as previously described [32], were used. The WT αB-crystallin and its three deamidated species were exposed to UV-A light (320–400 nm) at intensities of 20 or 50 J/cm(2). The UV-A-unexposed and UV-A-exposed preparations were examined for their chaperone activity, and their activities were correlated with the UV-A-induced structural changes. The structural properties studied included dimerization and degradation, intrinsic tryptophan (Trp) fluorescence, ANS (8-anilino-1-naphthalenesulfate)-binding, far ultraviolet circular dichroism (UV-CD) spectral analysis, molecular sizes by dynamic light scattering, and oxidation of Trp and methionine (Met) residues. RESULTS: The WT αB-crystallin and its three deamidated mutant proteins showed enhanced dimerization to 40 kDa species and partial degradation with increasing doses during UV-A-exposure. Compared to the deamidation of asparagines (Asn) 78 residue to aspartic acid (Asp) or both Asn78 and Asn146 residues to Asp, the deamidation of Asn146 residue to Asp resulted in a greater loss of chaperone activity. The UV-A-induced loss of chaperone activity due to structural changes was studied. The ANS-binding data suggested that the αB-Asn146Asp mutant protein had a relatively compact structure and an increase in surface hydrophobic patches compared to WT and two other deamidated proteins. Similarly, UV-A-exposure altered the Trp microenvironment in the deamidated mutant proteins compared to the WT αB-crystallin. Far-UV CD spectral analyses showed almost no changes among WT and deamidated species on UV-A-exposure except that the αB-Asn146Asp mutant protein showed maximum changes in the random coil structure relative to WT αB-crystallin and two other deamidated proteins. The UV-A-exposure also resulted in the aggregation of WT and the three deamidated mutant proteins with species of greater mass compared to the non-UV-A exposed species. Among the four spots recovered after two-dimensional (2D)-gel electrophoresis from WT and the three deamidated species, the Met and Trp residues of αB-Asn146Asp mutant showed maximum oxidation after UV-A exposure, which might account for its greater loss in chaperone activity compared to WT αB-crystallin and two other deamidated species. CONCLUSIONS: After UV-A-exposure, the deamidated αB-Asn146Asp mutant protein showed a complete loss of chaperone activity compared to WT αB and αB-Asn78Asp and αB-Asn78/146Asp deamidated species. Apparently, this loss of chaperone activity was due to oxidative changes leading to its greater structural alteration compared to other αB-species.