Distinct roles of αA- and αB-crystallins under thermal and UV stresses

Abstract

α-Crystallin, a major protein of all vertebrate lenses, consists of two subunits, αA and αB, which form polymeric aggregates with an average molecular mass of about 800 kDa. In this study, we have employed various biophysical methods to study aggregate sizes and conformational properties of purified αA, αB subunits, and cloned recombinant αB subunit. From far- and near-UV CD spectra, native α-, αA-, αB-, and recombinant αB-crystallins from porcine lenses all show similar β-sheet conformation to that from bovine and human lenses as reported previously. By means of gel-filtration chromatography and dynamic light scattering, we have found that the molecular sizes of all four crystallin aggregates are polydispersedly distributed in the following order of aggregate sizes, i.e., native α>αA>αB≈ recombinant αB. To investigate the structural and functional relationships, we have also compared the chaperone activities of all four α-crystallin aggregates at different temperatures. From the results of chaperone-activity assays, ANS (8-anilinonaphthalene-1-sulfonic acid) binding and thermal stability studies, there appeared to be at least two factors playing major roles in the chaperone-like activity of these lens proteins: one is the hydrophobicity of the exposed protein surface and the other is the structural stability associated with each protein. We showed that αA-crystallin is a better chaperone to protect γ-crystallin against UV irradiation than αB-crystallin, in contrast to the observation that αB is generally a better chaperoning protein than αA for enzyme protective assays at physiological temperatures.