Alpha B-crystallin exists as an independent protein in the heart and in the lens

Abstract

Alpha B-crystallin, a polypeptide of molecular mass 22 kDa, is considered to be one of two subunits (alpha A and alpha B) of the multimeric lens-specific protein, alpha-crystallin. Recent demonstrations of the extra-lenticular presence of alpha B-crystallin have suggested that outside of the lens, this polypeptide may have functions independent of alpha A. Within the lens however, as part of the protein alpha-crystallin, its function is assumed to be structural. In an effort to investigate the functional status of alpha B-crystallin in the lens, we have characterized this polypeptide in the rat heart and the human lens. Unequivocal identity of alpha B-crystallin in the rat heart and the rat lens was established by the sequence analyses of the respective cDNA clones. Size exclusion chromatography (FPLC) and immunoblotting showed that in the rat heart, alpha B-crystallin exists as an aggregate of 300-400 kDa average molecular mass, similar to that of purified alpha B-crystallin isolated from bovine lens. Interestingly, analysis of the human lens proteins by immunoblotting showed that, with age, unlike alpha A-crystallin, the alpha B subunit remains detectable in the soluble fractions derived from normal lenses as old as 82 years. Importantly, the average molecular mass of the alpha B subunit in the soluble fractions prepared from 60-80-year-old human lens nuclei was also found to be 300-400 kDa. These data lead to the conclusion that alpha B-crystallin may exist as an independent protein not only in non-lens tissues (e.g. heart) but in the lens as well.