αB‐crystallin exists as an independent protein in the heart and in the lens

Abstract

αB‐crystallin, a polypeptide of molecular mass 22 kDa, is considered to be one of two subunits (αA and αB) of the multimeric lens‐specific protein, α‐crystallin. Recent demonstrations of the extralenticular presence of αB‐crystallin have suggested that outside of the lens, this polypeptide may have functions independent of αA. Within the lens however, as part of the protein α‐crystallin, its function is assumed to be structural. In an effort to investigate the functional status of αB‐crystallin in the lens, we have characterized this polypeptide in the rat heart and the human lens. Unequivocal identity of αB‐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, αB‐crystallin exists as an aggregate of 300–400 kDa average molecular mass, similar to that of purified αB‐crystallin isolated from bovine lens. Interestingly, analysis of the human lens proteins by immunoblotting showed that, with age, unlike αA‐crystallin, the αB subunit remains detectable in the soluble fractions derived from normal lenses as old as 82 years. Importantly, the average molecular mass of the αB 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 αB‐crystallin may exist as an independent protein not only in non‐lens tissues (e.g. heart) but in the lens as well.