The Sequence of Human B1-Crystallin cDNA Allows Mass Spectrometric Detection of B1 Protein Missing Portions of Its N-terminal Extension

Abstract The sequence of human βB1-crystallin cDNA encoded a protein of 251 amino acids in length. Mass spectrometric analysis of intact βB1 from young human lens confirmed the deduced amino acid sequence. Lenses of human donors newborn to 27 years of age also contained partially degraded forms of βB1 missing 15, 33, 34, 35, 36, 39, 40, and 41 amino acid residues from their N-terminal extensions. The similarity of the cleavage site between residues 15 and 16 in human βB1 to the cleavage occurring in bovine βB1 suggested that lenses of both species may contain a similar proteolytic activity. The remaining cleavage sites occurring in human βB1 did not closely match those occurring in other species, possibly due to the widely divergent amino acid sequence of the N-terminal extension of βB1 amoung species. Results from animal models suggest that cleavage of the N-terminal extension of βB1-crystallin could enhance protein insolubilization and cataract in lens. However, the presence of partially degraded βB1-crystallins in both water-soluble and water-insoluble fractions of lenses of young donors suggested that the rate that proteolyzed βB1-crystallins become water-insoluble is relatively slow in humans.