Alpha-crystallins composed of alpha-A (~19kD) and alpha-B (~19.2kDa) subunits, are major water-soluble proteins accounting for almost 50% of the total protein of the mammalian transparent eye lens and they are also found in a variety of other tissues (1). The alpha-crystallins have sequence homology as well as structural and functional similarities with the small heat shock proteins such as Hsp25 (2). The alpha-crystallins and Hsp25 have been shown to act as molecular chaperones by suppression of heat-induced aggregation of other proteins and are efficient inhibitors of elastase. The alpha-B subunit like Hsp25 was also found to be a substrate for protein cross-linking by tissue-type transglutaminase (2). The alpha-crystallins may be involved in metabolic pathways important for the development, maintenance, or pathology of the lens (3). They prevent destabilized proteins from precipitating out of solution by forming a soluble high-molecular-weight (HMW) complex (4). The bovine HMW protein form of the lens is composed mostly of alpha-crystallin in a highly aggregated state which has markedly reduced chaperone ability compared to alpha-crystallin. NMR spectroscopy of the naturally occurring HMW complex shows that the short C-terminal extension of alpha-B subunit in HMW has lost its flexibility whereas the alpha-A subunit extension is still flexible. These results suggest that depletion of alpha-crystallin with age as it is converted to HMW, could be one of the contributing factors to cataract formation in the lens (5).
Note:
May contain small amounts of alphaB crystallin.
Source:
Bovine eye lens
Molecular
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