Alpha crystallin

Alpha crystallin is a water-soluble structural protein found in the lens of the eye. Compared to the other crystallins, it elutes earlier from a gel filtration chromatography column.

The crystallins are water-soluble structural proteins that occur in high concentration in the cytoplasm of eye lens fiber cells. Four major groups of crystallin have been distinguished on the basis of size, charge and immunological properties: alpha-, beta- and gamma-crystallins occur in all vertebrate classes (though gamma-crystallins are low or absent in avian lenses); and delta-crystallin is found exclusively in reptiles and birds.

Alpha-crystallin occurs as large aggregates, comprising two types of related subunits (A and B) that are highly similar to the small (15-30kDa) heat shock proteins (HSPs), particularly in their C-terminal halves. The relationship between these families is one of classic gene duplication and divergence, from the small HSP family, allowing adaptation to novel functions. Divergence probably occurred prior to evolution of the eye lens, alpha-crystallin being found in small amounts in tissues outside the lens.

Alpha-crystallin has chaperone-like properties including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress. It has been suggested that these functions are important for the maintenance of lens transparency and the prevention of cataracts. This is supported by the observation that alpha-crystallin mutations show an association with cataract formation.

The N-terminal domain of alpha-crystallin is not necessary for dimerisation or chaperone activity, but appears to be required for the formation of higher order aggregates.

Human proteins containing this domain
CRYAA;    CRYAB;