α-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed αA- and αB-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate.

The idea of organ-specificity had to be abandoned when it was discovered that α-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover αB-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases.

An earlier excitement arose when it was found that αB-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of α-crystallin could be demonstrated unequivocally.

All these unexpected findings make α-crystallin a subject of great interest far beyond the lens research field.

A survey of structural data about α-crystallin is presented here. Since α-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, α-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about α-crystallin are illuminative for the latter proteins as well.

This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of α-crystallin.