In biology, a crystallin is a water-soluble structural protein found in the lens of the eye, accounting for the transparency of the structure. It has also been identified in other places such as the heart  and aggressive breast cancer tumors . Since it has been shown that lens injury may promote nerve regeneration, crystallin has been an area of neural research. So far, it has been demonstrated that crystallin ß b2 (crybb2) may be a neurite promoting factor .
The main function of crystallins at least in the lens (anatomy) of the eye is probably to increase the refractive index while not obstructing light. However, this is not their only function. It is becoming increasingly clear that crystallins may have a several metabolic and regulatory functions, both within the lens and in other parts of the body .
Interestingly and perhaps excitingly from an evolutionary perspective, some crystallins are active enzymes. A number of crystallins are related to the serine and tyrosine proteases, and others to quinone oxidoreductases. Whether these crystallins are products of a happy accident of evolution, in that these particular enzymes happened to be transparent, or whether the enzymatic activity is a part of the protective machinery of the lens, is an active research topic.
Crystallins from a vertebrate eye lens are classified into three types: alpha, beta and gamma crystallins. These distinctions are based on the order in which they elute from a gel filtration chromatography column. These are also called ubiquitous crystallins. Beta- and gamma-crystallins are similar in sequence, structure and domains topology, and thus have been grouped together as a protein superfamily called βγ-Crystallins. The α-crystallin superfamily and βγ-crystallins compose the major superfamily of proteins present in the crystalline lens.
In addition to these crystallins there are other taxon-specific crystallins which are only found in the lens of some organisms; these include delta, epsilon, tau, and iota-crystallins. For example, alpha, beta, and delta crystallins are found in avian and reptilian lenses, and the alpha, beta, and gamma families are found in the lenses of all other vertebrates.
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