‘Crystallins’ are defined as the abundant water soluble proteins of the eye lens. The α‐, β‐ and γ‐crystallins are found in all vertebrate lenses, including those of the lamprey, and must have been recruited as lens proteins in the ancestral vertebrate. The two α‐crystallins are members of the small heat‐shock protein family. The six β‐crystallins and the eight (depending on species) γ‐crystallins are closely related in structure and probably evolved by gene duplication and fusion from a common ancestor. Expression of the ubiquitous crystallins is limited to the lens, with the exception of αB‐crystallin, which abounds in brain and muscle and is stress inducible. Additional and unrelated types of crystallins are phylogenetically restricted. These crystallins are identical to housekeeping proteins and usually encoded by the same genes.

Key Concepts:

  • Lens proteins must last the lifetime of the organism and must be highly stable.

  • The β‐ and γ‐crystallin gene families originated in the ancestral vertebrate.

  • The exact protein composition of a lens determines the optical properties and changes with an organism's habitat.

Keywords: crystallins; eye lens; small heat‐shock proteins; cataract; chaperones

Figure 1.

Structure of the α‐crystallin genes. Coding regions are indicated by wide rectangles, noncoding regions by narrow rectangles and introns by lines. Coding regions are drawn to scale, noncoding and intronic regions are not. The alternatively spliced αAins exon in the αA‐crystallin gene of some mammals is indicated by the stippled rectangle.

Figure 2.

(a) Structure of a β‐crystallin gene. Coding regions are indicated by wide rectangles, noncoding regions by narrow rectangles and introns by lines. The stippled rectangles in the first and second exons indicate that these can be coding or (partially) noncoding. Exons are drawn to scale except the second exon, whose length is variable. (b) Organisation of the two human β‐crystallin gene clusters on chromosome 22. The transcription units (arrows) and the intergenic regions (lines) are drawn to scale except where indicated by //. The direction of transcription is indicated by the direction of the arrow. Data compiled from information available on the NCBI website.

Figure 3.

(a) Structure of a γ‐crystallin gene. Coding regions are indicated by wide rectangles, noncoding regions by narrow rectangles and introns by lines. Exons are drawn to scale, introns are not as they vary in length. (b) Structure of the γN‐crystallin gene. Introns and exons are indicated as in (a). (c) Organisation of the human γ‐crystallin gene cluster on chromosome 2. The transcription units (arrows) and the intergenic regions (lines) are drawn to scale except where indicated by //. The direction of transcription is indicated by the direction of the arrow. Compiled using data from den Dunnen et al. and Meakin et al. and information available on the NCBI website.



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Further Reading

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Lamb TD, Collin SP and Pugh EN (2007) Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nature Reviews. Neuroscience 8: 960–976.

Land MF and Nilsson D‐E (2002) Animal Eyes. Oxford: Oxford University Press.

Wistow G, Peterson K, Gao J et al. (2008) NEIBank: genomics and bioinformatics resources for vision research. Molecular Vision 14: 1327–1337.

Web Links

NCBI website



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de Jong, Wilfried W, and Lubsen, Nicolette H(Mar 2011) Crystallins. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005906.pub2]