Abstract
Crystallins are the abundant, long-lived proteins of the eye lens. The major human crystallins belong to two different superfamilies: the small heat-shock proteins (α-crystallins) and the βγ-crystallins. During evolution, other proteins have sometimes been recruited as crystallins to modify the properties of the lens. In the developing human lens, the enzyme betaine-homocysteine methyltransferase serves such a role. Evolutionary modification has also resulted in loss of expression of some human crystallin genes or of specific splice forms. Crystallin organization is essential for lens transparency and mutations; even minor changes to surface residues can cause cataract and loss of vision.
Highlights
Crystallin is a functional term that originated as a description of the highly abundant soluble proteins of the ‘crystalline’ vertebrate eye lens [1,2]
The human genome contains genes corresponding to all the major crystallins that are expressed in mouse and most other mammals
The genes for human crystallins encode proteins which belong to two unrelated superfamilies, the α-crystallin/small heat-shock proteins (sHSP) superfamily and the βγ-crystallin superfamily
Summary
Crystallin is a functional term that originated as a description of the highly abundant soluble proteins of the ‘crystalline’ (clear) vertebrate eye lens [1,2]. In contrast to the genes of the Chr 2 cluster, CRYGS is well-conserved in evolution and is expressed at high levels in human lens, in adult in the cortical region [36,70]. CYRGGP In addition to the full-sized genes for γ-crystallins with orthologs in other mammals, the human genome contains an isolated fragment of sequence similar to that encoding γD motif 2 in CRYGD, apparently associated with a line located on Chr 2p [59]. This has been given a HUGO designation as CRYGGP, but it is not represented in most genome browsers. One of the more recently recruited enzyme crystallins, ζ-crystallin of the guinea pig (C. porcellus), illustrates dual promoter functionality quite nicely with an alternative promoter dependent on Pax and Maf elements, driving lensspecific expression [95]
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