Crystallin

Not to be confused with Crystalline.

In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure.^[1] It has also been identified in other places such as the heart, and in aggressive breast cancer tumors.^[2][3] Since it has been shown that lens injury may promote nerve regeneration,^[4] crystallin has been an area of neural research. So far, it has been demonstrated that crystallin β b2 (crybb2) may be a neurite promoting factor.^[5]

Function

The main function of crystallins at least in the lens 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 several metabolic and regulatory functions, both within the lens and in other parts of the body.^[6] More proteins containing βγ-crystallin domains have now been characterized as calcium binding proteins with Greek key motif as a novel calcium-binding motif.^[7]

Enzyme activity

Interestingly and perhaps excitingly from an evolutionary perspective, some crystallins are active enzymes, while others lack activity but show homology to other enzymes.^[8][9] The crystallins of different groups of organisms are related to a large number of different proteins, with those from birds and reptiles related to lactate dehydrogenase and argininosuccinate lyase, those of mammals to alcohol dehydrogenase and quinone reductase, and those of cephalopods to glutathione S-transferase and aldehyde dehydrogenase. Whether these crystallins are products of a fortuitous accident of evolution, in that these particular enzymes happened to be transparent and highly-soluble, or whether these diverse enzymatic activities are part of the protective machinery of the lens, is an active research topic.^[10] The recruitment of protein that originally evolved with one function to serve a second, unrelated function is an example of an exaptation.^[11]

Classification

Crystallins from a vertebrate eye lens are classified into three main 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 (such as CRYGC) are similar in sequence, structure and domains topology, and thus have been grouped together as a protein superfamily called βγ-Crystallins. The α-crystallin family and βγ-crystallins compose the major family 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.

External links

• Graw, J. (1997). "The crystallins: Genes, proteins and diseases". Biological chemistry 378 (11): 1331–1348. PMID 9426193.  edit
• MeSH Crystallins
• Lens Crystallin Crystal Structures by Christine Slingsby, Birkbeck College

References

1. ^ Jester, J. V. (2008). "Corneal crystallins and the development of cellular transparency". Seminars in Cell & Developmental Biology 19 (2): 82–93. doi:10.1016/j.semcdb.2007.09.015. PMC 2275913. PMID 17997336. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2275913.  edit
2. ^ Lutsch, G.; Vetter, R.; Offhauss, U.; Wieske, M.; Gröne, H. J.; Klemenz, R.; Schimke, I.; Stahl, J. et al. (1997). "Abundance and location of the small heat shock proteins HSP25 and alphaB-crystallin in rat and human heart". Circulation 96 (10): 3466–3476. PMID 9396443.  edit
3. ^ Moyano, J. V.; Evans, J. R.; Chen, F.; Lu, M.; Werner, M. E.; Yehiely, F.; Diaz, L. K.; Turbin, D. et al. (2005). " B-Crystallin is a novel oncoprotein that predicts poor clinical outcome in breast cancer". Journal of Clinical Investigation 116 (1): 261–270. doi:10.1172/JCI25888. PMC 1323258. PMID 16395408. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1323258.  edit
4. ^ Fischer, D.; Pavlidis, M.; Thanos, S. (2000). "Cataractogenic lens injury prevents traumatic ganglion cell death and promotes axonal regeneration both in vivo and in culture". Investigative ophthalmology & visual science 41 (12): 3943–3954. PMID 11053298.  edit
5. ^ Liedtke, T.; Schwamborn, J. C.; Schroer, U.; Thanos, S. (2007). "Elongation of Axons during Regeneration Involves Retinal Crystallin   b2 (crybb2)". Molecular & Cellular Proteomics 6 (5): 895–907. doi:10.1074/mcp.M600245-MCP200. PMID 17264069.  edit
6. ^ Bhat, S. P. (2003). "Crystallins, genes and cataract". Progress in drug research. Fortschritte der Arzneimittelforschung. Progres des recherches pharmaceutiques 60: 205–262. PMID 12790344.  edit
7. ^ betagamma-crystallin AND calcium - PubMed result
8. ^ Jörnvall, H.; Persson, B.; Du Bois, G. C.; Lavers, G. C.; Chen, J. H.; Gonzalez, P.; Rao, P. V.; Zigler Jr, J. S. (1993). "Zeta-crystallin versus other members of the alcohol dehydrogenase super-family. Variability as a functional characteristic". FEBS letters 322 (3): 240–244. doi:10.1016/0014-5793(93)81578-N. PMID 8486156.  edit
9. ^ Rao, P. V.; Krishna, C. M.; Zigler Jr, J. S. (1992). "Identification and characterization of the enzymatic activity of zeta-crystallin from guinea pig lens. A novel NADPH:quinone oxidoreductase". The Journal of biological chemistry 267 (1): 96–102. PMID 1370456.  edit
10. ^ Piatigorsky, J. (1993). "Puzzle of crystallin diversity in eye lenses". Developmental Dynamics 196 (4): 267–272. doi:10.1002/aja.1001960408. PMID 8219350.  edit
11. ^ Buss, D. M.; Haselton, M. G.; Shackelford, T. K.; Bleske, A. L.; Wakefield, J. C. (1998). "Adaptations, exaptations, and spandrels". The American psychologist 53 (5): 533–548. doi:10.1037/0003-066X.53.5.533. PMID 9612136.  edit