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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 [1] and aggressive breast cancer tumors [2]. Since it has been shown that lens injury may promote nerve regeneration[3], 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 [4].
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 a several metabolic and regulatory functions, both within the lens and in other parts of the body [5].
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.[6][7] 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 happy 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.[8] The recruitment of protein that originally evolved with one function to serve a second, unrelated function is an example of an exaptation.[9]
Classification
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 ß?-Crystallins. The a-crystallin superfamily and ß?-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.
External links
References
- ^ LUTSCH, G., R. VETTER, U. OFFHAUS, M. WIESKE, H.-J. GRÖNE, R. KLEMENZ, I. SCHIMKE, J. STAHL & R. BENNDORF (1997). "Abundance and location of the small heat shock protein HSP25 and aB-crystallin in rat and human heart". Circulation 96 (11): 3466–3476. PMID 9396443.
- ^ Jose V. Moyano, Joseph R. Evans, Feng Chen, Meiling Lu, Michael E. Werner, Fruma Yehiely, Leslie K. Diaz, Dmitry Turbin, Gamze Karaca, Elizabeth Wiley, Torsten O. Nielsen, Charles M. Perou and Vincent L. Cryns (2006). "aB-Crystallin is a novel oncoprotein that predicts poor clinical outcome in breast cancer". Journal of Clinical Investigation 116: 261–270. doi:10.1172/JCI25888.
- ^ Dietmar Fischer, Mitrofanis Pavlidis and Solon Thanos (2000). "Cataractogenic Lens Injury Prevents Traumatic Ganglion Cell Death and Promotes Axonal Regeneration Both In Vivo and in Culture". Investigative Ophthalmology and Visual Science 41: 3943–3954. PMID 11053298.
- ^ Thomas Liedtke, Jens Christian Schwamborn, Uwe Schröer and Solon Thanos (2007). "Elongation of Axons during Regeneration Involves Retinal Crystallin ß b2 (crybb2)". Molecular & Cellular Proteomics 6: 895–907. doi:10.1074/mcp.M600245-MCP200. PMID 17264069.
- ^ Bhat SP (2003). "Crystallins, genes and cataract". Progress in drug research. Fortschritte der Arzneimittelforschung. Progrès des recherches pharmaceutiques 60: 205–62. PMID 12790344.
- ^ H. Jörnvall, B. Persson, G. C. Du Bois, G. C. Lavers, J. H. Chen, P. Gonzalez, P. V. Rao and J. S. Zigler Jr. (1993). "?-Crystallin versus other members of the alcohol dehydrogenase super-family". Federation of European Biochemical Societies Letters 322 (3): 240–244. PMID 8486156.
- ^ P. Vasantha Rao, C. Murali Krishna and J. Samuel Zigler, Jr. (1992). "Identification and Characterization of the Enzymatic Activity of ?-Crystallin from Guinea Pig Lens". The Journal of Biological Chemistry 267 (1): 96–102. PMID 1370456.
- ^ Piatigorsky J (April 1993). "Puzzle of crystallin diversity in eye lenses". Dev. Dyn. 196 (4): 267–72. doi:10.1002/aja.1001960408. PMID 8219350.
- ^ Buss, David M., Martie G. Haselton, Todd K. Shackelford, et al. (1998) “Adaptations, Exaptations, and Spandrels,” American Psychologist, 53 (May):533-548.
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