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M. Posner, A. Smith, T.S. Vihtelic, G.J. Wistow; A Second Alpha B Crystallin From the Zebrafish (Danio rerio) . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3870.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We previously reported that zebrafish αB–crystallin is not constitutively expressed in nervous or muscular tissue. It also has reduced chaperone–like activity compared to its human ortholog. Here we report that zebrafish contains a second αB–crystallin that differs from the first in both expression pattern and chaperone activity. Methods: Messenger RNA from zebrafish lens was used to construct a cDNA library for the NEIBank project. Almost 4000 clones were sequenced and analyzed. A novel αB–crystallin (zαB2) was identified and the complete sequence was obtained. Reverse–transcriptase polymerase chain reaction (RT–PCR) using primers specific to zαB2 was used to determine its tissue specific expression. Recombinant zαB2 was expressed in E. coli and purified using ion exchange and size exclusion chromatography. The chaperone–like activity of the purified protein was assayed by measuring its ability to prevent the chemically induced aggregation of lactalbumin at temperatures between 25 and 40 degrees Celsius. Results: The complete zαB2 cDNA is 1,953bp in length and encodes a protein sequence of 165 amino acids that is 58.2% and 50.3% identical to human αB–crystallin and zebrafish αB1–crystallin, respectively. Semi–quantitative RT–PCR indicated expression in lens, heart, brain, skeletal muscle and liver. Expression was highest in lens and lowest in the liver. At 25 and 30 degrees Celsius zαB2 demonstrated greater chaperone–like activity than human αB–crystallin. At 35 and 40 degrees Celsius the human protein provided greater protection against aggregation. Conclusions: The zebrafish is now the first species known to express two different copies of αB–crystallin. The low level of identity between the two zebrafish αB–crystallins (50.3%) suggests that these two proteins are under different selection pressures. Divergent physiological roles for these two proteins would explain the differences in both expression and chaperone–like activity. It is possible that gene duplication has allowed a separation of functions, with zαB2 maintaining the widespread chaperone–like role of mammalian αB–crystallin, while zebrafish αB1–crystallin has acquired a more specialized function in the lens. The presence of two zebrafish αB–crystallins differing in expression and chaperone–like activity provides a unique model for studying the mechanisms behind α–crystallin function.
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