Koretz J F, Doss E W, Reid G H
Center for Biophysics, Rensselaer Polytechnic Institute Science Center, Troy, New York 12180-3590, USA.
Biochem Biophys Res Commun. 1997 Feb 13;231(2):270-6. doi: 10.1006/bbrc.1997.6079.
alpha-crystallin, the major protein component of the crystallin lens of mammalian eyes, is found in vivo as two separate gene products. Both isoforms are expressed in different major tissues of the body, with the lens the only location where both are found together. Both sequences can be phosphorylated, though at different locations. Both exhibit a high sequence homology to the small heat shock proteins, and it has been shown that alpha-crystallin also resists heat-induced denaturation. Horwitz [J. Horowitz (1992) Proc. Natl. Acad. Sci. USA 89, 10449-10453] demonstrated that alpha-crystallin can exhibit chaperone-like protection against heat-induced turbidity increases, and it has been suggested that this may be an in vivo function as well. However, neither isoform, when purified, shows the same overall level of chaperone-like activity as the native species, except for one phosphorylated species [M. A. M. van Boekel, S. E. A. Hoogakker, J. J. Harding, and W. W. de Jong (1996) Ophthalmic Res. 28(Suppl. 1), 32-38]. Experiments designed to determine the factors leading to loss of chaperone-like activity indicate that strong ionic conditions, such as those used in isoform separation and/or the presence of divalent cations reduce the efficiency of this function and that the presence of EDTA fully restores it irrespective of prior treatment or buffer conditions. Heat stability is essentially preserved under all conditions. These results suggest that alpha-crystallin may serve primarily as a heat shock protein in vivo and that the chaperone-like function may be inhibited under physiological conditions.
α-晶体蛋白是哺乳动物眼睛晶状体中的主要蛋白质成分，在体内以两种不同的基因产物形式存在。这两种异构体在身体的不同主要组织中表达，晶状体是唯一同时发现这两种异构体的部位。两种序列都可以被磷酸化，不过磷酸化位点不同。它们与小分子热休克蛋白都具有高度的序列同源性，并且已经表明α-晶体蛋白也能抵抗热诱导的变性。霍维茨[J. 霍维茨（1992年），《美国国家科学院院刊》89卷，10449 - 10453页]证明α-晶体蛋白可以表现出类似伴侣蛋白的保护作用，防止热诱导的浊度增加，并且有人提出这也可能是一种体内功能。然而，除了一种磷酸化形式外，纯化后的两种异构体都没有显示出与天然形式相同的整体伴侣蛋白样活性水平[M. A. M. 范·博克尔、S. E. A. 胡加克、J. J. 哈丁和W. W. 德容（1996年），《眼科研究》28（增刊1），32 - 38页]。旨在确定导致伴侣蛋白样活性丧失的因素的实验表明，强离子条件，如用于异构体分离的条件和/或二价阳离子的存在会降低这种功能的效率，并且无论先前的处理或缓冲条件如何，EDTA的存在都能完全恢复其活性。在所有条件下，热稳定性基本保持不变。这些结果表明，α-晶体蛋白在体内可能主要作为一种热休克蛋白，并且类似伴侣蛋白的功能在生理条件下可能会受到抑制。
