Ghahghaei A, Rekas A, Carver J A, Augusteyn R C
Biology Department, University of Sistan and Baluchestan, Zahedan, Iran.
Mol Vis. 2009 Nov 20;15:2411-20.
alpha-Crystallin is the major protein of the mammalian lens where it contributes to the refractive properties needed for vision and possibly to the stability of the tissue. The aim of this study was to determine whether the properties of alpha-crystallin have changed during the course of evolution.
Dogfish alpha-crystallin, which appeared over 420 million years ago, has been contrasted with bovine alpha-crystallin, which emerged around 160 million years later, by comparing their sizes, the microenvironments of their cysteine and tryptophan residues, their chaperone-like activities and the flexibility of their COOH-terminal extensions.
Dogfish alpha-crystallin consists of alphaA- and alphaB-polypeptides, in a 1:5 ratio, and has a molecular mass of around 400 kDa. By contrast, the bovine protein is around 600-800 kDa in mass and has a 3:1 subunit ratio. Cysteine residues in the proteins were equally accessible to reaction with 5,5'-dithiobis-(2-nitrobenzoic acid). Quenching of fluorescence with acrylamide indicated tryptophan residues in the two proteins were in similar environments. The chaperone activity of dogfish alpha-crystallin was comparable to that of bovine alpha-crystallin in preventing the heat-induced precipitation of beta(L)-crystallin but the dogfish protein was three times more effective at preventing insulin precipitation after reduction at 37 C. (1)H nuclear magnetic resonance spectroscopic studies showed that the last 17 amino acids of the dogfish alphaB polypeptide (V162-K178) have great conformational flexibility, are highly exposed to solvent and adopt little ordered conformation. This is comparable to, but slightly longer in length, than the COOH-terminal extension observed in mammalian alpha-crystallins.
The structure and properties of alpha-crystallin have changed relatively little during the evolutionary period from the emergence of sharks and mammals.
α-晶体蛋白是哺乳动物晶状体的主要蛋白质，它有助于形成视觉所需的屈光特性，并可能对组织的稳定性有作用。本研究的目的是确定α-晶体蛋白的特性在进化过程中是否发生了变化。
通过比较鲨鱼α-晶体蛋白（出现在4.2亿多年前）和牛α-晶体蛋白（大约在1.6亿年后出现）的大小、半胱氨酸和色氨酸残基的微环境、它们的伴侣样活性以及COOH末端延伸的灵活性，对两者进行了对比。
鲨鱼α-晶体蛋白由αA和αB多肽组成，比例为1:5，分子量约为400 kDa。相比之下，牛的这种蛋白质质量约为600 - 800 kDa，亚基比例为3:1。蛋白质中的半胱氨酸残基与5,5'-二硫代双（2-硝基苯甲酸）反应的可及性相同。用丙烯酰胺淬灭荧光表明两种蛋白质中的色氨酸残基处于相似的环境中。鲨鱼α-晶体蛋白在防止β(L)-晶体蛋白热诱导沉淀方面的伴侣活性与牛α-晶体蛋白相当，但在37℃还原后防止胰岛素沉淀方面，鲨鱼蛋白的效果是牛蛋白的三倍。氢核磁共振光谱研究表明，鲨鱼αB多肽的最后17个氨基酸（V162 - K178）具有很大的构象灵活性，高度暴露于溶剂中，几乎没有有序构象。这与在哺乳动物α-晶体蛋白中观察到的COOH末端延伸相当，但长度略长。
从鲨鱼和哺乳动物出现以来的进化时期，α-晶体蛋白的结构和特性变化相对较小。
