van Boekel M A, de Lange F, de Grip W J, de Jong W W
Department of Biochemistry, University of Nijmegen, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands.
Biochim Biophys Acta. 1999 Sep 14;1434(1):114-23. doi: 10.1016/s0167-4838(99)00178-8.
The major lens protein alpha-crystallin is composed of two related types of subunits, alphaA- and alphaB-crystallin, of which the former is essentially lens-restricted, while the latter also occurs in various other tissues. With regard to their respective chaperone capacities, it has been reported that homomultimeric alphaA-crystallin complexes perform better in preventing thermal aggregation of proteins, while alphaB-crystallin complexes protect more efficiently against reduction-induced aggregation of proteins. Here, we demonstrate that this seeming discrepancy is solved when the reduction assay is performed at increasing temperatures: above 50 degrees C alphaA- performs better than alphaB-crystallin also in this assay. This inversion in protective capacity might relate to the greater resistance of alphaA-crystallin to heat denaturation. Infrared spectroscopy, however, revealed that this is not due to a higher thermostability of alphaA-crystallin's secondary structure. Also the accessible hydrophobic surfaces do not account for the chaperoning differences of alphaA- and alphaB-crystallin, since regardless of the experimental temperature alphaB-crystallin displays a higher hydrophobicity. It is argued that the greater complex stability of alphaA-crystallin, as evident upon urea denaturation, and the higher chaperone capacity of alphaB-crystallin at physiological temperatures reflect the evolutionary compromise to obtain an optimal functioning of heteromeric alpha-crystallin as a lens protein.
主要的晶状体蛋白α-晶状体蛋白由两种相关类型的亚基αA-晶状体蛋白和αB-晶状体蛋白组成，其中前者主要局限于晶状体，而后者也存在于各种其他组织中。关于它们各自的伴侣能力，据报道，同多聚体αA-晶状体蛋白复合物在防止蛋白质热聚集方面表现更好，而αB-晶状体蛋白复合物在保护蛋白质免受还原诱导的聚集方面更有效。在这里，我们证明，当在升高的温度下进行还原测定时，这种明显的差异得到了解决：在50摄氏度以上，αA-晶状体蛋白在该测定中也比αB-晶状体蛋白表现更好。这种保护能力的反转可能与αA-晶状体蛋白对热变性的更大抗性有关。然而，红外光谱显示，这并不是由于αA-晶状体蛋白二级结构具有更高的热稳定性。可及的疏水表面也不能解释αA-和αB-晶状体蛋白的伴侣差异，因为无论实验温度如何，αB-晶状体蛋白都表现出更高的疏水性。有人认为，αA-晶状体蛋白在尿素变性时表现出的更大的复合物稳定性，以及αB-晶状体蛋白在生理温度下更高的伴侣能力，反映了为使异源α-晶状体蛋白作为晶状体蛋白获得最佳功能而做出的进化妥协。
