Save Sight Institute, Macquarie Street, Sydney, New South Wales 2001, Australia.
J Biol Chem. 2012 Nov 9;287(46):39012-20. doi: 10.1074/jbc.M112.391565. Epub 2012 Sep 20.
Long-lived proteins exist in a number of tissues in the human body; however, little is known about the reactions involved in their degradation over time. Lens proteins, which do not turn over, provide a useful system to examine such processes. Using a combination of Western blotting and proteomic methodology, age-related changes to a major protein, γS-crystallin, were studied. By teenage years, insoluble intact γS-crystallin was detected, indicative of protein denaturation. This was not the only change, however, because blots revealed evidence of significant cross-linking as well as cleavage of γS-crystallin in all adult lenses. Cleavage at a serine residue near the C terminus was a major reaction that caused the release of a 12-residue peptide, SPAVQSFRRIVE, which bound tightly to lens cell membranes. Several other crystallin-derived peptides with double basic residues also lodged in the cell membrane fraction. Model studies showed that once cleaved from γS-crystallin, SPAVQSFRRIVE adopts a markedly different shape from that in the intact protein. Further, the acquired helical conformation may explain why the peptide seems to affect water permeability. This observation may help explain the changes to cell membranes known to be associated with aging in human lenses. Age-related cleavage of long-lived proteins may therefore yield peptides with untoward biological activity.
人体内的许多组织中都存在长寿蛋白；然而，人们对其随时间降解过程中涉及的反应知之甚少。晶状体蛋白不会发生周转，为研究这些过程提供了一个有用的系统。本研究采用 Western blot 和蛋白质组学方法相结合，研究了与年龄相关的主要蛋白质 γS-晶体蛋白的变化。在青少年时期，就已经检测到不溶性完整的 γS-晶体蛋白，表明蛋白质变性。但这并不是唯一的变化，因为免疫印迹显示出明显的交联证据，以及所有成年晶状体中 γS-晶体蛋白的裂解。在 C 端附近丝氨酸残基的裂解是主要反应，导致释放出一个 12 个残基的肽，SPAVQSFRRIVE，它与晶状体细胞膜紧密结合。其他几个带有双碱性残基的晶体衍生肽也位于细胞膜部分。模型研究表明，一旦从 γS-晶体蛋白中裂解出来，SPAVQSFRRIVE 就会从完整蛋白中呈现出明显不同的形状。此外，获得的螺旋构象可能解释了为什么该肽似乎会影响水通透性。这一观察结果可能有助于解释与人类晶状体衰老相关的细胞膜变化。因此，长寿蛋白的与年龄相关的裂解可能会产生具有不良生物学活性的肽。
