Lakshmipathy U, Campbell C
University of Minnesota Medical School, Department of Pharmacology, 6-120 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA.
Nucleic Acids Res. 2000 Oct 15;28(20):3880-6. doi: 10.1093/nar/28.20.3880.
Hamster EM9 cells, which lack Xrcc1 protein, have reduced levels of DNA ligase III and are defective in nuclear base excision repair. The Xrcc1 protein stabilizes DNA ligase III and may even play a direct role in catalyzing base excision repair. Since DNA ligase III is also thought to function in mitochondrial base excision repair, it seemed likely that mitochondrial DNA ligase III function would also be dependent upon Xrcc1. However, several lines of evidence indicate that this is not the case. First, western blot analysis failed to detect Xrcc1 protein in mitochondrial extracts. Second, DNA ligase III levels present in mitochondrial protein extracts from EM9 cells were indistinguishable from those seen in similar extracts from wild-type (AA8) cells. Third, the mitochondrial DNA content of both cell lines was identical. Fourth, EM9 cells displayed no defect in their ability to repair spontaneous mitochondrial DNA damage. Fifth, while EM9 cells were far more sensitive to the cytotoxic effects of ionizing radiation due to a defect in nuclear DNA repair, there was no apparent difference in the ability of EM9 and AA8 cells to restore their mitochondrial DNA to pre-irradiation levels. Thus, mitochondrial DNA ligase III function is independent of the Xrcc1 protein.
缺乏Xrcc1蛋白的仓鼠EM9细胞，其DNA连接酶III水平降低，并且在核碱基切除修复方面存在缺陷。Xrcc1蛋白可使DNA连接酶III稳定，甚至可能在催化碱基切除修复中发挥直接作用。由于DNA连接酶III也被认为在线粒体碱基切除修复中起作用，因此线粒体DNA连接酶III的功能似乎也依赖于Xrcc1。然而，几条证据表明情况并非如此。首先，蛋白质免疫印迹分析未能在线粒体提取物中检测到Xrcc1蛋白。其次，EM9细胞线粒体蛋白提取物中存在的DNA连接酶III水平与野生型（AA8）细胞类似提取物中的水平没有区别。第三，两种细胞系的线粒体DNA含量相同。第四，EM9细胞在修复自发性线粒体DNA损伤的能力方面没有缺陷。第五，虽然由于核DNA修复缺陷，EM9细胞对电离辐射的细胞毒性作用更为敏感，但EM9细胞和AA8细胞将线粒体DNA恢复到辐射前水平的能力没有明显差异。因此，线粒体DNA连接酶III的功能独立于Xrcc1蛋白。
