Takao M, Aburatani H, Kobayashi K, Yasui A
Department of Molecular Genetics, Institute of Development, Aging and Cancer, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-77, Japan.
Nucleic Acids Res. 1998 Jun 15;26(12):2917-22. doi: 10.1093/nar/26.12.2917.
Oxidative damage to mitochondrial DNA has been implicated in human degenerative diseases and aging. Although removal of oxidative lesions from mitochondrial DNA occurs, the responsible DNA repair enzymes are poorly understood. By expressing the epitope-tagged proteins in COS-7 cells, we examined subcellular localizations of gene products of human DNA glycosylases: hOGG1, hMYH and hNTH1. A gene encoding for hOGG1 which excises 7,8-dihydro-8-oxoguanine (8-oxoG) from DNA generates four isoforms by alternative splicing (types 1a, 1b, 1c and 2). Three tagged isoforms (types 1b, 1c and 2) were localized in the mitochondria. Type 1a protein, which exclusively contains a putative nuclear localization signal, was sorted to the nucleus and lesser amount to the mitochondria. hMYH, a human homolog gene product of Escherichia coli mutY was mainly transported into the mitochondria. hNTH1 protein excising several pyrimidine lesions was transported into both the nucleus and mitochondria. In contrast to the three DNA glycosylases, translocation of the human major AP endonuclease (hAPE) into the mitochondria was hardly observed in COS-7 cells. These results suggest that the previously observed removal of oxidative base lesions in mitochondrial DNA is initiated by the above DNA glycosylases.
线粒体DNA的氧化损伤与人类退行性疾病和衰老有关。尽管线粒体DNA中的氧化损伤会被修复，但人们对负责该修复过程的DNA修复酶了解甚少。通过在COS-7细胞中表达带有表位标签的蛋白质，我们检测了人类DNA糖基化酶hOGG1、hMYH和hNTH1基因产物的亚细胞定位。编码hOGG1的基因可从DNA中切除7,8-二氢-8-氧代鸟嘌呤（8-氧代鸟嘌呤），通过可变剪接产生四种异构体（1a、1b、1c和2型）。三种带有标签的异构体（1b、1c和2型）定位于线粒体中。仅含有一个假定核定位信号的1a型蛋白被分选到细胞核中，少量进入线粒体。hMYH是大肠杆菌mutY的人类同源基因产物，主要转运到线粒体中。切除多个嘧啶损伤的hNTH1蛋白则被转运到细胞核和线粒体中。与这三种DNA糖基化酶不同，在COS-7细胞中几乎未观察到人类主要AP核酸内切酶（hAPE）转运到线粒体中。这些结果表明，先前观察到的线粒体DNA中氧化碱基损伤的修复是由上述DNA糖基化酶启动的。
