Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.
Proc Natl Acad Sci U S A. 2010 Apr 6;107(14):6228-33. doi: 10.1073/pnas.1001855107. Epub 2010 Mar 22.
Topoisomerase IIIalpha (topo IIIalpha), a member of the conserved Type IA subfamily of topoisomerases, is required for the cell proliferation in mitotic tissues, but has a lesser effect on DNA endoreplication. The top3alpha gene encodes two forms of protein by utilizing alternative translation initiation sites: one (short form) with the nuclear localization signal only, exclusively localized in the nuclei, and the other (long form), retaining a mitochondrial import sequence at the N-terminus and the nuclear localization sequence at the C-terminus, localized primarily in the mitochondria, though with a small portion in the nuclei. Both forms of topo IIIalpha can rescue the viability of null mutants of top3alpha. No apparent defect is associated with the flies rescued by the long form; short-form-rescued flies (referred to as M1L), however, exhibit defects in fertilities. M1L females are sterile. They can lay eggs but with mitochondrial DNA (mtDNA) copy number and ATP content decreased by 20- and 2- to 3-fold, respectively, and they fail to hatch. Of the newly eclosed M1L males, 33% are completely sterile, whereas the rest have residual fertilities that are quickly lost in 6 days. The fertility loss of M1L males is caused by the disruption of the individualization complex and a progressive loss of germ-line stem cells. This study implicates topo IIIalpha in the maintenance of mtDNA and male germ-line stem cells, and thus is a causative candidate for genetic disorders associated with mtDNA depletion.
拓扑异构酶 IIIalpha（topo IIIalpha）是保守的 I 型拓扑异构酶亚家族的成员，是有丝分裂组织细胞增殖所必需的，但对 DNA 内复制的影响较小。top3alpha 基因通过利用替代翻译起始位点编码两种形式的蛋白质：一种（短形式）只有核定位信号，仅定位于核内，另一种（长形式）保留 N 端的线粒体导入序列和 C 端的核定位序列，主要定位于线粒体，但有一小部分定位于核内。两种形式的 topo IIIalpha 都可以拯救 top3alpha 缺失突变体的活力。由长形式拯救的果蝇没有明显的缺陷；然而，由短形式拯救的果蝇（称为 M1L）表现出生育力缺陷。M1L 雌蝇不育。它们可以产卵，但线粒体 DNA（mtDNA）拷贝数和 ATP 含量分别降低 20 倍和 2 至 3 倍，而且它们无法孵化。新羽化的 M1L 雄蝇中，有 33%完全不育，其余的有残余生育力，但在 6 天内迅速丧失。M1L 雄蝇的生育力丧失是由个体发生复合物的破坏和生殖干细胞的逐渐丧失引起的。这项研究表明 topo IIIalpha 参与维持 mtDNA 和雄性生殖干细胞，因此是与 mtDNA 耗竭相关的遗传疾病的候选致病基因。
