Zhang S, Herrmann C, Grosse F
Department of Biochemistry and Department of Electron Microscopy and Molecular Cytology, Institute for Molecular Biotechnology, Beutenbergstrasse 11, D-07745 Jena, Germany.
J Cell Sci. 1999 Aug;112 ( Pt 16):2693-703. doi: 10.1242/jcs.112.16.2693.
Nuclear DNA helicase II (NDH II) is a highly conserved member of the DEXH superfamily of eukaryotic helicases, whose physiological role is still unclear. To explore the function of NDH II, we studied the intracellular distribution of NDH II of different mammalian species by immunofluorescence and compared these findings with the known role of the Drosophila homologue MLE that is involved in sex-specific gene dosage compensation. NDH II displayed an apparent nucleolar localization in murine cells, whereas in cells from all other mammalian species examined so far the protein was confined to the nucleoplasm and apparently excluded from the nucleoli. The nucleolar localization of mouse NDH II strongly suggests a role in ribosomal RNA biosynthesis. Immunoelectron microscopic studies revealed that the mouse NDH II was found at the dense fibrillar components of the nucleoli, and a significant percentage of NDH II molecules colocalized with the RNA polymerase I (Pol I) transcription factor UBF (upstream binding factor). Additionally, the nucleolar localization of NDH II coincided with a preferential immunolabeling pattern of nascent transcripts with bromouridine (BrUMP). Furthermore, mouse NDH II redistributed in mitosis in a manner highly correlated with Pol I activity. Conditions leading to the inhibition of Pol I activity in the interphase decreased the amount of NDH II in the nucleoli that diffused into the nucleoplasm and the cytosol. Contrary to the effect of inhibiting rRNA synthesis, treatment of mouse cells with the translation inhibitor cycloheximide did not compromise the nucleolar localization of murine NDH II.
核DNA解旋酶II（NDH II）是真核解旋酶DEXH超家族中高度保守的成员，其生理作用仍不清楚。为了探究NDH II的功能，我们通过免疫荧光研究了不同哺乳动物物种中NDH II的细胞内分布，并将这些结果与果蝇同源物MLE的已知作用进行了比较，MLE参与性别特异性基因剂量补偿。NDH II在鼠细胞中表现出明显的核仁定位，而在迄今为止检测的所有其他哺乳动物物种的细胞中，该蛋白局限于核质，明显排除在核仁之外。小鼠NDH II的核仁定位强烈表明其在核糖体RNA生物合成中起作用。免疫电子显微镜研究表明，小鼠NDH II存在于核仁的致密纤维成分中，并且相当一部分NDH II分子与RNA聚合酶I（Pol I）转录因子UBF（上游结合因子）共定位。此外，NDH II的核仁定位与新生转录本用溴尿苷（BrUMP）的优先免疫标记模式一致。此外，小鼠NDH II在有丝分裂中重新分布，其方式与Pol I活性高度相关。导致间期Pol I活性受到抑制的条件会减少扩散到核质和细胞质中的核仁中NDH II的量。与抑制rRNA合成的效果相反，用翻译抑制剂环己酰亚胺处理小鼠细胞不会损害小鼠NDH II的核仁定位。
