Ueshima Shuhei, Nagata Kyosuke, Okuwaki Mitsuru
Faculty of Medicine, University of Tsukuba, Tsukuba, Japan.
Faculty of Medicine, University of Tsukuba, Tsukuba, Japan
Mol Cell Biol. 2017 Oct 27;37(22). doi: 10.1128/MCB.00218-17. Print 2017 Nov 15.
Upstream binding factor (UBF) is a member of the high-mobility group (HMG) box protein family, characterized by multiple HMG boxes and a C-terminal acidic region (AR). UBF is an essential transcription factor for rRNA genes and mediates the formation of transcriptionally active chromatin in the nucleolus. However, it remains unknown how UBF is specifically localized to the nucleolus. Here, we examined the molecular mechanisms that localize UBF to the nucleolus. We found that the first HMG box (HMG box 1), the linker region (LR), and the AR cooperatively regulate the nucleolar localization of UBF1. We demonstrated that the AR intramolecularly associates with and attenuates the DNA binding activity of HMG boxes and confers the structured DNA preference to HMG box 1. In contrast, the LR was found to serve as a nuclear localization signal and compete with HMG boxes to bind the AR, permitting nucleolar localization of UBF1. The LR sequence binds DNA and assists the stable chromatin binding of UBF. We also showed that the phosphorylation status of the AR does not clearly affect the localization of UBF1. Our results strongly suggest that associations of the AR with HMG boxes and the LR regulate UBF nucleolar localization.
上游结合因子（UBF）是高迁移率族（HMG）盒蛋白家族的成员，其特征在于多个HMG盒和一个C端酸性区域（AR）。UBF是rRNA基因的必需转录因子，并介导核仁中转录活性染色质的形成。然而，UBF如何特异性定位于核仁仍不清楚。在这里，我们研究了将UBF定位于核仁的分子机制。我们发现第一个HMG盒（HMG盒1）、连接区（LR）和AR协同调节UBF1的核仁定位。我们证明AR在分子内与HMG盒结合并减弱其DNA结合活性，并赋予HMG盒1对结构化DNA的偏好。相反，LR被发现作为核定位信号并与HMG盒竞争以结合AR，从而使UBF1定位于核仁。LR序列结合DNA并协助UBF与染色质稳定结合。我们还表明AR的磷酸化状态并没有明显影响UBF1的定位。我们的结果强烈表明AR与HMG盒和LR的结合调节UBF的核仁定位。
