Voit R, Kuhn A, Sander E E, Grummt I
Division of Molecular Biology of the Cell II, German Cancer Research Center, Heidelberg.
Nucleic Acids Res. 1995 Jul 25;23(14):2593-9. doi: 10.1093/nar/23.14.2593.
The nucleolar factor UBF is phosphorylated by casein kinase II (CKII) at serine residues within the C-terminal acidic domain which is required for transcription activation. To investigate the biological significance of UBF modification, we have compared the trans-activating properties of cellular UBF and recombinant UBF expressed in Escherichia coli. Using a variety of assays we demonstrate that unphosphorylated UBF is transcriptionally inactive and has to be phosphorylated at multiple sites to stimulate transcription. Examination of cDNA mutants in which the serine residues within the C-terminal domain were altered by site-directed mutagenesis demonstrates that CKII-mediated phosphorylations of UBF contribute to, but are not sufficient for, transcriptional activation. Besides CKII, other cellular protein kinases phosphorylate UBF at distinct sites in a growth-dependent manner. The marked differences in the tryptic peptide maps of UBF from growing and serum-starved cells suggest that alterations in the degree of UBF phosphorylation may modulate rRNA synthetic activity in response to extracellular signals.
核仁因子UBF在其C末端酸性结构域内的丝氨酸残基处被酪蛋白激酶II（CKII）磷酸化，该结构域是转录激活所必需的。为了研究UBF修饰的生物学意义，我们比较了细胞UBF和在大肠杆菌中表达的重组UBF的反式激活特性。通过各种检测方法，我们证明未磷酸化的UBF转录无活性，必须在多个位点磷酸化才能刺激转录。对通过定点诱变改变C末端结构域内丝氨酸残基的cDNA突变体的检测表明，CKII介导的UBF磷酸化有助于转录激活，但并不充分。除了CKII外，其他细胞蛋白激酶以生长依赖的方式在不同位点使UBF磷酸化。来自生长细胞和血清饥饿细胞的UBF胰蛋白酶肽图谱的显著差异表明，UBF磷酸化程度的改变可能响应细胞外信号调节rRNA合成活性。
