Sarshad Aishe A, Corcoran Martin, Al-Muzzaini Bader, Borgonovo-Brandter Laura, Von Euler Anne, Lamont Douglas, Visa Neus, Percipalle Piergiorgio
Department of Cell and Molecular Biology, Karolinska Institute, Stockholm, Sweden.
Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.
PLoS Genet. 2014 Jun 5;10(6):e1004390. doi: 10.1371/journal.pgen.1004390. eCollection 2014 Jun.
Nuclear myosin 1c (NM1) mediates RNA polymerase I (pol I) transcription activation and cell cycle progression by facilitating PCAF-mediated H3K9 acetylation, but the molecular mechanism by which NM1 is regulated remains unclear. Here, we report that at early G1 the glycogen synthase kinase (GSK) 3β phosphorylates and stabilizes NM1, allowing for NM1 association with the chromatin. Genomic analysis by ChIP-Seq showed that this mechanism occurs on the rDNA as active GSK3β selectively occupies the gene. ChIP assays and transmission electron microscopy in GSK3β-/- mouse embryonic fibroblasts indicated that at G1 rRNA synthesis is suppressed due to decreased H3K9 acetylation leading to a chromatin state incompatible with transcription. We found that GSK3β directly phosphorylates the endogenous NM1 on a single serine residue (Ser-1020) located within the NM1 C-terminus. In G1 this phosphorylation event stabilizes NM1 and prevents NM1 polyubiquitination by the E3 ligase UBR5 and proteasome-mediated degradation. We conclude that GSK3β-mediated phosphorylation of NM1 is required for pol I transcription activation.
核肌球蛋白1c（NM1）通过促进PCAF介导的H3K9乙酰化来介导RNA聚合酶I（pol I）转录激活和细胞周期进程，但其调控的分子机制仍不清楚。在此，我们报告在G1早期，糖原合酶激酶（GSK）3β磷酸化并稳定NM1，使NM1与染色质结合。通过ChIP-Seq进行的基因组分析表明，这种机制发生在核糖体DNA（rDNA）上，因为活性GSK3β选择性占据该基因。在GSK3β基因敲除小鼠胚胎成纤维细胞中进行的ChIP分析和透射电子显微镜观察表明，在G1期，由于H3K9乙酰化减少导致染色质状态与转录不相容，rRNA合成受到抑制。我们发现，GSK3β直接磷酸化位于NM1 C末端的单个丝氨酸残基（Ser-1020）上的内源性NM1。在G1期，这一磷酸化事件稳定了NM1，并防止NM1被E3连接酶UBR5多聚泛素化以及蛋白酶体介导的降解。我们得出结论，GSK3β介导的NM1磷酸化是pol I转录激活所必需的。
