Department of Biochemistry, Yong Loo Lin School of Medicine, National University Health System, National University of Singapore, 8 Medical Drive, Singapore 117597.
J Biol Chem. 2010 Jul 2;285(27):20904-14. doi: 10.1074/jbc.M109.098558. Epub 2010 May 3.
The human mixed lineage leukemia-5 (MLL5) gene is frequently deleted in myeloid malignancies. Emerging evidence suggests that MLL5 has important functions in adult hematopoiesis and the chromatin regulatory network, and it participates in regulating the cell cycle machinery. Here, we demonstrate that MLL5 is tightly regulated through phosphorylation on its central domain at the G(2)/M phase of the cell cycle. Upon entry into mitosis, the phosphorylated MLL5 delocalizes from condensed chromosomes, whereas after mitotic exit, MLL5 becomes dephosphorylated and re-associates with the relaxed chromatin. We further identify that the mitotic phosphorylation and subcellular localization of MLL5 are dependent on Cdc2 kinase activity, and Thr-912 is the Cdc2-targeting site. Overexpression of the Cdc2-targeting MLL5 fragment obstructs mitotic entry by competitive inhibition of the phosphorylation of endogenous MLL5. In addition, G(2) phase arrest caused by depletion of endogenous MLL5 can be compensated by exogenously overexpressed full-length MLL5 but not the phosphodomain deletion or MLL5-T912A mutant. Our data provide evidence that MLL5 is a novel cellular target of Cdc2, and the phosphorylation of MLL5 may have an indispensable role in the mitotic progression.
人类混合谱系白血病 5（MLL5）基因在髓系恶性肿瘤中经常缺失。新出现的证据表明，MLL5 在成人造血和染色质调控网络中具有重要功能，它参与调节细胞周期机制。在这里，我们证明 MLL5 通过其细胞周期 G(2)/M 期的中央结构域上的磷酸化来进行严格调控。进入有丝分裂后，磷酸化的 MLL5 从浓缩染色体上脱定位，而有丝分裂后，MLL5 去磷酸化并重新与松弛的染色质结合。我们进一步确定，MLL5 的有丝分裂磷酸化和亚细胞定位依赖于 Cdc2 激酶活性，并且 Thr-912 是 Cdc2 的靶位。过表达 Cdc2 靶向的 MLL5 片段通过竞争性抑制内源性 MLL5 的磷酸化来阻碍有丝分裂的进入。此外，通过耗尽内源性 MLL5 引起的 G(2)期阻滞可以通过过表达全长 MLL5 来补偿，但不能通过磷酸化结构域缺失或 MLL5-T912A 突变体来补偿。我们的数据提供了证据表明 MLL5 是 Cdc2 的一个新的细胞靶标，MLL5 的磷酸化可能在有丝分裂进程中具有不可或缺的作用。
