Clijsters Linda, Wolthuis Rob
Division of Cell Biology (B5) and Division of Molecular Carcinogenesis (B7), The Netherlands Cancer Institute (NKI-AVL), Plesmanlaan 121, 1066 CX, Amsterdam, The Netherlands.
J Cell Sci. 2014 Mar 15;127(Pt 6):1336-45. doi: 10.1242/jcs.145862. Epub 2014 Jan 16.
Cdc6 and Cdt1 initiate DNA replication licensing when cells exit mitosis. In cycling cells, Cdc6 is efficiently degraded from anaphase onwards as a result of APC/C-Cdh1 activity. When APC/C-Cdh1 is switched off again, at the end of G1 phase, Cdc6 could thus re-accumulate, risking the re-licensing of DNA as long as Cdt1 is present. Here, we carefully investigated the dynamics of Cdt1 and Cdc6 in cycling cells. We reveal a novel APC/C-Cdh1-independent degradation pathway that prevents nuclear Cdc6 re-accumulation at the G1-S transition and during S phase. Similar to Cdt1, nuclear clearance of Cdc6 depends on an N-terminal PIP-box and the Cdt2-containing CRL4 complex. When cells reach G2 phase, Cdc6 rapidly re-accumulates but, at this time, Cdt1 is mostly absent and expression of Cdc6 is limited to the cytoplasm. We propose that Cdk1 contributes to the nuclear export of Cdc6 at the S-to-G2 transition. In summary, our results show that different control mechanisms of Cdc6 restrain erroneous licensing of DNA replication during G1, S and G2 phase.
细胞退出有丝分裂时，Cdc6和Cdt1启动DNA复制许可。在循环细胞中，由于后期促进复合物/细胞周期蛋白依赖性泛素连接酶1（APC/C-Cdh1）的活性，从后期开始Cdc6就会被有效降解。当APC/C-Cdh1在G1期结束时再次关闭时，只要Cdt1存在，Cdc6就可能重新积累，从而有DNA重新许可的风险。在这里，我们仔细研究了循环细胞中Cdt1和Cdc6的动态变化。我们发现了一种新的不依赖APC/C-Cdh1的降解途径，该途径可防止G1-S期转换和S期期间核内Cdc6的重新积累。与Cdt1类似，Cdc6的核清除取决于N端磷脂酰肌醇结合盒（PIP-box）和含Cdt2的Cullin4-RING泛素连接酶复合物（CRL4）。当细胞进入G2期时，Cdc6迅速重新积累，但此时Cdt1大多不存在，且Cdc6的表达仅限于细胞质。我们认为，细胞周期蛋白依赖性激酶1（Cdk1）在S期到G2期转换时促进Cdc6的核输出。总之，我们的结果表明，Cdc6的不同调控机制在G1期、S期和G2期抑制DNA复制的错误许可。
