Separation of sister chromatids is a drastic and irreversible step in the cell cycle. The key biochemistry behind this event is the proteolysis mediated by the ubiquitin ligase called the anaphase promoting complex, or APC/C. Securin and cyclin B1 are the two established substrates for APC/C whose degradation releases separase and inactivates cyclin B1-dependent kinase 1 (cdk1), respectively, at the metaphase-to-anaphase transition. In this study, we have combined biochemical quantifications with mathematical simulations to characterize the kinetic regulation of securin and cyclin B1, in the cytoplasmic and chromosomal compartments, and found that they are differentially distributed and degraded with different rates. Modeling their interaction with separase predicted that activation timing of separase well coincides with the decline of securin-separase concentration in the cytoplasm. Notably, it also coincides with the peak of cyclin B1-separase level on chromosomes, which appeared crucial to coordinate the timing for separase activation and cdk1 inhibition. We have also conducted phosphoproteomic analysis and identified Ki67 as a chromosomal cdk1 substrate whose dephosphorylation is facilitated by cyclin B1-separase interaction in anaphase.

中文翻译：

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中期到后期转变的定量分析揭示了对安全蛋白和细胞周期蛋白B1的动力学差异调节。

姊妹染色单体的分离是细胞周期中一个激烈且不可逆的步骤。该事件背后的关键生物化学是被称为后期促进复合物（APC / C）的泛素连接酶介导的蛋白水解作用。Securin和cyclin B1是APC / C的两个已确定的底物，在中期到后期的转换过程中，它们的降解分别释放分离酶并使cyclin B1依赖性激酶1（cdk1）失活。在这项研究中，我们将生化定量分析与数学模拟相结合，以表征在细胞质和染色体区室中securin和cyclin B1的动力学调控，发现它们以不同的速率差异分布和降解。模拟它们与Separase的相互作用，可以预测Separase的激活时间与细胞质中Securin-separase浓度的下降相吻合。值得注意的是，它也与染色体上细胞周期蛋白B1分离酶水平的峰值相符，这似乎对协调分离酶激活和cdk1抑制的时间至关重要。我们还进行了磷酸化蛋白质组学分析，并将Ki67鉴定为染色体cdk1底物，其后期磷酸化作用是通过细胞周期蛋白B1-分离酶的相互作用而促进的。