Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.
Nat Cell Biol. 2010 Feb;12(2):185-92. doi: 10.1038/ncb2018. Epub 2010 Jan 17.
The metaphase-anaphase transition is orchestrated through proteolysis of numerous proteins by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C). A crucial aspect of this process is sister chromatid separation, which is thought to be mediated by separase, a thiol protease activated by the APC/C. Separase cleaves cohesin, a ring-shaped complex that entraps sister DNAs. It is a matter of debate whether cohesin-independent forces also contribute to sister chromatid cohesion. Using 4D live-cell imaging of Drosophila melanogaster syncytial embryos blocked in metaphase (via APC/C inhibition), we show that artificial cohesin cleavage is sufficient to trigger chromosome disjunction. This is nevertheless insufficient for correct chromosome segregation. Kinetochore-microtubule attachments are rapidly destabilized by the loss of tension caused by cohesin cleavage in the presence of high Cdk1 (cyclin-dependent kinase 1) activity, as occurs when the APC/C cannot destroy mitotic cyclins. Metaphase chromosomes undergo a bona fide anaphase when cohesin cleavage is combined with Cdk1 inhibition. We conclude that only two key events, opening of cohesin rings and downregulation of Cdk1, are sufficient to drive proper segregation of chromosomes in anaphase.
有丝分裂-后期转换是通过一种称为后期促进复合物或周期蛋白体（APC/C）的泛素蛋白连接酶对许多蛋白质进行蛋白水解来协调的。这个过程的一个关键方面是姐妹染色单体分离，据认为这是由分离酶介导的，分离酶是一种由 APC/C 激活的硫醇蛋白酶。分离酶切割黏连蛋白，一种束缚姐妹 DNA 的环形复合物。黏连蛋白非依赖性力是否也有助于姐妹染色单体的黏合，这是一个有争议的问题。我们使用果蝇合胞胚胎中期（通过 APC/C 抑制）的 4D 活细胞成像表明，人工黏连蛋白切割足以引发染色体分离。然而，这对于正确的染色体分离仍然是不够的。在高 Cdk1（细胞周期蛋白依赖性激酶 1）活性存在下，由于黏连蛋白切割导致的张力丧失会迅速破坏动粒-微管附着，当 APC/C 不能破坏有丝分裂周期蛋白时就会发生这种情况。当黏连蛋白切割与 Cdk1 抑制结合时，有丝分裂染色体会经历真正的后期。我们得出结论，只有两个关键事件，即黏连蛋白环的打开和 Cdk1 的下调，足以驱动后期染色体的正确分离。
