Phosphorylation- and deacetylation-driven dissolution of sister chromatid cohesion in mitosis

点击翻译按钮获取中文摘要

Cell division is essential for human growth and reproduction. Before a cell begins to divide, the genetic information stored on the chromosomes is duplicated. After replication, each chromosome consists of two identical sister chromatids, which are topologically embraced by cohesin, a ring-shaped protein complex. Sister chromatid cohesion is enforced by acetylation of cohesin's Smc3 subunit and essential for error-free distributive halving of the genetic material. However, before chromosome segregation can take place, cohesion needs to be resolved. In metazoans, this occurs in two waves. Firstly, the concerted actions of protein kinases and Wapl open cohesin rings in a non-proteolytic manner. While this so-called prophase pathway removes cohesin from chromosome arms, cohesin at centromeres is protected by Sgo1-dependent dephosphorylation. Activation of separase results in cleavage of the Rad21 subunit of remaining cohesin, and it is this proteolysis-dependent, second wave of ring opening which triggers anaphase. In yeast, where the early mitotic upregulation of the cohesin release activity does not occur, all chromosomally bound cohesin is removed by Rad21 cleavage. While one study claims that dissociation of cohesin from chromosomes is pre-requisite for deacetylation of Smc3 in anaphase, this view is challenged by the prolonged chromatin association of (even cleaved) cohesin when deacetylation is prevented. We recently discovered that separase, once stripped off its inhibitors, forms a complex with HDAC8, the human Smc3 deacetylase. Using chromatin bound cohesin for enzymatic assays, we will clarify the temporal order and (inter)dependence of Rad21 cleavage and Smc3 deacetylation.Sgo1 leaves centromeric cohesin in prometaphase, which raises important as yet unresolved questions: Why is de-protected centromeric cohesin not removed by phosphorylation- and Wapl-dependent ring opening? Does this mean that the prophase pathway is no longer active in prometaphase, and - if yes - how is it switched off at the right time? How is Sgo1 targeted to centromeric cohesin (and later removed from it) on a mechanistic level? And why does Sgo1 leave the centromeres at all? Is this required because separase-dependent cleavage of cohesin would otherwise be impaired? Here, we propose a combination of biochemical reconstitutions, genome editing and cell biological experiments to answer all of these questions and obtain a comprehensive understanding of human chromosome segregation.