18.11: Separation of Sister Chromatids

At the transition from prophase to metaphase, there is a reduction in cohesion along the chromosomal arms, resulting in the resolution of sister chromatids. However, residual cohesin connections remain to hold the sister chromatids together until the transition from metaphase to anaphase. The residual connection prevents any premature separation of sister chromatids, blocking the risks of aneuploidy within the daughter cells.

At the onset of anaphase, separase, a proteolytic enzyme, is activated. The activated separase cleaves the Scc1 subunit of the residual cohesin rings, resulting in a total loss of cohesion. The sister-chromatids separate in the absence of force to hold them together. The lack of cohesion permits the poleward forces, straining along the microtubules, to pull the separated chromatids toward the spindle poles.

In early mitosis, the spindle assembly checkpoint (SAC) prohibits the anaphase-promoting complex or cyclosome (APC/C) from ubiquitinating proteins, such as securin and M-phase cyclins, whose degradation is necessary for anaphase. Only when each chromosome correctly aligns on the mitotic spindle, the SAC is deactivated to allow phosphorylation and activation of APC/C regulatory subunits- CDC20 and CDH1.

The CDC20 is phosphorylated by Cdk1/cyclin B to form active APC/C-CDC20. Active APC/C-CDC20 then catalyzes the degradation of securin, cyclin A, and cyclin B to promote anaphase transition. Because Cdk activity is cyclin-dependent, cyclin B degradation results in the loss of Cdk1 activity. Loss of Cdk1/cyclin B complex inactivates APC/C-CDC20 but activates another regulatory subunit, APC/C-CDH1, that signals completion of the metaphase-to-anaphase transition. The active APC/C-CDH1 facilitates mitotic exit and stabilizes the subsequent G1 phase by preventing the premature accumulation of mitotic cyclins.

The switch in the activities of APC/C-CDC20 and APC/C-CDH1 has two significant consequences. First, these regulatory subunits trigger overlapping yet distinct substrate specificities and thereby promote ordered cell cycle transition. Second, CDC20 and CDH1 are regulated through different mechanisms. When APC/C-CDC20 is active, CDH1 undergoes inhibitory phosphorylation by cyclin B/Cdk1, preventing it from binding to APC/C. In contrast, the CDC20 activity is inhibited by a mitotic checkpoint complex- a multiprotein (BUBR1, BUB3, CDC20, and MAD2) activated by the SAC.

At the end of metaphase, the bi-oriented chromosomes align at the metaphase plate. During this phase, the cohesin protein ring-complexes hold the sister chromatids together at the centromere region and prevent them from being pulled apart.

Metaphase to anaphase progression is triggered by the cyclin-Cdk-induced phosphorylation of a multisubunit, ubiquitin ligase enzyme – the anaphase-promoting complex, also known as the cyclosome or APC/C. Phosphorylated APC/C binds to a protein, Cdc20, forming an active complex.

The active APC/C complex recognizes an inhibitory protein called securin that is bound to a protease enzyme called separase. Prior to this recognition, securin inhibits the activity of separase.

The active APC/C complex tags securin with the protein ubiquitin, targeting it for proteasomal degradation. Destruction of securin releases separase.

The cyclin-Cdk complex also negatively regulates unbound separase activity through inhibitory phosphorylation.

The active APC/C complex causes the ubiquitination of cyclins, targeting them for proteasomal degradation. The destruction of cyclin removes the enzymatic activity of cyclin-dependent kinases or Cdks.

Inactivation of Cdks allows phosphatase enzymes to dephosphorylate separase and prevents Cdk-mediated inhibitory re-phosphorylation of separase. Dephosphorylation allows the separase enzyme to cleave the cohesin ring-complex that holds sister chromatids together.

Cohesin cleavage results in the absolute loss of sister-chromatid cohesion, marking the transition from metaphase to anaphase. The loss of cohesion permits the sister chromatids to separate and move to opposite poles of the spindle.