Figure 7.

Condensin-Dependent Chromosome Recoiling Facilitates Elimination of Residual Sister Chromatid Cohesion

(A) A larger amount of cohesin remains on chromosomes during anaphase when chromosome recoiling is defective. YCG1⁺ (T7802) and ycg1-2 (T7803) cells with Pgal-CDC20 SCC1-18×myc TetR-GFP tetOs (at CEN and TEL loci as in Figure 1A) were arrested at metaphase by Cdc20 depletion and subsequently released to anaphase synchronously by re-expression of Cdc20, as in Figure 3. During metaphase arrest and also 10 min after Cdc20 re-expression, chromosomes were fixed and immobilized on a slide glass immediately after cell lysis. Chromosomes were stained with DAPI and Scc1 was immunostained using an anti-myc antibody. (Ai) Representative cells. (Aii) The distance between sister CEN dots and the percentage of cells, in which TEL-dot segregation was completed (toward two distinct nuclear masses). (Aiii) The amount of Scc1, bound on chromosomes, was quantified and compared between the two strains. Bars and errors show means and SEMs, respectively. n.s. = not significantly different.

(B) Reduction of residual cohesion restores chromosome segregation in condensin-defective cells. smc2-aid (T8636) and smc2-aid scc1-aid (T8595) cells with osTIR1 Pmet-CDC20 TetR-GFP tetOs (integrated at three loci as in Figure 1A) were induced to synchronous anaphase by depletion of Cdc20 followed by its re-expression, as in Figure 4B. NAA was added concomitantly with Cdc20 re-expression and after 15 min GFP images were acquired every 4 s for 45 min. The graph shows the time (after the CEN-CEN distance became >3 μm) when TEL segregation occurred to the bud. As a control, metaphase was extended for T8595 cells without re-expression of Cdc20 but with addition of NAA in the same timing; CEN-CEN distance did not exceed 3 μm during image acquisition (data not shown).

(C) Summary for residual sister chromatid cohesion and its elimination by condensin-dependent chromosome recoiling. Separase cleaves the majority of cohesin rings at the onset of anaphase. However, due to a small amount of residual cohesins, weak sister chromatid cohesion is still present at some loci along chromosome arms, which transiently opposes sister chromatid separation and causes regional chromosome stretching. Stretched chromosome regions are recoiled by the action of condensins, leading to removal of residual cohesins, either by their cleavage facilitated by separase (Ci) or by their physical breakage/removal (Cii and Ciii) (see Discussion). Regional chromosome stretching/recoiling advances from para-centromere regions to telomeres, resulting in sister chromatid separation along the entire chromosome arms.