Stephens et al. describe how cohesin and condensin organize pericentric chromatin into a spring that resists the pull of mitotic spindle microtubules.
During metaphase, the chromatin surrounding each chromatid's centromere is thought to act as a spring that balances the microtubule-based forces pulling sister chromatids apart. Cohesin, best known as a protein that links sister chromatids together, and condensin, which helps compact DNA by organizing it into loops, are both enriched on pericentric chromatin, and both proteins have been implicated in generating tension on metaphase chromosomes.
Stephens et al. analyzed mutant yeast strains and found that yeast lacking condensin and cohesin had longer metaphase spindles that fluctuated greatly over time, indicating that pericentric chromatin offered less resistance to spindle-pulling forces in the absence of these proteins. By mapping the organization of pericentric chromatin in wild-type and mutant cells, the researchers found that cohesin and condensin have different functions in organizing the spring. Condensin localized in line with the mitotic spindle and kept pericentric chromatin compacted along the spindle axis, whereas cohesin localized around the spindle axis and prevented the chromatin from spreading out radially.
By confining pericentric chromatin in these different directions, condensin and cohesin generate a force that resists the pull from spindle microtubules. The resulting tension signals to cells that sister chromatids are correctly attached to opposite spindle poles. Now senior author Kerry Bloom wants to investigate whether the release of this tension at the beginning of anaphase helps to propel the chromatids apart from each other.