A mitotic chromatin phase transition prevents perforation by microtubules.

Dividing eukaryotic cells package extremely long chromosomal DNA molecules into discrete bodies to enable microtubule-mediated transport of one genome copy to each of the newly forming daughter cells1-3. Assembly of mitotic chromosomes involves DNA looping by condensin4-8 and chromatin compaction by global histone deacetylation9-13. Although condensin confers mechanical resistance to spindle pulling forces14-16, it is not known how histone deacetylation affects material properties and, as a consequence, segregation mechanics of mitotic chromosomes. Here we show how global histone deacetylation at the onset of mitosis induces a chromatin-intrinsic phase transition that endows chromosomes with the physical characteristics necessary for their precise movement during cell division. Deacetylation-mediated compaction of chromatin forms a structure dense in negative charge and allows mitotic chromosomes to resist perforation by microtubules as they are pushed to the metaphase plate. By contrast, hyperacetylated mitotic chromosomes lack a defined surface boundary, are frequently perforated by microtubules and are prone to missegregation. Our study highlights the different contributions of DNA loop formation and chromatin phase separation to genome segregation in dividing cells.

Chromosome clustering by Ki-67 excludes cytoplasm during nuclear assembly.

Gene expression in eukaryotes requires the effective separation of nuclear transcription and RNA processing from cytosolic translation1. This separation is achieved by the nuclear envelope, which controls the exchange of macromolecules through nuclear pores2. During mitosis, however, the nuclear envelope in animal and plant cells disassembles, allowing cytoplasmic and nuclear components to intermix3. When the nuclear envelope is reformed, cytoplasmic components are removed from the nucleus by receptor-mediated transport through nuclear pores2. These pores have a size limit of 39 nanometres4-7, which raises the question of how larger cytoplasmic molecules are cleared from the nucleus. Here we show in HeLa cells that large cytoplasmic components are displaced before nuclear envelope assembly by the movement of chromosomes to a dense cluster. This clustering occurs when chromosomes approach the poles of anaphase spindles, and is mediated by a microtubule-independent mechanism that involves the surfactant-like protein Ki-67. Ki-67 forms repulsive molecular brushes during the early stages of mitosis8, but during mitotic exit the brushes collapse and Ki-67 promotes chromosome clustering. We show that the exclusion of mature ribosomes from the nucleus after mitosis depends on Ki-67-regulated chromosome clustering. Thus, our study reveals that chromosome mechanics help to re-establish the compartmentalization of eukaryotic cells after open mitosis.

DNA Cross-Bridging Shapes a Single Nucleus from a Set of Mitotic Chromosomes.

Eukaryotic cells store their chromosomes in a single nucleus. This is important to maintain genomic integrity, as chromosomes packaged into separate nuclei (micronuclei) are prone to massive DNA damage. During mitosis, higher eukaryotes disassemble their nucleus and release individualized chromosomes for segregation. How numerous chromosomes subsequently reform a single nucleus has remained unclear. Using image-based screening of human cells, we identified barrier-to-autointegration factor (BAF) as a key factor guiding membranes to form a single nucleus. Unexpectedly, nuclear assembly does not require BAF's association with inner nuclear membrane proteins but instead relies on BAF's ability to bridge distant DNA sites. Live-cell imaging and in vitro reconstitution showed that BAF enriches around the mitotic chromosome ensemble to induce a densely cross-bridged chromatin layer that is mechanically stiff and limits membranes to the surface. Our study reveals that BAF-mediated changes in chromosome mechanics underlie nuclear assembly with broad implications for proper genome function.