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

Abstract

SummaryEukaryotic 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.

Local regulation of lipid synthesis controls ER sheet insertion into nuclear envelope holes to complete nuclear closure

“…It is plausible that transient changes in NE organization at NE rupture sites may signal or support membrane insertion (De Vos et al, 2011;Denais et al, 2016;Raab et al, 2016;Penfield et al, 2018). For instance, BAF crosslinks chromatin at the core region of chromatin that coincides with spindle microtubules (Samwer et al, 2017) and was recently shown to accumulate at rupture sites (Halfmann et al, 2019), providing a possible mechanism to direct membrane incorporation to this region. Identifying how ER sheets are delivered into NE holes in different contexts is an important next step.…”

Mechanosensitive recruitment of BAF to the nuclear membrane inhibits nuclear E2F1 and Yap levels

“…BAF is a small 89-aa protein, that forms dimmers, which are able to bind dsDNA, and thus capable of bridging between two strands of DNA, and thereby, to potentially affect DNA condensation (Bradley et al, 2005;Zheng et al, 2000). Alternatively, BAF dimers may bind to inner components of the nuclear membrane, including Lap-2, emerin, MAN1 (LEM) domain proteins, or to lamin A/C or B, as well as to dsDNA, and thus promote the association of dsDNA with the nuclear membrane (Cai et al, 2001;Lee et al, 2001;Mansharamani and Wilson, 2005;Samwer et al, 2017). Proteomic analysis of BAF partners indicated its potential association with additional proteins, including transcription factors, damage-specific DNA binding proteins, and histones (Holaska et al, 2003;Montes de Oca et al, 2009).…”

How chromosomes unite

“…However, many questions remain to be answered about the mechanisms underlying this cellular 'heartbeat' of life. Writing in Cell, Samwer et al 1 now shed light on the mystery of how chromosomes are packaged together in the same nucleus, an outcome known to help protect the integrity of the genome.…”

“…On the more applied side, they might even help chemists to create low-cost fertilizers and organic batteries (see go.nature.com/2jvwchw). In the long run, they could revolutionize our ability to design materials and drugs 1 . Today, however, quantum simulators are still at an early stage of development.…”

“…Samwer et al 1 investigated how groups of chromosomes coalesce within a single nucleus after cell division and found that BAF is essential to this process. a, The authors isolated purified chromosomes (pink) from cultured human cells, placed them on glass slides and incubated them in the presence or absence of purified BAF (blue).…”