The reformation of the NE (green) around chromatin (blue) is impaired (right) by the addition of pure DNA.

DNA calls for the return of the nuclear envelope (NE) after mitosis, according to Ulbert et al. (page 469). The abundance of DNA may thus be one reason why the NE reforms so quickly.

NE reformation occurs so quickly that breaking down the process in vivo has so far been impossible. Many scientists instead use in vitro reconstitution assays, which indicate that chromatin decondensation (as occurs at the end of mitosis) initiates the recruitment of vesicle populations to chromatin. In the new work, the authors aimed to identify membrane and chromatin components that mediate this recruitment and thus NE assembly.

The main component of chromatin is, of course, DNA. The group found that excess naked DNA prevented in vitro NE reformation by titrating vesicles away from chromatin at early stages. DNA alone was a more potent inhibitor than chromatin, although chromatin proteins might also contribute, particularly later on as the envelope matures.

DNA does not bind pure liposomes, so the authors next sought the membrane proteins that mediate vesicle recruitment. Two transmembrane proteins of the inner nuclear envelope were already known to bind to DNA in vitro, and the authors have now identified several more. Most of them had highly basic nucleoplasmic regions, which are well-suited to binding negatively charged DNA. In fact, large basic tails were found in half of the tested nuclear membrane proteins but only 4% of ER and Golgi membrane proteins.

Loss of any particular transmembrane NE protein in vivo does not interfere with envelope reformation, so maybe they all work together. This redundancy and the prevalence of DNA might explain the speed of NE reformation.

Premature reformation is probably prevented by the inaccessibility of DNA in highly condensed mitotic chromatin. Additionally, many NE proteins are phosphorylated during mitosis. The added negative charge probably hinders their interaction with DNA.