A cadre of chromatin-binding proteins helps rebuild the nuclear envelope after mitosis, Anderson et al. show. The proteins anchor the nascent envelope to the chromosomes and allow it to expand.
The nuclear envelope is an obstacle to chromosome separation. So before mitosis, the membrane dissolves and the lipids and some of the proteins it contained take refuge in the endoplasmic reticulum (ER). After segregation, ER membranes coalesce and surround the chromosomes, restoring the envelope. The researchers previously showed that reticulons, proteins that curl ER sheets into tubes, hamper envelope reconstruction. That suggests that other proteins must counteract this rolling tendency so that the reforming nuclear envelope bends just the right amount. Anderson et al.'s suspicions fell on a group of proteins from the inner nuclear membrane, including LBR and MAN1, that fasten the developing envelope to chromatin.
Cells work fast, rebuilding the nuclear envelope in about 10 minutes. The team demonstrated that trimming the levels of these inner membrane proteins one at a time slowed the reconstruction but didn't stop it. Restoration of the nuclear envelope occurs in two steps—first the ER membranes home in on the chromatin, and then they reshape into nuclear envelope sheets. Reducing levels of three of the inner membrane proteins hindered the second step, not the first. Two components of nuclear pores also took part in the rebuilding. The researchers suggest that by tacking sections of ER membrane to the chromosomes, the inner membrane proteins prevent ER membranes from rolling up and enable them to eventually enclose the chromosomes.