Nuclear envelope assembly relies on CHMP-7 in the absence of BAF-LEM-mediated hole closure.

Barrier-to-autointegration (BAF) is a DNA binding protein that crosslinks chromatin to allow mitotic nuclear envelope (NE) assembly. The Lap2b-Emerin-Man1(LEM)-domain protein LEMD2 and ESCRTII/III hybrid protein CHMP7 close NE holes surrounding spindle microtubules (MTs). BAF binds LEM-domain family proteins, which repairs NE ruptures in interphase, but whether BAF-LEM binding participates in NE hole closure around spindle MTs is not known. Here, we take advantage of the stereotypical event of NE formation in fertilized C. elegans oocytes to show that BAF-LEM binding and LEM-2LEMD2-CHMP-7 have distinct roles in NE closure around spindle MTs. LEM-2/EMR-1emerin function redundantly with BAF-1 in NE closure. Compromising BAF-LEM binding revealed an additional role for EMR-1emerin in maintenance of the NE permeability barrier. In the absence of BAF-LEM binding, LEM-2-CHMP-7 are required for NE assembly and embryo survival. The winged helix domain of LEM-2 recruits CHMP-7 to the NE in C. elegans and a LEM-2-independent nucleoplasmic pool of CHMP-7 also contributes to NE stability. Thus, NE hole closure surrounding spindle MTs requires redundant mechanisms that safeguard against failure in NE assembly to support embryogenesis.