An ESCRT grommet cooperates with a diffusion barrier to maintain nuclear integrity

The reformation of the nuclear envelope (NE) at the end of metazoan mitosis requires the sealing of numerous fenestrations that recruit the endosomal sorting complexes required for transport (ESCRT) machinery. The molecular mechanisms by which ESCRT proteins drive NE sealing and their necessity to the reestablishment of the nuclear compartment have yet to be fully defined. Here, we leveraged the single NE hole generated by mitotic extrusion of the Schizosaccharomyces pombe spindle pole body to reveal two modes of ESCRT function carried out by unique complements of ESCRT-III proteins, both of which depend on the NE-specific CHMP7/Cmp7. The first is a grommet-like function, where we tie the presence of specific ESCRTs to a constriction of the NE hole from ∼150 to ∼50 nm in anaphase B. Consistent with a direct role for ESCRTs in restricting this NE hole diameter, the hole dilates a remarkable five-fold in cells lacking Cmp7. Second, in the subsequent G1/S-phases, a sealing module of ESCRT proteins replaces Cmp7 and is required to drive closure of the NE. Surprisingly, in the absence of Cmp7, nucleocytoplasmic compartmentalization remains intact despite discontinuities in the NE as large as 400 nm, suggesting a mechanism to establish a diffusion barrier(s) over persistent NE holes. Indeed, we demonstrate that the ortholog of the pericentriolar material protein, pericentrin (Pcp1), establishes such a barrier, likely through its ability to form a biomolecular condensate. NE remodeling mechanisms therefore cooperate with proteinaceous diffusion barriers beyond nuclear pore complexes to protect the nuclear compartment.