A tubule-sheet continuum model for the mechanism of nuclear envelope assembly

Nuclear envelope (NE) assembly defects cause chromosome fragmentation, cancer, and aging. However, major questions about the mechanism of NE assembly and its relationship to nuclear pathology are unresolved. In particular, how cells efficiently assemble the NE starting from vastly different, cell type-specific endoplasmic reticulum (ER) morphologies is unclear. Here, we identify a NE assembly mechanism, “membrane infiltration,” that defines one end of a continuum with another NE assembly mechanism, “lateral sheet expansion,” in human cells. Membrane infiltration involves the recruitment of ER tubules or small sheets to the chromatin surface by mitotic actin filaments. Lateral sheet expansion involves actin-independent envelopment of peripheral chromatin by large ER sheets that then extend over chromatin within the spindle. We propose a “tubule-sheet continuum” model that explains the efficient NE assembly from any starting ER morphology, the cell type-specific patterns of nuclear pore complex (NPC) assembly, and the obligatory NPC assembly defect of micronuclei. [Display omitted] •Different cell types use different combinations of two mechanisms for NE assembly•Mitotic actin and ER infiltrating the spindle control one NE assembly mechanism•These mechanisms allow NE assembly from any starting mix of ER sheets or ER tubules•NE assembly on micronuclei is defective irrespective of mitotic ER morphology The nuclear envelope (NE) assembles from the mitotic endoplasmic reticulum (ER). Zhao and Liu et al. show that different cell types assemble the NE using one or a combination of two mechanisms, thus explaining how cells with any starting ER morphology assemble a functional NE and explaining the NE defects of micronuclei.