Cortical ER-PM contact sites promote nuclear ER membrane integrity and mitotic nuclear segregation

The endoplasmic reticulum (ER) extends from the nuclear envelope through the cytoplasm to the cell cortex, where it contacts the plasma membrane (PM) at ER-PM membrane contact sites (MCSs). We hypothesized that as contiguous extensions of nuclear ER these cortical ER (cER) attachments to the PM might dampen shearing stresses on the nuclear envelope and help orchestrate nuclear ER dynamics. Although individual cER-PM tether proteins promote cER inheritance during mitosis, our findings reveal that removing all cER-PM tethers collectively disrupts nuclear envelope membrane integrity. In yeast ∆-super-tether (∆-s-tether) cells lacking seven cER-PM tethers, cER detachment causes membrane blisters and nuclear envelope ruptures; however, individual tether deletions or ER stress do not disrupt nuclear containment. Despite the extensive nuclear envelope lesions in ∆-s-tether cells, ESCRT-III (Endosomal Sorting Complex Required for Transport-III)-mediated membrane repair sustains cell viability. The loss of nuclear integrity, however, appears to disrupt genomic repair and promotes spontaneous mutation; separately, nuclear inheritance is also disrupted. Without cER-PM MCSs, cortical actin depolarization and defects in nucleopodia impair mitotic nuclear dynamics and segregation, which triggers a Spindle Position Checkpoint (SPOC) cell-cycle delay. That cER-PM MCSs serve as dual-function hubs for the preservation of nuclear envelope integrity is indicated by two independent mechanisms capable of rescuing the observed nuclear membrane defects: the restoration of phospholipid metabolism or the re-establishment of nonspecific cER-PM contact.