LEM2 phase separation governs ESCRT-mediated nuclear envelope reformation

During cell division, remodeling of the nuclear envelope (NE) enables chromosome segregation by the mitotic spindle(1). The reformation of sealed nuclei requires Endosomal Sorting Complexes Required for Transport (ESCRTs) and LEM2, a transmembrane ESCRT adapter(2–4). Here, we show how LEM2’s ability to condense on microtubules governs ESCRT activation and coordinated spindle disassembly. The LEM motif of LEM2 binds barrier-to-autointegration factor (BAF), conferring affinity for chromatin(5,6), while an adjacent low complexity domain (LCD) confers the ability to phase separate. A proline-arginine-rich sequence within the LCD binds microtubules, targeting LEM2 condensation to spindle microtubules traversing the nascent NE. Furthermore, LEM2’s winged-helix (WH) domain activates the ESCRT-II/ESCRT-III hybrid protein, CHMP7, to form co-oligomeric rings. Disrupting these events in cells prevented the recruitment of downstream ESCRTs, compromised spindle disassembly, and led to nuclear integrity defects and DNA damage. We propose that during nuclear reassembly, LEM2 condenses into a liquid-like phase and coassembles with CHMP7 to form a macromolecular O-ring seal at the confluence between membranes, chromatin, and the spindle. The properties of LEM2 described here, and the homologous architectures of related inner nuclear membrane proteins(7,8), suggest that phase separation may contribute to other critical envelope functions, including interphase repair(8–13) and chromatin organization(14–17).