Molecular basis of ESCRT-mediated sealing of the nuclear envelope.
Biochemistry
Cell Biology
Final Report Abstract
The eukaryotic genome is compartmentalized in the nucleus; whose boundary is a doublemembrane structure called the nuclear envelope (NE). The NE segregates transcription from translation, regulates gene activity, and prevents DNA damage and malignant transformation as a consequence. Dividing cells of vertebrates go through the process of “open mitosis” in which the NE opens up to such a degree that nucleoplasmic identity is lost. NE membranes including many of their proteins disperse into the connected endoplasmic reticulum during chromosomes segregation. The process is reversed during mitotic exit when an intact and closed NE reforms. After each cell cycle, cells need to reform the nuclear envelope to maintain functionality. The reformation of sealed nuclei requires ESCRTs (endosomal sorting complexes required for transport). A clear mechanistically compelling model, by which the double membrane of the nuclear envelope reforms following each cell division did not exist, a problem that I began to address in this proposal. As part of my DFG-funded work, I established that LEM2, a two-pass transmembrane protein of the inner nuclear membrane, acts as ESCRT adaptor at the NE. Most surprisingly, I could show that LEM2 employs it’s cationic material properties to condense into a liquid-like state on the surface of spindle microtubules driven by its low complexity domain (LCD). The condensation of LEM2 spatiotemporally directs the cell’s ESCRT-machinery for spindle-disassembly and membrane fusion. My work revealed a novel mechanism, where the LEM motif of LEM2 binds BAF, conferring on LEM2 an affinity for chromatin, while an adjacent a proline–arginine-rich sequence within the LCD binds to microtubules and targets LEM2 to spindle microtubules that traverse the nascent NE. LEM2’s winged-helix (WH) domain activates the ESCRT protein CHMP7 to form co-oligomeric rings. In human cells, the disruption of LEM2’s modular architecture prevented the recruitment of downstream ESCRTs, compromised spindle disassembly, and led to defects in nuclear integrity and DNA damage. My work was published in the journal Nature. The properties of LEM2 described here, and the homologous architectures of related inner nuclear membrane (INM) proteins suggest that phase separation may contribute to other critical envelope functions, including interphase repair and chromatin organization. The recent links between the LEM2-ESCRT-pathway and age-related disease including cancer, neurodegeneration, heart disease and progeria suggest that deregulated LEM2 activity, and hence LEM2 phase separation, is a critical component of human pathology.
Publications
- LEM2 phase separation promotes ESCRT-mediated nuclear envelope reformation. Nature. 2020
von Appen A, LaJoie D, Johnson I, Pick SM, Trnka M, Ullman KS and Frost A
(See online at https://doi.org/10.1038/s41586-020-2232-x)
