Membrane shaping by reticulon homology domain proteins in nuclear envelope dynamics and mitotic progression

• Applicant: Dr. Anne-Lore Schlaitz
• Subject Area: Cell Biology
• Term: since 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 265090024

Membrane-bound organelles have characteristic shapes and intracellular distributions that correlate with their specific roles. Organelles are continuously remodeled, for instance in the course of cell division. The nuclear envelope disassembles at mitotic onset and its membrane-associated components merge with the endoplasmic reticulum (ER). In this process, ER membrane curvature increases and the ER becomes largely excluded from the mitotic spindle and chromatin. Simultaneously, nuclear pore complexes (NPCs) also disintegrate. After chromosome segregation, ER targets to chromatin to assemble the new nuclear envelope and NPCs are reconstructed into this forming nuclear envelope in a process that requires the local induction and stabilization of high membrane curvature. We found that the proteins REEP3 and REEP4 induce high curvature in the ER membrane and position the ER during mitosis but are not required for interphase morphogenesis of the organelle. REEP4 (but not REEP3) also localizes to the inner nuclear membrane and promotes NPC formation in late mitosis. I propose to analyze the mitosis-specific interaction partners and post-translational modifications of REEP3/4, as well as the molecular mechanisms of REEP4 targeting to the inner nuclear membrane to understand how the different mitotic functions of REEP3/4 are regulated (aim 1).We furthermore observed that ER tubules transiently enter the spindle area during mitosis and come into close proximity to metaphase chromosomes. This ER subdomain, which we term "spindle ER", is enriched in proteins that mediate early steps of nuclear envelope reassembly. I propose to investigate the characteristics and determinants of spindle ER and to test two hypotheses for the function of spindle ER: (1) it may represent an early precursor of nuclear envelope reformation that promotes the rapid targeting of ER membrane to the segregated chromosomes in anaphase, (2) it may provide a means of communication between the spindle and membrane-bound organelles that coordinates the organelle remodeling with chromosome segregation (aim 2).Overall, our proposed work aims to uncover how morphological transformations of the ER during mitosis are controlled and coordinated with the cell division program and how the ER interacts with the mitotic machinery. Our results will provide new insight into the regulation of membrane-shaping proteins, help to define the roles of membrane-bound organelles during mitosis and may provide a starting point to understand the control of other mitotic membrane remodeling events.

• DFG Programme: Research Grants