Current Biology
Volume 24, Issue 12, 16 June 2014, Pages 1421-1428
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An Arp2/3 Nucleated F-Actin Shell Fragments Nuclear Membranes at Nuclear Envelope Breakdown in Starfish Oocytes

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An F-actin “shell” assembles underneath the nuclear envelope at NEBD in oocytes

This transient F-actin shell is nucleated by the Arp2/3 complex

The F-actin shell functions to fragment nuclear membranes in NEBD

Prevention of F-actin shell formation in meiosis leads to formation of aneuploid eggs


Animal cells disassemble and reassemble their nuclear envelopes (NEs) upon each division [1, 2]. Nuclear envelope breakdown (NEBD) serves as a major regulatory mechanism by which mixing of cytoplasmic and nuclear compartments drives the complete reorganization of cellular architecture, committing the cell for division [2, 3]. Breakdown is initiated by phosphorylation-driven partial disassembly of the nuclear pore complexes (NPCs), increasing their permeability but leaving the overall NE structure intact [4, 5, 6, 7]. Subsequently, the NE is rapidly broken into membrane fragments, defining the transition from prophase to prometaphase and resulting in complete mixing of cyto- and nucleoplasm [6, 8]. However, the mechanism underlying this rapid NE fragmentation remains largely unknown. Here, we show that NE fragmentation during NEBD in starfish oocytes is driven by an Arp2/3 complex-nucleated F-actin “shell” that transiently polymerizes on the inner surface of the NE. Blocking the formation of this F-actin shell prevents membrane fragmentation and delays entry of large cytoplasmic molecules into the nucleus. We observe spike-like protrusions extending from the F-actin shell that appear to “pierce” the NE during the fragmentation process. Finally, we show that NE fragmentation is essential for successful reproduction, because blocking this process in meiosis leads to formation of aneuploid eggs.