Physical breakdown of the nuclear envelope is not necessary for breaking its barrier function.

During mitosis in higher eukaryotic cells, nuclear envelope breakdown (NEBD) occurs and leads to the disassembly of the nuclear membrane and nuclear pore complexes (NPC). This brings about a mixing of nuclear and cytoplasmic macromolecules (open mitosis). On the other hand, in many fungi, mitosis occurs without NEBD (closed mitosis). In a recent study, we reported a novel phenomenon in a closed mitosis organism, Schizosaccharomyces pombe: mixing of nuclear and cytoplasmic proteins occurred in meiosis without breakdown of the nuclear membrane or disassembly of nuclear pore complexes. We designated this event virtual nuclear envelope breakdown (V-NEBD). The key event in V-NEBD is nuclear translocation of Rna1, a RanGAP1 homologue in S. pombe. This leads to collapse of the Ran-GTP gradient across the nuclear envelope (NE) and occurs coincidently with V-NEBD. Thus, the barrier function of the NE can be abated without its physical breakdown through modulation of the Ran-GTP gradient.