Nuclear Compartmentalization Is Abolished during Fission Yeast Meiosis

In eukaryotic cells, the nuclear envelope partitions the nucleus from the cytoplasm. The fission yeast Schizosaccharomyces pombe undergoes closed mitosis in which the nuclear envelope persists rather than being broken down, as in higher eukaryotic cells [1]. It is therefore assumed that nucleocytoplasmic transport continues during the cell cycle [2]. Here we show that nuclear transport is, in fact, abolished specifically during anaphase of the second meiotic nuclear division. During that time, both nucleoplasmic and cytoplasmic proteins disperse throughout the cell, reminiscent of the open mitosis of higher eukaryotes, but the architecture of the S. pombe nuclear envelope itself persists. This functional alteration of the nucleocytoplasmic barrier is likely induced by spore wall formation, because ectopic induction of sporulation signaling leads to premature dispersion of nucleoplasmic proteins. A photobleaching assay demonstrated that nuclear envelope permeability increases abruptly at the onset of anaphase of the second meiotic division. The permeability was not altered when sporulation was inhibited by blocking the trafficking of forespore-membrane vesicles from the endoplasmic reticulum to the Golgi. The evidence indicates that yeast gametogenesis produces vesicle transport-mediated forespore membranes by inducing nuclear envelope permeabilization.