Abstract
Dictyostelium cells undergo a semi-closed mitosis, during which the nuclear envelope (NE) persists; however, free diffusion between the cytoplasm and the nucleus takes place. To permit the formation of the mitotic spindle, the nuclear envelope must be permeabilized in order to allow diffusion of tubulin dimers and spindle assembly factors into the nucleus. In Aspergillus, free diffusion of proteins between the cytoplasm and the nucleus is achieved by a partial disassembly of the nuclear pore complexes (NPCs) prior to spindle assembly. In order to determine whether this is also the case in Dictyostelium, we analysed components of the NPC by immunofluorescence microscopy and live cell imaging and studied their behaviour during interphase and mitosis. We observed that the NPCs are absent from the contact area of the nucleoli and that some nucleoporins also localize to the centrosome and the spindle poles. In addition, we could show that, during mitosis, the central FG protein NUP62, two inner ring components and Gle1 depart from the NPCs, while all other tested NUPs remained at the NE. This leads to the conclusion that indeed a partial disassembly of the NPCs takes place, which contributes to permeabilisation of the NE during semi-closed mitosis.
Highlights
The nucleus of Eukaryotes is surrounded by the nuclear envelope (NE)
We observed that the nuclear pore complexes (NPCs) are absent from the contact area of the nucleoli and that some nucleoporins localize to the centrosome and the spindle poles
We learned that the NPCs are excluded from the regions where the nucleoli are attached to the NE, but the mechanism behind this has yet to be elucidated
Summary
The nucleus of Eukaryotes is surrounded by the nuclear envelope (NE) It consists of two lipid bilayer membranes, the outer one being continuous with the endoplasmic reticulum. Prior to spindle assembly, nuclear envelope break down (NEBD) takes place in most animal cells [1]. There is no nuclear envelope break down in Dictyostelium, the nuclear lamina as well as several membrane proteins remain present at the NE throughout mitosis [3]. In order to assemble a mitotic spindle, the centrosome, which is situated outside of the nucleus in interphase and is tightly attached to the membrane, has to be inserted into the NE. The nuclear pore complexes (NPCs) are partially disassembled and provide a channel for free diffusion of proteins through the NE [7]
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