The interaction of two classes of nuclear vesicles is induced by the dissociation of soluble proteins from one class of vesicles

Abstract

The regulatory mechanism of fusion steps in nuclear envelope assembly was studied in vitro using cell-free extracts of Xenopus eggs. The nuclear envelope is formed only around the chromatin at the end of mitosis. Two kinds of vesicles are required for nuclear envelope assembly (J. Cell Biol. 112 (1991) 545). Light vesicles (LVs) have the chromatin-binding activity, but cannot fuse solely. On the other hand, heavy vesicles (HVs) neither bind to the chromatin nor LVs, but are needed for the fusion event. Therefore, the association of HVs with LVs that bind to the chromatin is required at a first step during the fusion of vesicles. We found that salt-treated HVs inhibited the binding of LVs to the chromatin. In addition, when salt-treated HVs were pretreated with the proteins in the residue of salt-treatment of HVs, LVs recovered the chromatin-binding activity. In contrast, salt-treatment of LVs did not influence the binding of LVs with chromatin. By using a fluorescence microscopy assay, we showed directly that salt-treated HVs associated with LVs or salt-treated LVs, but HVs did not. These results suggest that soluble mask proteins on HVs keep the two distinct vesicles separate. We propose that the dissociation of mask protein on HVs allows HVs to bind with LVs that are located on the chromatin, subsequently the fusion of each vesicle occurs and the nuclear envelope is formed.

Lamin B receptor plays a role in stimulating nuclear envelope production and targeting membrane vesicles to chromatin during nuclear envelope assembly through direct interaction with importin beta

“…Therefore, we propose that, in mediating NE precursor vesicles to bind to chromatin, LBR may first bind to a linker protein to mediate the membrane-chromatin attachment, as it was reported that the LBR-containing NE precursor vesicles can not bind directly to chromatin at the onset of nuclear assembly (Drummnond et al, 1999;Oke and Inoue, 2003). We propose that this linker protein is importin ␤, which is also very important for NE precursor vesicle recruitment (Zhang et al, 2002b).…”

Assembly of nuclear pore complexes mediated by major vault protein

“…Functionally distinct membrane fractions with chromatin targeting, membrane fusion and NPC-inducing activities have also been described by others (Vigers and Lohka, 1991;Sasagawa et al, 1999;Oke and Inoue, 2003). However, in contrast to our present results, formation of intact NEs with NPCs required the simultaneous presence of these membrane fractions in the nuclear reconstitution assays.…”