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Department of Cell and Molecular Biology, The Scripps Research Institute, La Jolla, California 92037
We have analyzed the fate of several integral
membrane proteins of the nuclear envelope during mitosis in cultured mammalian cells to determine whether
nuclear membrane proteins are present in a vesicle
population distinct from bulk ER membranes after mitotic nuclear envelope disassembly or are dispersed
throughout the ER. Using immunofluorescence staining and confocal microscopy, we compared the localization of two inner nuclear membrane proteins (laminaassociated polypeptides 1 and 2 [LAP1 and LAP2]) and
a nuclear pore membrane protein (gp210) to the distribution of bulk ER membranes, which was determined
with lipid dyes (DiOC6 and R6) and polyclonal antibodies. We found that at the resolution of this technique, the three nuclear envelope markers become
completely dispersed throughout ER membranes during mitosis. In agreement with these results, we detected LAP1 in most membranes containing ER markers by immunogold electron microscopy of metaphase
cells. Together, these findings indicate that nuclear
membranes lose their identity as a subcompartment of
the ER during mitosis. We found that nuclear lamins
begin to reassemble around chromosomes at the end of
mitosis at the same time as LAP1 and LAP2 and propose that reassembly of the nuclear envelope at the end
of mitosis involves sorting of integral membrane proteins to chromosome surfaces by binding interactions
with lamins and chromatin.
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