INTEGRAL MEMBRANE-PROTEINS OF THE NUCLEAR-ENVELOPE ARE DISPERSED THROUGHOUT THE ENDOPLASMIC-RETICULUM DURING MITOSIS

We have analyzed the fate of several integral membrane proteins of the nuclear envelope during mitosis in cultured mammalian cells to determ ine whether nuclear membrane proteins are present in a vesicle populat ion distinct from bulk ER membranes after mitotic nuclear envelope dis assembly or are dispersed throughout the ER. Using immunofluorescence staining and confocal microscopy, we compared the localization of two inner nuclear membrane proteins (lamina-associated 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 dye s (DiOC(6) and R6) and polyclonal antibodies. We found that at the res olution of this technique, the three nuclear envelope markers become c ompletely dispersed throughout ER membranes during mitosis. In agreeme nt with these results, we detected LAP1 in most membranes containing E R markers bi immunogold electron microscopy of metaphase cells. Togeth er, these findings indicate that nuclear membranes lose their identity as a subcompartment of the ER during mitosis. We found that nuclear l amins begin to reassemble around chromosomes at the end of mitosis at the same time as LAP1 and LAP2 and propose that reassembly of the nucl ear envelope at the end of mitosis involves sorting of integral membra ne proteins to chromosome surfaces by binding interactions with lamins and chromatin.