Vimentin-associated mitotic vesicles interact with chromosomes in a lamin B- and phosphorylation-dependent manner

We have assessed the involvement of the nuclear lamins in nuclear envelope reassembly. Analysis of perforated mitotic cells shows that A-type lamins are partly cytosolic and partly chromosome-bound, whereas B-type lamins are associated with vesicular structures throughout cell division. Lamin B-containing vesicles appear to dock on vimentin intermediate filaments during prometaphase, but dissociate from the cytoskeleton and assemble around chromatin at later phases of mitosis. Mitotic vesicles isolated from prometaphase cells en bloc with vimentin filaments can specifically capture chromosomes. Efficient chromosome capturing requires cytosolic factors and a dephosphorylating environment. Urea-stripping of the vesicles abolishes binding to chromosomes. However, reconstitution of the stripped membranes with purified B-type lamins restores their ability to bind to chromosomes in a cytosol- and dephosphorylation-dependent fashion. Vesicles reconstituted with B-type lamins form membraneous 'crescents' on the surfaces of chromosomes, but, unlike native vesicles, do not fuse into large sheets. From these observations we conclude that the initial targeting of mitotic vesicles to chromosomes is dependent on B-type lamins and on factors present in the mitotic cytoplasm. Apparently, further recruitment of membranes and fusion of chromosome-bound vesicles onto chromatin involves non-lamin peripheral membrane proteins.