VIMENTIN-ASSOCIATED MITOTIC VESICLES INTERACT WITH CHROMOSOMES IN A LAMIN B-DEPENDENT AND PHOSPHORYLATION-DEPENDENT MANNER

We have assessed the involvement of the nuclear lamins in nuclear enve lope reassembly, Analysis of perforated mitotic cells shows that A-typ e lamins are partly cytosolic and partly chromosome-bound, whereas B-t ype lamins are associated with vesicular stuctures throughout cell div ision, Lamin B-containing vesicles appear to dock on vimentin intermed iate filaments during prometaphase, but dissociate from the cyto-skele ton and assemble around chromatin at later phases of mitosis, Mitotic vesicles isolated from prometaphase cells en bloc with vimentin filame nts can specifically capture chromosomes, Efficient chromosome capturi ng requires cytosolic factors and a dephosphorylating environment, Ure a-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 dephos phorylation-dependent fashion. Vesicles reconstituted with B-type lami ns form membraneous 'crescents' on the surfaces of chromosomes, but, u nlike native vesicles, do not fuse into large sheets. From these obser vations we conclude that the initial targeting of mitotic vesicles to chromosomes is dependent on B-type lamins and on factors present in th e mitotic cytoplasm, apparently, further recruitment of membranes and fusion of chromosome-bound vesicles onto chromatin involves non-lamin peripheral membrane proteins.