REGULATED DOCKING OF NUCLEAR-MEMBRANE VESICLES TO VIMENTIN FILAMENTS DURING MITOSIS

During mitosis, several types of intermediate-sized filaments (IFs) un dergo an extensive remodelling in response to phosphorylation by cdc 2 and other protein kinases. However, unlike the nuclear lamins, the cy toplasmic IFs do not seem to follow a fixed disassembly stereotype and often retain their physical continuity without depolymerizing into so luble subunits. To investigate potential interactions between mitotica lly modified IFs and other cellular structures, we have examined prome taphase-arrested cells expressing the IF protein vimentin. We demonstr ate here that vimentin filaments associate in situ and co-fractionate with a distinct population of mitotic vesicles. These vesicles carry o n their surfaces nuclear lamin B, the inner nuclear membrane protein p 58, and wheat germ agglutinin (WGA)-binding proteins. Consistent with a tight interaction between the IFs and the mitotic membranes, vimenti n, nuclear lamin B, and a 180-kD WGA-binding protein are co-isolated w hen whole mitotic homogenates are incubated with anti-vimentin or anti -lamin B antibodies immobilized on magnetic beads. The vimentin-associ ated vesicles are essentially depleted of ER, Golgi and endosomal memb rane proteins. The interaction of vimentin with lamin B-carrying membr anes depends on phosphorylation and is weakened by dephosphorylation d uring nuclear reassembly in vitro. These observations reveal a novel i nteraction between IFs and cellular membranes and further suggest that the vimentin filaments may serve as a transient docking site for inne r nuclear membrane vesicles during mitosis.